LECTURE XVIII

THE GERM-PLASM THEORY (continued)

Structure of the germ-plasm—Vital affinities—Division—O. Hertwig's chief objections to this theory—Male and female eggs in the Phylloxera show differential division—Dispersal of the germ-plasm in the course of Ontogeny—Active and passive state of the determinants—Predetermination of cells—There are no determinants of characters—Liberation of the determinants—Accessory idioplasm—Herbst's lithium larvæ—Plant galls—Cells with several facultative determinants—Connective tissue in vertebrates—Mesoderm cells of Echinoderms—Sexual dimorphism—Female and male ids—Polymorphism (Papilio merope)—Ants.

I haveendeavoured to prove that the germ-substance proper must be looked for in the chromatin of the nucleus of the germ-cell, and more precisely still in those ids or chromosomes which we conceive of as containing the primary constituents (Anlagen) of a complete organism. Such ids in larger or smaller numbers make up the whole germ-plasm of a germ-cell, and each id in its turn consists of primary constituents or determinants, i.e. of vital units, each of which determines the origin and development of a particular part of the organism. We have now to make an attempt to picture to ourselves how these determinants predetermine those cells or cell-groups to which they correspond. In doing so we have to fall back upon mere hypotheses, and in stating any such hypothesis I wish expressly to emphasize that I am only following up one of the possibilities which our imaginative faculty suggests. Nevertheless, to endeavour to form such a conception is certainly not without use, for it is only by elaborating a theory to the utmost that we are able to use it in application to concrete cases, thus stimulating the search for corroboratory or contradictory facts, and leading gradually to a recognition of the gaps or mistakes in the theory.

The first condition that must be fulfilled in order that a determinant may be able to control a cell or cell-group is that it should succeed in getting into it. It must be guided through the numerous cell-divisions of ontogeny so that it shall ultimately come to lie in the cells which it is to control. This presupposes that each determinant has from the very beginning its definite position in relation to the rest, and that the germ-plasm, therefore, is not a mere loose aggregate of determinants, but that it possesses a structure, an architecture, inwhich the individual determinants have each their definite place. The position of the determinants in relation to one another cannot be due to chance, but depends partly on their historical development from earlier ancestral determinants, partly on internal forces, such as we have already assumed for keeping the determinants together. We may best designate these hypothetical forces 'affinities,' and in order to distinguish them from mere chemical affinities we may call them 'vital.' There must be forces interacting among the different determinants which bind them together into a living whole, the id, which can assimilate, grow, and multiply by division, in the same manner as we were forced to assume for the smaller units, the biophors and single determinants. In the ids, however, we can observe the working of these forces quite directly, since each chromosome splits into two halves of equal size at every nuclear division, and not through the agency of external forces, e.g. the attraction which we may assume to be exerted by the fibrils of the nuclear spindle, but through purely internal forces, often long before the nuclear spindle has been formed at all.

But if the determinants must separate from each other in the course of development so as to penetrate singly into the cells they are to control, the id must not only have the power of dividing into daughter-ids of identical composition, it must also possess the power of dividing under certain influences into dissimilar halves, so that the two daughter-ids contain different complexes of determinants. The first mode of division of the id, and with it of the nucleus and of the cell, I callerbgleich, or integral, the seconderbungleich, or differential. The first form of multiplication is the usual one, which we observe everywhere when unicellular organisms divide themselves into two equal daughter-units, or when the cells of multicellular bodies produce their like by division into two. The second is not directly observable, because a dissimilarity of the daughter-cells, as long as it lies only in the idioplasm, cannot be actually seen; it can only be inferred from the different rôle which the two daughter-cells play in the building up of the individual. When, for instance, one of two sister-cells of the embryo gives rise to the cells of the alimentary canal and the other to those of the skin and the nervous system, I infer that the mother-cell divided its nuclear substance in a differential way between the two daughter-cells, so that one contained the determinants of the endoderm, the other those of the ectoderm; or when a red and a black spot lie side by side and under exactly the same conditions on the wing of a butterfly, I conclude that the ancestral cells of these two spots have divided differentially, so thatone received the 'red,' the other the 'black' determinants. Our eyes can perceive no difference between the nuclear substance of the two cells, but the same is true of the chromosomes of the paternal and maternal nuclei in the fertilized ovum, although we know in this case that they contain different tendencies. In any case we are not justified in concluding from the apparent similarity of the chromosome-halves in nuclear division that there cannot be differential division. The theoretical possibility that there is such differential division cannot be disputed; indeed, I am inclined to say that it is more easily imagined than the division of the ids into absolutely similar halves. Both are only conceivable on the assumption that there are forces which control the mutual position of the determinants in the ids, that is, on the assumption of 'affinities.' I shall not follow this further, but that there are forces operative within the ids which are still entirely unknown to us is proved at every nuclear division by thespontaneoussplitting of the chromosomes.

