Book - Outline of Comparative Embryology 2-6

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Richards A Outline of Comparative Embryology. (1931)
1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types

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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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Chapter VI Paedogenesis And Neoteny

A series of conditions is known that have to do with the early sexual maturity and reproduction of certain animal forms which otherwise retain their youthful characteristics. These are to be taken up under the general heads of paedogenesis and neoteny although not all the cases included come under a strict definition of either of these terms. They occur in a great many widely scattered groups of the animal kingdom and present a great many variations some of them so wide that they are scarcely to be recognized as of related processes. When such diverse conditions appear, it goes without saying that confusion is to be found in the literature dealing with them. Regarding the processes under consideration in this chapter exactly such confusion is found, some writers using the terms loosely and failing to classify with sharpness the cases cited.


The biogenetic law comes into the discussion for the reason that one may regard a particular phenomenon as recapitulating past phylogenetic conditions, or he may look upon the animal showing it as exhibiting neoteny, at least of the characters in question, and perhaps generally. Thus the familiar axolotl which is the sexually mature, unmetamorphosed Amblystoma is the usual example of neotenous development, but it is also cited by the proponents of recapitulation as explained by the general progress in development of the Proteus, Necturus, Amblystoma, Salamandra series of urodele recapitulation. Most of the cases of this kind, however, are concerned with adult conditions quite as much as with those of immature forms and therefore would seem to be rather outside the scope of the present discussion.


The general subject of neoteny also calls up the problems of heterochrony, by which is meant a disturbance of the synchrony of development. That is, if one might take the normal series of developmental time stages with each organ in its proper relation to the others as a standard, representing perhaps the phylogenetic series, then any departure from this standard would be spoken of as heterochrony. (See also Chapter III, Part Two, for discussion of heterochrony.) Similarly, the sequence of embryonic events when disturbed, as in neotenous development, involves heterochrony. The principle is one with aninteresting history, recalling especially the work of two investigators, Oppel and Keibel, although many others have contributed to it, some independently and others in collaboration with Keibel in the publication of his famous “Normentafeln zur Entwickelungsgeschichte der Wirbeltiere.” Oppel began this study comparing the different developmental stages of different animals and arranging them in tabular form to show the comparative progress of the most important organs of vertebrates. He thought himself to have found similar ontogenetic stages and to be able to compare the young stages among themselves as well as the adults among themselves. The recapitulation doctrine found support in his Work in that the young stages of higher forms are similar to the older stages of the lower animals. Departures from the series thus established were to be attributed to heterochrony. Keibel as a result of his studies and tables concluded that the time of the appearance of an organ is dependent upon the time at which it will be required to function. Thus the order of appearance of organs in a developmental series is of itself of phylogenetic significance. But Keibel did not recognize the biogenetic law as valid in these cases.


Another early student of heterochrony in vertebrates was Mehnert, who devoted his attention particularly to the relation of the subject to recapitulation. His view on this matter is of interest, for he thought that only the early development of an organ had recapitulatory value in any precise fashion, whereas in later stages the processes are quite schematic. He studied also organs which undergo regression as well as those whose development is only progressive. Heterochrony he found to be due (1) to precocious development of an anlage, (2) to rapidity of growth, (3) to rapidity of histological differentiation, and (4) to abbreviation or omission of intermediate stages. Retardation of these processes as well as their acceleration call forth heterochrony.


Heterochrony is thus seen to deal with parts of organisms rather than the animal as a whole and to-show the effects of disturbances of developmental rate and rhythm upon the entire animal only through its effect on its component organs. At first thought paedogenesis might be considered as but very indirectly concerned with a matter such as this, but a reconsideration of the facts easily brings out a very clear relation, for we find paedogenesis to mean merely that, in an animal exhibiting this phenomenon, the organs of the body which have to do with reproduction have been much accelerated in development, while at least some other parts of the body have failed to keep up the pace set by these organs; or in some cases we must think that the reproductive organs and related parts have retained their customary pace while the others have been very considerably inhibited.


During the present century many investigators have contributed to the knowledge.of the general subject. It will perhaps serve as an illustration to point out the importance of Stockard’s very extensive study on the development of Fundulus, particularly with regard to the effect of changes in the rate and to the various means by which abnormalities may be produced, notably by temporary arrests of development.


It is thus clear that the matters which are the subject of this chapter really constitute but special cases of heterochrony. In assigning to these phenomena as a causal factor the disturbance of the speed of development, we bring them all into one category. It is sometimes difficult to decide whether one organ has undergone a retardation of development or whether another is accelerated with respect to the normal and so it has not always proven easy to separate sharply the processes which are involved. By some, especially Giard, Chun, Kollmann, and others, fine distinctions have been drawn, the need for which is not entirely clear if we but relate the different processes to heterochrony.


