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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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III Preformation and Epigenesis

In the long stretch of time from Aristotle to the nineteenth century there is little to interest us from the purely theoretical point of view save the origin and growth of the preformistic theories of development. Historically the later centuries are important, and if it were our intention to trace out the evolution of knowledge about heredity and development it would be necessary to treat them in full detail. We are not, however, primarily concerned with the history of discovery, but rather with those fundamental conceptions which are ageless and persist, however much they may be altered, extended, or transformed by the discovery of new facts. We are far from wishing to minimize the importance of the discovery of facts, of that vast extension of knowledge which is the essential achievement of nineteenth- and twentieth-century science. It is indeed solely due to this patient and long-continued investigation that an understanding of the essential facts about reproduction and development at last became possible about the middle years of the nineteenth century. Until the facts of organic continuity became manifest, until the egg and the spermatozoon were recognized to be cells, until the behaviour of the chromosomes in maturation and fertilization was discovered, the basis for the modern ideas about development and heredity simply did not exist. And all this did not become fully established till about the ’seventies. We are justified therefore in passing very lightly over all theories prior to the modern, paying attention only to those fundamental ideas which, being as it seems rooted in the constitution of the human mind, are ever alive and active even in the most modern and apparently novel theories.

In the main, Aristotle’s conception of development remained dominant right down to the seventeenth century. William Harvey, following up the embryological researches of his teacher Fabricius, departed not at all in his theoretical views from the doctrine of Aristotle . 1 He had no conception of genetic continuity; the famous dictum Omne vivum ex ovo, which is ascribed to him but does not occur in this form in his writings, did not mean that all living things were developed from ova as we understand the word, and it was perfectly consistent with the belief in spontaneous generation which Harvey shared with Aristotle. The term ovum for Harvey meant the undifferentiated embryo, and covered also the larva and pupa of insects.

Harvey was, of course, like Aristotle, an upholder of epigenesis, or the gradual and successive differentiation of the germ. The opposing doctrine of preformation arose almost in Harvey’s lifetime, and was first stated by Malpighi. It is difficult to allot any one cause for the appearance of this new doctrine; it seems, however, certain that one important contributory factor was the establishment in the seventeenth century of the mechanistic view of the universe as a result of the astronomical and physical researches of Kepler, Galileo, and Newton, and the philosophical criticism of Descartes. This view was directly opposed to Aristotle’s ‘biological’ theory of the universe, and led to an entirely new conception of the nature of living things. These came to be regarded as machine-like, as soulless automata whose actions were a mechanical consequence of their structure. At the same time the introduction of the microscope revealed the structural complexity of the lower animals, and what the eye could not see through the imperfection of the instrument the mind readily imagined.

This point is well treated by Wheeler . 2

‘The microscope,’ he writes, ‘invented in the sixteenth and bequeathed to the seventeenth century, had profoundly influenced speculation. Magnification had revealed, as if by magic, the existence of a great world of structures undreamed of by the greatest intellects the race had hitherto produced. . . . The mind, full of the great microscopic discoveries of the time, was carried away by its own inertia, and, outrunning the instrument, first dreamed of and then believed in the existence of structures too minute to be revealed by the available lenses. ... It was natural but erroneous to conceive unseen structures as diminutive duplicates of the seen. The verisimilitude of this error increased when it became apparent that the microscope was unable to resolve perfectly transparent structures even of considerable size’ (p. 270).

1 See E. RAdl, Gescbicbte der biologiscken Tbeorien in der Neuzeit , i, 2nd Ed., Leipzig and Berlin, 1913, pp. 1 34-9. Also C. O. Whitman, ‘Evolution and Epigenesis' in Woofs Holl Biological Lectures for 1894, Boston, 1895, pp. 216-19.

1 W. M. Wheeler, 'Caspar Friedrich Wolff and the Tbeoria Generations^ Woofs Holl Biological Lectures for 1898 , Boston, 1899.

It was natural then to assume in the germ a complicated machine-structure corresponding with the visible structure of the adult.

