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==Contents==
==Contents==
[[Book - Experimental Embryology (1909) 1|Chapter I Introductory]]
[[Book - Experimental Embryology (1909) 1|Chapter I Introductory]]


[[Book - Experimental Embryology (1909) 2|Chapter II Cell-Division And Growth]]
[[Book - Experimental Embryology (1909) 2|Chapter II Cell-Division And Growth]]
 
# Ce1l-division
1. Ce1l-division
# Growth
2. Growth


[[Book - Experimental Embryology (1909) 3|Chapter III External Factors]]
[[Book - Experimental Embryology (1909) 3|Chapter III External Factors]]
 
# Grravitation
1. Urrnvitation
# Mechanical agitation
 
# Electricity and magnetism
2. Mechanical agitation
# Light
 
# Heat
Electricity and magnetism
# Atmospheric pressure. The respiration of the embryo.
 
# Osmotic pressure. The role of water in growth
Light
# The chemical composition of the medium
 
# Summary
Heat
 
Atmospheric pressure. The respiration of the embryo.
 
Osmotic pressure. The role of water in growth
 
The chemical composition of the medium
 
Summary


[[Book - Experimental Embryology (1909) 4|Chapter IV Internal Factors]]
[[Book - Experimental Embryology (1909) 4|Chapter IV Internal Factors]]


(1) The initial structure of the germ as a cause of differentiation.
(1) The initial structure of the germ as a cause of differentiation.
# The modern form of the preformationist doctrine
# Amphibia
# Pisces
# Amphioxus
# Coe-lenterata
# Ecliinodcrmata
# Nemertinen
# Ctenophora
# Chaetopoda and Mollusca
# Ascidia
# General considerations and conclusions
# The part played by the spermatozoon in the determination of egg-strucure
# The part played by the nucleus in differentiation


1. The modern form of the prefurmationist doctrine
(2) The actions of the parts of the developing organism on one another
2. Amphibia
3. Pisces
4. Amphioxus
5. Coe-lenterata
6. Ecliinodcrmata
7. Nemertinen . . . . . . . . 204
8. (.‘tenopho1':i . . . . . . . . 208
9. Chaetopoda and Mollusca . . . . . . 213
10. Ascidia . . . . . . . . . 229
11. General consiileratious and conclusions . . . 240
12. The part. played by the spernmtozoon in the determination of egg-.<ztructm'e . . . . . 247
13. The part played by the nucleus in ilifl'e1'enti;iti0n . . 251
(2) The actions of the parts of the developing oiganism on
one another 271


[[Book - Experimental Embryology (1909) 5|Chapter V Driesch’s Theories Of Development - General Reflections And Conclusions]]
[[Book - Experimental Embryology (1909) 5|Chapter V Driesch’s Theories Of Development - General Reflections And Conclusions]]
Line 109: Line 97:


APPENDIX A
APPENDIX A
On the .’~y)1)l)lL'l2l'y of the egg, the symmetly of scglnentation, and the symmetry of the embryo in the Frog  
On the symmetry of the egg, the symmetry of segmentation, and the symmetry of the embryo in the Frog  
 


APPENDIX B
APPENDIX B


On the part played by the nucleus in (lifferenti:L’tion
On the part played by the nucleus in differentiation
 
I.\'m<;x or AUTl{0I{
 
Ixmzx or SUBJPJCTS
 
ADDENDA
 
==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.
 
===Literature===
 
ARISTOTLE. Dc gcneratione animalium, ed. Bekkcr, Oxford, 1837. De partibus animaliuin, ed. Bekker, Oxford, 1837. De anima, ed. Trendelenburg, Berlin, 1877.
 
C. BONNET. (Euvres, Neuchatel, 1781. Contemplation do In. nature, vii“‘° partie; Considerations sur les corps organises; Mémoire sur les gerlnes; Palingénésie philosophique.
 
G. BORN. Ueber den Einfluss der Schwere ant‘ das Froschei, Arch. mi/rr. Anaf. xxiv, 1885. I INTRODUCTORY 2 1
 
L. DE I1UFI~‘()N. Histoirc nuturelle, génc’-mle et. pzl1'ti('111ii:1‘e, vol. ii, 1’zu'is, 1749.
 
C. B. DAV!-:N1*oR'1‘. Studies‘ in morphogenesis: iv. A prelimimtry catalogue of the processes concerned in ontogeny, Bull. 1Iarmr«l Mus. xxvii, 1896.
 
H. DRIESOII. Anzrlytische Theorie der orgzmischen Entwicklung, Leipzig, 1894.
 
IL FABRICIUS AB AQUAPENDENTE. De formatione ovi et pulli. Opera. omnia, Lipsiae, 1687.
 
A. HALLER. Hernmnni Boerhanve pmelectiones ncndemicae: edidit at notas addidit Albertus Haller, v, Gottingne, 1744. Prinme lineae physiologiae, Gottingae, 1747. Elementa. physiologiae corporis humani, Lausannae, 1778.
 
W. HARVEY. Exercitationes dc genemtione :mima.1ium, London, 1651.
 
C. Ilmmsr. Ueber die Bedeutung dcr Reizphysiologie fiir die emisale A11f'f'zLss11ng' V011 Vorga"mgeu in der thierisclien Ontogenese, Biol. (‘mil-albl. xiv, xv, 1894, 1895.
 
\V. HIS. Unsere Korperforin nnd das physiologisclle Problem ihrer Entstehung, Leipzig, 1874.
 
G. W. LEIBNIZ. (Euvres philosophiques, ed. by 1’. Janet, Paris, 1900. Systeine nouvenu de 1:1. natlire, 1695. Principes de ]:1. nature at de 1:1. gr-free, 1714. Essnis do Théodicée. Préfmze, 1710.
 
N. MALEBRANCHE. Recherche dc la. vérité, Si1non’s edition, Paris, 1846.
 
M. MALPIGIII. De formatione pulli in ovo, London, 1673.
 
E. PFLUGER. Ueber den Einfluss der Schwerkmft uuf die Theilung der Zellen und auf die Entwieklung dos Embryo, Pflc7,«/m".s- An-h. xxxi, xxxii, 1883.
 
W. ROUX. Ein1eit.11n;;' zu den ‘ Bcitrfigen zur Entwicklungsmechunik des Embryo ’, Zeitsc-hr. Biol. xxi, 1885 ; also Ge.»-. Abh. 13, Leipzig, 1895.
 
W. Roux. Ueber die kiinstliehe Hervorbringung ‘ha.1ber’ Embryonen, V4'rrIum‘s Arch., 1888 ; also Gas-. Abh. 22.
 
W. Roux. Ueber Mosuikarbeit mid neuere Entwieklungslnypothesen, Anat. Ilrjffe, 1893 ; also G'(‘.s'. Abh. 27.
 
VV. Roux. Einleitung, Am-I1. Eut. Jllwclz. i, 1895.
 
VV. ROUX. Fiir unser 1’rogramn1 und seine Verwirkliehung, Arrlz. Ent. Mech. v, 1897.
 
VV. RUUX. Ueber die Ursuchen der Bestimmun,g' der H:lll])1.l'iC]|tnngen des Elnbryo im Frosclici, Anat. Anz. xxiii, 1903.
 
\V. ROUX. Vortri'Lgc und Aufsdtzc fiber Entwvicklungsliieelmnik (ler O1'ga.nismen, Leipzig, 1905.
 
J. SWAMMERDAM. Mimculum na.tura.c sive uteri muliebris fabrica, Lugdunulu Buta.vorun1, 1679. Histoire génémle des Insectes, Utrecht, 1682.
 
C. F. WOLFF. 'I‘heo1-in generationis, Hnlle a. d. Szmle, 1774.
 
==Appendix A==
 
FURTHER REMARKS ON  RELATION BETWEEN THE
SYMMETRY OF THE EGG, THE SYMMETRY OF SEG
MENTATION, AND THE SYMMETRY OF THE EMBRYO IN
THE FROG.
 