By paedogenesis is meant sexual reproductive maturity in a pre-adult stage; it is of two types, parthenogenetic and bisexual. As a special form of paedogenesis, Packard has given the term “chrysallogenesis” to a case in which the pupa of Chironomus has been found to lay eggs.


By progenesis is meant the permanent retention on the part of the somatic structures of the conditions reached at the time when sexual maturity is attained.


Neotenous organs have youthful characters although the animal possessing them has developed its adult condition. Or, to put it in another way, animals with neotenous organs retain the ancestral larval conditions in the particular structures which show neoteny.


Disogeny is the “sexual maturity of one and the same individual in two different conditions, between which a metamorphosis with retrogression of the sex products occurs” (Chun).


It is true that these definitions savor of dogmatism and of course require much elaboration. It would seem, however, that neoteny and progenesis are not far apart and that paedogenetic animals may easily exhibit disogeny. It will perhaps be best for the purposes of our present discussion if we limit ourselves fairly sharply to the cases which illustrate neoteny and paedogenesis. Of these there are certain classical ones which should receive especial attention. There are only a few of these cases in which the animals as a whole are spoken of as paedogenic or neotenous, but when we come to consider the organs to which the latter conception in particular may be applied, the illustrations abound and important explanations of obscure phenomena have been worked out on this basis.

Of all the cases undoubtedly the axolotl is the best known. As already stated, Amblystoma tigrinum occurs in two forms, the one of which is technically immature, although it is sexually active. This paedogenic form was first described from the lakes about Mexico City and was thought to belong to the genus Siredon (S. lichenoides). The relation of this form to the well-known salamander was only discovered accidentally in Paris in 1865. Sexual maturity is reached at six months of age and sex products are shed. This is paedogenesis, for the animals have not yet lived long enough for metamorphosis.

However, metamorphosis may never occur. In many high mountain lakes axolotls live their entire life without undergoing the necessary


Fro. 218. External views of an axolotl (A) and an Amblystoma showing difierences of gills. body form. and leg development.

changes to transform them into Amblystoma. The reason for this is not clear. If the animal be fed on thyroid, if it be forced gradually to leave the water and adopt a land life, if it be transferred from deep to shallow water, from cold to warm, or if it be brought into the presence of certain chemicals it will undergo metamorphosis promptly, but the causal mechanism is not yet understood. If it remains in the same environment as that in which it became paedogenetically mature, however, it will live its entire life without metamorphosis. This is neoteny, for the larval characters are retained long after the normal time.


The neotenous animal differs in a number of important characters from the type form. It has large external, red gills with gill slits, its tail is long and broad but flattened laterally so that it is adapted for swimming, and the body features are those of a water-living animal. Internal conditions likewise show immaturity, the skull bones, for example, never becoming properly developed. With metamorphosis the animal undergoes a very extensive transformation. It loses its gills, its legs develop for crawling upon land, and the tail becomes rounded and tapering. It is now a land salamander resorting to the water only for egg laying.


The neotexiy of axolotl is thus facultative, for with the proper conditions the animal does not remain neotenous but becomes an adult Amblystoma. Other urodeles, the perennibranchiates, correspond in their adult structure very nearly to axolotl and may be looked upon as permanently neotenous. Typhlomolge, of the underground streams of Europe and Texas, Proteus, Necturus, and Siren, belong to this group.


Another illustration from the chordates is the tunicate group called Appendicularia or Larvacea. These animals are small tadpole-like creatures averaging about half a centimeter in length and living perhaps one year. They swim in the surface waters of the sea of all parts of the world and the young have been taken in plankton between February and summer, but their development is almost unknown. There is no evidence of reproduction by budding, gemmation, or other modes of asexual type such as are found in the Salpidae, for example, nor is any metamorphosis known. Development is direct and the small appendicularia correspond in a general way to the tailed larvae of the ascidians; that is, they retain the tail portion of the body with its typical chordate characteristics which is lost in the metamorphosis of the more familiar forms such as Molgula, Ciona, Cynthia, etc. Hence the group has been named Larvacea and the animals have been looked upon as larval forms which have become sexually mature. They have also been looked upon by some as primitive forms from which other Tunicata have been derived, and, it must be noted, by some they have been thought of as larvae of some adult form which is pelagic. In any case the sex glands are developed and the products shed by the animals as known to us, and we have a clear case of neoteny.


