The first case of an “internal formative stimulus” in the proper sense, that is, of one embryonic part causing another to appear, was discovered by Herbst himself. The arms of the so-called pluteus of the sea-urchin are in formative dependence on the skeleton—no skeleton, no arms; so many skeletonprimordia,42in abnormal cases, so many arms; abnormal position of the skeleton, abnormal position of the arms: these three experimental observations form the proof of this morphogenetic relation.
It may be simple mechanical contact, or it may be some chemical influence that really constitutes the “stimulus” in this case; certainly, there exists a close and very specific relation of the localisation of one part of the embryo to another. Things are much the same in another case, which, after having been hypothetically stated by Herbst on the basis of pathological data, was proved experimentally by Spemann. The lens of the eye of certain Amphibia is formed of their skin in response to a formative stimulus proceeding from the so-called primary optic vesicle. If this vesicle fails to touch the skin, no lens appears; and, on the other hand, the lens may appear in quite abnormal parts of the skin if they come into contact with the optic vesicle after transplantation.
But formative dependence of parts may also be of different types.
We owe to Herbst the important discovery that the eyes of crayfishes, after being cut off, will be regenerated in the proper way, if the optic ganglion is present, but that an antenna will arise in their place if this ganglion has also been removed. There must in this case be some unknown influence of the formative kind on which depends, if not regeneration itself, at least its special character.
In other cases there seems to be an influence of the central nervous system on the regenerative power in general. Amphibia, for instance, are said to regenerate neither their legs (Wolff), nor their tail (Godlewski), if the nervous communications have been disturbed. But in other animals there is no such influence; and in yet others, as for instance, in Planarians, it must seem doubtful at present whether themorphogenetic influence of the nervous system upon processes of restoration is more than indirect; the movements of the animal, which become very much reduced by the extirpation of the ganglia, being one of the main conditions of a good regeneration.
Of course, all we have said about the importance of special materials in the ripe germ, as bearing on specifically localised organisations, might be discussed again in our present chapter, and our intimate polar-bilateral structure of germs may also be regarded as embracing formative stimuli, at any rate as far as the actual poles of this structure are concerned. This again would bring us to the problem of so-called “polarity” in general, and to the “inversion” of polarity, that is to a phenomenon well known in plants and in many hydroids and worms, viz., that morphogenetic processes, especially of the type of restitutions, occur differently, according as their point of origin represents, so to speak, the positive or the negative, the terminal or the basal end of an axis, but that under certain conditions the reverse may also be the case. But a fuller discussion of these important facts would lead us deeper and deeper into the science of morphogenesis proper, without being of much use for our future considerations.
And so we may close thissection43on formative stimulior “causes” of morphogenesis by shortly adding, more on account of its factual than of its logical interest, that the phenomenon of the determination ofsex,44according to the latest researches, seems to depend on cytological events occurring in the very earliest embryonic stages, say even before ontogeny, and not on formative stimuliproper45: it seems, indeed, as if the sexual products themselves would account for the sex of the individual produced by them, particularly if there were differences in theirchromatin.46
Let us now turn again to considerations of a more abstract kind: we have become acquainted with some morphogenetic interactions among the parts of a developing embryo; and, indeed, we can be sure that there exist far more of such interactions than we know at present.
But it is far from being true that the development of each embryonic part depends on the existence or development of every other one.
On the contrary, it is a very important and fundamental feature of organogenesis that it occurs in separate lines,that is to say, in lines of processes which may start from a common root, but which are absolutely independent of one another in their manner of differentiation. Roux has coined the term “self-differentiation” to denote this phenomenon, and we admit that this term may be conveniently used for the purpose, if only it can be kept in mind that its sense is always relative, and that it is also negative. Suppose a part,A, shows the phenomenon of self-differentiation: this means that the further development ofAis not dependent on certain other parts,B,C, andD; it doesnotmean at all thatAhas not been formatively dependent on some other parts,EorFat the time of its first appearance, nor does it imply that there might not be many formative actions among the constituents ofAitself.
We indeed are entitled to say that the ectoderm of Echinus shows “self-differentiation” with regard to the endoderm; it acquires its mouth, for instance, as has been shown by experiment, even in cases where no intestine is present at all (Fig. 10); but ectoderm and endoderm both are formatively dependent on the intimate and the material organisation of the blastoderm. It further seems from the most recent experiments that the nerves and the muscles of the vertebrates are independent of each other in their differentiation, but that their fate is probably determined by formative processes in the very earliest stages of ontogeny.
Fig. 10.—Pluteus-larva of Sphaerechinus.The Intestine (i) is developed outside instead of inside (by means of raising the temperature); but the mouth (r) is formed in its normal place. S = Skeleton.
Fig. 10.—Pluteus-larva of Sphaerechinus.
The Intestine (i) is developed outside instead of inside (by means of raising the temperature); but the mouth (r) is formed in its normal place. S = Skeleton.