IN the measurements, referred to above (pp. 165-8), of the
angles between the plane of symmetry of the egg (as determined by
the position of the grey crescent), the first furrow and the sagittal
plane of the embryo, it was found (1) that there was a certain
tendency for the first furrow and the sagittal plane to coincide,
since in a. large number of cases small angles preponderated over
large ones, the standard deviation of this angle from the mean
(which was practically = 0°) being a- = 40-39° i-65 ; (2) that
there was a much greater tendency for the plane of symmetry
and the sagittal plane to coincide, the standard deviation of the
angle between these two planes being o'=29-75° _-J; -63 ; (3) that
the first furrow tended either to coincide with or to lie at right
angles to the plane of symmetry, the standard deviation about 0°
being 18-70° i -60, that about 90° being 23-29° j-_ -86, the value
of 0' for all the observations being 47-90° 1- 1-19. The
correlation between the first furrow and the sagittal plane was
found to be p=-138i -031, that between the plane of symmetry
 
and the sagittal plane p=-372i -025, that between the plane of
symmetry and the first furrow p=-O87 i -032.
 
These results may be tabulated as follows :
rr :0
40-39° + -65. -138 i -031.
 
tal Plane.
 
Plane of Symmetry and
Sagittal Plane. l 2975 -t '63’
 
Plane of Symmetry and
First Furrow.
 
First Furrow and Sagit- }
 
.372 i .025.
 
} 47.9oi1.19. .os7¢_.o32.
 
Full details of these results will be found in a paper in
Biometrika V. 1906.
 
For the purpose of making these measurements the eggs were
placed in rows parallel to the [mat]; of glass slides, and the
angles measured between the various planes and lines ruled
across the slide. Such eggs compress one another by their jelly coats; further, the eggs taken ‘from the uterus were placed
haphazard on the slides with the axis making any direction with
the vertical. The egg takes about half-an-hour to turn into its
normal position with the axis vertical, and during this interval
gravity may possibly act upon the yolk and protoplasm, of
different specific gravities, and impress a plane of bilateral
gravitation symmetry upon the egg, as occurs when the egg is
permanently inverted (see above, pp. 82-87). This obliquity of
the axis may possibly afiect the relations between the planes,
and the mutual compression may also be a disturbing factor,
since it is known that in compressed eggs the nuclear spindle is
perpendicular to the direction of the pressure (pp. 34-36).
 
These angles have therefore now been measured under four
different conditions:
 
(a) The eggs are close to one another in the rows and the axis is
horizontal.‘ (Since the rows are parallel to the length of the slide
the pressure, if any, must be in the same direction, while the
surfaces of compression or contact are across the slide. The eggs
were always so placed that the vegetative poles faced in one
direction and the planes of ‘ gravitation symmetry ’ were at right
angles to the length of the slide. This holds good of all the
following experiments.)
 
(/3) The eggs close, but the. axis vertical with the white pole
below. In these there can be no gravitation plane of symmetry.
 
(y) The eggs spaced, but the axis horizontal. In these the
jellies do not touch.
 
(6) The eggs spaced and the axis vertical. In these, therefore,
both the supposedly disturbing factors are removed. The results
are given in the following table :—
 
A B C
 
First Furrow and Plane of Symmetry Plane of Symmetry
Sagittal Plane. and Sagittal Plane. and First Furrow.
 
(.7) .7 = 38-42 g._ -70. .7 = 31-86: -56. .7 = 41-591-_-84.
,7 = -201;-028. ,7 = -263;:-_-027. ,7 -= -118;-029.
(.9) .7 = 33-443-_-56. .7 = 30-17:51. .7 = 39-7l_-1;-61.
,7 = 3523-021. .7 = .27si.o22. ,7 = .o23¢.o24.
(-,) .7 —_- 33-49;:-_-96. .7 = 27-53¢-84. .7 = 36-60: 1-108.
,. = .292:-039. —_— -399:-036. p = .075:-043.
(a) .7 = 31.45133. .7 —_— 26-80¢-82. .7 = 34-46:1-065.
,7 —_- -364:-033. ,7 = -451 1.035. ,7 -_- -186;-043.
 
It is evident from this that gravity and ‘ mutual compression ’
(as I will for the moment term it, though it is doubtful whether
the pressure has anything at all to do with the result) do affect the
 
magnitude of the angles between these three planes, for in each case the standard deviation falls, while the correlation coeflicient
rises, when they are both removed. It will be observed that,
while gravitation (y) has less eflect than compression (3) upon the
angles B and C, the reverse is the case with the angle A. We
may be able to find a reason for this later on.
 
There is one point worth noticing. It is quite clear that
gravity is not indispensable for the development of a grey
crescent and plane of symmetry, though it is true that the position
of this plane may be aifected by gravity even in the short interval
that elapses before the egg turns over.
 
The values for the compressed eggs with horizontal axes (or)
compare fairly well with those previously obtained, except in the
case of the plane of symmetry and the first furrow. In the
former series the latter tended either to coincide with or to lie
at right angles to the former. In the present series this is not
the case. This diflerence is probably to be attributed to the
fact that many of the eggs in the first series must have been
placed on the slide with the white pole upwards: possibly also
the ‘ compression ’ was greater then than now.
 
It is fortunate that the same data enable us to study exactly
the relation between the first furrow and the plane of symmetry
on the one hand, and the direction of ‘compression’ and of the
gravitation symmetry plane on the other. It must be remembered that these two are at right angles to one another.
 
Consider first the first furrow.
 
(a) When the eggs are close but the axis horizontal the first
furrow tends to lie at right angles to the slide, that is, in the
direction of compression, but at right angles to the gravitation
symmetry plane. (a-=38-16 i -69.)
 
(fl) When the eggs are close but the axis vertical this tendency
is not quite so marked. (a'=46-67 i -7' 1.)
 
(y) When the eggs are spaced and the axis horizontal it is
still there, but slight. (o-=49-32 ;l-_ 1-40.)
 
(6) When the eggs are spaced and the axis vertical the
direction of the first furrow is random. (zr=52-76¢ 1-17.)
 
VVe may conclude, therefore, that the first furrow tends to lie
in the direction of the ‘ compression’ and at right angles to the
plane of gravitation symmetry. The latter tendency, we know,
exists in forcibly inverted eggs, together with a tendency to lie
in the plane of symmetry and at 45° to it (above, p. 84).
Pressure experiments alo show that division is in the direction
of pressure (p. 34 sqq.).
 
The direction taken up by the plane of symmetry under these
different circumstances is-quite distinct from that of the first
furrow. It appears to be determined in the first instance by gravitation, as it usually lies in the gravitation symmetry plane.
It is not, however, only so determined, for if the eggs (compressed
and with axi horizontal) be allowed to develop in the light the
plane of symmetry lies either in the gravitation symmetry plane,
or in the direction of the incident light (parallel to the length of
the slide in the experiment , while in the dark it lies only across
the slide. That this secon effect is due to the light and not to
the pressure is shown by the fact that it occurs when the eggs
are spaced, and that it may be made to vary in position by
varying the position of the slide with regard to the light.
Light, therefore (ordinary daylight), as well as gravity, can help
to determine the -position of the plane of symmetry, and when
the latter is excluded it appears that this plane is placed either
in or at right angles to the source of light.
 
Light appears to exert no effect u on the first furrow.
 
It is now intelligible why, when 1 these factors are operative,
the relation between the first furrow and the planes of symmetry
of egg and embryo should be disturbed, since, in the conditions
of the experiment, those factors which determine the position of
the former are at right angles to those on which the direction of
the latter depends.
 
It still remains for us to inquire into the internal causes of
the direction of these planes in the egg. Roux, as has been
pointed out, has asserted that the grey crescent appears on the
opposite side of the egg to that on which the spermatozoon has
entered (pp. 80, 165), and further that the point of entry of the
sperm also determines the meridian of the first furrow, since this
either includes the sperm-path, or is parallel to it, or, when it is
crooked, includes or is parallel to the inner portion or ‘ copulation ’
path, which is taken to represent the line of approximation of
the two pronuclei; the outer part being simply the ‘ penetration’
path. Roux also arbitrarily selected a fertilization meridian
(meridian of the sperm-entry), and showed that this became
the ventral side (opposite the grey crescent) later on, as well as
the ineridian of the first furrow (p. 248).
 
