Evolutio and Epigenesis in the old Sense
The organism is a specific body, built up by a typical combination of specific and different parts. It is implied in the words of this definition, that the organism is different, not only from crystals, as was mentioned in the last lecture, but also from all combinations of crystals, such as those called dendrites and others, which consist of a typical arrangement of identical units, the nature of their combination depending on the forces of every single one of their parts. For this reason dendrites, in spite of the typical features in their combination, must be called aggregates; but the organism is not an aggregate even from the most superficial point of view.
We have said before, what must have been familiar to you already, that the organism is not always the same in its individual life, that it has its development, leading from simpler to more complicated forms of combination of parts; there is a “production of visible manifoldness” carried out during development, to describe the chief character of thatprocess in the words of Wilhelm Roux. We leave it an open question in our present merely descriptive analysis, whether there was already a “manifoldness,” in an invisible state, before development, or whether the phrase “production of manifoldness” is to be understood in an absolute sense.
It has not always been granted in the history of biology, and of embryology especially, that production of visible manifoldness is the chief feature of what is called an organism’s embryology or ontogeny: the eighteenth century is full of determined scientific battles over the question. One school, with Albert von Haller and Bonnet as its leading men, maintained the view that there was no production of different parts at all in development, this process being a mere “evolutio,” that is, a growth of parts already existing from the beginning, yes, from the very beginning of life; whilst the other school, with C. F. Wolff and Blumenbach at its head, supported the opposite doctrine of so-called “epigenesis,” which has been proved to be the right one.
To some extent these differences of opinion were only the outcome of the rather imperfect state of the optical instruments of that period. But there were also deeper reasons beyond mere difficulties of description; there were theoretical convictions underlying them. It isimpossible, said the one party, that there is any real production of new parts; theremustbe such a production, said the other.
We ourselves shall have to deal with these questions of the theory of organic development; but at present our object is narrower, and merely descriptive. It certainly is of great importance to understand most clearly that there actuallyisa “production of visible manifoldness” duringontogenesis in the descriptive sense; the knowledge of the fact of this process must be the very foundation of all studies on the theory of development in any case, and therefore we shall devote this whole lecture to studies in merely descriptive embryology.
But descriptive embryology, even if it is to serve merely as an instance of the universality of the fact of epigenesis, can only be studied successfully with reference to a concrete case. We select the development of the common sea-urchin (Echinus microtuberculatus) as such a case, and we are the more entitled to select this organism rather than another, because most of the analytical experimental work, carried out in the interests of a real theory of development, has been done on the germs of this animal. Therefore, to know at least the outlines of the individual embryology of the Echinus may indeed be called theconditio sine qua nonfor a real understanding of what is to follow.
The Cell3
You are aware that all organisms consist of organs and that each of their organs has a different function: the brain, the liver, the eyes, the hands are types of organs in animals, as are the leaves and the pistils in plants.
You are also aware that, except in the lowest organisms, the so-called Protista, all organs are built up of cells. That is a simple fact of observation, and I therefore cannot agree with the common habit of giving to this plain fact the title of cell-“theory.” There is nothing theoretical in it; and,on the other hand, all attempts to conceive the organism as a mere aggregate of cells have proved to be wrong. It isthe wholethat uses the cells, as we shall see later on, or that may not use them: thus there is nothing like a “cell-theory,” even in a deeper meaning of the word.
The cell may have the most different forms: take a cell of the skin, of a muscle, of a gland, of the wood in plants as typical examples. But in every case two parts may be distinguished in a cell: an outside part, the protoplasm, and an inside part, the nucleus, to leave out of special account several others, which, by the way, may only be protoplasmatic modifications.
Protoplasm is a mere name for what is not the nucleus; in any case it is not a homogeneous chemical compound; it consists of many such compounds and has a sort of architecture; all organic functions are based upon its metabolism. The nucleus has a very typical structure, which stands in a close relation to its behaviour during the most characteristic morphological period of the cell: during its division. Let us devote a few words to a consideration of this division and the part the nucleus plays in it; it will directly bear on future theoretical considerations about development.
There is a certain substance in every nucleus of a cell which stains most markedly, whenever cells are treated with pigments: the name of “chromatin” has been given to it. The chromatin always gives the reaction of an acid, while protoplasm is basic; besides that it seems to be a centre of oxidation. Now, when a division of a cell is to occur, the chromatin, which had been diffusely distributed before, in the form of small grains, arranges itself into a long andvery much twisted thread. This thread breaks, as it were by sections, into almost equal parts, typical in number for each species, and each of these parts is split at full length.A certain number of pairs of small threads, the so-called “chromosomes,” are the ultimate result of this process, which intentionally has been described a little schematically, the breaking and the splitting in fact going on simultaneously or occasionally even in reverse order. While what we have described is performing in the nucleus, there have happened some typical modifications in protoplasm, and then, by an interaction of protoplasmatic and nuclear factors, the first step in the actual division of the cell begins. Of each pair of the small threads of chromatin one constituent is moved to one side of the cell, one to the other; two daughter-nuclei are formed in this way; the protoplasm itself at the same time forms a circular furrow between them; the furrow gets deeper and deeper; at last it cuts the cell in two, and the division of the cell is accomplished.
Not only is the growth of the already typically formed organism carried out by a series of cell-divisions, but also development proper in our sense, as a “production of visible manifoldness,” is realised to a great extent by the aid of such divisions, which therefore may indeed be said to be of very fundamental importance (Fig. 1).