The phenomenon of self-differentiation, properly understood, now may help to the discovery of one most general character of all development. If the phenomenon of self-differentiation really occurs in ontogeny in its most different aspects, and if, on the other hand, in spite of this relative morphogenetic independence of embryonic parts, the resulting organism is one whole in organisation and in function, some sort ofharmony of constellation, as it may properly be styled, must be said to be one of the most fundamental characters of all production of individual form. In establishing this harmony we do nothing more than describe exactly what happens: the harmony is shown by the fact that there is a whole organism at the end, in spite of the relative independence of the single events leading to it.
But still another sort of harmony is revealed in morphogenesis, by an analysis of the general conditions of the formative actions themselves. In order that these actions may go on properly the possibility must be guaranteed that the formative causes may always find something upon which to act, and that those parts which contain the potencies for the next ontogenetic stage may properly receive the stimuli awaking these potencies: otherwise there would be no typical production of form at all. This, the second species of harmonious relations to be described in ontogeny, may be calledcausal harmony; the term simply expresses theunfailing relative condition of formative causes and cause-recipients.
Finally, infunctional harmonywe have an expression descriptive of the unity of organic function, and so we may state, as the latest result of our analytical theory of development up to this point, that individual morphogenesis is marked by athreefold harmonyamong its parts.
At this stage we leave for a while our analytical studies of ontogeny proper. We must not forget that typical ontogenesis is not the only form in which morphogenesis can occur: the organic form is able to restore disturbances of its organisation, and it certainly is to be regarded as one of the chief problems of analytical morphogenesis to discover the specific and real stimulus which calls forth the restoring processes. For simply to say that the disturbance is the cause of the restoration would be to evade the problem instead of attacking it. But there are still some other problems peculiar to the doctrine of restitutions.
A few Remarks on Secondary Potencies and on Secondary Morphogenetic Regulations in General
We have only briefly mentioned in a previous chapter that there exist many kinds of potencies of what we call the secondary or truly restitutive type, and that their distribution may be most various and quite independentof all the potencies for the primary processes of ontogeny proper. Let us first add a few words about the concept of “secondary restitution” and about the distribution of secondary potencies in general.
Primary ontogenetic processes founded upon primary potencies mayimplyregulation, or more correctly, restitution in many cases: so it is, when fragments of the blastula form the whole organism, or when the mesenchyme cells of Echinus reach their normal final position by an attraction on the part of specific localities of the ectoderm in spite of a very abnormal original position enforced upon them by experiment. In these cases we speak of primary regulations or restitutions; disturbances are neutralised by the very nature of the process in question. We speak of secondary restitution whenever a disturbance of organisation is rectified by processes foreign to the realm of normality; and these abnormal lines of events are revealed to us in the first place by the activity of potencies which remain latent in ontogeny proper.
We know already that a certain kind of secondary restitution has been discovered lately, very contradictory to the theoretical views of Weismann; the process of restoration being carried out not by any definite part of the disturbed organisation, but by all the single elements of it. The problem of the distribution of secondary potencies in these cases of so-called “re-differentiation” is to form our special study in the next chapter. In all other cases restoration processes start from specific localities; if they occur on the site of the wound which caused the disturbance, we speak of regeneration; if they occur at some distance from the wound, we call them adventitiousprocesses. Besides these three types of processes of restitution there may be mentioned a fourth one, consisting in what is generally called compensatory hypertrophy; the most simple case of such a compensatory process is when one of a pair of organs, say a kidney, becomes larger after the other has beenremoved.48Finally, at least in plants, a change of the directive irritability, of so-called “geotropism” for instance, in certain parts may serve to restore other more important parts.
In two of these general types of restitution, in regeneration proper and in the production of adventitious organs, the potencies which underlie these processes may be said to be “complex.” It is a complicated series of events, a proper morphogenesis in itself, for which the potency has to account, if, for instance, a worm newly forms its head by regeneration, or if a plant restores a whole branch in the form of an adventitious bud.
Such generalisations as are possible about the distribution of complex potencies are reserved for a special part of our future discussion.
Secondary restitution is always, like ontogeny, a process of morphogenesis, and therefore all the questions about single formative stimuli, and about internal and external conditions or means, occur again. But of course we cannot enter into these problems a second time, and may onlysay that, especially in regeneration proper, the specific type of the regenerative formation of any part may differ very much from the ontogenetic type of its origin: the end of both is the same, but the way can be even fundamentally different in every respect.
The Stimuli of Restitutions49
But now we turn to the important question: what is the precisestimulus50that calls forth processes of restitution; or, in other words, what must have happened in order that restitution may occur?