It has been objected to my theory that such a complex whole as the id could not in any case multiply by division, since there is no apparatus present which can, in the division into two daughter-units, re-establish the architecture disturbed by the growth. But this objection is only valid if we refuse to admit the combining forces, the 'vital affinities' within the ids, and the same is true for the smaller vital units. An ordinary chemical molecule cannot increase by division; if it be forcibly divided it falls into different molecules altogether; it is only the living molecule, that is, the biophor, which possesses this marvellous property of growth and division into two halves similar to itself and to the ancestral molecule, and we may argue from this that in the division of the ids forces of attraction and repulsion must likewise be operative[19].

[19]In my bookThe Germ-PlasmI have already assumed the existence of 'forces of attraction' in the determinants and biophors, as in the cells. I did not, indeed, enter into details, but I argued on the same basis as now (Germ-Plasm, p. 64, English edition). My critics have overlooked this.

I see no reason why we should not assume the existence of such forces, when we make the assumption that the hundreds of atoms which, according to our modern conceptions, compose the molecule of albumen and determine its nature, are kept by affinities in this definite and exceedingly complex arrangement. Or must we suppose that between the atom-complex of the molecule and the next higher atom-complex of the biophor, determinant, and id there is an absolute line of demarcation, so that we must assume quite different forces in the latter from those we conceive of as operative in the former? Thebiophor is ultimately only a group of molecules, the determinants a group of biophors, the id a group of determinants, and all the three inferred stages of vital organization only become real units through the forces operating within them and combining them into a whole. What compels the chromatin granules of the resting nucleus to approach each other at the time of cell-division, to unite into a long, band-like thread, and what is it that subsequently causes this thread to break up again into a definite number of pieces? Obviously only internal forces of which we know nothing further than that they are operative.

We shall see later that this assumption of vital affinities must be made not only in regard to the cells, but also in regard to entire organisms whose parts are united by an internal bond, and whose co-ordination is regulated by forces of which we have as yet no secure knowledge. In the meantime we may designate these forces by the name of 'vital affinities.'

It must be admitted, however, that some objections of a fundamental nature have been urged against the assumption of a differential nuclear division of the hereditary substance. O. Hertwig holds that the assumption of differential division is essentially untenable, because it is contradictory to 'one of the first principles of reproduction,' for 'a physiologically fundamental character of every living being is the power of maintaining its species.'

This certainly seems so, but a closer examination shows that this 'principle,' although correct enough when taken in a very general sense, does not really cover the facts, and is therefore incapable of supporting the inferences drawn from it. If the proposition expressed the whole truth there could have been no evolution from the primitive organisms to higher ones, every living being must have simply reproduced exact copies of itself. Whether the transformations of species have been sudden or gradual, whether they have been brought about by large steps or by very small ones, they could only have come about by breaking through this so-called 'principle' of like begetting like. In fact, we may with more justice maintain the exact converse of the principle, and say that 'no living being is able to produce an exact copy of itself,' and this is true not only of sexual, but of asexual reproduction.

In ontogeny we see exactly the same thing. There are no two daughter-cells of a mother-cell which are exactly alike, and the differences between them, if they increase in the same direction, may lead in later descendants to entire differences of structure. Indeed the whole process of development depends on such an augmentationof the differences between two daughter-cells—on differences which proceed from within and are definitely pre-established. Here, again, the facts do not justify us in making a dogma of the proposition that it is a 'fundamental power' of every living being to maintain its species by producing replicas of itself. If we look at two directly successive cell-generations, we can hardly, it is true, in most cases, perceive any difference between them, just as in the generations of species; but if we compare the end of a long cell-lineage with the beginning, then the difference is marked, and we recognize that the difference is due to a gradual summing up of minute, invisible deviations. In my opinion these steps of difference cannot possibly depend merely on direct external influences; they proceed rather from the hereditary substance the cell receives from the ovum, which, therefore, in order to attain to such many-sided and far-reaching differentiation, must have undergone a frequently repeated splitting up of its qualities. That this splitting is not merely a variation to which the whole of the hereditary substance of the daughter-cells is uniformly subject, according to the influences dependent on their position in relation to other cells of the embryo, will be made clear from the case of the Ctenophora referred to in the next lecture. A scarcely less striking example is that of those animals in which the ova contain the primary constituents for only one sex, in which, in other words, there are 'male' ova and 'female' ova. This is the case, for instance, among Rotifers, and in plant-lice such as the vine-pest,Phylloxera. Here the eggs from which males develop are smaller than those which produce females. The primary constituents for both male and female are not, as in most animals, contained in the same ovum, to be liberated on one side or the other by influences unknown to us, but in each ovum there is only one of the two sets of primary constituents present, and in this case, therefore, the development of hermaphrodites, which not infrequently occur in other animals, would be impossible. But all these ova have been produced by one primitive reproductive cell, and consequently, at one of the divisions implied in the multiplication of this first cell, a separation of the male from the female primary constituents must have taken place, that is, a differential division of hereditary substance, for which no external and no intercellular influences can possibly account.