Fig. 1.—Diagram of Cell-Division(afterBoveri).a.Resting cell; the chromatin distributed in the form of small granules inside the nucleus. Outside the nucleus is the “centrosome,” not mentioned in the text.b.Beginning of division; the chromatin arranged in the form of a long thread. Centrosome divided in two.c.The thread of chromatin cut into four parts, the “chromosomes.”d.The four parts of the chromatin arranged symmetrically between the centrosomes and the star-like “spheres.”e.Each of the chromosomes split at full length.f.Beginning of division of protoplasm; the two parts of each chromosome separated.g.End of cell-division.
Fig. 1.—Diagram of Cell-Division(afterBoveri).
Each cell-division which promotes growth is followed by the enlargement of the two daughter-cells which result from it; these two daughter-elements attain the exact size of the mother-cell before division, and as soon as this size is reached a new division begins: so the growth of the whole is in the main the result of the growth of the elements. Cell-divisions during real organ-formation may behave differently, as will be described at a proper occasion.
The Egg: its Maturation and Fertilisation
We know that all the organs of an animal or plant consist of cells, and we know what acts a cell can perform. Now there is one very important organ in all living beings, which is devoted to reproduction. This organ, the so-called ovary in animals, is also built up of cells, and its single cells are called the eggs; the eggs originated by cell-division, and cell-division is to lead from them to the new adult.
But, with a very few exceptions, the egg in the ovary is not able to accomplish its functions, unless certain typical events have occurred, some of which are of a merely preparatory kind, whilst the others are the actual stimulus for development.
The preparatory ones are generally known under the name of “maturation.” The egg must be “mature,” in order that it may begin development, or even that it may be stimulated to it. Maturation consists of a rather complicated series of phenomena: later on we shall have occasion to mention, at least shortly, what happens in the protoplasm during its course; as to the nuclear changes during maturation it may be enough for our purposes to say, that there occur certain processes among the chromosomes, which lead to an extension of half of them in the form of two very small cells, the “directive cells” or “directive or polar bodies,” as they have been somewhat cautiously called.
The ripe or mature egg is capable of being fertilised.
Before turning to this important fact, which, by the way, will bring us to our specially chosen type, the Echinus, a few words may be devoted to the phenomenon of“parthenogenesis,” that is to say, the possibility of development without fertilisation, since owing to the brilliant discoveries of the American physiologist, Jacques Loeb, this topic forms one of the centres of biological interest at present. It has long been known that the eggs of certain bees, lice, crayfishes, and other animals and also plants, are capable of development without fertilisation at all. Now Richard Hertwig and T. H. Morgan already had shown, that at least nuclear division may occur in the eggs of other forms—in the egg of the sea-urchin for instance—when these eggs are exposed to some chemical injuries. ButLoeb4succeeded in obtaining a full development by treating the eggs of echinoderms with chloride of magnesium; thus artificial parthenogenesis had been discovered. Later researches have shown that artificial parthenogenesis may occur in all classes of the animal kingdom and may be provoked by all sorts of chemical or physical means. We do not know at present in what the proper stimulus consists that must be supposed here to take the place of fertilisation; it seems, of course, highly probable that it is always the same in the lastresort.5
But enough about processes, which at present are of a highly scientific, but hardly of any philosophic interest.
By fertilisation proper we understand the joining of the male element, the spermatozoon or the spermia, with the female element, the egg. Like the egg, the spermatozoon is but a cell, though the two differ very much from one anotherin the relation between their protoplasm and nucleus: in all eggs it is the protoplasm which is comparatively very large, if held together with somatic cells, in the spermatozoon it is the nucleus. A large amount of reserve material, destined for the growth of the future being, is the chief cause of the size of the egg-protoplasm. The egg is quite or almost devoid of the faculty of movement, while on the contrary, movement is the most typical feature of the spermia. Its whole organisation is adapted to movement in the most characteristic manner: indeed, most spermatozoa resemble a swimming infusorium, of the type of Flagellata, a so-called head and a moving tail are their two chief constituents; the head is formed almost entirely of nuclear substance.
It seems that in most cases the spermatozoa swim around at random and that their union with the eggs is assured only by their enormous number; only in a few cases in plants have there been discovered special stimuli of a chemical nature, which attract the spermia to the egg.
But we cannot enter here more fully into the physiology of fertilisation, and shall only remark that its real significance is by no meansclear.6
The First Development Process of Echinus
Turning now definitively to the special kind of organism, chosen of our type, the common sea-urchin, we properlybegin with a few words about the absolute size of its eggs and spermatozoa. All of you are familiar with the eggs of birds and possibly of frogs; these are abnormally large eggs, on account of the very high amount of reserve material they contain. The almost spherical egg of our Echinus only measures about a tenth of a millimetre in diameter; and the head of the spermatozoon has a volume which is only the four-hundred-thousandth part of the volume of the egg! The egg is about on the extreme limit of what can be seen without optical instruments; it is visible as a small white point. But the number of eggs produced by a single female is enormous and may amount to hundreds of thousands; this is one of the properties which render the eggs of Echinus so very suitable for experimental research; you can obtain them whenever and in any quantity you like; and, moreover, they happen to be very clear and transparent, even in later stages, and to bear all kinds of operations well.