The belief in spontaneous generation was also graduallyovercome in this country. Redi’s experiments showed that insects did not arise from decaying matter, as the ancients had believed, but by processes of sexual reproduction as in the higher animals. The idea of organic continuity — Omne vivum e vivo — therefore made headway, at the expense of the older Aristotelian view, and to the advantage of the preformationists.

Another factor which undoubtedly helped in the establishment of the preforma tionist view was the Biblical doctrine of creation. The early mechanistic view was easily reconcileable with the belief in a Divine Creator, who made the world-machine once for all and set it going, thereafter resting from His labours. The world and all the creatures therein were on this view called into existence by the creative Fiat, and all that happened subsequently was predestined and predetermined — the movements of the celestial spheres, all history, all development. No new creation ever took place. Hence also in the development of the living thing there could be no new formation ; the essential structure was there from the beginning, and development merely an unfolding. Preformation was essentially a derivative of the philosophical doctrine of determinism ; whether this original predetermination was due to a Divine act of creation or to ‘blind chance’ (as in the materialistic view) amounted to very much the same thing.

Wheeler, in the excellent paper already referred to, traces the preformationist view to the Tlatonizing’ tendency of the human mind. All thinkers, he considers, from the earliest times fall into one or other of two classes. The first are impressed by the flux of things, the succession of events. Theirs is the philosophy of change and their high priest is Heraclitus. The other group fasten on the recurrence of similar forms, the identities underlying change. Theirs is the philosophy of rest and fixity, and they go back to Parmenides.

The contrast in these two views reappears between Aristotle and Plato. This difference is seen in the all-pervading movement as conceived by Aristotle in his Physics, in contrast with the “ideas” of Plato. . . . Plato, under the influence of Parmenides and the philosophy of rest, emphasizes the forms and qualities that keep recurring to our minds in time and space, generalizing them into his “ideas” and endowing them with all the attributes of reality’ (p. 266).

This difference between the two types of mind comes out very clearly in the study of development :

‘Under the influence of the Christian church the Platonic conception seems to have led to the notion of the special creation of fixed types or forms. It culminated in that finished theory of predelincation in embryonic development known as emboitement. This was, in reality, the very negation of all development, since the theory held that all the individuals of a species had been created simultaneously for all time’ (pp. 267-8).

Even at the present day, Wheeler points out, the difference of attitude shows itself in our theories of development.

‘He who finds little difficulty in passing from the simple to the complex, from the homogeneous to the heterogeneous, will take an epigenetic view of development. The physiologist, who deals with processes, who is ever mindful of the Heraclitean flux, inclines naturally to this view. On the other hand, he who readily idealizes and schematizes, whose mind is endowed with a certain artistic keenness, an appetite for forms and structures, and a tendency to make these forms final patterns, eternal moulds, more permanent than the substance that is poured into them — such a one will find more difficulty in understanding how the homogeneous can become the heterogeneous. Of this type is the modern morphologist who is continually diagrammatizing, who has his eye fixed on complex static structures and conceives the continually changing form of the developing egg as a series of kinematograph pictures in three dimensions of space. He is as much inclined to Platonize as is the modern physiologist to reason along lines suggested by Aristotle. He is by nature a preformationist’ (p. 282).

That is an acute and profound analysis, and gets very near the essential truth. Epigenesis and preformation represent two different attitudes to the problem of development, arise from two fundamentally different philosophies. The epigenetic view is dynamic, vitalistic, physiological; the preformationist is static, deterministic, and morphological. The one stresses time or process, the other space and momentary state — the one emphasizes function, the other concentrates on form.

The preformistic theories, in the actual form which they took in the seventeenth and early eighteenth centuries, are of little importance or interest in relation to modern views of development and heredity. It is unnecessary for us to consider them in detail; the fascinations of the doctrines of emboitement shall not tempt us, nor the battles of the ovists and spermatists. 1

The preformationists contributed nothing of value to the understanding of our problems. Whitman sums the position up very neatly when he says: 2 ‘Evolution [preformation] was the absolute negation of both heredity and generation, while epigenesis upheld generation, but denied organic continuity, the essential foundation of heredity’ (p. 226). What is chiefly of interest to us is the general attitude of mind that produced the preformistic theories.