If there is, then, a differential division of the ids and with them of the whole idioplasm, the germ-plasm of the fertilized ovum must be broken up in the course of ontogeny into ever smaller groups of determinants. I conceive of this as happening in the following manner.

In many animals the fertilized ovum divides at the first segmentation into two cells, one of which gives rise predominantly to the outer, the other to the inner germinal layer, as in molluscs, for instance. Let us now assume that this is the case altogether, so that one of the first two blastomeres gives rise to the whole of the ectoderm, the other to the whole of the endoderm; we should here have a differential division, for the developmental import (the 'prospective' of Driesch) of the primitive ectoderm-cell is quite different from that of the primitive endoderm-cell, the former giving origin to the skin and the nervous system, with the sense organs, while the second gives rise to the alimentary canal, with the liver, &c. Through this step in segmentation, I conclude, the determinants of all the ectoderm-cells become separated from those of the endoderm-cells; the determinant architecture of the ids must be so constructed in such species that it can be segregated at the first egg-cleavage into ectodermal and endodermal groups of determinants. Such differential divisions will always occur in embryogenesis when it is necessary to divide a cell into two daughter-cells having dissimilar developmental import, and consequently they will continue to occur until the determinant architecture of the ids is completely analysed or segregated out into its different kinds of determinants, so that each cell ultimately contains only one kind of determinant, the one by which its own particular character is determined. This character of course consists not merely in its morphological structure and chemical content, but also in its collective physiological capacity, including its power of division and duration of life[20].

[20]Emery has lately called attention to another direct proof of the existence of differential cell- and nucleus-division. According to observations made by Giardina, in the water-beetle (Dytiscus), one primitive ovum-cell gives rise, through four successive divisions, to fifteen nutritive cells and one well-defined ovum-cell. But only half of the nuclear substance takes part in these divisions, the rest remains inactive in a condensed, cloudy condition. 'The meaning of the whole process is obviously that the germ-plasm mass as a whole is handed over to the ovum-cell, while the nutritive cells receive only the nuclear constituents which belong to them' (Biol. Centralbl., May 15, 1903).

But embryogenesis does not proceed by differential divisions alone, for integral divisions are often interpolated between them, always, for instance, when in a bilateral animal an embryonic cell has to produce by division into two a corresponding organ for the right and left sides of the body; for instance, in the division of the primitive genital cell into the rudiments of the right and left reproductive organs, or in the division of the primitive mesoderm-cell into the right and left initial mesoderm-cell, but also later on in the course of embryogenesis, when, for instance, the right or the left primitive reproductivecell multiplies into a large number of primitive germ-cells, or in the multiplication of the blood-cells, or of the epithelial cells of a particular region; in short, whenever mother and daughter-cells have the same developmental import, that is, when they are to become nothing more than they already are. In all such cases a similar group of determinants, or a similar single determinant, must in the nuclear division penetrate into each of the two daughter-cells.

It is in this way, it seems to me, that the determinants gain entrance into the cells they are to control, by a regulated splitting up of the ids into ever smaller groups of determinants, by a gradual analysis or segregation of the germ-plasm into the idioplasms of the different ontogenetic stages. When I first developed this idea I assumed that the splitting process would in all cases set in at the same time, namely, at the first division of the ovum. But since then, in the controversies excited by the theory, many facts have been brought to light which prove that the ova of the different animal groups behave differently, and that the splitting up of the aggregate of primary constituents may sometimes begin later—but I shall return to this later on.

If we accept the segregation hypothesis, which is similar in purport to that advanced by Roux as the' mosaic theory,' it must strike us as remarkable that the chromatin mass of the nucleus does not become notably smaller in the course of ontogeny, and even ultimately sink to invisibility. Determinants lie far below the limits of visibility, and if there were really only a single determinant to control each cell there would be no chromatin visible in such a case. This objection has in point of fact been urged against me, although I expressly emphasized in advance the assumption that the determinants are continually multiplying throughout the whole ontogeny, so that in proportion as the number of thekindsof determinants lying within a cell diminishes the number of resting determinants of each kind increases. When, finally, only one kind of determinant is present there is a whole army of determinants of that kind.

