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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)

Outline of Comparative Embryology

By

Aute Richards

Professor 0 7,00/ugy Univrr.r1' 0 0/c/ahomc:

New York John Wiley 8: Sons, Inc. London: Chapman & Hall, Limited 1931

1951

Preface

As the result of a number of years of experience in introducing students to the subject of embryology, the writer has found it desirable to ‘depart from the common practice of studying chiefly the special embryology of the chick, pig, and perhaps of amphioxus and the frog, and to devote much attention to the principles of general embryology. This procedure is based upon the assumption that a broad background of general knowledge forms a desirable starting point for the study of special embryology to be taken up later. As comparative anatomy has proven to be the best basis for the later study of anatomy, so comparative embryology forms a proper basis for a study of special embryology. Unfortunately there has been no book in our language which brings together in a small compass the principles of general comparative embryology.

In this book it is the aim to set up the general principles of development so that the student may see how his special case—chick, starfish, or other form—is merely one of several types and what its relation to the other types may be. The writer holds that the most effective presentation of the subject matter of embryology is the laboratory study of desirable types, supported by text-book and “quiz” work on these forms and accompanied by the consideration from a comparative standpoint of the broad principles of the subject, such as types of egg structure, of cleavage, of gastrulation, and the critical consideration of classical embryological doctrines, etc. To present the material in brief form for this general part of a course in Comparative Embryology this volume is designed. '

In Part One are taken up those phases of development which are passed through in the embryology of most higher forms of animals. A comparative study of these phases no doubt forms the basis of ninetenths of the courses in this subject. Of the vast amount of material from which choice must be made, the decision as to whether any certain topic should be discussed has been based always upon a single criterion. It is sought to show only how the single-celled fertilized egg arrives at the multicellular condition characteristic of the fully formed, but undifferentiated, young organism in which the organ systems are established. In an outline of comparative embryology it is not possible to follow through all the details nor to repeat descriptions whose general features apply to more than one kind of animal. Nor is it possible to follow through for these forms the later stages in their development and differentiation. If completeness were the goal, it would be necessary to include the special embryology of the many forms about which investigators would desire information, an obviously impossible task. It is hoped, however, that a basis is here laid for understanding the detailed embryology of any of the types and for working out later stages in whatever group the student’s interest may lie.

Part Two considers numerous miscellaneous problems that have arisen in connection with the history of embryology. Here the choice of topics has been governed largely by questions asked by students both beginning and more advanced. No attempt is made to cover all the classical topics of embryology, nor to arrange the ones chosen in an order to develop any particular thesis. It is simply thought to satisfy in some measure student interest in them and to make them easily available. Discussions of many of them are not available in brief form in any English text, although they concern matters about which students of zoology should be informed, for they are chiefly problems of normal development.

The embryology of the present day is experimental embryology. Its aim is to secure an insight into the real nature of those steps by which a single-celled egg becomes a completed adult. An understanding of development in its true sense is its goal. Many aids to experiment used only by chemists or physicists in years gone by are now every-day tools of the embryologists, for descriptive embryology alone is inadequate to give a complete understanding of development. Yet experimental embryology runs the danger of considering only processes and those out of relation to structures. It is in structures that functions are inherent, for one cannot understand the workings of a machine with which he is unfamiliar. Morphology in its descriptive phases is therefore basic to a proper attempt to work out functional relations. Our admiration for the attempts to secure an intelligent conception of the mystery which underlies the pageant of development must not allow us to disregard the importance of a foundation of knowledge of descriptive embryology. Perhaps the errors which may be avoided by the study of descriptive embryology in its comparative phases may serve to recompense the student for the time lost from experimental embryology.

Undoubtedly numerous errors are to be found in this volume. Much that is discussed has been the subject of life-long study by many investigators and has concerned material collected from all over the world. To understand all adequately so that each may be mentioned correctly and weighted properly in a brief outline, is a task hardly to be accomplished, and many inconsistencies of treatment must be apparent to the reader. It must be borne in mind, however, that not an extended treatise but an outline of principles is the intent of the book, and the intent places many limitations upon what can be undertaken.

Acknowledgments are due for assistance in almost every section. The material treated in a work of this kind is by no means new, and it would be impossible to present any one phase of it without finding parallel treatment in some source of information available to students of embryology of this day. There is no claim to originality in the book, not even in the treatment of many chapters. The book will have served its purpose, however, if it appears to have sufficient pedagogical background and if it presents in sufficiently compact form these principles, many of which I think are not briefly summarized elsewhere in the English language, and which are needed to give students a basis for work in special embryology,-either medicalor experimental.

Korschelt and Heider’s “Lehrbuch der vergleiehenden Entwicklungs— geschichte” is a great treasure house of embryological learning to which constant reference has been made. In many places (particularly in Chapters VII and VIII, Part One) even their arrangement has been so much the best that only minor departures from it are to be found here. In others an entirely different organization of the same material is here adopted. But the debt to Korschelt and Heider is a very great one. I appreciate very mucl_1-the gracious verbal permission to use their material freely which was given me by both Doctors Korschelt and Heider. .”_

Drawings to illustrate ,the text are from many sources and not many are new. The writer is not impressed with the desirability of replacing pictures made by investigators on specialforms with new ones when the former are well known and of proven value. The drawings in every case are accompanied by a legend giving the name of the investigators to whom indebtedness is due.

Permission has been granted by the following publishers to make use of the figures indicated either as modified, or in the case of numbers 1, 2, 3, 135, 136, 142, 153, 1-81 of the figures themselves. By Henry Holt & Co. permission has been given to use from Loey’s “Biology and Its Makers,” our figures 1, 2, and 3; to use from Kellicott’s “General Embryology” our figure 5; from Kellicott’s “Chordate Development” our figures 135, 136, 142, 144, and 181; and from Lillie’s “Embryology of the Chick” our figures 151, 152. By P. Blakiston’s Son & Co., Inc., permission has been given to use from Folsom’s “Entomology with Special Reference to Its Ecological Aspects,” 3rd edition, our figures 117, 161, and 162; and from Patten’s “Embryology of the Pig,” figure 7. The Oxford University Press has permitted the use from Jenkinson’s “Vertebrate Embryology” of our figures 81 and 173. The Princeton University Press has granted permission to use from Conklin’s “ Heredity and Environment” our figures 7 and 8. The U. S. Bureau of fisheries has given permission to use from Herrick’s “American Lobster” figures 113, and 1973.. And William Wood & Co. have permitted the modification of figures from Bailey and Miller’s “Textbook of Embryology” represented in figures 140 and 141. Macmillan & Co., Ltd., has sold us permission to make use of our figures 103, 109, 110 from McBride’s “Textbook of Embryology, Vol. I, Invertebrate Embryology,” and of our figures 167, 168, and 170 from Graham Kerr’s “Textbook of Embryology, Vol. II, Vertebrate Embryology.” The graciousness of the various publishing companies in thus facilitating our work is deeply appreciated.

Especial acknowledgment for help in many ways including aid with the materials used and the reading of parts of the text is due to 1ny wife, Mrs. Mildred Hoge Richards. My colleagues of the Department of Zoology of the University of Oklahoma, Doctor A. O. Weese and Doctor A. I. Ortenburger have assisted with advice and have read portions of the text. Professor J. F. Paxton of the Department of Greek has aided me with questions of terminology. And finally my assistants Mrs. Celeste Whaley Taft and Miss Kara J. Fullerton have given the most painstaking attention to a multitude of details. To all of these I am deeply grateful.

A. R.

Part Two Embryological Problems

Chapter II Germ-Layer Theory

It will be recalled that the old issue between preformation and epigenesis finally resulted in the overthrow of the cruder type of preforn1ation which held that development was simply an unfolding of parts already existing in egg or sperm. As better understanding of the facts of development was obtained it became clear that, although preformation did not occur, there were definite signs of organization within the egg. The developing organs could be traced farther and farther back until it was evident that in general they came from what might be called three plates or layers of tissue lying one over the other. These layers give rise by various processes of folding, outpocketing, unequal growth, etc., to the fundaments of the future organs. It was further obvious that there is a great similarity in regard to this method of origins from these layers in different kinds of embryos. These facts were known in whole or in part to many of the early embryologists, including von Baer (1828), and others, even back to the time of Wolff (1768). ()f course the proper interpretation of cells and cell structure in the animal kingdom had not as yet been made and it had proven impossible to explain the origin of the layers in terms of any other lower units of organization. It is not surprising, therefore, that these early embryologists should have looked upon the formation and subsequent development of the germ layers as the most critical processes of embryonic life.

The germ-layer theory, which is distinctly an attack on the problem of the organization of the embryo, has been one of the most fruitful of all biological speculations. After the true nature of cells and their methods of reproduction and growth came to be understood, the theory acquired new significance and, in the hands of Oscar and Richard Hertwig, had attained the status of a full-fledged theory of development by the early eighties. Through a great variety of forms, blastulae and gastrulae were identified, their methods of origin understood, and the remarkably uniform character of all the processes involved in germlayer formation and in the laying down of the fundaments of the organs was made out. Exceptions occur; indeed, some of them are of great importance, but considered as a whole there is striking uniformity in

282 GERM-LAYER THEORY 283

the relationships of the layers to each other and to the subsequently developing organs.

The essential point of the theory is that the three layers are homologous throughout the animal kingdom above the Porifera. In the coelenterates, as in the gastrulae, there are present only ectoderm and endoderm. Since the layers are homologous, the organs which arise from them are also homologous wherever they are found.

In all metazoa there normally develops following the blastula, a stage called the gastrula, of which the two component layers are known as primary germ layers, the outer being the primary ectoderm (epiblast, ectoblast), and the inner the primary endoderm (hypoblast or endoblast). The primary ectoderm in etenophores, turbellarians, rotifers, annelids, and molluses contributes with the primary endoderm to the formation of the middle germ layer, the mesoderm (mesoblast) giving rise to the distinction of ectomesodcrm and cndomesoderm respectively. From the outer layer also come: (a) the covering tissues with all their modifications and appendages including hair, horns, nails, scales, skin, glands, (b) the nervous system and sensory epithelium, and (c) in many cases the stomodaeum and proctodacum (extreme anterior and posterior regions of the alimentary canal).

The primary endoderm, in addition to cndomesoderm, produces the lining of the midgut and of all the organs which are derived from it, as pancreas, liver, etc. There is also exceptionally endoderm participation, for example, in the formation of a part of the nervous system in some coclentrates, but in some other cases, as in formation of blood cells and endothelium of blood vessels, the evidence is not yet clear as to whether it takes part.

The mesoderm may come, as already indicated, from the two primary germ layers, but except as mentioned above, only the endoderm seems to be involved in its production. There are several methods of mesoderm formation. Isolated endoderm cells may push into a space between the two layers at the same time with the secretion of a jelly-like substance, thus giving rise to a filling-in layer known as mesenchyme from which certain organs or parts of organs take their origin. In a second method the typical epithelial character of the primary germ layers is preserved and by outfoldings from the walls of the archenteron (primitive gut), which becomes entirely disconnected from it, lateral coelomic pouches are formed. This type of mesoderm is known as mesothelium. There are also other processes by which coelomic walls may in special cases be produced. The inner or splanchnic wall of the coelomic pouch unites with the endoderm to form the splanchnopleure, and the outer or somatic layer forms with the ectoderm the somatopleure. In some 284 GERM-LAYER THEORY

animals the middle germ layer is entirely mesenchymous; in others, entirely mesothelial; and in many, both types are present. Either one may develop first; in the echinoderms the mesenchyme arises before the mesothelium and in the vertebrates after it. The mesoderm gives rise to muscles, blood, skeletal and connective tissues, the excretory organs (at least in part) and usually to the sexual organs, but not the sex cells.

The surprising uniformity with which these relationships are held throughout the higher groups of animals has been responsible for the great importance of the germ-layer theory as outlined by the Hertwigs. Probably no other theory or working hypothesis, with the exception of Darwin’s own contributions, has been so fruitful as a stimulus for constructive zoological work. The germ-layer theory has to its credit much of the most important embryological accomplishment. As a working hypothesis it is of the greatest importance.

As an analysis of the problem of organization of the embryo, less can be said for the germ-layer theory. Researches of later years with careful technique have shown that the exceptions which formerly were overlooked are in many cases difficult to harmonize with the theory and, indeed, often operate decidedly to limit its usefulness.

Among the objections that have been cited are the following: inverse relationships of the layers found in the sponges; the fact that the outer layers of the (-estodes are in part or in toto thrown off leaving the covering tissues doubtfully related to the eetoderm; the varying derivation (depending upon the viewpoint of the observer) of the notochor(l and mesenchymous connective cells in different chordate groups; the composite nature of some organs, for example the nephridia, from more than one germ layer; the identification of the mesoderm as the 4d cell, not a layer at all, in the individuals with spiral cleavage; and especially the facts of budding, differentiation, and regeneration. Some of these objections have proven difficult to answer although this fact should not make us underestimate the value of this conception as a whole. However, it is now known that there are many evidences of organization appearing long before germ layers are formed. The promorphology of the ovum in the eggs with determinative cleavage and the possibility of tracing out the cell lineage during cleavage and organ formation in embryos of this class distinctly minimize the importance of the germ layers. Back of this the organization of the chromosomes and their part in shaping development as is now known from studies of cytology and genetics again detract from the value of the theory.

We have now come to view the germ layers as representing a stage in development just as we regard the blastula or gastrula as progressive GERM-LAYER THEORY 285

steps. It is an essential stage in most cases through which the course of development must pass, but in special cases adaptive modifications have arisen. Homologies exist between blastomeres—sometimes, as in the 4d cell and the “cross” of the annelids and molluscs, of very striking character; they also exist between germ layers, between organs and between organ systems. All are important, perhaps equally so. But the germ layers are probably best regarded as temporary embryonic organs which play their part and give rise to subsequent stages of development rather than as units of organization which determine the future course of development.

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.

Chapter Iv Asexual Reproduction

Asexual reproduction occurs in a great many plants and in many animal groups. Among the invertebrate phyla it is lacking only in the arthropods, molluscs, and nematodes of the more important groups; while among the chordates, the tunicates present many illustrations of this mode of reproduction. It should also be noted in passing that even in certain mammals a process of budding in the embryonic state occurs. This matter is discussed at length in the chapter on polyembryony where it is shown that the blastocyst of the armadillo regularly buds to produce four embryos. This gives asexual reproduction a place even among the highest groups.

In spite of the fact that this method of reproduction manifests itself in many different forms, it is essentially a very simple process. It occurs in organisms (or in portions of organisms) which have retained to a considerable extent their embryonic, undifferentiated character, and is really a mass division, due especially to the cells of some particular area undergoing a proliferation which presently results in the constriction and cutting ofi” of a greater or lesser portion of the animal. If the portions are approximately equal the process is spoken of as fission; if unequal, as budding. These, with sporulation, are the common forms of asexual reproduction, but each shows many variants in the animals in which it occurs.

Sexual reproduction, involving the participation of two individuals, is known as amphigony; asexual, since only a single individual is necessary, is monogony. In the former, germ cells or their equivalents are produced; in the latter, there are no special cells employed for the purpose. The alternation of a sexual with an asexual generation is called metagenesis. The alternation of a biparental sexual generation, that is, a case of amphigony, with a uniparental, a parthenogenetic, generation is heterogony. Sexual reproduction was further called gamocytogony or cytogony by Hartman, and asexual, agamocytogony (also agamogony).

fission is the division of an organism into two parts which are approximately equal and into whose formation very little new material has gone. The parent organism is lost in the production of the daughters,

which therefore can have no living ancestors, and can undergo only 295 296 ASEXUAL REPRODUCTION

what may be termed accidental death. It occurs when growth has taken place in excess of the needs of the individual. Contrasted to the condition in sexual reproduction, the new organism is at once provided with at least part of the organs which characterize the adult, and differentiation is always well advanced, considering that the organism is seldom high in the scale of animal life. Two categories were distinguished by F. von Wagner for organisms undergoing fission: in the one, paratomy, a special zone, in which the constriction will occur, is prepared before fission begins; in the other, architomy, no preparation is made, constriction taking place more primitively with little bodily reorganization. It should be noted that both of these categories may apply to either longitudinal or cross fission. Under the general head of fission may also be placed those cases of fragmentation, such as occur in oligochaetes and starfish, due to external influences, which may, if conditions are suitable, develop into mature individuals. These cases are also spoken of as autotomy, and sometimes as augmentation.

Budding, although not much more complex as a process than fission, is productive of much more complicated organisms and life cycles. It often is the means by which colonies are formed, and results in marked polymorphism with some of the individuals of the colony being specialized for nutritive and vegetative purposes and others for reproduction. Commonly where a considerable degree of specialization follows the budding processes the life cycle includes a sexual as well as an asexual phase and we have alternation of generations. A bud is a small portion of the parent organism which has begun to grow actively and to proliferate and from which will be derived a new individual having the full degree of differentiation characteristic of the species. There are buds in some triploblastic animals which involve only one of the germ layers, whereas in other animals the buds may include more than one layer. Furthermore budding may occur in embryonic, larval, or mature animals. Abundant illustrations of all these cases may be cited.

There are several types of budding. The most familiar are the external buds such as occur in Hydra for example in which a small portion of the parental tissue grows and constricts off to become a new, small hydra. In some of the hydroids, however, the separation is incomplete and the new individual remains as a permanent bud. The so-called “free buds” of certain forms are also of the external type. As buds they break off and become for a time at least free swimming. A second type includes the internal buds among which are the gemmulae of sponges, the statoblasts of the bryozoa, the germ balls of trematodes and others. A group of proliferating cells becomes isolated within the mother organism and in time results in a new organism. A third type involves the formation OCCURRENCE OF ASEXUAL REPRODUCTION 297

of a stolen or a “runner” from which numerous buds arise. This might seem to be really a kind of external budding but it is sufficiently distinct from the usual cases of this kind to warrant special mention. finally a fourth series of processes should be included under budding, although they are often overlooked; they are the processes known as frustulation and laceration in which small fragments separate off from the parent organism, when unfavorable conditions arise, and form new individuals.

Closely related to asexual reproduction and of much significance to the comparative embryologist are the phenomena of regeneration. Morphogenic processes are involved in the reorganization of portions of the old individuals to produce new ones which do not greatly differ from the preceding. These processes involve the regeneration of the organism or of portions of it. The student of comparative embryology should give careful thought to these processes, for in them are manifested fundamental capacities and characteristics of living protoplasm. Indeed some of these processes take us far into the innate organization and give an insight into the nature of living stuff which the Inorc usual studies fail to offer.

The formation of colonies is closely correlated with the occurrence of asexual reproduction. Colony formation occurs in certain phyla in a striking manner, and it is in these very phyla that asexual reproduction is a dominant method. It may be taken as a general rule (not, however, without exception), that whenever individuals are found organized into colonies, either asexual reproduction, or polyembryony, or parthenogenesis will be found to occur in the same groups. The simple sexual mode of reproduction seems not usually adequate to produce enough individuals to be associated together in a permanent colony, and one of those three accessory modes must be depended upon for the increased task.

OCCURRENCE or AsEx}JAL REPRODUCTION

The first examples of asexual reproduction to be found in the animal kingdom are in the protozoa, but the process here is one of single cells and perhaps is thus somewhat outside the general problems of embryology. We may begin our study therefore with the Porifera.

Porzfera. In the phylum Porifera, asexual reproduction predominates; here are to be found fission, budding, formation of free buds, and of gemmulae. Since most sponges exist as colonies it is easy to see how the dividing processes have lacked completion resulting in a degree of union that is more or less extensive. Asexual reproduction commonly leads to formation of colonies rather than to independence of organisms. But budding and fission are shown to advantage in Leucosolenia. Some of 298 ASEXUAL REPRODUCTION

the individuals indicate fission as their mode of reproduction for the products are equal in size although they remain attached at the basal end. However, the presence of small immature individuals on certain

Fm 194 Budding and fission in Leucosolema blanca. (From Korsehelt and Heider)

specimens as well as the mode of colonial formation shows that budding is often the actual method of their origin. When fission does occur it begins as a split at the osculum and progresses toward the base. When

fiG. 195. A, Budding in Leucosolema botryozdes B, A bud which has become free and attached to an algal filament. (From Korschelt and Heider, after Vasseur.)

it is not complete the beginning of a colony is seen and these colonies

often become quite complex. In another species of Leucosolenia (botrymdes), Vasseur long ago THE PORI FERA 299

found free bud formation. The buds form from an indifferent group of cells, grow irregularly from the parent individual, undergo differentiation to a considerable extent, and at length break off, the ruptured end forming the osculum of the new little sponge; they then settle down to produce young sponges.

