FOOTNOTES:

FOOTNOTES:[1]Wilhelm Roux inZeitschrift für Biologie, vol. xxi. (1885):Zür Orientirung ueber einige Probleme der Embryonalen Entwicklung.[2]See Weismann'sCollected Essays, Clarendon Press (2nd edit.), vol. i., 1891, vol. ii., 1892; and Weismann'sGermplasm, Walter Scott's Contemporary Science Series, 1893. The references in this translation are to the latter volume.[3]The Germplasm, p. 137.[4]Ibid., p. 138.[5]The ideas expressed in this book may be found, in an elementary condition, in various publications of my own, and written in conjunction with my brother, Richard Hertwig: Oscar and Richard Hertwig,Die Actinien; Jena, 1879 (pp. 203-217). Oscar Hertwig,Das Problem der Befruchtung und der Isotropie des Eies, eine Theorie der Vererbung; Jena, 1884. Oscar Hertwig,Vergleich der Ei- und Samenbildung bei Nematoden, Arch. f. Mikrosk. Anatomie, vol. xxxvi., 1890, pp. 77-128. Oscar Hertwig,Urmund und Spina bifida, Arch. f. Mikrosk. Anatomie, vol. xxxix., 1892, pp. 476-492. Oscar Hertwig,Aeltere und neuere Entwicklungstheorien; Berlin, 1892. Oscar Hertwig,The Cell: Sonnenschein; London, 1895. Oscar Hertwig,Ueber den Werth der ersten Furchungszellen für die Organbildung des Embryo, Arch. f. Mikrosk. Anatomie, vol. xlii., 1893. The chief other writers to whom I refer are: Herbert Spencer,Principles of Biology. Darwin,Pangenesis, a Provisional Hypothesis (in Variation of Plants and Animals under Domestication). Haeckel,Die Perigenesis der Plastidule. Weismann,loc. cit., p. 8. Naegeli,Mechanisch-physiologische Theorie der Abstammungslehre; München, 1884. Strasburger,Neue Untersuchungen ueber den Befruchtungsvorgang bei den Phanerogamen als Grundlage für eine Theorie der Zeugung, 1884. H. de Vries,Intracellulare Pangenesis. W. His,Unsere Körperform und das physiologische Problem ihrer Entstehung, 1874. W. Roux,loc. cit., p. 6. Driesch,loc. cit., p. 48.[6]An English translation,The Cell, was published by Swan Sonnenschein and Co. in 1895.

[1]Wilhelm Roux inZeitschrift für Biologie, vol. xxi. (1885):Zür Orientirung ueber einige Probleme der Embryonalen Entwicklung.

[2]See Weismann'sCollected Essays, Clarendon Press (2nd edit.), vol. i., 1891, vol. ii., 1892; and Weismann'sGermplasm, Walter Scott's Contemporary Science Series, 1893. The references in this translation are to the latter volume.

[3]The Germplasm, p. 137.

[4]Ibid., p. 138.

[5]The ideas expressed in this book may be found, in an elementary condition, in various publications of my own, and written in conjunction with my brother, Richard Hertwig: Oscar and Richard Hertwig,Die Actinien; Jena, 1879 (pp. 203-217). Oscar Hertwig,Das Problem der Befruchtung und der Isotropie des Eies, eine Theorie der Vererbung; Jena, 1884. Oscar Hertwig,Vergleich der Ei- und Samenbildung bei Nematoden, Arch. f. Mikrosk. Anatomie, vol. xxxvi., 1890, pp. 77-128. Oscar Hertwig,Urmund und Spina bifida, Arch. f. Mikrosk. Anatomie, vol. xxxix., 1892, pp. 476-492. Oscar Hertwig,Aeltere und neuere Entwicklungstheorien; Berlin, 1892. Oscar Hertwig,The Cell: Sonnenschein; London, 1895. Oscar Hertwig,Ueber den Werth der ersten Furchungszellen für die Organbildung des Embryo, Arch. f. Mikrosk. Anatomie, vol. xlii., 1893. The chief other writers to whom I refer are: Herbert Spencer,Principles of Biology. Darwin,Pangenesis, a Provisional Hypothesis (in Variation of Plants and Animals under Domestication). Haeckel,Die Perigenesis der Plastidule. Weismann,loc. cit., p. 8. Naegeli,Mechanisch-physiologische Theorie der Abstammungslehre; München, 1884. Strasburger,Neue Untersuchungen ueber den Befruchtungsvorgang bei den Phanerogamen als Grundlage für eine Theorie der Zeugung, 1884. H. de Vries,Intracellulare Pangenesis. W. His,Unsere Körperform und das physiologische Problem ihrer Entstehung, 1874. W. Roux,loc. cit., p. 6. Driesch,loc. cit., p. 48.

[6]An English translation,The Cell, was published by Swan Sonnenschein and Co. in 1895.

As may be seen in his essays,On Life and Death,On the Duration of Life, etc., Weismann believes himself to have established a fundamental distinction between unicellular and multicellular organisms. Unicellular organisms (he would have it) do not undergo natural death, but, since they are able to reproduce themselves continuously by a process of simple division, are immortal. Multicellular organisms, on the other hand, must perish after a definite duration of life, and so are mortal. He makes an exception of the sexual cells, which, like unicellular organisms, are able to multiply indefinitely, and so are immortal. Thus Weismann came to make a distinction between the mortal (somatic) cells and the immortal (germ) cells of multicellular organisms. The latter he regarded as arising directly from the egg-cell, and never from somatic cells.