It follows from this conception of the gradual segregation of the components of the id in the course of development that we must attribute to the determinants two different states, at least in regard to their effect upon the cell in which they lie: an active state, in which they control the cell, and a passive state, in which they exert no influence upon the cell, although they multiply. From the egg onwards, therefore, a mass of determinants is handed on by the cell-divisions of embryogenesis, which will only later become active.

My conception of the manner in which the determinants becomeactive is similar to that suggested by De Vries in regard to his 'Pangens,' very minute vital particles which play a determining part in his 'pangen theory,' similar to that filled by the determinants in my germ-plasm theory. It seems to me that the determinants must ultimately break up into the smallest vital elements of which they are composed, the biophors, and that these migrate through the nuclear membrane into the cell-substance. But there a struggle for food and space must take place between the protoplasmic elements already present and the newcomers, and this gives rise to a more or less marked modification of the cell-structure.

It might be supposed that the structure of these biophors corresponded in advance to certain constituent parts of the cell, that there were, for instance, muscle biophors, which make the muscle what it is, or that the plant-cells acquired their chlorophyll-making organs through chlorophyll biophors. De Vries gave expression to this view in his 'pangen theory,' and I confess that at the time there seemed to me much to be said for it, but I am now doubtful whether its general applicability can be admitted. In the first place, it does not seem to me theoretically necessary to assume that the particles which migrate into the cell-bodies should themselves be chlorophyll or muscle particles; they may quite well be only the architects of these, that is to say, particles which by their co-operation with the elements already present in the cell-body give rise to chlorophyll or muscle substance. As we are as yet unacquainted with the forces which dominate these smallest vital particles, as well as the processes which lead to the histological differentiation of the cells, it is useless in the meantime to make any further hypotheses in regard to them. But in any case the biophors which transform the general character of the embryonic cells into the specific character of a particular tissue-cell must themselves possess a specific structure different from that of other biophors, for they must keep up the continuity of the structures handed on from ancestors, chlorophyll and muscle-substance and the like, since we cannot assume that these structures, so peculiar and so complex in their chemical and physical constitution, are formed afresh, so to speak, by spontaneous generation in each new being, as De Vries has very rightly emphasized. A specific biophor, for instance, of muscle substance will produce this substance as soon as it has found its way into the appropriate cell-body, even though it may not be a contractile element itself.

To this must be added that the structure of the body and the distinctive features of an organism do not depend merely on the histological differentiation of the cells, but quite as much on theirnumber and arrangement, and on the size and on the frequency of repetition of certain parts. These distinctive characters are just as constant and as strictly transmissible, and may be as heritably variable as those which depend on specific cell-differentiation, and they must therefore likewise be determinable by definite elements of the germ-plasm. Obviously enough, however, these elements are not of the same nature as the known specific histological elementary particles; they can be neither nerve-, muscle-, nor gland-biophors. They must rather be vital units of such a kind that they communicate to the cells and lineage of cells, into whose bodies they migrate from within the nucleus, a definite vital power, that is, an organization which regulates the size, form, number of divisions, and so on, of these cells—in short their whole prospective significance. Always, however, they act in co-operation with the cell-body into which they have penetrated.

Throughout we must hold ourselves aloof from the idea that 'characters' are transmissible. It is customary, indeed, to speak as if this were so, and it is also necessary, because we can only recognize the 'characters' of a body, and not the essential 'nature' on which these characters depend; but the determinants are not seed-grains of individual characters, but co-determinants of the nature of the parts which they influence. There are not special determinants of the size of a cell, others of its specific histological differentiation, and still others of its duration of life, power of multiplication, and so on; there are only determinants of the whole physiological nature of a cell, on which all these and many other 'characters' depend. For this reason alone I should object to the assumption that the determinants of the germ are ready-made histological substances. That is as unlikely as that their groups in the germ-plasm are 'miniature models' of the finished parts of the body.

I conceive of the process of cell-differentiation as follows: at every cell-stage in the ontogeny determinants attain to maturity, and break up so that their biophors can migrate into the cell-bodies, so that the quality of each cell is thus kept continually under control, and may be more or less modified, or may remain the same. By the 'maturity' of a determinant I mean its condition when by continual division it has increased in number to such a point that its disintegration into biophors and their migration into the cell-substance can take place.