The sponges show what is perhaps their highest degree of asexual reproductive activity in the formation of gemmulae, seen to best advantage in the Hexactinellidae. They are derived from parenchymal cells which have wandered into the mesoglea and become separated from their original layers. Aggregates of these “archeocytes” (also called “sorites” by some) take on an oval or rounded shape, become

I‘it. 106 (:(‘lllI‘n1ll( forination in Eph;/dalm blrmbmata (From Korschelt and Heider. Liter Evans )

A, an early stage showing the aggregation of the ‘ germ" cells (g) from those which make up the covering membrane (m) B, the cutieular membrane (c) is beginning to form from the outer membrane

surrounded by a special membrane and thus make up a gemmulc. During conditions which are not favorable, this structure, which is really an internal bud with a protective covering, tides over the organism until a more suitable time. Thus this form of reproduction is a device which enables these sponges to adapt themselves to changing environments. The parental tissue degenerates and dies after the gemmules are formed. When the gemmules begin to grow, the cells multiply rapidly, those at the surface arrange themselves in a layer, and from the gemmules a larva issues which is strikingly similar to the sexually produced young in form, structure, and ciliation.

Division among sponges is often accidental so far as the organism is concerned, and the animals offer opportunity for experimental fragmentation as well as normal. H. V. Wilson’s work and that of J. S. 300 ASEXUAL REPRODUCTION

Huxley give illustrations of the extent to which sponges may be dismembered (in these cases by being squeezed through bolting cloth) and subsequent regeneration serve to produce new sponges. Often cell masses come together, fuse, and grow into a new individual from which a colony is formed. This is called concrescence and may also occur between larvae as they creep about on the bottom.

Coelenterata. By the eoelenterates many developmental experiments along the lines of asexual reproduction seem to have been tried as well as those which have already been shown for the various types of embryos, cleavage patterns, and methods of gastrulation. Nearly all types of asexual reproduction are exhibited somewhere in the phylum. In addi

Fm. 197. Trunsversc fission in Protohydra. (From Korschclt and Heider. ufter Aders.)

tion there is often manifested an extreme polymorphism in the forms which are thus produced, and the details of the manner of their production especially where a compound type of budding is involved are often extremely complicated. Mention here can be made of only a few cases which constitute a very meager outline indeed. fission, both transverse and longitudinal, budding both larval and adult, and colony formation by the production of permanent buds, metagenesis, stolen formation, fragmentation, frustulation, laceration, and the related, although in its results opposing, process of concrescence, occur in this group.

The Hydrozoa exhibit only a few types of asexual reproduction. Transverse fission occurs in Protohydra, according to Aders, and consists in THE HYDROZOA 301

the simple constriction of the animal around the region of its greatest diameter. The constriction cuts the animal into a proximal and a distal portion. The one develops a new base, the other a new oral region. A similar type of division may occur in Hydra; although undoubtedly very rare, this was one of the earliest cases to be described (Trembly, 1744; see also Koelitz, 1908). It is of essentially the same character as in Protohydra, the chief points of difference between the two being in the simpler structure and the lack of tentacles on the part of the latter. Longitudinal fission has been described for Polypodium only among hydrozoa.

Budding by larval as well as by adult hydroids is not uncommon. An example of budding in the larval condition occurs in Gonioncmus and in Ilaleremita as described by Schaudinn, Perkins, and others; the details of the life cycle in these forms have been lately worked out by Joseph. The egg hatches into a tiny planula larva. It develops a mouth and creeps about actively while feeding in Haleremita, although the planula of Gonionemus is sessile. On the sides of these larvae protuberances appear which grow into buds that gradually elongate to become like the planula, then constriet at their bases, and at length separate. The descriptions of the processes differ greatly in detail as described by the different investigators. For example, from one to six buds have been reported by the various students, according to Joseph, near the base of the larva, although Perkins found them about the middle rather than the base. In some cases the endoderm of the bud is said to be solid, in others to contain a cavity derived from the gastrovaseular cavity of the mother larva. There are also other differences in the details as given, some of which are doubtless to be attributed to the fact that both European and American forms have been used by the investigators. Lateral buds also have been described as occurring on Microhydra, where they may form tiny colonies of three or four polyps.

Among adult hydrozoans budding occurs in both polyps and medusae. In Hydra and the colonial hydroids it is of such common occurrence that mere reference to it is suflicient. The bud of Hydra consists of a protuberance which grows, develops tentacles, a hypostone, and at length a mouth, after which it is ready for separation from the parent. Both layers of the body wall participate in the formation of the bud, and the gastrevascular cavity is continuous between parent and bud until the time of complete constriction of the latter. Budding is much more common than sexual reproduction. In hydroids the process is essentially the same except that it is usually incomplete, the bud remaining in connection _with the parent stalk; in this manner a colony is formed. ’ 302 ASEXUAL REPRODUCTION

A special case of hydroid budding is that which gives rise to the medusa which is morphologically the equivalent of a bud from a polyp. There are cases, to be sure, where the ontogeny is so abridged that the polyp stage is extremely rudimentary or even entirely lacking, unless the planula be regarded as representing it, and the egg develops continuously into the medusa. But in general the medusoid generation is produced by budding from the hydroid generation or the polyp. The main stalk, or hydrocaulus, has attached to it in Obelia, modified hydranths, called gonangia, which bud to produce medusae. In Obelia these become detached to swim freely. In the Narcomedusae a proliferating stolon buds off medusae which may remain in clusters or may separate off completely. In the siphonophore, Halistemma, the planula develops an ectodermal thickening at the aboral pole which develops into the pneumatophore or float. Part of the planula becomes the eoenosarcal axis from which spring buds which become several different kinds of individuals. Those near the float are bell-shaped medusae, through whose efforts the colony is enabled to swim. Next come a series of covering scales which seem to be retrogressed medusae; they are protective in function. At various places along the coenosarc are feeding tubes in general similar to a hydranth. The tentacles and feelers also are probably to be looked upon as hydranths. finally there are the reproductive individuals which resemble certain types of medusae.

It will be seen from a consideration of all these cases that budding in the hydrozoa directly results in the formation of colonies. All hydroids which bud form colonies. Usually the planula develops directly into the first hydranth and the colony is formed by the subsequent branching and budding. This brings about in all except the simplest cases a very considerable degree of polymorphism. There are defensive and sensory individuals and in these the cnidoblasts are well developed. The gonanth or gonangium has for its purpose the function of reproduction, and various modifications of gonangia are to be found throughout the group. Medusae which are free swimming show a greater degree of differentiation than any of the other types of individuals produced in a hydroid colony. There are, however, numbers of medusa forms in which the development is incomplete, and some of them are quite simple in structure. As an accompaniment to the more complex polymorphism which we find in these colonies metagenesis, the alternation of generations, is perhaps shown here to a degree of completeness that scarcely exists elsewhere in the animal kingdom. The sexual functions are transferred to certain individuals while others specialize along the lines of feeding and protecting the colony. Hydroid colonies not uncommonly reproduce by another asexual method, namely stolonization. From the THE HYDROZOA 303

hydrorhiza of the original polyp cylindrical projections grow out which elongate, creep about on the bottom and may branch or even anastomose. These are stolons or runners upon which new hydranth buds appear to produce new individuals. Bougainvillia and Clavularia both serve as illustrations of colonies which grow by stolon formation.

In the second class of coelenterates, the Anthozoa, transverse fission occasionally occurs in young animals which have not yet developed sex organs. In Fungia as described by Bourne, the development of the larva at a certain stage results in the separation of its distal part and after a complicated series of processes two individuals are produced. Longitudinal fission is common among the Anthozoa. It is a slow process beginning at the oral pole in some cases, although in Actinia it may take place simultaneously from the aboral as well. In the adult Sagartia and in Paranemonia a constriction begins at the pedal disc and passes in the course of twenty-four hours to the oral region. In some others it is much more rapid. Furthermore the constriction of one individual to produce several at the same time may take place. In this case the resulting individuals will be of varying size, and it has been observed that one division may not even be completed before a new one begins. Strange complications thus arise in which the individuals may have several mouths or several systerrs of septa at the same time owing to this multiple fission. In the Anthozoa irregular longitudinal fission sometimes gives the appearance of budding. True budding, however, is rarely met with in the sea anemones. In the alcyonarians, however, much-branched colonies are formed by budding and the individuals undergo modification and even produce a very considerable polymorphism. The Zoantharia likewise owe their extremely complicated type of development to both fission and budding. In both these latter groups stolon formation and subsequent budding are commonly observed.

One of the most striking forms of asexual reproduction is that manifested by the scyphozoa in the process of strobilization. This has already been described in the chapter on “Types of Invertebrate Larvae.” The planula develops into a hydranth-like form called the scyphistoma. By a series of divisions which may be repeated perhaps a dozen times there are constricted off from the scyphistoma, ephyra larvae of which there may be one or several. If there is only a single ephyra strobilization is said to be monodiscal, if several are produced it is polydiscal. The question arises as to the nature of these divisions. The first ephyra is commonly said to be formed by terminal budding and it would seem that if the successive ones are produced only slowly they likewise are terminal buds. However, in certain well-fed scyphistomae the process of strobilization takes place so rapidly that the lower individuals are 304 ASEXUAL REPRODUCTION

already indicated by constrictions before the upper ones have progressed to any considerable degree of independence. It would seem that the distinction between terminal budding and transverse fission in this case is a difligult one to draw. The scyphistoma also for a considerable part of the year produces other scyphistomae by lateral budding in a manner similar to that in Hydra. All at length, however, undergo strobili— zation and produce ephyrae which gradually develop into adult jellyfish. In certain scyphozoans another type of asexual reproduction occurs in the formation of stolons from the original hydranth. They grow out from its base but remain in connection with it and their buds form a colony.

Before leaving the coelenterates the attention should be called to the other methods of asexual reproduction already discussed, namely, fragmentation or laceration and frustulation. In the actinians laceration is rather frequently seen, especially when the conditions of the water become unfavorable. A part of the basal rim begins to spread out, the ectoderm develops quite profusely and endodermal portions grow out into this new area. This then separates from the main body owing to the contractions of the latter and from the pieces so produced new anemones may at length regenerate, although the ability to do so is dependent upon the number of septa present. Frustulation occurs in the hydroids. Occasionally a small bud-like branch is observed to constrict off from the parent colony, to settle down on the bottom in which condition it is called the frustulum, to grow and to produce a hydranth. In hydranths also a process somewhat similar to laceration has been observed, although in this case it is spoken of as fragmentation, for a basal portion of the polyp is cut off and may develop into a young polyp.

A final question in connection with the coelenterates naturally arises as to which condition is the more primitive. There are three possibilities: one, that budding is the primitive type of asexual reproduction; two, that fission is primitive; and three, that both arose independently of each other. There have been interesting discussions of this question and transitions between them have been pointed out. It need only be said here that the evidence is not conclusive for either view.

Platyhelminthes. In the phylum Platyhelminthes the dominant type of asexual reproduction is fission, although there are also causes of budding. It is most common among the Turbellaria, although the polyclads have not been shown to reproduce asexually. Undoubtedly the phenomenon of asexual reproduction in this division of the animal kingdom is closely concerned with the ability which all members of the phylum have for extensive regeneration. In its simplest form, as for example, in the triclads, Planaria abissima and P. alpina, the reproducPLATYHELMINTHES 305

tion really consists in the separating of the animal by transverse fission into two portions each of which proceeds to regenerate the missing head or tail. In Planaria all the species described show that regeneration may take place at various levels proceeding from the anterior to the posterior. An area of regenerating tissue appears which enlarges and then differentiates. Sometimes a second fission makes its appearance before the complete regeneration following the first, and indeed there are recorded cases of several divisions with the appropriate organs already developing before the first is completed.

Among the rhabdocoels fission is an even more general phenomenon. The small fresh-water M zcrostoma reproduces mainly by fission. Commonly this process occurs before the young have developed sex organs in this form and in Stentostoma, and the divisions may take place in such rapid succession that chains of individuals incompletely divided are the result. Division takes place by paratomy very slowly, that is, a zone of division is prepared and the organs of the new portion of the animal are already developed to a considerable extent when constriction occurs. If the constrictions follow each other more rapidly they come under the category of architomy, no special region being A prepared in advance. The distinction pm 195

S\l(‘('(“§‘\l\(‘ lI‘f'lH§V€l'EaC fis betwcon pal-altolny and 3'1-chit/Oxny sions 111 (A) Stuwstmnum SM-boldi, and

. (B) '11 trrnslomum lmcarr (From Kormay perhaps be made clear by Saylng schelt and Holder after v Grofi )

that the regeneration Of the new Pharvnx is shown for each single inorgans precedes the actual fission in dividual, and the dn 131 111 planes of the

_ different ranks are numbered paratomy, no great change in the

organization of the animal taking place at the exact moment of division. On the other hand, in architomy, regeneration and subsequent reorganization follow the constriction. 306 ASEXU AL REPRODUCTION

In general, asexual reproduction in the trematodes is rare and certainly never occurs in any individuals which have a complete set of sex organs. The situation with regard to the germ balls which produce the rediae has already been referred to in connection with the chapter on in vertebrate larvae and also that on polyembryony. It is still looked upon as an open question as to whether parthenogenesis or asexual reproduction is the method by which the germ ball that forms the new rediae is produced. It has been pointed out that originally they were regarded as asexually produced, more recently as developed in parthenogcnie ova, and finally that the cytological study of these supposed ova has not yet shown evidence of a reduction division such as would be necessary to establish surely the fact of their parthenogenic nature. If we are to regard them as asexually produced then this type becomes of much more widespread occurrence among the trematodes than is usually considered.

Among the cestodes there are two aspects to the question of asexual reproduction. One has to do with the formation of the proglottids from the scolex, the other with multiplication of the eysticercus. The first of these involves a decision as to the fundamental nature of the proglottids. Is the tapeworm to be looked upon as a colony of individuals or as a single one? If the latter

fiG 199 * view is taken there eanof course be noquestion of asexual

fzvsli::;:::;;::)‘lf,? reproduction involved. If on the other hand the pro~ T‘”"“‘ "‘”““7’3 I ttid is re arded as an individual since each h‘ “ltll an accessory g 0 . ’ a v(..,,,.1e attached complete genital apparatus and since each has the ability S°°l°°°5 “"3 to live at least for some time after the separation from present in both _ _ (From Korschelt the remainder of the worm, then its method of forma;‘3';‘:tI){°'d°" “Mr tion must be looked upon as budding, it being thus

. produced from the scolex. It is the writer’s opinion that

the colonial view is not as generally held now as formerly.

There can be no question that in the development of the tapeworm from the eysticercus budding does occur. The egg develops into the onchosphere, the six-hooked embryo, in which condition it escapes from its adult host, and is taken in by the intermediate one. From it develops directly the eysticercus. The scolex of the eysticercus arises from a thickening in the wall of the bladder which becomes depressed into the cavity and later this bud-like structure is cverted to form the little worm.

It occasionally happens that two or more scoleces are formed by budPOLYZOA 307

like thickenings on the wall of the same bladder. This multiple production of scolices is rather rare but occurs regularly as is well known in Taenia echinococcus with disastrous results to the host. This is of course reproduction by budding. In some forms the bladders themselves may constrict and produce two vesicles each of which buds into a scolex. Polyzoa. Since the Bryozoa, or Polyzoa as it is now becoming custom~ ary to call them, commonly form colonies, it is to be expected that asexual reproduction will be found in this group, and the expectation is borne out. Three asexual methods, as well as polyembryony, are found here. They are budding, stolonization, and the formation of statoblasts. As a manifestation of the complicated condition which asexual reproduction may reach in a single group, the Bryozoa probably exceed any other branch of the animal kingdom. A student of the subject will find much to interest him in the group and may expect to see an everchanging variety of detail as he studies the different divisions of it. For

Fm. 200. Buds of ('ris!aleHa muredo in median section. (From Korschelt and Ileider. after Braem.) ee , ectoderm, m., mesoderm.

the general student of comparative embryology, however, it seems unnecessary to give more than a very brief consideration to the general types of asexual reproduction as they are developed here. From a consultation of the table of classification on page 260 the student will observe that the phylum Molluscoidea, of which the Polyzoa are a portion, consists of animals very diverse in structure and in their method of development. They range from rather simple colonies to aggregations of organizations which are complex in the extreme. Among the simplest genera is Cristatella, coming under the ectoproct order, Phylactolaemata. All the individuals of a colony of this form can be traced to the first one which develops from the larva. This individual produces buds commonly as thickenings of the pharynx on the oral side, both ectoderm and mesoderm participating in the process, although the ectoderm contributes a larger portion to the formation of the new individual. As the bud enlarges it grows out from the parent individual and its inner cell mass undergoes a very considerable differentiation. The intestinal tracts commonly remain in communication at least for 308 ASEXUAL REPRODUCTION

some time. An entire series of buds which in their turn repeat the process is involved in the production of the colony. The development of buds in the Endoprocta closely resembles that just described. The bud is first an enlargement of eetoderinal cells which receives migrations of meso W

0

F16. 201. A piece of Plumalella fungnsa showing the formation of the primary buds and of those which arise secondarily.

dermal cells as it develops. In this group a stalk makes the bud rather more independent than in the preceding. In some forms, notably the ectoproet groups Crissia and Tubulipora, budding takes place in the embryonic stages even before any differentiation of the blastomeres begins. The primary embryo produces a

32 B13 C D E F great number of buds in such a case and B, may become a stem and retain the others as secondary embryos or it may be en— tirely destroyed in their production. Of course the secondary embryos may also reproduce asexually. It is rather rare that the buds should separate naturally from each other, for it is by their uninterrupted relation to each other that the colony is Fm 202. Dmgmmmntic “_pm_ formed. They may branch laterally from

aem,m.,n of the method 0; i,m,ci.- the main stalk or may continue in a direct

in“ i" P“”"“"'”“ f"‘“‘”"*"“' (l“'°’“ line of the main branch. It is obvious that Korsehelt and Heider, after _ , . _ ’ Bmem) in such a highly organized type of animal as this, polymorphism of the individuals is to be expected. Special morphological changes take place so that some of the individuals of the colony become very much changed from the typical form and have very different functions. The Bryozoa. always reproduce by sending out stolons which then bud many times. The formation of the stolon itself is like that of the bud but it grows out as a stalk which

by repeated incomplete divisions produces a new portion of the colony.

A

I V AB3B‘~’BBC‘C D E 1‘ G POLYZOA 309

Some fresh-water forms produce hibernacula. which.‘ are essentially winter buds, club-like swellings on the stolons which enclose themselves in a cutaneous capsule during the period of severe winter. In the spring their development continues to form new colonies. Accidental subdivision of a colony is of course common, but the multiplication of the colony by fragmentation appears also to occur naturally.

fiG. 203. Production of zooecia ns buds from stolons (st.) of Pedicellina erhinala. (From Korschelt and Heider, after Ehlors.)

A final method of asexual reproduction is the formation of statoblasts. According to an old view a statoblast is supposed to arise from a single cell and upon the basis of this it was thought to be a sort of winter egg. It is now realized that the statoblasts are modified buds which have become changed into reproductive buds. They vary in shape, but are enclosed in a cutaneous capsule and contain several cells. They are 310 ASEXUAL REPRODUCTION

able to withstand extremes of temperature and thus serve to carry on

the colony during the cold of the winter. Amwlida. Asexual reproduction is of common occurrence in the two most important groups of the annelids, namely the polyehaetes and the

Fm. 204. Statoblasts of Cristatella mucedo. (From Korschelt and Heider, after Kraepelin.)

oligochaetes. This is due to the remarkable uniformity in the structure which the annelid body possesses, each segment being very similar to its neighbor. The growth of the annelid worm is accomplished by the

J __ ._5B.'5:'l!

w

C ‘SIN! _ .. " ifl‘-Jijurvl ‘ .u1l1“‘L4“ .

F10. 205. Developmental stages of the statoblasts of Crietazella. (From Korschelt and Heider, after Verworn.)

addition of the somites to the posterior end primarily, although anterior zones of budding are also found. In some instances the formation of new somites takes place more rapidly than their separation from the animal ANNELIDA 31 1

and thus a chain of individuals is formed. In a simple annelid chain in which the individuals of a series are spoken of as zooids in anticipation

fiG. 206. Germinating statoblasts of Cristatella. (From Korschelt and Heidcr, after Braem.)

