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


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


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


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


CELL-DIVISION AND GROWTH.
(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. Ce1l-division
(2) The actions of the parts of the developing organism on one another
2. Growth


CHAPTER III
[[Book - Experimental Embryology (1909) 5|Chapter V Driesch’s Theories Of Development - General Reflections And Conclusions]]


EXTERNAL FACTORS.
[[Book - Experimental Embryology (1909) 6|Appendices]]


1. Urrnvitation
APPENDIX A
 
On the symmetry of the egg, the  symmetry of segmentation, and the symmetry of the embryo in the Frog
2. Mechanical agitation
 
Electricity and magnetism
 
Light
 
Heat
 
Atmospheric pressure. The respiration of the embryo.
 
Osmotic pressure. The role of water in growth
 
The chemical composition of the medium


Summary
CHAPTER IV
INTERNAL FACTORS.
(1) The initial structure of the germ as a cause of differentiation.
1. The modern form of the prefurmationist doctrine
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
Cll.\l"l‘El{ V
Dmi~:scu‘s '1‘1n:on11a.x' or l)l'2\.'l-2l.()l‘M.l-INT. (}J«:.\'r;RAI. ll:-:i«‘m:c~
noxs AND (,.‘0Nc1.1.'s1o.\'s . . . . . . 27!)
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


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.
 
==Chapter II==
 
CELL-DIVISION AND G ROWTII
 
1. CELL-DIVISION
 
IN a future chapter we shall see that there is no necessary
connexion between segmentation and differentiation. Nevertheless, since ccl1~division is the first sign, or almost the first
sign, that a developing organism gives of its activity; since,
moreover, cell-division accompanies the later processes of growth
and differentiation, we may briefly discuss what is known of
those factors which determine the direction of division in general,
and in particular the pattern of segmentation.
 
We shall first presume that segmenting ova may be grouped
under several distinct types, as follows :—
 
1. Tim radial I_1//2e. Here the first division is meridional, the
second meridional and at right angles to the first, the third
equatorial—or more often latitudinal—-and at right angles to
both the preceding, the fourth meridional and at forty-five
degrees to the first two, the fifth latitudinal. What is characteristic above all of this type is, first, that four surfaces of
contact between cells meet in one line; for example, the four
surfaees between the first four blastomeres meet in the egg-axis,
while each pair of animal cells lies exactly over each pair of
vegetative cells after the third division; and secondly, the
blastomeres are radially arranged about the axis. This type has
been observed in Sponges (Syconl (Sehulze)), in Coelenterates,
in Crinoids, Holothurians, and Echinoids (Fig. 12) amongst the
Echinoderms, in Eetoproctous Polyzoa, in A111];/ziowz/.e and the
 
Vertebrates, and in some C1'11stacea—Cc/or/zilzts ((:‘rrobben),’
 
Luc-g'f'er (Brooks), Cyclops (Hiickcr), L’/‘awe/u'p11.v (Braucr) and
some Cirrhipedes. Certain of these cases present special
 
peculiarities.
In Echinoids micromeres are formed at the vegetative pole
 
by the division of the fourth phase (Fig. 12,
 
' In 831001: the third cleavage is meridional, the fourth latitudinal.
FIG. 12.-Normal development of the sea-urcliin SI;'ong3/Ionenlrolus
”l’i4Il(.\‘. (After Boveri, 1901.)
 
The animal pole is uppermost in all cases, and in the first two figures
the jelly with the canal (micropyle) is shown.
 
a, primary oocytc, the pigment is uniformly peripheral.
 
b, ovum after extrusion of polar bodies. The pigment now forms
a subequatorial band. The nucleus is ex-axial.
 
c, d, first division (meridional).
 
e, 8 cells, the pigment almost wholly in the vegetative blastomeres.
 
f, formation of mesomeres (animal cells) by meridional division: the
vegetative cells have divided into macromeres and micromeres.
 
_r/, blastula. h, mesenehyme hlastula.
 
1', j, k, imagination of the pigmented cells to form the archenteron of
the gastrula. In j the primary mesenchyme is separated into two
groups, in each of which, in Ir, a spicule has been secreted. In is the
secondary, pigmented mesenchyme is being budded off from the inner
end of the archenteron.
24 CELL-DIVISION AND GROVVTH II. I
 
In Asteroids and Ophiuroids the division is at first tetrahedral,
and to be classed, therefore, with those of the following type;
after the second furrow, however, the blastomeres are rearranged,
and division theneeforward is radial.
 
In Vertebiata segmentation is altered in megaleeithal eggs by
the amount of yolk present. It becomes meroblastic; still the
radial type is preserved, though the sequence of the furrows is
often altered, the third, for instance, being frequently meridional,
and the fourth latitudinal. Amongst the Ascidians Pg/rosoma
has a large-yolked, telolecithal, and radially segmenting egg.
In the Placental Mammals the first two (livisions may conform
to this type; but segmentation soon becomes irregular. The
accumulation of yolk in the Arthropod egg has resulted in
a totally different type of meroblastic segmentation. The yolk
is here uniformly distributed about the central protoplasm. In
the latter is placed the segmentation nucleus, and this central
mass divides into a number of cells, which subsequently migrate
to the surface and form a blastoderm ; the egg is then centrolecithal (Fig. 1). The stages of the development of this modification may be seen in the Crustaeea. In certain forms-——thosc
alluded to above (with the exception of the (‘irrhipedes)——
division is holoblastic and radial. In (fam/mru.\', Branc/zipux
(Brauer), Pellogastcr (Smith) segmentation is at_first total, but
the inner yolk-containing ends of the cells subsequently fuse. In
C’1'a7z.r/‘on, ]l[0i/ca, I)aj)/mella, ])a[//erzia, Ore/wstia segmentation is
superficial. In Isopods and in Deeapods segmentation is internal.
In all cases the result in the end is the same, a peripheral blastoderm, a central yolk. But the blastoderm is not always, though
it is often, formed simultaneously over the whole surface. There
are cases in which it appears first on the ventral side, and by
what may be described as a still more precocious formation of
the blastoderm, segmentation may begin at this, the future
ventral, point, as in Jlfysis and 0m'scu.9. In these cases the egg
is teloleeithal.
 
In the Insects, Arachnids, Myriapods, and Perz';)atus nomezealandiae, the segmentation is meroblastic and the egg comes to
be centrolceithal. In Peripatzw capevms and in some other species
it would appear that the yolk has been secondarily lost.
II. I CELL-DIVISION 25
 
2. The‘ second type is the so-called spiral form of cleavage
(Fig. 13). This is especially characteristic of the eggs of Polyclads, Nemert-ines, Molluscs (except Cephalopods), Annelids, and
Sipunculoids (P//((800/(I-_\'0I1l(I). The peculiarity of this mode of
division is that after the ’r'our-celled stage the blastomeres
usually known as the 1nacromeres—give elf ‘quartettes’ of
micromeres towards the animal pole, the first quartette being
given off’ dexiotropically (except in cases oi.’ reversed cleavage),
 
c B
 
 
ZD
 
FIG. 13.— Diagram of a ‘ spi1'a‘-Jy ’ segmenting egg in the 16-cell stage.
2 A-2 D macromeres; 2 «:2 d nueromeres of second quartette; ] n 1, 1 n LL
Id 1, 1 cl 2 nncronieres of first quartettc.
 
the second laeotropically, and so on in regular alternation, until
four quartcttes have been produced. The cells of each quartette
divide meanwhile in conformity with the same law of alternation of direction of cleavage. The direction of division is thus
always oblique to the egg-axis, and this ()l’)ll(lflll]_Y can be
observed in the division of the first two blastomcres, the result of
which is that of the two sister cells A and B A is nearer to the
animal pole than B, while in the other pair C is nearer that pole
than D; A being to the left of B and C to the left of I) (to an
observer standing in the axis with his head to the animal pole),
the division is laeotropie. The arrangement of cells approaches
the tetrahedral, especially when, as occurs very frequently,
A and C are united by a cross, or polar, furrow above, B and D
by a polar furrow at the vegetative pole, as in JVc=rci.v,
Icfinoc/zitou, Limaw, Plano)‘/153, Lejii/Ionolus, Dixcocelis, and others.
In Univ, however, it is B and D that are in contact at
the animal, A and C at the vegetative, pole. In other cases
26 CELL-DIVISION AND GROWTH II. I
 
(I/yauassu, Capi/cl/a, Umbra!/a, Clmyiirlzz/a, Amp/u'6m'te, A/‘euicolu,
for instance) the furrows are parallel, the same two blastomercs,
B and D, being in contact at both poles. In Troc/ms both
the ‘parallel furrows’ and the ‘ crossed furrows’ conditions
are found. A similar disposition is to be observed amongst the
mieromeres of the first quartette. These mieromeres, also, alternate with the macromeres. Not more than three contact surfaces
between blastomeres, therefore, meet in one line.
 
The eggs of certain Lamellibranchs—-Ykrerio, Cg/clas—i11 which
the ‘spiral’ arrangement is obscured by the large size of the D
maeromere, and possibly the ova of the Rotifera, are to be referred
to this type.
 
The tetrahedral arrangement of the first four cells is conspicuous in Asteroids and Ophiuroids, where the planes of
division of the first two cells are at right angles to one another.
Before the next division, however, the cells shift their positions
and come to lie in one plane, in which, however, the sister cells
are not adjacent, but opposite, to one another.
 
The eggs of Amp/u'0.mzs sometimes segment spirally (Wilson).
 
After the completion of the spiral period of division, segmentation beeomes radial and then bilateral.
 
3. The third type of cleavage is the bilateral. The first two
divisions intersect in the axis; the next. may he equatorial, as in
Aseidi-ans. In this case the hilaterality becomes evident in the
succeeding phase, in which the divisions in two adjacent cells of
the animal hemisphere meet the first furrow, while in the other
two they meet the second. The bilaterality is marked in the
reverse way in the vegetative half of the egg. The egg is thus
divided into what will be anterior and posterior, dorsal and
ventral, and right and left halves. In future divisions the
bilateral symmetry is retained.
 
The egg of ./Imp/ez'o.avzz.s' may divide in this fashion (Wilson), and
this is the normal method, according to Roux, in Ram: esculwz/a.
 
In the Teleostei and some Ganoids (Lepz'do.w‘ws) the bilaterality becomes evident in the third division, which is parallel
to the first, the fourth being parallel to the second division.
The egg is in fact iso-bilateral.
 
The Ctenophorc egg also possesses two planes of symmetry,
II. 1 CELL-DIVISION 27
 
for the third division is meridional and unequal in such a manner
that the next stag'e~—eig11t cells—is composed of two opposite
pairs of small and two opposite pairs of large cells.
 
The mesomeres in the sixteen—celled stage of Ecliinoids are
bilaterally arranged.
 
In the Cephalopoda the egg is large-yolked, and segmentation
consequently meroblastic. After the firsttwo meridional (livisions
 
V‘ I1.’ I 01' 1’
Flo. 14.—-Tln-ee segmentation stages in the blastoderm of S'I—'pi(I o_)_7i
4-inalis; the segmentation is of the bilateral type. I, left; 2-, right; I— V,
 
first to fifth cleavages. The top sides of the fig111‘es are anterior. (After
Vialleton, from Korschelt and Heider.)
 
the bilateral disposition sets in, for the furrows of the third
phase are unequally inclined to the first furrow in two halves——
the future anterior and posterior halves—oE the egg (Fig. 14).
The egg of A.s'aa7'2's megalocep/m/a also exhibits a bilateral
cleavage, but not on the plan just described. The iirst division
is equatorial. Then the animal cell divides meridionally, and,
as it will prove, transversely, the vegetative cell latitudinally.
28 CELL-DIVISION AND GROVVTII II. I
 
Betore the next division the most vegetative cell (P._,) slips round
to what will be the posterior side. All four cells are bilaterally arranged about the plane in which they all lie, and
this will become the sagittal plane of the embryo. The anterior
and posterior ends, and therewith the right and left sides, are
likewise now determined. The bilateral symmetry is preserved
in future divisions, at least in the vegetative hemisphere ; in the
animal part of the egg the blastomeres of the left side become t.ilted
forwards, those of the right side backwards (Fig. 155, p. 255).
 
4. In the Triclad Turbellarians, in Trematoda and Cestoda,
segmentation is irregular, the blastomeres separate from one
another and lie amongst the yolk-cells. The same phenomenon
may be witnessed in the Salps, and the separation and subsequent reunion of the blastomeres has also been described in
Coelenterates and in Asteroids.
 
Although these types of segmentation are distinct enough
from one another, intermediate conditions are readily found.
The radial easily passes into the spiral type for example, for
in many eggs of the former kind the ‘cross furrows ’ have been
observed at either one or both poles, while the animal blastomercs may be rotated slightly on the vegetative, and so lie
not over, but in between, them. The radial symmetry again may
become bilateral, as when the meridional furrows of the fourth
phase, instead of passing through the animal pole, meet the first
or second furrow, symmetrically on either side of one of these
divisions; this occurs as a variation in I?ana_/'/mew and (normally
(Roux)) in Ifamz excz/Zr’):/rt.
 
In ()phiuroids and Asteroids the tetrahedral arrangement is lost,
and the egg segments radially. In Amp/riuamr: all three types occur.
 
All three forms may therefore have been derived from one,
though what that one was we do not know. In any case,
however, one feature is common to them all; in all cases successive divisions are at right angles to one another. This is the
law formulated by Sachs long since for the divisions of the cells
of plants. It holds good for the segmenting animal ovum,
though exceptions may, of course, be found. The alternation of
dexiotropic and laeotropic divisions, for instance, in spirally segmenting ova continues for a long period with striking regularity,
II. I CELL-DIVISION 29
 
and it is comparatively rare for a cell to disobey the rule. The
rule is, however, no universal law of cell—division. Every embryologist will recollect the continued division of a teloblast in
the same direction to form a germ-band, which is such a coilspicuous fact in the development of Molluscs, Annelids, and
Arthropods. The four polar nuclei of Insect eggs, lying in one
straight line, may also be cited.
 
The direction of division and the size of the blasfomeres are
not, however, the only factors which determine the actual pattern
of segmentation. The cells can, and do, shift their positions on
one another. This is of common occurrence, and a few examples
will suflice. The rearrangement of the tetrahedrally disposed
cells in Asteroids and Ophiuroids has been ‘noticed already. In
many ‘spiral’ ova the micromeres have been observed to rotate
on the macromeres, or one quartette to be pushed out of position
by the cells of another. In z1.vca;'is the cell P._, slips to one side.
14‘urther, cells change their shape.
 
Two factors are therefore involved in the production of the
pattern of cleavage, the direction of division, and the movements
of the cells, and these factors in their turn demand explanation.
To these must be added the shape,
the size, and the rate of division
of the cells.
 
The two latter depend very
largely upon the amount of yolk
present in the egg ; yolk-cells are
large, the yolk divides slowly, or
not at all. This was expressed long
sinee by Balfour in the formula,
‘ The Velocity of segn'1enta,tiQn in ‘ FIG. I5.—' Segnientation Oi‘ the
 
, . Lro0“s one under the influence
any Part 01 the ovum 15) roughly of atbeentliijfiigal force (from Korspeaking, proportional to the eon- sc_he1t and Heider, after 0. Hertcentration of the protoplasm there ; ;l(')1(’l'2l'_l111 gzfilcolilmllsitvsijl 3131:
and the size of the segments is (yolk-syneytiuin) 2 I.-h,bln.stocoel;
inversely proportional to the eon- ”” y°lk'““°1°17 "’ y 011"’
centration of the protoplasm.’ ‘ The rule has been vindicated by
(). I-[ertwig experimentally. If the egg of the Frog be centri
‘ ("omp. Emb. i. C. 3.
30 CELL-DIVISION AND GROWTH II. I
 
fugalized with suflicient force the yolk is driven still more towards the vegetative pole, while the protoplasm is accumulated in
the animal half of the egg. Such eggs segment meroblastically,
a cap of cells or blastoderm being formed lying on the surface
of a nucleated but undivided yolk. The yolk-nuclei, moreover,
are enlarged, as in megalecithal fish eggs (Fig. 15).
 
The rule is, of course, only applicable to telolecithal eggs, and
for many of these it holds good, notably for Vertebrates. In
other classes there are, however, exceptions, which are best known
in those \vhose segmentation has been most carefully studied,
the ‘ spiral’ eggs of Turbellarians, Annelids, and Molluscs.
Large cells, in these ova, often divide more quickly than small
ones; the second quartette of micromeres, for instance, is formed
before the first quartette divides in 07-epitlula, Uuio, Limaw,
Troc/ms, Aplg/sia r/epilans, Discocelis, and the cells of the third
quartette before the first products of division have had time
to divide again in L[maa', Umbrella, and A/2/ysia limacina. 411 is
often formed before the corresponding cells in the other quadrants
(in Ifizio, for example), but in Crepirlula this is in accordance
with the rule, since 4 a, 4 /2, and 40 contain more yolk than 4« cl.
In Arem'coZa, though the yolk is uniformly distributed, the cells
are still unequal. Other exceptions are to be found in the
continued unequal division of teloblasts, in the formation of the
micromeres in Echinoids, and in the unequal division of
the blastomeres in the third and fourth phases in Ctenophors.
According to Ziegler the formation of the micromeres in
Ctenophors cannot be due to the presence of yolk, since
they are still formed when part of the vegetative hemisphere
is removed, as Drieseh and Morgan have also found.
 
Ziegler indeed puts forward another hypothesis to account for
unequal division; he supposes that the centrosomes are heterodynamic. So far there appears to be little evidence in support of
this view. It is quite true that in many cases of unequal division
the asters—not the centrosomes——-vary in size with the size of the
cells. This occurs, for instance, in the division of the first micromeres and of the first somatoblast in Nerei.9, in the formation of
the first and second quartettes, and in the division of the first
somatoblast in Uuio, in the division of the cell CD in /laplauc/ma,
II. 1 CE LL-DIVISION 31
 
and in the division of the pole cells of Annelids (Wilson and
Vejdovsky). It is doubtful, however, whether it is not the
inequality in the cells that is responsible for the inequality of
the asters, there being more room in a large cell for the outgrowth of the astral rays. At any rate, ‘there are many cases
of unequal clcavage——in polar body formation~——where the asters
are of the same size. Until evidence is brought forward of
a difference in the size of the centrosomes the hypothesis is no
more than a conjecture.
 
