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Russell ES. The interpretation of development and heredity. (1930) Oxford. Univ. Press.

   The interpretation of development and heredity (1930): 1 Introductory | 2 Aristotle’s ‘De Generatione Animalium’ | 3 Preformation and Epigenesis | 4 The Germ-Plasm Theory | 5 The Theory of the Gene | 6 Some Modern Epigenetic Theories | 7 Wilhelm Roux and the Mechanics of Development | 8 The Mnemic Theories | 9 Retrospect. The Use and Misuse of Abstraction | 10 The Organismal Point of View | 11 The Physiological Interpretation of the Cell Theory | 12 The Cell and the Organism | 13 The Cell in Relation to Development and Differentiation | 14 The Organism as a Whole in Development and Reproduction
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IV The Germ-Plasm Theory

In the charming preface to his Vortrdge uber Descendenztheorie (1002) Weismann remarks that a theory to be really valuable and stimulating should be clear cut and definite, so that if it be faulty it can be amended and if false rejected. The remark applies with special force to his own theory of the germ-plasm, which, though later shown to be incorrect in many respects, exercised through its precise and clear formulation a great and lasting influence on biological thought.

Weismann summed up in his theory many ideas which were just becoming ripe for expression, and he owed much to Darwin, Galton, Nageli, and H. de Vries; it is convenient to take his theory as typical of a method of thought which even at the present day retains a considerable measure of vitality.

The development of the cell-theory had by the ’fifties and ’sixties of the nineteenth century led to a clear conception of cellular continuity; it was known that both the ovum and the spermatozoon were single cells, and it was held that all development took place by means of division and growth of cells. The way was thus open for an explanation of heredity in terms of the transmission from germ-cell to germ-cell of the essential potentialities of development. A direct unchanged continuity between the germ-cells of one generation and the germ-cells of the next could, however, be demonstrated only in rare cases ; and his theory of heredity was from the beginning stated by Weismann in terms of germi nal substance, not in terms of germinal cells. In his essay 0^883, ‘Ueber die Vererbung’, after pointing out that the germ-cells become differentiated at very varying periods in the life-history of the different groups, he goes on : ‘Thus, as their development shows, a marked antithesis exists between the substance of the undying reproductive cells and that of the perishable body-cells. We cannot explain this fact except by the supposition that each reproductive cell potentially contains two hinds of substance, which at a variable time after the commencement of embryonic development, separate from one another, and finally produce two sharply contrasted groups of cells.’ 1

In this essay the essential lines of the germ-plasm theory are laid down, though in general terms, and the point is emphasized ‘that the understanding of the phenomena of heredity is only possible on~the fundarneiTrai yggp 5 sit ipiTb f the continuity ' oithe germ~plasm’ YiBiJ.. p. 106). 2

Itls, however, in the paper of 1885, ‘Die Kontinuitat des Keimplasmas’, that we find the clearest summary of the basic principles of his theory. Here he writes, after criticizing and r ejecting Darwin’s theory of pangenesis, that there remain “

‘only two other possible, physiologically conceivable, theories as to the origin of germ-cells, manifesting such powers as we know they possess. Either the substance of the parent germ-cell is capable of undergoing a series of changes which, after the building-up of a new individual, leads back again to identical germ-cells ; or the germ-cells are not derived at all, as far as their essential and characteristic substance is concerned, from the body of the individual, but they are derived directly from the parent germ-cell.

‘I believe that the latter view is the true one. ... I propose to call it the theory of “The Continuity of the Germ-plasm”, for it is founded upon the idea that heredity is broug ht abo ut by the trans fere nce from one g eneration to another , of a substance with a d efinit e chemical. aiTd above all, molecular constitution. I have called this substance “germ-plasm ”, and have assumed that it possesses a highly complex structure, conferring upon it the power of developing into a complex organism. | I have attempted to explain heredity by sup-| posing that in each ontogeny, a part of the specific germ-plasm is not| used up in the construction of the body of the offspring, but is reserved unchanged for the formation of the germ-cells of the follow- ( ing generation.

