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Jenkinson JW. Experimental Embryology. (1909) Claredon Press, Oxford.

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This is an historic 1909 embryology textbook. } Modern Notes: Historic Textbooks

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Chapter I Introductory

THAT living creatures reproduce their kind is a fact which is familiar to us all, but it is the peculiar privilege and province of the embryologist to observe and to reflect upon that marvellous series of changes whereby, out of a germ which is comparatively structureless and unformed, a new organism is developed which is, within the limits of variation, like the parents that gave it birth.

Development is the production of speciﬁc form. 1<‘rom a particular kind of germ only a particular kind of individual will normally arise, though unusual conditions may lead to the formation of an abnormality or monstrosity. Thus, while the germ is the material basis, development is the mechanism of inheritance. The student of heredity seeks to express in terms which shall be as exact as possible, ultimately mathematically exact, the degree of similarity between the offspring on the one hand, and parents and more remote ancestors on the other. The embryologist has under his very eyes the process by which that similarity is brought about, and even when the resemblance shall have been stated with all possible precision, it will still remain for him to give an explanation of those changes whereby the inheritable peculiarities of the species are handed on from one generation to the next.

Used in the widest sense of the word, development includes not merely the formation of a new individual from a single cell, whether fertilized or not, but also the phenomena of budding and regeneration. In a narrower sense, however, the term is restricted to the first of these processes, and a corresponding distinction is made, however artiﬁcially, between Experimental Embryology and Experimental Morphology, when the subject is treated from a physiological point of view.

In development two factors are obviously involved. One is growth, or increase of volume, more correctly increase of mass; the other is differentiation, or increase of structure; and, in multicellular organisms, both these factors are accompanied by division of the nucleus and the cell.

Segmentation is the first sign, or almost the first sign, the developing ovum gives of its activity; and this cutting up of the egg-cell into parts, which marks the beginning, is also continued during the later stages of ontogeny, and goes on as long as the life of the organism endures.

Growth is especially characteristic of the embryonic and of the adolescent organism. It occurs at different rates in the different cells, and indeed the growth of a group of cells is in itself often an act of differentiation. Growth may depend upon the absorption of water or the assimilation of other substances, and this may lead simply to an increase in the size of internal cavities, as in the blastula of lrlc-hinoderms or the Mammalian blastocyst; to an increase in the volume of the living protoplasm ; or to the secretion of intracellular or intercellular substances, either organic (for example, the notochordal vacuoles, the matrix of cartilage and bone) or inorganic (the skeletal spicules of Eehinoderm larvae, Sponges, and Coelenterata). This increase of mass is not only conditioned by the presence of food in the form of substances found in the environment, but depends on such external circumstances as temperature, atmospheric and osmotic pressure, and so forth.

But while the embryo is dividing up its material—a material which is already to a certain extent heterogeneous, composed, for example, of protoplasm and deutoplasm or yolk——while it is increasing its mass, it is also undergoing a process of differentia tion; and, as even a superficial acquaintance with embryology.

will inform us, one of the most characteristic features of differentiation is that it occurs in a series of stages which follow upon one another in regular order and with increasing complexity. When segmentation has been accomplished—sometimes, indeed, during segmentation——certain sets of cells, the germ-layers, become separated from one another. Each germlayer contains the material for the formation of a deﬁnite set of organs——the endoderm of a Vertebrate, for instance, contains the material for the alimentary tract and its derivatives-—gill-slits, lungs, liver, bladder, and the like ; the germ-layers are therefore not ultimate but elementary organs, and elementary organs of the first order. In the next stage these primary organs become subdivided into secondary organs——as the arehenteron of an Eehinoderm becomes portioned into gut and eoelom-sac, or the ectoderm of an Earthworm into epidermis, nervous system, and nephridia——and in subsequent stages these again become successively broken up into organs of the third and fourth orders and so on, until ﬁnally the ultimate organs or tissues are formed, each with special histological characters of its own. This end is, however, not necessarily reached by all the tissues at the same time. Indeed, it is no uncommon thing for certain of them to attain their ﬁnal structure while the others are yet in a rudimentary condition; thus, in some Sponges the scleroblasts begin to secrete spicules in the larval period, nematocysts may be formed in the Planula of the Coelenterates, notochordal tissue is differentiated in the newly hatched tadpole of the Frog; and, speaking generally, larval characters are developed at a very early stage.