That the operation in itself, by its removing of mechanical obstacles, cannot be the true stimulus of any restitutions, is simply shown by all those restitutions that do not happen at the place of the wound. If we took a narrower point of view, and if we only considered regeneration proper from the wound itself, we might probably at first be inclined to advocate the doctrine that the removing of some obstacles might in fact be the stimulus to the process of restoration; but, even then, why is it that just what is wanted grows out? Why is there not only growth, but specific growth, growth followed by specification? The removing of an obstacle could hardly account for that. But, of course, taking account of all the adventitiousrestitutions—that is, all restorations not beginning at the wound itself—the theory that the removing of obstacles is the stimulus to restoration becomes, as we have said, quiteimpossible.51
But where then is the stimulus to be found? There is another rather simple theory of the “Auslösung” ofrestitutions,52which starts from the phenomena of compensatory hypertrophy and some occurrences among plants. The removal of some parts of the organism, it is said, will bring its other parts into better conditions of nutrition, and therefore these parts, particularly if they are of the same kind, will become larger. Granted for the moment that such a view may hold in cases when one of a pair of glands becomes larger after the other has been removed, or when pruning of almost all the leaves of a tree leads to the rest becoming larger, it certainly must fail to explain the fact that in other cases truenewformations may arise in order to restore a damaged part, or that the latter may be regenerated in its proper way. Formerely quantitativedifferences in the mixture of the blood or of the nourishing sap in plants can never be a sufficient reason for the highly typical andqualitativestructure of newly-formed restitutions. And even in the most simple cases of a mere increase in the size of some parts, that is, in the simplest cases of so-called compensatoryhypertrophy,53it is at least doubtful,if not very improbable, that the compensation is accomplished in such a purely passive way, because we know that in other cases it is usually the growth of the young parts that actively attracts the nourishment: there is first differentiation and growth, andafterwardsthere is a change in the direction of the nourishing fluids.
The process of true regeneration, beginning at the locality of the wound itself, has been shown by Morgan, even as regards its rate, to occur quite irrespectively of the animal being fed ornot.54There could hardly be a better demonstration of the fundamental fact that food assists restitution, but does not “cause” it in any way.
But in spite of all we have said, there seems to be some truth in regarding the nutritive juices of animals and plants as somehow connected with the stimulus of restitutions: only in this very cautious form, however, may we make the hypothesis. It has been shown for both animals and plants, that morphogenesis of the restitutive type may be called forth even if the parts, now to be “regenerated” have not been actually removed;e.g.in the so-called super-regeneration of legs and tails in Amphibia, of the head in Planarians, of the root-tip in plants and in some other cases. Here it has always been a disturbance of thenormal connection of some parts with the rest of the organism which proved to be the reason of the new formation. This shows that something to do with the communication among parts is at least connected with restitution, and this communication may go on either by the unknown action of specific tissues or by the aid of the blood orsap.55But in what this change or break of specific communication consists, is absolutely unknown. One might suppose that each part of the organisation constantly adds some sort of ferment to the body fluids outside or inside the cells, that the removing of any part will change the composition of these fluids in this particular respect, and that this change acts as a sort of communication to summon the restituting parts of the whole to do theirduty.56
But I see quite well that such a theory is very littlesatisfactory; for what has to be done in restitution in each case is not a simple homogeneous act, for which one special material might account, but is a very complicated work in itself. It was the defect of the theory of “organ-forming substances” as advocated by Sachs, that it overlooked this point.
So all we know about the proper stimuli of restitutions is far from resting on any valid grounds at all; let us not forget that we are here on the uncertain ground of what may be called the newest and most up-to-date branch of the physiology of form. No doubt, there will be something discovered some day, and the idea of the “whole” in organisation will probably play some part in it. But in what manner that will happen we are quite unable to predict.
This is the first time that, hypothetically at least, the idea of the whole has entered into our discussion. The same idea may be said to have entered it already in a more implicit form in the statement of the threefold harmony in ontogeny.
Let us now see whether we can find the same problem of the “whole” elsewhere, and perhaps in more explicit and less hypothetical form. Let us see whether our analytical theory of development is in fact as complete as it seemed to be, whether there are no gaps left in it which will have to be filled up.
We have come to the central point of the first part of these lectures; we shall try in this chapter to decide a question which is to give life its place in Nature, and biology its place in the system of sciences. One of the foundation stones is to be laid upon which our future philosophy of the organism will rest.
The General Problem
Our analytical theory of morphogenesis has been founded upon three elementary concepts: the prospective potency, the means, and the formative stimulus. Its principal object has been to show that all morphogenesis may be resolved into the three phenomena expressed by those concepts; in other terms, that morphogenesis may be proved to consist simply and solely of what is expressed by them. Have we indeed succeeded in attaining this object? Has nothing been left out? Is it really possible to explain every morphogenetic event, at least in the most general way, by the aid of the terms potency, means, and stimulus?
All of these questions are apt to lead us to furtherconsiderations. Perhaps these considerations will give us a very clear and simple result by convincing us that it is indeed possible to analyse morphogenesis in our schematic way.
But if the answer were a negative one? What would that suggest?