The spermia enters the egg, and it does so in the open water—another of the experimental advantages of our type. Only one spermia enters the egg in normal cases, and only its head goes in, the tail is left outside. The moment that the head has penetrated the protoplasm of the egg a thin membrane is formed by the latter. This membrane is very soft at first, becoming much stronger later on; it is very important for all experimental work, that by shaking the egg in the first minutes of its existence the membrane can easily be destroyed without any damage to the egg itself.
And now occurs the chief phenomenon of fertilisation: the nucleus of the spermatozoon unites with the nucleus of the egg. When speaking of maturation, we mentioned thathalf of the chromatin was thrown out of the egg by that process: now this half is brought in again, but comes from another individual.
It is from this phenomenon of nuclear union as the main character of fertilisation that almost all theories of heredity assume their right to regard the nuclei of the sexual cells as the true “seat” of inheritance. Later on we shall have occasion to discuss this hypothesis from the point of view of logic and fact.
After the complete union of what are called the male and the female “pronuclei,” the egg begins its development; and this development, in its first steps, is simply pure cell-division. We know already the chief points of this process, and need only add to what has been described, that in the whole first series of the cell-divisions of the egg, or, to use the technical term, in the whole process of the “cleavage” or “segmentation” of it, there is never any growth of the daughter-elements after each division, such as we know to occur after all cell-divisions of later embryological stages. So it happens, that during cleavage the embryonic cells become smaller and smaller, until a certain limit is reached; the sum of the volumes of all the cleavage cells together is equal to the volume of the egg.
But our future studies will require a more thorough knowledge of the cleavage of our Echinus; the experimental data we shall have to describe later on could hardly be properly understood without such knowledge. The first division plane, or, as we shall say, the first cleavage plane, divides the eggs into equal parts; the second lies at right angles to the first and again divides equally: we now have a ring of four cells. The third cleavage plane stands atright angles to the first two; it may be called an equatorial plane, if we compare the egg with a globe; it also divides equally, and so we now find two rings, each consisting of four cells, and one above the other. But now the cell-divisions cease to be equal, at least in one part of the egg: the next division, which leads from the eight- to the sixteen-cell stage of cleavage, forms four rings, of four cells each, out of the two rings of the eight-cell stage. Only in one half of the germ, in which we shall call the upper one, or which we might call, in comparison with a globe, the northern hemisphere, are cells of equal size to be found; in the lower half of the egg four very small cells have been formed at one “pole” of the whole germ. We call these cells the “micromeres,” that is, the “small parts,” on the analogy of the term “blastomeres,” that is, parts of the germ, which is applied to all the cleavage cells in general. The place occupied by the micromeres is of great importance to the germ as a whole: the first formation of real organs will start from this point later on. It is sufficient thus fully to have studied the cleavage of our Echinus up to this stage: the later cleavage stages may be mentioned more shortly. All the following divisions are into equal parts; there are no other micromeres formed, though, of course, the cells derived from the micromeres of the sixteen-cell stage always remain smaller than the rest. All the divisions are tangential; radial cleavages never occur, and therefore the process of cleavage ends at last in the formation of one layer of cells, which forms the surface of a sphere; it is especially by the rounding-up of each blastomere, after its individual appearance, that this real surface layer of cells is formed, but, of course, the condition, thatno radial divisions occur, is the most important one in its formation. When 808 blastomeres have come into existence the process of cleavage is finished; a sphere with a wall of cells and an empty interior is the result. That only 808 cells are formed, and not, as might be expected, 1024, is due to the fact that the micromeres divide less often than the other elements; but speaking roughly, of course, we may say that there are ten steps of cleavage-divisions in our form; 1024 being equal to 210.
We have learned that the first process of development, the cleavage, is carried out by simple cell-division. A few cases are known, in which cell-division during cleavage is accompanied by a specific migration of parts of the protoplasm in the interior of the blastomeres, especially in the first two or first four; but in almost all instances cleavage is as simple a process of mere division as it is in our sea-urchin. Now the second step in development, at least in our form, is a typical histological performance: it gives a new histological feature to all of the blastomeres: they acquire small cilia on their outer side and with these cilia the young germ is able to swim about after it has left its membrane. The germ may be called a “blastula” at this stage, as it was first called by Haeckel, whose useful denominations of the first embryonic stages may conveniently be applied, even if one does not agree with most, or perhaps almost all, of his speculations (Fig. 2).
Fig. 2.—Early Development of Echinus, the Common Sea-urchin.a.Two cells.b.Four cells.c.Eight cells, arranged in two rings of four, above one another.d.Sixteen cells, four “micromeres” formed at the “vegetative” pole.e.Optical section of the “blastula,” a hollow sphere consisting of about one thousand cells, each of them with a small cilium.
Fig. 2.—Early Development of Echinus, the Common Sea-urchin.
It is important to notice that the formation of the “blastula” from the last cleavage stage is certainly a process of organisation, and may also be called a differentiation with regard to that stage. But there is in the blastula no trace of onepartof the germ becomingdifferent with respect to others of its parts. If development were to go on in this direction alone, high organisatory complications might occur: but there would always be only one sort of cells, arranged in a sphere; there would be only one kind of what is called “tissue.”
But in fact development very soon loads to true differences of the parts of the germ with respect to one another, and the next step of the process will enable us to apply different denominations to the different parts of the embryo.