I have been able to accurately investigate—by means of
sections-—the relation between the fertilization meridian, first
furrow, and sperm-path in a number of eggs in which the
direction of the symmetry plane had been previously determined,
and the results of the measurements of these angles are given
here. The eggs fall into two series, those which were compressed
and had their axes horizontal (a), and those which were spaced
and had their axes vertical, the white pole being below (6). In
(a) the gravitation symmetry plane and the direction of compression were at right angles to one another, as before.
 
8 a
Meridian of sperm entry a- = 21-02° 1-_ 1-63. o- = 31-04°: 1-34.
and first furrow. p = -435 3 -074. 9 =-613 i -038.
 
Meridian of sperm entry 0' = 25-67° i 1-35. 0 = 41-01° 3-_ 1-78.
and symmetry plane. p = -302 i -083. / P = -006 1 -061.
 
SP§;§§f:;l3(g;§‘}ff1,§§;“ } . .. .—. 17.94° : 1.15. o‘ = 21.47° 1 -93.
 
From this it is clear that there is a very close relation indeed
between the point of entry of the spermatozoon and the direction
of the first furrow, especially when the disturbing efiects of pressure and gravity are removed. There is, however, little relation
between the sperm meridian and the plane of symmetry even
under the most favourable circumstances, and when the conditiofis are not favourable the correlation is negligible. There is
however (in the 6 series) a considerable correlation (p = -479 i '070)
between the sperm-pat/l and the plane of symmetry. It should
be remembered, however, that all these eggs were exposed to the
light. From what we know of the eifect of this agent upon the
direction of the symmetry plane, it would not perhaps be too
hold a hazard to surmise that in darkness there would be a
correlation between the sperm entrance and the plane of symmetr .
 
Eiien after the removal of this disturbance there remain
factors which interfere with the completeness of the correlation
between these planes; these must probably be looked for in
the incomplete radial symmetry of certain eggs—due possibly
to pressure in the uterus—and to the slight squeezings and distortions the eggs may be subjected to when they are being taken
from the Frog.
 
It will be seen that the relation between the sperm-path and
first furrow is closer than that between the latter and the sperm
entrance. This is because though the furrow may be placed to
one side of the entrance point, it may still be parallel to the path ,
or, if not to the ‘penetration ’ path then to the inner or ‘copulation ’ path, as observed by Roux. This ‘ copulation’ path is
usually observed when the penetration path is turned away from
the first furrow, that i, when it has not been directed towards
the egg-axis.
 
The same data give the position of the point or of entrance
with regard to the direction of ‘pressure ’ and ‘gravitation
symmetry’. In the (a) series the sperm tends to enter in
the direction of ‘pressure’, that is, on that side of the egg on
which it is in contact with its neighbours. Hardly a single
spermatozoon enters on that side of the egg on which the white
pole had been turned up, and very few on the opposite side.
 
It is scarcely possible to suppose that either the compression of
the egg or the gravitation plane brings the spermatozoa round to
the side of compression, but it may be imagined that either by
capillarity or by some chemotactic stimulus the spermatozoa are
especially attracted to the point where the rapidly swelling coats
of adjacent eggs come into contact, and that therefore fertilization
is principally effected upon this side. This explains why the first
furrow lies so often in this direction. The pressure may of course
afiect the position of the planes in the egg later on.
 
When the eggs are spaced the sperm enters on any side at
random.
The deviation of the sperm entrance from the egg-axis (the
angle between sperm-entrance radius and egg-axis) varies in the
two series of observations. When the eggs are spaced and the
axes vertical, the sperm enters mainly near the equator, never
near the animal pole; when the eggs are compressed and the axis
horizontal, usually at about 45° from the axis, though it may
enter near the pole or near the equator. This difierence obviously
depends on the diiference in the initial position of the eggs on
the slide. The deviation has apparently very little effect on any
of the planes we have been considering.
 
Finally, let us try and gain some conception of the mechanism
by which the direction of the furrow depends on the point of sperm
entry. It is apparently quite simple, for the sperm-path is
directed usually towards the axis, the sperm nucleus travels along
that path to meet the female nucleus, which is also in the axis,
the centrosome of the sperm divides at right angles to that path,
the fertilization spindle is developed between the diverging
centrosomes and cell-division takes place in the equator of the
spindle ; the first furrow includes therefore the sperm-path.
Should, however, the ‘penetration ’ path not be exactly radial,
for whatever reason, the sperm nucleus turns aside to meet the
female pronucleus, there is a ‘ copulation’, as distinct from a
‘ penetration’ path, the centrosome divides at right angles to
the former, and this, then, is included in or parallel to the plane
of the furrow. In those cases in which the sperm-path is parallel
to the furrow it is always quite close to it, and we may suppose
perhaps that the first division "has not been quite equal. (The
division of the centrosomes has not, I believe, been observed in
the Frog, and the foregoing description has been taken from the
Axolotl. In this genus the definitive centrosome is formed from
the sperm nucleus, when the latter has already penetrated some
little way into the egg.) .
 
The causes of the formation of the grey crescent which marks
the symmetry plane are not so clear.
 
 
Roux describes it as being due to the immigration of superficial pigment. Now we have strong reason for believing that
both the entrance-funnel——produced when the spermatozoon first
touches the egg-—and the sperm-sphere are local aggregations of
watery substance. The accumulation of what appears to be a
more watery substance about the middle piece which has been
observed in the Axolotl,appears also to occur in the Frog: at least
the same formation of large clear vacuoles in the sperm-sphere may
be seen in the latter as in the former. Should this be actually so,
we may suppose that the streaming movement centred in the
entrance-funnel and sperm-sphere is responsible for drawing away
the pigment from a certain region of the surface; hence the grey
crescent. The sperm-sphere is on the inner side of the sperm
nucleus: hence the grey crescent would appear on that side of
the egg which is opposite to the entrance of the spermatozoon,
should no disturbance of the streaming movement have taken
place, and, since the sperm-path is radial, would be symmetrically
disposed with regard to it. In this case, fertilization meridian,
sperm-path, grey crescent and plane of symmetry, first furrow,
and, later on, sagittal plane, would all coincide. There is, as
we have seen, a very fair correlation between the sperm-entrance
and the first furrow, and again between the sperm-path and the
grey crescent. But should some other streaming movement of
the cytoplasm be set up by the gravitation of the heavy yolk
particles, or by pressure, or by light, then the relation between
the two processes, the division of the centrosome which determines the direction of the first furrow, on the one hand, and
on the other, the streaming movement towards the sperm-sphere
which determines the position of the grey crescent, would be
disturbed, and while the entrance point of the sperm might still
continue to determine, though not so completely, the position of
the furrow, it might come to be without relation to the symmetry
of the egg and of the embryo; and this is what is actually
observed.
 
Though it is diflicult to assign the exact cause of each and
every deviation from the rule, this much is certain, that however
they may coincide in ‘typical’ development (I use R0ux’s
expression), the factors which determine cell-division, and those
which determine differentiation, may be influenced by different
external causes in widely diifering ways, and are therefore presumably distinct. Nor does this artificial separation of the two
processes in any wise prejudice the complete normality of the
 
development of the embryo".
 
 
Lillie has shown (Jozmz. Esp. Z002. iii. 1906) that in the egg of
C’/Iaetopterus there are granules of difierent kinds which pass, in
segmentation, into definite cells. By means of the centrifuge
some of these--the endoplasmic—-may be driven to one side of
the egg, but in whatever position these organ-forming granules
may be thus artificially placed, the cleavage has the same relation
to the egg axis (as determined by the polar bodies) as in the
normal egg. The factors of cell-division are thus separable from
those of differentiation.
 
To the cases quoted in the summary on pp. 245, 246 might be
added the various instances in which an egg may be made, by
heat or pressure or shaking, or in artificial parthenogenesis, to
segment abnormally and yet give rise to a normal larva.
 
==Appendix B==
 
ON THE PART PLAYED BY THE NUCLEUS IN DIFFERENTIATION
 
(i) BOVERI has more recently (Zellen-Studim, vi, Jena, 1907)
published a very elaborate account of the irregularities produced
by dispermy in Echinoid eggs, in which are brought forward
 
still more facts in proof of the qualitative difference of the
chromosomes.
 