The full analysis of morphogenesis into a series of single formative occurrences, brought about by the use of given means and on the basis of given potencies, might assure us, perhaps, that, though not yet, still at some future time, a further sort of analysis will be possible: the analysis into the elemental facts studied by the sciences of inorganic nature. The organism might prove to be a machine, not only in its functions but also in its very origin.
But what are we to say if even the preliminary analysis, which possibly might lead to such an ultimate result, fails?
Let us then set to work. Let us try to consider most carefully the topic in which our concept of the formative cause or stimulus may be said to be centred, thelocalisationof all morphogenetic effects. Is it always possible in fact to account for the typical localisation of every morphogenetic effect by the discovery of a single specific formative stimulus? You will answer me, that such an analysis certainly is not possible at present. But I ask you again, are there any criteria that it is possible, at least in principle; or are there any criteria which will render such an aim of science impossible for all future time?
The Morphogenetic “System”
We know from our experimental work that many, if not all, of the elementary organs in ontogeny show oneand the same prospective potency distributed equally over their elements. If we now borrow a very convenient term from mechanics, and call any part of the organism which is considered as a unit from any morphogenetic point of view, a morphogenetic “system,” we may sum up what we have learnt by saying that both the blastoderm of the echinoderms, at least around its polar axis, and also the germ-layers of these animals, are “systems” possessing an equal potentiality in all of their elements, or, in short, that they areequipotential systems.
But such a term would not altogether indicate the real character of these systems.
Later on we shall analyse more carefully than before the distribution of potencies which are the foundation both of regeneration proper and of adventitious growth, and then we shall see that, in higher plants for instance, there is a certain “system” which may be called the organ proper of restitutions, and which also in each of its elements possesses the same restoring potency; I refer to the well-known cambium. This cambium, therefore, also deserves the name of an “equipotential system.” But we know already that its potencies are of the complex type, that they consist in the faculty of producing thewhole, of such a complicated organisation as a branch or a root, that the term “equipotential system” is here only to signify that such a complicated unit may arise out of each of the cells of the cambium.
The potencies we have been studying in the blastula or gastrula of echinoderms are not of the complex type: our systems are equipotential to the extent that each of their elements may play everysinglepart in the totality of whatwill occur in the whole system; it is to thissinglepart that the term “function of the position” relates. We therefore might call our systems equipotential systems with single potencies; or, more shortly, singular-equipotential systems.
But even this terminology would fail to touch precisely the very centre of facts: it is not only the simplicity or singularity of their potencies which characterises the rôle of our systems inmorphogenesis,57but far more important with respect to the production of form are two other leading results of the experimental researches. The proper act to be performed by every element in each actual case is in fact a single one, but the potency of any element as such consists in the possibility of many, nay of indefinitely many, single acts: that then might justify us in speaking of our systems as “indefinite equipotential,” were it not that another reason makes another title seem still more preferable. There are indeed indefinite singular potencies at work in all of our systems during ontogeny: but the sum of what happens to arise in every case out of the sum of the single acts performed by all of the single equipotential cells is not merely a sum but a unit; that is to say, there exists a sort of harmony in every case among thereal productsof our systems. The termharmonious-equipotential systemtherefore seems to be the right one to denote them.
We now shall try first to analyse to its very extremes the meaning of the statement that a morphogenetic system is harmonious-equipotential.
The “Harmonious-Equipotential System”
We have an ectoderm of the gastrula of a starfish here before us; we know that we may cut off any part of it in any direction, and that nevertheless the differentiation of the ectoderm may go on perfectly well and result in a typical little embryo, which is only smaller in its size than it would normally be. It is by studying the formation of the highly complicated ciliary band, that these phenomena can be most clearly understood.
Now let us imagine our ectoderm to be a cylinder instead of being approximately a sphere, and let us imagine the surface of this cylinder unrolled. It will give us a plane of two definite dimensions,aandb. And now we have all the means necessary for the analytical study of the differentiation of an harmonious-equipotential system.
Our plane of the dimensionsaandbis the basis of the normal, undisturbed development; taking the sides of the plane as fixed localities for orientation, we can say that the actual fate, the “prospective value” of every element of the plane stands in a fixed and definite correlation to the length of two lines, drawn at right angles to the bordering lines of the plane; or, to speak analytically, there is a definite actual fate corresponding to each possible value ofxand ofy. Now, we have been able to state by our experimental work, that the prospective value of the elements of our embryonic organ is not identical with their “prospective potency,” or their possible fate, this potency being very much richer in content than is shown by a single case of ontogeny. What will be the analytical expression of such a relation?
Let us put the question in the following way: on what factors does the fate of any element of our system depend in all possible cases of development obtainable by means of operations? We may express our results in the form of an equation:—
p.v. (X) = f( . . . )
i.e.“the prospective value of the elementXis a function of . . .”—of what?