A. the “germ disc" stage. B, its invagination preparatory to further growth.

of their future development to individual worms the anterior individual represents the original worm and is obviously the oldest. At its posterior

fiG. 207. Posterior end of Trypanosyllis misakiensis showing buds of difierent ranks. (From Korschelt and Heidor. after Johnson.)

end is a reproductive zone where the budding oil’ of new individuals takes place. The first individual budded off is of course the most posterior 3 12 ASEXUAL REPROD1 'C'l‘ION

of the chain, the others grading in age and size toward the middle of the worm where the budding zone is located. In the more complicated cases, of which the syllids are the extreme example, we may have not only terminal budding but also lateral and even ventral. In one of these forms, Trypanosyllis misakiensis, single budding soon produces numerous buds in all directions differing in age but ,.,:‘:.:-as *;:::r;..:" Merwisc giving the W231‘(Fmm Komheltamui ance of a rosette of small zooids. after v. Kenr}x]<:)l.) Between these two extremes lie gen‘: 0, S ‘V various degreesof complication in the forms produced. In many marine polychaetes a differentiation with respect to reproductive capacity is observed between the (lif ferent parts of the animal. The anterior portion of

the individual is sexless and is spoken of as the atoke. = H At the time of sexual maturity this slow-moving worm becomes very active, the hinder somites de- = 1

velop gonads, and special 1

‘W bristlesandparapodiagrow

El ‘ W _ out on them, developing

lllllllll them for rapid and exten sive movement. This sexual

portion is spoken of as the __

cpitoke, and individuals of "

this kind were earlier given

special systematic descrip tions. It is now clear, how ever, that N ereis, for ex— Fm- 209. Diu. grains to show the

ample; passes Over "Ito divisions zones in

H eteronereis or the epito— £0": hmlzaa. d(fir{or}1 - . . t kous stage. In many po1.v- dfflfnir c‘L'how..,°.3

chaetes, the epitokous por tion separates from the remainder and swims

: ' , ' ' d t th —

Fm. 210 The mlom about a(t1vely for a certain perio a e sur

worm. Eunice vimkiis, show- face of the sea. This is the swarming which

5”‘! ‘he ‘“”°’°“°° be‘“"~‘°" takes place in N ereis and is especially notable the anterior ntokous part , ' . . . . and the posterior epitokous in the palolo-worm, Eumce 1)27'Zd’l8, whose

portion. (From Korschelt . ‘ ' ' ' d and Heme“ aim W00d_ swarmxngm the south Paeificisoften observe

ward.) as a very unusual phenomenon. ANNELIDA 313

A further (-lassificatioii of tho asexual methods of reproduction in this group may he based upon the degree of preparation which precedes

fiG. 211. J! yriamda fumata with 29 zooids. (From Korscholt and Heider. after Maluquin.)

the division. In the simpler cases the division of the worm takes place between two segments with no special zone of separation having been developed. Following the division regeneration occurs producing a head 314 ASEXUAL REPRODYCTION

or tail, or in some cases where a portion of the middle of the worm is cut off from both ends both head and tail are regenerated. As has been pointed out this regeneration after division is known as architomy in contradistinction to paratomy where a. separation zone is developed before division. Such a zone is really the beginning of an early regeneration and may be present in variable degrees all the way from a simple region of constriction to the formation of a well—developed head and sensory apparatus.

Architomy is illustrated in Ctenodrzlus monostylus where without much preparation the worm divides into an anterior and posterior part by constriction and the lacking parts are regenerated. Regeneration begins at once, although the new worms continue to creep about actively in spite of the fact that they cannot for a short while take in food. Architomy may be accomplished by autotomous division of the worm. A number of polychaetes and oligochaetes have such great powers of regeneration that they are enabled to separate without external stimulation. Lumbnculus is an annelid of this type.

Paratomy is illustrated by certain species of Ctenodrzlas also, especially C. serratus, in which the regenerative powers are so great that even single segments may in certain cases produce new worms. Beginning a few segments

Fm 012 Tami”: mulomm with a back of the head, cellular prolrferations pnmaw" ,e;;(.,,e,a,,,,g Dome" and mo takeplacewhichgraduallyformthicken ings on the anterior side of each segment.

' They become the head folds and give rise to the organs of the head region in the new zooids. Thus a chain of zooids is formed asexually. Additions of this sort produce what is functionally an alternation of generations. It is scarcely developed with the regularity that characterizes this process in such forms as hydroids. The original individual was developed from the egg. It produces asexually a chain of zooids which separate from each other and when weather conditions become suitable develop further into individuals which reproduce sexually. Of course second and third asexual generations may be produced in the same manner as the first one. PTEROBRANCHIA 315

Echinodermata. Among the echinoderms asexual reproduction is described, but it is certainly of rare occurrence and one has difliculty in distinguishing it from an extreme type of regeneration. Spontaneous division of the arms and in some cases the splitting of the disc have been described in the asteroids, ophiuroids, and in the holothurians. Asteroids have been seen to split and separate, beginning at the gullet, and then

flu. 213. S1/Ilis ramosa showing anterior end of worm and the complex branching. The gut is stippled in the figure. (From Korsehelt and Heidor. after Maclntosh.)

the missing arms and organs of the disc to become gradually regenerated. It is of course well known that where accidental separation of the parts of an echinoderm is brought about extensive regenerations follow and theoretically there is no great difference between this and a natural type of division followed by regeneration. However, the latter is certainly of earlier occurrence.

Pterobranchia. The two genera Cephalodiscus and Rhabdopleura which compose the anomalous Pterobranchia both illustrate asexual reproduc316 ASEXUAL REPRODUCTION

tion by budding. Their doubtful position showing similarities both to Balanoglossus and to the Bryozoa is nevertheless in line with their type of reproduction. Cephalodiscus occurs as a single individual from the side of which a stalk grows out. This stalk bears a bud as is shown in the well-known figure often copied from Maclntosh. The bud arises from the apex of the ventral stalk and after a certain age breaks off. The animal lives in a gelatinous coenoecium where a large number of free individuals may be found. The animal possesses practically all the important organs found in Balanoglossus and structurally seems related to that form. Its method of asexual reproduction, however, much more strongly resembles that of the Polyzoa. Resemblance to the latter group is rather more striking in Rhabdopleura which lives in a tube and reproduces in a manner similar to that of the stoloniferous Bryozoa. The general anatomy, however, closely resembles that of Cephalodiscus. In Rhabdopleura the individuals derived from the buds remain close together and form small colonies living in branched tubes. They are con— nected by a muscular cord which passes back to join a common stem or stolen. The cord is the narrowed proximal portion of the body. By its contraction the animal is retracted into a stalk. The stolen is developed as a bud from the original zooid,

1«'.c.. 214. ('c1)Iw,lod£scus with but the details of the process are not well §:f:‘lt'i$;:f’)“‘M“°I“t°*h'“'i”"“°‘ll‘ understood. Part of the stolen loses its

formative power and becomes merely a connection between the creeping parts of the colony. The remainder, however, buds, and the buds give rise to branches. In the free-growing part there always appear two buds, an anterior, well—developed one and a posterior much younger.

Tunicata. Among the tunicates asexual reproduction is widespread and occurs as an important means of multiplication in two of the main divisions of this subphylum, the composite ascidians and the salps. The dominant type is budding but the details show various departures from the simple form and even instances of fission are observed within this group. It will perhaps serve our purpose to limit our discussion to the budding in the two cases mentioned, understanding that there are some other minor examples to be met with in this group. The embryology of the ascidians has been described in Part One of this book. The young embryo grows and in many cases begins its process of budding TUNICATA

317

before it has become attached and begun its metamorphosis. It is customary to speak of the individual which is produced from the egg as an oozooid whereas the individuals which are produced from buds are blastozooids. However, the individuals so called may become modified

in various ways and be given other names to indicate functional and morphological differences which later occur, for budding in the tunicates as in some other groups leads to polymorphism and to alternation of generations.

In the compound ascidians there are two main types of budding, palleal and stolonial, differentiated according to the position on the oozooid in which they occur. By palleal budding, which also goes under the name of peribranchial or atrial, or also cntero-epicardial, is meant the growth of a bud as an oesophageal or even intestinal outgrowth; it is thus derived near the atrial wall and under the mantle or pallium. It is common for the buds to be formed as lateral evaginations of the body Wall in symmetrical parts, but usually only one of them develops. Mesoderm and germ cells migrate from the tissues of the mother into the bud which gradually becomes a complete individual. The processes are the same in the asexually produced blastozooid and in the oozooid which are developed from the egg, but the oozooid undergoes a retardation in development and never reaches sexual maturity whereas the blastozooid develops fully although it is to be noted that both may reproduce asexually as well as sexually. The oozooid is short lived. After it has reached the typical form of the ascidian it undergoes retrogression and its tissues are used to build up new individuals. In the composite aseidians although the buds are independent of each other they yet remain enclosed in a common cellulose mantle, and after a number of generations have

‘ ‘~ .-..-.-I!l!‘."'€.;',.7,_,,’

Fm. 215. Budding in Rhabdopleura normanni.

(From a diagram by Korschelt and Heider.)

b. buds of different ranks from a common stolon (st.).

been produced the mass takes on the form of buds united into a very definite system built upon the plan of concentric circles of buds. As the system becomes more and more complicated some of the individuals become crowded out of their proper place and so become the center of a new irregular subdivision of buds. In this way a colony is formed. The place of origin of the buds varies in difierent genera, a fact which 318 ASEXUAL REPRODUCTION

gives rise to the marked differences in appearance of different colonies of compound ascidians. Thus we have pyloric budding which is also called oesophageal, epicardo-oesophageal, and epicardo-rectal budding. In this type when the blastozooid buds it is really of compound origin, the new process arising from abdominal and thoracic outgrowths, the abdominal bud being an invagination of the oesophagus and the thoracic formed largely from the epicardium. From a diverticulum of the left portion of the epicardium the nervous system is derived. It is thus endodermal in origin in the blastozooid, although in the oozooid it is derived in the regular manner from the ectoderm. Pyloric budding is described chiefly in the didemnids and the diplosomids. Among the modifications of the general type of budding described for the compound ascidians are the cases spoken of as pseudo-stolonial, occurring especially in the distomid Distaplia and in the polyclinids. Authorities differ in the manner in which they regard the budding of these forms, some tracing it to the formation of a stolon and others regarding it as more nearly related to the type already described. In Distaplia the formation of the “primordial buds” has been described as coming from the epicardium, but Salensky, Julin, Della Valle, and others have described this as a proliferating stolon from the intestine. The first generation of these buds never develops sex products but later generations of buds may reproduce sexually. It may be noted that the epicardium is wrongly named in that it does not participate in the formation of the pericardium and the heart, although it was so described by van Beneden and J ulin. Actually it is the endodermal element of each bud. In the polyclinids the budding of the posterior part of the animal resembles horizontal division, for the caudal part of the larva clongates and constricts, forming thorax, abdomen, and post-abdomen; gonads and heart develop in the latter which subsequently segments to form buds. By some this entire caudal portion is regarded as a stolon.

As an example of stolonial budding reference may be made to Perophora and to Clavellina. In Clavellina, after its attachment, root—like processes are sent out from the base giving it a more firm hold. At least one of these processes becomes the proliferating stolon (stolo prolzfer). The stolon consists of the three germ layers in addition to the cellulose tunic which covers it. From it arise buds as well as side branches. If a particular outgrowth contains an endodermal layer it would develop into a bud. Otherwise it is merely a sterile branch. Buds arise near the growing tip of the stolon, but simultaneously with their development the top continues to grow, so those near the base of the stolon are the more mature. Younger ones sometimes arise as a new generation of buds between the older ones. Stolonial budding takes place only from TUNICATA 319

the oézooid for the blastozooid can reproduce sexually. It should be noted in regard to the formation of colonies that although the usual condition is the production of an entire colony from a single oozooid, yet small colonies sometimes grow so close together that concrescence results and they fuse into a larger colony.

ooooo

g’

F16. 216 An adult of Clavellma with a stolen and .i young individual budded ofi from it (From Korschelt and Hcider, after Secliger )

In many respects the most interesting case of asexual reproduction in the tunicates is the formation of the Salpa chains from a proliferating stoloii. When investigators first began to study Salpa, solitary individuals were found, and also others which appeared to have a significantly different structure and were connected together like a chain or even a rosette were also occasionally discovered. At length the relation between the two was made out by the poet, Chamisso, who discovered that the solitary individuals can produce the chain salps and that certain individuals of the chains developed sex organs from which the 320 ASEXUAL REPRODUCTION

solitary individuals were again formed. The solitary individual always reproduces asexually but is itself produced from a fertilized egg. From the ventral side of the posterior half of such a solitary individual a bud, which first arose from the pharyngeal endostyle and is therefore in its beginning eiidodermal, grows out. As it enlarges, the mesoderm and ectoderm surround it, and finally it becomes a projection from the tunic as well. It is a direct continuation of the ectoderm and endoderm of the

""-‘Kw;

- 41;!!!

St Iliasc

fiG 217. Nearly adult stages of Dolwlum dentwulatum (From Korschclt and H0|d(‘r, after Neumann.)

cl , cloacu, d c , dorsal horn or cadophore to which buds Will be uttuchcd, st , \CIlU‘dl stolon from whuh buds are produced, t , mil with chorda

parent individual and its parts are likewise related to the organs which develop in its buds derived from it. This is the ventral proliferating stolen and the individual producing it is the oozooid developed from the single egg of the gonozooid.

The history of the ventral stolon differs somewhat in Salpa and Doliolum. Perhaps the latter, as described by Uljanin, is the one more familiar to students. As in the compound ascidians the oozooid produces no sexual organs but a great many buds arise from it in a complicated manner and the chain thus produced exhibits a high degree of polyVERTEBRATA 321

morphism. Indeed, the oozooid itself degenerates until it becomes nothing but a locomotor organ of the rest of the chain. The buds are produced continuously for a considerable time from the ventral stolon. They do not remain in that part of the body, however, but migrate dorsally to a posterior dorsal horn, or cadophore, which is an outgrowth resembling somewhat a stolen but does not of itself bud. Its epithelial cells become specially suited for the attachment and nourishment of the migrating buds which will presently reach it. As a matter of fact, however, there are more buds in the dorsal cadophore than were produced from the ventral stolon, multiplication in the form of a simple division having taken place during the migration. The cadophore continues its growth and more and more buds are attached to it. They arrange themselves in two lateral rows and one dorsal median row. The lateral blastozooids which are attached by a short stalk are nutritive and respiratory. They are apparently unable to reproduce and are morphologically independent of the rest of the colony. The median blastozooids are spoken of as nurse individuals, or phorozooids. They are rounded and have a stalk on the ventral side. It is this stalk which bears the buds that in their turn are to give rise to the sexual forms. Like the lateral blastozooids, the nurses are non-reproductive. The buds which arise from the stalk of the phorozooids are usually spoken of as protogonozooids. From these the sexual blastozooids, that is, the gonozooids, are produced. They are free swimming, undergo a considerable change in their general structure, develop sex organs, and the cycle is completed, for from the fertilized egg comes the new zooid ready to begin the asexual cycle again. Thus there are two distinct asexual generations, the oozooid and the protogonozooid, and some of the blastozooids have also multiplied by simple division in passing to the dorsal cadophore. These two asexual generations alternate with the one sexual generation. The development of all forms of Salpa is not understood yet and in some cases it is difficult to interpret the forms that have been found. The life cycle of Doliolum, however, illustrates satisfactorily the degree to which asexual reproduction is developed in these forms.

Vertebrate. In forms higher than the tunicates asexual reproduction does not commonly occur. However, the case of the armadillo which is described in detail in the chapter on polyembryony must be mentioned. Here a blastocyst is formed as a result of the cleavage of the egg which produces by budding four individuals. This is, of course, an asexual method of multiplication, and may be of general significance, as is suggested by the production of identical twins in various other forms of mammals.

Chapter V Parthenogenesis

I. NORMAL PARTHENOGENESIS

A. GENERAL DISCUSSION

As an embryological problem the phenomenon of parthenogcnesis has been of interest since its discovery in 1762 when Bonnet found that the summer generations of aphids reproduce by means of unfertilized eggs. The literature dealing with the subject is a vast one, for parthenogencsis is of wide occurrence in the animal kingdom and is also known among the plants. In spite of the vast amount of research on the problem, however, it has only recently become possible to bring the known cases into any kind of systematic order. The problem also has a great deal 01 cytological and genetical interest for we are here dealing with uniparcntal inheritance and it is obvious that the chromosomal mechanism which explains the inheritance of bisexual forms must undergo some modification to account for the appearances in unisexual organisms. While the cytological and genetieal aspects of parthenogenesis are closely related to the embryological, we can treat them only briefly and must devote ourselves chiefly to the embryological problems.

Confusion sometimes develops in the mind of the student in relation to parthenogencsis for at first thought one is apt to class all types of uniparental inheritance together and think of parthenogcnesis as a modified asexual type. The fallacy of this conclusion is easily seen, however, for parthenogenesis deals with the development of an egg although without the cooperation of a spermatozoon. It is clearly a gametic form of reproduction; its relationships are perhaps more easily understood if we speak consistently of unisexual and bisexual reproduction. It would perhaps be still clearer to use the terms monogametic and digametie, although these are not commonly found in the literature. The critical point in the evidence that we are here dealing with a second type of sexual reproduction is seen in the formation of polar bodies and the behavior of the chromosome during the maturation division. In some cases of parthenogenesis polar bodies are often seen to be formed and in other cases the behavior of the chromosomes in reduction is

well known. In addition to this evidence, the sporadic occurrence of 322 TERM INOLOGY 323

parthenogenesis in diverse classes of the animal kingdom, the occasional occurrence of parthenogenesis along with the bisexual method in the same species of animal, as for example the bee, and the not uncommon alternation of the parthenogenetic mode with typical sexual reproduction in the course of several generations within the same animal all bear out the same conclusion. Furthermore, the fact of artificial parthenogenesis, that is, the activation of an egg which would otherwise develop only by fertilization, shows that we are dealing with what must be a modification of the usual type of bisexual reproduction. Artificial parthenogenesis means the initiation of development on the part of the egg by chemical or physical stimulation artificially induced to take the place of the type of initiation usually to be seen only when the sperm enters the egg. It is really a study in experimental embryology which has been of the greatest value in giving an insight into the true nature of the process of fertilization.

a. Terminology. Throughout the animal kingdom parthenogenesis occurs in a wide variety of types and under many different conditions. Owing to this fact the literature concerning it contains many descriptive terms that are of special application and about which some confusion of meaning has grown up, as has been the case in certain other processes that are familiar but not clearly understood by the majority of students. In some species parthenogenesis is of rare occurrence and the eggs simply possess the power of developing without the intervention of the sperm, although they usually develop after fertilization. Such cases are known as facultative or optional and are of such irregular occurrence in the series as to be of no great embryological significance. Obligatory parthenogenesis occurs regularly in the life cycle of organisms which show it as a constant feature or only at certain intervals. Its importance is manifested by the fact that parthenogenetic eggs are often structurally different from those that are to be fertilized and there are morphological differences between the animals which are produced parthenogenetically and those which are produced bisexually. For instance, the eggs of daphnids which are fertilized usually develop only during the colder seasons of the year; they are known as winter eggs and have a protective shell that is very different from the summer eggs. Oftentimes cases of obligatory parthenogenesis show an alternation of generations although not of an asexual generation with a sexual, but of a unisexual generation or a series of them, with a bisexual generation. Where this type of development is firmly established a series of par thenogenetic generations may be developed to such an extent that it becomes the predominant method of reproduction and sexual genera3 24 PARTHENOGENESIS

tions appear only at infrequent intervals. Many of the more familiar cases of parthenogenesis are of this type.

Even in the extreme obligatory type it is often possible by experimental means to induce the sexual process, for instance, by a change of the environmental conditions. The rotifer, Hydatina senta, as shown by Shull and by Whitney, in old culture infusions will continue parthenogenetically for long periods of time, but if a fresh culture fluid is used bisexual animals soon make their appearance. Conversely, it is possible by maintaining appropriate conditions to postpone or even entirely to suppress the sexual process, as in the case of the rose aphid when cultivated through a series of years in a greenhouse.

In cases where structural differences are to be observed between the parthenogenetic and the bisexually developed eggs, a functional difference also is frequently seen in that the eggs of certain individuals are exclusively male producing, that is arrhenotokous, and of others female producing, or thelytokous. Where both males and females are produced in one brood, parthenogenesis is said to be amphoterotokous. Where the number of parthenogenetic generations which occurs before the sexual animals are produced is indefinite we say that the species has an “open life cycle.” Such cases occur in the aphids and in some species of phylloxerans. In others the female which produces the parthenogenetic eggs and hence is known as the stem mother must develop directly from the previous fertilized egg. This is the “ closed ” type, and it occurs in certain other phylloxerans and in the gall flies.