Nussbaum has given utterance to similar views, holding that the dividing egg at a very early period cleaves into the cells from which the individualgrows and the cells for the maintenance of the species. He has enunciated the proposition that, when the sexual cells have been separated from the cells of the young embryo, the material of the germ has been divided into shares for the individual and shares for the species; that the sexual cells take no part in the formation of the body, and that body-cells never give rise to ova or spermatozoa.

Weismann differs from Nussbaum in one important point. He lays no stress on the direct origin of the sexual cells, as cells, from the egg at the beginning of its development. He found, for instance, that, in the case of hydroids, the sexual cells did not arise in such a fashion. He considers, therefore, that the chain of events is as follows: The whole of the protoplasm of an egg-cell is not required to build up the new being, and the superfluous part remains unaltered to form the sexual cells of the new generation. Unlike Nussbaum, then, he asserts a continuity, not for the sexual cells, but for the germinal protoplasm which he believes to pass along definite cell-tracks until it forms the sexual cells. From this germinal protoplasm, which makes the germ-cells, he distinguishes the somatic protoplasm which makes the mortal, somatic cells.

The germplasm theory entered a new phase in the year 1885, after the independent appearance in 1884 of essays by Strasburger and by me, in which we gave reason for thinking that the cell nucleus was, as I expressed it, the bearer of the characters which were transmitted by parents to their offspring;that, in fact, the nucleus was the material basis of heredity.

Weismann laid hold of this idea, but transmuted it to fit in with his original theory of the germplasm. Shortly put, his view is as follows: The whole of the nuclear material is not hereditary material, but only a definite part is such, and this part, throughout the development of the individual, remains unaltered in composition, and finally becomes the starting-point for the generations to come. The remaining and greater part of the nuclear material does not remain in an unaltered condition. The layers of cells, first formed in the embryo, grow unlike each other, and give rise to different organs and tissues; Weismann draws the inference that the nuclear substance as well alters during the process of development, transforming itself in a regular, orderly fashion, until, finally, each different kind of cell in the whole body contains a specific nuclearplasm. This segregation and transformation begins with the process of cleavage itself, and thus 'the two daughter-cells that arise from the first cleavage of the egg-cell become different, so that the one contains all the hereditary characters for the ectoderm, the other for the endoderm. In further course the ectodermal nuclearplasm divides into that containing the primary germs of the nervous system, and that containing the similar constituents for the outer skin. By further cellular and nuclear divisions the inherited germs for the nervous system separate into those for the sense organs, those for the centralnervous system, and so forth, until there are separated the germs for all the separate organs, and for the production of the minutest histological differentiation.'

Weismann calls the diverging nuclearplasms into which the primitive germplasm is gradually transformedhistogenous, because they determine the specific characters of the tissues. He assumes that the primitive, original germplasm has a most complicated molecular structure, while the histogenous nuclearplasms for tissue-cells, like muscle-cells, nerve-cells, sense-cells, gland-cells, and so forth, have relatively simpler structures. As, during the growth of the embryo, the germplasm becomes transformed into the histogenous plasms, its molecular structure becomes simpler in proportion to the fewer different possibilities of development each separated portion of it comes to contain.

Following out this chain of ideas, Weismann attributes only to those cells which contain unaltered germplasm the power of giving rise to complete new individuals, while cells with histogenous nuclearplasm, whether these be embryonal cells or cells of the ectoderm or of the endoderm, he regards as having lost this capacity, because nuclearplasm of a simpler molecular structure cannot retransform itself into that with the more complicated structure. The further conclusion is necessary that a part of the nuclearplasm of the original nucleus of the fertilised egg-cell must remain unaltered throughout the various nuclear divisions, although it may be mingled with thenuclearplasms of certain series of cells. For these reasons, ova and spermatozoa can arise only when the germplasm which has been handed on from the original nucleus to certain cells is able to overcome the histogenous plasm of these cells. In this respect Weismann has amended his original proposition that the germ-cells were immortal, like unicellular organisms. In a strict and literal interpretation such a proposition would be incorrect, for the germ-cells are immortal only so far as they contain the germplasm, the immortal part of the organism.

In its further elaboration Weismann's conception was influenced considerably by publications of Naegeli, De Vries, and Wiesner. These dealt with the composition of the hereditary material, and they contained new hypotheses concerning the primary structure of the cell-body. Weismann avowedly accepted the suggestion of De Vries, who had rehabilitated and modernized Darwin's doctrine of pangenesis, according to which gemmules, small particles endowed with the power of division, were the material bearers of hereditary characters.

From these different sources Weismann has now worked out, in minutest detail, a theory to which he considers his former writings but as the preface; none the less, he has taken from his own writings the most essential and characteristic sequences of idea, in a fashion but slightly modified. Let me give the most important parts of his conception.

The substance which is the bearer of the hereditary character of a species (the idioplasm ofNaegeli) lies not in the general protoplasm of the ovum and spermatozoon, but in their nuclear matter (hypothesis of Hertwig and Strasburger). Weismann calls this the germplasm, so altering the previous connotation of the word. The germplasm of every species has an extremely complicated, stable architecture, an architecture that has been elaborated gradually in the course of past time. In this he distinguishes simple and complex component parts, the biophores, determinants, ids, and idants.