We know that we may take off any part of the whole, as to quantity, and that a proportionate embryo will result, unless the part removed is of a very large size. This means that the prospective value of any element certainly depends on, certainly is a function of, theabsolute sizeof the actually existing part of our system in the particular case. Letsbe the absolute size of the system in any actual experimental case of morphogenesis: then we may writep.v. (X) = f(s . . . ). But we shall have to add still some other letter to thiss.
The operation of section was without restriction either as to the amount of the material removed from the germ, or as to the direction of the cut. Of course, in almost every actual case there will be both a definite size of the actual system and a definite direction of the cut going hand-in-hand. But in order to study independently the importance of the variable direction alone, let us imagine that we have isolated at one time that part of our system which is bounded by the linesa1b1, and at another time an equal amount of it which has the linesa2b2as its boundaries. Now since in both cases a typical small organism may result on development, we see that, in spite of their equal sizethe prospective value of every element of the two pieces cut out of the germ may vary even in relation to the direction of the cut itself. Our element,X, may belong to both of these pieces of the same size: its actual fate nevertheless will be different. Analytically, it may be said to change in correspondence to the actual position of the actual boundary lines of the piece itself with regard to the fundamental lines of orientation,aandb; let this actual position be expressed by the letterl,lmarking the distance ofone58of the actual boundary lines of our piece fromaorb: then we are entitled to improve our formula by writingp.v. (X) = f(s, l . . . )(Fig. 11).
Fig. 11.—Diagram to show the Characteristics of an “Harmonious-equipotential System.”The elementXforms part of the systemsa bora1b1ora2b2; its prospective value is different in each case.
Fig. 11.—Diagram to show the Characteristics of an “Harmonious-equipotential System.”
The elementXforms part of the systemsa bora1b1ora2b2; its prospective value is different in each case.
But the formula is not yet complete:sandlare what the mathematicians call variables: they may have any actual value and there will always be a definite value ofp.v.,i.e.of the actual fate which is being considered; to every value ofsandl, which as we know are independent of each other, there corresponds a definite value of the actual prospectivity. Now, of course, there is also a certain factor at work in every actual case of experimental or normal development, which isnota variable, but which is the same in all cases. This factor is a something embraced in the prospective potency of our system, though not properly identical with it.
The prospective potency of our system, that is to say of each of its elements, is the sum total of what can be done by all; but the fact that a typically proportionate development occurs in every possible case, proves that this sum comes into account, not merely as a sum, but as a sort oforder: we may call this order the “relation of localities in the absolutely normal case.” If we keep in mind that the term “prospective potency” is always to contain this order, or, as we may also call it, this “relative proportionality,” which, indeed, was the reason for calling our systems “harmonious,” then we may apply it without further explanation in order to signify thenon-variablefactor on which the prospective value of any element of our systems depends, and, if we denote the prospective potency, embracing order, by the letterE, we are now able to complete our formula by sayingp.v. (X) = f(s, l, E).
So far the merely analytical study of the differentiation of harmonious-equipotentialsystems.59
Instances of “Harmonious-Equipotential Systems”
We must try at first to learn a few more positive facts about our systems, in order that we may know how important is the part which they play in the whole animal kingdom, and in order that our rather abstract analysis may become a little more familiar to us. We know already that many of the elementary morphogenetic organs have been really proved to be harmonious-equipotential systems, and that the same probably is true of many others; we also know that the immature egg of almost all animals belongs to this type, even if a fixed determination of its parts may be established just after maturation. Moreover, we said, when speaking about some new discoveries on form-restitution, that there are many cases in which the processes of restitution do not proceed from single localities, the seat of complex potencies in the organism, but in which eachsinglepart of the truncated organism left by the operation has to perform onesingleact of restoration, the full restitution being the result of the totality of all. These cases must now be submitted to a full analysis.
All of you have seen common sea-anemones or sea-roses, and many of you will also be familiar with the so-called hydroid polyps.Tubulariais one genus of them: it looks like a sea-anemone in miniature placed on the top of a stem like a flower. It was known already to Allman thatTubulariais able to restore its flower-like head when that is lost, but this process was taken to be an ordinary regeneration, until an American zoologist, Miss Bickford, succeeded in showing that there was no regeneration process at all, in the proper sense of the word, no budding of themissing part from the wound, but that the new tubularian head was restored by the combined work of many parts of the stem. Further analysis then taught us thatTubulariaindeed is to be regarded as the perfect type of an harmonious-equipotential system: you may cut the stem at whatever level you like: a certain length of the stem will always restore the new head by the co-operation of its parts. As the point of section is of course absolutely at our choice, it is clear, without any further discussion, that the prospective value of each part of the restoring stem is a “function of its position,” that it varies with its distance from the end of the stem; and so at once we discover one of the chief characteristics of our systems. But also the second point which enters into our formula can be demonstrated inTubularia: the dependence of the fate of every element on the actual size of the system. You would not be able to demonstrate this on very long stems, but if you cut out of aTubulariastem pieces which are less than ten millimetres in length, you will find the absolute size of the head restored to be in close relation to the length of the stem piece, and this dependence, of course, includes the second sort of dependence expressed in our formula.