One of the later preformationists, however, Charles Bonnet, who belongs to the middle and later years of the eighteenth century, merits rather fuller treatment, since he was the first originator of certain ideas which play a great part in some later theories, notably Weismann’s. I refer particularly to the notion of determinants.

1 On which see R. C. Punnett, ‘Ovists and Animalculists’, American Naturalist , lxii, 1928, pp. 481-507.

2 'Bonnet's Theory of Evolution’, Wood's Holl Biological Lectures for i 8 g 4 , Boston, 1895.

Bonnet’s views have been discussed in a very adequate way by Whitman, 1 whom we shall follow in our treatment of the subject.

While some of the earlier preformationists thought of the germ as an adult in miniature, and imaginative souls saw in the spermatozoon a tiny homunculus with head and arms and feet, Bonnet makes it clear that while all the parts are represented in the germ, they have not necessarily the same form, arrangement, and appearance as in the adult. ‘Tandis que le poulet est encore dans l’etat de germe, toutes ses parties ont des formes, des proportions, des situations qui different extremement de celles que devolution leur fera revetir. Cela va au point, que si nous pouvions voir ce germe en grand, tel qu’il est en petit, il nous serait impossible de la reconnaitre pour un poulet’ (quoted by Whitman, p. 235, f.n.). The organs pre-exist as ‘organic points’ forming, as it were, the nucleus of future organization.

‘The principal difference’, he writes, ‘between the germ and the developed animal is that the first is composed of elementary particles alone, and that the meshes which they form are as narrow as possible; while, in the second, the elementary particles arc joined to an infinite number of other particles which nutrition has associated with them, and the meshes of the simple fibres are enlarged as much as the nature and the arrangement of their principles will admit’ (Whitman, p. 245).

The following passages translated by Whitman from his ‘Palingenesie philosophique’ (1769) give a good idea of Bonnet’s position.

1 first assumed, as a fundamental principle, that nothing was generated; that everything was originally preformed, and that what we call generation was but the simple development of what preexisted under an invisible form and more or less different from that which becomes manifest to our senses.

‘I postulated that all organized bodies derived their origin from a germ which contained tres en fetit the elements of all the organic parts.

1 Paper above mentioned, and ‘The Palingenesia and the Germ Doctrine of Bonnet/ in same volume, pp. 241-72.

‘I conceived the elements of the germ as the primordial foundation, on which the nutritive molecules went to work to increase in every direction the dimensions of the parts. . . . The primitive organization of the germs determines the arrangement which the nourishing atoms must take in order to become parts of the organic whole. ... I considered the organic whole, attained to its full growth, as a composite of original or elementary parts and of foreign substances which nutrition had associated with them during the entire course of life’ (Whitman, pp. 246-7).

What was preformed was not the organs in miniature, but organic particles corresponding to and determining the growth of the organs. The analogy with the ‘seeds’ of Hippocrates (the ‘semen’ coming from all the parts 1 ) and the determinants of Weismann 2 is unmistakable; nor can we miss the obvious foreshadowing of the distinction drawn by Nageli between idioplasm and trophoplasm (see below, p. 42).

Bonnet conceived these representative particles to be as a rule combined into complete germs (like Weismann’s ids), but the study of regeneration in Hydra and other forms led him to admit that they might also exist separately or in groups, forming thus germs of parts (Whitman, pp. 266-71). The explanation of regeneration, as due to germs held in reserve (previously suggested by Reaumur), is clearly the same in principle as that put forward by Weismann at a much later date. Bonnet’s views indeed bridge over the gap between the old preformation doctrine and its modern equivalent.

