Talk:Paper - The thirty-first Long Fox memorial lecture - The experimental study of development
the Bristol Medico-Chirurgical Journal
““ Scire est nescire, nist 1d me Scire alius sciret.”
THE THIRTY-FIRST LONG FOX MEMORIAL LECTURE: BY Prorrssor C. M. Yonaz, D.Sc., Professor of Zoology in the University of Bristol.
DELIVERED IN THE UNIVERSITY OF BRISTOL ON TUESDAY, 7th JULY, 1942,
on THE EXPERIMENTAL STUDY OF DEVELOPMENT.
ae Problem of development—the manner in which an organism ene the size and elaboration of its adult structure—was not aturally a matter which early excited the interest of man. rae exist, indeed, few more fascinating subjects than the change, the ae period of hours, days, weeks, at most a few months, from Sree ny. simple structure of the fertilized egg to the immense an ural and functional complexity of the insect, fish, bird or dey, fat which it develops. Yet so commonplace is the fact of “opment that we tend to take for granted a process occupying or weeks, while the occurrence of evolutionary processes over teds of millions of years remains a subject of dispute. biolo 8 foundations of the science of embryology, that branch of moe, which deals with development, were laid by the Greeks. deyalee’ the Hippocratic writings there are many allusions to °pment, although mainly in connexion with obstetrical and Cente sical problems. But even at this early date, in the fourth imp Ty B.c., there are attempts at causal explanations and the °rtant fact was noted that the embryo “ dries up,” 7.e. loses water
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> 50 Proressor C. M. YONGE
during development. But it is from Aristotle, the first great biologist, in some respects the greatest of all biologists, that the bulk of classical knowledge on development comes. His work oD The Generation of Animals is the first text-book on embryology: beyond which little further advance was made until modern times- Yet, in the absence of the essential aids of microscope and microtome: the more fundamental facts inevitably escaped him. Thus, although he distinguished between oviparity, ovo-viviparity and viviparity: described later stages in development and allotted the correct functions to the placenta and the umbilical cord in the mammals; he knew nothing of the essential nature of the egg and the spermatozoon. Adopting what appears to have been an Egyptia? view, he regarded the human embryo as arising from the menstru@ blood under the moulding and dynamic agency of the male secretion®» although he admitted the possibility of the operation of som? previously existing mechanism which was thus set in motion. % one important matter he anticipated modern views. The Hippocrat!® belief was that development was no more than a revelation ° previously existing complexity, 7.e. essentially merely an increas? in size, a belief later known as preformation : but Aristotle conclude that development represents an ever-increasing degree of complexit¥ of structure ; he was an upholder of the opposing view of epigenes'*:
Galen, the great physician of the Roman world, made comparatively minor contributions to embryology and we have if await the revival of the spirit of scientific enquiry at the Renaissan® for further significant advance. Leonardo da Vinci, one of t supreme minds of all times, left in his amazing note-books beaut U drawings of the human foetus in utero, but, more significant of t new age of which he was the precursor, he made measurements embryos at various stages and, by his mathematical analysis of th processes of development and growth, has claims to being conside! the first experimental embryologist. William Harvey, whose fa as an embryologist has been overshadowed by his still greater fa as discoverer of the circulation of the blood, maintained in his gt work on The Generation of Animals that “all animals are in 8° sort produced from eggs ” wherein “no part of the future organs exists de facto, but all parts lie here in potentia.”
Here was a clear statement of embryological fact and - such a beginning, with the help of the recently-discovered mich t
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scope, rapid progress might have been expected. But, m <6 unfortunately, a controversy developed which impeded progr de for more than a century. Malpighi, the first great histologist, ee microscopical examination of the developing egg of the hen, but 4 so in the heat of the Italian summer when development may P¥0° pe without incubation. Hence he found, in what he took to be me undeveloped egg, clear traces of the future adult and so clay ed to have demonstrated the fact of preformation which Harvey denied. About the same time the Dutch microscopist, Leeuwenhoek, \dentified spermatozoa from the seminal fluid and, seeing faint Indications of internal structure, claimed that they were miniatures of the adult. Thus there arose two schools amongst the pre- oOrmationists ; the ovists who proclaimed that the adult structure Was present in the egg and the animalculists who claimed that it Tesided within the sperm. And until the end of the eighteenth ‘entury the history of embryology is largely the record of the Wordy battles between the upholders of these views and between €m and the supporters of epigenesis. Such progress as was Made concerned such matters as the identification of the ovaries and the testes. ti The light of scientific truth re-emerges in 1759 with the publica- lon by Caspar Friedrich Wolff of his T'heoria Generationis, in which me Stated that the parts of the embryo of the chick appeared in urn and could be seen being formed. His work leads directly to that
of the greatest of embryologists, the Esthonian, Karl Ernst von ey whose work in descriptive and comparative embryology Mer us almost to modern times ; he died at a great age in 1876.