As has been stated above, p. 263, dispermy is induced by
the simple expedient of adding a large quantity of sperm to the
eggs. The following types of dispermy are distinguished.
 
A. Tetracentric, i. e. each sperm centre divides.
(i) 'I‘etraster, with four spindles.
 
(ii) Double spindle, i. e. the female and one male pronucleus
lie in one spindle, the other male lies aside in its spindle.
 
B. Tricentric, one sperm centre remaining undivided.
(i) Triaster, a tripolar figure with three spindles.
 
(ii) Monaster-amphiaster, the undivided sperm centre remaining apart with one sperm nucleus.
 
C. Dicentric, neither sperm centre dividing.
(i) Amphiaster, a spindle is formed between the two centres.
 
(ii) Double monaster: the centres remain apart, one with
one male, the other with the other male and the female
pronucleus.
 
The segmentation of these eggs is as follows.
 
The tetraster divides simultaneously into four, which may
either lie in one plane if the divisions are meridional, or be tetrahedrally arranged. In the first case another meridional division
ensues, followed by an equatorial, then ‘eight micromeres are
formed, eight macromeres, and sixteen mesomeres. In the latter
case not more than three cells can share in the micromere region
and only four or six of these are produced. The triaster eggs,
having divided simultaneously into three (meridionally), subsequently show six micromeres, six macromeres, and twelve
mesomeres.
 
The segmentation of the double spindle eggs is interesting and
important. Usually the egg divides across the two spindles
312 APPENDIX B
 
into two binucleate cells, but it may divide at once into four, or
into three, one of which is binucleate. The interest lies in the
binucleate cells, for they continue to produce uni-nucleate and binucleate cells until the latter divide simultaneously into four,
and this simultaneous division may sometimes involve an irregular
distribution of the chromosomes, with fatal consequences to the
cell. Bovcri had already produced evidence of the evil effects of
an irregular distribution of the 3 n x 2 chromosomes present in
triasters and tetrasters. A more detailed account is now given.
 
Of the tripartite (triaster) ova about 8 % on an average produced Plutei. In these larvae three regions may be distinguished
in the egg by the size of the nuclei (proportional to the number
of chromosomes) and the boundaries between them may be shown
to correspond to the divisions between the three blastomeres.
The form is asymmetrical in skeleton and pigment, but Bovcri
shows that both sides are normal, as though the larva had been
compounded of two types such as occur, as individual variations,
in any culture. It is suggested therefore that the slight differences in the two sides are due to difierences in the two sperms.
 
Some of the larvae have partial defects in skeleton or pigment,
or the skeleton may be much reduced on one side, or one-third of
the cells may be pathological, i. e. disintegrate in the segmentation
cavity, while the remaining two-thirds are sound and sometimes
symmetrical. In this case it is supposed that the degenerate
cells had separated from the others at an early stage, and that
the remainder had had time to recuperate. In others two-thirds
are degenerate, one-third normal, or all three degenerate. When
the three blastomeres are isolated and allowed to develop independently, segmentation is partial, with two micromeres, two
macromeres, and four mesomeres, and often all three develop
normally up to the blastula stage. After that only one or two,
rarely all three, become Plutei, the rest giving rise to stereoblastulae or stereogastrulae, full of degenerating cells.
 
The isolated quarters of tetrasters also segment partially
and normally, but few give rise to Plutei. The whole simultaneously quadripartite eggs only rarely give rise to what may be
called a Pluteus (2 cases in 1500) ; but very degenerate larvae
are found, with masses of disintegrating cells inside, which are
assigned to one of the four blastomeres. Stereogastrulae-—with
nuclei of all the same size--are frequent.
 
As has been alread mentioned, Bovcri points out that the
probability of each cell’ of a triaster receiving a complete set of
the 71. chromosomes of the species when there are 3 n x_2 to be distributed must be greater than‘ that of each cell of tetraster
obtaining a full complement, and the probability for one isolated cell must be greater than that for the whole egg. What the
mathematical values of these probabilities are Boveri does not
know, though he makes an attempt to reckon them—not
theoretically, but by means of a mechanical apparatus; the
attempt is not quite successful. The fact, however, remains that
eight per cent. of the triasters produce normal Plutei, only -06 per
cent. of the tetrasters. This does not depend on the cells receiving
too much or too little chromatin (see p. 265), nor again on the
fact that the ratio between size of nucleus and size of cytoplasm
(see pp. 268, 269) can only be satisfied by certain definite
numbers of chromosomes, and the only explanation remaining is
that for normal development of each and every part the nucleus of
each cell must contain a complete set of the specific chrosomomes ;
from which it follows that the chromosomes are qualitatively
unlike.
 
A word may be said about the double-spindled eggs (Type
A. i). The larvae from these sometimes show abnormal regions,
and this is attributed to one or more of the binucleate cells
having divided with a tetraster and irregular distribution of
chromosomes. Of all such eggs 50 % gave rise to normal Plutei.
 
The degenerative changes undergone by the nuclei of these
larvae are of several types, to be associated again with differences
in the combinations of chromosomes.
 
(ii) Boveri’s experimental proof of the qualitative difference
of the chromosomes does not of course of itself involve a belief
in the individuality of these bodies, for if the chromatin is
concerned in inheritance, it is necessary to suppose that the
number of qualitatively distinct bodies is far greater than the
number of chromosomes, and these bodies may be differently
grouped during each successive resting stage.
 
The hypothesis of the individuality of the chromosomes, i.e. of
a constancy in the manner of grouping of these particles, rests
in the first instance on such facts as those observed by Sutton in
B2-ac/:3/stola, where in the spermatogonia the chromosomes are of
dilferent sizes, which may however be arranged in pairs, together
with an odd one or accessory chromosome. 1 In the resting stage
the accessory chromosome remains apart in a separate vesicle,
while the large chromosomes lie in separate pockets of the
nuclear membrane, the small ones, each as a separate reticulum,
in the main body of the nucleus. In the spermatocyte a number
of bivalent spiremes appear, which show the same dilferences of
sizes a the pairs of chromosomes previously, and the accessory
chromosome.
 
The accessory chromosome passes into two only of the four
spermatids and is supposed to be a sex-determinant.
 
 
Similar facts have been reported by Wilson for several Insects
(see Joum. Esp. Zool. ii, iii, 1905, 1906). '
 
Wilson finds constant size differences between pairs of chromosomes, and either an accessory odd chromosome (which passes
into only one half of the germ cells) or a pair of idio-chromosomes of unequal size (one of which goes to one half, the other to
the other half of the spermatozoa), or both the accessory and the
idio-chromosomes (giving four kinds of spermatozoa). The idiochromosomes are supposed, again, to play a part in sex-determination. Several other observers have found these accessory
chromosomes, idio-chromosomes, and pairs of chromosomes of
difierent sizes in various Insects (Boring, Journ. E211. Zool. iv.
1907 ; Stevens, ibid. ii. 1905, v. 1908; McClung, Biol. Bull. iii.
1902, ix. 1905; Montgomery, Biol. Bull., vi. 1904; Baumgartner, Biol. Bull. viii. 1904-5 ,- Zweiger, Zool. Anz. xxx. 1906;
Nowlin, Jomw. Exp. Zool. iii. 1906); in Spiders (Wallace, Biol.
Bull. viii. 1904»-5 ; Berry, Biol. Bull. xi. 1906); and in Myriapods (Blackman, Biol. Bull. v. 1903 ; Medes, Biol. Bull.
ix. 1905).
 
It is a noteworthy fact that the accessory chromosome retains
its individuality in the resting stage (looking like a chromatin
nucleolus), while the others break up. The belief in the individuality of these others rests therefore on the constancy of the relative sizes from generation to generation.
 
