Book - Outline of Comparative Embryology 2-3

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This historic 1931 embryology textbook by Richards was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.

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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)

Chapter III The Recapitulation Theory

The corner-stone of the science of embryology as it developed during the last half of the nineteenth century was the recapitulation doctrine. This doctrine is also called the biogenetic law, or in Haeckel’s term, the fundamental law of biogenesis. To it was due much laborious and painstaking research, and the interpretations based upon it have had far-reaching significance. Yet during the last decade critical studies have dethroned this doctrine so that as a “law” it is now of value only historically. As a tendency, however, recapitulation is still a useful conception for it unquestionably expresses a partial truth, a fact to which is due the long-continued interest in the doctrine and the stimulating effect it has had upon embryological studies.

Recapltulation refers to the parallel which exists between the history of the race, in its largest sense, and the development which an individual organism belonging to that race goes through. “Ontogeny rccapitulates phylogeny.” The life cycle of an individual is a brief summary of racial history. The animal kingdom presents innumerable instances which illustrate this principle. The development of the frog tadpole, through stages showing successively external gills, hind legs, front legs appearing later, and the disappearing tail and gills, closely parallels the taxonomic series of legless apoda and the salamanders. This series begins with the lower urodeles which have external gills and weak legs, then comes Amphzuma with small posterior legs, the Salamandridae in which external gills are lost, the legs are about equally developed, and the tail is about equal to the body in length, and finally the Anura which have no gills or tail and the hind legs are the better developed. (fig. 190.)

The development of the decapod Crustacea, in which the larvae pass through stages comparable to those characteristic of some other orders of the Malacostraca (fig. 191), is an often-cited case to illustrate the biogenetic law. Another illustration involves the ascidians (fig. 192) which superficially resemble shapeless jelly-like masses, and whose relationships are therefore difficult to decipher. A study of their embryology revealed the fact that they pass through a tadpole stage with dorsal nerve chord, notochord, etc., all recognizable as vertebrate char acters, but when they settle down and become sessile these structures 286 THE RECAPITULATION THEORY 287

gradually undergo degeneration and the creature loses all recognizable signs of its former organization. An analogous situation obtains in the

d 8 fiG. 190. Early stages in the development of a frog tadpole compared with adults of Siren (e), Amphiuma. (f), Desmoonathus (0), and an adult frog (h).

Cirripedia or barnacles. Barnacles have heavy shells, are sessile, and in general are quite unlike the ordinary Crustacea to which their developZ66 'l‘l'1l!2 RECAPITULAT ION THEORY

inent clearly shows them to be allied. A second suborder of the Cirripedia doubtless presents the most extreme case of modification to be found in the animal kingdom. This is Rhizocephala to which the parasitic Sacculina belongs. (fig. 193.) During its larval stages Sacculina is a free-swimming form having about the usual external features of a young braiichiopod crustacean. It attaches itself to the abdomen of a crab, sends root-like branches into the body by means of which it obtains nourishment, and completes its development to the adult stage. The

F1G- 191 -\. \ young lobster at the time of the third molt (after Herrick), and Ii. An adult A1’,[”37-'3 (after Verrill).

(~x., exopodite, en., endopodite.

adult is so degenerate that it has no crustacean characteristics but is merely a tumor-like sac on the abdomen of the crab in which even the usual internal organs are scarcely present, the body being chiefly filled with the gonads and their products. Only its life cycle reveals its relation to the other Cirripedia. The anatomical relationships by means of which its phylogenetic position is determined are made clear by a study of its cmbryological development.

Vaguely foreshadowed by the writings of Meckel and others of the period, the recapitulation principle is first directly hinted at in the writings of von Baer. Indeed, by some von Baer’s laws have been conTHE RECAPITULATION THEORY 289

fused with the recapitulation theory. These laws are, however, Inore properly regarded as alternative to certain phases of recapitulation. They were formulated by Von Baer in 1828 as follows:

“1. The more general features of a large division of animals arise in the embryo earlier than the special features.

“2. From the most general features of structure arise those that are less general, and so on until the most specific features arise.

“3. The embryo of a definite species tends away from the specific forms of other species, instead of passing through them.

In. 19.2 An adult mcidmn (\) ((}Illl)LlrUd \\lli| 1 lirxal {min of the same type (B) ( Uter Deluge and Huounrd )

“4. Fundamentally, therefore, the embryo of any higher species is never like the lower species, but only its e1nbryo”—(Lillie).

It will be noted that these laws antedate the cell doctrine and the publication of the “()rigin of Species,” and that the simultaneously enunciated germ-layer theory represented the last word in emhryological knowledge of the time in the matter of early structural organization. The advances made possible by the cell doctrine resulted in the enunciation by Fritz Muller in 1863 of the recapitulation doctrine as such. Haeckel developed the theory and applied it in his famous blasteagastrea hypothesis.