ej anwhile Spallazani, the pioneer experimental zoologist of the ‘ehteenth century, had shown that sperms were essential to “velopment. But it was not until the middle of the last century hae the union of the egg and the sperm was observed and still later re it was realized that this involved the union of the nuclei. . 0m this discovery, in turn, there dates the beginning of those
- Vestigations which have culminated in the discovery, first of the
thee somes within the nucleus, and then of the genes borne upon - Se which, linking up with Mendel’s experimental study of inheritance, have been revealed as the controlling agencies in heredity.
in To Tevert to the descriptive work of von Baer and his followers histor nineteenth century. It was shown that the developmental the A of any animal was an orderly series of stages, beginning with ails ertilized egg which divided repeatedly to form typically a on 6 W sphere known as the blastula, which by intucking of the cells ee Side, or overgrowth by the future outer cells, if the former sta, es was mechanically impossible, produced a two-layered ects iY the gastrula. The outer layer of cells became known as the erm, and gave rise to the skin, nervous system and sense Wi th ih the inner, which formed the future gut, as the endoderm. Organ ¢ formation of the gastrula the first indications of the future Axia] S—in the vertebrates the nervous system and the primitive two skeleton or notochord—made their appearance. Between the laye Primary cell layers was introduced an intermediate mesoderm tk, 0m which were derived in development the body cavity, ©x¢p, *emainder of the skeleton, and the muscular, circulatory, Story and reproductive systems. After this initial blocking 52 Prorressor C. M. YONGE
out in the rough of the future organs there followed a period of tissue differentiation, at the conclusion of which the developing organism became a functional individual.
The development of great numbers of animals, both vertebrates and invertebrates, was studied in detail during the latter half of the last century and a most imposing mass of knowledge acquired, much of it in connection with the study of evolutionary processes initiated by Darwin who had published his Origin of Species in 1859.
But in any science description is only the first stage ; it is the prelude to analysis by measurement and experiment. The distinguished American embryologist, Professor F. G. Conklin, who played a notable part in the initiation of the experimental period in embryology summarized development as a series of responses t0 stimuli, the responses being not merely repetitive as in the case of the tissue cells. He likened the egg to a complex machine s0 constructed that it transforms itself at every critical stage into another machine with its own peculiar mode of action. The problems to be solved concerned the nature of the mechanisi§ within this continually changing machine.
Fertilization of the egg became recognized as a dual process: one of activation, 7.e. the initiation of cleavage, and the othe! consisting of the union of the male and female nuclei, from the sperm and egg respectively. The latter process ensured the passag® into the new organism, so initiated, of hereditary material fro” two sources, but, although in the majority of cases entrance of th? sperm is also necessary for activation, important exceptions reve that the egg can develop without the presence of the sperm. representatives of various groups of animals, notably in the Insect® and Crustacea, eggs frequently develop parthenogenetically, without the assistance of the males which may be absent duri0é certain periods of the life-history (in a few cases no males have bee? found at any stage). Normally, however, a series of parthen” genetic generations is followed by one in which males appear. other groups, notably the Echinodermata, artificial activation possible. Jacques Loeb, one of the greatest of Jewish emigrant from Germany to the United States, showed how the eggs of starfis can be stimulated to develop by treatment, causing first a tempor f increase in the permeability of the egg, followed by the removal © some of the water from within it. As a result of activation potentialities of the quiescent egg cell are dramatically convert? t into kinetic energy, revealed in some cases by an immediate grea increase in oxygen consumption, but invariably by the subseque? initiation of the process of cleavage. a
A problem which early presented itself in this period of oxP ie mental analysis was the fate of the individual cells into which single activated egg cell gave rise in early cleavage. Did cleavage 18 was asked, divide the egg into portions corresponding to the pat Ture Lone Fox MrmoriaL LECTURE 53