Further support for the hypothesis may be derived from theoretical speculations. VVe know that only 2; (one-half the normal
number) chromosomes are necessary for normal development
provided that they comprise a complete set. In sexual reproduction n maternal unite with n paternal. A study of the reducing division shows that 1: whole chromosomes first pair with
and are then separated from or whole chromosomes, and that
when they dilfer in size those of the same size pair together, and
it looks as though paternal were here separated from maternal,
though the distribution of paternal and maternal to the two cells
will difier, almost certainly, in diiferent cases.
 
If the particles of which the chromosomes are composed are
also to be paired and separated, it would appear to be necessary
that their groupin should be constant, in other words that the
chromosomes shou d retain their individuality.
 
(iii) A case of heterogeneous fertilization between eggs of Seaurchins and the sperm of Anletlon has been described above
(p. 262). Loeb has recently succeeded in rearing Plutei from
the eggs of Slrongylocmlrolue fertilized by the sperm of a
Mollusc (0/lloroaloma). Cytological details are not given (Arc/E.
Eul. Mecfi. xxvi. 1908). ‘
 
 
==Index Of Authors==
 
Agassiz: effects of fertilization in Ctenophors, 250.
 
Aristotle: theory of development, 13.
 
— the soul in function and development, 292 sqq.
 
— mechanism and teleology, 296.
 
Auerbach :' segmentation of Ascuris
nigrovenosa, 33.
 
von Baer, 16.
 
Balfour: effect of yolk on segmentation, 29, 88.
 
Bataillon: monstrosities
osmotic pressure, 120, 135.
 
—- artificial parthenogenesis, 124.
 
Bergh: cell-division in germ-bands
of Crustacea, 34.
 
Berthold: surface-tension and celldivision, 41, 42.
 
Bischofl‘, 16.
 
Blane: effect of light upon the
development of the Chick, 94, 96.
 
Boas: rate of growth in man, 63.
 
— change of variability, 73, 74.
 
— diminution of correlation coefiicient, 75.
 
Bonnet : emboitement, 14.
 
— preformation, 15.
Bonnevie : diminution of chromosomes in Ascaris lumbricoidcs, 258.
Born : gravity and development, 18,
88-85.
 
— pressure experiments on Frogs’
eggs, 34, 35.
 
Boveri : early development of Slrongylocentrotus, 23, 183-185.
 
— egg of Strongylocentrotus stretched,
39.
 
— suppression of micromeres in
Strongylocentrotus, 186.
 
-— causes of the pattern of segmentation, 197.
 
— karyokinetic plane, sperm path,
:11 ng first furrow in Strongylocentrotus,
 
8 .
 
— potentialities of? animal and vegetative cells, 192.
 
— stratification of cytoplasmic substances, 242, 280.
 
-- characters dependent on cytonlmam in Flnhinnid larvae. 261.
 
due to
 
Boveri : diminution of chromosomes
in Ascaris megalocephala, 252, 255-257.
 
— due to a difference in the cytoplasm, 257.
 
— hybrid larva from enucleate egg
fragment with characters of male
parent, 253, 258-260.
 
— irregular distribution of chromosomes a cause of abnormality, 253,
263-266.
 
— individuality of chromosomes and
chromatin, 256, 263.
 
—part played by nucleus in differentiation, 266, 285.
 
—possiblo significance of reducing
divisions, 266.
 
— number of chromosomes, size of
nucleus, and size of cell, 68, 267,
268.
 
—2méclear division not qualitative,
 
6 .
 
Bowditch: rate of growth in man,
63.
 
-- change of variability, 73.
 
Brauer : Branchipus, 22, 24.
 
Brooks: Lucifer, 22.
 
de Butfon : Preformation, 15.
 
Bullzt: artificial parthenogenesis,
12 .
 
Bumpus: change of variability in
Litlorina, 71, 72.
 
Bunge: respiration of Ascaris, 112.
 
Castle : see Davenpofl: and Castle.
 
Chabry: segmentation furrows and
embryonic axes in Ascidians, 229.
 
—- development of isolated blastemeres in Ascidians, 229, 230.
 
Child : critique of Driesch’s vitalism,
292, note.
 
Chun : isolated blastomeres of Ctenophora, 209.
 
Conklin: maturation, fertilization,
and development of Cynthia, 230236.
 
— development of isolated blastemeres in Oyntlzia, 237.
 
— development of pieces of gastrula
in Cynthia, 238.
 
— streaming movements of protonlnsm. 40.
316 INDEX OF
 
Crampton : isolated blastomeres of
Ilycmesaa, 215, 216.
 
— efieot of removal of the polar lobe,
217.
 
Dareste: mechanical agitation of the
Hen’s egg, 89.
 
— electricity, 91.
 
Davenport : catalogue of ontogenetic
processes, 4 sqq.
 
— definition of growth, 58.
 
— rate of growth, 69.
 
— the role of water in growth, 58,
59, 115, 116.
 
- and Castle : acclimatization of eggs
of Bufo to heat, 100.
 
Delage : causes of artificial parthenogenesis, 124.
 
-- number of chromosomes in artificial parthenogenesis and in merogony, 125.
De Vries : importance of potassium
for turgor of plant-cells, 146.
 
Doncaster: hybrid Echinoid larvae,
26].
 
Driesch: effect of light in development, 94.
 
— abnormal segmentation in Erhinus
produced by heat, 105.
 
— Anenteria, produced by heat,
106.
 
—- segmentation made irregular by
dilution of sea-water, 118.
 
—— pressure experiments on Echinoid
eggs, 37, 38, 185, 240.
 
—- cell-division suppressed by pressure and dilute sea-water, 55; and
by heat, 105.
 
—nuclear division not qualitative,
186.
 
— blastomeres disarranged, 187, 188.
 
— isolated blastomeres of Echinoids,
190, 191, 193, 194.
 
— potentialities of animal and vegetative cells, 193, 194, 201, 242, 243.
 
— fragments of blastulae and gastrulae in Echinoderms, 194.
 
— potentialities of ectoderm and
agghenteron, and their limitations,
1 .
 
— development of egg fragments of
Echinoids, 195, 196.
 
— germinal value, surface-area of
larvae, and number of cells, 197199, 269.
 
— one larva from two blastulae, 202.
 
— and Morgan : isolated blastomeres
of Ctenophora, 210, 211.
 
—2e1gg-fragments of Ctenophora, 30,
 
2!
 
AUTHORS
 
Drgggchz development of Myzostoma,
 
— isolated blastomeres and parts of
larvae in Phallusia, 288, 289.
 
— first furrow and sagittal plane in
Echinoids, 250.
 
— characters which depend on cytoplasm in Echinoid larvae, 261, 262.
 
— number of organ-forming substances in cytoplasm, 246, 284,
286.
 
—— theory of egg-structure, 281, 286,
292.
 
— reason for limitation of potentialities, 192-194, 201, 212, 242, 243,
281, 282, 284, 291.
 
--fate a function of position, 188,
282.
 
—- return of displaced mesenchyme
cells in Echinus, 274.
 
- stimuli in ontogeny, 20, 277, 28"284.
 
— part played by nucleus in differentiation, 266, 284, 285.
 
—— equipotential and inequipotentiul
systems, 176, 277, 285.
 
— rhythm of development, 3.
 
—- harmony of development, 284.
 
—- composition in development, 3,
285.
 
— self-difierentiation, 284.
 
—- teleology, static, 286, 291, 292,
297.
 
— —- dynamic, 291, 292, 297.
 
— vitalism, 20, 289 sqq.
 
Edwards : physiological zero for
Home egg, 102.
 
-- growth without differentiation,
104.
 
Endres and Walter : post-generation
of missing half-embryo, 171.
 
Eycleshymer: first furrow
sagittal plane in Necturus, 168.
 
and
 
Fabricius : views on development,
13.
 
Fasola : electric currents, 91.
 
Fehling : growth of the human
embryo, 59, 60, 63.
 
Feré : effect of sound-vibrations upon
the Chick, 90.
 
_ ._ of light, 96.
 
— malformations due to high temperatures, 105. .
 
—- need of oxygen for the Chick, 109.
 
—— monstrosities produced by various
chemical reagents, 18,2.
INDEX OF AUTHORS
 
Fischel, A. : hybrid Echinoid larvae,
261.
 
— variability of Duck embryos, 71.
 