To this regular sequence of ontogenetie events Driesch has applied the term ‘rhythm ’, the rhythm of development. The organs of the body are, however, by no means all formed of single tissues~—-bone, epithelium, blood, and the rest——but are compounded, frequently of very many tissues, and this ‘ composition ’, to quote a term of Driesch’s again, is another of the obvious features of organogeny.

VVhile, therefore, in the last resort all diiterentiation is histological, that ﬁnal result, the assumption by the cells of their definitive form, is only achieved after many changes have taken place in the position of the parts relatively to one another while the organs are being compounded, and so its speciﬁc shape conferred upon the whole body.

It is possible to find a few general expressions for the manifold changes that take place in the relative positions of the parts. Several years ago, in 1874, His compared the various layers of the chick embryo to elastic plates and tubes; out of these he suggested that some of the principal organs might be moulded by mere local inequalities of growth——the ventricles of the brain, for instance, the alimentary canal, the heart—-and he further succeeded in imitating the formation of these organs by folding, pinching, and cutting india-rubber tubes and plates in various ways. This analysis, however, deals only with the foldings of ﬂat layers, and must be supplemented by a more exhaustive catalogue of the processes concerned in ontogeny, such as that more recently suggested by Davenport. Davenport resolves the changes in question into the movements of cells or cell aggregates, the latter being linear, superﬁcial, or massive, and within the limits of these categories the phenomena are susceptible of further classiﬁcation. The catalogue proceeds as follows :——

I. THE Movmtaxrs or SINGLE Cams.

1. Migration of nodal thickenings in a network of protoplasm: e. g. the migration of the ‘ cells ’ to the surface of the Arthropod ovum to form a blastoderm, the movements of vitellophags, and yolk-nuclei (Fig. 1).

FIG. 1. — Sections of the egg of Geophilus ferrrlyiuezls showing two stages in the formation of the blastoderiu: bl, blastoderm; dp, yolk pyramids; gr, groups of blastoderni cells on what will be the dorsal side ; L-, nuclei surrounded by masses of protoplasm. (After Sograff, from Korsehelt and Heider.)

2. Migration of free amoeboid bodies: e. g. the mesenchyme cells in the Eehinoderm gastrula, the lower layer cells of Elasmobranchs, the blastomeres amongst the yolk-cells in Triclads and Salps.

3. Aggregation of isolated cells.

a. Linear aggregates: e. g. the kidney of Lamellibranchs, the yolk-gland of Turbellaria, capillary blood-vessels.

6. Superﬁcial aggregates: e. g. the blastoderm of Arthropods, the formation of the imaginal gut-epithelium in some Insects.

0. Massive aggregates : e. g. the gemmule of Sponges, the spleen of Vertebrates.

4. Attachment of isolated cells to another body: e. g. the union of muscles to the shell in Mollusca and Arthropoda, of tendon to bone in Vertebrates, the application of skeletal cells to the notochord.

5. Investment and penetration by isolated cells: e. g. the follicle cells between the blastomeres in Tnnicata, the muscles of the gut in va.rious animals, the septa of the corpus luteum, the formative cells of the vitreous body of the Vertebrate eye, the immigration of the nephric cells in the Earthworm.

6. '1‘ransportation of bodies by wandering cells: e. g. of the buds in Doliolidae.

7. Absorption by wandering cells: e. g. phagocytosis in Insect pupae a11d in the ta(lpole’s tail.

8. We may place here. the frequent alterations in the shapes of cells, which do not apparently involve growth: e. g. when {lat cells become columnar.

II. Tm»; Movi~:.\mNTs or (J1~:r.r. A(:eu1«;(:A'rEs. A. Linear Aggregates.

1. Growth in length: e. g. the growth of the roots and stems of plants, of the stolons and hydranths of Hydroids, the outgrowth of nerves, of the necks of unicellular glands, the growth of the blood-vessels from the area vaseulosa into the body of the Chick embryo, of blood-vessels towards a. parasite, the growth of mesoblastie and other germ-bands in Annelids, the back-growth of the Vertebrate segmental duct, and the like.