Considering the embryological aspects we should classify known cases according to the grade or degree of parthenogenesis shown. It is possible to distinguish seven or eight different grades of parthenogenesis as follows: (a) Pathological. In the ova of birds it is frequently observed that a few cleavages take place resembling the normal but not leading to the formation of the blastula. Similar phenomena have been observed in some other eggs as well. (b) Casual parthenogenesis. In the case of the silk moths under exceptional conditions aspermic development takes place. (0) Occasional parthenogenesis. In the ants, bees, and wasps, the development of the egg without fertilization always produces a male. (d) Partial parthenogenesis. The queen bee by the copulation on the nuptial flight receives all of the sperm which she will ever use, and afterwards she can fertilize the egg to produce a female, that is, a worker or a queen, or leave it to develop parthenogenetically into a male, since she has control over the process. (e) Seasonal parthenogenesis. In the case of Daphnia already mentioned and other Entomostraca and also the aphids the summer eggs are parthenogenetic and the winter fertilized. (f) Larval parthenogenesis. The case of Miastor is described CYTOLOGICAL RELATIONSHIPS 325

in the chapter on paedogenesis. Some larvae are produced parthenogenetically and inside of them others through a number of generations. each eating its way out and producing yet others. finally the last larvae pupate, undergo metamorphosis, and develop bisexual forms. (g) Total

parthenogenesis. Only parthenogenetic eggs occur through many generations as is especially illustrated by the rotifers.

b. Cytological Relationships. A consideration of the cases of parthenogenesis upon the basis of their cytological conditions leads to the common classification of haploid and diploid parthenogenesis. By haploid parthenogenesis is meant the development without fertilization of an egg which has gone through the process of chromosome reduction. Haploid eggs in general are sexual, and in most cases in which aspermic development is possible the eggs are found to be capable of development either with or without fertilization. Hymenoptera, some of the Hemiptera, and the arachnids often exhibit haploid parthenogenesis. One case, the nematode Rhabdites (R. aberrans according to Kruger and R. pellio according to P. Hertwig), is worthy of note, however. Here the eggs develop parthcnogcnetically and are of the haploid type but the penetration of the sperm is necessary to initiate the development although it takes no further part and no true fertilization is accomplished. After it has penetrated the cortex of the egg, and thus set going the developmental processes, the sperm degenerates and the further development of the egg is as much parthenogenetic as if no sperm had been involved.

Diploid parthenogenesis is much more frequent, occurring in most of the other groups where parthenogenesis is found at all. Copepods, ostraeods, and the Hemiptera-Homoptera have this type characteristically whereas trematodes, nematodes, echinoderms, the phyllopods, the Orthoptera, and the Lepidoptera exhibit it in certain forms. An intermediate condition is found in the rotifers, both diploid and haploid parthenogenesis having been described for these forms. Triploid parthenogenesis is also known but is not common. The best understood case is that of the plant genus Hieracium which has been described by Rosenberg. There are two sections of this genus which differ in their cytological behavior. The sexual form of H. umbulatum has a diploid chromosome number of 18 whereas the variety linearifolium has a diploid number of 27. There are also some in which the diploid number is 36. The parthenogenetic Archieracium, which is closely related to this species, shows a chromosome number of 27. Rosenberg explains these facts upon the basis of hybridization. The triploid races, Rosenberg maintains, cannot easily beproduced by the usual bisexual processes. Triploid parthenogenesis, however, is of rare occurrence, and the triploidy in 326 PARTHENOGENESIS

animals which result from fertilization of a diploid egg, if that can occur, certainly produces only an abnormal zygote.

Usually the number of polar bodies which is given off by the animal egg permits, the observer to distinguish whether he is dealing with the haploid or diploid mode of development; in the haploid case two polar bodies are produced in the haploid manner, whereas in the diploid a

single maturation division produces only one polar body and no reduc-'

tion of chromosomes can take place. There is one special case of parthenogenesis to which was early ascribed theoretical significance in working out a theory of fertilization. O. Hertwig in 1890 described a process in the maturation of the starfish Astropecten which has been many times referred to since, namely, the reunion of the nucleus of the second polar body with that of the egg. A similar condition was reported by Lefevre for the egg of Thalessema in which the second polar body is not extruded, the spindle remaining “submerged” in the cytoplasm of the egg. Upon the basis of these observations parthenogenesis was for a long time supposed to be explained as a kind of fertilization by the polar body. It is now known, however, that such cases seldom occur. A somewhat analogous case is that of the wasp H abrobracon in which a cytological study shows parthenogcnetieally produced males to be haploid on account of the fact that the first spermatocyte division is abortive while the second is equational (A. Whiting, 1927). Of course cases in which only one maturation division occurs are not rare.

c. Parthenogenesis and Sex. Much of the work upon parthenogenesis has been due to the bearing of the problem upon sex determination. Many experiments have been made for the purpose of discovering what factors can influence the production of males or females within the parthenogenetic strains. Especially have the rotifers been used for experiments of this kind. Punnett observed various kinds of females as follows: arrhenotokous females or those which produce males; thelytokous females (female producers) of three kinds, namely, those which produce, respectively, a large percentage, a small percentage, and those which produce no arrhenotokous females. In the attempt to influence the sex of the progeny of these forms conflicting results have been obtained. The studies of rotifers have been most extensively carried on by Whitney and by Shull. In general Shull’s experiments on the modification of sex have yielded positive results, for he has found a relationship between metabolism and sex determination. When the quantity of food given H ydatina senta is small the number of male producers is increased; old food reduces the number. Creatin, ammonium salts, beef extract, and low temperature prevent the formation of male producers. Longcontinued parthenogenesis results in a decrease in male production. INTERSEXES 327

Oxygen in the water increases it, although osmotic pressure, alkalinity, and acidity were not found to be effective. Whether the egg is to develop into a male or a female producer is decided during the growth period but not in the early oogonial stages. Sex is determined a generation in advance.

Whitney, on the other hand, has found less evidence that male production is a response to external conditions, although he did find that crowding in H. senta permitted the development of fewer male producers in direct proportion to the degree of the crowding. He found, however, that neither oxygen, temperature, nor starvation is a factor. One of his strains when fed continuously upon a diet of Polytonia produced nearly all females for twenty-five generations, only three per cent of the daughters being male producing. But a change of diet to Dunaliella increased the percentage to 57. When they were fed again upon Polytonia, the percentage fell off. The influence of the food was found to be upon the grandmother.

In addition to the work of Shull and of Whitney, Noyes has reported a series of experiments on Proales decipiens lasting through 250 generations in which no males appeared although the animals were subjected to various environmental changes including changes of chemical composition, of temperature, and of feed. More found Brachionus to give increased arrhenotekous females upon treatment with chemicals such as ferric chloride (FeCl), whereas constant diet and temperature gave only thelytokous individuals.

Banta has studied the sex production in the parthenogenetic Cladocera, especially in M oina macrocopa. His studies with Brown upon sex control showed that experimental crowding brought about male production along with lowered metabolism in the females. The amount of food was not found to affect the results but a change in its character did have a profound effect. Male production was associated with an accumulation of excretory substances. Aeration with oxygen decreased the production of males even in crowded cultures. Similarly carbon dioxide, urea, and ammonium salts as well as other products related to excretion reduced the number of males.

d. Intersexes. Another problem in relation to sex has appeared in some of the parthenogenetie strains of Cladocera. It is the condition of intersexes or, as originally described by Banta, sex intergrades. The intersex condition manifests itself through the secondary sex characteristics in that a typical female has a series of eight secondary sex characters which are identified with her sex, and similarly for a typical male. The eight characters are as follows: (1) body size, the females are the larger at maturity; (2) size and position of the eye, the eyes of the 328 PARTHENOG EN ESIS

females being smaller and crowding the margin of the head less than those of the males; (3) outline of the head, the angle of the anteroventral margins of the head being more acute in the male; (4) absence in the female of the nuchal protuberance; (5) and (6) character of first antennae, the males having swollen basal portions and two lateral stylets; and (7) and (8) the outline and armature of the lateral postabdominal margins, these being more concave and not serrated in the female. The intersex individuals may have the sex organs of either sex accompanied by from one to several of the secondary sex characters of the other sex. There are also hermaphroditic forms with various combinations of the male and female secondary characters. It is Banta’s opinion that these intersex conditions in Cladocera and other parthenogenetically reproducing forms are probably determined by environmental factors.

A relation of hermaphroditism to parthenogenesis appears in certain cases and it has been suggested by Winkler that it is one of transition, hermaphroditism leading over into parthenogenesis. At least the case of Rhabdites aberrans as described by Kruger points in that direction. These animals are free-living nematodes found in moist earth and are almost exclusively females, two counts showing four males out of 10,000 females and thirty-two males out of 2026 individuals, respectively. But the females are hermaphroditic and the entrance of sperm into the egg is necessary for the initiation of development, although it takes no further part in the process and the cleavage of the egg is as truly parthenogenetie as if no sperm had penetrated into it. In addition to this relationship, hermaphroditism may be superimposed upon parthenogenesis at least in one family of the Phyllopoda where it appears that Lepidurus which under favorable conditions reproduces parthenogcnetically “may become hermaphroditic when food is scarce ” (Bcrnard,1896).

e. Geographical Races. Another matter of interest is the occurrence of geographical races which are parthenogcnetic. The phyllopod, Artemia salina, according to Artom, reproduces differently in different localities. Races which are exclusively parthenogcnetic occur in Marseilles, at Capodistria and in certain other Italian localities. Bisexual races occur in Cagliari in Italy and in Utah; and a third class in which both bisexual and parthenogcnetic modes occur is found at Odessa in Russia. Vandel described the conditions in the isopod, Tr2‘chom'scus provisorius, in which there are two distinct races, one bisexual in the usual sense, and the other a female-producing parthenogcnetic race. These occupy distinct regions, the latter occurring in northern Europe, but at some points the two may co-exist. These two races are looked upon by Vandel as incipient species. Geographical parthenogenesis has also been observed CAUSES OF PARTHENOGENESIS 329

in the moth Solenobia by Seiler. Solenobza triquetrella and S. znfleti occur in both bisexual and parthenogenetic forms, the latter being permanent and thelytokous. Of S. triquetrella the parthenogenetic form is widely distributed in Germany, Austria, and Switzerland, but the bisexual form has been found in a single locality only. In S. pineti the bisexual form is common but the parthenogenetic is restricted to a very few localities. finally Trialeurodes vaporariorum, a white-fly has two races, according to Schrader, that are unlike in their parthenogenesis, in that the English race produces females with only three per cent males and the American parthenogenetic females produce males only.

f. Causes of Parthenogenesis. Of the causes of natural parthenogenesis little is known. It would seem that the developmental mechanism of the egg is capable of activation in more than one manner. Usually the entrance of a sperm into the egg sets off the processes which are involved. This initiatory action is capable of imitation by both chemical and physical means as in the cases of artificial parthenogenesis. There are also conditions in which, it would appear, development may go forward automatically, the participation of the sperm being entirely dispensed with. In the case of the normal fertilization of any egg there is no reason for regarding the developmental processes of the egg as brought to a. standstill before the sperm enters. Rather they are slowed down to a speed from which they will not recover unless external influences are operative, usually in the form of the sperm. Perhaps monogametie development is possible only where the inhibition is less intense, thus giving automatically normal parthenogenesis. On this hypothesis the failure to induce the development of sperm, by placing them in suitable media as has been reported by Locb and others, may mean nothing more than that the developmental processes in the sperm are reduced to a still lower speed from which no means thus far tried have been effective to return them to a functional level.

Other suggestions have also been ‘made as to the factors which are responsible for parthenogenesis. Issakowitsch held that in Cladoeera the alternation of bisexual and parthenogenetic stages was governed by changes in the nucleo-cytoplasmic ratio. Papanicolau reached similar conclusions. Shull, however, investigated the nucleo-cytoplasmic ratios in the rotifer, H ydatina senta, without finding any correlation between it and the mode of reproduction. Although it has been claimed that food, oxygen content, and other similar external factors influence the sex ratios of parthenogenetically produced animals, there is no good evidence that these factors are responsible. Banta thought the formation of an ephippium (winter egg, which develops upon fertilization) to be 330 PARTHENOGENESIS

due either to unintentional change in the media, in which parthenogenetic daphnids had been developing, or to overcrowding.

Another suggestion (which is so far merely record of an observation rather than a cause) relates to the lagging chromosome of the phylloxerans which is observed to accompany parthenogenesis. Peacock and Harrison working upon lepidopteran crosses reached the conclusion that parthenogenesis is a result of hybridity and later supported this conclusion by a study of the data of other crosses including Nabours’ experiments on parthenogencsis in Apotettix. finally Nabours, working upon Parateltirc, studied genetically two strains one of which was highly parthenogenetic and the other had never so reproduced. He obtained genetic evidence that parthcnogenesis is a character which segregates in a Mendelian manner and believes it due to certain genes. Robertson’s cytological studies on Nabours’ material fall into line with this hypothesis. The entrance of the sperm is necessary for the second polocyte division, and when it is lacking the diploid chromosome number is retained. In this latter case if the specific genes for parthenogenesis are present development is initiated. It is to be noted that Agar working upon Daphnia and Whiting upon Habrobracon failed to find genetic evidence of the cause of parthenogenesis. But Punnett as long ago as 1906 had suggested the probability that the question is concerned with the zygotic constitution of the egg and thought that it might be a matter of Mendelian segregation. These different hypotheses show the need of much further study of this most elusive subject.

B. THE OCCURRENCE or NORMAL PARTHENOGENESIS

Natural parthenogenesis is of much wider occurrence in the animal kingdom than is generally recognized, and it is not rare among the plants. Examples are found in the following groups of animals: rotifers, nematodes, trematodes, Dinophilus, three orders of Crustacea, myriapods, arachnids, and ten orders of insects. It seems desirable to point out some of the important studies in each of these groups, although no attempt can be made toward inclusiveness. For references and a more nearly complete listing of cases the reader should consult Winkler.

a. Rotzfera. Among the rotifers, as already described, typical arrheno— tokous parthenogenesis occurs in the same groups of animals with thelytokous parthenogenesis. The studies on Hydatina senta are among the best known of all studies on the type of reproduction. Three types of females, as previously indicated, are found here, and by experimental means it is possible to change the strains from parthenogenetic to bisexual. CRUSTACEA 331

b. Nematode. For the nematodes the case of Rhabdites aberrans as described by Kriiger has already been mentioned. The trematodes are commonly described as giving rise to rediae from cells which are segregated at one end of the body of the sporocyst and become an ovary. The new rediae arise from these so-called “ova,” which must be thought to develop parthcnogenetically if the view of their origin is maintained that these are true eggs. This controversial subject is discussed in other chapters in this work.

c. Annelida. Among the annelids parthenogenesis is suspected in Dinophilus conklini from the preponderance of females. In the leech Ilemiclepsis marginata the early finding of parthenogenesis has failed of confirmation in more recent experiments (Brumpt, 1900).

d. Crustacea. Two orders of the Entomostraca exhibit parthenogenesis and include in their number some cases that have been given a great amount of study, while a third offers a case which is suspected to be parthenogenetic. Among the Phyllopoda examples may be cited from both the Branchiopoda and the Cladocera. Of the former, Apus, Lepidurus, and Artemia are well known. Reference has already been made to Lepidurus because of its hermaphroditic relationships. Thelytokous parthenogenesis occurs in these forms. Artemia salina likewise has been mentioned in connection with geographic parthenogenesis. On this form the most recent studies are those of Artom (1912). Two polocytes are formed in the development of the egg. Regularly a reduction of chromosomes from 42 to 21 takes place. The parthenogenetic forms are tetraploid and of a type called somatic. There are therefore two strains of Artemia, one diploid and bisexual, the other tetraploid and thelytokous parthcnogenetic. The following explanation has been upon the basis of the very slight evidence that in the parthenogenetic strain only one polocyte is formed (Brauer) and that without reduction. If the egg and polocyte nuclei, both of which retained their somatic number of 42 chromosomes, should fuse, then an individual with the tetraploid number would be produced. As yet this explanation must be looked upon as purely speculative. Numerous others of the true phyllo« pods, such as Branchipus streptocephalus, have been observed to repro duce parthenogenetically and still others are suspected because of the scarcity or absence of males.

Of the work on the Cladoeera, which dates from Weismann’s studies in 1879, the general facts are well known. Bisexual generations are followed by parthenogenetic which may continue indefinitely. The various races differ greatly in the number of generations produced within a year and in the frequency of the bisexual forms. Apparently most if not all forms are parthenogenetic at some stage of their life cycles, but 332 PARTHENOGENESIS

those genera most studied have been Daphnia, Simocephalus, M oina, Bosmina, Chydorus, Polyphemus, and Leptodora, the first four being best known. The more recent students of the group include Olofsson (1918), Woltereck (1909-1911), Langhans (1911), Kuehn (1908), Chambers‘ (1913), Taylor (1914), Thiebaud (1913), and especially Banta and his co-workers, whose work is published in a series of papers beginning in 1914 and is not yet completed. In all forms the males are rarely seen and in some species are unknown. As already stated much of the work concerns the control of sex and the intersex conditions in these

forms. There is good evidence that sex ratios and intersex conditions are both subject to environmental influences.

In the order Ostracoda, parthenogenesis was discovered in 1880 by W. Mueller and by Weismann for a number of genera, it having been shown that virgin females produced eggs that developed into females; that is, produced a thelytokous race. No reduction division was found to occur by Woltereck (1898) or by Schleip (1909) for a considerable number of forms which they studied and parthenogenesis for these animals is therefore of the diploid or somatic type. In 1914 Wohlgemuth studied carefully the reproduction of numerous fresh-water ostracods and thought they fall into two groups, one of a purely bisexual and the other of a purely parthenogenetic type, although there are transitional forms between the two. Other investigators of the Ostracoda have been Menzel (1911), Alm (1916), and Olofsson (1918). Their chief concerns have been to describe the facts of parthenogenesis, the relation of the bisexual generation to the parthenogenetic, the duration of each and the conditions under which they occur. Not much of importance to an embryological account is gained from these forms, although SchIeip’s work on the various species of C3/pris offers perhaps the best cytological description of parthenogenesis in any crustacean form. His observation of synapsis not followed by reduction is worthy of note. It may be remarked that some of the studies of ostracods have taken up the question, raised for many Crustacea, as to the possible impairment of vigor of a strain due to long-continued parthenogenesis. Here as elsewhere among the members of the class the general consensus of opinion seems to be that no loss of vigor can be noted.

Mention should be made of the possibility of parthenogenesis in the third order of the Entomostraca, the Rhizocephala. In the genus Sylon belonging to that suborder Smith reported his complete failure to find males and so suspected that parthenogenesis is the method of reproduction.

In one order of Malacostraca parthenogenesis has been reported, INSECTA 333

namely in the Isopoda. Vandel (1928) has given an account of geographical parthenogenesis, already mentioned in Trichoniscus. Two races occupying different localities were described, one bisexual, the other exclusively parthenogenetic. In some places they may exist side by side, but mating does not occur between the two races. Physiological factors tend to keep the two races distinct and to increase the divergence between them. The parthenogenetic race is found to be triploid in its chromosome relations. The eggs of the parthenogenetic female do not undergo synapsis.

For both of the two Inyriapod groups, the Chilopoda and the Diplopoda, cases of suspected parthenogenesis are recorded. Of the former Geophilus and Lomyctes, and of the latter N opoiulus and Polyxenus, have been studied. All of these include races which either lack males entirely or in which they are to be found in certain localities only. In addition observations were long ago made by Sograff (1882) upon Geophilus proximus of which unmated females laid eggs that began to develop.

Among the spiders accounts of parthenogenesis have been reported for nearly a century and it may still be suspected, but Montgomery in 1903 concluded that it is “very rare among spiders, and it is probable that most species do not show it at all.” Since Montgomery’s finding, it does not appear that any work has been done which successfully establishes parthenogenesis in these animals.

Among the mites there is no doubt that parthenogenesis occurs, for Ewing (1914) described the process in the bisexual Tetranychus telarius and showed that the unfertilized eggs give rise to males exclusively. It is arrhenotokous parthenogenesis. Among the ticks, the Brazilian form, Amblyomma agamum, has been shown by Aragas (1912) to multiply by thelytokous parthenogenesis.

It has been conjectured by Hennecke (1911) that the tardigrade. M acrobiotus macronyx, has an alternation between parthenogenesis and bisexual generations.

0. I nsecta. Parthenogenesis is of very general distribution in the insects, having been described in the ten orders. The following table, compiled in part from Winkler, summarizes the occurrence of the phenomenon. For references to most of these cases the reader is referred to Winkler, but a few of especial significance are mentioned following the table.

Aptera

M achilzls. Heymons, 1-905 ; doubtful but could find no males. Forbicina. Verhoeff, 1912; all females, so no doubt parthenogenesis occurs. 334 PARTHENOGENESIS

Orthoptera Mantidae

Mantis religiosa Przibram, 1909; induced experimental parthenogenesis, Sphodromantis not natural.

Phasmidad

Eurycnema. v. Wuelfing, 1899; three generations of parthenogenetic females in Java. Hanitsch, 1902; two generations of parthenogenetic females.