Fischel, H. : isolated blastomeres of
Ctenophora, 210, 211.
 
-— derangement of blastomeres in
Ctenophora, 211.
 
Fischer: artificial parthenogenesis,
124. ’
Foot : polar rings in Allolobophom,
 
251.
 
Garbowski : function of pigment
ring in Strongylocentrotus egg, 192.
— first furrow and sagittal plane in
 
Echinoids, 260.
 
— grafting of blastulae fragments of
Echinus, 202.
 
Gerassimow: size of nucleus and
cells in Spirogyra, 269.
 
Giacomini: need of oxygen for the
Chick, efiect of low atmospheric
pressure, 109, 110.
 
Giardina : difierentiation of chromatin in female cells of Dytiscus.
 
Godlewski : the respiration of the
Frog’s eg, 110, 112, 113.
 
-— heterogeneous cross-fertilization,
262.
 
Graf : fusion of blastomeres, 56.
 
Greeley: artificial parthenogenesis
produced by cold, 108.
 
— low temperatures and absorption
of water, 108.
 
Grobben : Cetochilus, 22.
 
Groom : effect of fertilization in
Cirripedes, 250.
 
Gigiber: regeneration in Protozoa,
 
54.
 
Gurwitsch : monstrosities produced
in Amphibian embryos by chemical
reagents, 120, 123.
 
Hacker : Cyclops, 22.
 
Haeckel: recapitulation, 16.
 
— development of fragments of
blastulao of Crystallodes, 181, note.
Hr;ller : preformation and epigenesis,
 
5.
 
Harvey: epigenesis, 13.
 
— metamorphosis, 14.
 
Hecker: growth of the human embryo, 62, 63.
 
Hansen: growth of guinea-pig embryos, 62.
 
Herbst : potassium, sodium, and
lithium larvae of Echinoderms,
136-140.
 
—- significance of monsters for origin
of variatiops, 141.
 
317
 
Herbst : necessity of elements present
in sea-water for normal development of Echinoid larvae, 141 sqq.
 
—— separation of blastomeres of Seaurchins in calcium-free sea-water,
 
45.
 
— stimuli in ontogeny, 20, 272, 273,
285.
 
— formation of Arthropod blastederm oxygenotactic, 114.
 
—— arms of Plutous due to presence of
skeleton, 187, 138, 144, 149, 274, 275.
 
I-Ierl itzka, development of half-blastomeres of Newt, 173.
 
Hertwig, 0. : centrifugalized Frog’s
egg, 29, 87.
 
—- rules for nuclear and cell division,
31, 32, 85.
 
— — confirmed by pressure experiments, 34-36.
 
— gravity and Echinoderm eggs, 78.
 
—— insemination of Frog's egg, 79.
 
— cardinal temperatures for Rana
 
fusca. and csculenta, 97.
 
— monstrosities produced by high
and by low temperatures, 99.
 
— temperature and rate of development, 100.
 
—— monstrosities produced in Amphibian embryos by sodium chloride,
119, 135.
 
— first furrow and sagittal plane in
Frog's egg, 165.
 
— compressedeggs: disproof of qualitative nuclear division, 34—86, 168,
169, 240.
 
— development of half-blastomere of
Frog’s egg, 169.
 
— mutual interactions of developing parts, 271, 285.
 
Hertwig, 0. and R. : fertilization
processes altered by heat and cold,
107.
 
— — by alkaloids, 126 sqq., 263.
 
His: mechanical explanation of
development, 3.
 
—- germinal localization, 17, 158.
 
— the blastoderm oxygenoti-opic,114.
 
Hunter: artificial parthenogenesis
by concentrated sea-water, 124.
 
Iijima: spiral asters in Nephelis egg,
40.
 
Jenkinson: pressure experiments on
eggs of Antedon, 37, note.
 
— abnormalities of Frog embryos
produced by various solutions not
due to increased osmotic pressure,
120, 133-136.
318
 
Jenkinson: plane of symmetry, first
furrow and sagittal plane in Frog's
egg, 165-168.
 
Jennings: fertilization spindle in
Asplanclma, 34.
 
Kaestner: cardinal temperature
points for the Hen‘s egg, 102.
 
— malformations due to low tem~
peratures, 104. '
 
Kant : teleology, 286-289, 292, 297.
 
Kastschenko: injuries to blastoporic
lip in Elasmobranchs, 178.
 
Kathariner: gravity and the gray
crescent of the Frog's egg, 86.
 
King : cause of differentiation of lens,
276, 276.
 
Knowlton : sec Lillie and Knowlton.
 
Kolliker: 16.
 
Kopsch : first furrow and sagittal
plane in Frog's egg, 165, 168.
 
—— efl'ect of injuries to blastoporic lip,
178.
 
Korschelt: fusion of ova in Ophryotmcha, 202.
 
— nucleus of egg-cell in Dyfiscus, 252. .
 
Kostanecki and Wierzejski: efi'ect of
fertilization in Physa, 250.
 
Kowalewsky: 16.
 
Kraus : the role of water
growth of plants, 58.
 
Lang : effect of fertilization in Polyclads, 250.
 
Leibnitz : preformation, 15.
 
Lewis: causes of formation of lens
and cornea, 275, 276.
Lillie and Knowlton: eflect of low
temperatures in Amphibia, 100.
— temperature and rate of development, 101.
 
Lillie: effects of salts on ciliary
movement, 135.
 
— ghysiologically balanced solutions,
1 6.
 
in the
 
— toxicity and valency, 136.
 
Loeb : suppression of cell-division
in Echinoids and Fishes, 56, 117.
-— eflect of light in development, 94.
—the respiration of Otmolabrua and
 
Fundulua eggs, 111.
 
—— the respiration of the ova of
Echinoids, 112.
 
— function of oxygen in regeneration
of Tubular-ia head and other processes, 114, 278, 274.
 
-— efi'ect of hypertonic solutions on
Fundulus and Arbacia eggs, 117.
 
--exovates produced by dilute seawater, 118, 190, 194, 195.
 
INDEX or AUTHORS
 
Loeb: artificial parthenogenesis,
121, 124.
 
—- etfect of potassium cyanide in prolonging life of ova, 131, 132.
 
— eflect of certain salts on Fundulus
embryos and on Plutei, 135.
 
— toxicity and antitoxicity functions
of valency, 186.
 
-— effect of alkalies, 151.
 
— effect of gravity on Anmmularia,
272, 273.
 
-gégterogeneous cross-fertilization,
 
Lombardini : electric currents, 91.
 
Lyon : need of oxygen for the eggs of
Arbacia, 112.
 
— action of potassium cyanide, 132.
 
Malebranche : preformation, 15.
 
Malpighi: preformation, 14, 15.
 
Marcacci : mechanical agitation of
Hen's eggs, 90.
 
Mark: spiral asters in eggof Lz‘maac,40.
 
Mathews: artificial parthenogenesis
by mechanical agitation, 90.
 
—— effects of atropine and pilocarpine
on Echinoderm eggs, 131.
 
—toxicity and decomposition tension,
136.
 
— see also Wilson (E.B.)and Mathews.
 
Mencl : formation of lensin SaImo,276.
 
Metsclinikoif : separation of blastemeres of Oceania, 181.
 
-—fusion of blastulae in Mitrocoma, 202.
 
Minot : rate of growth defined, 60.
 
—— change of rate of growth of guineapigs, 61.
 
— - of rabbits, 62, 68.
 
— — ofchickens, 67.
 
— coeflicients of growth, 65.
 
— senescence, 65.
 
-- increase of cytoplasm, decrease of
mitotic index, 65.
 
— change of variability in guineapigs, 71. _
— genetic restriction, 246, 277.
Mitrophanow: malformations due to
low and high temperatures, 104.
— necessity of oxygen for the Chick,
109.
 
Moore : sodium sulphate an antidote
to sodium chloride, 135, 186.
 
Morgan : suppression of cell-division
in Arbacia, 56, 118.
 
- gravity and the gray crescent of
the Frog's egg, 86.
 
-— monstrosities produced by low
temperatures in Ranapaluslris, 100.
 