2. Splitting.

a. At the end, that is, branching: e.g'. of nerves, bloodvessels, kidney tubules, glands, tentacles.

3. Throughout the length: e.g. the segmental duct of Elasmobranehs, the truncus arteriosus of Mammalia.

3. Anastomoses: e. g. of the dorsal and ventral roots of the spinal nerves, of nerve plexuses, of capillaries, of bile capillaries, of the excretory tubules of Platyhelmia.

4«. Fusion with other org'a11s: e. g-. of a nerve with its endorgan, of the vasa eiferenti-.1. with inesonephrie tubules, of nephriclia with the coelom in Annelida.

B. Superﬁcial aggregates. i. Increase of area. (1.. Growth of a sphere. 1. Equal in all dire(-tions: e.g. the blastula of ]*l(-hinoderms. 2. Unequal.

oz. Unequal in dilferent axes: e.g-. the conversion of a spherical blastula into an ellipsoid Planula in Coelenterata, or into an ellipsoid Sponge larva, or of the spherical into the ellipsoid blastocyst in l\Ia1mnalia.

,8. Unequal at dilferent poles: e. g. the formation of ovoid forms, such as 1’lannlae, the club-shaped glaild of .122/19/ziamzs, the auditory vesicle of Vertebrata.

6. Growth of a plane surface. 1. Equal in all directions: e.;,;'. the growtli of the blastodcrm over the yolk in Sauropsida, or Cephalopoda.

2. Unequal. a. When parts lying in one plane move out of that

plane: c. g. inva,t,;‘i11ations and evaginations of all descriptions.

(Fig. 2).

B. When parts—e. a row of eells——lying in one plane are moved in that plane: e. g. the germ-bands of C/cpsinc, by the growth of the epiblast (Fig. 3).

ii. Alterations of thickness. 1/. Increase: thickenings: e. g. the formation of the central nervous system in Teleostei, the formation of gonads from the I INTRODUCTORY 7

coelomic epithelium, the development of hair follicles, the trophoblast in the Mammalian placenta (Fig. 4).

Fm. 2.—-Three stages of an invagination or evagination. (After Korschelt and Ileider.)

FIG. 3.——Displacement of a row of cells in an epithelium. (After Kor- qQp§§ag!d9“’gQ‘éDO

schelt and Heider.) PI 4 F t _ tlef t, < .——— ours ages 1n 1 orma 1on

uf an epithelial thickening of many layers. (After Korsehelt and Heicler.)

6. Decrease: thinnings: e. g. in the roof of the thalamencephalon a11d medulla, the outer layer of the lens, the trophoblast of the Mammalian blastocyst. 8 INTRODUCTORY I

iii. Interruptions of continuity. a. By the atrophy of part of a, layer: e. g. when the ﬂoor of the archenteron tog-ether with the underlying paraderm dis appears in Amniota. (Fig. 5). X . ° UODCIDDUUDDDUDDU °

3 « <2 vacuum

5 canon» «aaaaaan

Fig. 5.——Three stages in the development of am interruption of von~ tinnity pcrpemliculzmr to the surfztce of am epithelium. (l'erfomtion.) (After Korschelt and Heidcr.)

/1. By the detachment of a part: c. g. of the medullary plate from the eetoderm in Amp//ioxus (Fig-. 6), of the notochord from the roof of the archenteron in Urodela. and I’e/r0myzo7z.

FIG 6. - Scheme of the formation of the medullary canal in Amphi0.rIts. (After Korschclt and Heidcr.) I INTRODUCTORY 9

iv. Conerescence of layers. a. By their margins: e. g. the edges of the eetoderm over the medullary plate, the edges of the embryonic eetoderm inside

Fm. 7.- Fusion of two cell plates by their nmrgins. (After Korschclt and Heider.)

the serosa of Si/nmczzlzts, the embryonic plate with the trophohlast in some Mammals (Figs. 7, 9).