Bacillus. Dominique, 1896-99; two thelytokous generations of parthenogenesis. Males are rare. Also Stadelmann, v. Bachr, Cameron, Daiber.

Dixippus. Pantel and de Sinety, 1908. Parthenogenesis established. Schmitz, 1906. Four generations of thelytokous partheno genesis.

Hammerschmidt, 1910. five generations of thelytokous parthenogenesis.

Jeziorski, 1918. Four generations of thelytokous parthenogenesis.

Phasma. Thurau, 1899. Only females.

M onandroptera P . Raphidems }Bordoz, 1913. arthenogenetic. Leptinia. de Sinety, 1900. Thelytokous parthenogenesis. Pantel and

Ucles also found parthenogenesis. Phyllium. Bordas, 1898. Suspected parthenogenesis.

Locustidae Saga pedo. Claus-Grobben, 1917. Report parthenogenesis. Gryllidae M yrmecophila acervorum. Schimmer, 1909, suspected parthenogenesis. Tettigidae fig Nabour and colleagues, 1919-1929. Thelytokous parthenogen' . B . Telmatemx esis est understood cases

Coleoptera. Parthenogenesis is of rare occurrence and most of the older accounts are not trustworthy.

Tropiphorus carinatus. Calwer, 1916, reported parthenogenesis. Otiorrhynchus turca. Ssilantjew, 1906 0. ligustici. Wassiliew, 1909 Thelytokous parthenogenesis. 0. cribricollis. Grandi, 1913 Calandra oryzae. Hinds and Turner, 1911, report parthenogenesis common. Eggs produce both sexes.

Strepsiptera Stylops}Brues, 1903 Xenos Nassonov, 1910 Elenchus. Muir, 1906

Parthenogenesis probable due to morphology of female genitalia. IN SECTA 335

Thysanoptera

Most species are bisexual. Many may multiply parthenogenetically. The1ytokous parthenogenesis is established for Parthenothrips, Anaphothrips, Heliothrips, Taeniothrips, and Liothrips; for a number of others it is probable. Arrhenotokous parthenogenesis is experimentally shown for Antlwthrips verbasci.

Corrodentia Ectopsocus. Ribaga, 1904. Probably parthenogenetic.

Hemiptera

Aleurodes. Morrill and Back, 1911. Arrhenotokous parthenogenesis.

Trialeurodes. Schrader, 1920. English race thelytokous. American race ar rhenotokous.

Aphididae. Many species of Aphis are known to be parthenogenetic and types vary. In north they reproduce bisexually during colder seasons but in south may be exclusively parthenogenetic.

Other genera include Callipterus, Ceratophis, M acrosiphum, M yzus, Pemphigus, Rhopalosiphum, Schizoneura, Toxoptera; Chermcs, Pincus,Phyllozera.

Coccidae. Many genera of scale insects are described as parthenogenetic although some cases are not known. Included are Aspidiotus, Ceroplastes, Cryptococcus, Diaspis, Eriopeltis, Lecanium, Lep1.'dosaphes, Orthezia, Parthenolecanium, Pseudococcus, Pulvinaria, Saissetia.

Lepidoptera All proven cases belong to family Psychidae or are closely related to it. Acanthopsyche, Pachythelia, Psyche, Sterrhopteryx, Phalacropteryx, Apterona, Cochlophora, and Lufia are included. The best known is Solerwbia liclwnella. Hoffmann, 1859.

Solenobia triquetrella. Hofimann, 1859-1869. Usually bisexual. Both bisexual

and parthenogenetic modes described. Geographical parthenogenesis. v. Seibold, 1871; Rolph, 1884; Rebel, _1906; Dampf, 1907; Seiler, 1918. Solenobia pineti

Diptera. Parthenogenesis seldom found in this group.

Chirmwmus. Godlewski, 1914. Paedogenesis and parthenogenesis intermingled. Corynoneura. Goetghebeur, 1913. Rarely parthenogenetic. M etriocnemus. Picado, 1913. Males few in number.

Hymenoptera. Six families show parthenogenesis.

Tenthredinoidea The following show thelytokous parthenogenesis: Albia fasciala ' Allantus pallipes

Allantus canadensis Amauronematus puniceus 336 PARTHENOGENESIS

Amauronematus semilacteus Phyllotoma aceris Caliroa limaama Phyllotoma nemorata Cimbex connata Pontania capreae Croesus varus Pontania viminalis Emprfu abdominalis Pristiphora pallipes Empria pulverata Pristiphora fulvipes H emichroa alml Pteronidea spiraeae Hemichroa crocea Pteronidea tibialis Nematus erichsoni Thrimax mixta

Pachynematus obductus The following are probably thelytokous:

Caliroa aethiops Selandria stramineipes Phyllotoma vagans Strzmglylogaster lineata Producing both males and females are: Ametastegia equiseti Pseudoclavellaria amerinae The following are arrhenotokous:

Albia nitens N ematus luteus

Allantus cinetus Periclista albida Allantus viennensis Phymatocera aterrima Ametastigia glabrata Priophorus padi

Arge, all species Pristiphora conjugate Caliora annulipes Pristophora crassicornis Cladius Pristiphora geniculata Croesus brischkei Pristiphora testacea Croesus litipes Pristiphora alnivom Croesus septentrionalis Pteronidea, 15 species Lophyrus, all species Trichiocampus viminalis Nematus coeruleocarpus Trichiocampus lucorum

Cynipoidea.

The following genera contain species which are parthenogenetic and in most cases the males are unknown, so they are probably thelytokousz

Andricus ' Cynips Rhodites

Aulacidea Drastrophus Dryophania Ceroptres Phanacis N euroterus

Ichneumonoidea Chalcids

The following are arrhenotokous:

Ageniaspis Entedon Paracopidoswnopsis Anaphmldea Litomastix Pentarthron Copidosoma M elittobia Pteromalws Encarsia M icrtmzelus Schedius

Encyrtus M icroterys Tropidopria IN SECTA 337

The following are thelytokous:

Asphclinus Coccophagus Tetrastichus Aspidiotiphagus Odtetrastichus Tripoctenus

The following contain species that produce both sexes or are uncertain as to sex although parthenogenetic:

Eupelmus I sosoma Scutellista H abrocytus Paniscus Trichogramma

Braconids: All parthenogenetic species are described as arrhenotokous.

Ichneumonids: All parthenogenetic species produce mixed broods.

Proctrotrupids: Anagrus, Gonatopus are thelytokous. Balus, Paranagrus, Phanurus produce mixed broods, or first females then later males.

Doubtless many more parasitic hymenoptera are parthcnogenetic.

Formicoidea

Arrhenotokous parthenogenesis is very widespread among the ants. Many workers are fertile and lay unfertilized eggs from which males develop. Some hold the view that both males and females may develop from parthenogenetic eggs. A few cases are recorded in which thelytoky is claimed to be established. Certainly it is much less common than arrhenotoky.

Vespoidea

Parthenogenetic eggs of queens and workers develop into males only. Examples are Polistes gallicus, v. Seibold, 1871 Vespa germanica, Marchal, 1896

Apoidea For many bees arrhenotokous parthenogenesis is conclusively established. Haploid males are so produced, and diploid males probably do not occur, that is, males produced from fertilized eggs, although that possibility has not yet been excluded. In a few genera both males and females are produced

parthenogenetically but this is not usual. Queens and workers are females produced bisexually.

As is to be seen from the preceding table, a great deal of work has

been done upon insect parthenogenesis. It has, however, been to a large extent concerned either with the general life cycle or with the cytological and genetical aspects of the problem. Many studies of very fine character have been made upon gametogenesis and sex determination, and upon the factors involved in the control of sex. Of recent years the problems which are bound up with uniparental inheritance have attracted much attention. It must be admitted that much of this work has not advanced our general embryological knowledge of the insects to any great extent.

Among the many investigations may be mentioned a few to which 338 PARTHENOGENESIS

the reader is referred if he would pursue further these aspects of the problem. The work of Nabours and his collaborators upon the various forms of grouse locut promises much in the cytological analysis since grasshopper chromosomes are among the most favorable for study. The saw flies have been studied especially by Doncaster and by Peacock and Harrison. Of the aphids and phylloxerans a very careful cytological study was made by Morgan. An earlier study by Tannreuther is a good account of the general embryology of the aphids. Parthenogenesis in the moths is most recently made known to us by the studies of Goldschmidt and of Seiler. Of the Hymenoptera the extensive work done by Patterson on Paracopidosomopsis will serve to introduce the student to the relation of parthenogenesis and polyembryony, and the best detailed account of the cytology of the bee is undoubtedly that of Nachtsheim. Other investigations have already been mentioned in connection with other aspects of the general parthenogenesis problems.

f. Plants. Among plants a considerable number of cases of parthenogenesis are already known and it is thought of interest to mention the conditions found there in this discussion although the problems involved are outside the scope of this work. It is difficult without careful study among the plants to distinguish between cases of parthenogenesis and those of the simple vegetative apogamy. Among higher plants haploid parthenogenesis is exceedingly rare, although several cases have been described for the lower forms. Haploid parthenogenesis is also described as generative parthenogenesis by Winkler and as true parthenogenesis by Strasburger. Diploid parthenogenesis is less common among the lower forms but more so among the vascular plants. It is due to the failure of the maturation process to occur to completion. The forms which are known to be parthenogenetic are as follows:

Haploid parthenogenesis: Spirogyra, Ernst, 1918; Vaucheria, von Wettstein, 1920; Ectocarpus, Kylin, 1918; Fucus, Overton, 1912 (probably haploid); Gastrodia, Kusans, 1915; Oenothera, Haberlandt, 1921, 1922; Datura, Blakeslee and Belling, 1922, evidence from breeding experiments; Nicotiana, Clausen and Mann, 1924.

Diploid parthenogenesis: Chara crinita, Ernst, 1918; Athyrium fel2':cfoemina, var. clarissima, Farmer and Digby, 1907; Scolopendrium vulgare, Farmer and Digby, 1907; Marsilia Drummondii, Strasburger, 1907 ; Allium odomm, Haberlandt, 1923 ; Atamosco texamz, Pace, 1913; Calycanthus, Schiirhofi, 1923; Alchemilla, Murbeck, 1901, Strasburger, 1905, Boos, 1917; Wikstroemia, Winkler, 1906; Eupatorium glandulosum, Holmgren, 1919 ; Erigeron annuus, Tahara, 1921; Antennaria alpina, Juel, 1900; Chondrilla, Rosenberg, 1912; Taraxacum, Juel, 1904, Osswa, 1913, Sears, 1922; Archieracium, Rosenberg, 1917. ARTIfiCIAL PARTHENOGENESIS 339

II. ARTIfiCIAL PARTHENOGENESIS

Of great embryological significance is the phenomenon of artificial or experimental parthenogenesis. It has been the subject of much of the work on experimental embryology of the present century and its chief consideration should be a matter for a treatise on experimental embryology. However, its importance for an understanding of matters which have to do with the normal embryological process is so great that a brief account should be included here. For details and bibliography the reader is referred to the extensive accounts which have been published by Loeb (see his “Artificial Parthenogenesis and Fertilization”) and by Morgan in his “Experimental Embryology.” Wilson has also critically considered the cytological aspects of the problems concerned with artificial parthenogenesis in “The Cell.”

The student should be reminded of the double function which fertilization serves in normal development; the one is hereditary in character in that fertilization provides the mechanism for conveying the contribution of the male to the offspring. The other is strictly developmental in that it sets in motion those processes which have been inhibited or are latent in the egg at the time of maturation. It is with this second aspect of fertilization that experimental or artificial parthenogenesis deals. These experiments have sought to imitate and to interpret the normal processes of development as initiated by fertilization, and because of that a much clearer understanding of the normal process has been reached. The experiments have shown that it is possible to cause the unfertilized egg of a great many animals to develop into larvae largely under the influence of chemical treatment, although physical means have also been used to accomplish the same end less perfectly.

The work on parthenogenesis is chiefly a monument to the insight of Jacques Loeb and his analytical experiments have been of the most importance in solving the many problems involved. However, numerous other students have contributed to the progress of the work both before and since his announcement of the discovery that it was possible to induce eggs to develop artificially. Among the preliminary studies which antedated Loeb’s discovery in 1899 may be mentioned the paper of Loeb himself in 1892 in which he studied the effect of addition of sodium chloride to the sea water in which the eggs of Arbacia developed; Richard Hertwig’s observation (1896) on the effect of treating unfertilized sea-urchin eggs with strychnine; a series of experiments by Morgan (1896, 1899, 1900) on the effect of salt solutions on the segmentation of eggs and the formation of artificial asters; the work of Mead (1896-98) showing that unfertilized eggs of Chaetopterus will 340 PARTHENOGENESIS

form polar bodies in certain solutions in which the amount of sodium chloride is increased. Loeb’s papers upon this subject are many, and to him is due the chief credit for the developing of our knowledge of the subject. Ampng those who early worked upon the problem was Delage, who in a series of papers from 1900 to 1913 added a great deal to the understanding of this difficult subject; he succeeded in rearing through metamorphosis the larvae of both sea-urchins and starfish which had been induced to develop by artificial parthenogenesis. Loeb and Bancroft (1913) brought a parthenogenetic frog through metamorphosis and found that its sex glands contained eggs. Shearer and Lloyd (1913) succeeded even better than Delage in bringing parthenogenetic larvae of Echinus through metamorphosis, and more recently (1918) Loeb has brought a considerable number of parthenogenetic tadpoles to the adult stage. It is to be noted, however, that only in the case of frogs has it been possible to imitate in the laboratory the conditions of nature sufficiently to bring animals developed by parthenogenesis to sexual maturity. Since Loeb’s work, Bataillon, R. S. and F. R. Lillie, Goldschmidt, Gray, Harvey, Heilbrunn, Herbst, Herlant, and Just are among the many who have made extensive studies into the embryological phases of artificial parthenogenesis.

VVhen one begins to consider the significance of artificial parthenogenesis he is at once led to inquire as to how general the phenomenon is or how generally it may be expected to be possible in the animal kingdom. It may well be that eggs are so constituted as to permit their development without fertilization, at least into the cleavage stages, and that we are merely unable to understand the requirements in each case and to devise the correct procedure. A broad generalization of this character is hardly safe, however, for up to the present time the only forms in which it has been possible to induce experimental parthenogenesis are those in which the experiment is relatively easy to perform. In every case the eggs are of the type which is shed freely into the water. The difficulties of the experimental work which is involved may be the reason for this limitation or it may be that a deeper limitation is operative so that eggs which are adapted for other kinds of development may not be stimulated to cleave artificially. Thus far the following groups have responded to the treatments that have been devised to induce parthenogenesis artificially. By far the most work has been done on the sea-urchins. Arbacia, Strongylocentrotus, T0:copneus'es, and Paracentrotus have all shown themselves excellently adapted to experiments of this type and they have been used for much important work, especially the first two mentioned. The sand dollar, Echinarachnius, has proved adaptable for similar experiments. Among the starfishes ARTIfiCIAL PARTHENOGENESIS 341

the eggs of the commoner species of Asterias as well as some of the more unusual starfishes, as Asterina gibbosa, have been used successfully. It is to be noted, however, that the methods which have proven most successful with the sea—urchin give but very poor results with the starfish, and conversely little success has been obtained by applying the two best methods of inducement for the development of the starfish egg, namely, carbon dioxide and mechanical shaking, to eggs of the sea-urchins. The eggs of the frog, Rana fusca, have often been induced to develop by parthenogenesis, and the most successful treatment has been again a mechanical one, namely the simple pricking of the surface of the egg. Various annelids have been studied, especially Chaetopterus, Amphitrite, Thalessema, but the best results in this group have unquestionably been gotten for the eggs of Nereis. Here again the means of inducing parthenogenesis is physical rather than chemical, namely, the subjecting of the egg to heat.

A second question arises in attempting an embryological interpretation of the experiments in artificial parthenogenesis, namely, do these experiments produce animals that are fairly normal. As already pointed out Loeb’s own studies led him to the belief that “parthenogenetic larvae may be normal and apparently healthy,” and indeed, he said, “if the raising of the larvae was not such a tedious process parthenogenetic animals would exist today in large numbers.” His methods have certainly produced, both in his own hands and in those of other investigators as well, larvae which resemble the normal in every particular, and Delage, Shearer, and also Fuchs were able to bring parthenogenetic larvae to adulthood. The chromosome relations of the individuals as produced, however, have not been made out with certainty as yet and it is still doubtful whether haploid larvae produce normal adults. Parmenter (1925) has shown that in the larval and adult stages Loeb’s parthenogenetic frogs have the diploid chromosome number. The direct evidence is entirely lacking as to whether the adults produced from diploid larvae can themselves produce offspring. It should be said that most investigators are of the opinion that the parthenogenetic larvae which are experimentally produced are not merely normal in appearance but that they really are in every respect what they seem, that is, normal animals.

‘Loeb developed a theory of parthenogenesis as a result of his experiments, and what he spoke of as his improved method is based upon that theory. According to his View the formation of the egg membrane is the deciding criterion by which the initiation of development may be recognized. Indeed he traced a causal relation between the formation of the egg membrane and the subsequent development of the egg and 342 PARTHENOGENESIS

attached much more importance to this process than had previously been done. Membrane formation is the deciding condition of development. Not all later workers have agreed with Loeb in this view, but unquestionably his explanations have not been out of harmony with the facts which he observed.

The so-called improved method of Loeb for inducing artificial parthenogenesis consists of two steps. Its details as to concentration and length of exposure must be worked out for the particular species of sea—urchin eggs used and indeed different individuals will respond differently so far as the duration of the exposures are concerned. With regard to these points the method is entirely an empirical one. The

procedure as worked out for Arbacia eggs is as follows: Unfertilized

. . . . N . eggs are placed in a sea water mixture containing 2 cc. of T6 butyric

acid to 50 cc. of sea water. Apparently any monobasic fatty acid would

serve equally well for this purpose but Loeb found butyric to be satis N factory for the experiments (2.8 cc. of I5 butyric were necessary in the

case of Strongylocentrotus purpuratus eggs). The duration of the exposure must be brief, from 1% to 3 mintues. The eggs are then transferred to normal sea water and after 10 or 15 minutes to hypertonic sea water consisting of 8 cc. of 2% M NaC1 to 50 cc. of sea water. At a temperature of 23° the eggs must remain in this solution from 17% to 25 minutes after which they are transferred to normal sea water. The effect of this double treatment has been shown many times and in many different localities. A very large percentage, usually about as great as is obtained from the development of fertilized eggs, go ahead with the cleavage process and develop to larvae; doubtless the inability to rear the larvae is not in any way due to the fact that they were parthenogenetically produced.

The explanation of this double procedure has been a matter of some discussion. Loeb believed that membrane formation sets going certain chemical reactions upon which the future development of the egg depends. These chemical reactions, however, leave the egg in a condition from which it will not itself recover, and unless a second factor is employed in the process it will disintegrate rapidly. These reactions are on the nature of oxidations and are at least to a certain extent cytolytic in nature. The second factor is therefore necessary as a corrective to regulate the extensive oxidization which is produced by the exposure to the first solution. Loeb’s conclusion that cytolysis must take place unless corrected by the hypotonic solution has been questioned by some ARTIfiCIAL PARTHENOGENESIS 343

later experimenters, notably by Just, who has been able to induce the development of normal plutei of Arbacia by the use of hypertonic solutions alone. The exact preparations of the salts which are used to render the sea water hypertonic and the durations of the exposure to this mixture must be regulated with a great exactness if successful results are obtained. His optimum solution is made up of 22 parts of 2% M NaCl or KCI plus 78 parts of sea water, although variations from this mixture were also used. Of course great care was used to control the experiment in every way necessary. Eggs subjected to a treatment with this solution form membranes while still in the solution. Subsequently the eggs are returned to sea water and their development proceeds. Upon the basis of these experiments Just holds that the treatment with the fatty acid is not necessary and therefore that the hypertonic sea water is not serving as a corrective to stop the cytolysis induced by the unusual step in Loeb’s procedure. He is disposed to relate the activation of the sea-urchin eggs by this treatment to the egg secretion, fertilizin, which is given off by the unfertilized eggs into the sea water and which has been shown to be a necessary intermediary to normal fertilization with sperm. Just suggests that the activating agent which will accomplish experimental parthenogenesis serves to bind the fertilizin produced by the egg and thus to complete the necessary cortical changes whic}

must take place if development is to proceed. The full explanation or the artificially induced changes has perhaps not yet been offered with regard to the experimental facts. However, there is no question that the

entire subject has been one of the most successfully studied of all problems of experimental embryology.