Haeckel’s hypothesis lays stress on the importance of the universal occurrence in the embryology of animals of the single-celled egg, the 290 THE RECAPITULATION THEORY

blastula, and the gastrula, and supposes that the phylogenetic development has taken a similar course. The protozoa represent the first step and are comparable to the egg stage in individual development. No animal is known which exactly corresponds to the blastula or the gastrula, although there are protozoan colonies quite like the former in some respects and simple eoelenterates have features in common with the latter. Haeckel, therefore, supposed that there must have been stages corresponding to each of these forms which had now ceased to exist,

and to these hypothetical forms he gave the names “Blasted” and “Gastrea.”

Fm. 193. A nauphus (A) and u (‘ypris larva (B) compared with an adult ((‘) Saeculma carcmt. (After Deluge.)

Haeckel also pointed out the significance of certain characters as indicating phylogenetic history whereas others are to be regarded as embryological adaptations of importance to the individual organism, but not to the race. Characters of the first class are spoken of as palingenetic; of the second, as cenogenetic. A chick embryo, for instance, has a good many characters which recall fish or amphibian conditions, but it could not exist in the environment where these forms dwell, and is different in numbers of other characteristics including the important one that it will develop not into a fish or amphibian, but a bird. It cannot, therefore, in its development represent with entire correctness the ancestral forms. Some of its characters are those which adapt it to development in a shell at a temperature of 37° C. and enable it to live in the peculiarities of its own environment. These are new or ceno— genetic characters. Other features, as the aortic arches, gill slits, brain THE RECAPITULATION THEORY 291

vesicles, and others, are supposed to be of ancestral significance and are, therefore, palingenetic.

A critical consideration of many cases which may be cited, however, shows that the facts do not all easily fit into the biogenetic interpretation. When one attempts to classify clear cases such as the shell tooth on the one hand or the aortic arches on the other as adaptive or ancestral little difliculty is experienced, but the problem of applying the same classification to a doubtful ease becomes much more complex, and as a rule the results cannot be certainly relied upon. Indeed, recognition of a particular character as palingenetic or cenogenetic may be quite an impossible task.

Other difficulties may be cited which require modification of the strict biogenetic way of looking at the animal kingdom. For example, there is a general tendency which becomes more marked the higher one goes in the animal kingdom to shorten and condense the ancestral phases of an organism’s development in favor of the specializations which relate it to its own environment. This tendency, in addition to complete changes of developmental conditions (for example, the long placental life of mammals, or the abundance of yolk in bird eggs), may result in the entire omission of certain stages (as in the case of embryos where gills do not develop in connection with gill arches).

This abbreviation or contraction of development is called by some writers tachygenesis. In the brief period of embryonic development, if reeapitulation occurs even in the most sketchy fashion, an immense contraction of stages must be thought to take place so that the necessary abbreviation may be accomplished. To this shortening of the developmental time, or tachygenesis, there are two aspects, the actual quickening of the rate of development and the fusion and omission of certain stages. The one aspect may occur without the other in special cases as in the response of the developmental rate to rise in temperature, where the rate is hastened without fusion or omission of stages.

Hcterochrony also presents an argument against the reeapitulation doctrine, as was pointed out by Keibel, Mehnert, and others. By heterochrony is meant the disturbances, which now are known to be of quite common occurrence, in the time of appearance of structures in a known sequence of stages.

These modifications in the rates of development bring about results which cannot be interpreted either as of phyletic or adaptive significance. Stockard has lately pointed out the importance of the arrest of development, even over a small part of the germ ring, whereby various types of monsters are produced. Evidently, here are structural modifications not to be related to recapitulation. Furthermore, the normal develop292 THE RECAPITULATION THEORY

ment of the organ systems of the embryos of different classes does not preserve the same synchrony as may be noted by comparing tables of the development, let us say, of the chick and pig (Keibel, N ormentafeln). In addition, there are extreme cases in which the actual order of development is the reverse of what would be demanded by recapitulation, as, for example, in those cases in which the joints become rounded off before movement is acquired. Another illustration is the formation of the tongue and the teeth. There is no question that in the phylogenetic series teeth made their appearance before tongues, but in the embryological development of mammals the teeth are much later developed than the tongue. Ontogeny not only fails to recapitulate phylogeny in this case, but completely misrepresents the facts.

In a recent discussion De Beer has summarized the possibilities of variation which heterochrony offers in comparing the appearance of structures in the individual with that in the ancestral forms. This summary with no further elaboration is sufficient to make clear that some of the possible variations not only do not fit in with the conception of recapitulation but are actually opposed to it. The summary is as follows 2*

“A character which is present or makes its appearance in the young stage of an ancestral animal may in the ontogeny of a descendant

appear:

“A. In the young stage only, producing youthful adaptations or caenogenesis,T not affecting phylogeny.

“B. In the young and adult stage, producing a substitution of the new adult condition for the old, resulting in a progressive denation in the ontogeny of the descendant from that of the ancestor.