At one pole of the swimming blastula, exactly at the point where the descendants of the micromeres are situated,about fifty cells lose contact with their neighbours and leave the surface of the globe, being driven into the interior space of it. Not very much is known about the exact manner in which these changes of cellular arrangement are carried out, whether the cells are passively pressed by their neighbours, or whether, perhaps, in a more active manner, they change their surface conditions; therefore, as in most ontogenetic processes, the description had best be made cautiously in fairly neutral or figurative words.
The cells which in the above manner have entered the interior of the blastula are to be the foundation of important parts of the future organism; they are to form its connective tissue, many of its muscles, and the skeleton. “Mesenchyme,”i.e.“what has been infused into the other parts,” is the technical name usually applied to these cells. We now have to learn their definite arrangement. At first they lie as a sort of heap inside the cell wall of the blastula, inside the “blastoderm,”i.e.skin of the germ. But soon they move from one another, to form a ring round the pole at which they entered, and on this ring a process takes place which has a very important bearing upon the whole type of the organisation of the germ. You will have noticed that hitherto the germ with regard to its symmetry has been a monaxial or radial formation; the cleavage stages and the blastula with its mesenchyme were forms with two different poles, lying at the ends of one single line, and round this line everything was arranged concentrically. But now what is called “bilateral symmetry” is established; the mesenchyme ring assumes a structure which can be symmetrically divided only by one plane, but divided in such a way, that one-half of it is the mirror image of theother. A figure shows best what has occurred, and you will notice (Fig. 3) two masses of cells in this figure, which have the forms of spherical triangles: it is in the midst of these triangles that the skeleton of the larvaoriginates. The germ had an upper and a lower side before: it now has got an upper and lower, front and back,right and lefthalf; it now has acquired that symmetry of organisation which our own body has; at least it has got it as far as its mesenchyme is concerned.
Fig. 3.—Formation of Mesenchyme in Echinus.a.Outlines of blastula, side-view; mesenchyme forms a heap of cells at the “vegetative” pole.a1.Heap of mesenchyme-cells from above.b.Mesenchyme-cells arranged in a ring round the vegetative pole.c.Mesenchyme-cells arranged in a bilateral-symmetrical figure; primordia of skeleton in the midst of two spherical triangles.
Fig. 3.—Formation of Mesenchyme in Echinus.
We leave the mesenchyme for a while and study another kind of organogenesis. At the very same pole of the germ where the mesenchyme cells originated there is a long and narrow tube of cell growing in, and this tube, getting longer and longer, after a few hours of growth touches the opposite pole of the larva. The growth of this cellular tube marksthe beginning of the formation of the intestine, with all that is to be derived from it. The larva now is no longer a blastula, but receives the name of “gastrula” in Haeckel’s terminology; it is built up of the three “germ-layers” in this stage. The remaining part of the blastoderm is called “ectoderm,” or outer layer; the newly-formed tube, “endoderm,” or inner layer; while the third layer is the “mesenchyme” already known to us.
The endoderm itself is a radial structure at first, as was the whole germ in a former stage, but soon its free end bends and moves against one of the sides of the ectoderm, against that side of it where the two triangles of the mesenchyme are to be found also. Thus the endoderm has acquired bilateral symmetry just as the mesenchyme before, and as in this stage the ectoderm also assumes a bilateral symmetry in its form, corresponding with the symmetrical relations in the endoderm and the mesenchyme, we now may call the whole of our larva a bilateral-symmetrical organisation.
It cannot be our task to follow all the points of organogenesis of Echinus in detail. It must suffice to state briefly that ere long a second portion of the mesenchyme is formed in the larva, starting from the free end of its intestine tube; that the formation of the so-called “coelum” occurs by a sort of splitting off from this same original organ; and that the intestine itself is divided into three parts of different size and aspect by two circular sections.
But we must not, I think, dismiss the formation of the skeleton so quickly. I told you already that the skeleton has its first origin in the midst of the two triangularcell-masses of the mesenchyme; but what are the steps before it attains its typical and complicated structure? At the beginning a very small tetrahedron, consisting of carbonate of calcium, is formed in each of the triangles; the four edges of the tetrahedron are produced into thin rods, and by means of a different organogenesis along each of these rods the typical formation of the skeleton proceeds. But the manner in which it is carried out is very strange and peculiar. About thirty of the mesenchyme cells are occupied in the formation of skeleton substance on each side of the larva. They wander through the interior space of the gastrula—which at this stage is not filled with sea water but with a sort of gelatinous material—and wander in such a manner that they always come to the right places, where a part of the skeleton is to be formed; they form it by a process of secretion, quite unknown in detail; one of them forms one part, one the other, but what they form altogether, is one whole.
When the formation of the skeleton is accomplished, the typical larva of our Echinus is built up; it is called the “pluteus” (Fig. 4). Though it is far from being the perfect adult animal, it has an independent life of its own; it feeds and moves about and does not go through any important changes of form for weeks. But after a certain period of this species of independent life as a “larva,” the changes of form it undergoes again are most fundamental: it must be transformed into the adult sea-urchin, as all of you know. There are hundreds and hundreds of single operations of organogenesis to be accomplished before that end is reached; and perhaps the strangest of all these operations is a certain sort of growth, by which the symmetryof the animal, at least in certain of its parts—not in all of them—is changed again from bilateral to radial, just the opposite of what happened in the very early stages.