(2. By their surfa.c-es (Figs. 8, 9, 10) : e. g. when the stomodaeum or proctodaenm open into the gut, when the mcdullary

FIG. 8. — Fusion of two cell plates by their surfaces. (Aft-er Korsehelt and Heider.) folds meet, when the edges of the peritoneal groove close to form the canal of the oviduct in Amphihia and Amniota.

This concrescence is commonly followed by a communication of the cavities on opposite sides of the adherent layers, as when the stomodaeum opens into the gut, or the amnion-folds unite ; but not necessarily, as when the somatopleure fuses with the trophoblast, or the allantois with the somatopleure in Mammalia.

v. Splitting of a. layer into two: e. g. the inner wall of the pineal vesicle in Lacertilia (Fig. 11). 10

FIG 11. — Three stages in the development of an interruption of continuity parallel to the surface of an epithelium. (Delanlination) (After K01-schclt and Heider.)

0. Massive aggregates.

i. Changes in volume.

a. Unequal in different axes : c. g. when the spherical larva becomes cylindrical in Dicyemidae.

3. Unequal at diﬂerent points : e. g. the outgrowth of limbbuds of Vertebrates and other forms, of the buds of plants.

ii. Rearrangement of material.

a. Simple rear1‘angement of cells: e.g. in the formation of the concentric corpuscles of the thymus, in the development of kidney tubules in the metanephric. blastema of Amniotn, in the grouping of the cells to form ectoderm, gut and atrium in the Salps.

b. Development of an internal cavity: e.g. segmentation cavities, lumina of ducts and blood-vessels, of the coelom and many generative organs.

c. Dispersion ol’ the elements of an aggregate: e. g. in gcmmule formation in certain Sponges, in unipolar immigration in some Sponges and some (‘oelcntcrates, in the liberation of the germ—celis.

iii. Division of masses.

a. By constriction : c. g. the segmentation of the mesodcrm and neural crest.

b. By splitting: e. the nervous system from the ectodcrm in Teleostei and many Invertebrates, the notochord from the roof of the {l.1'('l1(3lltCl'011.

iv. Fusion of masses : e. g. of originally separate nerve ganglia (Vertebrates, Arthropods, Annelids), of myotomes, of somites in Artln'opods.

v. Attachment of one mass to another: e. g. of selerotome to notoehord.

It will be seen that this ;'¢7.s-/rzzr/‘"3 17 1C principal kinds of movement executed by the dev 11111:) ‘arts extends Ilis’s principle of the local inequality of growth from ﬂat layers to linear and massive aggregates and at the same time includes the movements of isolated cells. Davenport, however, is not content merely to give a simple classiﬁcation ol the phenomena; he goes further, and endeavours to express them in terms of responses to stimuli, an idea due in the first instance to Ilerbst. 12 INTRODUCTORY I

Thus he suggests that the migrations of vitellophags and mesenchyme cells, the thickenings, thinnings, and perforations of ﬂat layers, the rearrangements of cells in a massive aggregate, their dispersion, the constriction, and splitting and fusion may be regarded as tactic responses, the growth in various ways of linear aggregates, the eoncrescence of layers and masses as so many tropic responses to stimuli which may be positive or negative and exerted by other organs or by agents in the world outside.

Now it is clear that the analyses both of His and Davenport aim at something more than a mere description of ontogenctic events, for a serious attempt is here made to give a causal, if you will a mechanical, explanation of those events, and the subject thereby raised at once from the level of mere morphology or morphography to a loftier, aetiological point of view.

There are, indeed, two methods by which embryology, like any other branch of zoology, may be investigated. One is purely descriptive, anatomical, morphological. By this method, truly, great results have been achieved. The life—l1i_stories of members of all the most important groups of the animal kingdom have been worked out, and the science of Comparative Embryology has been built up. Nor has an explanation of the process been lacking. For ontogeny is, the fundamental Biogenetic Law assures us, a recapitulation of and therefore explicable in terms of phylogeny; and since on this principle the individual repeats in its development the ancestry of its race, embryology affords a means of tracing out the relationships of the organism and establishing the homologies of its parts.