“C. In the adult, by a relative retardation of the development of the bodily structures as compared with the reproductive organs, resulting in paedogenesis and neoteny.

“A character which is present in the young and adult stage of an ancestral animal may in the ontogeny of a descendant appear:

“D. In the young stage only, resulting in the reduction of the character to a vestige.

“A character which is present or makes its appearance in the adult stage of an ancestor may in the ontogeny of a descendant appear:

“E. In the adult stage, resulting in those differences which distinguish individuals, varieties, and races: adult bariation.

Reprinted from De Beer's “Embryology and Evolution,” by permission of Oxford University Press. ’{ The same as cenogenesis. THE RECAPITULATION THEORY 293

“F. In the late adult stage, 1'.e., too late, resulting in the reduction of the character to a vestige by retardation.

“G. In the same stage, which is no longer adult, the new adult stage being relatively delayed, resulting in overstepping the previous ontogenies or hypermorphosis.

“H. In the young stage, producing precocious appearance of the ancestral character and acceleration.

“Cases B and C which produce phylogenetic efiects by introducing youthful characters into the line of adults may be combined under the term paedonzorphosis. Cases E, G, and H which produce phylogenetic eliects by modifying characters which were already present in the line of adults may conveniently be included under the term geront0morphos2's.”

In Inany forms the problem is further complicated by the prolongation of certain stages while internal processes are going on. If the gradual internal development of a caterpillar were accompanied by gradual conversion of the biting type of mouth parts to the sucking type, the insect would starve, for during the transition the mouth parts would be adapted to neither the larval nor the adult method of nutrition. By the prolongation of the biting stage until the internal parts are ready for the metamorphosis, the animal’s relation to its surroundings are unchanged until the internal conditions are right when a sudden metamorphosis brings about the necessary adjustment to a new environment without loss to the insect. These facts are not in keeping with the biogenetic law as usually interpreted.

Experimental embryology and genetics have not been without their bearing upon the doctrine of recapitulation. It is now possible to show that external factors reacting with the internal, inherited constitution of the organism can produce such effects upon the oflspring that its entire ontogeny including the adult is modified. As stated elsewhere, Stockard has shown, for example, that developmental arrests will produce many types of modifications in Fundulus, including two heads, trunks, or even two individuals where only one would have otherwise been formed, and conversely that the age-old tendency of the egg to produce two eyes on the fishes’ head can be changed by the simple addition of a little magnesium chloride. The internal, inherited control of development is thus capable of modification. And Muller has shown that by exposure to X-rays the developing Drosophzla can be induced to react in such a manner that entirely new mutations, that is, selfperpetuating strains, can be produced, even at will. Thus is phylogeny deceived by ontogeny’. For these reacting ontogenies have produced an entirely new series of adults which can undoubtedly become an294 THE RECAPITULATION THEORY

cestral to others later to appear. Now phylogeny means to most thinkers just a series of adult forms having racial significance. Hence it would appear that such experiments as that of Muller have brought us face to face with an exact reversal of the usual causal role attributed to phylogeny, for here phylogeny is the direct result of ontogenetic modification and certainly not the controlling cause of it, as it is usually considered to be by the adherents of recapitulation.

From the criticisms which have been given the student can understand why the so-called “fundamental law of biogenesis” has passed from the high position of a natural law, and is little more than a general tendency. It is a tendency, furthermore, which is more valuable in retrospect than as a means of prediction. And since prediction upon the basis of known facts and postulates is the goal of science and the fundamental test of a “natural law,” this doctrine does not measure up to scientific expectation and cannot be regarded as a law.

Yet the partial truths which are involved are of sufficient value in view of the widespread occurrence of the tendency represented to warrant a revaluation of the essential idea of the doctrine. We find an attempt of this kind expressed some years ago by Lillie in his work on the embryology of the chick. His views appear to go far in the direction of harmonizing the useful parts of recapitulation and at the same time to avoid some of the contradictions which have been mentioned. He conceived that the entire life history of an organism is as necessary for the definition of species as any other character. Ontogenies are inherited also and are subjected to variation with resulting modification. Ontogenies of closely related species are more nearly alike than those less related. In the evolution of a species those stages of ontogeny latest to have arisen (those found in the adult) are the ones most easily to be modified and hence the embryo as a rule retains the ancestral resemblances the longest. This is not because these have phylogenetic, that is palingenetic, significance, but because the embryo is less susceptible to outside influences and therefore its characters are less the material upon which selection may operate. Embryonic resemblances being more conservative and actually older are nearer the ancestral condition. They are statements of historical facts rather than causes of recapitulation. But because of the chronological sequence of stages in a great many observed cases, the older recapitulationists came to regard each stage as strictly causal to the succeeding ones and thus to look upon the tendency exhibited as a law of nature.

Historic Disclaimer - information about historic embryology pages
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)

Cite this page: Hill, M.A. (2024, April 18) Embryology Book - Outline of Comparative Embryology 2-3. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology_2-3

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