Fig. 4.—Larval Development of Echinus.A.The gastrula.B.Later stage, bilateral-symmetrical. Intestine begins to divide into three parts.C.Pluteus larva. S = Skeleton. I = Intestine.
Fig. 4.—Larval Development of Echinus.
But we cannot follow the embryology of our Echinus further here; and indeed we are the less obliged to do so, since in all our experimental work we shall have to deal with it only as far as to the pluteus larva. It is impossible under ordinary conditions to rear the germs up to the adult stages in captivity.
You now, I hope, will have a general idea at least of the processes of which the individual development of an animal consists. Of course the specific features leading from the egg to the adult are different in each specific case, and, inorder to make this point as clear as possible, I shall now add to our description a few words about what may be called a comparative descriptive embryology.
Comparative Embryology
Even the cleavage may present rather different aspects. There may be a compact blastula, not one surrounded by only one layer of cells as in Echinus; or bilaterality may be established as early as the cleavage stage—as in many worms and in ascidians—and not so late as in Echinus. The formation of the germ layers may go on in a different order and under very different conditions: a rather close relative of our Echinus, for instance, the starfish, forms first the endoderm and afterwards the mesenchyme. In many cases there is no tube of cells forming the “endoderm,” but a flat layer of cells is the first foundation of all the intestinal organs: so it is in all birds and in the cuttlefish. And, as all of you know, of course, there are very many animal forms which have no proper “larval” stage: there is one in the frog, the well-known “tadpole,” but the birds and mammals have no larvae; that is to say, there is no special stage in the ontogeny of these forms which leads an independent life for a certain time, as if it were a species by itself, but all the ontogenetical stages are properly “embryonic”—the germ is always an “embryo” until it becomes the perfect young organism. And you also know that not all skeletons consist of carbonate of calcium, but, that there are skeletons of silicates, as in Radiolaria, and of horny substance, as in many sponges. And, indeed, if we were to glance at the development of plants also, the differenceswould seem to us probably so great that all the similarities would seem to disappear.
But there are similarities, nevertheless, in all development, and we shall now proceed to examine what they are. As a matter of fact, it was especially for their sake that we studied the ontogeny of a special form in such detail; one always sees generalities better if one knows the specific features of at least one case. What then are the features of most general and far-reaching importance, which may be abstracted from the individual history of our sea-urchin, checked always by the teachings of other ontogenies, including those of plants?
The First Steps of Analytical Morphogenesis
If we look back upon the long fight of the schools of embryologists in the eighteenth century about the question whether individual development was to be regarded as a real production of visible manifoldness or as a simple growth of visibly pre-existing manifoldness, whether it was “epigenesis” or “evolutio,” there can be no doubt, if we rely on all the investigations of the last hundred and fifty years, that, taken in the descriptive sense, the theory of epigenesis is right. Descriptively speaking thereisa production of visible manifoldness in the course of embryology: that is our first and main result. Any one possessed of an average microscope may any day convince himself personally that it is true.
In fact, true epigenesis, in the descriptive sense of the term, does exist. One thing is formed “after” the other; there is not a mere “unfolding” of what existed already,though in a smaller form; there is no “evolutio” in the old meaning of the word.
The word “evolution” in English usually serves to denote the theory of descent, that is of a real relationship of all organisms. Of course we are not thinking here of this modern and specifically English meaning of the Latin wordevolutio. In its ancient sense it means to a certain degree just the opposite; it says that there is no formation of anything new, no transformation, but simply growth, and this is promoted not for the race but for the individual. Keeping well in mind these historical differences in the meaning of the word “evolutio,” no mistakes, it seems to me, can occur from its use. We now shall try to obtain a few more particular results from our descriptive study of morphogenesis, which are nevertheless of a general bearing, being real characteristics of organic individual development, and which, though not calculated of themselves to further the problem, will in any case serve to prepare for a more profound study of it.
The totality of the line of morphogenetic facts can easily be resolved into a great number of distinct processes. We propose to call these “elementary morphogenetic processes”; the turning in of the endoderm and its division into three typical parts are examples of them. If we give the name “elementary organs” to the distinct parts of every stage of ontogeny which are uniform in themselves and are each the result of one elementary process in our sense, we are entitled to say that each embryological stage consists of a certain number of elementary organs. The mesenchyme ring, the coelum, the middle-intestine, are instances of such organs. It is important to notice well that the word elementary isalways understood here with regard to visible morphogenesis proper and does not apply to what may be called elementary in the physiological sense. An elementary process in our sense is a very distinct act of form-building, and an elementary organ is the result of every one of such acts.
The elementary organs are typical with regard to their position and with regard to their histological properties. In many cases they are of a very clearly different histological type, as for instance, the cells of the three so-called germ-layers; and in other cases, though apparently almost identical histologically, they can be proved to be different by their different power of resisting injuries or by other means. But there are not as many different types of histological structure as there are typically placed organs: on the contrary there are many elementary organs of the same type in different typical parts of the organism, as all of you know to be the case with nerves and muscles. It will not be without importance for our future theory of development, carefully to notice this fact, that specialisation in thepositionof embryonic parts is more strict than in their histology.