Unfortunately a more intimate acquaintance with the facts has made it abundantly clear that development is no mere repetition of the aneert ‘é~\(_‘ "e1,tliat the organism has manifold ways of attaining its sin k i" ‘.',(\at those resemblances in early stages which were held to .,.:sm. bthe most triumphant vindication of the Biogenetic Law bear no constant relation to the similarities of adult organization, that the attempt to ﬁnd in development an absolute criterion of homology is vain.

The facts thus remain unexplained, as in truth it was only to be supposed that they would. A method, however comparative, which relies on mere observation, and is content to wait for Nature's own experiments, cannot hope to arrive at sound inductions, or to establish general laws of causation.

There is, however, another \vay. Development, the production of form, may be regarded as one of the activities, one of the functions of the organism, to be investigated. like any other function by the ordinary physiological method of experiment; and the ideal of the experimental or physiological embryologist is to give a complete causal account, whether the causes are external or internal, of each stage, and so of the whole series of ontogenctie changes, his weapon, to borrow Roux’s splendid phrase, ‘die Geistesanatomie, das analytische eausale Denken.’ 1

This effort is, of course, no modern one. Speculation into the nature and essence of development begins, indeed, with the Greeks, and theories of fertilization and development are to be found in the writings of Aristotle? In fertilization the male element, which, according to Aristotle, provides the formal and efficient causes in providing the necessary perceptive soul, acts upon the mere matter, endowed only with a nutritive soul, which is given by the female, in the same sort of way, to use his own illustration, as rennet coagulates milk. In the germ thus formed the parts of the embryo, which can only be said to pre-exist potentially, arise not simultaneously but in gradual succession, first the heart, then the blood, the veins from the heart and the various organs about the veins by a process of condensation and coagulation, the anterior parts of the body being built up first.

This Aristotelian doctrine appears to have persisted through the Middle Ages ; it reappears in the seventeenth century in the pages of Hieronymus Fabricius ab Aquapendentc and his pupil VVilliam Harvey in essentially the same form, although both authors diifer from Aristotle in certain matters of observational detail. Thus l*‘abricius3 states that ‘ ope generatricis facultatis pulli partes, quae prius non erant, produci atque ita ovum in pulli corpus migrate’, while Harvey“ gives to development as thus conceived of the name of ‘Epigenesin sivc partium superadditionem’, though he believes that in some cases (Insects) the

1 Roux, 1885. __ '2 Aristotle, De Gem, i. 20. 18; ii. 4. 43; 5. 2, 3, 10; 6. DeAM., 11.4. 2, 15 ; 5; 6. 3 Fabricius, 1. c., p. 22. ‘ Harvey, l. c., Ex. 44.

process is one of ‘metamorphosis’ or the simultaneous origin of all parts. In generation properly so called, however——in the development of the Chick, for exa1nple——tl1e process ‘a parte aliqua, tanquam ab origine, incipit; eiusque ope reliqua membra adseiscuntur: atque haec per epigenesin ﬁeri dieimus: sensim nempe, partem post partem’, and this ‘pars prima genitalis’ Harvey held, in opposition to Aristotle, to be the blood.‘

But in spite of the exact observation and brilliant exposition of his followers, the teaching of Aristotle was destined to be overshadowed and eclipsed, tempoiarily at least, by a new l1ypo— thesis which, appearing ﬁrst towards the end of the seventeenth century, swept the schools and universities, and dominated biolog-ieal speculation for a hundred years.

This was the theory of Evolution or Preformation. According to it the future animal or plant is already present in miniature in the. germ with all its parts complete, invisible or hardly visible it may be, but still there, and not merely ‘ potenti-ft’; and in development there is no such thing as ‘generation’, but only growth, whereby that which was before impalpable and invisible becomes tangible and manifest to our eyes. A further and logical outcome of the hypothesis was the doctrine of ‘emboitemcnt’, enthusiastically described by Bonnet as ‘une (les plus belles vietoires que l’entendement pur ait remporté sur les sens ’.3 The organism present already in the germ, with all its parts complete, possesses of necessity the germs of the next generation, and so on in indefinite though not in inﬁnite regress, for as Bonnet is careful to tell us, ‘ Il ne faut pas supposer un emboitcment in l’inﬁni, ce qui scroit absurde. La divisibilité de la matiére 51 l’inﬁni par laquelle on prétendroit soutenir eet emboitement est une vérité géométrique et une erreur physiqne.’3 Swammerdam solved the diﬂiculty in another way. All the’ germs of the human race must have been present. in the bodies of our ﬁrst parents, and ‘exhaustis his ovis humani generis ﬁnem adesse ’.“

The theory became widely held. First put forward by Marcello

‘ Harvey, l. e., EX. 50.