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 an

344 PAEDOGENESIS AND NEOTENY 345

interesting 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. 346 PAEDOGENESIS AND NEOTENY

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 PAEDOGENESIS AND NEOTENY 347

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 348 PAEDOGENESIS AND NEOTENY

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 PAEDOGENESIS AND NEOTEN Y 349

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. 350 PAEDOGENESIS AND NEOTENY

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 352 PAEDOGENFSIS AND NEOTENY

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 sug— gested 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.

Chapter Vii Polyembryony

By polyembryony is meant among zoologists the production of multiple embryos from a single egg. (This definition does not apply to botanical nomenclature.) The number of individuals produced from a single egg ranges from hundreds as in the parasitic Hymenoptera down to two, although in the latter case the condition is usually known as twinning, and not all types of twins are properly thought of as polyembryonic.- Polyembryony occurs very widely distributed through the animal kingdom in groups which are totally unrelated to each other. Because of this wide distribution the literature to which reference can be made in this connection is really very extensive and the significance of some of the cases extends beyond the realm of embryology. Of the numerous investigators who have studied the question only a few need be mentioned here, but it may be noted that a reference to their work will serve as an introduction to the much broader literature which is not included in this discussion. Patterson has summarized the knowledge of polyembryony in a paper in Volume II of the Quarterly Review of Biology and it is suggested that the student consult this paper in beginning a more extensive study of these problems.

Patterson recognized three types of polyembryony: “ (1) experimental polyembryony, or the production of multiple embryos by artificial means; (2) accidental or sporadic polyembryony, or the occasional production of multiple embryos in the species which is typically monembryonic; (3) specific polyembryony, or the habitual production of multiple embryos in a given species.” The general student of comparative embryology is interested in the last type much more than in the first two, although cases of the first two throw much light upon many fundamental problems of organization of the egg and embryo and offer a very stimulating field for the experimental embryologist. Many types of eggs whose normal development includes no hint of polyembryony can be induced to produce double monsters and even complete embryos by experimental means.

Cases of occasional or sporadic polyembryony are recorded for coelenterates, cestodes, annelids, echinoderms, arthropods, and vertebrates. Perhaps because the latter group is so much more studied, there

353 354 POLYEM BRYONY

have been more cases reported for the vertebrates than for the others and, as Patterson points out, it is here that the occasional identical twins of the human species should be classified.

Specific polyembryony, to which our attention is drawn as the special problem of the comparative embryologist, is found in the following groups of animals: flatworms, bryozoa belonging to the Cyclostomata, earthworms, parasitic Hymenoptera, and the mammals, especially as illustrated by the armadillo. It is of more widespread distribution, however, than is indicated by these groups, but here are included the more important cases.

An inspection of this list is sufficient to show that polyembryony has no phylogenetic or taxonomic significance, for the groups are too widely separated and too diverse in structure to admit of an interpretation of this kind. Moreover, the structural differences both of adults and embryos in these various groups are so great that it is not possible to look for the causes of the phenomena in any simple embryological process common to all. Doubtless in the final analysis all these processes have a common underlying causal factor which is bound up with the innate protoplasmic organization of the egg substance.

As Patterson has pointed out, it is really not much more strange for an egg to give rise to two embryos than to one. The remarkable fact is that a new organism can be produced from an egg at all, and those qualities of the living substance which make possible its reproduction are basic to the one case no more than to the other. We must recognize that specific polyembryony is as much a characteristic of certain animals as is the formation of a coelome. At a certain stage in development there occurs a series of events which lead perhaps to two buds, or more, from each of which individuals grow. Very little thought is necessary to convince oneself that the directive forces behind this type of budding are not much more astonishing than those which cause, in the other case, the archenteron to bud out to form an enterocoele.

A. THE Causns or POLYEMBRYONY

Of the explanations of polyembryony, probably the most prevalent is the theory of blastotomy, according to which the blastomeres of the cleaving egg become separated in the 2-, 4-, 8-cell stage or later, and lead entirely independent existences, each arriving at length at the stage of a completely formed embryo. This theory is the basis of the familiar explanation of the origin of identical twins in human beings, that the blastomeres in the 2-cell stage become separated and each gives rise to an embryo. This is a view with few observational data to support it, but it may be presumed to find considerable support in the THE CAUSES OF POLYEMBRYONY 355

experimentally induced development of isolated blastomeres. Numerous experimental devices are well known by which blastomeres can be separated. To mention only two of these devices as examples, it is possible more or less successfully to cut apart the blastomeres with a knife or delicate thread drawn about the egg, or to keep them apart by subjecting the eggs to calcium—free sea water in which the blastomeres do not cohere to one another. In some of these experiments the isolated blastomeres have lived and developed for a time; in others they have produced partial larvae (ascidians etc.); and in others they have developed into whole larvae (amphioxus, Cerebratulus). But in forms where direct evidence for blastotomy is possible none has been found which forces one to this view for cases of polyembryony. As will be seen presently, the facts in the armadillo make the explanation based on blastotomy untenable there, and render it highly doubtful elsewhere.

A second explanation takes the form of an assumption that polyovular follicles, that is, follicles containing several ova fused together, may provide the mechanism by which polyembryony is accomplished. It is only necessary to say that the evidence for this supposition is entirely insufficient and the theory may now be regarded as abandoned.

In the budding theory we have what is perhaps a closer correspondence with the observed facts, although it is more of a descriptive explanation than an attempt at developmental analysis. After cleavage there arise in the blastulae or gastrulae of lower animals or in the blastocyst of mammals certain areas or “growing points” which are essentially buds. If a single one is produced it becomes the apical or head end of a normal embryo and assumes dominance over the remaining parts. If two are formed they are the beginnings of twin embryos. Similarly secondary buds may be formed and four or more embryos result, depending upon the number of buds. The facts observed in the armadillo will be seen to harmonize well with this explanation.

This ‘theory of budding has been criticized rather severely, particularly by Assheton, on the grounds that budding cannot take place unless there is a stock from which the buds may arise and the presence of a stock Assheton claims was not demonstrated in the cases to which the theory was applied. Rather he interprets the described facts as cases of fission. The blastocyst of several mammals, for he was particularly considering the theory as applied to the armadillo, he described as undergoing direct fission into two embryonic rudiments. As a matter of fact the fission hypothesis does not differ from the idea of budding in relation to the facts as described, but is primarily a redefinition of them, for in both cases the embryonic vesicle is regarded as divided into several primordia, each of which is the beginning of a distinct formative area 356 POLYEM BRYONY

and consequently will give rise to a separate embryo. Whether the emphasis is placed upon the embryonic mass which is thought to bud or upon the separate areas which arise from it as would be the case in fission does not seem to involve any particular difference in the result nor to carry the explanation of the environmental processes much further toward a final analysis.

As an explanation of the process a physiological interpretation must be called on. Stockard has pointed out that if a developmental pause occurs at the critical moment in the formative stages of an embryo, a series of consequences ranging from simple malformation up to the production of double monsters or even two individuals may result. He applies this conception to the explanation of polyembryony, supposing that the reason for the loss of dominance of the first formative region of the embryo and the subsequent ascendency of the primary and secondary buds is connected in some way with the developmental pause. He points out that in the armadillo a period of physiological isolation intervenes at one of the critical moments, making possible the four resultant buds. Certain observed facts bear out this conclusion. In any case within the growing germ the isolation of formative areas occurs and from these formative areas the multiple embryos are produced.

An obvious connection exists between the phenomenon of polyembryony and that of metagenesis which involves at least the alternation of one sexually produced generation with an asexually produced one. It is usually the practice to limit alternation of generations to the lower forms of animal life and to say that it does not occur in higher forms, including the vertebrates. Stockard, however, speaking in terms of the budding hypothesis, has pointed out the fact that the embryonic mass of cells may be looked upon as one generation, namely the one produced from the fertilized egg, and that it is completed when it becomes a stock from which other buds, that is growing points, begin their development. This point of view has its application even to mammals whether only a single organism results or several as in the case of the armadillo. Here the blastocyst would be regarded as the sexually produced individual and the buds which give rise to the four young as those asexually produced from it.

Stockard says, “From a general biological standpoint the adult body of higher animals may be very correctly considered to be derived from a sexually produced embryonic axis, the stock which gives rise by an asexual method of budding to the various special organs. The vertebrate body is thus composed of a group of different zooids, the organs. There are seeing, hearing, excretory zooids, and so on, comparable to the zooids of a siphonophore colony. OBSERVED CASES OF POLYEMBRYONY 357

“Alternation of generations is here considered a phenomenon, not limited as is generally taught to lower forms, but occurring throughout the animal kingdom.”

If the blastocyst is to be looked upon as a sexually produced individual from which arises asexually another or several others of a second generation, it is suggested that in this fact lies the explanation

fiG. 219. The development of the ovicell in Crista remosa. (From Patteron, after Harmer.)

A, external view. B, median longitudinal section of young ovicell. fol., follicle formed from polypide-bud; ov., ovary; ovi., oviccll.

of the difliculties that have met every attempt to homologize the early development of the mammals with that of the lower vertebrate classes. These differences have been discussed in Chapter X on the formation of the mammalian embryo; they constitute a problem of major importance to the comparative embryologist.

B. Onsnnvnn CASES or POLYEMBRYONY

We may now devote ourselves to the consideration of the groups of organisms in which specific polyembryony has already been noted to 358 POLYEM BRYON Y

occur, bearing in mind the different types of explanations that have been offered and attempting to discover how the facts conform to the suggested explanations.

flatworms. Among the flatworms are many cases which depart from the usual methods of reproduction. Of these some which do not fully fit into the classification have been described from time to time as polyembryony. There is one flatworm in which polyembryony undoubtedly occurs, however; it is the cestode Taema echmococcus, or Echmococcus coenurus as it is sometimes known. This small tapeworm produces eggs which develop into the usual hooked embryo, the onchosphere, and these in their turn produce a cyst which becomes the bladder worm or cysticercus. As is usual in the further development of a cysticcrcus, an invagination from the outside wall of the bladder into its cavity develops a new scolex and neck region which will later become inverted and form the new tapeworm. However, the matter is complicated in this case in that many scolices may be produced within a single bladder by the budding process, and even secondary bladders which produce multiple scolices in their turn are described. Thus the encysted worm may grow to an enormous size to endanger the life of the host and oftentimes bring about its death.

Bryozoa. Polyembryony occurs in the cyclostomatous bryozoa belonging to the Gymnolaemata of the Endoprocta. These animals live in colonies made up of zooecia

fiG 220- Section through which are in general tubular with densely

a follicle of Cnaza remosa show- - mg primary embryo (1) which calcareous walls. The circular orifices of the

§>ré>dt:)c1I(IiL(zisecorZ<I‘i‘ary erlgnbgnis zooecia give the group their name. At the time son, Xftei’ H$fi;e,_,r°m 3 e ' of the breeding season a curious specialized zooecium develops which is called the oecium

or ovicell. In this is developed the young embryo. Fertilization of a rather unusual sort having been successful, the oecium serves as a brood chamber about the developing embryos and larvae. The primary embryo undergoes a process of budding, and secondary and tertiary buds are EARTHWORM 359

produced all of which may develop into larvae, as many as 150 being on record from a single egg. They develop into ciliated larvae and each is capable of forming a new colony upon escape from the oecium. Earthworm. In the earthworm of the species Lumbricus trapczoides (Helodrilus caliginosus trapezoides) polyembryony results in the production of twins, a process which by Kleinenberg was regarded as universal for the species, but by Vejdovsky was held to be abnormal. It is due to a process of fission of the embryo. The cleavage of the egg

C D

fiG. 221. Twinning in the earthworm, Lumbncus trapezoides. (From Patterson, A, B, C. after Kleinenberg.)

A, Section of young twin embryos. the right one being the more developed. B, Double embryo, in section. C, Double embryo about to break apart. D, Type of double monster formed when embryos fail to separate.

is much modified from the typical spiral form of the annelids and indeed is variable. A blastula is formed, the endoderm and mesoblast cells pass into the cavity and the entire mass begins to elongate. Across the equator of the elongating mass a transverse furrow now appears from one side. As it deepens, the embryo is divided into two hemispheres held together by a few ectodermal cells only. Each half is destined to form one of the twins. Differentiation, gastrulation, and the completion of the internal organization go ahead while the worms remain connected. But at length they separate after a series of rotations which breaks them apart, except in a certain proportion of cases which become double 360 POLYEMBRYONY

monsters of various degrees of union. Thus from the single egg twin earthworms are produced.

In two other genera of oligochaetes double monsters have been described, and one infers that the twinning phenomena, though rare, may occur in a number of families. These cases were Tubifea: tubifex, found by Welch, Tubzfex rivulorum by Penner, and Sparganophilus eiseni by Hague.

Parasitic Hymenoptera. In the parasitic wasps we find some of the most important and complex cases of polyembryony in the entire animal kingdom, and certainly here the results of the process are exemplified in the most striking manner. In a single brood hatching from one parasitized caterpillar hundreds of individuals may be seen to issue all developed from one egg. Our knowledge of polyembryony in insects dates from the work of Marchal in 1898 and of Silvestri in 1906. Although a number of important points still remain to be satisfactorily cleared up, a number of investigators have contributed to this subject since then. Among them are Martin (1914), Patterson (1915, 1917, 1921, 1927), Hill (1922, 1923), and Leiby and Hill (1923, 1924), as well as others. The important genera studied include Litomastix truncatellus, Ageniaspis fuscicollis, several species of Copidosoma, Paracopz'dosomopsis floridanus, and three species of Platygaster. The most recent study is that of Parker (1931) upon the braconid, M acrocentrus. Undoubtedly many other forms of insects show polyembryony, but the ones mentioned include the more important cases studied.

The manner by which a single egg produces multiple embryos in these forms is perhaps best understood by beginning as Patterson has done in the paper earlier referred to with a description of Platygaster hiemalis, a parasite of the Hessian fly, which usually produces two individuals from one egg. From this comparatively simple case we have a gradual increase in complexity which at the end of the series is only incompletely understood, but the simpler cases at least are suggestive of the methods by which the more complicated probably develop.

In this species the parasite lays from four to eight eggs either in the egg or young larva of the host. If fertilized, the egg nucleus divides twice, producing two polar body nuclei; these are not passed to the outside of the egg but remain in the cytoplasm where after a time they come together to form a large polar nucleus called the paranucleus. With the fusion of the male and female pronuclei the egg becomes differentiated into two separate regions. The cleavage nucleus and cytoplasm surrounding it become cut off from the remainder and constitute the embryonic region, for only from it comes the material which will go into the formation of the developing embryos. The rePARASITIC HYMENOPTERA 36 I

mainder of the egg containing the paranucleus and its own cytoplasm entirely surrounding the embryonic region functions to absorb and

fiG 222. Polyembryonie development of Platyoaster hwmalw (From Patterson, after Leiby and Hill )

A. Egg of four hours, showing sperm head and first maturation division B, Egg showing two polocytes. and both pronuclei C, Egg with pronuclei in comunction and polar nucleus D. Embryo, two polar nuclei and a. parasitic body showing four nuclei E, Enclosed within a cyst formed from tissue of the host are two embryonic bodies each surrounded by trophamnion F. A later stage in the development of two embryos G, A thirteen-day-old polygerm in section, showing several embryos.

elaborate the tissues of the host for the nourishment of the young embryos and it is therefore spoken of as the trophamnion. In this species, although it is not characteristic of all, the host tissues form a 362 ‘ POLYEMBRYON Y

cyst wall around this developing body which then is spoken of as the parasitic body.

The paranucleus as development proceeds divides amitotically twice and the daughters distribute themselves in the trophamnion surrounding the embryonic region. The zygotic nucleus of the embryonic region divides first into two, then four, embryonic nuclei. Thereupon, the embryonic region becomes constricted and presently separated into two regions, each provided with two nuclei, and the trophamnion with its paranuclei also divides into two corresponding regions. The two halves of the parasitic body thus differentiated remain held together within a single cyst of host tissue, but develop independently. Four, eight, and sixteen nuclei arise from the division of each of these germs, as they are called, and these arrange themselves into the form of a typical spherical blastula, the cell walls being cut off around each nucleus. The remainder of the development is unimportant, for the process characteristic of these blastulae go forward through regular stages. There is produced from each a new individual which hatches in the next year.

In another species of this genus, P. vernalis, eight embryos are regularly developed. Here the process has a fundamental similarity to that just described, but the details of development differ. The parasitic body develops a number of embryonic nuclei surrounded by an appropriate amount of cytoplasm and a cell membrane. Each of these becomes a germ for the production of the later embryo and the entire mass is spoken of as a polygerm. It happens that the more complex cases were studied before the development of Platygaster was understood and some of the problems that were not clearly worked out would now probably be more easily followed through. It is known now that in Copidosoma gelechiae in the formation of the polygerm many primary germs are produced in a manner similar to the eight of Platygaster vernalis. These separate germs of the last polygerm stage all divide and give rise to two which develop into separate embryos.

Leiby and Hill believe that in Paracopidosomopsis, which is even more complicated, a secondary germ divides to form tertiary ones before the larval differentiation begins.

Armadillo. The final case of polyembryony to be discussed is perhaps the one in which the greatest interest lies. The Texas nine-banded armadillo Dasypus (Tatusia) novemcinctus normally produces four young from a single egg, the quadruplets being identical in respect to sex and to most of their morphological features. The work on the armadillo from the standpoint of polyembryony is of rather recent date, Fernandez concluding in 1909 that it occurs in a South American ARMADILLO 363

species Dasypus hybridus, and Newman and Patterson in the same year publishing the beginning of their important studies on the Texas species. Subsequent publications by these authors working independently have made available an account of the development of this form which is practically complete. The processes have been shown to be identical in respect to all important features in the two species which have been studied. The reader is referred to Chapter X for a discussion of the formation of the blastocyst in mammals.

In the armadillo as in other mammals, a monodermic blastocyst is formed which becomes differentiated into a trophoblastic portion and a formative portion, the inner cell mass. Then from the inner cell mass the endoderm is differentiated off, the remainder becoming ectoderm. Up to this point the blastocyst has remained free in the uterine cavity, but now attaches to the uterine mucosa. Attachment takes place directly over the embryonic ectoderm and the Trdiger forms at this point; then the inward growth of the spherical endodermal mass begins which brings about the so-called inversion of the germ layers. Within the ectodermal mass the formation of the ectodermic vesicle marks the beginning of the amniotic cavity. Above it an extraembryonic cavity is formed in the mesoderm. With progressive development the shift of the ectodermal cells takes place, resulting in the formation of the em~ bryonic shield with its thick ectoderm and the true amnion above it. In this stage the first sign of polyembryony makes its appearance, for from opposite sides of the ectodermal vesicle thus formed blunt projections extend laterally. Patterson called these primary buds. By this time that portion of the trophoblast which has not become involved in the attachment to the uterine wall disappears and the yolk sac endoderm is directly exposed in the uterine cavity. The two primary buds now divide each to form two secondary buds, making in all four buds which are rudiments of the four embryos subsequently to arise. Although they arise in this bilateral fashion and develop in the right and left halves of the uterus respectively, they later come to occupy positions that are about equally spaced from each other.

Meanwhile the vesicle grows, and the endoderm of the yolk sac, including those portions with which the embryonic ectoderm of each bud is in contact and which will form the gut endoderm of the embryo, faces the uterine cavity. The anterior ends of all the embryos point toward the apex of the common ectodermal vesicle, that is, toward the original amniotic cavity, and the entire vesicle may now be spoken of as the common amniotic vesicle. The entire structure now grows very rapidly, especially that portion of it which originated from the trophoblastic knob or the Trdger. This growth finally comes to occupy most 364 POLYEMBRYONY

of the space in the fundus of the uterus. The further history is especially concerned with the embryonic buds. Their posterior ends lengthen out with the growth of the vesicle and finally unite at a point opposite the original attachment and from their point of union the umbilicus later arises. The common amniotic vesicle by this growth procedure is left

fiG. 223. The development of the blastocyst of the armadillo. (After Patterson.) ec., ectoderm; en., endoderm; icm., inner cell mass; mes.. mesoderm; tr., trager; tro., trophoblast; u.. uterus.

as a small structure at the lower apex, in contact with each bud, while most of the remaining portion of the cyst mass with its developing embryos is derived from the growth of the Trdger. The buds meanwhile continue their growth, each forming a primitive streak, which is a substitution for the embryonic shield as described for other mammals. The further development of the embryos and the subsequent changes which lead to the production of the four foetuses need not be traced ARMADILLO 365

since it is simply a problem of organogenesis and of the steps normally following. The relations which are necessary for the understanding of

fiG. 224. A, The uterus and entire blastocyst of the armadillo showing paired origin 01' embryos. B, Half-grown foetuses spread out from uterus wall. (After Patterson.)

the polyembryonic condition all grow out of the budding processes of the embryonic vesicle and are completed with the establishment of the four embryos. There are many interesting problems connected with 366 POLYEMBRYONY

the later stages from the standpoint of later embryology and hereditary correlations of the characters of the four offspring as well as from other points of view, but the special problem of polyembryony is explained by these earlier stages.