One further remark about Bonnet. It appears from an interesting passage quoted by Whitman that one of his main reasons for accepting the theory of preformation was that epigenetic development appeared to him to be inexplicable on mechanical grounds. ‘If organized bodies are not preformed, then they must be formed every day, in virtue of the laws of a special mechanics. Now, I beg you to tell me what mechanics will preside over the formation of a brain, a heart, a lung, and so many other organs ?’ He goes on to elaborate this difficulty, adducing the complexity, co-ordination, and harmony of structure which characterize the higher animals — and even the lower animals also, as Lyonet had recently shown for the goat-moth caterpillar.

x As ‘germs’ of these parts, cf. Aristotle, p. 17 above. a See p. 43 below.

‘I only ask one favour of those who are fond of mechanical explanations ; that is, that they will cast a glance at the wonders produced by the graver of the celebrated Lyonet. They will not behold without profound astonishment those four thousand muscles employed in the construction of a caterpillar, their admirable co-ordination, and that of the tracheae, which is no less admirable. And I am fain to persuade myself they will then feel that a whole so marvellously composed and yet so harmonious, so essentially one, cannot have been formed, like a watch, of related pieces, or by the ingraining of an infinitude of diverse molecules united by successive opposition. They will admit, I hope, that such a whole bears the indelible imprint of a work done at a single stroke. ... I limit myself to saying that, in the actual state of our knowledge of the physical world, we do not discover any rational way of explaining mechanically the formation of an animal, or even the least organ. I therefore think it more consonant with sound philosophy, because it is more consonant with facts, to admit, as at least highly probable, that organized bodies pre-existed from the beginning’ (Whitman, p. 259).

The difficulty of accounting for differentiation, the production of new form, on mechanistic principles, has driven many to deny like Bonnet the reality of development, as the only way of escaping the non-mechanical theory of epigenesis. But it is only the convinced mechanist who finds it necessary so to take refuge in the comfortable doctrine of preformation. Those who do not accept the mechanistic scheme are under no such compulsion. Here then is another proof of the close relation which exists between materialism and the preformistic theory.

With Bonnet the doctrine of preformation reached its fullest development, ripened, and fell. During his lifetime the way back to the Aristotelian doctrine of epigenesis was pointed out by C. F. Wolff, whose thesis, Theoria generations (1759), published when he was only twenty-six, is justly regarded as one of the classical writings on embryology. Wolff avoided the facile speculations about development which were popular in his day; he built up his views on a sound basis of painstaking observation, following out the development of the chick with as great accuracy as his instruments permitted, and thinking clearly and logically about what he saw. His actual theory of development is of little practical interest now; it relied upon the interaction of two forces, the V is essentialis , which was frankly vitalistic, and the ‘coagulative power’ (Solidescibilitas ). 1 He made a bold attempt to explain the development both of plants and of animals by means of these two principles. It is difficult for us nowadays to grasp what Wolff really meant ; one gets the impression in reading him that he was striving towards a physiological theory of development, but never quite getting there, on account of the almost complete lack of real physiological knowledge in his time. He is constantly trying to connect up assimilation, growth, and differentiation. Though he refers to the theory of ‘predelineation’ only here and there, his whole book is an attack upon the preformistic theory. He refutes it on the ground of simple observation. When we examine a very early chick embryo we find that ‘It consists of a mass, characterized only by its external form and condition, and for the rest composed of globules showing little coherence and simply heaped together; it is transparent, capable of movement, and almost liquid, and shows no trace of heart or vessels, nor of red blood ’. 2 He goes on: ‘In general we cannot say that what cannot be perceived by the senses does not therefore exist. This principle is more subtle than true when applied to these observations. The particles which constitute all animal organs in their earliest beginnings are little globules, which may always be distinguished under a microscope of moderate magnification. How, then, can it be maintained that a body is invisible because it is too small, when the -parts of which it is composed are easily distinguishable ?’ When we magnify the chick blastoderm we do not bring into view a miniature of the adult; the microscope reveals a totally different picture — an assemblage of globules or cells which bear no resemblance to the parts of the future embryo.