But elementary organs are not only typical in position and histology, they are typical also with regard to their form and their relative size. It agrees with what has been said about histology being independent of typical position, that there may be a number of organs in an embryonic stage, all in their most typical positions, which though all possessing the same histology, may have different forms or different sizes or both: the single bones of the skeleton of vertebrates or of adult echinoderms are the very best instances of this most important feature of organogenesis. If we lookback from elementary organs to elementary processes, the specialisation of the size of those organs may also be said to be the consequence of a typical duration of the elementary morphogenetic process leading tothem.7
I hardly need to say, that the histology, form, and size of elementary organs are equally an expression of their present or future physiological function. At least they prepare for this function by a specific sort of metabolism which sets in very early.
The whole sequence of individual morphogenesis has been divided by some embryologists into two different periods; there is a first period, during which the foundations of the organisation of the “type” are laid down, and a second period, during which the histo-physiological specifications are modelled out (von Baer, Götte, Roux). Such a discrimination is certainly justified, if not taken too strictly; but its practical application would encounter certain difficulties in many larval forms, and also, of course, in all plants.
Our mention of plants leads us to the last of our analytical results. If an animal germ proceeds in its development from a stagedto the stageg, passing througheandf, we may say that the whole ofdhas become the whole off, but we cannot say that there is a certain part offwhich isd, we cannot say thatfisd+a. But in plants we can: the stagefis indeed equal toa+b+c+d+e+ain vegetable organisms; all earlier stages are actually visible as parts of the last one. The great embryologist, Carl Ernstvon Baer, most clearly appreciated these analytical differences between animal and vegetable morphogenesis. They become a little less marked if we remember that plants, in a certain respect, are not simple individuals but colonies, and that among the corals, hydroids, bryozoa, and ascidia, we find analogies to plants in the animal kingdom; but nevertheless the differences we have stated are not extinguished by such reasoning. It seems almost wholly due to the occurrence of so many foldings and bendings and migrations of cells and complexes of cells in animal morphogenesis, that an earlier stage of their development seemslostin the later one; those processes are almost entirely wanting in plants, even if we study their very first ontogenetic stages. If we say that almost all production of surfaces goes on outside in plants, inside in animals, we shall have adequately described the difference. And this feature again leads to the further diversity between animals and plants which is best expressed by calling the former “closed,” the latter “open” forms: animals reach a point where they are finished, plants never are finished, at least in most cases.
I hope you will allow that I have tried to draw from descriptive and comparative embryology as many general analytical results as are possibly to be obtained. It is not my fault if there are not any more, nor is it my fault if the results reached are not of the most satisfactory character. You may say that these results perhaps enable you to see a little more clearly and markedly than before a few of the characters of development, but that you have not really learnt anything new. Your disappointment—my own disappointment—in our analysis is due to the use of pure description and comparison as scientific methods.
The Limits of Pure Description in Science
We have analysed our descriptions as far as we could, and now we must confess that what we have found cannot be the last thing knowable about individual morphogenesis. There must be something deeper to be discovered: we only have been on the surface of the phenomena, we now want to get to the very bottom of them. Why then occurs all that folding, and bending, and histogenesis, and all the other processes we have described? There must be something that drives them out, so to say.
There is a very famous dictum in theTreatise on Mechanicsby the late Gustav Kirchhoff, that it is the task of mechanics to describe completely and in the most simple manner all the motions that occur in nature. These words, which may appear problematic even in mechanics, have had a really pernicious influence on biology. People were extremely pleased with them. “‘Describing’—that is just what we always have done,” they said; “now we see that we have done just what was right; a famous physicist has told us so.” They did not see that Kirchhoff had added the words “completely and in the most simple manner”; and moreover, they did not consider that Kirchhoff never regarded it as the ultimate aim of physics to describe thunderstorms or volcanic eruptions or denudations; yet it only is with such “descriptions” that biological descriptions ofgivenbodies and processes are to be compared!
Physicists always have used both experiment and hypothetical construction—Kirchhoff himself did so in the most gifted manner. With these aids they have gone through the whole of the phenomena, and what they found to be ultimateand truly elemental, that alone may they be said to have “described”; but they have “explained” by the aid of elementalities what proved to be not elemental initself.8
It is themethodof the physicists—not their results—that morphogenesis has to apply in order to make progress; and this method we shall begin to apply in our next lectures. Physiology proper has never been so short-sighted and self-satisfied as not to learn from other sciences, from which indeed there was very much to be learned; but morphology has: the bare describing and comparing of descriptions has been its only aim for about forty years or more, and lines of descent of a very problematic character were its only general results. It was not seen that science had to begin, not with problematic events of the past, but with what actually happens before our eyes.
But before saying any more about the exact rational and experimental method in morphology, which indeed may be regarded as a new method, since its prevalence in the eighteenth century had been really forgotten, we first shall have to analyse shortly some general attempts to understand morphogenesis by means of hypothetic construction exclusively. Such attempts have become very important as points of issue for really exact research, and, moreover, they deserve attention, because they prove that their authors at least had not quite forgotten that there were still other problems to be solved in morphology than only phylogenetical ones.
THE THEORY OF WEISMANN
Of all the purely hypothetic theories on morphogenesis that of AugustWeismann9can claim to have had the greatest influence, and to be at the same time the most logical and the most elaborated. The “germ-plasma” theory of the German author is generally considered as being a theory of heredity, and that is true inasmuch as problems of inheritance proper have been the starting-point of all his hypothetic speculations, and also form in some respect the most valuable part of them. But, rightly understood, Weismann’s theory consists of two independent parts, which relate to morphogenesis and to heredity separately, and it is only the first which we shall have to take into consideration at present; what is generally known as the doctrine of the “continuity of the germ-plasm” will be discussed in a later chapter.