C. EXAMPLES or DOUBTFUL POLYEMBRYONY

Two other cases which resemble polyembryony in some respects should be mentioned. As a matter of fact there are some departures in each of them which do not justify their inclusion with this type of development.

Among the trematodes the egg of Fasciola hepatica, the sheep fluke, is fertilized in the body of the adult fluke, makes its way down the bile duct and out of the intestine of the sheep and hatches when it rains as a tiny ciliated larva, a miricidium. Entering the intermediate host, the snail, the miricidium becomes a sporocyst and within it parthenogenetic ova appear which develop into rediae, the next larval form. Within the body of the rediae the process may be repeated but at length cercariae are developed which escape from the snail and bring about the reinfestation of the adult host. This case has been called polyembryony by some, but by others it is regarded as paedogenesis, for here the new larvae are said to be produced not asexually but from parthenogenetic ova, a difierence in method which, if true, would seem to be fundamental. However, the reservation must be made that if the germ masses from which the new larvae arise are not ova, but are produced asexually (as F. G. Brooks believes, having found no maturation phenomena or other egg—like behavior), then we have to do here merely with asexual reproduction, and not even paedogenesis,

A second case which at first thought suggests polyembryony occurs in the tunicates. Among the Thalaceae as represented by the genus Salpa it has long been known that two forms of individuals are to be expected, one a solitary form and the other a colony which is usually found as a chain of individuals. The first of these develops asexually, although produced from a fertilized egg. The second is known to be budded ofi’ from a stolon which grows out from the asexual individual, and among the colonial forms some at least are sexual, producing the eggs and sperm which in their turn start the cycle over again. The genus Dolielum is rather more complicated than Salpa but it seems to illustrate the situation quite well and it is here that the type of development of particular interest to us may be said to occur.

In Doliolum an oozoid is developed from the fertilized egg and it reproduces asexually. By some it is regarded as a larval zooid, and certainly it has not yet reached its final character. If it is correct to EXAMPLES OF DOUBTFUL POLYE-MBRYON Y 367

regard the oozoid as a larval form, this case is very closely akin to polyembryony. From the ventral posterior part of the oozoid a proliferating stolon is formed as a protrusion of the ectoderm into which the mesoderm penetrates. From the stolon buds arise which break loose in a very immature condition and Worm their way upward and dorsally over the surface of the parent. Since the number given off from the ventral stolon is less than the number later found on the dorsal process, it is assumed that the buds must divide en route, and some evidence of this process has been found. Arrived at the dorsal side of the oozoid, the buds attach themselves to a posterior process of the test which arises as a middorsal projection (occasionally spoken of incorrectly as a dorsal stolon). These bodies attach themselves in three longitudinal rows to the outgrowth. The two lateral rows of buds are known as trophozooids or gasterozooids and their only function is to nourish the colony. The dorsal row of buds undergoes metamorphosis into several kinds of zooids. Some become phorozooids or nurse zooids, and according to some investigators they produce from a ventral stalk buds which are protogonozooids or primary sex buds. These latter either by budding or fission produce the sexual animals, the gonozooids or blastozooids, in which with further development sex organs are formed and the cycle is completed. Here an alternation of generations occurs in which there are three asexual generations and one sexual generation in one cycle. Whether it is to be regarded as polyembryony depends upon the interpretation of certain stages. It is very complicated in the extreme, and is at least close to true polyembryony.

Chapter VIII The Determination Problem

A. INTRODUCTION AND STATEMENT or PROBLEM

A problem which has run through the entire history of embryology and is yet a live question at the present time concerns the extent to which the egg represents a fixed and definite system whose course is set when its development has begun. This is the determination problem. It first became a live issue during the controversy between the preformationists and the epigenesists of the seventeenth and eighteenth centuries and was even in that crude form an attempt to solve the question of the organization of the egg. The one school held that development was

simply an unfolding of an organism already present in an infolded con—'

dition; that is, organization was complete and development brought forth nothing new. The other held that the egg was unorganized and that everything in development was new. The history of embryology since these crude beginnings, at least of that part of the science which has concerned itself with the earliest stages in ontogeny, has been an attempt to make clear the manner in which the organization of the egg becomes manifest. Except for historical reasons it is not necessary to trace the course of these studies nor would it be possible except in a much more extensive treatment of the subject than is given here, for the literature is voluminous. It is not even possible to summarize in a short discussion the work which has been done. All that can be attempted is to bring together briefly the lines of work which have been responsible for the present conceptions in regard to determination.

Two sets of facts confront the student who would consider the determination problem. Neither bears directly upon the question at issue, but both serve to limit it. In the first place an egg goes through certain steps with surprising uniformity and produces an individual which is structurally very complex and very much differentiated. It is organized in the highest conceivable degree. In the second place modern genetics has shown that the egg to begin with possesses in the mechanism by which the genes are controlled and distributed a type of organization likewise complex in the extreme; and there is no doubt that this mechanism operates to produce the conditions found in later ontogeny. Yet differentiation is a matter of the cytoplasm of the cells. The repeated

368 INTRODUCTION AND STATEMENT OF PROBLEM 369

mitoses do not affect in a differential fashion the elements of the nucleus and it is clear that the nucleus contains in the chromatin the non differentiating material, the germ plasm, while the cytoplasmic portion of the cell is the basis of specialization and is indeed the somatoplasm which gives rise to all the differentiations later seen in the course of ontogeny. Between these two sets of facts must lie the third set, little understood and often little appreciated at the present time. Almost nothing can be offered in the way of explanation as to the method by which the gene mechanism produces its fundamental effect upon differentiation. The gene mechanism concerns heredity, that is, by it is explained the similarity existing between generations, but as yet its study has offered little which enables us to understand its relation to the differentiating soma. It was earlier said that heredity was the central problem of biology. Differentiation is certainly a central problem of development, and, let it be said, at the present time one of the most elusive. The questions of differentiation and of determination have dominated a surprisingly large part of the embryological work of the last century.

These two questions are really two aspects of the problem of organization, for differentiation is the manifestation of the operative mechanism of the egg as modified by extrinsic factors. To discover the extreme to which this operative mechanism is already present in the early stages and to which it gives a definite “set” to the differentiating embryo is the aim of the work on determination. If differentiations become visible early and are little modified by extrinsic factors with the result that the course of development is orderly in a high degree, we say that the egg is strongly determinative. But if the egg retains its embryonic plasticity and easily adapts itself to environmental disturbances, that is, is easily modified by extrinsic factors, it is said to be indeterminative or of the regulatory type. If differentiations manifest themselves early, little regulatory capacity is retained by the egg. In other words, the capacity of the egg for regulation is inversely proportional to the progress of differentiation.

It must be borne in mind also in considering the problem of differentiation (and hence of determination) that the chemico-physical organization of the cell limits the conclusions which may be reached. Differentiation manifests itself through colloidal materials. The chemistry of the protoplasmic system is so imperfectly known that whatever conclusions are reached regarding these problems must be subject to revision when a better understanding shall have been attained, for example of the phenomena of gelation which plays a very important part in the physical expression of mitotic phenomena. It has been shown by the studies of 370 THE DETERMINATION PROBLEM

Conklin and of others that the position and size of the spindle are matters of great importance in differentiation. Yet the physico-chemical factors which are involved in spindle behavior have not yet been adequately analyzed. Other problems of physico-chemical nature similarly affect our"conclusions. It is not too much to say that in spite of the vast amount of embryological research differentiation and its mechanism is one of the least understood fields of biology.

It has been pointed out that the function of cleavage is to sort out the materials of the egg into cells so that differentiation may progress. The uncleaved egg represents all of the materials present, although not necessarily all that are later to appear. Yet the progressive development of the organism in its usual course at least as indicated by our present knowledge depends upon getting these materials separated from each other. It does not follow, however, that this sorting-out process of cleavage is the cause of the subsequent development. Rather there must be a fundamental underlying mechanism of organization, the manifestations of which are the cleavage phenomena. Primarily determination concerns itself with the manifestations of these underlying methods of organization. It seeks for evidence of pre—1ocalization of areas or substances within the egg or even of an early promorphology which gives visible expression to the underlying mechanism. Some of the questions involved may perhaps be formulated in the following manner: Are there special structures or substances or fundaments which exist in the egg at the very beginning of development and which are independent of each other and of other parts of the egg? If so, how are they formed? Must all appear at the same time, or may some appear later on? Conversely, are the definite structures of the embryo never independent of each other? Are the parts always influenced by the whole of which they are constituents? Or finally, are there some eggs which are determinative in their character and others which are not, or are some parts of the individual embryo determinative and other parts not? Perhaps all these questions may be said to depend most largely for their answers upon this one. At what time and from what sources do differentiations begin in the embryo? It may be noted that the answers to these questions are not the same for all organisms.

B. CLASSICAL THEoR1Es or THE NATURE or DEVELOPMENT

Historically there have been a number of theories developed in response to these questions. They represent the various forms which the answers of different investigators have taken and must be considered from the standpoint of the information available when they were formulated. One of them was the theory of organ-forming regions of CLASSICAL THEORIES OF THE NATURE OF DEVELOPMENT 371

Wilhelm His (1874). This is the doctrine that definitely localized areas perhaps even in the unsegmented egg are the forerunners of the'organs and parts of the embryo, that “the germinal disc [of the chick] contains the preformed germs of the organs spread out over a flat surface, and conversely . . . every point of the germinal disc is found again in a later organ.” Ray Lankester supported His in his views, saying that it was quite possible for the cell to “contain already formed and individualized, various kinds of physiological molecules. The visible process of segregation is only the sequel of a differentiation already established and not visible.” Whitman as a result of his work on Clepsine concluded that the embryo is predetermined, even if not predelineated. Rabl and van Beneden likewise were among the early supporters of the view that even in the unsegmented egg an organization of protoplasmic particles which predetermines the development of the embryo is present. In the bald form in which this doctrine was proposed by His it can scarcely be accepted, however, and as a matter of fact His had no great amount of observational and experimental evidence on which to base it. The study of cell lineage offers what of evidence there is in support of this doctrine but even at best it does not force as rigid an interpretation as His placed upon it. Many authors have denied it in its entirety and there are few who do not criticize it.

Another form of answer to the question of determination is the theory of organ-forming substances, according to which the materials within the egg rather than the areas which they occupy receive the chief attention. Here again it is obvious that not all materials within the egg, even though they be definitely localized, are formative. Yolk, pigment, oil globules, and other inclusions in the cytoplasm are definitely located in many eggs, yet clearly are not formative. In some instances visibly differentiated substances within the unsegmented egg have been shown to have no formative function. At least they have been displaced by pressure or by centrifugal force, and normal embryos have nevertheless resulted. Yet in other forms the materials of the egg have been traced in a precise manner to the organs of the embryo. Surely a principle of determination is here involved which is too important to deny because of a particular formulation of a doctrine. Perhaps the importance of the visible organ-forming substances has been overemphasized and these are but manifestations of a more fundamental mechanism. Yet in very many eggs the mechanism undoubtedly exists, and later researches have certainly demonstrated the essential correctness of the principle involved in the conception that the egg is not without an organization before cleavage begins which conditions the future course of its development. 372 THE DETERMINATION PROBLEM

As a result of the early work on cell lineage by Whitman, Rabl, van Beneden and others the former view that blastomeres were indifferent both in position and significance was gradually replaced by another to the effect that the cleavage pattern was in the nature of a mosaic, and that a blastomere occupied its particular place because of the material contained in it, that is because of its mosaic character. The views were supported not only by the extensive studies on cell lineage, but also by the experiments of Roux, Chabry, Conklin, Wilson, and others. These experiments are discussed later in this chapter.

The mosaic theory of development, if understood in a sense not too rigid to take account of the fundamental plasticity of living protoplasm, certainly expresses one of the underlying truths of embryology. One may, however, raise the question of the manner and the time of origin of the mosaic work and also whether it is so fixed that it is not subject to the regulatory processes of which the organism is capable.

The question of regulation brings up yet another theory of historical significance which We owe to Driesch. His terms prospective significance, prospective potency, his equipotential system, are all well known to those who are familiar with the history of the determination problem. By prospective significance of a cell he meant the actual destiny in the developmental process. By prospective potency he meant the possible fate which the cell might attain. Prospective potency includes the sum of the different developmental possibilities. As the result of numerous experiments, Driesch reached the conclusion which he thought of general application, that the egg is essentially an equipotential system, for the cells of a blastula of a sea-urchin, for example, are of uniform material and might be interchanged like balls in a pile without affecting the result in the development of the embryo; in other words that the fate, the prospective significance, of a particular blastomere is a “function of its position.” Driesch’s theory was in opposition to the mosaic theory of development according to which the fate of the blastomeres did not depend upon their position alone but upon their organization. It was based upon the conception current at the time when work on cell lineage first rose to an important place. Pfliiger had held that the egg and its blastomeres were homogeneous throughout and that cleavage simply multiplied the units out of which diiferentiations were later to arise. This view was also favored by Oscar Hertwig. Driesch, who although chiefly interested in philosophy, set himself to carry out experiments in development upon which certain philosophical decisions might be based, made this conception the starting point of his theory of development.

Some experimental evidence is at hand upon which to a certain MORPHOLOGICAL EVIDENCE 373

extent the theory of Driesch may be based, while there is also evidence upon which the mosaic conception may rest. On the one hand certain eggs possess a high degree of regulatory capacity and are hence indeterminative in their cleavage. On the other hand certain other eggs show the marks of differentiation appearing at a very early stage in the

development of the egg; these eggs possess but slight regulatory ability and are hence determinative in character. Between the mosaic type

and the regulatory type no sharp boundary line can be drawn. Rather there is a gradation from one extreme of the series to the other. If we sum up the results of these researches we may arrange the animal series from the regulatory to the mosaic forms in about this order: amphioxus, teleosts, mammals, nemerteans, urodeles, anura, ctenophores, annelids, molluscs, arthropods, nematodes, and ascidians. But it is to be noted that purely regulatory and purely mosaic types do not occur. In this we are merely emphasizing what has been said before, that the time at which differentiation appears is the real criterion upon which we may recognize the differences between determinative and indeterminative types of eggs. In other words if differentiation sets in early the regulatory capacity of the egg is correspondingly reduced. Conversely, eggs with high regulatory capacity show little evidences of early differentiation.

C. EVIDENCE BEARING ON DETERMINATION

The present status of opinion in regard to the determination problem has been reached as the result of two different lines of evidence. One of these is the morphological study of normal embryological stages. The other line of evidence embodies the results of many experiments which

have sought an insight into the fundamental nature of developmental problems.

1. Morphological Evidence

Of the morphological evidencesthat some type of determination exists to some extent in all eggs reference should perhaps be made first to the contributions of genetics. The breeding experiments of this century have very clearly shown the existence of hereditary units within the germ cells. Although we do not by any means identify these units with visible structures in the egg cell nor do we know how they express their influence upon the developing organism, yet any theory of particulate inheritance (that is inheritance based upon the presence of organized particles within the germ cell) is, in so far as it is justified, evidence in favor of a morphological basis of determinate development. This evidence bears out the conclusion stated above that, although 374 THE DETERMINATION PROBLEM

the regulatory processes may dominate in certain types of eggs, there are none which are purely regulatory.

The first signs of differentiation in the developing egg have to do with the general features of polarity, symmetry, and pattern of the egg. From these’ general features more detailed diiferentiations with respect to polarity, symmetry, location, and pattern of the constituent cells and parts of the embryo arise and later the differentiations of tissues and the beginning of organs. Development is thus progressive, and differentiation, at first relating only to the most general features, gradually passes to specialized details. Since polarity, symmetry, and cleavage pattern are general in character their importance is sometimes overlocked by the student, yet very little thought is required to convince one that they are really basic. Our morphological knowledge with regard to the development of polarity and symmetry has been discussed in the chapters which have to do with the different types of cleavages and reference should be made to those chapters. These observations have been checked by many experiments of the greatest importance and it will perhaps suflice to refer to these experiments under the second line of evidence as listed here.

Morphological studies of the cleavage pattern, as has already been indicated, became the foundation for the mosaic theory of development in the studies at the hands of the students of cell lineage. It early became clear that certain types of cleavage were predominately determinative. Some eggs with bilateral cleavage, certain types of eggs with superficial cleavage, those with spiral cleavage, with disymmetrical, and even an occasional form having radial cleavage, such as Strongylocentrotus, were shown at that time to be determinative in their development. Most forms with radial cleavage, some with bilateral, and even occasional examples of eggs with spiral cleavage show the regulatory type, however. For a discussion of these the student is referred to the chapters on cleavage types.

2. Experimental Evidence

The second line of evidence which bears upon the problem of determination is perhaps no more important than the morphological evidence but is certainly not less so and is much more recent. It is obtained from those special experiments* which have to do with this problem.

‘ As pointed out in the chapter which deals with history of embryology, the present is the period of acperimental embryology. Much of morphological character remains to be learned, but nevertheless the dominant note in embryological research of the present day is the experimental one. It is now sought to discover those underlying principles which account for the form changes by which the structure of the organism is produced rather than simply to give a descriptive account of those characters. EXPERIMENTAL EVIDENCE 375

The first series of experiments of significance for study of determination is concerned with the localization of the median plama. Here as with the other experiments to be summarized the student is referred to Morgan’s critical discussion as well as to the original papers which describe them. It would be impossible in a work of this size to give even a brief account of the experiments themselves. Reference can only be made to those which seem most significant.

The matter of the localization of the median plane is of importance because it indicates the early determination of symmetry and polarity on the part of the egg. According to some the polarity of the egg may be traced back even into an ovarian condition, and some believe that the type of symmetry is determined in the egg cytoplasm before fertilization. Although the evidence of these relationships seems good in some cases it has not been made clear how much the appearance of symmetry_in these early stages is related to the cleavage plane or the symmetry of the embryo. No mechanism for bringing this about has

Experimental embryology has seemed to cover a wide range of topics and the results of the investigations in this field are scattered throughout many journals. There have been but few attempts to bring this material into the scope of a single volume. The first section of Korschelt and Heider’s “Lehrbuch der vergleichenden Entwicklungsgeschichte der wirbcllosen Thiere” deals with the experimental results obtained up to that time (1902). In 1909 Jenkinson published his book entitled “Experimental Embryology” and summarized much of the work available at that time. The contribution to experimental embryology most important now, however, is the volume by Morgan entitled “Experimental Embryology” and published in 1927. Morgan has promised a further volume which will deal with such topics as growth, reflex reactions, tropisms of the larvae, the influence of environment upon the embryos, the source of energy of development, etc. It is to be hoped that this volume will appear at no distant date.

In spite of the diversity of topics which have engaged the attention of experimental embryologists it is possible to classify them into a few divisions. (1) The early students of this field were specially interested in the discovery of the effect that external factors have upon the developing embryo. Among these factors are gravitation, mechanical agitation, electricity, light, heat, atmospheric pressure, osmotic pressure, and the chemical composition of. the medium in which development takes place. (2) In addition to this the growth problem, the problems centering around fertilization, have occupied much attention during the present century. Something of the nature of these problems is suggested in the section of this book which has to do with artificial parthenogenesis, although this is but one of the many topics that have been studied. (3) finally a great deal of attention has been given by experimental embryologists to those problems which are concerned with the forrna— tive changes of the egg. These factors are internal in their nature and seem to take the experimenter further in his attempts at the analysis of the innate, vital character of the organism than do the other problems enumerated. The experiments which bear on the problem of determination come under this head. Those experiments which have sought light on this problem fall chiefly within a few animal groups, namely: the ctenophores, some hydromedusae, the nematodes, annelids and molluscs, echinodcrms, ascidians. It may be remarked that not only are these the forms which seem to have yielded results most capable of analysis but they are also the forms upon which it is easiest to experiment. 376 THE DETERMINATION PROBLEM

been demonstrated. In many cases where a relationship between the first cleavage plane and the planes of symmetry of the later embryo have been made out it is thought that the point of entrance of the spermatozoon and the path traveled by the sperm pronucleus in approaching the egg pronucleus determine the position of the plane. Suggestions have also been made that the pressure of the enveloping membrane of the egg or pressure from the oviduct or other external factors may be related to the position of the plane. Certainly the causal character of such factors has not been demonstrated. It may be said, however, that so long as the possibility remains of a relationship for example at the point of entrance of the spermatozoon it is necessary to assume that the symmetry of the embryo goes back to a predetermined symmetry of the egg.