1 An account of the theory is given by Ridl, pp. 244-7, and by Wheeler, op. cit., pp. 273 ff.

Tbeoria generation «, para. 166 (p. 3 of Part II in edition published in Ostwald’s Klassiker der exakten fFissenschaf ten, Leipzig, 1896).

Wolff was constitutionally a vitalist of the Aristotelian stamp; he emphasized always the driving force behind development, and regarded form as subsidiary to function. Goethe, who links up with Wolff in an interesting way through his theory of the metamorphosis of plants, also expressed belief in a formative force or Bildungstrieb , which works put the ‘idea’ of the organism. 1

By the early days of the nineteenth century the theory of preformation had sunk to the level of a popular belief, and in his masterwork on development 2 K. E. von Baer pays little attention to it. In refuting it he takes the same line as C. F. Wolff — the direct appeal to observation. His comments on the mentality of its upholders are interesting and deserve quotation. ‘It will ever be found’, he writes, ‘that, of all possible courses we can imagine, Nature chooses the simplest and most obvious. . . . But our fantasy so easily oversteps the simple way of Nature!’ (i, p. ix). Or again, with more direct reference to the theory of emboUernent — ‘Although this hypothesis borders on the insane, it has been advocated by very distinguished naturalists, and it affords a striking example of the aberrations into which one can fall if we systematically follow assumption rather than observation. . . . But Nature gives her observer cause only for admiration at the simplicity with which she works, and for astonishment at the proneness of the human wit to explain any phenomenon which appears remarkable, by means of infinitely greater and more incomprehensible wonders’ (ii, pp. 6-7).

Baer was the founder of modern embryology, and his book was the finest work of its kind since Aristotle. Its main interest is perhaps morphological, 3 but we can glean from it a clear idea of Baer’s views on the nature of development. Baer was not a materialist. He considered that development could not be fully explained as a succession of stages causally related to one another ; there was something more involved, namely the ‘Wesenheit’, the essential nature, of the animal. It is necessary to quote his own words to make this point clear — ‘Although it is self evident that, while each step in development is only rendered possible by the preceding state, the whole course of development is nevertheless ruled and guided by the essential nature of the future organism, and that any one state is not the sole and absolute conditioning factor of the future, it is not without interest to be able to demonstrate these facts by observation’ (i, p. 147). Baer goes on to illustrate this by showing that the large variations which may occur in early embryonic life are smoothed out by a process of regulation as development proceeds. Whereas nowadays it is widely believed, under the influence of the materialistic conception, that each stage of development is the sufficient cause of the next following stage, Baer held that one stage, while it conditioned the next, was not in any full sense its cause. The real cause was, in his view, the future and final form to which development converged. As against the materialist explanation he maintained that ‘it is not the matter , in its mere arrangement, but the essential nature (the Idea according to the new school) of the procreating organism that rules the development of the offspring’ (i, p. 148). A vitalistic and teleological point of view, no doubt, but containing, as we shall see later, a truth essential to the proper understanding of development. The analogy is striking with Aristotle’s view that development is not explicable in terms of material properties such as ‘heat’ and ‘cold’, but must be regarded as due to the ‘actuality’ of the procreating male imposing a certain course of development upon the formless germinal material.

1 See Form and Function , 1916, pp. 50-1.

2 Ueber Entwickelungsgescbicbte der Tbiere, Beobacbtung und Reflexion , Konigsberg, i, 1828, and ii, 1837.

2 I have dealt with this aspect in Form and Function , 1916, chap. IX.

The main characteristics of development were, in Baer’s view, first, the increasing autonomy and independence of the developing organism, and second, the fact that development is essentially a process of differentiation proceeding from the general to the special. He distinguished three processes of differentiation in the early stages, namely, ‘primary differentiation 5 or the formation of germ-layers, ‘histological differentiation’, and the ‘morphological differentiation’ of the elementary organs. The fact that development proceeds from the general to the special he used to refute the crude form of the recapitulation theory which was current in his day, summarizing his conclusions in the following laws :

  1. That the general characters of the large group to which the embryo belongs appear in development earlier than the special characters.
  2. From the more general structural relations the less general develop, and this process continues till the most specialized appear.
  3. The embryo of any given form, instead of recapitulating other definite forms, on the contrary separates itself from them.
  4. Fundamentally the embryo of a higher animal never resembles the adult of another animal, but only its embryo’ (i, p. 224).