Among the insects a number of cases are known, and some of these are among the most important of all for they illustrate parthenogenetic paedogenesis. The example usually given is M iastor, a cecidomyiid fly, but certain species of the genus Cecidomyia also show it. The case of M iastor has long been known as an illustration of paedogenesis, and is given by Hertwig along with certain other Diptera as an example proving his statement that paedogencsis is parthenogenesis in an immature organism. As is already seen, this statement is much too restricted, for the term paedogenesis is of equal application to cases in which inheritance is biparental.


In Miastor paedogenesis occurs normally during the spring, early summer, and autumn, according to Hegner who has been the principal student of this genus in America. No reproduction takes place during the winter, and the process is interrupted in midsummer by the appearance of male and female adults. The larva of Miastor possesses two ovaries in the tenth and eleventh segments. In each are thirty-two


oocytes each with nurse cells and follicular epithelium. After a time one of these oocytes with its nurse cells and its follicular epithelium is separated off from the rest of the ovary and in a distant part of the body grows and develops at the expense of the tissues of the mother larva. This process is repeated until five to seventeen separated growing oocytes are thus produced from one mother larva. Then one division takes place, the polar body which is given off divides again and both products degenerate. Parthenogenetic cleavage follows with chromatin diminution as previously described in connection with the history of the germ cells, the pole plasm is segregated and the embryo gradually takes on its characteristic form. No oviducts are present in the mother larva nor is there provision for the escape of the young thus paedogenetically produced. They escape by rupturing the body wall of the mother larva, which is left to die. After the production of a number of generations in this manner, the last larvae pupate and emerge as normal adult males and females.


In the spring of 1869 Grimm found a pupa of Chironomus laying eggs. To this form of paedogenesis in the chrysalis Packard gave the name chrysallogenesis, although the differences from other types of paedogenesis are so slight as to make the retention of the term of doubtful necessity. In the autumn other pupa change to flies without laying eggs and these adults are more prolific than the spring pupae were. The process was described as a seasonal phenomenon depending upon temperature.


Pacdogenetic reproduction has also been reported for the Tenthredinidae, the saw flies, thus extending this phenomenon to a second order of the insects, the Hymenoptera. '

The molluscs show one case of neoteny in the shell-less snail, Stamedorsia verrucosa, which according to Cuénot reproduces long before true adulthood is reached.

Among the trematodes paedogenesis is found in the most striking form. Two excellent illustrations occur in the suborder Monogena, and conditions in the Digenea offer material for interesting speculation. In the former group the genus Gyrodactylus exhibits what is perhaps the nearest approach to the old preformationist theory of “emboitement” to be found in the animal kingdom. The young individual comes to sexual maturity before it is born and produces young in its own uterus.


This process is repeated and as many as four generations have been seen, one within the other.

The second case of this group is that of Polystoma integerrimum originally described by Zeller in 1872. The embryo hatches in the water and swims Freely. It seeks for a young frog tadpole which it must find within twenty-four hours or die. If one is found it creeps over the surface until it finds the branchial opening, which it quickly enters; it undergoes metamorphosis, and passes down the alimentary canal to the rectum and thence to the urinary bladder. Here it remains for three years to become sexually mature. However, it may happen that the young worm has attacked a very young tadpole which still has external gills. In this case it remains in the gill chamber where nutriment is abundant, grows rapidly, and becomes sexually mature in the short space of five weeks. It does not then pass further along the alimentary tract but dies before the metamorphosis of its host. It differs in its structure as well as its life cycle from the usual form, in that it develops but one male gland instead of several, and it lacks entirely the intromittent organ, vagina, and uterus, or they are developed only to rudimentary vestiges. It is of interest to note that Polystoma ocellatum is structurally quite similar to the paedogenetic P. integerrimum.


Among the digenetic trematodes the life cycle of the liver-flukes involves questions which are of interest in this connection. The main facts are well known and are referred to in several chapters of this work. In both rediae and cercariae reproduction may take place and daughter rediae, and daughter cercariae may be produced. If it can be shown that these daughter forms are produced from eggs, either parthenogenetically or bisexually, then this is a case of paedogenesis. However, it has lately been shown for some flukes that the germ balls from which the daughter larvae develop are budded off asexually and never undergo any chromosome reduction (F. G. Brooks), so that for these forms at least it cannot be said that paedogenesis occurs in the flukes. It has been the usual view, however, that there is here the production of parthenogenetic ova, and hence paedogenesis.