Weismann assumes that a very complicated organised structure, below the limits of visibility even with thehighest optical powers, is the foundation of all morphogenetic processes, in such a way that, whilst part of this structure is handed over from generation to generation as the basis of heredity, another part of it is disintegrated during the individual development, and directs development by being disintegrated. The expression, “part” of the structure, first calls for some explanation. Weismann supposes several examples, several copies, as it were, of his structure to be present in the germ cells, and it is to these copies that the word “part” has been applied by us: at least one copy has to be disintegrated during ontogeny.
The morphogenetic structure is assumed to be present in the nucleus of the germ cells, and Weismann supposes the disintegration of his hypothetic structure to be accomplished by nuclear division. By the cleavage of the egg, the mostfundamentalparts of it are separated one from the other. The word “fundamental” must be understood as applying not to proper elements or complexes of elements of the organisation, but to the chief relations of symmetry; the first cleavage, for instance, may separate the right and the left part of the structure, the second one its upper and lower parts, and after the third or equatorial cleavage all the principal eighths of our minute organisation are divided off: for the minute organisation, it must now be added, had been supposed to be built up differently in the three directions of space, just as the adult organism is. Weismann concedes it to be absolutely unknown in what manner the proper relation between the parts of the disintegrated fundamental morphogenetic structure and the real processes of morphogenesis is realised; enough that there may be imagined such a relation.
At the end of organogenesis the structure is assumed to have been broken up into its elements, and these elements, which may be chemical compounds, determine the fate of the single cells of the adult organism.
Here let us pause for a moment. There cannot be any doubt that Weismann’s theory resembles to a very high degree the old “evolutio” doctrines of the eighteenth century, except that it is a little less crude. The chick itself is not supposed to be present in the hen’s egg before development, and ontogeny is not regarded as a mere growth of that chick in miniature, but what really is supposed to be present in the egg is nevertheless a something that in all its parts corresponds to all the parts of the chick, only under a somewhat different aspect, while all the relations of the parts of the one correspond to the relations of the parts of the other. Indeed, only on such an hypothesis of a fairly fixed and rigid relation between the parts of the morphogenetic structure could it be possible for the disintegration of the structure to go on, not by parts of organisation, but by parts of symmetry; which, indeed, is a very strange, but not an illogical, feature of Weismann’s doctrine.
Weismann is absolutely convinced that there must be a theory of “evolutio,” in the old sense of the word, to account for the ontogenetic facts; that “epigenesis” has its place only in descriptive embryology, where, indeed, as we know, manifoldness in thevisiblesense is produced, but that epigenesis can never form the foundation of a real morphogenetictheory: theoretically one pre-existing manifoldness is transformed into the other. An epigenetic theory would lead right beyond natural science, Weismannthinks, as in fact, all such theories, if fully worked out, have carried their authors to vitalistic views. But vitalism is regarded by him as dethroned for ever.
Under these circumstances we have a good right, it seems to me, to speak of adogmaticbasis of Weismann’s theory of development.
But to complete the outlines of the theory itself: Weismann was well aware that there were some grave difficulties attaching to his statements: all the facts of so-called adventitious morphogenesis in plants, of regeneration in animals, proved that the morphogenetic organisation could not be fully disintegrated during ontogeny. But these difficulties were not absolute: they could be overcome: indeed, Weismann assumes, that in certain specific cases—and he regarded all cases of restoration of a destroyed organisation as due to specific properties of the subjects, originated by roundabout variations and natural selection—that in specific cases, specific arrangements of minute parts were formed during the process of disintegration, and were surrendered to specific cells during development, from which regeneration or adventitious budding could originate if required. “Plasma of reserve” was the name bestowed on these hypothetic arrangements.
Almost independently another German author, WilhelmRoux,10has advocated a theoretical view of morphogenesis which very closely resembles the hypothesis of Weismann. According to Roux a minute ultimate structure is present in the nucleus of the germ and directs development by being divided into its parts during the series of nuclear divisions.
But in spite of this similarity of the outset, we enter analtogether different field of biological investigation on mentioning Roux’s name: we are leaving hypothetic construction, at least in its absoluteness, and are entering the realms of scientific experiment in morphology.
EXPERIMENTAL MORPHOLOGY
I have told you already in the last lecture that, while in the eighteenth century individual morphogenesis had formed the centre of biological interest and been studied experimentally in a thoroughly adequate manner, that interest gradually diminished, until at last the physiology of form as an exact separate science was almost wholly forgotten. At least that was the state of affairs as regards zoological biology; botanists, it must be granted, have never lost the historical continuity to such a degree; botany has never ceased to be regarded as one science and never was broken up into parts as zoology was. Zoological physiology and zoological morphology indeed were for many years in a relationship to one another not very much closer than the relation between philology and chemistry.