‘It is clear that this view of the origin of germ-cells explains the phenomena of heredity very simply, inasmuch as heredity becomes thus a question of growth and assimilation, the most fundamental of all vital phenomena. If the germ-cells of successive generations are directly continuous, and thus only form, as it were, different parts of the same substance, it follows that these cells must, or at any rate may, possess the same molecular constitution, and that they would therefore pass through exactly the same stages under certain conditions of development, and would form the same final product. The hypothesis of the continuity of the germ-plasm gives an identical starting-point to each successive generation, and thus explains how it is that an identical product arises from all of them’ (ibid., p. 170).

1 A. Wcismann, Essays upon Heredity , Eng. Trans., 2nd ed., vol. i, Oxford, 1891, P- 74

The fundamental idea of the continuity of germinal substance had already been expressed by Galton (1875), J ae 8 er (1876), and Nussbaum (1880).

There are two fundamental points to note in this confession of faith; first^ that heredity is due to the transmission from one generation to another of a peculiar subs tance of complex molecular structure, which though subject to metabolism remai ns e ssentia lly un cha nged ; second, that this substance exercises a di rect, determining, influence upon development^ The fact that development takes the same course in successive generations is explained quite simply by the supposition that its starting-point is always the same. The environment of the developing organism is assumed to be constant and is thereafter left out of the picture.

In the essay of 1883 the germ-plasm is not identified with any particular constituent of the cell; it is treated as a hypothetical substance whose existence logic forces us to postulate. In 1885, however, Weismann identifies it with the nuclear substance of the germ-cells. The brilliant cytological work of O. Hertwig, E. Strasburger, E. van Beneden, and others, which was bearing fruit about this time, showed clearly that the essential thing in normal fertilization was the union of the maternal and paternal nuclei. Pfliiger’s discovery of the apparent isotropism of the frog’s egg seemed to indicate that the cytoplasm took no part in the transmission of the hereditary characters (Weismann, 1885, p. 179), and Strasburger ’s studies of fertilization in Phanerogams appeared to afford proof that only the nucleus of the male cell is here concerned in the process of fertilization. As the contribution of the male and the female parents to the hereditary equipment appears as a rule to be equal, and as the nuclei are the only equivalent structures in the male and the female gametes, the deduction seemed obvious that the hereditary tendencies must be carried by the nuclear substance alone. J_Nageli, whose conception of a determinant idioplasm (1884) exercised considerable influence on Weismann’s thought, was apparently the first to draw the conclusion that the hereditary substance must be present in equal amounts in the egg and the sperm, and must therefore be present in minimal quantity^ His idioplasm, however, was a hypothet ical substanc e extending as a network throughout all the cells of the body. 1 It was left to O. Hertwig, Strasburger, Kolliker, and Weismann to enunciate, all about the same time, the modern theory that the physical basis of heredity and development is to be sought in the nuclear substance, and more particularly in the chromosomes.

It is not necessary for our particular purpose here to follow up the further history of cytological discovery, 2 nor to enter into full details concerning Weismann’s theory. We shall limit ourselves to considering such points as are of methodological interest, particularly in connexion with his theory of development.

Of fundamental importance is his conception of the germinal substance as the determiner of development. The chromatin granules, he tells us, are the most important constituents of the nucleus,

‘for we must assume that it is their influence which determines the nature of the cell, which, so to speak, impresses it with the specific stamp, and makes the young cell a muscle-cell or a nerve-cell, which even gives the germ-cell the power of producing, by continued multiplication through division, a whole multicellular organism of a particular structure and definite differentiation, in short, a new individual of the particular species to which the parents belong. We

Nageli was also the first to take the dubious step of distinguishing the hereditysubstance or idioplasm from the nutritive substance or trophoplasm — a distinction accepted and approved by Weismann, and by him explained on the principle of the division of labour (1902, i, p. 374).