One more point I must touch upon here, the question of the 'liberation' or 'stimulation' of the determinants. The activity of an organ never depends on itself alone; the contraction of a muscle is induced by a nerve stimulus or by an electric current; the activityof the nerve-cells of the brain requires the continual stimulus of the blood-stream, and cannot continue to exist without it; the specific sensory-nerves and sense-cells of the eye, ear, olfactory organ, and so on, are all prompted to activity by adequate stimuli. The same is true in regard to the determinants, they must be 'liberated' if they are to distribute themselves and migrate into the cell-body; and we have to ask how that happens, whether it is possibly due only to their own internal condition, which again would, of course, depend on the nutritive conditions of the cell in which they lie, or whether it is perhaps due to some specific stimulus which is necessary in addition to the fact of 'maturity,' just as a muscle is always ready to contract, yet only does so when it is affected by a specific stimulus.

From the very first, therefore, I have considered whether it would not be better to elaborate the determinant theory in such a way that it would not be necessary to assume a disintegration of the id in the course of ontogeny, but simply to conceive of every expression of activity on the part of a determinant as dependent on a specific stimulus, which in many cases can only be supplied by a definite cell, that is, by internal influences, and in other cases may be due to external influences.

Darwin assumed the first of these alternatives in his theory of Pangenesis, which we have still to outline. In it he attributes to his 'gemmules' the power of giving rise to particular cells, which, however, they can only accomplish when they reach the cells which are the genetic antecedents of those which the gemmules are to control. Translated into the language of our theory this view would read as follows: the whole complex of determinants is contained within every cell, as it is contained in the germ-cell, but at every stage of ontogeny, that is, in each of the developing cells, only the determinant which is to control the immediately successive cells is 'liberated,' and that through the stimulus which the specific nature of the cell supplies to the determinant. In that case there would necessarily be in every species of animal as many specific stimuli for determinants as there are determinants. This appeared to me improbable, and I rejected the hypothesis because of the enormous number of specific stimuli which it demands, but also on other grounds which will be touched upon in the course of these lectures.

Although the assumption of an autonomic dissolution of the determinant complexes of the id in the course of ontogeny seems to me imperative, I do not by any means reject the interposition of liberating stimuli, indeed I regard their co-operation as indispensable. Later on we shall discuss cases in which it is definitelydemonstrable that there may be two alternative sets of homologous determinants present in a cell, but that on any occasion only one of these becomes active, a fact which we can only explain on the assumption that only one of these is affected by the specific liberating stimulus. The phenomena of regeneration, of polymorphism, of germ-cell formation, &c., compel us to the assumption that numerous cells, even after the completion of the building up of the body, contain two or more kinds of determinants, as in a sense inactive 'accessory idioplasm,' each of which could control the cell alone, though in reality it only does control it when it is affected by the appropriate liberating stimulus. I stated this view some years ago when I attempted to define more precisely the rôle played by 'external influences as developmental stimuli[21]'. It is not, then, that I underrate the importance of external influences on the organism, but I believe that a still larger part of the determination of what shall happen at a particular point depends on the primary constituents, and that these are not alike at all parts of the body.

[21]Äussere Einflüsse als Entwicklungsreize[External Influences as Stimuli to Development], Jena, 1894.

All living processes, therefore, both those of growing and of differentiation, depend always upon the interaction of external and internal factors, of the environment and the living substance, and the resultants of the interaction, namely, the structure of the body and its parts must necessarily turn out differently, not only when the germ-substance is different, but when the essential conditions of development are changed. But that the constitution of the germ is by far the most potent factor, and that the nature of the results of development depends on it in a much greater degree than on the external conditions, has long been known. The conditions, such as warmth, may vary within certain limits, and yet the frog's egg becomes a frog; though it does not follow that the result of development may not be modified through certain changes in the conditions. The interesting experiments made by Herbst with the eggs of sea-urchins have shown that, in artificially altered sea-water in which sodium-salts are to a slight extent replaced by lithium-salts, these eggs develop into larvæ which only remotely suggest the normal structure, and diverge widely from it both in external shape and in the form of the skeleton.