Before quitting this subject we should refer to a rule which
Zur Strassen has found to hold good for the rate of segmentation
of Ascaris megalocep/zala. The cells do not all divide at the
same rate, but in certain groups of cells division is found to
occur simultaneously. These cells are related, descended from
some one cell, and the more nearly related the cells are, the more
nearly together do they divide. Coincidence in time of division
depends therefore on the degree of cell-relationship.
 
The direction of division of the cell depends upon that of the
nucleus, since, speaking generally, the division occurs in the
equatorial plane of the spindle, or, in other words, the plane
of division is at right angles to the direction of elongation of
the spindle or separation of the ccntrosomes. The latter again
depends on the relation between the nuclear spindle and centresomes on the one hand, and on the other the cytoplasm and its
contents, more particularly the yolk. The relation between
the (resting) nucleus and the cytoplasm has been expressed by
O. Hertwig in the following empirical rule: ‘The nucleus always
seeks to place itself in the centre of its sphere of activity.’
The sphere of its activity being not the inert yolk but the
cytoplasm, we find, in accordance with this rule, that the nucleus
places itself in the centre of the egg where the yolk is uniformly
distributed (isoleeithal), nearer the animal pole but still in the
axis where the yolk is on one side (telolecithal). Examples of
the former condition are to be found in Eehinoids (the fertilization nucleus is nearly, but not quite, central) and large-yolked
Arthropod ova, of the latter in the eggs of Vertebrates, Molluscs,
 
and many others.
The nucleus, however, may wander from this position, as occurs,
32 CELL-DIVISION AND GROWTH 11.1
 
for instance, in the egg of Eehinoids after the expulsion of the
polar bodies and before fertilization. Apart from such exceptions, due very likely to some temporary alteration in the relations
of yolk and cytoplasm, the rule is a reliable one.
 
The relation between the dividing nucleus, the spindle and
centrosomes and the cytoplasm has been stated by O. Hertwig
in his second empirical rule ‘that the two poles of the division
figure come to lie in the direction of the greatest protoplasmic
mass ', by Pfliiger in the l'o1-mula, ‘the dividing nucleus elongatcs
in the direction of least resistance.’
 
The objection that has been urged to this latter expression,
that in a fluid the pressure is equal in all directions, may be
set aside. For though the cytoplasm is fluid it is an extremely
viscid fluid, and the presence of the suspended yolk granules
 
 
FIG. 16.——Diagr-am oi the segmentation of the Frog's egg (after
0. llertwig, from Korschclt and Heider). A, first (meridional); B, third
(latitudinal) phase of segmentation; p, superficial pigment of animal
hemisphere; pr, protoplasm; y, yolk; sp, spindle.
must certainly ofl'er a greater resistance than the fluid itself,
and greater in proportion to their number and size. Pfliigcr’s
formula, therefore, if not merely a truism, resolves itself into
a restatement of Hertwig-’s rule. This rule certainly holds
good for a large number of cases, for it explains, for instance,
the two first meridional divisions of all spherical telolecithal
and radially segmenting eggs, the third, latitudinal (in smallyolked eggs 1), and possibly also subsequent meridional and latitudinal divisions (Fig. 16). It will not, however, in the present
state of our knowledge, explain the obliquity of the spindles to the
egg-axis in spirally dividing ova, nor cases of bilateral division ;
 
1 ln Sycon the third is meridional, the fourth latitudinal.
II. I CELL-DIVISION 33
 
here, it is evident, other factors must come into play, in the second
case probably a bilateral symmetry in the constitution of the
cytoplasm. These exceptions may, however, ultimately prove to
be special cases of Hertwig’s rule.
 
A very striking confirmation of the rule is to be found in the
division of the egg of Arcane m7_//roreuoxa (Figs. 17, 18). The
 
 
- FIG. 17.- Four stages in the fertilization of the egg of A.»-z-(n-is
m'_qroveuosa. (After Auerbach, from Korschelt and Heider.)
 
egg of this worm is ellipsoid. At one end (that turned towards
the upper end of the ovary) the polar bodies are extruded, and here
the female pronucleus is placed. The spcrmatozoon enters at the
 
 
Fm. 18.-—Tl1ree diagrams of the rotation of the fertilization spindle in
the egg of Ascaris nigrovenosa. e, s, the directions in which the female
and male pronuclei approached one another in A; 1, 2, 3, successive
positions of the spindle. (From Korschelt and Heider, after 0. Hertwig.)
 
muxisson D
34 CELL-DIVISION AND GROWTH II. I
 
opposite end. The line of union of the two pronuclei therefore
lies in the long axis of the egg. Nevertheless the fertilization
spindle is not formed in the minor axis of the ellipsoid as one
might expect. The two pronuclei rotate together through 90°,
the spindle is developed, as usual, in a direction at right angles
to their line of union, that is to say the axis of the spindle lies
in the major axis of the egg, and the rule is confirmed. There
is a similar rotation of the fertilization spindle in the egg of
another Nematode, Diployaster (Ziegler), and in the Rotifer
Airplane/ma the spindle, at first oblique, becomes later coincident
with the long axis of the ovum (Jennings).
 
Curiously enough, this rotation of the pronuclei does not occur
in another ellipsoid egg, that of the Rotifer Callidiua. According
to Zelinka, the polar body is formed at one end of the long axis,
but the fertilization spindle lies in the minor axis, the first
division includes the major axis, and the law is disobeyed. After
the division, however, the cells rotate, and the plane of contact is
then, as in Ascaris m'groveuo.m, transverse.
 
Again, all polar divisions violate the rule, as also does the first
division of the fertilized egg of Away-is megalocep/zala, and the
division of the cells of the germ-bands of Crustacca parallel to
their length (Bergh).
 
On the other hand, Ilertwig has brought forward experimental
evidence in support of his generalization. In the eggs of the
Frog the directions of some of the divisions were altered by compression between glass plates. The eggs were just allowed to
assume their normal position with the axis vertical. They were
then placed between glass plates and compressed.
 
In the first series of experiments the plates were horizontal. In
such eggs the first furrow was meridional and vertical, the second
meridional and vertical and at right angles to the first. So far,
therefore, division was as in the normal egg. In the third division
the furrows were, however, not latitudinal and horizontal, but
nearly vertical, being parallel to the first furrow above, to the second
furrow below. The surface of contact, therefore, formed by the
furrows of this phase must pass through a meridional position in
the interior of the egg. The fourth furrows are latitudinal. Born
has repeated the experiment and confirmed this result (Fig. 19).
He adds, however, that the furrows of the third division pass
II. I CELL-DIVISION 35
 
towards the vegetative pole below, or may even remain parallel to
the first furrow throughout. The fourth furrows, Born says, are
parallel to the second. I have myself observed that this division
may be either parallel to the second, or latitudinal, even in different
 
I
will Ill  l [ill ?
 
5' 3 3
 
   
 
   
 
/ / ' /
FIG. 19.—- Segmentation of the Frog's egg under pressure.
The compression is in the direction of tlie axis.
A. v1ew of the egg between horizontal plates; the animal part IS
 
islraxled. II, (I, I), first (1), second (2), third (3), and fourth (4) divisions
 
as seen from the animal pole. (Alter Born, from Korsehelt and lleider )
 
 
FIG. 20.—-The first four divisions (I, II, III, IV) in a F10g’s egg compressed between horizontal plates in the direction of the axis. The
third furrow is more or less meridional and vertical in three quadrants,
horizontal in the fourth, and this a smaller quadrant. 'l‘he fourth furrow
is meridional in this quadrant, horizontal in the remaining three.
 
quadrants of the same egg (Fig. 20). It will be observed that
the quadrant in which the third furrow is latitudinal is smaller
than the others. It is of great interest to observe the striking
similarity between the direction of the third and fourth furrows
in these eggs and the corresponding divisions in the Teleostean
egg where the blastodisc is compressed by the chorion.
 
In the second series of experiments made by Hertwig the glass
36 CELL-DIVISION AND GROWTII II. 1
 
plates were vertical, the eggs, therefore, compressed not, as before,
in, but at right angles to, the axis.
 
The first furrow was meridional, and therefore vertical, and at
right angles to the plates. The second was latitudinal and horizontal, and also at right angles to the plates. The furrows of
the third phase were parallel to the first, those of the fourth, in
the four upper animal cells, parallel to the plates. Born again
 
,1
 
/ 1 7
 
F1G.2l.—Segmentation of the Frog's egg under p1'ess1u'e.
The pressure is at right angles to the axis.
 
A, view of the colnpressed egg. The piglnented animal portion is shaded.
 
If, C, 1), views of the egg from the animal pole after the first (1), the
second (2), and the third (3) divisions.
 
E, 11‘, 0, views of the egg from the compressed side after the first (1),
the second (2), the third (3), and the fourth (4) divisions. The first
furrow may pass as (1') in E.
 
(From Korschelt and Heider, after Born.)
 
confirms this account (Fig. 21). The direction of the furrows of
the third phase is, however, variable ; it may be not parallel to the
first, but perpendicular to it. In this case it may be parallel to the
second, or so oblique to it as to become nearly parallel to the glass
plates. The direction of the fourth division depends on that of the
third, to which it is at right angles. It may, therefore, be either
oblique and nearly parallel to the plates, as described by Hertwig,
or parallel to the second furrow and perpendicular to the plates.
In a third series of experiments I-Iertwig placed the plates
II. I CELL-DIVISION 37
 
obliquely, at 45°. In these eggs the yolk sinks slightly from
the upper to the lower side, while the cytoplasm rises in the
opposite direction; in other words, a bilateral symmetry is conferred upon the egg by the combined action of pressure and
gravity. The plane of this symmetry is midway between and
parallel to the plates. The first furrow is at right angles to the
plates and to the plane of symmetry.
 
VVe are indebted to Drieseh for a similar series of experiments
on Echinoderm eggs. Drieseh compressed the eggs of 15?:/ri/u/.s~
under a cover-glass supported by a bristle. The direction of the
egg—axis with regard to the pressure was not known, but the
 
 
FIG. 22.——I9'rIu')ms: segmentation under pressure.
 
(1, preparation for third division (radial); b, preparation for fourth
 
division (tangential); 11', after fourth division; 0, another form of the
8-cell stage (third division pa1'allcl to first); «I, the same after removal
of the pressure. (After Driesch, 1893.)
Echinoid egg is nearly isolecithal. VVhen the egg membrane
remained intact the first two furrows were vertical, that is, in
the direction of the pressure, since the slide and cover-glass were
horizontal, and at right angles to one another.
 
The spindles for the next division are again horizontal, and
usually tangential, sometimes, however, radial. The eight-celled
stage consists, therel'ore,o[’ a flat plate of cells. At the next division
the formation of mieromeres —which would ordinarily occur at this
moment——is suppressed; the spindles are horizontal and radial,
the furrows, therefore, vertical and tangential (Fig. 22 a, /1, /2’).
 
In certain cases cell-formation is wholly or partially suppressed. When the pressure is less (in those eggs which lie
nearer the bristle) the micromeres may be, but generally are not,
formed. The spindles are no longer horizontal. Similar results
are obtained when the eggs are released from strong compression.
 
In another experiment the eggs were first deprived of their
membranes. The first and second furrows are vertical and
generally at right angles to one another. Sometimes,-however,
38 CELL-DIVISION AND GROWTII II. I
 
the second is parallel to the first, or one blastomere may lie apart
from the other three. Should the eggs be now released from
the pressure, each blastomere becomes rounded off, and——after
two more cleavages—-the sixteen-celled stage consists of two
plates of eight eells lying over one another. But if the pressure
is maintained, the spindles are horizontal and the blastomeres lie
all, or nearly all, in one plane (Fig. 22 c, (7).
 
 
Flu. 23.--Segnieiitatioii of the egg of E(‘7li1lII8 micI'oh¢bcrcuIulus
under pressure. (After Ziegler, 1894.)
 
(V, 8 cells in one plane; 1;, 16 cells, the last division having been
tangential ; c, (I, 16-32 cells: the direction of the spindles in c is shown
by the line: it is in the greatest length of each cell; c, 64 cells: a cross
signifies a vertical or oblique division, a line a horizontal division.
 
Ziegler has followed the segmentation of the compressed eggs a
step further (Fig. 23). As the figures show, the first two divisions
are at right angles to one another, while the furrows of the next
two phases are, roughly, parallel to the first and second. In the
next division-—sixteen to thirty-two cells—the outer cells divide
radially, the inner more or less tangentially, these divisions
being, like the previous ones, at right angles to the compressing
plates. In the following phase, some cells (those marked with
a line) still divide in the same direction as before; but in others
(distinguished by a cross) the spindle is perpendicular to the
II. 1 JELL-DIVISION 39
 
plates and the division horizontal. Ziegler points out that, in
the former cases, the cells have greater dimensions in the horizontal plane than in the la.tter. This, however, may be the
efieet, not the cause, of the direction of the spindle-axis.
 
Two other pressure experiments maybe mentioned here. In
Nereis Wilson produced a flat plate of eight equal cells by
applying pressure in the direction of the axis. The formation
of the first quartette of micromeres was thus suppressed. On
relieving the pressure eight micromcres were formed. For the
Ctenophora (]ierb'z') Ziegler has shown that the normal inequality
of the third and fourth divisions is not altered by pressure.‘
 
The foregoing experiments all agree in demonstrating the
perfectly definite eft'cct produced by pressure upon the segmenting
cgg. The nuclear spindles place themselves at right angles to
the direction of pressure, the divisions fall at right angles to the
compressing plates. This holds good for the first three or four
divisions, at least, and sometimes for later phases still. In all
these cases, therefore, the nuclear spindle elongates in a direction
of least resistance, and, in the normal uncompressed egg, we may
argue, with Ilertwig, the least resistance is offered by the greatest
protoplasmic mass.
 
Even in the compressed eggs, however, the greatest extension
of the protoplasm, or the least extent of the yolk, is a factor
which must in some cases come into play. When the egg of the
Frog is compressed between vertical plates, the nuclear spindle
does not elongate in any direction at right angles to the pressure, but in one only, a horizontal ; and this is the direction of
the greatest protoplasmic mass, since the egg-axis is vertical.
 
Speaking generally, therefore, experiment has upheld Hertwig’s contention that the direction of nuclear division, and
therefore of cell-division, is determined by the relation between
the nucleus with its centrosomes and the cytoplasm with its yolk.
 
There are one or two experiments which do not support
Hertwig’s view. Boveri stretched the eggs of the sea.-urchin
St7'o2z_9;yloceul/'0!/1.9 in the direction of the axis. The fertilization
spindle lay in the usual equatorial position, occupied, that is, the
 
minor axis of the ellipsoid.
 
‘ I have recently had occasion to notice that when the egg of Anfedm
is compressed in the direction of the axis the third division is meridional
instead of latitudinal.
40, CELL-DIVISION AND GROWTH II. I
 
Again, Roux observed that Frogs’ eggs sucked up into a tube
with a narrow bore became elongated either parallel or transverse to the length of the tube, the axis of the egg lying in each
case lengthways. In the first case the division was at right
angles to, in the second usually parallel to, the tube in accordance with the rule; but exceptionally, in the transversely
stretched eggs, the division was not perpendicular to, but coincided with, the extent of the greatest protoplasmic mass.
 
However important a factor the disposition of the yolk may
thus be in deciding the direction of cell-division, it is certainly
not the only factor. In the eggs pressed between horizontal
plates there are many—an infinite number—of' directions of
least resistance. In one of these the segmentation spindle elongates, and at right angles to this the first furrow falls. This is
probably determined-——-as it is determined in the normal Frog
and Sea-urchin egg-——by the point of entrance of the spermatozoon, or at least by the direction of the sperm-path in the egg.
The second division is at right angles to the first, and here the
direction may very possibly be decided on lIertwig’s principle.
But why, in the next phase, should the furrows be at right
angles to the second rather than to the first, for the extent of the
protoplasmic mass is as great in each of the four cells, in a direction parallel to the first as to the second furrow? Here, it is
clear, some other reason must be found for this succession of
divisions at right angles to one another. The cause is probably
to be sought for in the direction of division of the centrosomes ;
for these divide——frequently soon after the telophase—at right
angles to the axis of the previous figure. VVe thus gain a new
expression for Sachs’ Law.
 
The original direction of divergence of the centrosomes is,
 
however, by no means always the ultimate one, for the growing ‘
 
spindle may be twisted out of its original position. Conklin has
made a careful study of this phenomenon in C’)'¢j[Ii(]l(ltl, in which
egg he finds that vortical movements are set up in the cytoplasm by
the escape of nuclear sap at the beginning of mitosis. The movements are in opposite directions in sister cells, centre in the spindle
poles, and often carry both nucleus and spindle into a fresh place.
These currents, which had been noticed previously by other
observers (by Mark in Limam and by Iijima in Ale]//zelis), may
II. 1 CELL-DIVISION 41
 
thus play an important part in the production of the cell pattern.
We shall see elsewhere that they, and other protoplasmic movements, are also of the very greatest significance in difierentiation.
 
There remains now to be noticed another principle, which is
especially applicable to plant-cells with fixed walls, though it
may possibly be used for the phenomena of animal segmentation
as well. Berthold has pointed out that when a newly formed
cell-wall places itself perpendicular to the previously existing
walls it is——at least in a good many instances—simply obeying
tlie laws of capillarity, it merely conforms to the principle of least
surfaces formulated by Plateau. This principle is as follows:
‘ Homogeneous systems of fluid lamellae so arrange themselves, the
individual lamellac adopt a curvature such that the sum of the
(external) surfaces of all is under the given conditions a minimum.’
 
A fluid lamella, of soap solution, for example, placed across
the interior of a hollow, rigid cylinder, or parallelepiped, or
cube, is, with the film coating the internal surface of the vessel
 
-in which it lies, a special case of such a system of lamellae, and,
 
in obedience to the principle, the lamella places itself at right
angles to the walls of the cavity and transverse to the long axis.
 
In the ease of the plant-cell, the cell-plate, formed by solidification of the spindle fibres in the equator of the mitotic figiii'e,
represents the soap-lamella, and like the latter in its parallelepiped, the cell-platc, or new cell—wall, places itself perpendicular
to the old one, and transverse to its length.
 