1 This is well treated in outline by E. B. Wilson, The Cell in Development and Heredity , 3rd ed., New York, 1925, chap. I.

call the substance of which these chromatin granules consist by the name first introduced into science by Nageli, though only to designate a postulated substance which had not at that time been observed, but which he imagined to be contained within the cell-body — by the name Idioplasm , that is to say, a living substance determining the individual nature (etSos — form)’. 1

The idioplasm accordingly exercises a direct formative influence upon the cell containing it, determining what sort of cell it will become. (This idea of active determiners working upon the more or less inert cell dates back to Darwin's theory of pang enesis (1868) and is prominent also in De Vries’ theory o f intra- celljular pangenesis (1889)). The idioplasm of the germ-cell — the hereditary or germinal substance proper — is conceived to be of a complex and orderly architecture, built up of self-propagating units or determinants, each of which is destined to be the formative agent of some particular part of the organism or of some particular group of cells. Weismann had been greatly impressed by the evidence that many single characters of the organism can vary quite independently of the rest and can be separately transmitted. 2 It seemed to him a necessary corollary that such independently heritable characters, or their constituent structural elements, must be represented in the germ-plasm by separate determinants. He concluded therefore that the germ-plasm must be compounded of determinant units, and that there must be a point-to-point correspondence between these units and the parts of the developing and adult organism which they produce. The theory of determinants is clearly stated in the following passage :

‘I assume that the germ-plasm consists of a large number of different living particles, each of which stands in a definite relation to particular cells or kinds of cells in the organism to be developed, that is, they are “primary constituents” in the sense that their co-operation in the production of a particular part of the organism is indispensable, the part being determined both as to its existence and its nature by the predestined particles of the germ-plasm. I therefore call these last Determinants (Bestimmungsstticke), and the parts of the complete organism which they determine Determinates , or hereditary parts’ (ibid., p. 355).

1 English trans. of the Vortrdge by J. Arthur Thomson and M. R. Thomson, under the title The Evolution Theory , London, 1904, vol. i, p. 287.

The conception of ‘unit characters’, due originally to Nageli, was elaborated particularly by H. de Vries in his Intracellular Pattgenests , Jena, 1889.

The de terminants are to be regarded not as specific chemical substances, however complex, but as living unit s, capable of ass i milation, growth, and r ep rodu ction by division. Weismann here follows the physiologist Brucke 1 and others (H. Spencer and Darwin, for example) in supposing that all living matter is composed of invisibly minute units or biophors, each of which manifests the essential characteristics of life. Determinants may sometimes be single biophors, notably in unicellular organisms, but more often they are groups of biophors bound together in a higher unity. In their turn the determinants for the whole organism are bound up together to form an Id.

✓TIow do the determinants act ? As they are essentially cell-determiners it is necessary that in the course of development they reach the cells or cell-groups on which they are to impose their particular character. For this to be possible it must be assumed that the Id has a definite and complex architecture, in which each determinant and each group of determinants has its proper and definite position. In the course of development the Id must be divided up in such a way that the determinants for the different regions of the body become separated and sorted out at the proper times and places. There must in other words be a progressive disintegration of the Id-complex by division, which proceeds in an orderly and predetermined fashion, until finally each separate kind of determinant is sorted out into the cells which it is predestined to transform. It is implied in this process that the Id can divide not only into identical parts (by ‘Erbgleich’ division), but also into unequal and dissimilar parts (by ‘Erbungleich’ division).

1 ‘Die Elementarorganismen', S. B. Akad. IViss, Wien, xliv, 1861.

We may quote Weismann’s own description of the process :

‘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 rise 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 each 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’ (ibid., pp. 377—8).

When as a result of this orderly disintegration of the original complexity of the Id each kind of determinant reaches its allotted station in a particular group of cells, it becomes active and proceeds to shape the cells to their predestined form. To quote Weismann again:

‘My conception of the manner in which the determinants become active 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 chlorophyllmaking 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 cooperation with the elements already present in the cell-body give rise to chlorophyll or muscle substance’ (ibid., pp. 379-80).

A little later on in the same discussion Weismann makes it clear that the determinants are not ‘seed-grains’ of individual characters, but co-determinants of the nature of the parts which they influence. In sum, he conceived the process of cell-differentiation to be 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’ (ibid., p. 381).

The theory of the qualitative division of the nucleus as the basic phenomenon in differentiation, which Weismann worked out in such detail, had of course already been put forward by W. Roux, 1 on the strength of his own and Pfliiger’s experiments on the egg of the frog, and the early results of work on experimental embryology appeared for a time to corroborate it.