The biophores are his smallest material units, and to them are due the fundamental qualities of life—assimilation, metabolism, and reproduction by division. Thus, they correspond to Herbert Spencer's physiological units, Darwin's gemmules, De Vries' pangenes, and Hertwig's idioblasts. They are the bearers of the various characters of cells, and there are present in the germplasm a very large multitude of different kinds of them, corresponding to the number of cells with different characters.

The determinants are units of the rank next higher; they have qualities of their own, but are composed of groups of several kinds of biophores. They, too, have the power of division which is associated with, and comes about by, multiplication of the coherent company of biophores which lies within them.

The histological character of every cell in a multicellular organism is determined by a single determinant (cell-determinants). Weismann has framed his conception of determinants so as toavoid the supposition that every single cell is represented in the germplasm by its own biophores. There are small parts in the body in which the cells are all alike, and for these parts a single determinant suffices, afterwards multiplying by division. On the other hand, each cell or cell-group in the body, that is independently variable, must have its special determinant in the germplasm. And so the germplasm of a species must possess as many determinants, or guiding particles, as there are in the organism cells or cell-groups that are independently variable in the germ or in later stages (hereditary pieces or determinates).

As every cell or group of cells which corresponds to determinants has a definite position in the body, Weismann infers that the determinants are definitely placed in the germplasm, and form an ordered, complicated community. He has given the name id to these communities, which are higher units with definite constitution and with complicated architecture. Theseidsare bodies containing all the determinants necessary to build up the individual of a species, and correspond to what Weismann previously called ancestral plasms. Every id must be able to grow and multiply, for it is by their multiplication that the germplasm for new individuals is formed.

A singleidwould suffice for the conduct of a single life-history; Weismann, however, in the pursuit of a chain of thought connected with the relation of sexual reproduction to heredity, andwhich I shall not discuss here, regards the germplasm as being still more complicated, and consisting of many, sometimes more than a hundred, ancestral plasms orids, which have been derived from near or distant ancestors, the peculiarities of whose structures they retain, and may at some time actually produce (explanation of atavism).

But how does this fabric, endowed with an architecture so complicated, actually produce the development of the adult from the egg? The natural mechanism for this purpose is cell division and nuclear division.

According to Weismann's supposition—a supposition which forms, as we shall see, a chief corner-stone of his system—there are two kinds of nuclear division, the difference between which has not been observed, but is a corollary from the difference between their results. The one kind is denoted as integral, or doubling division; the other as differential, or differentiating division. The first method has only an incidental importance in Weismann's hypothesis: it consists of the doubling by growth of the rudiments, and of a perfectly fair division of them between the half-chromosomes; it occurs in tissues-cells, where parent-cells divide into daughter-cells exactly similar to each other and to their parents.

On the other hand, in differentiating division the rudiments become irregularly grouped during their growth; consequently, on division of the ids, which are composed of determinants, totally different combinations of the determinants are included in thedaughter-ids. This method of division of the germplasm plays the chief part in the transformation of the egg into the adult. It has to take place so that the numberless determinants, or guiding particles, of the germplasm may be disentangled and brought forward at the time and place necessary for them to guide the formations of thedeterminates, or independently variable parts of the adult body.

To take an example: Weismann's hypothesis requires that when the egg first divides into two, the germplasm should divide into two halves, each containing only one half of the total assemblage of determinants. In each subsequent cell-division this process of segregation is continued, so that the ids, as the phases of embryonic growth occur, contain more and more few different kinds of determinants. Supposing the germplasm to be composed of a million determinants at one stage, in the next it would contain only half a million, and in the next, again, only a quarter-million. In this manner the architecture of the ids becomes simpler and simpler, reaching the simplest conceivable condition in the active cells of the adult body. In these the germplasm consists only of the kind of determinants peculiar to the cells in which they lie; and these determinants are broken up intobiophores, or bearers of cell qualities.

'The disintegration of the germplasm,' says Weismann,[7]'is a wonderfully complicated process; it is a true "development," in which the idic stagesnecessarily follow one another in a regular order, and thus the thousands and hundreds of thousands of hereditary parts are gradually formed, each in its right place, and each provided with the proper determinants. The construction of the whole body, as well as its differentiation into parts, its segmentation, and the formation of its organs, and even the size of these organs—determined by the number of cells composing them—depend upon this complicated disintegration of the determinants in the id of germplasm. The transmission of characters of the most general kind—that is to say, those which determine the structure of an animal as well as those characterising the class, order, family, and genus to which it belongs—are due exclusively to this process.'

This mechanism of differentiating division fails to explain the phenomena of reproduction and of regeneration. For these Weismann has the following ancillary suppositions:

The first is the already-described hypothesis of continuity of the germplasm. As the disintegration of the germplasm into determinants, occurring in the development of an egg into an organism, is a process which cannot be retraced, and, as the future reproductive cells of the organism must contain undisintegrated, perfect germplasm, it follows that the germplasm in the germ-cells of the child must have come directly from the original germplasm of the parent. During the development, as Weismann assumes, only a few of the ids, each of which contains all the necessary germs, break up by differentiating divisioninto the determinants which control the course of the ontogeny, and decide the final characters of the cells. Another set of ids remains undisintegrated, with their determinants fast bound together, and, in the cell divisions, is not broken up into dissimilar groups. The first set of ids is the active, disintegrating germplasm; the second set is a passive, latent germplasm, which may be described as accessory germplasm (Nebenkeimplasma). The active ids are his explanation of the embryonic events, which they direct; the accessory germplasm is reserved to form the germ-cells, and, in fast-bound condition, is handed on through a short or long series of cell-divisions alongside the active germplasm. Handed on in this passive state, it finally reaches a group of cells which may be many or few generations distant from the original egg-cell, and impresses upon them the character of sexual cells. This transfer of germplasm from the egg to the sexual cell occurs in orderly fashion, along prescribed series of cells which Weismann has called the germ-tracks. Only these cells, which contain part of the perfect, undisintegrated germplasm, serve for the preservation of the species and are immortal; the other cells, since, from the disintegration due to differentiating division, they contain only fragments of the perfect plasm (groups of determinants or single determinants), are mortal, somatic cells.