of the future embyro, i.e. were definite organ-forming substances present in different parts of the egg and so became separated from One another when the egg divided ? Experiments were carried out I which the early cleavage products, or blastomes, were separated Tom one another and their subsequent fate followed. These revealed that eggs could be divided into two types. There were mosaic eggs which any portion of the products of early cleavage formed only that portion of the embyro which it would have formed had it "mained in contact with the other product or products of egg Ivision. But there were other, so-called regulative, eggs in which Separated blastomeres formed complete but small embryos. Partial Separation of the blastomeres produced double monsters ; the union ° two activated eggs led to the formation of an unusally large, but ‘ingle, embyro. Thus in eggs of this type, which included those of € Echinodermata and, to a more limited extent, of the Amphibia, € early stages of development are plastic ; the greater part, though Not actually all, of the egg has no predestined fate. But invariably 1S plastic stage is succeeded by a mosaic stage in which the fate of © various parts is determined. This comes, in the case of the decenibia, at about mid-gastrulation. And it is upon amphibian ®velopment that much of the significant work of the past twenty ®ars has been done. the he amphibian egg is not homogeneous. It is rich in yolk, but pol Sreater part of this collects at one end, the so-called “ vegetative ” ®, the bulk of the protoplasm with the nucleus lying near the PPosite ‘‘animal”’ pole. It was discovered at a relatively early tine in embryological investigation that, although before fertilization whi the egg is apparently radially symmetrical about an axis ‘ch passes from the animal to the vegetative pole, bilateral Metry appears immediately after fertilization. The animal i 1s dark in colour, the vegetative pole light, but between the x after the entrance of the sperm there extends a grey crescent. of gepostvion of this has been regarded as controlled by the position th epee of the sperm because it usually appears exactly opposite nwa > but there is some doubt on this point. What is certain, evelan’ 1s that from this stage onwards the orientation of the The Oping egg and so of the future embyro and adult is determined. Now fees area of the grey crescent is mid-dorsal and the egg is midal Materally symmetrical about a plane passing through the eog «© Of the crescent and the two poles. The first division of the ie Normally in this plane. The result of later cleavage is to smal about the formation of a hollow blastula composed of many ole Cells on one side, derivatives of the micromeres from the animal of rec on the other, of larger cells from the yolk-laden macromeres Tp vegetable pole. aoe fate of the cells of the blastula has been followed out by the lous method of staining them in life with harmless dyes such as neutral red or Nile blue. In this way the superficial cells at this stage in the developing amphibian—frog or newt—have been mapped out into “ presumptive” areas. The term presumptive is important. It implies that, under normal developmental conditions, these areas will develop into certain specified structures, e.g. endoderm, mesoderm, notochord, neural plate, or epidermis. But actually most are still fundamentally undetermined and, as we shall see, unde! certain experimental conditions, or as a result of damage, their fate may be changed. Of one region only is the destiny irrevocably determined at this stage and that is the area occupied by the cells of the grey crescent.
The extraordinary significance of this region was discovered by the late Professor H. Spemann, who was subsequently awarded thé Nobel Prize for his work. He developed a very beautiful techniqu® whereby he was able to excise portions of these early embryos an graft them into other embryos. Carefully executed experiment® and critical analysis of the results obtained led him to one ° the really significant advances in the experimental analys!§ of development.
Spemann used the embryos of newts, which provide betté experimental material than those of frogs. When the gastrulé is formed—by a combination of overgrowth and inpushing in tb? amphibia —the opening of the new internal cavity, lined bY endoderm, constitutes the blastopore. In animals where little y° is present in the egg this represents a rounded opening, but in tb® -amphibia, where the egg is yolky, this is blocked by a plug of yolk laden cells. The blastopore comes to lie on the posterior side of the ( embryo and from its upper, dorsal margin there extends forward th® ; shallow medullary groove which later forms the neural tube. Fro” this, by subsequent specialization of the cells, is formed + central nervous system characteristic of all vertebrates. Speman removed the upper lip of the blastopore from one embryo 2! grafted this into the side of another embryo at the same stage development and established the remarkable fact that in front? ; the grafted cells there appeared a medullary groove in additio? ‘ the one which appeared in the normal position. Meanwhile in the original embryo from which the dorsal lip had been removed ts medullary groove appeared. He further showed that these rest could only be obtained if the tissue was removed and the one established in young gastrule. Later removal did not afiect t subsequent appearance of the medullary groove and grafting }® no effect. At the same time the experiment had not to be car? out too soon or no effect was produced by the grafted cells.