Such larvæ are not able to survive, but soon perish; they are, however, of great interest from the point of view of our theory, for they show that determinants do not bring forth the same structure under all circumstances, but that, as I have already said, they are vital units of specific composition, which play a part in the course ofdevelopment, and give rise under normal external influences to normal parts, while under unusual influences, if these are not such as to prohibit development altogether, they may give rise to an abnormally formed part. It must not be forgotten that most composite parts—indeed, strictly speaking, all the parts—of an animal are not controlled by a single determinant, but by the many which successively determine the character of the cells and define the path of development of the part in question. There are no determinants of 'characters,' but only of parts; the germ-plasm no more contains the determinants of a 'crooked nose' than it does those of a butterfly's tailed wing, but it contains a number of determinants which so control the whole cell-group in all its successive stages, leading on to the development of the nose, that ultimately the crooked nose must result, just as the butterfly's wing with all its veins, membranes, tracheæ, glandular cells, scales, pigment deposits, and pointed tail arises through the successive interposition of numerous determinants in the course of cell-multiplication.

But in both processes we must presupposenormal conditions of development. In regard to the butterfly we know that abnormal conditions, such as cold during the pupal period, can cause considerable variation in the colour and marking of the wing, and in regard to the nose it can scarcely be doubted that, for instance, persistent pressure on the nasal region would result in a considerable deviation from the hereditary form.

The case of the lithium-larvæ is similar. Here the chemical conditions of the first segmentation-cells are modified by the presence of the lithium-salts, and the determinants which make their way out of the nucleus in the first and in subsequent cell-generations find an unusual soil for their activity, which diverges further and further from the normal with each successive cell-generation. Thus the whole animal is abnormally formed. The process may perhaps be compared to a plant which is negatively geotropic and positively heliotropic, that is, the stem of which tends to grow straight upwards, while all its green parts grow towards the light. If a plant of this kind have light shed on it from one side only, the stem with its leaves will grow obliquely towards that side. If the plant be then turned round so that it receives light from the other side, the stem in its further growth will curve in a direction opposite to that which it took before, and so by continually changing the position of the plant in relation to the light one could—theoretically at least—produce a plant with a zigzag stem. But this would not furnish any evidence against the presence of determinants; there are no 'upright determinants' any more than there are 'zigzag determinants' or 'crooked nose determinants,' but there are determinants controlling the nature of the cells which give rise, under normal conditions of development, to the straight stem, under abnormal conditions to the zigzag stem, or to a flat nose instead of a crooked one, and so on.

This consideration should make it clear that plant-galls are not in the remotest degree a stone of stumbling for the determinant theory, as some have supposed. Of course there can be no 'gall-determinants,' for galls are not transmissible adaptations of the plants on which they occur; they arise solely through the larvæ of the gall-insect which has laid its eggs within the tissues of the plant. But the specific nature of the different kinds of plant-cells, predetermined by their determinants, is such that, through the abnormal influences exercised upon them by the larvæ, they are compelled to a special reaction which results in the formation of galls. It is marvellous enough that these abnormal stimuli should be so precisely graded and adjusted that such a specifically definite structure should result, and in this case there is obviously a very different state of matters from that obtaining in most other processes of development, in which the chief determining factor is rather implied in the nature of the idioplasm, that is, of the determinants, than in the nature of the external influences. Here, however, the specific structure of the gall depends mainly on the quality, variety, and successive effects of the external influences or stimuli. In discussing the influences of surroundings I shall return once more to the galls.

My determinants have generally been regarded as if they were like grains of seed, from which either nothing may arise, under unfavourable conditions, or just the particular kind of plant from which the seed itself originated.

This simile is, however, to be takencum grano salis. The whole ovum is certainly comparable to a grain of seed, but single determinants or groups of determinants will always be able to adapt themselves to different influences, and to remain active even under slightly abnormal conditions, though in that case the resulting structures may be somewhat divergent. This relative plasticity is indispensable even in relation to the ceaseless mutual adaptations of the growing parts of the organism. Not only do the cells which live beside each other at the same time influence each other mutually, but the influence extends to the whole cell-lineage. No cell or group of cells develops independently of all the others in the body, but each has its ancestral series of cells on whose determinants it is so far dependent, since these have taken part in determining its own nature,in, so to speak, supplying the soil in which ultimately its own determinants will be sown from the nucleus, and whose influence modifies these last according to its quality. We might therefore say that every part is determined by all the determinants of its cell-ancestors.

If there be urged against the doctrine of determinants the undoubted fact of the dependence of individual development on external conditions, or the capacity that organisms have of functional adaptation, or especially the power that some parts of the organism have of taking a different form in response to different stimuli, I can only say that I see no reason why certain cells and masses of cells should not be adapted from the first for responding differently to different stimuli.