The formation of buds is explained in much the same way as the origin of germ-cells. There is handed along from the egg, through prescribedseries of cells, a quantity of accessory, or bud, idioplasm.

The phenomena of alternation of generations require the supposition that in those animals and plants in which it occurs 'two kinds of germplasm exist, both of which always are present in the egg or in the bud, but of which one only is active at any time and rules the ontogeny, while the other remains inactive.' The alternating activity of these two produces the alternation of generations. So also dimorphism, which is exhibited most frequently as differences between the sexes, is explained by the assumption that 'double determinants' are present in the germplasm for all the cells, cell-groups, or entire organisms which have different characters in the male and female. One set of these double determinants remains latent, the other becomes active.

Finally, to explain the phenomena of regeneration, it is assumed that in the complicated cases where large parts of the body, like the head, the tail, or a bone, can be replaced after accidental loss, the cells with this power of regeneration contain, in addition to the determinants proper to them,supplementary determinants, which contain the germs needed for regeneration of the lost parts. These were handed on, during the ontogeny, through definite series of cells, in a passive condition, to become active when the conditions for their growth are supplied by the loss of the parts they can replace.

At first sight, much of Weismann's fabric of hypotheses gives the impression of being a closed system, thought out as a whole, and it has been treated as such in most of the notices and criticisms which I have seen. As a matter of fact, Weismann has spared no pains in the elaboration of his system, and has attempted to bring under his theory the many different phenomena of heredity and development, as well as alternation of generations, regeneration, atavism, and so forth. But, on the other hand, he has been careless in testing the stability and security of the foundations upon which he has built. It is on solid foundations that lie deep in the earth, and that avoid all reproach of being scamped or superficial work, that the durability of a structure depends. In this criticism the details of the superstructure will be disregarded, but the foundation will be tested thoroughly.

Cells and cell-properties are essential parts of Weismann's theory; while Naegeli has attempted to make his theory of the idioplasm independent of the whole conception of cells. In this matter I agree with Weismann, as, indeed, with De Vries and others, and I consider that the course taken by Naegeli has made his position untenable.

Naegeli would make his theory of the idioplasmquite independent of the theory of cells, because, while cells are important units in morphological structure, independently of this they cannot be regarded as important units. 'By a unit,' he insists, 'we must understand, in a physical sense, a system of material particles. In the organic world there are very many kinds of higher and lower units; vegetable and animal individuals, organs, tissues, groups of cells (in the vegetable kingdom, for instance, vessels and sieve-tubes), cells, parts of cells (plant cell-membranes, plasma, granules, and crystalloids, starch—grains, fat-globules, and so forth), micellæ, molecules, atoms. In morphology and physiology, sometimes one kind of unit, sometimes another, comes characteristically and notably into evidence. That being so, there is no reason why a special kind of unit should be exalted in a general theory.'

Although, with Naegeli, we must recognise and keep in view the presence of a large number of higher and lower units in the organic world, a fact upon which I shall lay considerable emphasis later, we must none the less recognise that, among all elementary units, cells are most the conspicuous, morphologically and physiologically, in the whole organic realm. In actual research this is avowed very practically, as a glance at the biological literature of the last thirty years will show. Especially in the study of heredity, the cell is a unit that cannot be neglected, for it has been established that spores, ova, and spermatozoa, the units by which species are preserved in reproduction, both in theanimal and in the vegetable kingdom, have the morphological value of cells.

In this point I am in opposition to Naegeli, although otherwise I agree with much in his conceptions.

A theory of heredity must be reconciled with the cell theory. In investigating Darwin's pangenesis, Galton's doctrine of the stirp, Naegeli's idioplasm, Weismann's germplasm, the intracellular pangenesis of De Vries, His' doctrinal of germinal foci for the formation of organs, or Roux's mosaic theory, I believe that one must face the question: How far do these doctrines agree with what we know about the structure and function of the cell? Moreover, in deciding between the alternatives—preformation and epigenesis—I believe that it will profit us to start our critical investigation with the cell itself. With this object, I shall now sum up in a few sentences as much of our present knowledge of the life of cells as, I believe, must be reckoned with in any theory of propagation.

The cell, which consists of protoplasm and a nucleus, is an elementary organism, that, by itself, or in combination with other cells, forms the basis of all animal and vegetable organisation. In minute structure it is so extraordinarily complicated that its essential constitution (its micellar or molecular structure) eludes our observation. It is a medley, composed of numerous, chemically distinct particles that may be divided into two groups, organised and unorganised. The latter are free, or in solution; they are such as albuminates, fats,carbo-hydrates, water, salts, and they serve as material for the nutrition and growth of the cell. The former make up the living cell body (in the narrow sense). They are able to multiply by growth and division, and they are therefore the elementary parts, units of life of lower rank, of which the cell, a unit of higher rank, is composed. They are the gemmules of Darwin, the physiological units of Spencer, the bioblasts of Altmann, the pangenes of De Vries, the plasomes of Wiesner, the idioblasts of Hertwig, and the biophores of Weismann.