These results were explained by Spemann by postulating he presence of an “ organizer ” in the dorsal lip of the blastopore: & ; presence of which induced the formation of the medullary grote An invisible, chemical change in the cells preceded the V
Structural differentiation, so that if the dorsal lip was removed after a certain stage in development the medullary groove would Still appear. In the same way the cells of the embryo into which the excised dorsal lip was grafted had to be “‘ competent ” ; if the graft was made too late the fate of these cells, which would normally develop into skin, had been determined and the organizer could not therefore induce the formation of an additional medullary groove. Later work, some of it in this country, notably by Needham and Waddington, has abundantly confirmed these results. An Organizer or “‘ organization centre ”’ has been located in the embryos of other amphibia and other groups of vertebrates, such as mammals, irds and fishes, but also in the embryos of invertebrates such as ®chinoderms and insects. It was early shown that induction of the °rmation of a medullary groove would take place, not only after the cells had been narcotized but also that the organizing substance, °t evocator, as it is now frequently termed, persists after the death Of the cells containing it. It is resistant to heat, to freezing and to € action of a wide range of organic substances ; indeed it is only €stroyed when the cells are reduced to ash. That a chemical Substance is involved is indicated by the fact that small pieces of gar after being placed in contact with the dorsal lip of the blastopore or a short time acquire the power of inducing a medullary groove €n applied to other embryos, indicating that some specific Substance had diffused into the agar from the cells of the original st Tyo. <A variety of artificial evocators such as methylene blue,
- rols and fatty acids have a similar effect, and it is probable that
re, produce their effect by causing the cells of the embryo to ‘ fase the normal evocating substance. Attempts to isolate the ae substance, or substances, involved have not, as yet, been thine tely successful. In view of the minute quantities concerned ie 18 not surprising. But the evocator does appear to be soluble the wt and petrol-ether and it may be one of the sterols. If this is of ee then it belongs to the same group of substances as certain will ih. Sex hormones and cancer-producing agents. All of these, it e € noted, are in some way or other concerned with development 8towth—normal or pathological. pe car © organizer is not specific ; cells from the primary organization will te of the embryo of one group of animals, for instance birds, . cag ae the formation of a medullary groove in the embryos of the «wt? Such as amphibia. But there is something inherent within i Cells of the particular embryo which insures that it is amphibian ke Wes which are induced by the evocator and not avian. It may be eed that it is the genes resident on the chromosomes which are fhe for the particular reactions of the cells—the stimulating comples sone identical in all cases. aura is een Saar acti interplay between the stimu ating evocator an e ing cells, the analysis of which will involve much further work.
There is evidence that the pattern which the induced tissues assume is influenced both by the evocator and the tissues affected.
The position of the primary organization centre was determined,
-as we have seen, at fertilization. It invariably appears within the region of the grey crescent. The actual position of this would seem to be determined by what, in our present state of knowledge, we can only describe as the interaction of certain gradients within the egg. There is a gradient from the vegetative pole to the animal pole and apparently at right angles to this a second gradient, the organization centre appearing at the intersection of the two.
Although the medullary groove, the first indication of the future central nervous system, is the first organ rudiment to be induced it is actually not the first to be formed. Beneath it, along the upper side of the archenteric, or primitive gut, cavity which is formed at gastrulation by the intucking of the cells around the blastopore, tw other organ rudiments make their appearance. These are the notochord, the primitive axial skeleton of the vertebrates, and the segmented mesodermal somites which give rise to the body cavity> body musculature, excretory system and so forth. The notochord is situated in the antero-posterior axis of the embryo, the mesodermal somites appearing bilaterally on either side of this. These structures arise spontaneously at a certain critical stage in development, namely mid-gastrulation, and it is their presence which induces thé appearance of the medullary groove above them. The dorsal lip ° the blastopore actually consists of cells which will, by the continual process of intucking, form notochord and mesoderm.
This induction can be proved in two ways. First by grafting early notochordal and mesodermal cells below ‘competent ectoderm, following which a medullary plate invariably makes it® appearance above the region of the graft. Second by reariDé suitably susceptible embryos in dilute saline solutions. This treat ment has the effect on converting the normal gastrulatio? invagination into an evagination, i.e. the cells which form th® primitive gut, notochord and mesoderm are pushed out away fro” the rest of the embryo instead of being tucked in under the sup, ficial ectoderm. Under these conditions a notochord aD mesodermal plates lateral to this appear within the mass of ¢cé Is which has been pushed out, but no medullary plate is formed aloné the mid-dorsal line of the ectoderm because there is no chord4” mesoderm below it and so no inductive action. We speak, therefor® of the self-differentiation of the chorda-mesoderm and of the inductio” of the medullary plate which normally appears above it.