— need of oxygen for the Frog's egg,
110.
INDEX OF AUTHORS
 
Morgan :lithium salts used to produce
alzlgéiormalities in Frog's eggs, 120,
 
— attempts to induce
parthenogenesis, 124.
 
— number of chromosomes in artificial parthenogenesis, 125.
 
— artificial parthenogenesis produced
by cold, 108.
— first furrow, plane of symmetry,
and sagittal plane in Frog's egg,
165,168.
 
— development of half-blastomere of
 
Frpg's egg ; post-generation, 170,
 
17 .
 
— development of vegetative cells of
Frog’s egg, 173.
 
— potentialities of half-blastomeres
in Teleostei, relation of flrstfurrow
tn sagittal plane, effect of removal
of yolk, 178.
 
— effect of injuries to blastoporic lip,
179.
 
— number of cells in partial larvae
of Amphioxus, 181.
 
— potentialities of ectoderm in
Echinoids, 195.
 
— development of egg-fragments of
Echinoids, 197.
 
— number of cells in partial larvae
of Echinoids, 198.
 
— fusion of blastulae of Sphaerechinua,
201.
 
— and Driesch: isolated blastomeres
and egg-fragments of Ctenophora,
210-212.
 
— micromercs of Ctenophore egg, 30.
 
—- characters of hybrid Echinoid
larvae, 260.
 
Moscowski : gravity and the gray
crescent of the Frog's egg, 86.
 
Miihlmann : prenatal growth-rate
in man, 64.
 
artificial
 
Nfigeli : permutations of original
elements in development, 286.
 
Pander: 16.
 
Pearson : variability in man, 73.
 
Pfliiger: isotropy of the cytoplasm,
18, 158.
 
—--influence oi’ gravity on development, 18, 78, 81-83, 168.
 
-- rule for direction of nuclear
division, 32, 85.
 
Plateau : principle of least surfaces,
41, 43.
 
Platnerz 280.
 
Pott : growth of the Chick, 59, 60, 67.
 
319
 
Pott and Preyer: respiration of the
Chick, 112.
— loss of weight of Hen’s egg due to
evaporation from albumen, 115.
Preyer : rate of growth, 60.
 
Quetelet: change of rate of
in man (weight), 68.
 
— — (stature), 69.
 
— — (other dimensions), 90.
 
growth
 
Rauber : efiect of reduced atmospheric pressure on the Frog’s egg,
110.
 
— elfect of pure oxygen on the eggs
and tadpoles of the Frog, 118, 114.
 
Reichert: 16.
 
Remak : 16.
 
Robert : mechanics of spiral segmentation, 45-47.
 
— rate of growth in man, 68.
 
—-— change of variability, 73.
 
Rossi : efi‘ect of electricity on
Amphibian eggs, 91.
 
Roux : aims of experimental embryology, 13.
 
— ‘Mosaik-Theorie ’ of self-differentiation, 17, 158, 279, 286, 297.
 
— qualitative nuclear division abandoned, 19, 159, 240.
 
— idioplasm and reserve-idioplasm,
159, 266.
 
— a half-embryo from one of first
two blastomeres and post-generation of missing half, 159, 162.
 
— coincidence of first furrow and
sagittal plane in Frog's egg, 17, 159,
165. '
 
— the spermatozoon and symmetry
of the Frog's egg and embryo, 80,
165, 247, 248.
 
— meaning of karyokinesis, 252.
 
— dependent diflerentiation, 17, 158,
277, 286.
 
— functional adaptation, 290.
 
-— specific gravity of contents of
Frog’s eg, 79.
 
—- gray crescent of Frog's egg, 80, 165.
 
— influence of gravity on the Frog's
egg, 85-87.
 
— effect of electricity upon the Frog’s
egg, &c., 92.
 
— light and development, 93.
 
— segmentation of Rana esculenta, 26.
 
—- Frog's eggs compressed in small
tubes, 39, 40.
 
— comparison of systems of oil drops
and segmenting ova, 49-58.
 
— cytotropism, 55, 278.
320
 
Roux: cytotaxis, 55.
 
— cytochorismus, 45.
 
-— cytarme, 45, 53.
 
— cytolisthesis, 58.
 
— ‘ Framboisia’, 135.
 
Ruseoni : electric currents, 91.
 
Sachs : law of direction of cell
division, 28.
 
Sala: fertilization processes altered
by cold, 108.
 
- fusion of the eggs of Ascaris, 202.
 
Samassa: effect of pure oxygen at
pressures on the Frog's egg,
 
— effect of lack of oxygen on the
Frog's egg, 119.
 
— effect of various gases on the eggs
of Ascaris, 112.
 
—development of animal cells of
Frog's egg, 173.
 
— Schaper: development of tadpoles
after removal of brain and eyes,
175.
 
—- cause of differentiation of lens,
275.
 
Schulze, F. E. :
Sponges, 22.
Schulze, 0.: gray crescent of Frog’s
 
eg, 80, 247.
 
—— gravity and the Frog’s egg, 86.
 
—- effect of low temperatures on the
Frog's egg, 100.
 
—— first furrow and sagittal plane in
Frog's egg, 165.
 
— double monsters from Frog’s egg,
171.
 
Seeliger : hybrid Echinoderm larvae,
260, 269.
 
Selenka: first furrow and sagittal
plane in Echinoids, 250.
 
Semper: rate of growth in Limnaea, 67.
 
Smith: Peltogaster, 24.
 
Sollmann : after effects of hypertonic
solutions, 124.
 
Spemann : development ofconstricted
Newt's eggs, and embryos, 174, 175.
 
— causes of formation of lens and
cornea, 275, 276.
 
Sumner: injuries to blastoporic lip
of Teleostei, 178, 246.
 
Sutton {individuality of chromosomes
in Brachyslola, 256.
 
Swammerdam : preformation, 14, 15.
 
segmentation of
 
Vejdovsky : unequal centrosomes in
dividing pole-cells, 31.
 
— polar rings in Rhym.-hclmis, 251.
 
Vernon: rate of growth in Strongmlocmtrotus, 67, 70.
 
INDEX or AUTHORS
 
Vernon : alteration of variability in
Echinoid larvae, 71, 74.
 
-— effect of light on Echinoid larvae,
95, 96. '
 
— effects of change of temperature
on Echinoid larvae, 106, 107.
 
-— change of variability produced
by heat, 107.
 
— and by chemical agency, 141, 156.
 
—poisonousness of carbon dioxide
to Sea-urchin eggs, 112.
 
— characters of hybrid Echinoid
larvae, 261.
 
Verworn : behaviour of Protozoa in
an electric current, 93.
 
— regeneration in Protozoa, 254,
note.
 
Walter, sec Endres and Walter.
 
Weber : law of stimuli, 272.
 
Weismann: qualitative
division, 19, 297.
 
— idioplasm, and reserve—idioplasm,
159.
 
Weldon : growth-rate in Carcinus, 71.
 
— change of variability in Carcinus,
72.
 
— — in Clausilia, 73.
 
Wetzel : double monsters
Frog’s egg, 172, 245.
 
Whitman : polar rings in Clepsine,
251.
 
Wierzejski, see
Wierzejski, 250.
 
Wilson, 0. B. : malformations of
Amphibian embryos, 120.
 
— acclimatizution to salt-solution,
136.
 
Wilson, E. B. :
phioxus, 26.
 
—— segmentation of Renilla, 55, note.
 
— unequal centrosomes in dividing
pole-cells, 31.
 
—pressure experiments on eggs of
Nareis, 39, 213, 240.
 
- cytology of artificial parthenogenesis, 124.
 
— development of isolated blastemeres in Amphioxus, 179, 180.
 
—— isolated blastomeres of Oerebratulus,
and fragments of blastulae, 205,
206.
 
— isolated blastomeres of Patella,
218-222.
 
—- of Dentalium, 225, 226.
 
—— removal of polar lobe, 224.
 
— effect of fertilization, 222, 223.
 
— development of egg-fragments,
226, 227.
 
nuclear
 
from
 
Kostanecki and segmentation of Am
 
Wilson (E. B.) and Mathews : spermpath, egg axis, fix-st furrow, and
embryonic axes of Toacopneustes,
185, 249, 250. ‘
 
Windle: effect of magnetism and
electricity on development, 91.
 