There are very numerous cases in which the law is obeyed,
but it is not so in all. Under certain conditions the. lamella
should be not at right angles, but oblique to the wall of the
chamber across which it is stretched. If, to take a concrete
case, the lamella be made to move (by abstracting air) towards
one end of its receptacle (a cube or parallelepiped), it will reach
a critical position in which the principle of least surface can
only be satisfied by its occupying an oblique position. The
 
point at which this occurs is when the lamella is distant 3 from
7r
 
the end, where a is the length of the side of the cube (short side
of the parallelepiped). The lamella new forms the fifth side
 
to a wedge-sliaped space (quadrant of a cylinder, whose radius
 
. 4 . . . .
is 1- = 9-a), but as more air is abstracted, and it moves still
4-2 CELL-DIVISION AND GROWTH II. I
 
further toward the end, it comes to another critical position
when it must lie across one corner, forming so the base of
a pyramid, or octant of a sphere. This position is defined by
the equation 2-, =11, where 1', is the radius of this sphere.
It is impossible, therefore, for a very fiat cell, or short cylinder, to
be divided in conformity with the principle parallel to its longest
side, and yet this occurs, as, for instance, in cambium cells.
 
It will also be noticed that this principle does not explain why
one particular direction is selected when many are apparently
equally possible.
 
We turn now to a consideration of the remaining factor which
assists in determining the shape of the cells and so the geometrical pattern of segmentation ; this is the movement of the
cells upon one another.
 
That such movement does occur we have already seen; the
question which immediately suggests itself is whether in taking
 
up their new positions the cells obey the laws of capillarity as
enunciated for systems of fluid lamellac such as soap-bubbles by
Plateau in his principle of least surfaces.
 
This principle, as we have seen, demands that the sum of
the external surfaces should be, under the conditions, a minimum,
or, expressed in physical rather than in geometrical language,
that the total surface energy should be minimal. In accordance
with this doctrine of minimal surface energy a drop of fluid
floating in a fluid medium assumes, as need hardly be said, the
form of a sphere. In a system of drops contact surfaces will be
formed between the drops, provided that each possesses a coating
film which has a positive energy with the media it separates;
a film, that is, of such a nature that the total surface energy
 
would be diminished by apposition, without, however, involving ‘
 
the disappearance of the separating film and fusion of the drops.
In other words, the film must be insoluble in both the external
and the internal media. A simple example of this is afforded
by the behaviour of the spheres of jelly covering the eggs of the
Frog, when taken from water and floated between chloroform
and benzole. Two or more such drops of jelly cohere by their
coating films, and form systems of lamellae —the films, that is,
at the external surfaces and between the opposed surfaces of the
II. I CELL-DIVISION 43
 
drops———in which the principle of least surfaces is obeyed. Soapbubbles form similar systems. But where this condition is not
fulfilled, as in oil-drops floated, for instance, between alcohol and
water, the drops either unite or separate, each retaining its
spherical form.
 
The geometrical analysis of such systems given by Plateau
is as follows. In a system of two bubbles the curvature of the
 
surface of contact is given by the equation r = £7, where 7' is
the radius of that surface, p, p’ the radii of the larger and
smaller bubbles. Since the pressure varies inversely with the
radius, the surface of contact is convex towards the larger
bubble. When p = p’, 7- = a, and this surface is plane. Since
there is equilibrium the external surfaces of the bubbles and
their common surface meet at angles of 120°.
 
In a system of three bubbles there are three contact surfaces;
 
these meet in one line and make angles of 120° with one
 
.another. When there are four bubbles, however, the four con
tact surfaces cannot meet in one line except for an inappreciable
instant; they immediately shift their positions in such a way
that two opposite bubbles meet and separate the other two from
one another. There are thus five surfaces of contact, and these
make angles of 120° with one another as before. This is the
arrangement when four bubbles—whether equal or unequal is no
matter—are placed side by side in the same plane. When,
however, one bubble is placed in a different plane to the remaining three, four surfaces are formed and disposed in such
a manner that the four lines, each formed by the intersection
of three of these surfaces, meet in one point, making with one
another angles of 109° 28’ 16", the angles at the centre of a
tetrahedron. In short, the four are now tetrahedral] y arranged.
The systems of drops of jelly alluded to above arrange themselves as do soap-bubbles under similar circumstances. What
holds good of four holds good of an assemblage of any number
of bubbles. The size of the bubbles is a matter of indifference,
except to the curvature of the surfaces of contact, and, to
a certain extent, to the arrangement. Thus, if four equal
bubbles be placed in a plane, they will form together five
surfaces of contact, one of which will be between two opposite
44 CELL-DIVISION AND GROWTH II. I
 
bubbles. If these two be now diminished, or the opposite two
enlarged, the surface of contact will be between the opposite
pair of larger bubbles. On the other hand, it is possible
to bring smaller opposite bubbles into contact, while the
larger ones remain apart. Again, on four bubbles lying in one
plane, four small ones may be superimposed in such a fashion
that while two lie at either end of the surface of contact, the other
two lie over between the two opposite large bubbles below. If
now the two latter small bubbles be enlarged, they will displace
the other two until all four come to lie not over but between the
 
 
FIG. 24.—Diugra1ns of systems of soap-bubbles.
 
A-0, four small bubbles superimposed on four large ones. In A and B
the bubbles are not compressed ; in C the lower bubbles have been
circumscribed by a. cylindrical vessel. In B the upper bubbles are small
enough to show the surfaces of contact between each and the two
adjacent large bubbles below. These surfaces are invisible in A and C.
 
D is a system of eight bubbles in one plane, four forming a cross in
the centre.
 
In all figures notice the fifth contact surface or ‘polar furrow ’.
 
bubbles below, the usual arrangement when four are superimposed on four (Fig. 24 A—C).
 
The final disposition must depend, therefore, not merely on the
principles of least surfaces, but also, provided that the conditions
of that principle are fulfilled, on the sizes and initial arrangement of the bubbles.
 
It will hardly need pointing out that very many ova adopt
the form which presents the least external surface, that of a
II. I CELL-DIVISION 45
 
sphere, when placed in a fluid medium, and it is also a familiar
fact that after the first (and subsequent) divisions the blastemeres are flattened against one another (Cytarme, to use Roux’s
term), and that whether they are compressed by an egg
membrane or not (examples of the second alternative are to be
found in Unio, .D1'eis3eu8z'a, Umbrella, C'7'q2i(I'/(la, /lp/yxia limecimz, /late;-iae), though the surface of contact is not always
curved when the cells are unequal. The two cells, however,
often become rounded of and partially separated from one
another prior to the next division. Such a separation (Cytochorismus) has also been observed by Roux in the ease of cells
of the Frog’s egg, which, having been isolated in albumen or
salt solution, have subsequently reunited.
 
That the cells flatten against instead of repelling one another,
as free oil-drops would do, suggests that they, like soap-bubbles,
are provided with an insoluble coating-film, while their subsequent separation may be provisionally explained by supposing
that this coating-film becomes temporarily dissolved under the
action of some substance formed in the cell. This idea is borne
out by a striking experiment of Herbst’s, who found that in
sea-water deprived of its calcium the blastomeres of the seaurchin egg came apart and resumed their spherical shape. At
the same time the surface membrane underwent a visible alteration, becoming radially striated. It seems reasonable to conclude
that there is a membrane by which contact is normally effected,
and that this is soluble in sea-water devoid of calcium. On the
addition of calcium the cells eohere again.
 
It may be mentioned that when systems of drops of jelly,
floating in a medium of oil and united by their coating-films
of water, are removed to alcohol, in which both oil and water
are soluble, the films disappear and the drops separate.
 
In the next stage (four cells) the type of segmentation in which
the laws of capillarity are most strictly obeyed is obviously
that which we have distinguished above as the spiral or tetrahedral type, and Robert has been able to show that successful
imitations of the four-, eight-, twelve-, and sixteen-celled stages
of the egg of 2’7'oc/we may be made with soap-bubbles.
 
Four equal bubbles were placed in a porcelain cup, which held
them together in the same way that the actual cells are held
46 CELIi-DIVISION AND GROWTH II. I
 
together by the vitelline membrane. Five surfaces of contact
were formed, that between two opposite bubbles representing
the cross furrow or polar furrow in the egg. In the fl’/'oc/ms
egg, however, the polar furrows need not be parallel at the
animal and vegetative pole; they may be at right angles to one
another, and this tetrahedral arrangement of crossed polar
furrows may be imitated by lifting up one of the bubbles and
bringing it into contact with its opposite, one pair of bubbles
being new in contact below, the other pair above. This arrangement is, however, unstable whilc the four bubbles remain in one
plane, the two bubbles soon coming into contact both above and
below. When the bubbles are not confined within a cup the
instability of the ‘ crossed-furrow ’ condition is extreme.
 
By reducing the volume of the bubbles that are in contact
the other two may be brought together; as the polar furrow
changes positions there is at least a temporary condition when
they are crossed.
 
As we have already pointed out, both conditions—the ‘ parallel
furrows’ and the ‘ crossed furrows ’—-are met with in the eggs
at the four-celled stage of Molluscs, Annelids, and marine
Turbellarians. Whether both opposite pairs or only one opposite
pair of blastomeres are in contact does not, however, appear to
depend upon whether the vitelline membrane is close to and
compresses the egg or not. In most cases of crossed furrows
the membrane fits, it is true, quite closely (Nereis, Io/moo/titou,
Porlar/cc, Lcpidouotus, Jjiscocelis, P/(yea, and possibly Li)/may and
Planorbis, if there is in these two, as in P/13/ea, a very fine membrane between the albumen and the ovum); so also, speaking
generally, where the furrows are parallel the membrane is absent
(Umbrella, ./ljzlysia, Dreisseueia, Crepirlu/a), but in Am];/aitrile and
C/gmzenella it is lightly applied to the egg.
 
It is remarkable that when the furrows are crossed, it is the
A and C cells which meet at the animal pole, the B and D cells
at the vegetative (except only in U/tio), and this must depend on
other properties of the cells than their surface tensions. But it
may be very plausibly suggested that the explanation of the fact
that it is the cells B and D which meet to make the ‘parallel’
furrows is to be looked for simply in the large size of D.
 
Robert has indeed shown that by simply altering the sizes of
II. 1 CELL-DIVISION 47
 
the bubbles the conditions observed in the four-celled stage of
other types—Nereis, A/-euicola, Uuio, zlplysia, I)isc-ocelz'.s°-may be
faithfully copied.
 
It only remains to be added that the contact surfaces of the cells,
like those of the bubbles, make angles of 120° with one another.
 
Robert has also imitated the eig-ht—eelled stage (the four
micromeres alternating with the four maeromercs), the stage of
twelve cells (division of the micromeres), and that of sixteen
cells (second quartettc formed). The bubbles of the second
quartette may be made to slide in between the maeromeres and
so rotate the whole first quartette, as happens in the egg. The
division of the micromeres in the egg results in the arrangement
of four cells crosswise in the centre, four others occupying the
spaces between the arms of the cross. The bubbles behave in
the same manner.
 
In the eigl1t—celled stage the micromeres alternate with the
macrorneres. In the case of the bubbles this is not necessarily
so; the two sets of bubbles may be superposed if the ‘polar
furrow’ in one tier is at right angles to that in the other, or if,
as pointed out above, the upper bubbles are small. Otherwise
superposition is a very unstable condition.
 
It would appear then that many of the patterns exhibited by
eggs with a spiral cleavage are explicable by reference to the
laws of surface tension. The principle of least surfaces may
be extended to other cases. The first four blastomeres of
Ophiuroids and Asteroids form a perfect tetrahedron, though
this arrangement is subsequently discarded for one which could
not be imitated with soap-bubbles (we may notice in passing that
in the first case the egg is tightly invested by its membrane,
in the second it is perfectly free). In zlscarzlv megalocep/aala
the four cells come to lie, as do four bubbles, in one plane,
and polar furrows have been seen in many eggs which belong
to another type of segmentation (in Coelenterates (llyrlractiuia),
Sponges (Spongillu), Crustacea (Brauc/z2'pu.v, Luci/‘er, 0rc/Iestia),
Vertebrates (Petra/1z_yzo:2, Rana), Ascidians, and Am/2/u'oama).
 
The principle of least surfaces——not more than three surfaces
meeting in a line, not more than four lines meeting in a point—
is, however, not of itself suflicient to explain the whole of the
phenomena even in this most favourable tetrahedral type;
48 CELL-DIVISION AND GROWTH II. I
 
other factors must intervene, just as other factors intervene in
a mass of soap-bubbles—their size and initial arrangement~—
in the determination of the actual pattern. These other factors
are the direction of cell-, that is of nuclear, division, and the
magnitude of the cells; and these, as we have seen, in turn
depend upon the relation between the nucleus and the cytoplasm
with its included yolk. Thus it is the direction of the spindles
which determines whether the mieromeres of the first quartette
shall be given ofi laeotropieally or dexiotropically ; the direction
of division, oblique to the egg-axis, again determines that the
mieromeres shall alternate with the macromeres and not be
superimposed upon them ,- the size of the cells and the direction
 
 
FIG. 25.-— Mitotic division with elongation of the cell-body in a protozoon,Acanthor_1/stis aculeuta. (After Schaudinn, from Korsehelt and Heider.)
 
of division may determine the position of the polar furrow,
while the rate of division will also not be without effect, since
the whole arrangement at any stage depends in part on the
disposition at the stage before.
 
There is one other point that is worthy of notice. The mitotic
spindle possesses considerable rigidity, and is able as it elongates
to materially alter the shape of the cell. This may be seen in
many cases in Annelid, Mollusean and other eggs—the division of
the first mieromeres in Nereis is an instance—and in the Protozoa
(Fig. 25). Another interesting case is the Rotifer Asplcmc/ma,
where, preparatory to the fourth division, the shortest axis of
the cells——in which the spindles are placed-—becomes by the
elongation of the spindles the longest. This alteration of shape
is itself an important factor in deciding the positions to be taken
up by the daughter cells.
II. I CELL-DIVIS ION 49
 
In the other types——radial and bilateral——thc principle of least
surfaces is obviously disobeyed, for here four or more surfaces
meet in one line and at angles other than 120°.
 
Roux (1897) has, however, shown that if a certain condition
be imposed on the system of lamellac, figures may be produced
which very closely resemble the patterns presented by radially
and bilaterally segmenting ova. This indispensable condition
is that the system shall be surrounded by a rigid boundary, as
the eggs themselves are by a membrane. Roux’s system was
made by dividing into two, four, and eight a drop of paraflin oil
suspended in a closely fitting cylindrical vessel between alcohol
and water. To this medium was added calcium acetate to prevent
the drops reuniting. The drop was divided with a glass rod.
 
C9®f®
®a@
 
Fm. 2(5.~ltoux's oil-drops. A and B, the drop divided equally; U and
 
D, unequally. Each of the two equal drops divided equally in E,
unequally in 1". (From Korschelt and Heider, after Roux.)
 
When the two drops formed by the first division were equal
the surface of contact was flat, when unequal convex towards
the larger one, in accordance with the rule (Fig. 26 A—l)).
 
When the second was also equal, four drops were formed with
four surfaces of contact meeting in one line, or enclosing between
them a small ‘segmentation’ cavity. If the division of the
two equal drops was unequal, and the smaller cells adjacent, they
pushed into the larger ones; the result, in fact, was the same
 
Jicnxnsnv E
50 CELL-DIVISION AND GROWTH II. 1
 
as would have been produced by an equal following on an unequal
division, the four surfaces meeting in one line as before (Fig. 26
E, F). The appearance presented is like a side view of a radially
 
 
Fm. 27.—-Arrangement of four oil-drops produced by unequal division of
two equal drops, the small and large drops alternating. The first division
is shown by I: the second (II) may pass as in a or in b, but the result is
always as in c, the two large drops meeting in a polar furrow and excluding
the small drops from the centre; the system is symmetrical (iso~bilateral)
about the dotted lines in c. (After Roux, from Korschelt and Heider.)
 
segmenting egg after the third division. When, however, the
smaller drops were 11ot adjacent, but opposite, five surfaces of con
” I 3 /. tact were formed,
 
a polar furrow
 
appearing between
 
A A the two larger and
 
I L 1 joining the centres
\ /7 W  of mass of the
two smaller drops,
 
I I whether these are
unequal or not.
 
C’ . .
The direction 1n
fi which the division
of the drops is per” V 3’ formed isirrelevant;
the final result is
4% always the same.
 
/ Should two adjacent
 
_lfIG. 2S.-— A_and B are diag1':1111s of an oil—drop drops be equal, the
divided into four and eight to explain Roux’s 1 f _ - - 1
notation. C is a figure of the oil-drop divided into P0 M m row 15 Sh]
 
eight equal parts. (From Korsclielt and Heider.) formed by the union
of those two which have together the larger mass (Fig. 27).
II. I CELL-DIVISION 51
 
The length of the polar furrow varies directly with the size
of the drops which unite to form it; its direction makes an
angle with the plane separating the first two, which varies
 
 
FIG. 29.~Arrangenicnts of six oil-drops. In all cases A = B = a = b.
In A, rt’ = a", b’ = b". In B, a’ > a", b’ > b”. In C, a’ < a”, b’ < b”.
I, first furrow; II, second furrow. (From Korsclielt and Heidcr, after
 
Roux.)
 
 
FIG. 30.-—Various arrangements of eight oil-clrops, all bilaterally
syiiiinetrical about the first furrow (1). In all cases the first division has
been equal. In A and B the second division (II) has also been equal,
but in C 0, b are smaller than A, B. In A, a”, la”, A”, B" < rs’, b’, A’, B’.
In B and C, a”, b" < a’, b’, but A", B” = A’, B’; hence a”, I)" < A", B".
(From Korschelt and lleider, after Roux.)
 
 
_Ei_G. 31. —~ Arrangement of six (A) and eight (13) oil-drops, after iineqnal
division of four equal drops (A = B = a = 1;), the smu.llcr and linger
drops regularly alternating. (From Korschelt and Hcider, after Roux.)
 
E2
52 CE LL-DIVISION AND GROWTH II. 1
 
inversely with its length, so that when all the drops are equal
the cross furrow lies in the same plane with the first division,
and so disappears. .
 
By another division it is possible to make a ring of eight drops
whose surfaces of contact all meet in one line, or in a ‘segmentation’ cavity (Fig. 28). To realize this condition, however,
it is necessary that the division should be equal, and its direction
accurately radial. If unequal, the larger drop invariably passes
towards or wholly into the inside. If oblique or tangential the
inner drop passes into the segmentation cavity (Fig. 32).
 