The figures will serve to show you a little more concretely what has been described. The head ofTubulariaconsists of a sort of broad base with a thin proboscis upon it, both bearing a large number of tentacles; these tentacles are the first things to be seen as primordia (“Anlagen”) in the process of restitution. You notice two rings of longitudinal lines inside the stem; the lines will become walls and then will separate from the stem until they are only connected with it at their basal ends; the new tentacles are ready assoon as that has happened, and a process of growth at the end will serve to drive the new head out of the so-called perisarc or horny skeleton, which surrounds the stem. By comparing the two figures, 12e, andg, you easily find out that the absolute lengths of the two tentacle rings are very different, and that both are inproportion60to the actual size of the stem (Fig. 12).
Fig. 12.—Tubularia.a.Diagram of the “Hydranth,” with its short and long tentacles.b.Restitution of a new hydranth inside the perisarc (p).c.The same—later stage; the tentacles are complete; the whole hydranth will be driven out of the perisarc by a process of growth that occurs at the locality marked ↑.d.A stem ofTubulariacut either ata1b1or ata2b2or ata1c.e.Position of tentacles in the piece cut ata1b1.f.Position of tentacles in the piece cut ata2b2which is equal in length toa1b1.g.Position of tentacles in the piece cut ata1c, which is half as long asa1b1.
Fig. 12.—Tubularia.
So we find our formulap.v. (X) = f(s, l, E)very well illustrated inTubularia. The formula indeed may help us to predict, in any case, where a certain part of the polyp’s organisation is to originate, at least if we know all that is included under our letterE,i.e.the normal proportion of our form. Of course such prediction would not have much practical importance in all our cases of morphogenesis, but nevertheless I should like to state here that it is possible; for many scientific authors of recent times have urged the opinion that prediction of, and domination over, what will happen, can be the only true aims of sciences at all. I myself judge these aims to be of second or third-rate importance only, but, if they may be reached by what our purely theoretical study teaches, so much the better.
Fig. 13.—Clavellina.a.Diagram of the normal animal:EandJ= openings;K= branchial apparatus;D= intestine;M= stomach;H= heart.b.The isolated branchial apparatus.c-e.Different stages of reduction of the branchial apparatus.f.The newwholelittle ascidian.
Fig. 13.—Clavellina.
Another very typical case of a morphogenetic system of the harmonious type is supplied by the phenomena of restoration in the ascidianClavellina. I cannot fully describe the organisation of this form (Fig. 13a), and it must suffice to say that it is very complicated, consisting of two very different chief parts, the branchial apparatus and the so-called intestinal sac; if these two parts of the body ofClavellinaare separated one from the other, each may regenerate the other in the typical way, by budding processes from the wound. But, as to the branchial apparatus, there may happen something very different: it may lose almost all of its organisation and become a small white sphere, consisting only of epithelia corresponding to the germ-layers, and of mesenchyme between them, and then, after a certain period of rest, a new organisation will appear. Now this new organisation is not that of a branchial apparatus but represents a very small but complete ascidian (Fig. 13). Such a fact certainly seems to be very important, not to say very surprising; but still another phenomena may be demonstrated on the animal which seems to be even more important. You first isolate the branchial apparatus from the other part of the body, and then you cut it in two, in whatever direction you please. Provided they survive and do not die, as indeed many of them do, the pieces obtained by this operation will each lose their organisation, as did the whole branchial apparatus, and then will each acquire another one, and this new organisation is also that of acompletelittleClavellina. So we see that not only is the branchial apparatus of our animal capable of being transformed into a whole animal by the co-operative work of all its parts, but even each part of it may be transformed into a smallwhole, and it is quite at our disposal how large this part shall be, and what sort of a fragment of the original branchial apparatus it shall represent.
We could hardly imagine a better instance of an harmonious-equipotential system.
I cannot give you a description of all the other types of our systems subservient to restitution, and I can only mention here that the common hydra and the flatwormPlanariaare very fine examples of them. But to one special case of harmonious equipotentiality you must allow me to direct your further attention.
It has been known for many years that the Protozoa are also capable of a restoration of their form and organisation after disturbances, if at least they contain a certain amount of their nuclear substance. This process of restoration used to be regarded as belonging to the common typeof regeneration proper, until T. H. Morgan succeeded in showing that in the genusStentorit follows just the very lines which we know already from our study of embryonic organs or fromTubularia; that an harmonious-equipotential system is at the basis of what goes on. Now, you know that all Protozoa are but one highly organised cell: we have therefore here an instance where the so-called “elements” of our harmonious-morphogenetic system are not cells, but something inside of cells; and this feature must appear to be of very great moment, for it first shows, as we have already pointed out on another occasion, that morphogenesis is not dependent on cell-division, and it states at the same time that our concept of the harmonious-equipotential system may cover a very great area—that, in fact, it is a scheme of a very wide extent.