1 Ueber die Zeit der Bestimmung der Hauptrichtungen der Froscbembryo , Leipzig, 1883. See account by J. W. Jenkinson, Experimental Embryology , Oxford, 1909, pp. 17-19, and 158-62.

But, as is well known, the further progress of experimental research brought to light facts which were irreconcilable with the theory, 1 and it is now no longer tenable. Its place has been taken by the more definitely substantiated view that the ‘germ-plasm’ is represented by the complete set of chromosomes, and that the complete (double or single) set is present in all the cells of the organism, somatic and germinal alike. We shall consider this modern form of the germplasm theory — the theory of the gene — in the next Chapter. The theory of a germ-plasm present in all cells was also developed by de Vries and O. Hertwig, prior to the modern gene theory. 2 It is interesting to note in this connexion that in his paper of 1885 Weismann declared the theory of an omnipresent germ-plasm to be untenable. A propos of Nageli’s idioplasm, he pointed out how improbable it was that this substance could have the same constitution everywhere in the organism and during every stage of its ontogeny. For if this were so, how could the idioplasm effect the great differences which obtain in the formation of the various parts of the organism ? 3 A very pertinent objection, which it is difficult to meet.

Although Weismann’s theory of development can no longer be upheld in the present state of knowledge, the fundamental assumptions and conceptions underlying it are, many of them, still commonplaces of biological thought. We may summarize them as follows :

(1) For Weismann the ultimate goal was the explanation of vital phenomena in terms of physico-chemical action: he was a child of his time, and the materialist philosophy seemed to him the only possible foundation for scientific biology. Hence the emphasis laid on continuity of substance , and the formal solution offered of the problem of heredity on the basis of identity of starting-point. If the complex material configuration a brings about under standard conditions the development of the specific organism A, and if this configuration is transmitted unchanged to serve as the starting-point of the next generation, it follows on the general principles of ‘Mechanism’ that there will be developed a replica of organism A, provided that the environment remains reasonably constant. So runs the materialistic argument. One main underlying assumption of Weismann’s theory of heredity and development is then the correctness and adequacy of the mechanistic conception of life.

1 See Wilson, 1925, pp. 1059-62.

a See Delage, 1895, sections on de Vries and Hertwig.

3 Essays upon Heredity , 2nd ed., i, p. 184. He adopts, however, a less intransigeant attitude in 1902, see Vortrage y i, pp. 419 and 445 (Eng. Trans., i ,pp. 382 and 407).

(2) Weismann realized, however, that the time was not ripe, nor knowledge sufficient, for the elaboration of a purely physico-chemical theory of heredity and development — he had some pertinent criticisms to make of the chemical epigenesis theory of Delage (1895). 1 He put forward his determinant theory as a first approach to a physiological analysis of the problems, believing himself justified in taking as given the properties of the elementary vital units which he conceived to be the components of all living substance. But he thought of the properties of his biophors and determinants as being the direct outcome of their material configuration.

(3) We must regard as fundamental to his theory the particulate conception of living things, according to which all living structures are built up of ultra-microscopic units or biophors, each of which manifests the essential characteristics of life — metabolism, growth, and movement. The origin of this general conception is obscure, 2 but it has a very long history; it has obvious relationship with the atomistic views both of matter and of living substance which can be traced back certainly as far as the Greeks ; it is in a sense a biological atomism. The particulate conception is of significance even at the present day, for more than a trace of it remains in the modern concept of the gene. It exemplifies very clearly a common methodological error, that of ascribing to an abstract part or component of an organism or a cell functions and capabilities which belong in reality only to the cell or organism as a whole (see below, pp. 153, 155).

1 Vortrdge , i, pp. 439-40 (Eng. Trans., pp. 401-2).

2 See historical account in E. Ridl, Gescbichte der biologiscbett Tbeorien , ii, Leipzig, 1909, pp. 386-9. Wilson’ 8 account of the modern evolution of the idea is as follows: ‘Briicke’s suggestion, that the cell might be a congeries of bodies more elementary than itself, found a much fuller expression in Herbert Spencer’s theory of physiological units, but it was Darwin’s theory of pangcnesis that laid the real basis for what followed in the works of de Vries, Wiesner, Weismann, and Hertwig,’ Wood's Holl Biological Lectures for 1898 , Boston, 1899, p. 3.