 
FIG. 3'Z.——'l‘hree stages in the passage of a large drop (a") into the
centre of the system. The fiist stage extremely unstable. (From
Korschelt and Heider, after Roux.)
 
Unequal division of all four equa.l drops produces very interesting patterns, some of which recall the appearance of bilaterally segmenting ova, when the divisions are corresponding-ly
unequal on each side of the first or second division (Figs. 29, 30),
while others resemble certain phases of ‘ spiral’ division when small
and large cells regularly alternate (Fig. 3]). It is a rule for the
smaller of the two drops to go to the periphery, while the larger
assumes an oblong or wedge shape, passing towards the centre if
it does not slip entirely inside. The latter occurs with clean oil,
when the large drop is flanked by small ones on both sides.
 
It is also possible to divide four equal drops horizontally
into two tiers. The upper drops, however-—-unless absolutely
undistnrbed~quiekly come to alternate with the lower.
 
In these systems of drops the final arrangement is due to,
first, the principle of least surfaces; secondly, the circumscribing boundary; thirdly, the size of the drops; and fourthly,
in some cases, the direction in which they are divided.
 
It only remains for us to consider, with Roux, to what extent
II. I CELL-DIVISION 53
 
the cells of a radially segmenting egg, such as that of Rana
fusca, are governed by the same influences as determine the
pattern of the drops.
 
The resemblances, it will be conceded, are often very close.
There are also important differences. The polar furrow, which
is often present in the Frog’s egg, is not necessarily between the
cells with the greatest mass. Again if, in the four-celled stage,
with no polar furrow, one of the cells be diminished by puncture,
a polar furrow does not always appear, as it would with oil-drops,
nor, if it does, is it always formed by the union of the larger
cells. 01', if when a polar furrow is present between the larger
cells, one of these is diminished by puncture until it, together
with its opposite, is less than the other two, the polar furrow
nevertheless retains its position.
 
In the sixteen-celled stage the animal cells together form a
ring of eight around the axis. The cells are not necessarily equal,
and a small cell may be compressed by, instead of compressing,
adjacent large ones, while they, not it, move away to the periphery.
 
Other differences are that large cells bulge into small, that
cells are elongated tangentially instead of radially, that there
are amoeboid processes at the inner ends of the cells, and intercellular spaces between them.
 
Further, Roux has examined the behaviour of the isolated
cells of the Frog's egg in the morula stage. The cells were
separated in a medium of albumen, or salt-solution, or a mixture
of the two. They first approach and then flatten against one
another (Cytarme), as do the blastomeres in the egg, completely
or incompletely. The contact surface is generally symmetrical
to the line joining the centres of mass of the two cells ; it may be
concave towards either the small or the large cell. More than
two cells may unite to form rows or heaps. The angles made by
the surfaces of contact may be 120°, or have other values. Four
surfaces may meet in one line ,- at other times the arrangement
is tetrahedral. In a 1-25 Z solution of salt the cells are
elongated, and united end to end in long branching strings.
The pigment, diffused through the cell, later returns to its
original position at the surface, or usually to the middle of the
free surface of each.
 
The cells may also move over one another (Cytolisthcsis) by
54- CELL-DIVISION AND GROVVTH II. I
 
sliding or rotation, or both. Even two cells will glide on one
another, as two soap-bubbles will not. In complexes two threesurface lines may unite to form one four-surface line, a behaviour
the very opposite of that exhibited by soap-bubbles.
 
It appears, then, that in the living egg of the Frog (and other
radial and bilateral types) there are factors which overcompensate, to use Roux’s expression, the purely physical factors by
which the behaviour of the oil-drops is governed. These organic
factors are that division is slow, and begins on the outside ; that
the direction of division—determined by the yolk—is persistent;
that the cell contents are neither perfectly fluid nor perfectly
structureless ; that the cells being different, their surface tensions
may be of dilferent magnitudes, and the whole system, therefore,
not homogeneous; and that the cells possess a more or less solid
rind or membrane, the rind which becomes wrinkled transversely
to the furrow when the cell divides.
 
It would seem that this rind is an important factor, for if
Roux’s experiments be repeated with drops of albumen suspended
between xylol and oil of cloves, to which a little alcohol has been
added, it will be seen that each drop gets a su1:erficial membrane,
and that by these membranes adjacent drops adhere. In fact,
such drops behave more like the cells of the egg‘ than do the oildrops. Thus, a small cell goes towards the inside, or the outside,
according" to the way in which the division is made, and, after
a horizontal division of four equal cells, the upper remain superimposed upon the lower.
 
At the same time, it is apparently because the cells have this
surface film, which the oil-drops have not, that they are able to
flatten against one another as soap-bubbles do; while, on the
other hand, it is because the film is solid that the cells are
unable to move upon one another and adopt the geometrical
arrangement seen in systems of soap-bubbles.
 
There is still another kind of cell-movement to which brief
reference must here be made, since it is found in one type of
segmentation at least. In the segmenting eggs of some Platyhelmia (Triclads), Ascidians (Salps), Echinoderms (Asteroids), and
Coelenterates (0ceam'a), the blastomeres have been seen to completely separate from one another, afterwards reuniting. Roux
II. I CELL-DIVISION 55
 
has observed a similar reunion of the artificially isolated cells of
the Frog’s egg. This Cytotropism, as Roux calls it (Cytotaxis
would be a preferable term), is noticed when the slide is kept
perfectly horizontal and streaming movements of the medium
(albumen) are rigidly excluded. The cells become rounded, and
then approach one another in, more or less, a straight line, oscillating slightly backwards and forwards. The cells must not be
too far apart, not further than a radius of small, or less of large
cells. Groups of two or more cells behave in the same way.
 
The movement may be simply a surface-tension phenomenon,
or, as Roux suggests, more complex, of the nature of a response
to a mutual ehcmotactic stimulus.
 
These various kinds of cell-motion are also an important
feature in such processes of differentiation as the union of cells
to form muscles, tendon, epithelia, and so forth.
 
A review of all the facts thus leads us to conclude that while
some of the phenomena of segmentation-—-the flattening of
cells against one another, the pattern made by the cells in
cleavage, especially of the spiral type—are largely referable to
the action of the purely physical laws of surface tension, there
are many cases, the radial and bilateral types, and the radial and
bilateral periods of spirally segmenting eggs, in which the operation of these laws is restricted and confined by other causes.
But in any case those laws can only co-operate with other factors,
which are to be looked for in the rate and direction of division,
and in the magnitude of the cells, factors which themselves are
dependent on the relation between the cell and its nucleus.
 
Before concluding this section we have to call attention to
some experiments which may possibly throw some light on an
event of fairly frequent occurrence in ontogeny—the division of
the nucleus without the division of the cell,‘ as in the formation
of coenocytia such as striated muscle fibres and the trophoblast of
the placenta ; or the fusion of distinct cells into a syncytium, as
in the trophoblast again; or the secondary union of yolk-cells.
 
‘ In the Alcyonaria the nucleus may divide three, four, or five times
before the egg simultaneously breaks up into eight, sixteen, or thirty-two
 
cells. See especially E. B. Wilson, ‘On the development of Renilla,’
Phil. Trans. Roy. Soc, clxxiv, 1883.
56 CELL-DIVISION AND GROWTH II. I
 
Driesch has observed that in the egg of Ea/Linus cell-division
may be wholly or partially suppressed by pressure, and also by
diluting the sea-water. Nuclear division continues (Fig. 33).
 
Morgan has found that the egg of another sea-urchin (Arbacia)
will not segment in a 2 Z solution of salt in sea-water; on
replacing the eggs in sea—water, however, the nucleus divides
with great rapidity several times, and this is followed by celldivision. So Loch notices that the eggs when treated in this
way, and brought back to their normal medium, divide simul
taneously into four. The egg
 
es of the fish Cteuolabm.s (accord
O“ o«° ing to the same author) behaves
in a similar fashion when first
f I deprived of, and then restored
 
,, 1, to oxygen. .
FIG. 33.—Echinus: suppression of lmf’ again’ has Seen the re
cell-division by 1)1'essure, I), and by union Of sister cells and nuclei
 
heat, a. Nuclear division continues. ' . ' .
(After Driesch, 1893.) in the eggs of Avbacza leleased
 
 
from pressure.
 
Three distinct agencics——mechanical pressure, increase of osmotic pressure, and decrease of osmotic prcssure—arc all capable
of effecting this interesting change in the usual relations of cell
and nucleus. We can only guess at the real cause, and surmise
that it will be found in an alteration of internal and external
surface tensions.
 
LITERATURE.
 
No'rE.—For a complete bibliography of segmentation the well-known
textbooks of 0. Hertwig and Korsehelt and Heider must be consulted.
The literature of ‘spiral’ segmentation is given by Robert (quoted below).
 
F. M. BALI-‘OUR. Comparative Embryology, London, 1885.
 
G. BERTHOLD. Studien fiber Protoplasmameehanik. VII. Theilungsrichtungen und Thcilungsfolge, Leipzig, 1886.
 
G. BORN. Ucber Druckversuche an Froscheiern, Anat. Anz. viii, 1893.
 
T. BOVERI. Die Entwiekelung von Asca;-is megalorephala mit beson—
derer Riicksieht auf die Kernverhaltnisse, Fesfschr. Xupflkr, Jena, 1899.
 
E. G. CONKLIN. P1-otoplasniic movement as a factor of differentiation,
Woods Hell Biol. Lech, 1898.
II. I CELL-DIVISION 57
 
H. DRIESCH. Entwicklungsmechanische Studien, IV, Zeilschr. wiss.
Zool. lv, 1893.
 
H. DRIESCH. Entwicklungsinechanische Studien, VIII, Mitt. Zool.
Stat. Neapel, xi, 1895.
 
A. FISCIIEL. Zur Entwicklungsgeschichte der Echinodermen. I. Zur
Mechanik der Zelltheilung. II. Versuche mit vitaler Fiirbung, Arch.
Ent. Mech. xxii, 1906.
 
J. H. GEROULD. Studies on the Embryology of the Sipunculidae,
Mark Anniversary Volume, New York, 1903. _
 
A. GRAF. Eine 1-iickgitngig gemachte Furchung, Zool. Anz. xvii, 1894.
 
C. HERBST. Ueber dais Auseinandergehen von Furchungs- und Gewebezellen in ka.lkfrciem Medium, Arch. Ent. M¢'c7z. ix, 1900.
 
O. HERTWIG. Die Zelle und die Gewebe, Jena, 1893.
 
O. HERTWIG. Ueber den Worth der ersten Furchungszellen fiir die
Organbildung des Embryo, Arch. nu'l.-r. Anal. xlii, 1893.
 
O. HERTWIG. Ueber einige mu befruehteten Froschci durch Centrifugalkraft hervorgerufene Mcclnmomorphosen, S.-B. Kimigl. prcuss.
All-ad. 11733., Berlin, 1897.
 
J. LOEB. Investigations in physiological morphology, Joum. Morph.
vii, 1892.
 
J. LOEB. Untersuchungen fiber die physiologischen Wirkungen des
Sauerstoffnmngels, I7l:2ger‘s Arch. lxii, 1896.
 
J. LOEB. Ueber Kerntheilung ohne Zellt-heilung, Arch. Ent. Mech.
ii, 1896.
 
T. H. MORGAN. The action of salt solutions on the unfertilized and
fertilized eggs of Arbacia and of other aniimtls, Arch. Ent. Mech. viii, 1899.
 
E. PFLi'IGER. Ueber die Einwirkung (ler Schwerkra.i't und andere
Bedingungen nuf die Richtung der Zclltheilung, 1_’flc'«'ger‘s Arch. xxxiv,
1884.
 
J. PLATEAU. Statique des liquides, Paris, 1873.
 
A. RAUBER. Der karyokiuetische Process bei erhohtem und vermin(lertexn Atmosphiirendruck, Vcrs. Deutsch. Naimf. u. Aerzte, Magdaburg, 1884.
 
A. ROBERT. Rec-herchcs sur le développement des Troques, Arch.
Zool. E.1'p. et G6». (3), x, 1902.
 
W. Roux. Ueber die Zeit der Bestimmung der Hauptrichtungen
des Froschembryo, Leipzig, 1883, also Ges. Abh. 16.
 
W. Roux. Ueber den ‘Cytotropismus’ der Furchungszellen des
Grasfrosches (Rmmfusca), Arch. Ent. Mech. i, 1894.
 
W. ROUX. Ueber die Selbstordnung (Cytotztxis) sich ‘beriihrendcr'
Furchungszellen des Froscheies durch Zellonzusammenfiigung, Zellentrennung und Zellengleiten, Arch. Ent. Mech. iii, 1896.
 
W. Roux. Ueber die Bedeutung ‘geringei-' Verschiedenheiten der
relativen Grfisse der Furchungszellen fur den Clim-akter des Furchungsschemas, Arch. Ent. Mech. iv, 1897.
58 CELL-DIVISION AND GROWTH II. 2
 
G. SMITH. Fauna und Flora. des Golfes von Neapel: Rhizocephala,
 
Berlin, 1906.
O. ZUR STRASSEN. Embryonalentwicklung des Ascarisnmgalorephala,
 
Arch. Em‘. Jlfcah. iii, 1896.
E. B. VVILSON. (_1leava.ge and mosaic work, An-7:. Ivlul. lilo:-72. iii, 1896.
H. E. ZIEGLER. Ueber Furchung unter Pressung, Vcrh. Aunt. Gesell.
 
viii. 1894.
H. E. ZIEGLER. Untersuehungen fiber die eisten Entwicklungsvor
gétnge der Nematoden, Zeifschr. W1'.s-s. Zool. lx, 1895.
H. E. ZIEGLER. Experimentelle Studien fiber die Zelltheilung, Arch.
Elli. llferh. vii, 1898.
 
2. GROWTH
 
Following Davenport we define growth as increase in size or
volume. Since, therefore, growth is increase in all three
dimensions of space, it is most accurately measured not by
increase in some one dimension—such as stature——but by increase
of mass or weight.
 
Growth depends upon the intake of food and the absorption
of water and exhibits itself in the form of increase in the amount
of living matter or of secretions of watery or other substances,
organic or inorganic, intra-cellular or extra-cellular, such as
ehondrin, fat, muein, cellulose, calcium phosphate, and the like.
 
That growth depends———in later stages at least—upon the
intake of food is obvious. That it is due to the absorption of
water has been demonstrated effectively by Davenport for the
tadpoles of Amphibia (zlmtlys/oma, Rana, I311/2)). The method
employed was to weigh known numbers of the tadpoles at
difl:'erent ages, desieeate and weigh again. The results of the
investigation are shown in the accompanying figure (Fig. 34-),
from which it will be seen that the percentage of water rises with
remarkable rapidity———from 56% to 96% during the first fortnight
after hatching. After that point the amount of water present
slightly but steadily declines.
 
The same result is brought out by an analysis of the terminal
buds and successive internodes of plants. It is found in I/ele7'oceutrou (Kraus) that the percentage of water rises rapidly from
the terminal bud to the first internode, more slowly from the
first to the second internode, and then remains constant.
II. 2 . GROWTII 59
 
It would thus appear that during the period of most rapid
growth, growth is efifected by imbibition of water rather than
by assimilation, since the weight of dry substance in the tadpole
during this period does not increase at all.
 
In later development the proportion of water slowly falls.
This may be seen not only in Davcnport’s table of the growth
of Frogs but in the data furnished by Potts for the Chick and by
 
 
FIG. 34.— Curve showing change in percentage of water in Frog tadpoles from the first to the eighty-fourth day after hatching. Abscissae,
days; ordinates, percentages. (After Davenport, from Korschelt and
IIe1der.)
 
Fehling for the human embryo. These data. are given in the
accompanying tables (Tables I, II). The percentage of water,
at first high, slowly falls in both cases; conversely, the percentage of other substances increases.
 
‘ These results indicate that during later development growth
is largely effected by excessive assimilation or by storing up
formed substance’ (Davenport).
 
There are other external agencies by which growth may be
affected in various ways—-such as heat, light, and atmospheric
pressure. These will be discussed in another chapter. For the
present let us confine our attention to certain features which are
characteristic of growth in general, of the growth of the animal
organism under normal conditions. These are the changes that
60 CELL-DIVISION AND GROWTH II. 2
 
take place during growth in the rate of growth itself, in the
variability of the organism and in the magnitude of the correlations between its various parts.
 
TABLE I
 
Showing the percentage of water in Chick embryos at various stages up
to hatching. ( From Davenport, 1899 (2), after Potts.) ’1_‘he table also
shows the hourly and daily percentage increments of weight.
 
Absolute Hourly Daily
 
H f . . P r entage
   
48 0-06 83
54 0-20 0-14 38-3 919-2 90
58 0-33 0-13 16-0 384-0 88
91 1-20 0-87 7-9 189-6 83
96 1-30 0-10 1-7 40-8 68
124 2-03 0-73 2 0 48-0 69
264 6-72 4-69 1 6 38-4 59
TABLE II
 
Showing the percentage of water in the Human embryo at various stages
up to birth. (From Davenport, 1899 (2), after Fehling.) The table also
shows the weekly percentage increments of weight.
 
Age in Absolute weight \Vec-kly per- Percentage
 
weeks. in grammes. Increase‘ ccntnge increment. of water.
 
6 0-975 97-5
17 36-5 35-525 331-2 91-8
22 100-0 63 5 34-8 92-0
24 242-0 142-0 71-0 89-9
26 569-0 327-0 67 6 86-4
30 924-0 355-0 15-6 83-7
35 928-0 4 0 0-1 82 9
39 1640-0 712-0 19-2 74-2
 
VVe follow Minot and Preyer in measuring the rate of growth
by the percentage increments of weight (or of other measurements where weight is not available) during a given interval of
time ; that is to say, by expressing the increase in weight during
a given period as a. percentage of the weight at the beginning
(or end) of that period. The change of rate, if any, is found
by taking such percentage increments for successive equal
increments of time.
 
As a first example let us consider the data furnished by Minot
II. 9. GROWTH 61
 
himself for the rate of growth, after birth, of guinea.-pigs
(Table III, Fig. 35).
 
TABLE III
 
Showing the change of rate of growth in male and female Guinea.-pigs. as
measured by daily percentage increments of weight. (From Minot, 1891.)
 