The cells of every organic species possess a proper, specific organisation, more or less complicated, and, in correspondence with this, they are composed of more or less numerous and varied organised particles.

The nucleus is a special organ of cells, which is always present. It displays a collection of numerous, peculiar, elementary living units, the idioblasts. These show chemical, morphological, and functional differences from the plasomes, the living units of the protoplasm; but perhaps the idioblasts, by absorption of different material, may transform themselves into the plasomes, just as these last, by a similar process, may produce the plasma-products. In my view, the nucleus is the bearer of the idioplasm or hereditary material, that is to say, of a substance that is more stable than protoplasm, and, because it is less subject to influences of the outer world, it stamps its specific character upon the organism.[9]

A mass of protoplasm with several nuclei (like the myxomycetes, cœloblasts, etc.) has the morphological value of a number of cells (synergides), corresponding to the number of the nuclei.

The means by which the continuity of life is maintained is the capacity of the cell to manifold itself by division, so forming two or more separate pieces. The process, which in most cases is associated with complicated changes of the nuclear contents, appears essentially to consist of the following: The elementary units of the cell (centrosomes, chromatin bodies in the case of nuclear division), being endowed with special energy resulting from the processes of growth, divide, and the elementary products of division separate into two groups, which move from the middle line;upon this there follows a division of the general body of the cell,i.e., of the protoplasm and its contents.

From the point of view of cells, I believe myself compelled to raise several objections to most important bases of Weismann's germplasm theory. For convenience of exposition these may be divided into two groups: Objections to the hypothesis of differentiating division; objections to Weismann's doctrine of determinants.

A corner-stone of Weismann's theory is his assumption of nuclear divisions which are differentiating. Proof of this fundamental assumption may be sought in vain in Weismann's writings. Instead of that, a series of abstract arguments are brought forward in favour of it. Thus on p. 31 (of the English translation) Weismann treats the chromatin in the nucleus of the fertilised egg as the substance which accomplishes inheritance, and he denotes all the nuclei of the organism arising from the nucleus of the egg by divisions as the chromatin-tree, and then goes on to ask whether or no the pieces of hereditary material that make up the chromatin-tree of an organism are like each other or different. 'It can easily be shown,' the answer runs, 'that the latter must be the case.' For 'the chromatin is in a condition to impress the specific character on the cell in thenucleus of which it is contained. As the thousands of cells which constitute an organism possess very different properties, the chromatin which controls them cannot be uniform; it must be different in each kind of cell.'

Moreover, on p. 45 (of the English edition), 'The fact itself' (the capacity on the part of the idioplasm for regular and spontaneous change) 'is beyond doubt. When once it is established that the morphoplasm of each cell is controlled, and its character decided, by the idioplasm of the nucleus, the regular changes occurring in the egg-cell, and the products of its division in each embryogeny, must then be referred to the corresponding changes of the idioplasm.'

Finally, on p. 205 (of the English edition), 'The cells of the segmenting ovum are completely dissimilar as regards their hereditary value, although they are all young and embryonic, and are not infrequently quite similar in appearance. It therefore seems to me to follow from this, as a logical necessity, that the hereditary substance of the egg-cell, which contains all the hereditary tendencies of the species, does not transmit themin tototo the segmentation cells, but separates them into various combinations, and transmits them in groups to the cells. I have taken account of these facts in considering the regular distribution of the determinants of the germplasm, and the conversion of the latter into the idioplasm of the cells in the different stages of ontogeny.'

In the different propositions I have quoted, wehave to deal with what is merely a fallacy in rhetorical disguise. For, from the premiss that the chromatin has the power of impressing specific character upon the protoplasm of the cell, it by no means follows that two cells, distinguishable by the nature of their plasma-products, must therefore contain different kinds of protoplasm. There are other possibilities to be reckoned with. Weismann himself knows that there is no logical necessity for the conclusion, for he himself suggests another possibility in the following: 'If we wished to assume that the whole of the determinants of the germplasm are supplied to all the cells of the entogeny, we should have to suppose that differentiation of the body is due to all the determinants exceptoneparticular one remaining dormant in a regular order, and that, apart from special adaptations, only one determinant reaches the cell, viz., that which has to control it. If, however, we do make the assumption,' etc. (p. 63, English edition).

Here, then, Weismann himself points out that what in other places he has attempted to represent as a necessary conclusion is but one of two alternatives.

Not only does he grant the possibility of the alternative, but uses it himself in explanation of the phenomena of reproduction and development. He attributes to certain series of cells, in addition to the active rudiments controlling the normal characters of their protoplasm, the possession of numerous latent rudiments which become active when opportunity presents itself.

Thisnon sequiturin his argument Weismann excuses with the remark that the presence of latent rudiments in special cases 'depends, as I believe, upon special adaptations, and is not primitive, at any rate not in higher animals and plants. Why should Nature, who always manages with economy, indulge in the luxury of always providing all the cells of the body with the whole of the determinants of the germplasm, if a single kind of them is sufficient? Such an arrangement will presumably have occurred only in cases where it serves definite purposes' (p. 63, English edition). Here, again, is a rhetorical flourish instead of a proof.