There is a very interesting passage in the V ortrage , bearing on this point, which is worth while quoting in full :

‘Some modern biologists deny that there is any hereditary substance per se , and believe that the whole of the germ-cell, cell-body, and nucleus together, effects transmission. But though it must be admitted that the nucleus without the cell-body cannot express inheritance any more than the cell-body without the nucleus, this is dependent on the fact that the nucleus cannot live without the cellbody; if it be removed from the cell and put, say, into water, it bursts and is dissolved. But the cell-body without the nucleus lives on, though of course only for a few hours or days, and its metabolism ceases only when it is brought to a standstill by the failure to replace by nutrition the used-up material. Thus the argument used by those who deny the existence of a hereditary substance would be paralleled if we denied that Man possesses a thinking substance, and maintained that he thinks with his whole body, and even that the brain cannot think by itself without the body.’ 1

Actually of course a ‘heredity substance’ is as much an abstraction as a ‘thinking substance’, and a man does think with his whole body, in the sense that the integrity of the body as a whole is an essential condition of the life and activity of the brain, the sense organs, and the neuromuscular system generally. As we shall see in more detail later (below, p. 155), elementary vital units such as biophors are likewise purely hypothetical and abstract constructions or figments of the intelligence; the only objects manifesting the full powers of life are living organisms, unicellular and multicellular. It is unjustifiable to fragment the living organism into lower units, to ignore the problem of ‘composition’ or wholeness, and to ascribe to these units the powers and capabilities which we know only as belonging to the organism as a whole. Weismann’s determinants, by the way, by means of which he purports to explain development and heredity, themselves manifest these phenomena (as do the chromosomes also) — a point which Aristotle made with regard to the ‘seeds’ of the pangenesists. 1

1 P- 3735 Eng. Trans., p. 340.

1 See above, p. 17.

Cf. Weismann’s own expression ‘Werde-Bedeutung’ ( Vortrdge , i, p. 414).

(4) The peculiar weakness of the particulate conception of living things comes out very clearly when we consider the presumed activities of the determinants, which are themselves biophors or groups of biophors. They are conceived to be controllers or shapers of form, exerting upon the cytoplasm a modifying influence which cannot be clearly defined. With justice they might, though material, be likened to the entelechies of Driesch (see below, p. 102), for their mode of action is equally mysterious and equally far divorced from any known physiological process. The notion of determinant (in so far as it is taken to mean something more than a conditioning factor) is in fact a confused and unclear notion, deriving by way of the particulate conception from an underlying conviction of the reality of material determinism.

(5) Turning now to Weismann’s theory of development regarded as a whole, we note at once that it is purely preformistic. To the developed organism there corresponds, point for point, the complex architecture of the Id. Development is brought about by the orderly disintegration of this complexity, so that, so far as the Id is concerned, development is a process of simplification which proceeds fari j passu with the visible increase in complexity of the organism itself. The two processes are exactly complementary. What Weismann has done is to give an inverted redescription of the process of development in terms of a purely hypothetical complexity which simplifies itself. To translate the Idtheory into terms of the visible events it is only necessary to read ‘potentiality’ 2 of development for ‘complexity’ of the Id-architecture. The richer the potentiality, the less advanced the process of differentiation of visible structure, the more complex the structure of the idioplasm. The correspondence is strictly one of logical relation — hence the perfection of the theory. As is well known, Weismann had no difficulty in accounting for even the most astounding cases of regeneration by the simple and logical method of postulating a reserve idioplasm of the proper complexity. The theory is indefinitely extensible, and can surmount any difficulty; all that is necessary is to postulate for every new potentiality a new determinant or complex of determinants. But just in this logical perfection lies the essential weakness of the whole theory.

Why this type of reasoning makes so great an appeal to certain minds is an interesting psychological problem. It would seem that the mind jibs at the thought of a real evolution of complexity from simple beginnings, and seeks somehow to insert the complexity there at the beginning. But the plain evidence of the senses is that visible complexity does arise from visible simplicity, and there is nothing to be gained by a hypothetical inversion of the process.