Average daily per cent. Average daily per cent.
 
Ago in Age in
 
d“Y‘- Ma1§§°'°m§'§§$1cs. “‘°““‘s- Ma1§§.°r°m°§§:.a1..s.
1-3 0.0 2.1 8 0.05 0.2
4-6 5.6 5.5 9 0.3 0.2
7-9 5.5 5.4 10 0.1 0.1
 
10-12 4.7 4.7 11 0.04 0.1
 
13-15 5.0 5.0 12 0.1 0.05
 
16-18 4.1 4.3 13 -0.2 0.3
 
19-21 3.9 3.5 14 0.5 -0.03
 
22-24 3.1 1.7 15 0.2 0.00
 
25-27 2.3 1.9 16 0.07 0.2
 
23-30 2.3 2-6 17 -0.1 -0.02
 
31-33 1.9 1.3 13 -0.05 -0.2
 
34-36 1.7 1-6 19-21 0.006 -0.1
 
37-39 1.9 1.3 22-24 0.02 -0.05
 
40-50 1.2 1.1
 
55-65 1.3 1.3
 
70-80 1.2 0.3
 
35-95 0.9 0.9
 
100-110 0.7 0.3
115-125 0.6 0.5
130-140 0.1 0.2
145-155 0 4 -0.03
160-170 0.3 0.5
175-135 0.2 0.2
190-200 0.2 0.2
205-215 0.4 0.3
 
 
|||||||i| I'I_
||||||| II it
 
15Il| I7 D E II N 75 I50
 
FIG. 3-'3.—Curve showing the daily percentage increments in weight
of female Guine-.1-pigs. (From Mxnot, 1907 )
62 CELL-DIVISION AND GROWTH II. 2
 
An inspection of tlie accompanying table and figure in which
Minot’s results are reproduced will show at once that there is
in both sexes, almost from the moment of birth, a. decline in the
growth-rate. The decline is not, however, uniform. The rate
falls rapidly between about the fifth day (when it is from 5% to
6%) and the fiftieth, from the fiftieth day onwards more slowly,
becoming eventually very small, zero or even negative. The
younger the animal, therefore, the faster it grows; the more
developed it is the more slowly it grows. The rate of growth
in fact varies inversely with the degree of difierentiation. A
mammal, therefore, which is born in a less developed condition
than is the guinea-pig ought to grow at first more rapidly still.
The rabbit is such an animal, and Minot has been able to show
that on the fourth day after birth the young rabbit adds 17 %
to its weight. The curve also shows the same rapid decline in
the growth-rate as was observed in the guinea-pig, followed by
a period of gentle decrease.
 
Accurate observations on the prenatal rate of growth of these
two mammals are lacking, but Henscn’s few observations (quoted
by Preyer) on the weight of guinea-pig embryos show that the
daily percentage increase descends from 220% on the twenty-first
day to 116% on the twenty-ninth day, to 33% on the forty-third
(lay, and again to 6% on the sixty-fourth day, that is just after
birth, the moment at which Minot’s observations begin. Again,
Minot has found, as a result of the investigation of the weight
 
TABLE IV
 
Showing the decrease in the rate of growth of the Human embryo before
birth. Percentage increments calculated from the figures given by
 
Hecker, Toldt, and Hennig. (From Preyer.)
 
Average monthly percentage increments of
Month. Weight. Length.
 
(Hecken) (Toldt.) (Hennig.)
1 _ _ __
2 — 133-3 433 0
3 — 100-0 110-0
4 418-2 71-4 92-8
5 398-2 66-7 69-8
6 123-2 50-0 28-2
7 92-2 16-7 14-2
8 28 8 14-3 11-9
9 25-6 12-5 6-3
10 — 11-1 4 3
II. 2 GROWTH 63
 
of spirit specimens of rabbit embryos that the mean daily
percentage increment is 704 between the ninth and fifteenth
days, but between the fifteenth and twentieth days only 212.
 
The postnatal decline in the
growth-rate is therefore only a
continuation of a process which has
been going on for some time, perhaps from the first moment at
which growth began.
 
The human being forms no exception to this rule. Data of the
growth of the human embryo before birth are somewhat meagre,
but an inspection of the tables
will show that whatever the discrepancies may be between the results obtained by Fehlin g (Table II)
and Hecker (Table IV), they agree
in this, that the growth-rate falls
with great rapidity between the
fourth and the sixth months,thereafter more slowly till the end of
pregnancy. This is graphically
represented in the curve (Fig. 36).
It will be observed from the
table (Table IV) that the rate of
increase of stature also declines,
but less abruptly. This is a poi11t
to which we shall return. For
the study of the postnatal growth
of man very numerous data have FIG’ 36' —' Curve Sh°‘”il‘<‘=’
 
_ monthly prenatal percentage 1nbecn collected byvarious observers. cmments in Man. (From Minot,
Measurements of the bed y weight 1907-)
have been made on Belgians by Quetelet, on Boston school children
by Bowditch, on the school children of Worcester, Mass., and
Oakland, Mass., by Boas, and on English of the artisan and the
well-to—do classes by Roberts. It is unnecessary to reproduce all
these data here, for they all show the same decline in the growth
Monnvso-zaascrogm
64 CELL-DIVISION AND GROWTH II. 2
 
rate, but Quetelet’s measurements for males, being the completest
series, are given in the accompanying figure (Fig. 37). The
figure shows that at the end of the first year after birth the per.
 
2005‘;
 
   
 
any 71/‘ I
Mwmtfiaez;
 
500%
 
E-§
 
._.-..
YEMISI 2 3 0 5 6 7 8 0 Ion Iz I3I4I5 I617 uamzozuzzaaaozs
 
FIG. 37.——Curve showing the yearly percentage increments in weight
of Boys. (From Minot, 1907.)
 
centage increment is as high as 200% (or nearly), but that then
this increment drops to just over 20% at the end of the second year.
From this point the decline is slow but sure, until at the thirtieth
year the annual percentage increase is only 0-1 %. The change of
rate of growth in females is practically the same as in males.
The monthly percentage increment immediately before birth is
about 20% according to Miihlmann’ s curve (Fig. 36) ; this represents an annual percentage increment of, say, 250 %, and the annual
increase at the end of the first year is about 200 %. The postnatal deerease of growth is, therefore, as in other mammals, a continuation of the prenatal change. Further, there are two points
at which the rate diminishes with great rapidity-—between the
fourth and sixth months of pregnancy and between the first and
second years after birth. It would be of the greatest interest
to discover the causes of these sudden decreases. Elsewhere the
II. 2 GROWTH 65
 
diminution is gradual. A point of importance is that in both
years there is a slight temporary rise in the growth-rate about the
time of puberty (see the curve, Fig. 37'). This has been noticed by
all observers, but the actual time of its occurrence ditfers in diEercnt cases ; the rise is invariably earlier in females than in males.
A comparison of the growth of the three mammals considered
is interesting.
A Guinea-pig reaches 775 grammes in 43.2 days.
A Rabbit ,, 2,500 ,, 395 ,,
A Man ,, 63,000 ,, 9,428 ,,
or the average daily increment is for a
Guinea-pig L82 grammes.
Rabbit 6-30 ,,
Man 669 ,,
 
Hence ‘men are larger than rabbits because they grow longer,
but rabbits are larger than guinea—pigs because they grow
faster’. Minot, however, distinguishes between the ‘rapidity’
of growth, the average actual increment, and the ‘ rate’ of
growth, the percentage increment. The average percentage
increments for these mammals are
 
Guinea-pig 0-4-7
 
Rabbit 0-50
 
Man 0-02
 
The rate is, therefore, much slower in man than in the otln r
two. These percentages Minot calls the coefficients of growth.
Together with the duration of growth they determine the ultimate
size of the organisms.
 
The progressive loss of growth-power Minot speaks of as
‘senescence’, and compares to the loss of the power of celldivision in the ‘senile decay’ of Protozoa. The same author
has also brought forward evidence to show that during differentiation there is an increase in the amount of cytoplasm in the
cell, a decrease in the size of the nucleus, and a decrease in the
‘mitotic index’, that is in the proportion, in any tissue, of
dividing cells. During segmentation, of course, the reverse of
this is taking place, since cell-division is rapid and the protoplasm
per cell is being constantly diminished until a fixed ratio between
 
umxixsox 1'‘
66 CELL-DIVISION AND GROWTH II. 2
 
nucleus and cytoplasm is reached (Boveri) (see below, p. 268).
Minot suggests that ‘ senescence ’ and differentiation alike depend
on an increase in the protoplasm.
 
10 Z Aboral Arm-Length.
 
...... _- Oral Arm-Length.
Body-Length.
 
   
 
 
Pluteus of Slrongylomrirotus lividus.
 
120 z
110 z
100 %
90 %
so %
70 z
60 Z
50 z
40 z
30 z
20 z
10 %
 
Shell-length of Linmaea stagnalis.
 
Days. 10 15 20 25 30 35 40 45 50_ 55 60 65 70 75 80 85
 
_ FIG. 38.— Curves showing the change with age of the rate of growth
In the larva of the sen.-urchin Strongylocentrotus (from Vernon's data),
and the pond-snail Limnaea (from Semper's figures). The nbscissae are
days, the ordinates percentage increments.
II. 2 GROWTH 67
 
The decline of the growth-rate may also be seen in Pott's
weighings of the Chick embryo before hatching (Table I) and
Minot’ s Weights of young chickens. It appears from these that
the daily percentage increment is 919 % at the beginning of the
third day of incubation, 189% at the end of the fourth day; at
this point there is a sudden drop to 40 Z, which is still the rate of
growth after eleven days of incubation; eight days after hatching the ra.te is 9% in the male, not quite 9% in the female, and
then comes a period of more or less gradual decline, until when
the chicken is 342 days old it is able to add less than 0-5 % to its
weight per diem.
 
Sempei-’s observations on the pond-snail, Limizaea, and Vernon’s
on the sea-urchin, St1'0ngyloce7m'ot1m, are other examples which
 
 
FIG. 39.—Dai1y percentage increments of weight in tadpoles: the continuous line (a) gives the whole weight, the broken line (b) the dry
weight. (After Davenport, 1899.)
 
may be mentioned. The results of these autl101's are shown in
the accompanying charts (Fig. 38). Their measurements are of
lengths, not of weights.
 
So far we have found no exception to the law of the decline
in the rate of growth as development proceeds. Davenport's
measurements of tadpoles will not, however, conform to the
generalization. As the figure shows (Fig. 39), the daily percentage increments, whether of the whole weight or of the
weight of dry substance only, first rise abruptly, then descend
and then rise again. An explanation of this anomaly may
possibly be found in the fact that Davenport/s measurements are
68 CELL-DIVISION AND GROWTH II. 2
 
taken during that early period when growth is due in the main
to absorption of water, the other measurements (as may be seen
from Tables I, II) during the later period when the percentage of
water has already begun to decline and growth is effected by
other means. '
 
It is_, of course, a commonplace of embryology that the growth
of all the organs of the body does not occur at the same rate.
 
50 %
40 X Chest-girth.
~ - - ~- Stature.
 
'—-——- Weight.
 
30 Z
 
20%
 
10%
 
 
Years. 1 2 3 4 5 6 7 8 9 1011l21314151617l8l920
 
FIG. 40.«—C1n-ves showing the alteration during the first twenty years
 
of life of the rate of growth as measured by weight, stature, and chestgirth in the human being (males). (Constructed from Quetelc-t’s data.)
The abscissae are years, the ordinates percentage increments. (The
percentage increment of weight for the first year could not be included
in the figure. It is given in Fig. 37.)
There are nevertheless few cases in which the exact difference
in rate has been ascertained. From those few cases, however,
it appears that the individual parts, though they do not grow
with equal rapidity, still obey the same law as the whole.
 
Thus human stature exhibits the same loss of growth-power
 
as is shown by the weight of the whole body, with this
11. 2 GROWTH 69
 
difference, however, that the rate is not so high in early stages,
the descent in later stages less abrupt. This will be seen in
Table IV, in which such figures as are obtainable for the prenatal growth-rate are given, and in Fig. 40, in which the
curve of change of growth-rate in human stature has been constructed from Q.uetelet’s data (male Belgians). The percentage
 
in %
l
l
I
I
'.
I
-10 Z  I[e:ul-Length.
ll — — — — — —- Leg-Lengtln.
I -—j—- Stature.
I - - — - - -- Vertebral Column.
I
'3oz- ‘l
I
l
I
I
1
20 %-
 
 
 
“1jI—l"I“T"'T%—l'_‘
Y(':l1'S. I 2 3 4 5 I‘) 7 S 9 1011]2l314151(';1718l‘.)‘.’.0
 
FIG. 41.— Curves showing the alteration during the first twenty years
of life of the rate of growth of stature, length of head, length of vertebral
column, and length of leg in the human being (males). (Constructed
from Quetelet’s data.) Ordinates, percentage increments; abseissae, years.
 
increment in the first year is only 39-6 as against 190-3 for
weight, in the second year 13-3 us against 22-2 for weight.
Thenceforward the rate slowly declines, until at the fortieth year
it is zero, and after the fiftieth year increasingly negative. The
rate of increase of stature is always slightly less than that of
70 CELL-DIVISION AND GROWTH II. 2
 
weight. Q.uetelet’s figures do not show the rise in rate about
the time of puberty, but this phenomenon is apparent in the
data furnished by Bowditch, Boas, and Roberts (see Fig. 4-2).
The change in the growth-rate is practically the same in women
as in men. As with weight, the rise of rate at the time of
puberty is earlier.
 
The decline in the growth-rate of chest-girth is shown in the
same figure (Fig. 40). It will be noticed at once that in this case
the drop in the first year is very great indeed, from nearly 50 % to
nearly 5 %, and that the rate is only diminished by another 24 ‘Z
in the next nineteen years. The weight will depend upon the
volume and the volume on both stature and girth; in. fact
a rough weight-curve might be constructed from the measurements of stature and girth. It is evident that the sudden loss
in the rate of total growth after the first year is due to the very
rapid decrease in the percentage increment of girth.
 
It maybe mentioned that other measurements of girth—girth
by the sternum, the waist, the hips, the neck, the biceps, the
thigh——show the same exceedingly abrupt decrease, almost to
the minimum rate, between the first and second years.
 
In other cases——distance between the eyes, width of mouth,
length of hand, length of foot, arm-length, leg-length, length
and breadth of head, distance from the crown of the head to
the first vertebra, length of the vertebral column—the change
is more gradual ,- the rate of change, however, diifers in different
cases. As an instance of this let us consider the measurements——
from the crown to the first vertebra, the length of the vertebral
column, and the leg-length——whieh together make up the total
stature. The growth-curves of these three and of the whole
stature are presented in the figure (Fig. 41), from which it will be
seen that the growth of the leg is faster than that of the vertebral column (until the eighteenth year), and this than that of the
head. Increase in stature takes place at nearly the same rate as
that of the vertebral column , but is on the whole a little faster.
 
There are few cases—besides man—in which we possess information as to the growth of the parts. In the sea-urchin,
S57'07I_q,I/[Odell/7'0tl(8, Vernon has shown that the growth-rate of the
oral and aboral arms of the Pluteus diminishes rapidly from
II. 2 GROWTH 71
 
the third to the fifth days, more slowly from the fifth to the
eighth days. After this the rate becomes negative, as the skeleton
of the Pluteus is used up by the developing urchin. The
curves of change of rate of growth-—as constructed from
Vernon’s figures—-are shown in the chart (Fig. 38).
 
In Oarcirzm mamas a gradual decrease in the growth-rate of
the frontal breadth can be ascertained from Weldon’s data.
 
We have next to consider another feature of growth, the
alteration of variability. The facts at our command are derived
from a study of Echinoid larvae (Vernon), Duck embryos
(Fisehcl), Guinea-pigs (Minot), the Periwinkle (Bumpus), the
Crab, C'arcz'mts (Weldon), and the human being (Bowditch,
Pearson, Robeits, and Boas). Vernon has shown that in the
Pluteus of Stronyy/ocentrolus the variability of the body-length
increases regularly up to the fifth day, and then decreases
regularly again to the sixteenth day. So Fischel’s measurements
of Duck embryos seem to establish a greater variability in
younger than in older stages. This is true of the whole length,
the head (as far back as the first somitc), the hand and trunk
together, and the total length exclusive of the primitive streak.
The data are, however, too few to be treated statistically; the
variability can only be roughly estimated from the extent of the
limits within which the part varies at each stage.
 
Minot, who expresses the variability of guinea-pigs by the
difference between the mean weight and the mean weight of the
individuals above, and of those below, the mean, likewise finds
that the range in variation diminishes with age, and further
that, in the case of the males, there is a pcriod—-from about the
fourth to the ninth months—when the variability is very much
less than at any other time. No such sudden fall is observed in
the female, only a steady diminution. ‘
 
A more satisfactory calculation of the altera.tion of variability
may be made from the measurements taken by Bumpus of the
‘ventricosity’ (ratio of breadth to length) of the shell of the
Periwinkle, Lit/orina lit/area. The series of observations is very
large, and includes both British and American forms. In the
accompanying table (Table V) the coeflicients of variability (the
standard deviation expressed as a percentage of the mean) are
72 , CELL—DIVISION AND GROWTH 11.2
 
given for each age, as determined by length of shell, for the
English and American periwinkles separately and also for the
complete series. It will be seen that the variability increases
slightly and then diminishes again. This is the case also in
the American examples, where the fall at the end of growth is
greater still, but in the British specimens there is only a slight
fall, at 20-21 mm., followed by a considerable rise. The
possible significance of this diiference in the behaviour of the
same species on the two sides of the Atlantic we shall discuss
 
in a moment.
TABLE V
Showing the alteration in the variability of the ventrieosity of the shell
of the Periwinkle (Liltorina littorea.) during growth.
 
Coefficient of variability (E; x 100).
 
Length in mm. British. American. All.
-15 2-77 3-27 3-25
 
16-17 2-94 3-41 3-34
18-19 3-02 3-39 3-35
20-21 2-93 3-03 3-13
 
22 3-25 2-84 3-02
 
In the meantime let us consider another case, the Crab,
Cr/rcimc.v mocnas, the variability of the frontal breadth of
which was examined by the late Professor VVelden. Weldon
found that the variability, as measured by the quartile error ,
first increased and then suddenly diminished with age (as determined by carapaee length). If the variability is measured by
the coefficient of variability (easily calculated from VVel(lon’s
data) the result is the same. This will appear from the table
 
(Table VI).
TABLE VI
 
Showing the change in the variability of the frontal breaulth with age in
Curr-inns momms. (After Weldon.)
 