The five ‘Scholia’ in which Baer states and discusses the main deductions to be drawn from his description of the development of the chick, some of which we have considered briefly above, merit fuller treatment than can be given here. We shall content ourselves with quoting his own final summing up :

‘If we consider the purport of the Scholia as a whole, a general conclusion can be drawn therefrom. We found that the effect of procreation lies in this, that the part is raised to the rank of a whole; during development independence grows in relation to environment, just as the form becomes more definite; in the evolution of internal structure, more specialized parts grow out from the general parts and their differentiation increases; the individual as the bearer of a definite organic form gradually passes from the general to the more special, so that the general conclusion from these investigations and observations can thus be expressed :

The history of the development of the individual is the history of the growing individuality in all its relations ’ (i, p. 213).

Reproduction is essentially the formation of a new individuality, a new whole; the ovarian egg is under the control of the parent organism; through fertilization a whole develops from this which was originally a part (i, p. 150). Development is essentially a process of differentiation, accompanied by an increase in the independence and autonomy of the developing organism over against its environment — in sum, a growth of individuality or wholeness. We have seen above that Baer recognized also the importance of regulatory processes.

Baer’s philosophical standpoint is definitely anti-materialistic. He is clearly influenced by Kant’s teleology, but appears to go farther in the direction of a sort of universal vitalism. He refers in the second volume of his great work to the attempts being made in his own time to explain life in physico-chemical terms, and remarks that such attempts must always remain incomplete and unsatisfactory, since life can be explained only in terms of itself. The passage may be quoted:

‘But all explanations of this [materialistic] kind the physiologist finds soon to be highly incomplete, since they touch only one single side of life; and he comes to see, above everything, that life cannot be explained from something else, but must be conceived and understood in itself. The time is approaching when even the physicist must admit that in his investigations he merely puts together the isolated physical antecedents of the totality of life, and thereby fashions for himself an artificial beginning’ (ii, p. 3).

His vitalistic and teleological tendency is strongly expressed in certain of the essays republished in his Lectures. 1 Thus in an essay (dating from 1834) on ‘Nature’s most general law in development and evolution’, after discussing the evidence for evolution and considering the facts of development, he concludes that the ultimate law of all creative processes is ‘the progressive victory of spirit over matter’ {Reden, i, p. 71). In another essay in the same volume the following remarkable passage occurs: ‘Therefore instinct seems to me to be the completion of the lifeprocess. We do not regard the life-process as a result of its organic constitution but as the rhythm, almost the melody, according to which the organic body builds and rebuilds itself. Naturally the means must be found in the organism by which the individual functions of the life-process can express themselves. But from these the life-process does not come, otherwise they would have no unity’ 1 {Reden, i, p. 280).

1 Reden, St. Petersburg, i, 1864; ii, 1876.

What attitude Baer adopted towards the dualism which almost inevitably attaches to a vitalistic conception of living things is not quite clear. He was almost more a poet than a philosopher. There is a definite hint however, in the following passage, that the rigid distinction generally made between matter and spirit might be due to the limitations of the human intellect, and that this dualism could be overcome. ‘Is it not owing to our intellectual limitations that we are compelled to regard the past, present, and future as always separated, that we can form no clear idea of an existence without a beginning and conditioning causes, because we must be always demanding such causes, and that we treat force and matter, spirit and body as separate in their essence, although they are spatially united ?’ {Reden, ii, pp. 79-80). Truly a penetrating and prophetic passage!

1 Cf. Aristotle, p. 17 above.

   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
Historic Disclaimer - information about historic embryology pages 
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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)
Historic Disclaimer - information about historic embryology pages 
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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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