Therefore I see no contradiction of the determinant theory when, for instance, among the higher vertebrates, the cells of the connective tissue exhibit a great diversity of form, becoming a loose 'filling' connective tissue in one place, a tense fascia, ligament, or tendon tissue in another, according as they are subjected to slight pressure on all sides or to stronger pressure on one side. I see no difficulty in the fact that this connective tissue forms in one case bone-tissue with the most accurate adaptation of its microscopic structure to the conditions of stress and pressure which affect the relevant spot, or in another case cartilaginous tissue, when the cells are exposed to varying pressure (as on the surface of joints), or even that it gives rise to blood-vessels when the pressure of the circulating blood and the tension of the surrounding tissues supply the necessary stimulus. It is easy to see how important, indeed how necessary, the many-sidedness of these cells is for the organism, even leaving out of account such violent interference as the breaking of a bone, the irregular healing of broken ends of bones, new joint formation, and so on, and thinking only of the normal phenomena of growth. While the bone grows it is continually breaking up in the inside and forming anew on the surface, and this occurs through the power of the connective tissue-cells to form different tissues under different influences or stimuli.

We must therefore assume that there are side by side in the connective cells of higher vertebrates determinants of bone, of cartilage, of connective tissue in the narrower sense, and of blood-vessels, and that one or other of these is liberated to activity according to the stimulus affecting it. Phenomena occur also in the development of lower animals which lead us to the same assumption.

Among these is the remarkable behaviour of the primary mesoderm-cells in the young embryo (gastrula) of the Echinoderms (Fig. 92). At the point where the primitive gut or archenteroninvaginates into the interior of the hitherto single-layered blastula (Fig. 92,A), some cells are separated off (M), and move independently, constantly multiplying the while, into the clear gelatinous fluid (G) which fills the cavity of the larva, and there they fix themselves, some on the outer ectodermic layer, others to the various regions and outgrowths of the archenteron (Ms). According as these cells have established themselves at one or another point, they become connective tissue, muscle, or skeleton cells of the dermis, or contribute to the muscular layer of the food-canal and water-vascular system, or, finally, become skeleton-forming cells of the calcareous ring which surrounds the gullet of the sea-cucumber. In all this there is nothing to indicate a determination of the cells in one direction; on the contrary it seems as if the fate of the individual cells depended on the chance conditions which may lead them to one place or to another.

Fig. 92.Echinoderm-larvæ.A, blastula-stage; the primary mesoderm-cells (M) are being formed at the subsequent invagination-area of the endoderm (Ent).Ekt, the ectoderm.B, gastrula-stage; the archenteron (UD) has been invaginated (Ent), and between it and the ectoderm (Ekt) the mesoderm-cells (Ms) migrate into the gelatinous fluid which fills this cavity. There they attach themselves partly to the ectoderm, and partly to the endoderm. After Selenka.

There are thus three possibilities of development, three kinds of reaction, implied in these cells, which are all outwardly alike, and we can only understand their rôle in the building up of this very symmetrical animal if we assume that of these three only one is in each case liberated, by the specific stimulus exerted by the immediate surroundings of the cell, so that it may become, according to the chance position it takes up after its migration, either a skin-cell, a muscle-cell, or a skeleton-forming cell.

This case may be compared in some respects with the permanent colour-adaptation of those caterpillars, in regard to which Poulton demonstrated that they become almost black if they are reared on blackish-brown bark, light brown on light bark, and green if they are kept among leaves, and in all cases permanently so. In this case also the implicated pigment-cells of the skin may develop in three ways, according to whether this or that quality of the light releases this or that determinant.

But in many cases we do not know the quality of the liberating stimulus, and must content ourselves with imagining it. This is so in the case of dimorphism of the sexes. It is clear that in the males of a species the germ-cells develop quite otherwise than they do in the females, that different determining elements attain to activity in each sex, and since the primary constituents of both sexes must be contained in most animals in the ovum and in the spermatozoon, we must assume that in both there are at once 'ovogenic' and 'spermogenic' determinants, of which, however, onlyonekind becomes active in a given individual. There are, however, both among plants and animals hermaphrodite individuals, in which both kinds of sexual products are developed simultaneously or successively.

It is not only the primary sexual characters, however, that compel us to the assumption of double determinants in the germ-plasm, the secondary sexual characters do so too. We know very well in relation to ourselves that 'the beautiful soprano voice of the mother may be transmitted through the son to the grand-daughter, and that the black beard of the father may pass through the daughter to the grandson.' Thus both kinds of sexual charactersmust be present in every sexually differentiated being, some visible, others latent. In animals the determinants are sometimes handed on from germ-plasm to germ-plasm through several generations in a latent state, and only make their appearance again in a subsequent generation. This is the case in the water-fleas (Daphnids) and the plant-lice (Aphides), in which several exclusively female generations succeed one another, and only in the last of them do males occur again side by side with the females.