Of the examples usually recognized, the final one is that of the lobate ctenophore, Bolina hydatina. Here the cydippid larvae become sexually mature, producing eggs and sperm. Fertilization follows and the eggs develop in the regular manner. The larval gonads subsequently degenerate, metamorphosis takes place, a new set of gonads appear, the animals again attain sexual maturity, producing eggs and sperm, this time as adults.

These are the classical cases, and they illustrate both neoteny and paedogenesis, both parthenogenetic and bisexual. PAEDOGENESIS AND NEOT ENY 351

In addition to these, experimentally produced delay or acceleration of development are well known. Frogs transferred as larvae to alpine heights where the winters come early have remained in the larval condition over the winter. High temperatures hasten sexual maturity. Brackish water or fresh water will often hasten the maturity of oceanic forms. Hunger in some forms and overfeeding in others result in heterochronic growth. Termites which are fed in a certain manner mature very early while the wings are still undeveloped and eyes have not yet appeared (Grassi). Parasitism may be looked upon as a strong factor accelerating maturity because of the abundant food supply.


It may be observed that there have been omitted many cases of larval budding and other forms of asexual reproduction such as may be so commonly found, for example, among jellyfishcs, liver—flukes, bryozoa, and tunicates. These cases, though not a far step from the types discussed in this chapter, are not properly considered here, for paedogenesis and neoteny are matters of sexual reproduction. It is admitted that in some cases, as already shown for the 1iver—flukes, the distinctions are hard to draw, but for the sake of clarity it is usually thought wiser to adhere to the definitions given.


Thus far we have discussed this subject from the point of view of the entire organism. There still remains the matter of neotenous organs rather than organisms which deserves mention before the subject is closed. This phase of the subject was developed by Garstang and by Bolk and recently discussed by De Beer. There are many instances in the animal kingdom of forms having organs that retain embryonic characters although the organism as awhole has passed on to a new adult condition. Organs of this kind are neotenous. In the succession of somatic stages the organs in question have fallen behind the others, in short they have become distinctly heterochronic. Bolk has discussed the features of man which resemble the structure of embryos of anthropoid apes, assuming that the latter‘are nearer the ancestral forms of structure. Among the features of man which show resemblance to the embryonic structures of the ancestral types, for which he uses the term foetalization, are the relatively high weight of the brain, the retention of the embryonic cranial flexure with the resulting erect posture as Bolk thinks to have demonstrated, the position of the foramen magnum, the late closure of the skull sutures, the flatness of the face, lack of hair on the body, and others. Bolk’s study of cranial flexure and human posture comparing both adult and ancestral conditions is particularly interesting. He finds as a result that the flat face as compared with the elongated muzzle of other mammals is largely responsible for the trend of human evolution with regard to vision and other special senses, the character of the teeth and of the anterior end of the alimentary canal. And all the characteristics which are distinctly human are clearly neotenous and related to the embryological derivatives from the ancestors.


One other illustration, from those given by De Beer, may be cited to show the part that neoteny may have played in evolution. It is a comparison between the important structural characteristics of adult insects and larval myriapods. The larva of Iulus has a head composed of six or seven segments, an elongated segmented body, the first three metameres of which bear pairs of legs whereas posterior segments bear only rudimentary legs at the time of hatching or shortly thereafter (Metchnikoff). These features are quite insect like, in that the insect head has six or seven segments, the thorax three, each bearing a pair of legs, and the abdomen of about ten segments, legless or bearing appendages only as a larva. Insects are known among the lower orders whose structure corresponds more closely with this immature myriapod than does the structure of insects of the very specialized higher orders, and it is suggested by students of these matters that the ‘insect derivation passes through these intermediate forms from neotenous larvae at least not unlike those of the myriapods.


From these cases it would appear that the study of neoteny from the standpoint of embryology offers a productive field of quite a new order for investigations and that the conclusions reached from researches of this kind may profoundly affect our ideas of phylogeny.

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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)

1931 Richards: Part One General Embryology 1 Historical Development of Embryology | 2 The Germ-Cell Cycle | 3 Egg and Cleavage Types | 4 Holoblastic Types of Cleavage | 5 Meroblastic Types of Cleavage | 6 Types of Blastulae | 7 Endoderm Formation | 8 Mesoderm Formation | 9 Types of Invertebrate Larvae | 10 Formation of the Mammalian Embryo | 11 Egg and Embryonic Membranes | Part Two Embryological Problems 1 The Origin And Development Of Germ Cells | 2 Germ-Layer Theory | 3 The Recapitulation Theory | 4 Asexual Reproduction | 5 Parthenogenesis | 6 Paedogenesis And Neoteny | 7 Polyembryony | 8 The Determination Problem | 9 Ecological Control Of Invertebrate Larval Types


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