There were always a few men, of course, who strove against the current. The late WilhelmHis,11for instance, described the embryology of the chick in an original manner, in order to find out the mechanical relations of embryonic parts, by which passive deformation, as an integrating part of morphogenesis, might be induced. He also most clearly stated the ultimate aim of embryology to be the mathematical derivation of the adult form from the distribution of growth in the germ. To AlexanderGoette12we owe another set of analytical considerations about ontogeny. Newport, as early as 1850, and in later years Pflüger and Rauber, carried out experiments on the eggs of the frog, which may truly be called anticipatory of what was to follow. But it was WilhelmRoux,13now professor of anatomy at Halle, who entered the field with a thoroughly elaborated programme, who knew not only how to state the problem analytically, but also how to attack it, fully convinced of the importance of what he did. “Entwickelungsmechanik,”—mechanics of development—he called the “new branch of anatomical science” of which he tried to lay the foundations.
I cannot let this occasion pass without emphasising in the most decided manner how highly in my opinion Roux’s services to the systematic exploration of morphogenesis must be esteemed. I feel the more obliged to do so, because later on I shall have to contradict not only many of his positive statements but also most of his theoretical views. He himself has lately given up much of what he most strongly advocated only ten years ago. But Roux’s place in the history of biological science can never be altered, let science take what path it will.
It is not the place here to develop the logic of experiment; least of all is it necessary in the country of John Stuart Mill. All of you know that experiment, by its method of isolating the single constituents of complicated phenomena, is the principal aid in the discovery of so-called causal relations. Let us try then to see what causalrelations Wilhelm Roux established with the aid of morphogenetic experiment.
THE WORK OF WILHELM ROUX
We know already that an hypothesis about the foundation of individual development was his starting-point. Like Weismann he supposed that there exists a very complicated structure in the germ, and that nuclear division leads to the disintegration of that structure. He next tried to bring forward what might be called a number of indicia supporting his view.
A close relation had been found to exist in many cases between the direction of the first cleavage furrows of the germ and the direction of the chief planes of symmetry in the adult: the first cleavage, for instance, very often corresponds to the median plane, or stands at right angles to it. And in other instances, such as have been worked out into the doctrine of so-called “cell-lineages,” typical cleavage cells were found to correspond to typical organs. Was not that a strong support for a theory which regarded cellular division as the principal means of differentiation? It is true, the close relations between cleavage and symmetry did not exist in every case, but then there had always happened some specific experimental disturbances,e.g.influences of an abnormal direction of gravity on account of a turning over of the egg, and it was easy to reconcile such cases with the generally accepted theory on the assumption of what was called “anachronism” of cleavage.
But Roux was not satisfied with mere indicia, hewanted a proof, and with this intention he carried out an experiment which has become verycelebrated.14With a hot needle he killed one of the first two blastomeres of the frog’s egg after the full accomplishment of its first cleavage, and then watched the development of the surviving cell. A typical half-embryo was seen to emerge—an organism indeed, which was as much a half as if a fully formed embryo of a certain stage had been cut in two by a razor. It was especially in the anterior part of the embryo that its “halfness” could most clearly be demonstrated.
That seemed to be a proof of Weismann’s and Roux’s theory of development, a proof of the hypothesis that there is a very complicated structure which promotes ontogeny by its disintegration, carried out during the cell divisions of embryology by the aid of the process of nuclear division, the so-called “karyokinesis.”
To the dispassionate observer it will appear, I suppose, that the conclusions drawn by Roux from his experiment go a little beyond their legitimate length. Certainly some sort of “evolutio” is proved by rearing half the frog from half the egg. But is anything proved, is there anything discovered at all about the nucleus? It was only on account of the common opinion about the part it played in morphogenesis that the nucleus had been taken into consideration.
Things soon became still more ambiguous.
THE EXPERIMENTS ON THE EGG OF THE SEA-URCHIN
Roux’s results were published for the first time in 1888; three years later I tried to repeat his fundamentalexperiment on another subject and by a somewhat different method. It was known from the cytological researches of the brothers Hertwig and Boveri that the eggs of the common sea-urchin (Echinus microtuberculatus) are able to stand well all sorts of rough treatment, and that, in particular, when broken into pieces by shaking, their fragments will survive and continue to segment. I took advantage of these facts for my purposes. I shook the germs rather violently during their two-cell stage, and in several instances I succeeded in killing one of the blastomeres, while the other one was not damaged, or in separating the two blastomeres from oneanother.15
Let us now follow the development of the isolated surviving cell. It went through cleavage just as it would have done in contact with its sister-cell, and there occurred cleavage stages which were just half of the normal ones. The stage, for instance, which corresponded to the normal sixteen-cell stage, and which, of course, in my subjects was built up of eight elements only, showed two micromeres, two macromeres and four cells of medium size, exactly as if a normal sixteen-cell stage had been cut in two; and the form of the whole was that of a hemisphere. So far there was no divergence from Roux’s results.
The development of our Echinus proceeds rather rapidly, the cleavage being accomplished in about fifteen hours. I now noticed on the evening of the first day of the experiment, when the half-germ was composed of about two hundred elements, that the margin of the hemispherical germ bent together a little, as if it were about to form a whole sphere of smaller size, and, indeed, the next morning awholediminutiveblastula was swimming about. I was so much convinced that I should get Roux’s morphogenetical result in all its features that, even in spite of this whole blastula, I now expected that the next morning would reveal to me the half-organisation of my subject once more; the intestine, I supposed, might come out quite on one side of it, as a half-tube, and the mesenchyme ring might be a half one also.
But things turned out as they were bound to do and not as I had expected; there was a typicallywholegastrula on my dish the next morning, differing only by its small size from a normal one; and thissmall but wholegastrula was followed by a whole and typical small pluteus-larva (Fig. 5).