The germ-plasm of the ovum which is ripe for development must thus contain not only the determinants of the specific ova and sperms of the species, but also those of all the male and female sexual characters, which we discussed at length in the section on sexual selection. I then showed that these secondary sexual characters differ greatly in range and in strength, that among lower animals they are almost entirely absent, and that among higher forms, suchCrustaceans, Insects, and Birds, they attain to very different grades of development even among the same species. Thus the birds of Paradise are in most species brilliantly coloured and adorned with decorative feathers only in the male sex, while the females are simply blackish-grey, but there is a single species in which the males are almost as soberly coloured as the females. Conversely, too, we find that in parrots both sexes are usually coloured alike, but a few species exhibit a totally different colouring in the two sexes. In the same way the secondary sex differences may affect only a few parts of the animal or many, while in a few species the sexes are so divergent in structure that almost everything about them may be called different. Examples of this are the dwarf males of most Rotifers, and the males, more minute still in proportion to the females, of the marine wormBonellia viridis(p. 227).

We have now to inquire what theoretical explanation of these facts we can arrive at in accordance with the germ-plasm theory. That double determinants, male and female, for the differently formed parts of the two sexes must be assumed to exist in the germ-plasm has been already said, and we have to suppose that the same stimulus—usually unknown to us—which incites the determinants of the primary sexual characters to activity also liberates those of the secondary characters. But we may safely go a step further and conclude that there are male and femaleids, that is, that the male and female determinants belong to different ids. I infer this from the fact that in some groups, such as the Rotifers and certain plant-lice, the ova are sexually differentiated even at the time of their origin. Males and females of these animals arise from different kinds of eggs, which are even externally recognizable. Both develop parthenogenetically, so that fertilization has nothing to do with it; from the first, therefore, they must contain ids which consist of determinants of one sex alone.

If this conclusion be correct, then the sexual equipment of the determinants of the sexual characters must have taken place in the course of phylogeny in such a way that each id was affected in one direction only, and we should thus have to assume male and female ids, even before the separation of the sexes as males and females, and the same conclusion must be extended to the primary sexual characters. Only in this way can we understand the fact that differences between the sexes, at first small, have increased in the course of phylogeny to such complete divergence of structure as is now exhibited in the forms we have named,Bonellia, the Rotifers, and some parasitic worms.

But there is not only sexual dimorphism, there is also dimorphism of larvæ, e.g. green and brown caterpillars in certain species of hawk-moth (Sphinx), and there are sometimes not only two but three or more forms of a species; and in all these cases determinants of the differential parts must be represented twice, thrice, or several times in each germ-plasm, in each fertilized ovum, at least in all cases in which the different forms live together on the same area. In discussing mimicry we spoke of species of butterfly which were everywhere alike or nearly so in the male sex, while the females were not only quite different from the males, but differed greatly in many respects among themselves. Three different forms of females ofPapilio meropeoccur in the same region of Cape Colony, each of these resembling a protected model. All three forms have been obtained from the eggs of one female. In this case the female ids of the germ-plasm must be represented by three different sets, one of which, when it is in the majority in the fertilized ovum, gives rise to theDanais-form, the second to theNiavius-form, and the third to theEcheria-form of the species. Phylogenetically considered, it is probable that each of these three kinds of ids originated by itself, on a more limited area on which the protected model lived in abundance; but with a wider distribution the different female ids mingled together, were united through the males into a single germ-plasm, and now occasionally exhibit all three forms on the same area. I doubt whether there is any other theory that can offer an interpretation of these facts, and I regard them, therefore, as affording further evidence of the real existence of ids.

The polymorphism of social insects must be thought of as similarly based in the germ-plasm.

In bees there are in addition to the males and females the so-called workers, and this can only depend on the existence of special kinds of ids. Those of the workers were originally truly female, but as many of their determinants underwent variations advantageous for the maintenance of the species, they were modified into special 'worker-ids.' I postpone for the present any inquiry into the causes by which these ids come to dominate the ontogeny; obviously it cannot be by the mere fact of being in a majority over the rest of the ids, as I indicated in the case of the butterflies with polymorphic females.

In many ants the division of labour goes further still; there are two kinds of workers in the colony, the ordinary workers and the so-called 'soldiers,' and in this case the worker-id must have developed in two different directions in the course of phylogeny, and haveseparated into two kinds of ids, so that the germ-plasm of these species must contain four kinds of ids.

I might cite many more cases in regard to which the assumption of two or more kinds of determinants seems imperative, but I believe that what has been said is enough to enable any one to think out other cases for himself.

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