But the dilemma which we are examining is not yet at an end. Supposing for the moment that we accept the assumption that different character in cells implies different character in their nuclear matter, we have at once a new and important decision to make. Does the nuclear matter in the different cells, that has arisen by division from the nuclear matter of the egg-cell, become unlike by the process of division itself? or is it only after the division that it becomes different, and in consequence of the action of outer forces upon the nuclei?

Weismann decides boldly—but again without bringing forward proof—in favour of the former interpretation. 'For the chromatin,' he remarks,[10]'cannotbecomedifferent in the cells of the fully formed organism; the differences in the chromatin controlling the cells must begin with the developmentof the egg-cell and must increase as development proceeds; for otherwise the different products of the division of the egg-cell could not give rise to entirely different hereditary tendencies. This is, however, the case.' Weismann represents to himself that[11]'the changes of the idioplasm depend on purely internal causes, which lie in the physical nature of the idioplasm. In obedience to these, a division of the nucleus accompanies each qualitative change in the idioplasm, in which process the different qualities are distributed between the two resulting halves of the chromatin rods.'

I shall proceed to show that this conception involves material difficulties and contradictions. It will be found that characters totally contradictory are ascribed to Weismann's idioplasm. On the one hand, it is credited with being a stable substance, possessing a coherent, complicated architecture; in the form of ancestral plasms it is supposed to be handed on, from one individual to another, unchanged through many generations; on the other hand there is ascribed to it a labile architecture, that allows a free and perpetual casting loose of rudiments, of such a kind that at each division there is caused a complete rearrangement and unequal division of these rudiments. In the one case, the inner forces produce a reciprocal, coherent bond between the numerous rudiments; in the other case, permit change of their position and relations to one another, and this not only once but in orderly, definite fashion, different in each of manysuccessive divisions, so that theidcomes to possess a completely altered architecture. 'Eachidin every stage' (p. 77 of the English edition), has its definitely inherited architecture; its structure is a complex, but a perfectly definite one, which, originating in theidof germplasm, is transferred by regular changes to the subsequent idic stages. The structure exhibited in all these stages exists potentially in the architecture of theidof germplasm: to this architecture is due, not only the regular distribution of the determinants—that is to say the entire construction of the body from its primary form.'

Unfortunately, Weismann's hypothesis tells us nothing at all about these internal causes, that depend upon the physical nature of the idioplasm; that is to say, nothing at all about the causes which, working in a fashion so contradictory and astonishing, really produce the whole development.

In such a state of affairs it is better to turn to Nature herself; and to see whether or no the occurrence of differentiating division of the nucleus in the organic world is at all supported by the actual observations and investigations of those who study cells.

We shall examine (1) Unicellular organisms; (2) Lower multicellular organisms; (3) The phenomena of generation and regeneration; (4) alteration of structural growth due to external interferences (heteromorphosis); (5) A number of physiological indications that cells and tissues, inaddition to their patent characters, contain latent characters which have reached them by doubling division, and which are representative of the species.

Doubling division alone exists, or could exist, among unicellular organisms. The maintenance of the species depends upon this. Our belief that a species produces only its own species, that like begets only like, a belief that finds continual confirmation all through the study of systematic and embryological natural history, would disappear, were it possible that in the division of unicellular organisms the hereditary mass should be split into two unequal components and be bestowed unequally upon the daughter-cells. All research shows that unicellular fungi, algæ, infusoria, and so forth, in dividing, transmit specific characters so strongly and in detail so minute that their descendants, a million generations off, resemble them in every respect. No one has doubted the fact, and Weismann himself recognises that division, among unicellular organisms, is always doubling. The process of division, as such, appears never to be the means by which new species are called into existence among unicellular organisms. This is a fundamental proposition of cell-life, not to be doubted, and to be taken into account in the presentation of theories of heredity.

From the proposition that like begets only likethe corollary by no means follows that mother- and daughter-cells must appear identical from the beginning. For the identity under consideration belongs only to the substance that is the bearer of specific characters, to the hereditary mass; besides that, a unicellular organism contains other substances, substances that change from time to time during its life. Many unicellular organisms pass through a regular series of developmental stages; the stages themselves being inherited, and following each other as infallibly as in the case of embryonic stages of higher animals.

The following will serve as examples of this.Podophrya gemmipara, an Acinetan, in the adult condition is attached by a long stalk, while the free end, at which is the mouth, is provided with suctorial tentacles. It reproduces by giving rise to many little buds, ciliated on the upper surface like free-swimming, hypotrichous infusoria. These, in appearance, are quite unlike the parent organism, and, after a vagrant existence in the water for some time, they attach themselves to a surface and produce a stalk, tentacles with suctorial pseudopodia, and so for the first time attain the maternal form.

Some Gregarines are large, jointed cells, divided into two pieces, a protomerite and a deutomerite; they are clad with a cuticle, under which lies a layer of muscular fibrils. After conjugation they encyst, the nucleus divides, and they break up into numerous peculiarly-shaped boat-like structures, (pseudonavicellæ), which afterwards are set free as small, sickle-shaped embryos. These exceedinglysmall germ-cells afterwards develop into the very different, adult gregarine-cells.

If the characters of a species be associated with a hereditary mass, an actual substance that is handed on from the parent-cell to the offspring, it is clear that the infusoria-like vagrant young of the Acinetan, and the sickle-shaped embryos of the Gregarine possess it, although for some time they are quite unlike the parent organism. For at last they become an Acinetan or a Gregarine, exactly like the parent-cell from which they arose as embryos.