Cara ace Len th Q. 0'
 
E, ,,,m_ 3 51- x 100.
7-5 9-42 1-64
8-5 9-83 1-76
9-5 9-51 1-73
10-5 9-58 1-78
11-5 10-25 1-93
12-5 10-79 2-06
13-5 10-09 1-95
 
For the calculation of the variability at different ages in man
data have been provided by Roberts, Bowditch, and Boas. Some
II. 2 GROWTH 73
 
of these results are collected in the following table (Table VII),
from which it may be gathered that the variability diminishes at
first, then rises until it attains a maximum at about the time
of puberty, and then diminishes again, reaching finally a value
which is lower than the original. The values for the coeflicient
obtained by the diifercnt investigators are fairly similar, and
agree very well with those first given by Pearson (for male
new—born infants, weight 15-66, stature 6-50; for male adults,
weight 10-83, stature 3-66). It may be seen from the values for
the newborn that the variability has already undergone adiminution before the age at which the other observations begin.
 
TABLE VII
 
Showing the change in the value of the coefiieient of variability in the
male Human being during growth.
 
Coeffioieiit of variability (1% X 100).
 
Vtleight. Stature.
Years. Boston Worcester, English Boston Toronto \Vorecstor,
(Bowditch). Mass. Artisans (Bowditch \. (Boas). Mass.
(Boas). (Roberts). (Boas)
4 14-00
5 11-56 11-48 4-76 4-82
6 10-28 12-04 10-08 4-60 4-34 5-40
7 11-08 11-87 10-29 4-42 4-35 4-24
8 9-92 11-83 10-78 4-49 4-58 4-32
9 11-04 12-29 10-85 4-40 4-41 4-30
10 11-60 12-92 11-06 4-55 4-68 4-44
11 11-76 14-45 11-90 4-70 4-53 4-51
12 13-72 15-56 11-48 4-90 4-85 4-49
13 13-60 18-07 11-76 5-47 5-36 5-21
14 16-80 16-80 12-74 5-79 5-64 5-43
15 15-32 18-28 14-00 5-57 5-71 5-19
16 13-28 13-95 12-95 4-50 3-92
17 12-96 11-23 11-55 4-55 3-32
18 10-40 12-18 3 69
19 10-29
20 9-03
10-50
10-92
12-04
 
Further, the variability does not merely diminish as the
animal grows older. Its diminution accompanies the diminution in the rate of growth, and when—as at the time of puberty
in man—that rate increases, the variability increases too.
 
The variability of such parts as have been examined for the
74 CELL-DIVISION AND GROWTH II. 2
 
purpose alters in the same way as that of the whole body.
Besides weight and stature Boas has recorded measurements of
height sitting, head-length, head-width, length of fore-arm,
and hand-width.
 
Though the evidence, it must be admitted, is scanty, it is
none the less a remarkable fact that in all the cases we have
examined the variability, whether of the whole organism or of
its parts, decreases with the decrease in the rate of growth.
We seem to be in the presence of a phenomenon of general
occurrence, though what the significance of the phenomenon is
is not at present clear.
 
As is well known, Weldon has argued that the decline in the
variability of the older crabs is due to a selective death-rate, an
argument which is supported by the same author’s observations
on the snail Clausilia, since in this form the variability of the
adult was found to be the same as the variability of the same
individuals when young, but less than the variability of the
general population of young. It is possible that the marked
decrease in the variability of the American as compared with
the British periwinkles may also be attributable to the same
cause, since this animal has only recently been introduced into
America, and may, therefore, be subjected to a more severe
struggle for existence in its new environment.
 
It is doubtful, however, whether this explanation will fit
all cases.
 
Vernon has suggested that at periods of rapid growth the
effect produced upon the organism by a change in its environment must be much greater than at other times, and, since he
has further shown that one of the effects of an adverse change
of circumstances is an increased variability, he argues that an
increase in variability would naturally accompany a high growthrate.
 
Lastly, Boas points out that the rate of growth is itself
a variable magnitude, and this ‘ variation in period’ may, with
other causes, be a factor in producing the actual variation at
each stage. Should that be so, the variability would necessarily
increase and diminish with an increasing and diminishing growthrate, since those that are above the mean would tend to remove
II. 2 GROWTH 75
 
themselves further from the mean than those that are below
could approach it, and the more so the faster they were growing,
and conversely.
 
We have finally to consider very briefly what little is known
of the alteration with growth in the value of the correlation
between various organs. Such data as we have indicate that,
like variability, this value rises and falls with the growth-rate.
 
Boas has ascertained the correlation coefficicnt (p) in man
between weight, stature, height sitting, length and width of
head at diiferent ages. Some of these results are tabulated
below (Table VIII) ; from this table it will be evident that the
value of p decreases, increases, and decreases again. The values
are for girls, and the period of increase is earlier than that found
for boys. In the chart (Fig. 42) are given the successive values
of growth-rate (stature), variability (height), and correlation coefficient (height sitting and head-length) for boys; the three
magnitudes rise at about the time of puberty, and subsequently
decline together. Boas urges that if the actual variability is in
part the efi’cct of variation in period, this eifect will be greater
during periods of rapid development. It follows from this that
if the various organs of the body are equally affected by a change
in the growth-rate, correlations would be closer during periods of
rapid growth than at other periods.
 
TABLE VIII
 
Values of the correlation coefficient, p, during growth for four diflbrent
correlations. Girls, Worcester, Mass. (Boas).
 
Years. Stature and Stature and Stature and Stature and
Weight. Height sitting. Length of Head. Width of Head.
7 -73 -74 -30 -21
8 -76 -79 -36 -15
9 -80 -82 -35 -16
10 ~83 -83 -37 -16
ll -81 -84 -37 -25
12 -77 -82 -38 -27
13 -73 -83 .38 -37
14 -67 -82 -30 -25
15 -6!’) -79 -26 -22
16 -60 -74 -25 -10
 
It will be noticed that the value of p for the difi'erent organs
is different, being greater between axial organs—stature and
height sitting, stature and length of head-—than between longi76 CELL-DIVISION AND GROWTH II. 2
 
tudinal and transverse parts, such as stature and width of head.
The correlation between stature and weight is liigli.
 
To whatever cause it may be due this diminution of correlation
with age is of the greatest interest, since it points to an increasing power of self-difierentiation in the parts of the body.
From other sources also there is evidence of a. progressive loss
 
.10
Correlation
(p). -30 Height sitting
and length of -90
 
head. .10
6
5
Variability 4
(I
— 100 (M X ) 3
Stature. 0
 
 
 
Rate of Growth 4 A
(Percentage 3 7
increments of °
stature). 3 Z
 
12
 
l "T |‘"’l""'l'jl—‘__l'_—‘l—""lj'l
Years. 7 s 9 10 11 12 1:; 14 15 16
 
FIG.42.-—'Figu1'e to show how the rate of growth (percentage increments of stature), the variability (of stature) and the correlation
coefficient (between height sitting and length of head) rise together at
the time of puberty in man and then fall together. (Constructed from
 
the tables of Boas.)
 
of totipotentiality of the parts, of an inereasing- independence
of the parts, of a tendency to be increasingly governed in their
development by factors that reside wholly within themselves.
But this evidence must be discussed elsewhere.
II. 2 GROWTH 77
 
LITERATURE
 
F. BOAS. The growth of’ Toronto children, U.S.A. Report of the
Commissioner of Ed'ucatz'on, ii, 1897.
 
F. BoAs and C. WISSLER. Statistics of growth, Unitecl Slates Education
Commission, i, 1904.
 
H. P. BOWDITCII. The growth of children, Massachusetts Sfale Board
of Iloalih, 1877.
 
II. C. BUMPUS. The variations and mutations of the introduced
Littorina., Zool. Bull. i, 1898.
 
C. B. DAVENPORT. The role of water in growth, Proc. Boslou Soc.
Nat. Hist. xxviii, 1899 (1).
 
C. B. DAVENPORT. Experimental morphology, New York, 1899 (2).
 
A. FISCIIEL. Ueber Varhibilitiit und VVa.ehstl1um des embryonalen
Kéirpers, lllurph. Jahrb. xxiv, 1896.
 
G. KRAUS. Ueber die Wa.sse1-vertheilung in der Pflmize, Fests-rhr.
Fem‘ Iuuulertjiilu-. Best. Natmf. Ges., Halle, 1879.
 
C. S. MINOT. Seneseence and rejuvenation, Jouru. I’h_2/s. xii, 189].
 
C. S. MINOT. The problem of age, growth and death, Pop. Sci. Jllonthlg/,
1907.
 
K. PEARSON. Data. for the 1)l‘Ol)1el1l of evolution in man. Ill. On the
magnitude of certain coeflicients of correlation in mam. 1’rac. Roy. Soc.
lxvi, 1900.
 
W. PREYER. Spezielle Physiologie (les Embryo. VIII. Dns embryona.le Wachsthum. Leipzig, 1885.
 
A. QUETELET. Anthropometric, Bruxelles, 1870.
 
C. ROBERTS. Manual of a.nthropomet1'y, London, 1878.
 
K. SEMPER. Animal Life, 5th ed., London, 1906.
 
H. M. VERNON. '1‘he effect of environment on the development of
Echinoderm larvae : an experimental enquiry into the causes of variation,
Phil. Trams. Roy. Soc. elxxxvi, B, 1895.
 
II. M. VERNON. Variation, London, 1898.
 
W. F. R. WELDON. An attempt to measure the death-ra.te due to the
selective destruction of (larcinus mamas with respect to a, pu.rticul:|.r
dimension, 1’roc. Roy. Soc. lvii, 1894-5.
 
W. I‘‘. R. WELDON. A first study of na.tura,l selection in (L'luu.x-iliu
Imm'nalu, Biometrilca, i, 1901-2.
 
 
==Chapter III==
EXTERNAL FACTORS
 
1. GRAVITATION
 
IN the large majority of cases there is no definite relation
between the vertical and either the axis of the egg, the planes
of its segmentation furrows, or the position of the development
of the embryo in it. Thus the eggs of insects are laid with the
axis making any angle with the vertical, and the same may be
said of Crustacean ova. In such eggs as develop freely in the
sea (some Mollnsca, for example, Polyehaeta, Coelenterata, Ctenophora) the axis and the planes of segmentation undergo a perpetual
change of position, and Oscar Hertwig has shown that in the
eggs of Echinoderms there is no necessary fixed relation between
the direction of the planes of segmentation and the vertical. In
these cases it is clear that the features of development referred
to cannot depend upon the force of gravitation.
 
There are, however, instances in which it seems possible that
the directions of the planes of segmentation—bearing as they do
a constant relation to the axis of the egg-—may depend upon
gravity, since the axis is normally vertical. It was Emil Pfliiger
who in 1883 first brought forward experimental evidence to show
that this was indeed the case.
 
It is well known that the yolk of the hen’s egg always turns
over so that the germinal disc is uppermost, and the egg of the
Frog, free to rotate inside its jelly membrane, invariably takes up
a position with the black pole uppermost, the white pole below.
 
This property of the Frog-’s ovum is exhibited alike by the
ovarian, the coelomic, the uterine, and the freshly laid egg, by
the living egg and the dead egg, by the whole egg, and by
portions that contain both kinds of egg-substance, the yolk and
the cytoplasm, as Roux showed by floating eggs or fragments
in a medium of the same specific gravity. It is simply due to
the fact that in the spherical telolecithal egg the heavier yolk
III. I GRAVITATION 79
 
is placed mainly on one side, while on the other the lighter
protoplasm is more abundant, the yolk granules far smaller and
more sparse. The distribution of these two substances determines
indeed the axis about which the egg has a ‘ rotation structure ’
or is radially symmetrical. The symmetry is further marked by
the disposition of the pigment and the position of the nucleus.
The pigment is placed in a thick superficial layer in the protoplasmic portion, it extends over rather more, sometimes considerably more, than a hemisphere, for there is much variation
in this respect, and its boundary is a circle whose plane is at
right angles to the egg-axis—the line which passes through the
centre of the egg, the centre of the pigmented portion or animal
pole and the centre of the unpigmented portion or vegetative
pole. There is also an axial, less-deeply pigmented plug in the
animal hemisphere. The nucleus is placed axially, b11t excentrically, very much nearer the animal than the vegetative pole, in
a pigment-free spot or ‘ fovea germinativa ’.
 
The egg is invested with a layer of jelly (muein), inside which
it becomes eventually free to rotate. This, however, is not possible
when the egg is first laid, for the jelly is at that time closely
adherent to it. In water, however, the jelly swells up, and
a narrow cavity is formed in about three hours between it and
the egg, and the egg then turns over until its axis is vertical.
The formation of the cavity is much more rapid if fertilization
(insemination) has taken place; in this case the egg turns over
in half an hour. The rapid formation of the perivitelline fluid
is the first effect of insemination, and is due to some substance
secreted by or accompanying the sperm, since the spermatozoon
does not reach the egg for another quarter of an hour (0.
Hertwig). A second effect is an alteration in the viscidity or
cohesion of the egg-contents ; for while in the ovarian or uterine
egg no alteration occurs apparently in the disposition of yolk,
cytoplasm, and pigment, although the egg-axis may make any
angle with the vertical, such an alteration is undoubtedly produced by gravitation (see below) after fertilization has occurred.
Another effect noted by Roux is that fertilized (not merely
inseminated) eggs turn over more rapidly in a medium of like
specific gravity than do unfertilized.
80 _ EXTERNAL FACTORS III. I
 
The most important first result of fertilization is, however,
the replacement of the radial by a bilateral symmetry. About
two or three hours after insemination a certain portion of the
border of the pigmented area, crescentic in shape and extending;
over about half its periphery, becomes grey by retreat of the
pigment into the interior (Roux). The egg can now be divided
into similar halves by only one plane, the plane of bilateral
symmetry, which includes the axis and the middle of this grey
crescent (Fig. 43).
 
 
Fm. 43.—Formation of the grey crescent in the Frog's egg (IE. femparuriu). A, B from the side; 0, D from the vegetative pole. In A, C
there is no crescent, in B, D a. part of the border of the pigmented area
has become grey.
 
The middle of the grey crescent is always diametrically
opposite to the point of entry of the sperm (Roux and Schulze) ;
the crescent has hence been held by Roux to be directly caused
by that entrance.
 
The plane of symmetry, as we shall see in another connexion,
becomes in most cases the sagittal plane of the embryo, since the
dorsal lip of the blastopore arises in the region of the grey
III. 1 GRAVITATION 81
 
crescent. This side becomes the dorsal side of the embryo, while
the animal pole marks, approximately, the anterior end.
 
By complete disappearance of the pigment the grey crescent
becomes added to the white vegetative area of the egg.
 
The foregoing account applies in particular to Rana temporaria
and R. fusca; I{.11aZusm'n appears to be similar, but in R. escaZenta it is stated that the egg—axis is eventually not vertical but
oblique (Fig. 44). It seems, however, doubtful whether this
obliquity is not rather apparent than real. The grey crescent has
apparently not been recognized as such—-the pigment is not so
deep as in the other species—but included, nevertheless, in the
white area, with the result that the centre of this, the definitive
 
 
FIG. 44.——Egg of Rana esculenta after fertilization, in its normal
position with the axis oblique (‘?). A, from the side; B, from above;
an’, egg-axis ; mm, plane of first furrow. (After Korsehelt and Heider.)
white area, has been confounded with the centre of the original
unpigmented area or vegetative pole of the vertical egg-axis.
(Compare Fig. 44 A with Fig. 43 B.)
 
As is well known, the planes of division during the first few
regular phases of segmentation bear a perfectly definite relation
to the axis. The first two, at right angles to one another, are
meridional and therefore also vertical, the third furrows are parallel
to the equator, therefore also horizontal; the furrows of the
fourth phase are again meridional, and hence vertical, those of
the fifth once more latitudinal and horizontal. It is this obvious
relation of the planes of cleavage to the direction of gravity
which has raised the question whether there is not a causal connexion between the two, the question which Pfliiger attempted
 
mnxmson G
82 EXTERNAL morons 111. 1
 
to answer by experiments, performed, however, not on the eggs
of the Frog but on those of a Toad, Bombinator igneus.
 
The close adherence of the unlaid egg to the glutinous jelly,
which in its turn could be easily fixed to some object, provided
a simple method of keeping the egg in any desired position.
The eggs were removed from the uterus, attached with the axis
at various angles to the vertical to watch glasses and fertilized
with just enough sperm-water to allow of development, but not
enough to permit of the formation of the pcrivitelline space and
rotation of the egg into the normal position with the axis
vertical. In these forcibly inverted eggs it was found that the
furrows of segmentation bore the same relation to the vertical
as in the normal egg; that is to say, the first was vertical, the
second vertical and at right angles to the first, the third horizontal and nearer the upper pole, whatever the inclination of the
egg-axis to the vertical, except in the extreme case where the
white pole was exactly uppermost (180° of inversion), when
segmentation did not occur at all. There was, however, no
definite relation between the plane of the first (and therefore of
subsequent) furrows and the original axis of the egg; the angle
between this axis and the plane of the first furrow, as also that
between the first furrow and the plane including the original
and the actual vertical axes of the egg, might, it was found, have
any value.
 
Except in a few cases, and where the white area was nearly
exactly uppermost, these eggs gave rise to normal embryos.
The upper smaller cells divided more rapidly than the lower ones,
whether pigmented or unpigmented, and the blastula stage was
reached; the dorsal lip of the blastopore appeared on one side
a little below the (actual) equator, and the lower surface was
covered over by the blastoporic fold in the ordinary Way. Only
in the failure of the whole egg to rotate after the closure of the
blastopore (owing to the close adherence of the jelly) and in
the irregular pigmentation (according to the original degree of
inclination) did these embryos differ from the normal. One
other point is worthy of notice. In the majority of cases the
dorsal lip of the blastopore, marking the sagittal plane, appeared
on the unpigmented side and lay in the plane including the
III. 1 GRAVITATION 83
 
original, now inclined, and the actual axis, or vertical line of
intersection of the first two furrows. While, therefore, the
cleavage planes are definitely related to the vertical but not to
the original axis of the egg, the median plane of the embryo
appears to be jointly determined by both.
 