2 Bonnet, Cont. de la nm‘., 71"“ partie, c. ix ; (1a'uLv'cs, vol. iv, p. 270. 3 Bonnet, Consid. sur les rorps 0131., c. viii; (lﬁuvres, vol. iii, 1). 74.

‘ Swammerdam, 1679, pp. 21, 22.

Malpighi in the memoir, ‘De formatione pulli in ovo,’ which he presented to the Royal Society of London in 1673, it was not only adopted by biologists of prestige, by Swammerdam, Haller, who in his early days had been an advocate of Epigenesis, dc Buffon, and Bonnet, but secured the adherence of philosophers of such eminence as Malebranche and Leibniz.

In some cases it was accepted as a result of observation. Thus Malpighi,‘ in the treatise referred to, asserted that he had himself observed the chick in the unineubated egg, ‘ inelusum foetum animadvertebam, cuius caput cum appensae carinae staminibns patenter emergebat,’ while de Buifonz expresses himself even more categorically. ‘ J ’ai ouvert,’ he says, ‘ une grande quantité d’eeufs a diﬁérens temps, avant et apres Pineubation, et je me suis eonvaincu par mes yeux que le poulet existe en entier dans le milieu de la eicatricule a11 moment qu’il sorte (lu corps de la poule.’ To others, however, it was rather a matter of theoretical necessity. Ilaller explains his conversion from the contrary opinion by asking‘ the very pertinent question, ‘(‘ur vis ea essentialis quae sit unica tam divorsas in animali partes semper eodem loco, semper ad eun(lem arehetypum struit ; si materies inorganica mutabilis et ad omnem ﬁguram rccipiendam apta est ? Cur absque ullo errore ex gallinac mista materie ea vis semper pullum, ex pavo11e pavonem fabricatur? ’

‘Nil nisi vis dilatans et progrediens reeipitur. Ab ea nihil sperarem nisi vasorum rete tamdiu continue amplius futurum quamdiu vis expandens resistentiae superandae par est. Cur loeo eius retis cor, eaput, eerebrum, ren struuntur? Cur in singulo animali suus ordo partium? Ad eas quaestiones nulla. datur responsio,’ a charge which is, of course, perfectly just.“

Bonnet’s argument is different. The heart of the chick, he points out, is already present in the egg‘; and since anatomy teaches that all the parts of an animal ‘doivent avoir toujours coexisté ensemble’, preformation follows as a matter of course.“

The belief in preformation continued paramount till towards the end of the eighteenth ceutur , nor was it till the publication in 1774 of Caspar Friedrich Wolff's T/Ieoria Gczze/'atz'om'.9 that the evolutionists were aroused from their dogmatic slumbers. Putting speculation on one side, Wolff returned to the method of Harvey, Fabricius, and may we not say also of Aristotle, the method of exact observation. He demdnstrated the presence in the unincubated egg not of a complete organism, but of ‘globules’; ‘ partes enim eonstitutivae, ex quibus omnes corporis animalis partes in primis initiis componuntur, sunt globuli," and described the epigenetic formation of the heart and blood-vessels, the central nervous system, the limbs and the ‘ Wolffian’ bodies from these primary elements.

‘ Malpighi, 1. e., p. 4. 2 do Buffon, 1. c., p. 351. 3 Haller, 1778, VIII. i. 29, p. 121. _ ‘ Bonnet, Cont. de la nat., 7""? partie, c. ix; (lc'm‘)‘ﬂs, vol. 1v, 1). 261.

Development thus consists of the gradual production and organization of parts; ‘embryonis partes sensim produci, mea observata suadent,’ 2 and again, ‘ suppeditari prius partem, deinde cam organisari iutelligitur ’.3

The ground was thus taken from beneath the feet of the preformationists, and Epigenesis restored to its former place of honour as the fundamental expression of developmental fact.