The question of a relationship of the median plane and the first cleavage plane arises in the history of embryology in relation to the frog egg. As long ago as 1851 Newport had reported that the two coincide, and from that time the question has been often discussed. In the frog egg at the time of cleavage there is already present a bilaterality as shown by the presence of the gray crescent. The real question involved in determining the symmetry relationships is: what in the uncleaved egg causes the material to take the position of the gray crescent, and why does the first cleavage plane cut through the middle of it? Experiments have shown that its position is determined after fertilization and that any meridian of the unfertilized egg may become the median plane. After fertilization the meridians are not equivalent in this respect, however, for usually the crescent forms opposite the point of entrance of the sperm. A long series of experiments can be cited to show that in nearly two-thirds of the cases the first cleavage plane does coincide with the middle of the gray crescent. It is clear, however, that if 30 per cent or more of the eggs fail to show this relationship the mechanism of determination is by no means a fixed one. In other amphibia the experiments of Jordan indicate that the first cleavage plane is at right angles to the axis of the egg of Diemyctylus, and Spemann has found for Triton that the second cleavage plane coincides with the median plane of the embryo. Of course it is immaterial from the standpoint of determination as to whether it is the first or second plane which shows the relationship. In the teleost fishes according to the work of Morgan and of Clapp no definite relation between the second plane and the median plane of the body can be made out. For the sea-urchin a considerable amount of evidence is available. In Toxopneustes, Wilson and Mathews related the appearance of the first cleavage plane to the entrance of the spermatozoon, and Boveri held that it coincided with the median plane of the EXPERIMENTAL EVIDENCE 377

embryo. In the eggs of Echinus, Driesch found the second plane to correspond to the median plane and Runnstrém assumed that the first cleavage coincides with the median plane. Recently Von Ubisch has performed a very ingenious experiment by which certain portions of the egg were stained with intra vitam stains; in three species of seaurchins he obtained results which are in agreement. He was able to demonstrate no fixed relation between the planes of symmetry and planes of cleavage, but found some evidence that the first cleavage plane more nearly coincides with the median plane. On the whole for the sea-urchins perhaps this is the most acceptable conclusion. In his study of ascidian embryology Conklin determined that the first cleavage plane is the median plane of the embryo, and that in Cynthia its position is indicated before the pronuclei meet. In the nematodes, according to Boveri’s work on Ascaris, the third cleavage plane of the dorsal cells is the median plane of the embryo. In Nereis, Just has found that the second cleavage plane usually corresponds to the median plane of the embryo but the first cleavage plane is determined by the position of the entering spermatozoon. Insect eggs are distinctly bilateral so far as their orientation is concerned, the position of the egg as laid corresponding to the symmetry of the mother’s body and definitely indicating the position of the embryo in the egg. For the chick and pigeon Bartelmez has held that bilaterality is present even in the small ovarian eggs.

A second series of experiments related to the problem of determination has sought to discover to what extent localization of germinal areas is present before cleavage, using as a method the development of egg fragments. Eggs of Cerebratulus were out along definite planes by Wilson, Zeleny, and Yatsu. Cuts were made both before and after fertilization and development followed without difficulty. The cleavage of the fragments in general corresponded to the type of cleavage of the whole egg but upon a much reduced scale. It was not found to make a difference from which part of the egg the fragment was taken. The experiments show that either factors which determined cleavage were not yet definitely localized at the time of maturation or the egg is capable of very extensive regulation. According to Yatsu if the operation occurred between the formation of the first and second polar bodies the cleavage was entirely regular but if it was after the second division irregularities developed in some of the cases. Wilson experimented with fragments of the egg of Dentalium. Here the presence of the yolk lobe complicates the result. If the yolk lobe is entirely present and only the apical portion of the egg cut off, the resulting larva is nearly normal; likewise an entirely symmetrical division of the yolk lobe seems not seriously to interfere with the course of development. If the cuts are made in any other manner, however, irregularities appear. The ctenophore Beroé when out into fragments in the unsegmented condition produces in some cases partial embryos and in others whole ones depending on whether the cut is symmetrical or oblique. Driesch, Morgan, Yatsu, and fischel have experimented with this form. With sea-urchin eggs conflicting results have been obtained from experiments with the development of fragments. Taylor and Tennant using accurate methods of cutting with a micro-dissecting machine obtained small pluteae from the developing fragments which were like the normal ones. But Harnley obtained evidence that the materials of the egg were qualitatively different. finally experiments on the development of parts of Triton eggs have been performed by Spemann and Baltzer. Non-nucleated fragments, obtained by separating the egg by means of a hair tied around it, developed but rarely and the nucleated fragments produced dwarf larvae which did not live long enough to undergo metamorphosis although normal in most particulars.

An interesting result of these experiments is the conclusion that the entrance of the sperm into the egg is not of itself sufficient to start development, for, if an egg is cut just after the penetration of the sperm so that the female pronucleus is in one half and the male pronucleus in the other, it is the half containing the sperm pronucleus that develops but not the other. Something else besides the mere initiation of division is accomplished by the entrance of the spermatozoon. The cleavage of egg fragments demonstrates that for many types of eggs the pattern develops along with the mitotic figure.

Another series of experiments has sought to discover to what extent it is possible for a whole embryo to develop from an isolated blastomere. In some few eggs it is possible to cut the blastomeres apart at the time when they are most widely separated from each other as the first cleavage is closing. In others it has been found possible to separate them by shaking them in a tube of water, or by squirting them from a pipette. Again eggs of echinoderms permit the easy separation of the blastomeres if they are kept in calcium-free sea water while the first cleavage is taking place. Isolation experiments have been performed on sea-urchins, the hydroid Clytia flavidula, Cerebratulus, amphioxus, teleost fishes, Triton, and the frog, and in all these cases whole embryos were obtained from the isolated blastomere. On the other hand blastomeres of ctenophores, molluscs, and ascidians when isolated give rise to half embryos only. It would seem that this emphasizes again the distinction between determinative and regulatory eggs.

The question arises as to whether the development of the isolated blastomere is strictly comparable with the results that would be obtained if the material of the missing portion of the egg were present. Experiments have been performed in which the single blastomere has been allowed to develop in contact with the material of its sister cell, which owing to various kinds of injury was prevented from any active participation in the normal result. Experimenting upon the frog, Roux injured one blastomere with a hot needle but did not kill it. He thought that the subsequent development of the uninjured blastomere in contact with the inactive one was in the nature of regulation and that gradually the missing portion of the embryo was restored. This conclusion seems doubtful as there is now good evidence that the resulting embryo is more nearly one-half than a whole. Later studies have shown that, in experiments of this kind, if the plane of the first cleavage goes through the middle of the gray crescent a half embryo results. If the plane is parallel to the gray crescent and the injured blastomere is the one which contains it, nothing recognizable is produced from the opposite one. But if the opposite blastomere is injured the one containing the gray crescent will produce the anterior end of the embryo. Even if the cleavage plane is at other angles the blastomere containing most of the gray crescent will produce an anterior end.

McClendon working on the tree frog, Chorophylus (Pseudacris), sucked out the injured blastomere to determine whether its presence has any effect. In his experiments the remaining blastomere produced a normal whole embryo of one—half size. Injuries to one blastomere of the egg of Ascaris led to the conclusion in the experiments of Stevens, of Boveri, and of Schleip that each individual cell contains the factors which are responsible for its own development; in short, that there is little self-regulation on the part of the blastomeres. The egg of Cyclops after the injury to one blastomere has been studied by Fuchs and by Miss Jacobs. The evidence goes to show that for this egg the contact of the injured blastomere with the uninjured one does not materially affect the result of the development of the latter. This results in a partial embryo. In this same connection the experiments of a number of investigators, of whom Hegner and Reith are typical, on the effect of injury to the eggs of insects should be mentioned. Insect eggs apparently may be among the most determinative with which we have to deal, for these experiments indicate that different cytoplasmic areas on the surface of the egg are fixed in their prospective significance even before the cleaving nuclei with their surrounding cytoplasmic islands migrate to the surface and they have no great powers of readjustment. There is indeed a considerable degree of independence in the development of the respective parts. If the injury is not too severe to a certain localized portion of the egg the remainder will go ahead and develop without much dependence upon the injured portion, but will produce only that part of the embryo which was to be expected. Reith found that, if the posterior end of the egg of the house fly is injured, the parts normally resulting from the anterior end will develop. If the anterior end is the location of the injury, a larva without a head end develops, and in some cases injury to the middle portion of the egg was not so severe but that both anterior and posterior organs developed.

Hegner’s experiments on the eggs of the chrysomelid beetles Calligrapha multipunctata and Leptinotarsa decemlineata are Well known. Here the posterior end of the egg is the location of the pole plasm which Hegner found to pass into the germ cells in development. By injuring this region with a hot needle he was able to secure embryos without germ cells. Evidently the cytoplasmic regions in the eggs of these insects are very early set aside to produce definite parts of the embryo. Yet it must not be inferred that the regions in question retain no powers of adjustment. As cleavage progresses the possibilities of readjustment are lessened but are not entirely lacking.

A very important series of experiments carried out in the laboratory of Spemann on the embryos of Triton have shown that the future of certain ectodermal areas in the gastrula are much modified by their position. The method by which these experiments were carried out was that of transplanting a portion of the ectoderm by means of a micro-pipette from one portion of the body to another and studying its relationship to the development of the neural plate. From this study it appeared that the presence of the endomesoderm beneath the surface of the ectoderm is immediately necessary for the neural plate to be formed. Spemann and his collaborators have shown that the prospective ectoderm is capable of producing entirely different organs upon transplantation. For instance ectoderm from the top of the young blastula or gastrula implanted on the lip of the blastopore and carried to the interior may become notochord, mesoblastic somites, pronephros, or perhaps other organs. Ectoderm taken from a slightly later stage, that is, after the closure of the blastopore, if carried into the mesoderm becomes mesodermal somites; if carried into the endoderm goes to the formation of the archenteron. It should be pointed out that particular organs which differentiate from given substances may through the process of rearrangement be induced in an entirely different direction from that which their normal determination indicated. Throughout the animal kingdom there are many cases which serve to emphasize the difference between the determination of an organ and the actual differentiation which sometimes results from the modification of normal processes. What is shown by such cases, however, is not that the unusual condition is one of indetermination but that the organism has responded to modi— fying factors to produce a result different from that which would have occurred had not the unusual factors been present. Determination of an embryo does not always correspond to its differentiation.

A problem which has given rise to a great deal of experimentation in relation to the question of determination is the influence of pressure upon cleaving eggs. The work began with the experiments of Pflfiger in 1884 in compressing the eggs of frogs between two glass plates. The direction of the first three cleavage planes was found to be at right angles to the plane of compression. When the compression was released, normal embryos developed. Similar experiments have been performed in numerous other eggs with the result that normal development is found to follow the release of pressure on eggs of hydroids, sea-urchins, frogs, and Cerebralulus. In eggs of N ereis, Ciona, and molluscs, abnormal development follows compression. In the first group of eggs it is to be noted that differentiation does not begin until there is relatively a large number of cells present. In the second group signs of differentiation are to be noted very early. This series of experiments was used by 0. Hertwig and by Driesch in support of their opposition to the mosaic theory of development. Perhaps the experiments are less crucial than was formerly believed and merely emphasize the distinction previously insisted upon that there are two types of eggs with respect to their capacities for regulation as distinguished from determination.

finally the literature of experimental embryology contains many records of attempts to bring about the redistribution by centrifuging of egg substances, particularly those which are visibly different. These materials as well as the formed materials of the egg yolk, pigment, fat, and other inclusions are often of different specific gravity and hence respond to centrifuging by redistributing themselves in different zones or strata. This transfer of materials throughout the egg takes place as a rule without injury to its living substance. The use of the centrifuge in experimental embryology began with the work of Lyon in 1906, although it had been utilized for the study of the constitution of the cytoplasm by Gurwitsch in 1904. Lyon’s paper is a classical one in embryology both because of the introduction of a new method and because of his discovery that the redistribution of the egg constituents does not afiect the development of the eggs from the standpoint of determination. Many other investigators have used this method for the study of eggs through a wide range of animal forms. The results of the experiments with the centrifuge have rather uniformly indicated that the visibly stratified substances in the egg are not determinative in the sense that they are organ forming. It would seem that the method, though productive of results which have great value from other standpoints, has failed to give critical evidence as to the determinative character of development in the eggs to which it has been applied.

As a conclusion to this long catalogue of evidences, the position earlier talfen in this chapter may be reiterated. The animal series can be arranged in such a manner as to show a transition from eggs which are highly regulatory to those that are highly determinative in character. N 0 eggs are known which are purely regulatory or purely determinative, but at one end of the series differentiation sets in very late and the regulatory capacity is high. At the other end the marks of difierentiation begin even before cleavage and the powers of regulation are correspondingly lessened.

Chapter IX Ecological Control Of Invertebrate Larval Types

Throughout the animal kingdom many diverse environmental relations surround the eggs and young so that many responses might be catalogued. Environmental influences find expression in the places where the eggs are laid, in the amount and kind of food provided for the young during the early development, in the different devices for their protection, in specialized modifications for accomplishing locomotion and other vital activities, and in many cases in the production of forms of larvae ‘which are totally different from the adult organisms. These special types of larvae present a great range of variations from the modes of direct development with which the student of the embryology of vertebrates is familiar. These larval types involve a metamorphosis to the adult form which may be more or less complete and which in such cases brings the growing organism into an environment quite unlike that of its earlier development; metamorphosis is necessary in those forms in which the food and habits of the adult are unsuitable for developing young. If we assume the sea to have been the original home of most primitive stocks, an assumption which is more or less common and seems in line with the fact that very many types of animals develop in a moist atmosphere or liquid medium, we should expect the majority of unusual larvae to be found in the species living in salt water. The transition from salt water to fresh or to the terrestrial forms of life has presented many difficulties to the developing young and as a result those animals which live in fresh water or are terrestrial are more often characterized by direct development than are the salt-water forms. It may be regarded as an axiom of development that sessile adults have active young. This activity has two obvious results; first, the species secures its dispersal through the migration of the young, and econd, it extends the range. The migration is commonly passive but over a period of time serves to extend the range of the species as successfully as if the adults themselves were able to move. To insure dispersal, vast numbers of gametes must be produced, for the chance method of fertilization that usually accompanies such cases results in a failure of very many eggs and sperm to become further activated, and of those eggs which are fertilized only a few can come to maturity. The larval mortality is very high indeed, for these young are the food of many species as well as the victims of physical forces and of their own inability to find continuously satisfactory conditions for growth. The vast number of eggs produced may be realized by reference to the turbot which produces 9,000,000 in a single season, the cod with 5,000,000, and to the flounder with 1,000,000. In higher animals the reproductive energy is conserved by various means for caring for the young during early life, but in marine forms this is usually not the case. The second consequence of larval activity is to bring the growing forms into an entirely difierent range. This is illustrated in the case of the lobster, which is a bottom feeder, lurking in the crevices between stones and elsewhere to capture whatever prey may come within its reach, or feeding upon such dead forms as may be found nearby. The larvae, up to the time of the fourth molt, however, swim at the surface of the sea water. In addition to this, they are positively phototropic to light of the intensity of ordinary daylight, but the adults are negative. Thus the pelagic larvae are brought into a range of environment where their food, which consists of plankton, especially copepods, is abundant. Special structural modifications adapt these larvae to their surface pelagic life. They are the better enabled to swim on the surface of the water because of the possession of exopods, the outer branches of the walking legs characteristic of lower crustaceans but not present in the adults of the higher forms. These are retained to the fourth molt, when they are cast off, being no longer useful; the larva then goes to the bottom. Herrick makes note of several changes in structure and instincts which take place at the beginning of the fourth stage, which marks the most surprising leap in the whole history of development. Among these are the following: the primitive swimming branches of the thoracic appendages are lost; the cuticle becomes shell— like, containing more lime; the pigments are denser, the colors brilliant, and the color pattern variable; otocysts are present and orientation is perfect; rotation of the great forceps is complete; the animal, during at least a part of this stage, moves toward the light and swims steadily at the surface with the great claws directed forward and held close together; the preying instinct is more marked; the fighting instinct, the instinct of fear, “feigning,” and hiding are all developed by the close of the fourth stage or in the fifth, when the animal goes to the bottom to stay. In many other animals similar gain is accomplished by a specialized form of larval structure. The larva finds itself adapted to-securing food which is suitable for it, and structures which have only temporary employment are present and useful. It follows, however, that the gain to the larva would become a loss if the subsequent stages in the life cycle were not radically changed in such a way as to enable the organism to undertake a new mode of life. In other words, metamorphosis in such cases is a necessity for bringing about those adaptations which fit the organism to live in its permanent environment and

there to undergo those further changes which look toward the production of new germ cells.

It is not usual to find metamorphosis in animals which live in fresh water, although there are some outstanding cases in which larvae totally unlike the adult are developed in species inhabiting fresh water. It may, however, be taken as a general rule that development in freshwater forms with the exception of certain few highly specialized cases is direct. Fresh water presents a great lack of constancy in living conditions as compared with those of the sea. With but few exceptions the bodies of fresh water lack sufficient depth and area to maintain even a low degree of constancy. Fresh water becomes heated much more easily than the sea, it freezes comparatively quickly, its streams are subjected to periods of flood during which the water runs rapidly and scours out the beds or overflows, and when it recedes leaves innumerable forms of life stranded to perish. Every stream has narrows and depths where it runs rapidly and flats where its current is slow, an environment in which the fragile larvae would scarcely be able to maintain themselves, and there are but few animals which go through an independent larval history in fresh water. The lack of suitability of fresh water to larval development without doubt explains why so many groups of marine animals have not been able to gain a foothold in fresh water. A very few sponges, almost no coelenterates, occur in this environment, and many groups of marine fish and ascidians, cephalopods, king crabs, and some of the worm groups have not a single representative in fresh water. Animals that cannot produce eggs which develop into young like the adult are prevented from gaining ‘a. foothold by such a changeable medium as fresh water. In addition to the difficulties of fresh-water life already noted, a zone of brackish water between the fresh water and the sea is itself an almost impassable barrier to the entrance of marine forms.

Terrestrial life presents to developing larvae even more dangers than fresh water. To this situation is due the fact that in land forms development is either direct and fairly simple or else specialized with complicated devices for caring for the young during their immature period. The latter condition is illustrated by many forms of insects in which metamorphosis is complete, often involving very complicated life histories. Some of the disadvantageous features of the terrestrial 386 ECOLOGICAL CONTROL OF INVERTEBRATE LARVAL TYPES

environment are the following: temperature undergoes a wide range of variation, often passing quickly between the extremes. This condition is quite unknown in the ocean and is much less in fresh water. The very changeableness as well as the extremes of heat and cold are hardships for the developing larvae. The weight of the body is no longer buoyed up by water and must be supported at every moment. This involves consumption of much energy as compared with the almost passive drifting of larvae in aquatic habitats, and renders movement much more difficult and restricted. Food no longer streams by as in the contrasting case, but must be actively sought, is often very much more restricted as to kind, and presents an increased toughness since in the composition of land forms the percentage of water is much less, and the food is necessarily more difficult to find. The glare of daylight and the consequent difficulty of avoiding enemies puts the terrestrial larvae to a disadvantage which forms living in the subdued shadows of the water do not share. finally the rapid evaporation to which terrestrial forms are subjected requires special devices for protecting the soft exterior of the organism. A heavy shell, mucous glands, and other similar devices show what a serious drain upon the organism requirements of this kind cause.

These considerations account in a large measure for the lack of uniformity in the occurrence of larval types throughout the different groups of the animal kingdom. The particular forms of larvae within a group are of adaptive rather than taxonomic significance.

The general subject of the care of young animals was briefly but instructively discussed by Gamble in “The Animal World” (Holt and Company). Some of the ideas mentioned on the three preceding pages are suggested by his discussion. Gamble sums up the responses of animals to different environmental conditions in which the young must be produced as follows:* “We find isolated examples of the retention of the young by diminution in the size of the family. Speaking generally, marine animals pour their eggs broadcast, and leave their minute larvae to complete their metamorphosis unaided and unsheltered, but in every group there are malthusian species which not only restrict their families in number, but enclose them by protective envelopes. The varied experience of larval life in this curtailed and direct development supplants metamorphosis. The eggs become larger and the young stronger at birth.

“In fresh water the limitation and protection of the family is more generally the rule. Insects and Amphibia are the only large classes

‘ Quoted by permission of Henry Holt & Co.

which go through their larval life in fresh water. But the protection

is usually of the simplest kind and is confined to the earlier stages of development.

“On land the insects form the only class in which the majority still pursues a free larval history. Others develop directly, either growing up into full stature without guidance or protection, or carried and fed by their parents for some time both before and after hatching.”

Cite this page: Hill, M.A. (2026, September 8) Embryology Book - Outline of Comparative Embryology. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology

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