Wolff : epigenesis, 16. '
 
Yatgu: egg-fragments of Cerebratulus,
 
2 7.
 
Yung: effect of light on tadpoles,
etc., 94.
 
Zeleny : egg-fragments of Cerebratulus,
206, 207.
 
Zelinka : fertilization
Callidma, 34.
 
spindle in
 
Jnxntsonr’ Y
 
Ziegler : heterodynamic centrosomes, 80.
 
.— formation of micromeres in Cteno
phora, 209, note.
 
-— pressure experiments on egg
gaéiinoids and Ctenophora,
 
— fertilization of Diplogaster, 84.
 
— egg and embryonic axes, 250.
 
Zoja : isolated blastomeres of Hydromedusae, 181, 182.
 
—— animal and vegetative cells of
Strongylocentrotus, 198.
 
Zur Strassen : segmentation of
Asoaiis, 81.


— fusion of the eggs of Ascaris.
Index of Authors


s of
Addenda
88,


==Addenda Et Corrigenda==
==Addenda Et Corrigenda==

Latest revision as of 16:06, 2 April 2019

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

Jenkinson (1909): 1 Introductory | 2 Cell-Division and Growth | 3 External Factors | 4 Internal Factors | 5 Driesch’s Theories - General Conclusions | 6 Appendices
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Experimental Embryology

Experimental Embryoijogy

By

J. W. Jenkinson. M.A.. D.Sc.

Lecturer in Embryology in the University of Oxford

(1909)

Preface

For the biologist there are, I conceive, in the main two problems. One is to give an account of those activities or functions by means of which an organism maintains its specific form in an environment. The other is to find the causes which determine the production of that form, whether in the race or in the individual. The solution of the first of these problems is the business of physiology, in the usual sense of the term. The second falls to morphology.


It is with the origin of form that we are here concerned, and in particular with its origin in the individual. The endeavour to discover by experiment the causes of this process — as distinct from the mere description of the process - is a comparatively new branch of biological science, for Experimental Embryology, or, as some prefer to call it, the Mechanics of Development (Entwicklungsmechanik), or the Physiology of Development, really dates from Roux's production of a half-embryo from a. half-blaatomere, and the consequent formulation of the ‘ Mosaik-Theorie’ of self-differentiation. That hypothesis has been the focus of much fruitful criticism and controversy, the experiment has been followed by many others of the same kind, and the present volume is an attempt to sketch the progress of these researches and speculations on the nature and essence of differentiation, as well as of those which deal with growth, cell-division, and the external conditions of development.


In writing this review I have had the very great advantage of an excellent model in the textbook of Korsehelt and Heider (Lehrbuch cler fucrgleichemleat Entwio/cluugsgeschiclzte (Zer 1ve'rbelZo.~e'n T/u'c7'e, Allgemeiner Theil, Jena, 1902). I have indeed followed the general arrangement adopted by these authors fairly closely except in one respect. I believe so strongly that the processes of growth and cell-division, though they always (in the Metazoa) accompany, are yet distinct from, differentiation, that I have felt justified in treating them in a chapter apart from the other internal factors of development. The external factors—whether of growth, celhdivision, or differentiation - are discussed in Chapter III, and the ground is thus cleared for a consideration of the real problem — the differentiation of specific form.

The last chapter is devoted to the theories, scientific and philosophical, of Hans Driesch. I sincerely hope that Herr Driesch will allow my great admiration for the former to atone in some measure for my inability to accept the tenets of nee-vitalism.


It is a very great pleasure to me to acknowledge my indebtedness to the Delegates and Secretaries of the Clarendon Press, and in particular to Professor Osler, for undertaking the publication of this book, as well as for the pains which have been expended in its preparation. Dr. Osler also took the trouble to read through the whole of the manuscript, and Mr. G. V. Smith and Dr. Haldane have been kind enough to look through certain chapters.


To Dr. Ramsden I am under great obligations for his assistance in that part of Chapter II, Section 1, in which surface-tensions are discussed; to Dr. Vernon for calling my attention to Roberts’s work on Anthropometry, and to Mr. Grosvenor for the information embodied in the foot-note on p. 89. Mr. A. D. Lindsay has given me invaluable assistance in those sections of Chapter V which deal with the philosophy of Kant, while, for Aristotle, I was fortunately able to attend Professor Bywater’s lectures on the De Anima.

I can hardly express the debt I owe to Mr. J. A. Smith for much friendly counsel and criticism, although he is, of course, in no way responsible for the philosophical speculations in which I have ventured to indulge.


The illustrations are largely borrowed from Korschelt and Heider’s work, and I must thank Herr Gustav Fischer, of Jena, for his readiness in supplying the blocks. Others are from the original publications‘, and I am obliged to the proprietors for permission to make use of them. A few are my own.


In the appendices will he found an account of some recent work on the relation between the symmetry of the egg and that of the embryo in the Frog, and on the part played by the nucleus in ditt'c1-entiation.

Proceedings of the Boston Society of Natural History, the Journal of Experimental Zoology (Williams 8; Wilkins, Baltimore), the Anm'ir(rn Journal of I‘hysz'ulo_'/_I/ (Ginn & C0., Boston), ZeIIrn~Sfu(Iim (Fischer, Jena), l’erhamIlmI_r/en 410;" A/mlumis-1-hm G(‘.s'¢'”N(‘7I((fl (Fischer, Jena), Er;/cbnisse fiber din Ii'on.m'tzm'ou dcr cIu'onmta'scIzm Kernsubslmz: (Fischer, Jena), .[r¢-kin fiir mik)'osk0])i.s¢*7¢1: .»lm¢tomi(' (Cohen, Bonn), Archizv ff/"r Entwiclcluuysnwvlzanik (Engelinunn, Leipzig), and the Popular Science .llontM3/ (Appleton & Co., New York).

Contents

Chapter I Introductory

Chapter II Cell-Division And Growth

  1. Ce1l-division
  2. Growth

Chapter III External Factors

  1. Grravitation
  2. Mechanical agitation
  3. Electricity and magnetism
  4. Light
  5. Heat
  6. Atmospheric pressure. The respiration of the embryo.
  7. Osmotic pressure. The role of water in growth
  8. The chemical composition of the medium
  9. Summary

Chapter IV Internal Factors

(1) The initial structure of the germ as a cause of differentiation.

  1. The modern form of the preformationist doctrine
  2. Amphibia
  3. Pisces
  4. Amphioxus
  5. Coe-lenterata
  6. Ecliinodcrmata
  7. Nemertinen
  8. Ctenophora
  9. Chaetopoda and Mollusca
  10. Ascidia
  11. General considerations and conclusions
  12. The part played by the spermatozoon in the determination of egg-strucure
  13. The part played by the nucleus in differentiation

(2) The actions of the parts of the developing organism on one another

Chapter V Driesch’s Theories Of Development - General Reflections And Conclusions

Appendices

APPENDIX A On the symmetry of the egg, the symmetry of segmentation, and the symmetry of the embryo in the Frog


APPENDIX B

On the part played by the nucleus in differentiation

Index of Authors

Addenda

Addenda Et Corrigenda

P. 5, 5 lines from bottom, for unicellular read multicellular. P. 28, line 10, after irregular, insert and in Triclads.

P. 57. To Literature acid J. Sacns. Die Anordnung den-Zellen in jiingsten Pflanzentheilen, Arb. Bot. Inst. Wurzburg, ii, 1882. _

P. 114. To Literature add G. BUNGE. Weitere Untersuchungen iiber die Athmung der Wiirmer, Zeitsc-hr. physiol. Chem. xiv, 1890.

P. 140, line 22, for prospective potentialities read prospective significanoes.

P. 225, 2 lines from bottom, for is now placed in road has now moved into.

P. 271. To Literature add W. S. Surrox. On the morphology of the chromosome group in Brachyslola magna, Biol. Bull. iv, 1902.

P. 278. To Literature add J. W. Jnxxmsox. On the effect of certain solutions upon the development of the Frog's egg, Arch. Ent. Mech. xxi, 1906.


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