Tacitly accepted by all the great embryologists of the nineteenth century—~Pander, von Baer, Reichert, Bischoif, Remak, Kolliker, Kowalewsky, IIaeckel——the epigenetie idea continued to control the progress of research. These were men who set themselves to describe the sequence of changes that the embryo passes through with all possible accuracy, and over as wide a range as might be of animal form. 'l‘hey made Comparative Embryology. On the facts that they discovered new light was shed by the doctrine of descent with m0(lilicati0n, or evolution in the wider sense of the word. Von Baer had pointed out that in any group of animals the embryos were more like one another than were the adult organisms, and this now became easily translated by Haeekel into the idea that the form which is in every g-roup——ultimately in all groups—thc common startingpoint of individual development is representative of the common ancestor of the race. Ontogeny was thus not merely expressed but explained in phylogenetic terms.

Now that, as we have already seen, the proposed explanation has very‘ largely broken down, Epigenesis, taken by itself, remains, not a theory in terms of cause and effect, but a mere description of what occurs, and it is the crying need for such a theory that has given birth to modern experimental embryology.

‘ Wolf, 1. c., Praemonenda, lxviii. 2 Id. ib. lxxiii. 3 Id. ib., De Gen. An., § 240.

The new era opens with the publication by Wilhelm His in 1874, just a hundred years after the appearance of the '1’/eeoria G'e2wratiom's, of a remarkable series of essays entitled Uusere K5f])87f07'7)l marl Jae 1;/1_'/siologisc/10 Problem 2'/zrer Eutste/nm_q. In these essays His, who was already in revolt against the ‘ Biogenctie Law’, not only sought to give a mechanical explanation of differentiation, but also laid down his famous ‘Prinzip der organbildenden Keimbezirke.’ According to this principle of ‘germinal localization’, every spot in the blastoderm corresponds to some future organ : ‘ (las Material zur Anlage ist schon in der ebencn Keimseheibe vorhanden, abcr morphologisch nieht abgcglie(lert, und somit als solchesnicht ohne Weiteres erkennbar.’ 1 Conversely, every organ is represented by some region in the blastoderm, and ‘ wenn wir consequent sein wollen’ in the fertilized, or even unfertilized, egg. In other words, although the parts of the embryo cannot be said to be preformed in the germ, the materials for those parts are already there, prelocalized, arranged roughly, at least, as the parts themselves will be later on. In this material unequal growth produces the form of the parts, and so of the whole body. Whether there is a strict causal connexion between each material rudiment and the organ which arises from it, whet-her these rudiments could be interchanged without prejudice to the normality of subsequent development, is a question which is not touched upon by His. It was reserved for another anatomist, Wilhelm Roux, to raise what in IIis’s hands had been merely a principle to the rank of a theory, the ‘ Mosaiktheoric ’, or theory of self-diiferentiation.

1 His, 1. c., §ii.

For Roux, no doubt, the ‘Mosaik-theoric’ was in part the outcome of the theoretical necessity of explaining the speciﬁc nature of development ; but it rests also upon a basis of observation and experiment. The coincidence in a majority of Frogs’ eggs of the first furrow with the sagittal plane, the production of local defects in the embryo by local injuries to the egg, th occurrence of certain natural monsters (Hemit/leria anteriora, for example) in which one half of the body is normally developed, the other entirely suppressed, and the experimental demonstration of the formation of a half-embryo from one of the first two blastomercs of the Frog’ s egg when its fellow had been killed, all led Roux to regard the development of the whole and of each part as essentially a process of self-differentiation, a process, that is to say, of which the causes reside wholly within the fertilized ovum and within each part as it is formed, though allowance was made for the possible formative inﬂuence of the parts on one another in later stages. External conditions, though they may be necessary in the same sense as they are generally necessary to the maintena.nce of life, are yet of no importance for diiferentiation regarded as a speciﬁc activity of the organism.