From these experiments Pfliiger drew remarkable and farreachingconclusions. He conceived of the eggas beingmeridionally
polarized, composed of a large number of rows of molecules placed
meridional] y with regard to the original egg-axis. Each row is
equivalent developmentally to every other row, but within the
limits of each the molecules are of difierent value, since one end,
for example, is anterior, the other posterior. .Which of these
equipotential rows shall lie in the sagittal plane of the embryo is
decided by gravity, and by gravity alone. Similarly the vertical
direction of the first two furrows, the horizontal direction of
the third, is due to the operation of some general, though at
present unknown, law, in accordance with which ‘ (lie Schwerkraft
die Organisation beherrseht ’. 1
 
The original structure of the egg, on the other hand, has no
definite relation either to segmentation or to the symmetry of
the embryo, except, of course, in so far as the original axis
together with the actual vertical axis determines its sagittal
plane, the white side its dorsal side.
 
‘. . . das befruchtete Ei gar keine wescntliche Beziehung zu
der spiiteren Organisation des 'l‘hieres besitzt, sowenig als die
Sehneeflocke in einer wesentliehen Beziehung zu der Grosse
und Gestalt der Lawine steht, die unter Umstiinden aus ihr sich
entwickclt. Dass aus dem Keime immer dasselbe entsteht,
kommt daher, dass er immer unter dieselben iiusseren Bedingungen gebracht ist.’ 2
 
It will certainly be agreed that so sweeping and revolutionary
a dogma as this is in need of very substantial support; and
though the facts as stated by Pfliiger are incontrovertible, as
the repetition of his experiments has shown, it is unfortunate
that he did not also take into consideration the internal changes
occurring in his forcibly inverted eggs. The deficiency has been
made good by Born. Like Pfliiger, Born found that in the
 
’ 1. C. i11fl'a, XXxii- p. 24. 3 I. c. infra, xxxii. p. 64.
G 2
84 EXTERNAL FACTORS III. I
 
forcibly inverted eggs the cleavage planes had the normal relation
to the vertical, but not to the egg-axis; he observed, however,
that the first furrow was usually in or at right angles to the
streaming meridian, the plane, that is, including ‘the original and
the secondary vertical axes. The recent examination of 215
cases by the writer has shown that the first furrow tends to lie
either in, or at right angles to, or at an angle of 45° to this
plane. Subsequent development was normal, and the sagittal
plane coincided with the streaming meridian. The dorsal lip
appeared on the white side, which is thus anterior (antc1'o(l0rsa.l).
The examination of sections, however, showed that in these
 
 
FIG. 45.-—Seetions through forcibly inverted Frog's eggs. In A the egg
has just been inverted, in B the streaming of protoplasm upwards and
yolk downwards has begun. Both sections are in the streaming meridian
or gravitation symmetry plane, including both the axis (a a’) and the
vertical. bD, pigmented animal protoplasm; wD, unpigmented vegetative yolk. ct, animal pole; a’, vegetative pole; 1', pigment-free clear
area; A-, nucleus; 1), superficial pigment. (After Born, from Korsehelt
and Heider.)
 
inverted eggs there had been a redistribution of the contents,
the heavy yolk sinking to the lower side, the lighter protoplasm,
with the pigment and the nucleus, and the spermatozoon rising
to the upper side (Fig. 45). The movement is rotatory, the cytoplasm and yolk ascending and descending in opposite directions ;
and it also takes place naturally parallel to, and in a similar
manner on each side of, the plane in which the primary and
secondary axes lie, hence known as the streaming meridian. That
end of this plane towards which the protoplasm moves in its
ascent, the end, that is, marked by the primary vegetative pole,
III. I GRAVITATION 85
 
is anterior (more correctly, anterodorsal), for it is here that the
dorsal lip of the blastopore appears; the opposite end is posterior
(more correctly, postero-ventral).
 
The pigment moves with the cytoplasm; it is, however,
unable to completely displace that yolk which remains at the
upper surface in consequence of the greater viscosity of the
superficial rind, and here a ‘white plate’ or ‘grey patch’ is
formed. Similarly, at the lower surface the pigment is not
necessarily wholly displaced by the descending yolk.
 
There is one special case that may be noticed. When the inversion is complete (l80°) the yolk flows radially and peripherally
away from the upper pole while the cytoplasm ascends in the axis.
 
Born’s observations make it perfectly clear that gravity rearranges the eontents of these inverted eggs, and so confers
upon them a secondary structure like that of the normal, and
symmetrical about a secondary axis which is likewise vertical.
To this secondary axis the direction of elongation of the karyokinetic spindles, and consequently the cleavage planes, bears the
same relation as in the normal egg; and there is certainly no
more need to explain these directions by reference to gravity,
to suppose, in fact, a causal connexion between the two, in the
one case than in the other. The planes, indeed, may fall where
they do simply because the mitotic figures elongate in the direction of least resistance (Pfliiger) or (O. Hertwig) in that of the
greatest protoplasmic mass, or may be related, in some similar
way, to the structure of the egg alone.
 
The point can only be determined by an experiment in which
the directive influence of gravity is eliminated. This experiment has been made by Roux. The eggs were fastened in
small vessels, at distances of from one to eight centimetres
from the centre, to a wheel rotating continually about a horizontal axis, but so slowly (one revolution in from one to two
minutes) that the centrifugal force developed was insullicient to
make the eggs turn with the white pole outwards, and therefore
negligible. The direction of the force exerted by gravity upon
them from moment to moment was thus not constant. Of the
eggs some were free to rotate inside their jelly, others were
fixed. To anticipate the objection that the plane of rotation,
86 EXTERNAL FACTORS III. I
 
the plane of the wheel, is constant, a third set were packed
loosely in test-tubes, and so able to roll over one another in all
directions as they fell from one end of the tube to the other
with each revolution. The first furrow appeared in all these
eggs at the normal time and it was meridional, as in the normal
egg; similarly the second was meridional, the third latitudinal;
but the egg-axes exhibited no definite relation either to the
vertical or to the plane of the wheel. The eggs were allowed
to continue their development on the apparatus, and gave rise
to normal tadpoles.
 
From this experiment Roux drew the conclusion that it is not
gravity which determines the direction of the planes of cleavage,
and that gravity is not an indispensable necessity for the normal
development of the egg of the Frog.
 
Incontrovcrtible though this conclusion appears to be on the
evidence, it has nevertheless been disputed by certain embryologists, Sehulze and Moszcowski, the controversy between whom
and Roux upon the subject has new extended over many years.
 
Schulze has urged (1) that eggs placed on such a machine
do not develop normally, and  that the rotation of the eggs
in their jelly exactly compensates for the rotation of the wheel.
With regard to the latter point Roux has replied that on this
supposition the egg‘-axes ought to be, at any moment, vertical,
which is not the case. To the first objection it is a sufficient
answer that not only Roux, but subsequent investigators
(Morgan, Kathariner and the present writer) have been able
to produce normal tadpoles from such rotated eggs.
 
It may be noticed here that Kathariner has repeated Roux’s
experiment with a slight variation. The eggs were kept constantly rotating, not in one but in an indefinite number of
planes by a stream of air-bubbles passing through a glass vessel
filled with water. Development was normal. This result does not
differ materially from that obtained by Roux with the test-tube
eggs referred to above, which has indeed been also independently
corroborated by Morgan.
 
The criticisms of Moszcowski take a different form. This
author urges that gravity always exercises an influence upon
the egg in determining the bilateral symmetry of both egg and
III. 1 GRAVITATION 87
 
embryo. The grey crescent which appears soon after fertilization and is regarded by Roux as a direct effect of this process, is
supposed by Moszcowski to be produced by the action of gravity
upon the egg-contents during the short interval before the
perivitellinc space is formed and the egg able to turn over,
to be comparable, in fact, to the grey area or white plate
described by Born in his forcibly inverted eggs. Every normal
egg, therefore, has a ‘gravitation’ plane of symmetry which
later on becomes, as in inverted eggs, the median plane of the
embryo ; nor are the eggs on the rotatory apparatus exempt, for
it is held that the work of gravity can be accomplished on them
even in the few moments before they are placed on the machine.
 
With regard to the latter point both Katharincr and Morgan
have demonstrated that eggs kept in a state of perpetual
rotation in all directions, from the very moment of insemination
develop into perfectly normal, bilaterally symmetrical embryos,
while Roux has replied to the first part of the criticism by
pointing out that the grey area observed by Moszcowski was
not the normal grey crescent produced by the entering spermatozoon, but the ‘white plate’ of Born due to the incomplete
rearrangement of yolk and cytoplasm in an egg which had been
quite unintentionally prevented from assuming its normal
position. The grey crescent, indeed, Roux argues, could not
possibly be due to gravity in a normal egg, for it does not
appear until some time after the axis has become vertical.
 
There seems, therefore, to be little room for doubt that Roux’s
original contention, that gravity does not determine the symmetry of the egg and embryo in the Frog, is correct, although
it remains a result of considerable importance that this external
factor may be made artificially to induce a bilaterality in the
egg which is sullieiently strong to persist as the symmetry of the
embryo.
 
There is one other matter of interest in this connexion. It is
obvious, and has been experimentally shown by O. Hertwig,
that a centrifugal force can replace gravity. On a. wheel rotated
with sufficient velocity the eggs turn with their axes radial, their
white poles outermost. If the velocity is great enough (145 revolutions a minute, radius from 24- to 32 cm.) the yolk is driven
88 EXTERNAL FACTORS III. I
 
inside the egg towards the vegetative pole, and the distinction
between it and the protoplasm accentuated. The segmentation
of such eggs is mcroblastic ; a cap
of small cells is formed, a blastederm, resting upon an undivided,
though nucleated, yolk, and these
yolk-nuclei are large and irregular,
resembling the giant nuclei of
the large-yolked eggs of Elasmobranchs and other forms (Fig. 46).
An experimental confirmation is
 
FIG- 46-—S8gm6nt9:ti911 Of We thus aiforded of Balfour’s hypoFmgis egg under the Influence thesis tf 'v' rd on com arative
 
, P11 0).‘: d I)
 
of a centrifugal force (from KorSchelt and Heldelu after 0- Hert- grounds, that it is on the varying
 
wi ). The egg consists of a blas- - - _
togerm and an undivided yolk quantity of yolk that differences
 
(yolk-syncytium): Ich,blastocoel; in the segmentation of eggs prim, yolk-nuclei; d, yolk. marily depend_
 
If removed from the centrifuge in time, such eggs may continue to develop, though they frequently give rise to monstrosities
 
(Spina bifizla).
 
 
LITERATURE
 
G. BORN. Ueber den Einfluss der Schwerc auf das Frosehei, Arch.
mikr. Auat. xxiv, 1885.
 
0. HERTWIG. Welchen Einfluss iibt die Schwerkraft auf die Teilung
der Zellen ‘P Jen. Zeitschr. xviii, 1885.
 
0. HERTWIG. Ueber einige am befruchteten Froschei durch Centrifugalkraft hervorgerufene Mechanomorphosen, S.-B. lci1m'g. preuss. All-ad.
Wiss. Berlin, 1897.
 
L. KATHARINER. Ueber die bedingte Unabliiingigkeit der Entwicklung des polar differenzirten Eies von der Schwerkraft, Arch. Em.
Mech. xii, 1901.
 
L. KATHARINER. Weitere Versuche fiber die Sclbstdifferelizirung dcs
Froschcies, Arch. Ent. Mech. xiv, 1902.
 
F. KEIBEL. Bemerkungen zu Roux’s Aufsatz ‘ Das Niclitiietliigsein der
Schwerkraft fiir die Entwicklung des Froscheies ', Amzf. Anz. xxi, 1902.
 
T. H. MORGAN. The dispensability of gravity in the development of
the Toad’s egg, Auat. Anz. xxi, 1902.
 
T. H. MORGAN. The dispensability of the constant action of gravity
and of a. centrifugal force in the development of the Toad‘s egg, Anat.
Anz. xxv, 1904.
 
M. MOSZCOWSKI. Ueber den Einfluss der Schwerkraft auf die Entstehung und Erhaltung der bilateralen Symmetric des Froscheies, Arch.
 
milcr. Anal. 11:, 1902.
III. I GRAVI'1‘ATION 89
 
M. MOSZCOWSKI. Zur Analysis der Schwerkraftswirkung auf die
Entwicklung des Froscheies, Arch. milcr. Amzt. lxi, 1903.
 
E. P1«‘LiiG1«:R. Ueber den Einfluss der Schwerkraft auf die Teilung
der Zellcn, Pfliigeris Amh. xxxi, xxxii, xxxiv, 1883.
 
W. Roux. Ueber die Entwicklung des Froseheies bei Aufhebung der
riehtenden Wirkung dcr Schwerc, Breslau drtz. Zeitschu, 1884 ; also
Ges. Abh. 19.
 
W. RoUx. Bemerkung zu O. Schu1ze's Arbeit iiber die Nothwendigkeit, etc., Arch. Ent. Mech. ix, 1900.
 
W. RoUx. Das Nichtnothigsein der Schwerkraft fiir die Entwicklung
des Froscheies, Arch. Ent. Mech. xiv, 1902.
 
W. ROUx. Ueber die Ursachen der Bestimmung der Hauptrichtungen
des Embryo im Froschei, Auat. Auz. xxiii, 1903.
 
O. SCHULZE. Ueber die unbedingte Abhiingigkeit normaler tierischer
Gestaltung von der Wirkung der Schwerkraft, Verh. Anat. Ges. viii, 1894.
 
O. SCIIULZE. Ueber die Nothwendigkeit der freien Entwicklung des
Embryo, Arch. 1m'kr. Amtt. lv, 1900.
 
O. SCHULZE. Ueber das crste Auftreten der bilateralen Symmetric
im Verlauf der Entwicklung, Arch. milrr. Anal. lv, 1900.
 
2. MECHANICAL AGITATION
 
The necessity of perpetual and violent agitation for the very
numerous pelagic ova which are ordinarily exposed to the stress
of wind and weather is well known to every zoologist who has
attempted to rear such forms in an aquarium, and need not be
further insisted on.’
 
There are also other eggs which require a small amount of
movement. The Hen turns her eggs every day, and the operation has to be artificially performed in an incubator. Its omission
leads to serious consequences, for, as Dareste has shown, the
allantois sticks to and ruptures the yolk-sac in unturned eggs, the
ruptured yolk-sac cannot be withdrawn into the abdomen, and
the Chick cannot hatch out. Death may ens11e at an early stage.
 
A violent agitation of the IIen’s egg, on the other hand, is
equally fatal.
 
Dareste subjected the unincubated eggs to violent shocks at
the rate of 27 a second for varying periods (from %' hour to
1 hour). The percentage of monstrosities observed after three
or four days of incubation was very high indeed, except when
 
‘ It seems probable that the principal value of the mechanical agita
tion to the larvae is to prevent the Diatoms and Algae, of which their
food consists, from sinking to the bottom.
90 EXTERNAL FACTORS III. 2
 
the eggs were placed vertically with the blunt pole uppermost,
the blastoderm therefore resting against the shell membra.ne.
 
Marcacci has exposed the eggs, inside the incubator, to continual rotation for 48 hours. The eggs were fastened to horizontal and vertical wheels rotating 40, 80, and 60 times a minute.
At the last mentioned rate of revolution, the direction of rotation
was reversed half-way through the experiment.
 
Many of the eggs actually hatched out, but the chickens were
feeble and liable to disease, and exhibited malformations of the
muscles or skeleton. Others, however, died before hatching, in
some cases at an early stage, and death seems to have been due
to rupture of the vitelline membrane; this was always fatal.
The vertical motion was, on the whole, more harmful than the
horizontal, owing to the perpetual see-saw.
 
It may be noted here that Féré has succeeded in producing
retardation and abnormality of development in the Chick by
means of short exposures to sound-vibrations.
 
Mathews has shown that mechanical agitation —violent
shaking in a test-tube—is sufficient to provoke development
(artificial parthenogenesis) of the unfertilized eggs in As/arias,
but not in /lrlacia (see, however, below, p. 124).
 
LITERATURE
 
C. DARESTE. Rccherches sur la production des monstruosités par les
secousses imprimées aux ueufs de poules, Comptcs Rendus, xcvi, 1883.
 
C. DARESTE. Sur le role physiologique du retournement dcs oeufs
pendant l‘incubation, Comptes Remlus, c, 1885.
 
C. DARESTE. Non velles rccherches concern-ant Pinfluence des secousses
sur le gei-me de 1'wuf de la. poule pendant la période qui sépare la pontc
de la mise en incubation, Comptes Remlus, ci, 1885.
 
C. DARESTE. Note sur 1‘évolution de l’embryon de la poule soumis
pendant Pincubation in. un mouvenient de rotation continu, Comptes Ilumlus,
cxv, 1892.
 
C. F1~':R£':. Note sur les differences des effets des vibrations mécaniques
sur l'évo1ution de l’embryon de poulet suivant 1‘époque oil elles agissent,
C. R. Soc. Biol. (10) i, 1894.
 
C. Ffiné. Note sur Pincubation dc l'oeuf dc poule dans la position
verticalc, C. R. Soc. Biol. (10) iv, 1897.
 
A. MARCACCI. Influence du mouvement sur le développement (les
ceufs de poule, Arch. Ital. Biol. xi, 1888.
 
A. P. MATHEWS. Artificial parthenogenesis produced by mechanical
agitation, Amer. Jomw. Phys. vi, 1901-2.
91
 
3. ELECTRICITY AND MAGNETISM
 
An external agent, to which all eggs are inevitably exposed, is
the natural magnetism of the earth. No evidence has, however,
as yet been brought forward that this agent exercises any directive
influence upon them, although their development may be distorted
by excessive exposure to it.
 
Thus Windle placed a number of Hens’ eggs between the
poles of a large horse-shoe magnet. Over 50 ‘A of these, when
incubated, gave rise to abnormalities, the area vasculosa being
affected in most cases. No relation could be detected between
the position of the egg in the magnetic field and the kind of
monstrosity produced.
 
In the case of Trout ova similarly treated a very high deathratc was observed, but this was attributed by the experimenter
to the action of the electric currents set up by the running water
between the poles of the magnet. VVealc electric currents had
less effect.
 
Silkworms’ eggs, however, suffered no harm.
 