It is perhaps hardly adequate to describe Weismann’s theory of development as preformationist. The early preformationists regarded development as a simple unfolding of complexity already present ; Weismann holds that development is essentially the progressive simplification of an original complexity ; to the appearance of visible complexity there exactly corresponds a loss of complexity in the invisible formative germ-plasm.

(6) It is interesting to note how Weismann’s theory enables him to deal with the problem of the harmony of development, of the proper co-ordination of developmental processes in time and space. For Weismann this is a preestablished harmony, fixed in its main lines by the orderly architecture of the idioplasm. The harmony or wholeness is there at the beginning and persists throughout development, being maintained by the structural orderliness of the germ-plasm. The normal equipment of germ-plasm suffices for normal development; if the conditions change and the organism must adapt itself, the emergency is met by groups of determinants held in readiness at the proper place for all likely eventualities. Again a redescription or translation in terms of hypothetical entities rather than a real explanation.


(7) It is hardly necessary to point out that Weismann’s theory is essentially a morphologist’s theory. He deals in terms of parts, of structures, and pays little attention to function or environment. The organism is regarded as a co-ordinated assemblage of unitary parts, each of which can vary independently of the others, each being represented in the germ-plasm by a special determinant or group of determinants, occupying its proper place in the germinal architecture. The relation between determinant and determinate is never stated in physiological terms, never linked up with known physiological processes. The theory is therefore nonphysiological, and determinants are not merely abstractions, but morphological abstractions.

If we now sum up this elementary discussion and attempt to appraise the general value of Weismann’s theory in its relation to the major problems, we may state our conclusions as follows : the theory is based explicitly upon a mechanistic conception of life; it offers a purely formal solution of the problems of differentiation, harmony, regulation, and heredity, in terms of the postulated activities of a hypothetical particulate substance, the idioplasm or germ-plasm, but this solution amounts to nothing more than a translation of the facts into what may appear to be, but is not, a simpler and more understandable form; it takes no account of the changing relationship between form and function during development, and it is pre-eminently a morphological, nonphysiological theory, working in terms of abstract structural units of different grade, and their relations one to another.

Weismann does, however, take into consideration the relation between phylogeny and ontogeny which finds expression in the fact of recapitulation. His theory allows for the origin of new determinants in the course of evolution, and the consequent appearance of new characters in ontogeny. Speaking of his determinants and groups of determinants, he writes :

‘That they do not enter into activity all at once, but successively take their part in development, seems to me a necessary consequence of their successive origin in the phylogeny ; and the ontogeny, as we shall see later, arises through a modified condensation of the phylogeny. Now since every new determinant that arises in the course of phylogeny can only develop by division and subsequent variation from the determinants which were previously active at the same place in the organism, it is quite intelligible that later on, when the phylogeny has been condensed in the ontogeny, they should not enter upon their active stage at the same time as their phyletic predecessors, but after them.’ 1

In the foregoing discussion we have dealt only with the broad outlines of Weismann’s theory, and mainly with the theory as set out in the first edition of the V or tr age (1902). Much detail has accordingly been omitted, and it should be noted that Weismann modified some of his views in the light of advancing knowledge. 2 The main underlying ideas remain however the same, and it is his earlier writings that are historically important.

1 Eng. Trans., i, p. 405,

2 See the third edition of the Vortrage , 1913.

   The interpretation of development and heredity (1930): 1 Introductory | 2 Aristotle’s ‘De Generatione Animalium’ | 3 Preformation and Epigenesis | 4 The Germ-Plasm Theory | 5 The Theory of the Gene | 6 Some Modern Epigenetic Theories | 7 Wilhelm Roux and the Mechanics of Development | 8 The Mnemic Theories | 9 Retrospect. The Use and Misuse of Abstraction | 10 The Organismal Point of View | 11 The Physiological Interpretation of the Cell Theory | 12 The Cell and the Organism | 13 The Cell in Relation to Development and Differentiation | 14 The Organism as a Whole in Development and Reproduction
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

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