In the meantime, an experiment of Pfluger’s had apparently shown that, however obviously each part of the egg-cell might be related to the production of a particular organ, the relation was no necessary one, but that, on the contrary, the parts were all equivalent and the ovum ‘isotropic’. Pfliigcr demonstrated that in a Frog-’s egg which had been prevented from assuming its normal position with the axis vertical, the planes of the segmentation furrows bore no constant relation to the original egg-axis, that is to say to the structure of the egg, though they exhibited the same relation to the vertical as when de— veloping in the normal position. Further, in such forcibly upturned eggs the plane which included the original egg-axis and the present vertical axis became the median plane of the embryo, whose axes were disposed with regard to the vertical as in normal cases. Any part of the egg, therefore, might give rise to any part of the embryo, according to the extent to which and the direction in which the egg-axis had been diverted out of its original vertical position, and hence the egg-substance was ‘isotropic’ ; the planes of segmentation and the embryonic axes being determined by gravity. In fact, Pﬂiiger went so far as to say that an egg only becomes what it does become because it is always placed under the same external conditions.

Nor was this conception of the isotropy of the ovum invalidated by Born’s proof that in these eggs there is a redistribution of yolk and protoplasm owing to the sinking of the former to the lower, the rising of the latter to the upper side of the egg. For though the egg thus comes to acquire a secondary structure about an axis which is vertical, still the arrangement of the parts of the supposed rudiments of the organs must have been disturbed. Yet from such ova normal tadpoles are developed.

It became necessary, therefore, to locate the self-differentiating substance, the ‘idioplasma’, mainly, at any rate, in the nucleus; and this idioplasma was imagined as composed of dissimilar determinant units, each representative of some part or character of the organism and arranged according to a plan or architecture which corresponds in some way with the architecture in the embryo of the organs represented. All that is necessary, then, and all that happens, at least in the early stages of development, is the gradual sundering of these units from one another by successive qualitative divisions of the nucleus and their distribution to the cytoplasm, where each determines the assumption by the cell to which it is allocated of that character which it represents.

Roux’s ‘Mosaik—theoric’ (Roux, 1903) and Weismann’s very similar but more elaborate hypothesis of the constitution and behaviour of the germ-plasm both frankly involve the belief that every separately inheritable quality of the body has its own representative in the germ, with the difference, however, that this preformation, extended by Weismann to the adult characters, is limited by Roux to those of the embryo. The renewed inquiry into the nature and essence of development has thus simply resulted in the resuscitation of the eighteenth-century doctrine of evolution, though in a far more subtle form. Once again we ﬁnd ourselves face to face with the old alternative, Preformation or Epigenesis ,- and it is to the desire of solving this problem that a very considerable proportion of modern experimental research is attributable. Though much of this has been directed against and been destructive of the ‘ Mosaik-theorie’, which as far as the nucleus is concerned has now been abandoned by Roux himself} renewed investigation has proved the existence in many cases of definite and necessary organ-forming substances in the cytoplasm, while the necessity for ﬁnding a causal explanation of what is obviously in some sense a predetermined process, without presupposing the preformation in the germ of morphological units representing every possible iuheritable character, has issued in Herbst’s and Driesclfs conception of the events of ontogeny as so many responses to stimuli exerted by the developing parts on one another. At the same time the need for inquiry into the external conditions and the part they may play in growth and differentiation has not been forgotten.

Thus though it would be vain to pretend that the ideal of a complete causal explanation has yet been realized, still some material has been gathered for an answer to each of the two main questions: what are the internal, what the external conditions that determine the course of development? These questions we shall discuss in the following pages. It will then only remain to inquire whether a causal explanation—-in the accepted sense of the phrase—a mechanics of ontogeny which resolves the single occurrences ﬁrst into general physiological laws, and these in the last resort into the generalizations of physics and chemistry, can ever afford a theory which may be said to be complete either from a scientific or from a philosophical point of view. Should the mechanical explanation prove to be scientiﬁcally insufficient, it may be necessary, with Driesch and the neo-vitalists, to invoke a consciousness of the end to be realized to guide and govern the merely material elements; but even were this not so it would still be incumbent upon us to consider whether the end itself—not the consciousness of it—is not the final, and yet none the less the principal determining cause of the whole process.

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Cite this page: Hill, M.A. (2024, April 23) Embryology Book - Experimental Embryology (1909) 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Experimental_Embryology_(1909)_1

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