The result of the appearance of the chorda-mesoderm ani the inductions which this primary organization centre brings abou is thus the formation of a zone of constructive activity along 5 antero-posterior axis of the embryo. This has become know? the primary organ field. This conception of fields of dynam
activity is at present no more than a description of what we observe and isin no way an explanation, although there are many indications lat some underlying physical mechanism will eventually be disclosed. The primary organ field gives rise to a series of secondary Organ fields or sub-fields which control the development of the Notochord along the mid-line and of the first-formed mesodermal Structures which produce body cavity, excretory and muscular Systems on either side of this. It also, by way of the medullary Plate, induces the formation of the sub-fields concerned with the ormation of the nose, the ear and the eye, anterior to and lateral © the brain region.
In this way the embryo becomes a mosaic of different fields, each ® centre of dynamic energy and concerned with the development of some particular organ system, eye, ear, heart, limbs, tail and so forth. This establishment of local control in different areas of the developing rbryo is spoken of as emancipation, indicating some measure of
edom in these various areas, although all work together for the ®Ventual formation of the unified organism. The, as yet, “specialized embryonic cells provide the material on which the ynamic energy of the various fields exerts its effect. It does not “Ppear unreasonable to postulate a competition between the Activity of the various fields of activity, but there is no compromise ; Cells lying between two such fields must come completely under the
- Mination of one or the other; once they have done so there is
° turning back, they are inevitably committed to the formation of a Particular organ, eye or ear or limb as the case may be. Meantime © Original unitary primary field persists especially in connexion sub; the direction of the primary axis of growth and of the various “Ubsidiary organ fields. - It will be profitable at this stage to consider briefly a few
- amples of these secondary fields and of their mode of operation.
y Classic case is that of the eye which will be dealt with first. Att as long been known that the vertebrate eye has a dual origin. on ‘t the primitive brain has made its appearance, there develop
Sither side of the fore-brain a pair of projections known as the ‘ 1¢ vesicles, which grow outwards towards the side of the head. ee each assumes the form of a hollow bulb attached by a itself’ The latter gives rise to the optic nerve, the former to the eye orm; The outer wall becomes pushed in against the inner wall so the uo the optic cup, the outer cells of which eventually constitute
. retina. But at the same time the superficial ectodermal cells arest to the centre of the optic cup thicken, project down into the pecning of the cup and then separate from the superficial ectoderm a “rm the rudiment of the future lens. The completed eye, there- ®; Tepresents the intimate functional association of material from
fre is an organ rudiment admirably suited for experimental treatment and experimental embryologists have been quick to avail themselves of the opportunity. In the Amphibia the results obtained have varied according to the species studied. In the grass frog; Rana fusca, if the eye rudiment is removed not only does no opti¢ vesicle appear but also no lens, although the cells which normally form this are uninjured. This indicates that the optic vesicle induces the formation of the lens and this conclusion is confirmed by the results of experiments involving the removal of the opti¢ rudiment and its transplantation under the ectoderm of the body, when a lens appears, for instance, in the middle of the under side of the body. In the same way if the ectoderm which normally overlies the optic vesicle is removed and replaced by grafted cells from other parts of the body a lens is formed. In other words theré is, in this species, nothing inherent in the ectoderm which bring§ about the formation of a lens; this is induced by the specifi¢ organizing action of the cells which form the optic vesicle. On the other hand in the allied edible frog, Rana esculenta, or in the axolotl, the lens does appear even after the removal of the vesicle below it: while grafting of the eye rudiment into other regions of the body does not induce the appearance of a lens in those regions. Othe! examples could be quoted where conditions are intermediaté between these two extremes, and the explanation of the difference: although the evidence is not yet complete, appears to reside in thé time at which chemical, as apart from structural, differentiation ° the ectodermal cells occurs in different species. Once this bé* occurred there is no going back ; lens formation of certain cells, sk? formation by other cells is from that moment irrevocably determine®: | In the same way the formation of the capsule of skeletal mater