These circumstances, among unicellular organisms, are a weighty indication of the error of concluding, with Weismann, in the case of multicellular forms, that because cells are unlike in outward appearance, the hereditary mass, or, as I call it, the nuclear matter, within them is also unlike. Such an assumption would involve us in the greatest contradictions. For the supposition that the nucleus is the hereditary mass transmitting the characters of the species necessitates the conclusion, in the case of unicellular forms, that the hereditary mass remains in possession of all the rudiments of the cell while it passes through the various phases of its cycle of development. Otherwise, these phases would have to be acquired anew in each case. We must, therefore, represent the possibilities of exchange between the nucleus, in its capacity of bearer of the hereditary mass, and the protoplasm as being such that all the rudiments are not simultaneously in activity, but that some of them can remain latent for a time.

Although in the development of unicellular organisms the way by which like begets like is plain and intelligible enough, at least in the cases dealt with, it is different with multicellular organisms, which have reached a higher grade of development. Among them we have to do with a continuous process of development, in which the highly-differentiated, multicellular organism arises from an egg, and in turn gives rise to an egg, and so on in unending sequence. But the succeeding stages of the sequence are so exceedingly dissimilar in appearance that the question how one step of the series turns into the next, and, above all, the question how the similarity of organisms, separated by the egg-stage, can be transmitted through the egg-stage, form the deepest riddle offered to biological investigation. Here, in a completeness so wonderful that our intelligence can hardly apprehend it, are presented to us the qualities of the organic material of which cells are made. Here lies that dark secret into which the various theories of generation try to direct a beam of light, and seek to find out the direction in which explanation may be found.

An intermediate stage which may serve towards the explanation of these circumstances is presented by the lower multicellular organisms, such as threadlike algæ, fungi, and other simple creatures. In them cells arise by division from the egg or from the spore, and become united into an individualof a higher rank; these cells resemble one another so completely in appearance and in qualities that there can be as little doubt as in the case of unicellular organisms that they arose by doubling division.

It is certain, then, that there exist multicellular bodies, often consisting of many thousand cells, in which each part retains the qualities of the egg from which it arose by doubling division, and which, as that method implies, possess the rudiments of the whole of which each is a part.

In this category there naturally fall the multinucleated masses of protoplasm, sometimes highly organised, in which every nucleus, surrounded by a shell of protoplasm, is capable of reproducing the whole. I am thinking of the slime-fungi (Myxomycetes), with their peculiar formation of reproductive bodies; of the 'acellular plants,' which in some cases closely resemble multicellular species in their formation of leaf and root, and in their mode of growth, as, for instance,Caulerpa, the multinucleatedForaminiferaandRadiolarians. For, according to our definition of the cell, a multinucleated organism potentially is a multicellular organism.

In this matter Weismann has assumed a position which leads to peculiar consequences. In his opinion, somatic cells and germ-cells were sharply distinct at their first appearance in evolution, and have remained so ever since. Transitional forms between them are nowhere to be found. It would be inconsistent with his theory of the germplasm had somatic cells contained germplasm as theiridioplasm, even when the soma first came into existence. The phyletic origin of the somatic cells depended directly upon an unequal separation of the determinants contained in the germplasm. It would totally contradict his presentation if the somatic cells, even at their first origin in phylogeny, contained, in addition to their patent special qualities, the qualities common to the whole species in a latent condition.

Weismann's conception, therefore, implies that many of the lower multicellular organisms, having no somatic-cells, have no body. Take the closely-allied creaturesPandorina morumandVolvox globator, which Weismann himself brings forward as instances for his view; the latter has a body, the former has no body, as all its cells are able to serve for reproduction!

It is enough to have pointed out how contradictory are the interpretations in this matter. Enlarging upon them may be postponed at present, for we are concerned here not with the interpretation of individual cases, but with the principles involved in the question, and, therefore, we must pass on to show further reason for considering the existence of differentiating division highly improbable in the whole organic world.

The numerous phenomena of reproduction and regeneration appear to support the principle ofdoubling division—that is, of division in which the germinal substance is handed on to every part of the organism. Our review may be short, as the phenomena are matters of common knowledge.

In nearly all plants there exist, widely spread through the body, cells and cell-groups, which may be induced, by inner or outer influences, to give rise to a bud; the bud grows out into a shoot, ultimately producing flowers and genital products. Such happens both in parts of the plant above the ground and below it; in the latter case shoots arise from roots, and reproduce the species in the ordinary sexual fashion by bearing sexual products.

Thus, in the case ofFunaria hygrometrica, a little moss, one may chop up the plant into tiny fragments, scatter these on damp earth, and see numerous moss-plants reproduced from the little groups of cells. By cutting little pieces from a willow, an experimenter may cause the production from slips of thousands of willow-trees, each with all the characters of the species, so that there must have been contained in each of the little pieces of tissue hereditary masses with the characters of the whole plant. Separate pieces of the leaves of many plants, as of the begonia, produce buds from which the whole plant may grow out.

An aptitude for reproduction like that in plants exists in many cœlenterates, worms, and tunicates. The polyps of hydroids and of bryozoa, the stolons of an ascidian (Clavellina lepadiformis), may give rise to buds in many places, and these grow up into the perfect hydroid, bryozoon, or ascidian.There must, then, be contained in the cells of the bud the germinal rudiments of the whole animal; this conclusion is more necessary as the individuals, produced from the buds, in due course bear sexual products.