The Problem of the FactorE
We turn back again to considerations of a more abstract form. We left our analysis of the differentiation of the harmonious-equipotential systems, and particularly of the phenomena of localisation during this differentiation, at the point where we had succeeded in obtaining an equation as the expression of all those factors on which the prospective value, the actual fate, of any element of our systems depends,p.v. (X) = f(s, l, E)was the short expression of all the relations involved;sandl, the absolute size of the system and the relative position of the element with respect to some fixed points, were independent variables;Ewas a constant, namely, the prospective potency, with special regard to the proportions embraced by it.
We shall now study the significance of the factorE.
What does thisEmean? Is it a short expression merely for an actual sum of elemental agents having a common resultant? And, if so, of what kind are these agents? Or what mayEmean, if it can be shownnotto be a short sign for a mere sum?
No Explanation Offered by “Means” or “Formative Stimuli”
For practical purposes it seems better if we modify the statement of our question. Let us put it thus:Eis one of the factors responsible, among variables, for the localisation of organic differentiation; what then do we actually know about the causal factors which play a localising partin organogenesis? We, of course, have to look back to our well-studied “formative stimuli.” These stimuli, be they “external” or “internal,” come from without with respect to the elementary organ in which any sort of differentiation, and therefore of localisation, occurs: but in our harmonious systems no localising stimulus comes from without, as was the case, for instance, in the formation of the lens of the eye in response to the optical vesicle touching the skin. We know absolutely that it is so, not to speak of the self-evident fact that the general “means” of organogenesis have no localising value atall.61
So we see there is nothing to be done, either with the means or with the formative stimuli; both are entirely unable to account for those kinds of localisation during differentiation which appear in our harmonious systems.
But is there no possibility of explaining the phenomena of organogenetic localisation by any other sort of interaction of parts? Two such possibilities may at the first glance seem to exist.
No Explanation Offered by a Chemical Theory of Morphogenesis
Though never set forth, in the form of a properly worked-out theory, the view has sometimes been advocated by biologists, that a chemical compound of a very high degree of complication might be the very basis of both development and inheritance, and that such a chemical compound by its disintegration might direct morphogenesis.
Let us first examine if such a view may hold for the most general features of organic morphogenesis. It seems to me that from the very beginning there exists one very serious objection to every chemical theory of form-building, in the mere fact of the possibility of the restoration of form starting from atypical localities. The mere fact, indeed, that there is such a thing as the regeneration of a leg of a newt—to say nothing about restitution of the harmonious type—simplycontradicts,62it seems to me, the hypothesis, that chemical disintegration of one compound may govern the course of morphogenetic events: for whence comes the re-existence of the hypothetical compound, newly to be disintegrated, after disintegrationhasbeen completed once already? And we even know that regeneration may go on several times running from the same locality!
But, if we intentionally disregard this difficulty, in spite of its fundamental character, how could the hypothesis of chemical disintegration give the reason for the differentiation of our harmonious-equipotential systems, with special regard to the localisation of it; how could it account, in other words, for the appearance of typically localised specifications in an organ for which no external localising causes can be predicated?
Let us remember that a few original intimate differences exist in our harmonious systems: the main directions of the intimate protoplasmic structure including polarity and bilaterality. There are therefore three times two specified poles in each of these systems, at least in bilateral organisms, but no other differences are present in them. A few very simple cases of harmonious differentiation might indeed be understood on the theory of a disintegrating chemical compound in connection with these few differences. Imagine that the original compound, of the quantitya, is disintegrated to the amount ofa1; froma1are formed the two more simple compounds,bandc, both of them in definite quantities; then we have the three chemical individuals,a-a1,bandc, as the constituents of our harmonious system; and it now might be assumed, without any serious difficulty, though with the introduction of some new hypotheses, that the two poles of one of the fundamental axes of symmetry attractbandcrespectively,a-a1remaining unattracted between them. We thus should have the three elementary constituents of the system separated into three parts, and as they all three are of a definite quantity, their separation would mean that the system had been divided into threeparts,a-a1,bandc, also with regard to its proper form. It is clear, that by taking away any part of the original system, by means of operations, there would be taken away a certain amount of the original compound; say thata/nis left; then, of course, the three constituents after the partial disintegration would bea-a1/n,b/nandc/n, and so it follows that the proportionality of localisation would really be preserved in any case.