which surrounds the ear does not occur if the rudiment of the ¢ vesicle is extirpated in the early embryo. That the capsule is form? as a result of inductive action is proved by its formation in unusual regions of the embryo into which the rudiment of the ear vesiolé have been transplanted. Although each organ field is normally an entity it may combit® with another of the same type, or, on the other hand, if a particulat | organ rudiment is divided it may give rise to two or more similar | structures instead of the usual one. A few examples will make thes? points clear. If an eye rudiment in a sufficiently early stage is tral® planted close to another at about the same stage in development, due attention being paid to the orientation of the graft, the two org” fields will fuse with the consequent formation of a single, althous rather large, eye. Such a fusion of separate fields would appeat 0 occur in the normal development of the vertebrate heart. Ths organ arises in the trunk region from the mesodermal plates op either side of the mid-line ventrally. The rudiments grow togethe and eventually form a single tubular structure which divides UP into the four primitive chambers, which early begin those rhythm!c
pulsations which will be continued throughout the life of the animal. It is now some three hundred years ago since Harvey spoke of the “capering bloody point” of the embryonic heart. If the early eart rudiments are transplanted a four-chambered pulsating heart May be formed in altogether abnormal parts of the body. But if the mesodermal rudiments from the two sides are prevented from growing together, by the introduction between them of foreign tissue or the cutting of a wide incision, then each will proceed to form a Separate heart with, in due course, circulatory powers. Thus both M normal development and as a result of experimental] grafting two ike centres of developmental activity may blend with the resultant formation of a single structure. _ Subdivision of organ fields has been brought about in many Mstances. With the heart, the two original components may be €pt separate and each subdivided with the consequent formation of as many as five hearts in a single embryo. Limb rudiments may be split and twin limbs produced. It is probable that abnormalities i development which cause separation of organ rudiments are Tesponsible for the birth of animals with additional limbs, eyes and €ven, on occasion, heads. It is only possible to consider one further point which emerges Tom the experimental analysis of limb formation. This is controlled Y a bud of mesodermal cells which contain the specific organizing eld, the outer, ectodermal, cells merely conforming to the growth Pattern assumed by these underlying cells. Transplantations of €se mesodermal buds beneath the ectoderm elsewhere and the Consequent formation there of a limb proves this. But in the case of the limb we are dealing with an organ having considerable €xtension in length and which demonstrates very beautifully that Redient in activity to which reference was made earlier. If the ‘mb of a newt is amputated at the base, the wound heals and then, ter a few weeks, a mass of cells accumulates under the epidermal “overing. This cell mass, or blastema, grows and gradually assumes © form of a limb with all the internal equipment of bones and Muscles and surrounded by skin. Into this new limb penetrate od vessels and nerves to nourish and control it. th ow if the limb be severed in the region of the forearm and then bls humerus be removed from the portion which is left, the lon ome which subsequently appears is able to regenerate the " Wer half of the limb including the digits, but is incapable of forming era humerus behind the cut. This surely indicates the presence i 4 field of activity possessing a gradient of activity from the base utwards, Regeneration from the base produces a complete new > Tegeneration from an intermediate position along the limb Produces only the distal portion, even although constituents from e More proximal regions have been removed, because only a part the organ field is concerned. There is much in the facts of regeneration amongst the simpler invertebrates which supports this theory of gradients in activity, which was originally developed by Professor C. M. Child as a result of work on regeneration which largely preceded modern developments in experimental embryology- Regeneration essentially represents embryonic development during adult life and the incorporation of its study within the sphere of experimental embryology has led to an enrichment of the content of that branch of zoology.
It is impossible to proceed further with this subject and t0 discuss the specialization of cells to form tissues and the later appearance of functional organs. Here both the organ field and the interaction of tissues play their part, while functional activity itself finally assumes some control over maintenance or “ conservation ‘a of fully constituted organs or tissues. But despite the hetero- geneous nature of the adult structure produced by this mosaic of developmental fields, there is finally produced that unit in functio? and behaviour which we designate an organism.
In this brief survey of recent developments in experimental embryology an attempt has been made to indicate the range av! far-reaching significance of modern investigations. Embryology: which at the beginning of the century was little more than descriptiv® repetition, has now become one of the most active branches 2 zoology. Modern work has, in some measure, reconciled the ol conflict between Preformation and Epigenisis. There is an unde!- lying preformation, although not of the type postulated by the old Preformationists, but development itself is epigenetic representing a progressive increase in complexity from egg to adult. The experimental analysis of development is one of the most hopeft approaches to that fuller interpretation of living things and liviD8 processes which is the essential aim of the science of zoology. that way it is adding to the basic raw material of knowledge which the art and science of medicine applies to the amelioration of hum@? suffering and the promotion of the health and happiness of mankind: