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

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Chapter 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.

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Cite this page: Hill, M.A. (2024, April 23) Embryology Book - Outline of Comparative Embryology 2-8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Outline_of_Comparative_Embryology_2-8

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