Although in many higher animals and plants one sees that cells with the capacity for reproduction are limited to special areas, still, the capacity for regeneration often is very great. In a wonderful fashion animals will reproduce lost parts, sometimes of most complicated structure; just as a crystal, from which a corner has been chipped, will perfect itself again when brought into a solution of its own salt. AHydra, from which the oral disc and tentacles have been cut off, aNaisdeprived of its head or of its tail, a snail of which a tentacle with its terminal eye has been amputated, will reproduce the lost parts, sometimes in a very short time. The cells lying at the wounded spot begin to bud, producing a layer or lump, the cells of which resemble embryonic cells. From this embryonic mass of cells the lost organs and tissues arise—inHydra, the oral disc with its tentacles; inNais, the anterior end with its sense-organs and special groups of muscles; in the snail, the tentacle with its compound eye built up of elements so different as retinal-rods, pigment-cells, nerve-cells, lens, and so forth.

Even among vertebrates, in which the capacity for regeneration is the least, as in the restoration of the wounded parts small defects occur, lizards can reproduce a lost tail, tritons an amputatedlimb. From a bud of embryonic tissue there are elaborated in the one case whole vertebræ, with their muscles and tendons, and part of the spinal cord with its ganglia and nerves, in the other case, the numerous, differently-shaped, skeletal pieces of the hand or foot, with their appropriate muscles and nerves. The regeneration, moreover, is in strict conformity with the characters of the species concerned. Thus, from the facts of regeneration also, we must infer that cells in the vicinity of these casual wounds possess not only the special qualities which they possess as definite parts of a definite whole, but also the characters of the whole, and thus have the power of becoming buds, from which a complicated part of the body may be reproduced with the appropriate characters of the species.

Of all the facts brought forward here, the phenomena of heteromorphosis perhaps bear most strongly in favour of my conception, and offerdifficulties most irreconcilable with Weismann's theory.

Loeb uses the word 'heteromorphosis' to denote the ability possessed by organisms, under the stimulus of external forces, to produce organs on parts of the organism where such do not occur normally, or the power to replace lost parts by parts unsimilar to them in form and function. Regeneration is the reproduction of parts like those lost; heteromorphosis is the reproduction of parts unlike those lost.

Heteromorphoses are well known in plant physiology. When one cuts a slip from a willow, one may make the cut at the bottom of the slip and the cut at the top in any part of the willow-twig, yet still the lower end of the slip always produces rootlets, which are organs not normal to that part of the twig, while shoots will rise from the upper end. Moreover, either end of the slip may be made the root portion, and it is clear, therefore, that in every small area there are cell-groups present able to bear roots or shoots according to the determining conditions; and therefore that, in addition to the characters active at any time, there are present the germinal rudiments for shoots and roots, and, indeed, for the whole organism, since the shoots ultimately may bear genital products.

When the prothallus of a fern has developed normally, it is a flattened leaf-like structure which bears rootlets and male and female genital organs on the lower surface,i.e., on that turned from the light. But the experimenter may reverse thisorder, by artificially shading the upper surface, and strongly illuminating the lower surface.

Among the most interesting heteromorphoses are the galls, produced upon young plants when certain insects lay eggs on them, or when plant-lice irritate their tissues. From these abnormal stimuli there result active masses of cells which grow into organs of definite form and of complex structure. The galls, moreover, differ widely, in correspondence with the specific stimulus which was their initial cause, and with the specific substance, the stimulation of which resulted in the formation of a gall. By the action of different insects upon the same plant different galls are produced, and the galls of different plants may be distinguished systematically.

Blumenbach has already brought forward the existence of galls as an argument against preformation, holding them to be structures produced epigenetically, and, therefore, unrepresented by rudiments in the germ. I, also, consider them witnesses against Weismann's germplasm. They teach us that the cells of the plant-body may serve purposes quite different from those arranged for in the course of development; that cells modify their form in correspondence with novel conditions, and that they are forced into forming special structures, not by special determinants in the germ, but by external stimulants.

Galls exhibit yet another instructive kind of heteromorphosis.

Even the tissue of a leaf, turned into a gall bypathological conditions, retains the power of producing roots. Beyerinck has shown that galls ofSalix purpurea, planted in moist earth, bear rootlets identical with those of the normal plant. As the roots of all woody plants are able to bear adventitious buds, De Vries thinks it probable that one could rear a whole willow-tree from a gall. That would imply that all the inheritable characters of the willow were contained even in the gall.

Loeb has produced heteromorphoses experimentally upon many lower animals, among which wereTubularia,Cerianthus, andCione intestinalis.

InTubularia mesembryanthemum, a hydroid polyp, there are stalk, root, and polyp-head. If one cut off the head, a new head will be formed in a few days, this being a case of regeneration. On the other hand, a heteromorphosis may be produced by modifying the experiment as follows: Both root and head must be cut from the stem; if the lopped piece of the stem be stuck in the sand of the aquarium by the end that bore the head, then the original aboral pole in a few days produces a head; if the lopped piece of stem be supported horizontally in the water, then each end of it produces a head.

In aCerianthus membranaceus(Fig. 1), the body was opened by a cut some distance below the mouth, whereupon buds appeared on the lower edge of the slit, where the experimenter had prevented coalescent growth. These buds gave rise to inner and outer tentacles, and an oral disc was produced. Thus, artificially, an animal with two mouth-openings or two heads was produced; and, similarly, animals with a row of three or more heads may be produced.

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