But these considerations, evident as they seem to be in the most simple case, fail to satisfy in a really general sense: for two different reasons. First, they could never account for the fact that the differentiated organism by no means consists of so many different compounds as it shows single parts of its differentiation, but that, on the contrary, it only consists, as we know, of a certain rather limited number of true different morphogenetic elements, these elements occurring again and again—as for instance, nervous or muscular elements—but typical each time in locality, quantity, and form. And in the second place, the veryformof elementary organs, their form as such, does not at all go hand-in-hand with chemical differences; this feature alone would absolutely overthrow any sort of a chemical morphogenetic theory to account for the problem of localisation. Take the typically arranged ring of the mesenchyme cells in our Echinus-gastrula, with its two spherical triangles, so typically localised; look at any sort of skeleton, in Radiolaria, or in starfishes, or in vertebrates: here you have form, real form, but form consisting of only one material. Not only is the arrangement of the elements of form typical here,e.g.the arrangement of the singleparts of the skeleton of the hand or foot, but also the special form of each element is typical,e.g.the form of each single bone of the foot; and, on a purely chemical theory of morphogenesis the sufficient reason for the production of typical form in such a sense would be wanting. For atoms or molecules by themselves can only account for form which is arranged, so to speak, according to spatial geometry—as in fact they do in crystallography; but they can never account for form such as the skeleton of the nose, or hand, or foot. You will answer me perhaps, that there may be non-chemical agents in thegerm,63responsible for typical form-localisation, but by such reasoning you would be departing from a purely chemical theory. Our next paragraph will be devoted to this side of the question.
That is the principal reason for rejecting all sorts of chemical morphogenetic theories put forward to explain the problem of localisation; it is more explicit, and therefore, I suppose, still more convincing than the more general consideration that the very fact of restitutions in itself must contradict the hypothesis that a disintegration of compounds might be the directive agency in morphogenesis. To sum up: Specificity of organic form does not go hand-in-hand with specificity of chemical composition, and therefore cannot depend on it; and besides that, specific organic form is such that it can never be explained by atomic or molecular arrangement in the chemical sense; for, to state it in a short but expressive manner, the “form” of an atom or molecule can never be that of a lion or a monkey. Toassume that would be to go beyond the limits of chemistry in chemistry itself.
No Machine Possible Inside the Harmonious Systems
And now we turn to the last possibility which is left to us in our endeavour to “understand” the localisation of the differentiation in our harmonious-equipotential systems by the means of physics and chemistry. Outside causes have failed to account for it, chemical disintegration of a compound has failed too. But could there not exist some sort of complicated interactions amongst the parts of the harmonious system themselves? Could there not exist some kind of a real machine in the system, which, if once set going, would result in the differentiations that are to take place? Then we might say that the “prospective potency” of the system is in fact that machine; we should know what the letterEof our equation stood for: viz., a resultant action of many complicated elemental interactions, and nothing more.
Weismann, we know already, had assumed that a sort of machine was the prime mover of morphogenesis. We have seen that his theory cannot be true; the results of experiments most strongly contradict it. But, of course, the experiments only showed us thatsucha machine ashehad imagined to exist could not be there, that development could not be governed by the disintegration of a given complicated structure into its simplest parts. But might not some other machine be imaginable?
We shall understand the word “machine” in a most general sense. A machine is a typical configuration ofphysical and of chemical constituents, by the acting of which a typical effect is attained. We, in fact, lay much stress upon embracing in our definition of a machine the existence of chemical constituents also; we therefore understand by the word “machine” a configuration of a much higher degree of complication than for instance a steam-engine is. Of course a machine, whose acting is to be typical with regard to the three dimensions in space, has to be typically constructed with regard to these three dimensions itself; a machine that was an arrangement of elements in a strict plane could never have typical effects at right angles to that plane. This is a point which must well be kept in mind in all hypothetical considerations about machines that claim to explain morphogenesis.
It must be granted that a machine, as we understand the word, might very well be the motive force of organogenesis in general, if only normal, that is to say, if only undisturbed development existed, and if a taking away of parts of our systems led to fragmental development.
But we know that, at least in our harmonious-equipotential systems, quite another process occurs after parts have been taken away: the development that occurs is not fragmental but whole, only on a smaller scale.
And we know, further, that this truly whole development sets in irrespective of the amount and direction of the separation. Let us first consider the second of these points. There may be a whole development out of each portion of the system—above certain limits—which is, say, of the volumeV. Good! Then there ought to exist a machine, like that which exists in the whole undisturbed system, in this portionValso, only of smaller dimensions; but it alsoought to exist in the portionV1which is equal toVin amount, and also inV2, inV3,V4and so on. Indeed, there do exist almost indefinitely manyVnall of which can perform the whole morphogenesis, and all of which therefore ought to possess the machine. But these different portionsVnare only partly different from each other in spatial relation. Many parts ofV2are also parts ofV1and ofV3and ofV4and so on; that is to say, the different volumesVnoverlap each other successively and in such a manner that each following one exceeds the preceding one in the line by a very small amount only. But what then about our machines? Every volume which may perform morphogenesis completely must possess the machine in its totality. As now every element of one volume may play any possible elemental rôle in every other, it follows that each part of the whole harmonious system possesses any possible elemental part of the machine equally well, all parts of the system at the same time being constituents of different machines.
A very strange sort of machine indeed, which is the same in all its parts (Fig. 14)!