The effects of the electric current upon the eggs of Amphibia
and Birds were tested by some of the older observers. Rusconi,
Lombardini, and Fasola all found that the development of the
Frog's egg could be accelerated by weak currents. Lombardini
produced monstrosities in the ease of the Chick by this method.
More modern experiments are due to VVindle, Dareste, Rossi,
and Roux.
 
Windle observed a fairly high death-rate amongst Trout eggs
exposed to the action of the current. Dareste has found a large
percentage of monsters among embryos developed from Hens’
eggs subjected for from one to three minutes to the electric
spark (12 cm. long from Bonnetty’s machine, 3-35 cm. long
from a Rhumkorif coil). Development was, however, normal in
the case of eggs placed for an hour in a Tesla’s solenoid traversed
by a discharge of 500,000 periods a second. Rossi employed
a continuous current passing through the eggs (of Salm/lamb-iua
perspicilla/a) in the direction of the axis. Both yolk and
pigment became aggregated at the animal pole, leading to the
formation there of a grey raised area surrounded more or less
92 EXTERNAL FACTORS III. 3
 
completely by a furrow. When segmentation occurred the first
two blastomeres were unequal and detached; the vegetative
hemisphere was hardly segmented at all in later sta.ges, the
previous divisions having disappeared. The nuclei were affected
in various ways, and the directions of the cleavage spindles
altered. The capacity for resistance to these evil effects was
noticed to increase as development advanced.
 
The polar area produced in these experiments recalls the polar
areas observed by Roux in Frogs’ eggs exposed to a horizontal current, at right angles, therefore, to the axis. Alternating currents
of 50 and 100 volts were employed. The eggs were fertilized
two or three hours before the commencement of the experiment.
In from fifteen to thirty seconds after exposure two polar
areas appeared in each egg. The polar areas were turned
towards the electrodes. They were marked, dotted in various
ways, and flocked with white extruded drops of yolk, and
separated by furrows from a middle or ‘equatorial’ zone, the
width of which varied directly with the distance of the egg
from the electrode, inversely with the strength of the current
and the duration of exposure.
 
Unfertilized ova were found to react in the same way. So
also eggs in which segmentation had begun, and in those cases
where the furrow cut the equatorial zone obliquely, the two
halves of the latter turned away from one another.
 
The polar areas appear too in eggs which are exposed in the
‘ morula ’ stage, each cell having in addition a polar area of its
own. The latter, however, do not appear in enfeebled eggs, but
only the former.
 
In the gastrula and later stages the reaction occurs, but less
markedly.
 
None of the eggs which have been exposed to the current
develops any further. They stick to the jelly, and consequently
lose their power of rotation.
 
Similar results were obtained by the use of the continuous
current (43 volts), but the anodic and the kathodie areas usually
differed from one another in certain details.
 
It is important to notice that neither in these experiments,
nor in another in which the eggs were placed inside a glass
III. 3 ELECTRICITY AND MAGNETISM 93
 
tube surrounded by a coil, could any definite relation be satisfactorily made out between the direction of the first furrow and
that of the current. Indeed, though intrinsically interesting,
the experiments throw no particular light upon the problem of
development. Rather should they be classed with the investigations of Verworn and others upon the behaviour of Protozoa
in the electric current, investigationswhich promise to contribute
to the understanding of the structure and movements of living
substanee. It may be noted here that Roux has himself
produced these polar areas on such structures as the heart and
gall-bladder of the Frog and other vertebrates.
 
LITERATURE
 
C. DARESTE. Rechcrches sur l‘iniluence de l’électi-icité sur 1'4.’-volution
dc l‘embr_yon de la poule, Comples Bemlus, cxxi, 1895.
 
U. ROSSI. Sull‘ azione dell‘ elettricita nello sviluppo dclle uova
(legli Anfibi, Arch. Ent. Mech. iv, 1897.
 
W. RoUx. Ueber die morphologisehe Polarisation von Eiern uml
Embryonen durch den electrischen Strom, sowie fiber die Wirkung des
eleetrischen Stroms auf die Riehtung (ler elsten Teilung des Eies,
S’.-B. Icais. Alrud. IViss. Wien, ci, 1891, also Ges. Abh. 25.
 
B. C. A. WINDLE. On certain early ma.lformn.tions of the embryo,
Jouru. Aunt. and Phys. xxvii, 1892-3.
 
B. C. A. W1NDLE. The effects of electricity and magnetism on
development, Journ. Aunt. and Phys. xxix, 1895.
 
4. LIGHT
 
As Roux pointed out long ago in the case of the Frog, light
exercises no directive influence upon the development of the
ovum. Blane, indeed, has attempted to prove that the direction
of the embryonic axis in the egg of the Hen may be made to
depend upon the direction of the incident light-rays, but the
experiments are hardly conclusive. The method employed was
to blaeken the shell of the horizontally placed egg with the
exception of one spot to right or left of the blastoderm. On
this spot a beam of light was kept directed during incubation.
In some cases, but not in all, the embryonic axis was found to
deviate from its normal position at right angles to the long
axis of the shell. Further, the head of the embryo might be
turned towards or away from the source of light. There was
94 EXTERNAL FACTORS 111. 4
 
no relation between the amplitude of the deviation and the
length of the exposure.
 
Nor are the processes of growth and differentiation necessarily
affected in any way at all by the presence or absence of light,
or by the kind of light to which the eggs are subjected.
 
Thus Driesch, who has experimented with the eggs of
I9’o/Mme, P/auorbis, and Rana, maintains that neither red, yellow,
green, blue, nor violet light has the slightest eifect upon the eggs
during the early stages of segmentation and gastrulation, in
what he calls the organ-forming period of development; and
Loeb has asserted that the development of the embryos of the
lish Fzmthzlus is as rapid in darkness as in the light, except that
on the yolk-sac (not in the embryo) far fewer pigment-forming
cells are produced.
 
Yung, on the other hand, has brought forward evidence to
show that in later stages, at any rate, the embryos of the Frog
react difierently to lights of various wave-lengths, some of which
are harmful, others, apparently, beneficial.
 
Yung obtained his colours from solutions of fuchsin (red),
potassium bichromate (yellow), nickel nitrate (green), bleu de
Lyon (blue), and viole de Parme (violet). The colours, it may
be noticed, are not absolutely monochromatic.
 
Freshly laid eggs of Rana tem17omria were placed under the
influence of these lights. After one month, samples of the tadpoles were measured, with the following result in millimetres :
TABLE IX
Red. Yellow. Green. Blue. Violet. White.
Length 2158 25-91 18-83 26-83 29-66 25-75
Breadth 4-83 5-58 4-16 5-75 6-83 5-25
 
The mortality in the green light was great.
After two months the dimensions were as follows :—
 
TABLE X
Red. Yellow. Green. Blue. Violet. White.
Length 26-25 31-83 All ‘ 33-50 41-30 31-00
Breadth 6-00 7-50 dead. 8-00 10-16 7-33
 
All the tadpoles in the red light eventually died.
White and yellow light gave the greatest number of perfect
frogs, but, as will be seen, those in the violet were larger. They
III. 4 LIGHT 95
 
were, however, less differentiated, for they did not acquire their
hind legs so soon as did those in the white light. It may be
mentioned, however, that when the tadpoles reared under these
conditions are replaced in ordinary light and starved, those from
the violet exhibit a. greater power of resistance.
 
Experiments with Rana ésczzlenta gave the same result. In
this case the effect of darkness was also tried and found to be
distinctly unfavourable. Thus after one month the lengths in
darkness and white light were respectively 19-66 mm. and
23-10 mm., the breadths 4-66 mm. and 5-50 mm. ,- after two
months the difference was intensified, the lengths being 21-50 mm.
and 32-16 mm. , the breadths 7-16 mm. and 7-66 mm. The deathrate in the dark was exceedingly high.
 
The eggs and embryos of the Trout were likewise found by
Yung to be highly sensitive to green and red light, while the
larvae reared in violet hatched out rather more quickly than
those from yellow, blue, or white light.
 
In an experiment on the eggs of I/inmaca stayizalie, due to the
same investigator, the effect is measured by the time required
for the young to hatch out, as the following table shows :—
 
TABLE XI
 
Light. Time to hatching in days.
 
Red . . . . . . 36
 
Yellow . . . . . . 25
 
Green . . The heart is formed, then death occurs.
Blue . . . . . . 19
 
Violet . . . . . . 17
 
White . . . . . . 27
 
Dark . . . . . . 33
 
Green light is evidently fatal; development is retarded in red
light, less so in darkness ; yellow has about the same clfcet as
white light, while there is a considerable acceleration in blue and
violet.
 
The relative effect produced by the various lights is as in the
preceding experiments.
 
The results obtained by Vernon for Eehinoid larvae are,
however, not quite consonant with this, as may be seen in the
table (Table XII), where the colours are arranged in the order
of the efiect they produce. It will be observed that yellow is
96 EXTERNAL FACTORS III. 4
 
more harmful than red, while green exerts about the same effect
as blue (copper sulphate). The author states, however, that in
two other experiments the larvae were entirely killed off by
the green light though developing perfectly in the white. He
also adds that in violet light no development was possible owing
to the swarms of bacteria.
 
TABLE XII
Percentage change of size.
 
Semi-darkness . . . . . + 2-5 ‘
Absolute darkness . . . . . — 1-3*
 
Blue (copper sulphate) . . . . -46
 
Green . . . . . . . -4-8
 
Red . . . . . . . -6-9
 
Blue (bleu de Lyon) . . . . —-7-4
 
Yellow . . . . — 8-9
 
* Almost within the limits of experimental error.
 
In the Pluteus Vernon found that both the oral and the
aboral arm-length decreased in darkness, green and blue (bleu
de Lyon) lights, while blue (copper sulphate), yellow, and red
light exerted little influence on this magnitude.
 
It only remains to be added here that Blanc and Féré have
brought forward some not very satisfactory evidence to show
that white light is favourable to the development of the Chick.
Féré has also stated that red and orange lights are more
harmful than white, while violet has about the same elfect.
The experiments are, however, vitiated by the fact that the
eggs were not turned over.
 
LITERATURE
 
L. BLANO. Note sur l‘influence de la lumiere sur l'o1-ientation de
l'embryon dans l’oeuf de poule, C. R. Soc. Biol. (9) iv, 1892.
 
L. BLANC. Note sur les effets tératogéniques de la lumiere blanche
sur l’oeuf de poule, C. R. Soc. Biol. (9) iv, 1892.
 
H. Dnmscn. Entwicklungsmechanische Studien II, Zeitschr. wiss.
Zool. liii, 1892.
 
C. Feat}. Note sur l'influence de la lnmiere blanche et de la lumiere
colorée sur l’ineuba.tion des oeufs de poule, C’. R. Soc. Biol. (9) v, 1893.
 
'1‘. LIST. Ueber den Einfluss des Lichtes auf die Ablagerung von
Pigment, Arch. Ent. Mech. viii, 1899.
 
J. LOEB. A contribution to the physiology of coloration in animals,
Journ. Morph. viii, 1893.
 
J. Lorna. Ueber den Einfluss des Lichtes auf die Organbildung bei
Thieren, Pfl12ger's Arch. lxiii, 1896.
III. 4 LIGHT 97
 
H. M. VERNON. The effect of environment on'the development of
Echinoderni larvae: an experimental enquiry into the causes of variation,
 
Phil. Trans. Roy. Soc. clxxxvi, 1895.
E. YUNG. De 1’influence des milieux physiques sur les étres vivants,
 
Arch. Zool. Exp. et G'e'n. vii, 1878.
E. YUNG. De Pinfiuence des lumieres colorécs sur le développement
 
des aniinaux, Mitt. Zoo]. Stat. Neapel, ii, 1881.
 
5. HEAT
 
As is very well known, those activities by which every
organism maintains its specific form can only be carried on
within certain definite limits of temperature. So also a certain
degree of heat is necessary for the due performance of the
functions of growth and difierentiation ; above or below certain
limits—more or less definite for each organism, but varying in
different organisms—-development is unduly accelerated or retarded, or brought to a standstill, while its form is frequently
distorted as well.
 
’l‘o Oscar Hertwig we are indebted for a careful inquiry into
the conditions of temperature under which the development of
the Frog-’s egg takes place.
 
In the case of Ifanaflcsccz Hertwig has found the cardinal
temperature-points to be as follows :—The normal is about
15°—16° C.; above this up to 20°—22° C. development is accelerated without being otherwise altered ; this temperature is therefore the optimum (Fig. 51). Above this point the form of development is altered, a11d at such a high temperature as 30° C. death
follows very quickly. At low temperatures (6°-1° C.) there is considerable retardation, and at the zero-point a complete cessation
of segmentation ; the eggs are often permanently injured.
 
At the high temperatures referred to—-from 23° C. upwards—
it is the yolk—cells which are primarily affected. At from 29-6‘’
to 27-5" the yolk is unable to divide, though it is nucleated, and
segmentation is confined to the animal hemisphere, and soon
ceases even there (Fig. 47). At 26-5° the first furrow indeed
passes through the yolk, but subsequent segmentation is mereblastic, with the resulting formation of a cap of cells or blastoderm
 
lying upon and separated by a segmentation cavity from the
 
JEN xmson 11
98 EX'1‘ERNAL FACTORS 111. 5
 
nucleated yolk. The eggs then die. At lower temperatures25°-23°—the yolk is also affected, and many eggs die in the
‘ morula’ stage; such as do survive give rise to distortions or
monstrosities (Figs. 48, 49). The injury to the yolk interferes
 
’9&o
 
, °\
 
 
A 11
 
Flo. 47.——Meridional sections of eggs of Rmmfusm developed (A) at
29-5° 0., (B) 26-5“ 0. Five hours ten minutes after fertilization. 7;, nuclei;
p, pigment.
 
 
F100.’ 48.—Meridiona.l sections of eggs of Rama fusca developed at
._‘2 -5 C. Qne day after fertilization. /.-, nuclei; kk, blastocoel; 2, cells
imbedded in unsegmented yolk.
 
 
FIG. 49.——Abnorma1 embryos of Rauafusca, produced by heat. A, embryo
two days old developed at a. temperature of 24° C. ; 13, embryo three days
old, reared at a. temperature of about 25° 0. br, brain; y.p, yo1k—plug;
Lb, tail-bud ; t,ta.1l; s, sucker ; g, gill.
III. 5 HEAT 99
 
with the proper closure of the blastopore ,- there is consequently
a. large, persistent yolk-plug surrounded by a thickened blasteporic rim into which the separated halves of the medullary plate
and notoehord are differentiated (spina bifida) (Fig. 50). The
 
 
FIG. 50.-~’l‘wo transverse sections through the embyro shown in
Fig. 49 A. A, passes through the blastopore and yolk-plug; B, through
the anterior end. (I, yolk-plug ; m.p, medullary plate; ch, notochord;
mk, mesoblast.
 
front end of the arehenteron is, however, normally developed if
the temperature is not too high, and in this case the anterior
portion of the nervous system and
notochord are undivided ; posteriorly, however, their right and
left halves diverge round the blastepore, and are continued into the
halves of the double tail when the
latter is formed. Gill slits, protovertebrae, striated muscle—fibres,
the pronephros and its duct, and Fm.51.__Mel_idi0m1section
 
the tail {in may all be differentiated. of an egg of Rum, f,,_m,, de_
 
The development of the organs Veloped at 9: t01"Pe1‘°«t“1‘6 0f
f t] t .d f t] b d . f_ 22° C. Six hours fifty minutes
0 1e W0 S‘ es 0 1° 0 Y 1s 1c‘ after fertilization. kh, blaste
quently unequal. coel.
 
At low temperatures segmentation and the closure of the
blastopore take place very slowly, and at ()° cease altogether.
The eggs are not, however, dead, but will resume their development when replaced under ordinary temperature conditions.
They show abnormalities, however, due to injury of the yolk;
 
ll 2
100 EXTERNAL FACTORS III. 5
 
Morgan has similarly found that the fertilized (not, however,
the unfertilized) eggs of an American species (If. palust/'18) which
ve been subjected to a temperature of 1° C. and then allowed
elop under normal circumstances exhibit spina bifida and
 
 
 
   
 
 
lze has also observed these abnormalities as the result of
e cold. On one point, however, this author is not quite
ement with O. Hertwig, for he states that eggs and
s exposed to 0° in various stages do continue to develop,
 
thld of course very sl0\vly. ’.l‘hus, in the ease of eggs exposed
sl8§ after fertilization, the blastula stage was only reached in
 
ays, while a month elapsed before the bl-astopore was formed.
ie and Knowlton, however, state that in la’. rireecens and
.4m0l_y.s*to1/ta liqrinum segmentation is totally inhibited at 0°.
In another species of Frog (If. esculenla) which spawns much
later in the year——in May and J une——the cardinal points were
found by Ilertwig to be much higher, and the eggs endured
a temperature of 33° C. without injury. They are, in fact,
acclimatized to a higher temperature, and it is very interesting
to notice that Davenport and Castle have succeeded in artificially
acclimatizing the eggs of another Amphibian (Bag/‘o le7zli_qz'/zoszw)
to a considerable degree of heat. Eggs were reared at 15° C.
and 24°—25° C. After four weeks the heat rigor temperature
was 40° C. for the former, 43-2° for the latter; and in another
experiment the temperature was raised to 43-3’ by allowing the
eggs to develop at 33°—34.«° for seventeen days.
 
A similar lowering of the minimum seems to have been observed
by Lillie and Knowlton in the case of /lmM_y&lo1/za tigrimzi/I. In
this form, which spawns much earlier than Rana vi7'c.vc¢m.v, there
is considerably less retardation of development at 4°.
 
That temperature markedly influences the rate of development,
or, as Hertwig puts it, that the quantity of developmental work
performed per unit of time is a function of the temperature, is
abundantly clear, and is well shown in the annexed diagrams
(from Hertwig), in which the curves show the times taken to
reach various stages at various temperatures (Figs. 52, 53).
 
It will be seen that as the temperature sinks the rate of
development, or rather of differentiation, decreases, but at an
The rate of growth, however, may increase at an increasing
(or decrease at a) decreasing) rate, as Lillie and Knowlton found
 
FIG. 52.—Curves showing
 
development of the Frog (Ramtfusca). The abs '
 
ture in degrees Centigrade, the ordinates the
 
each of the stages I to ]X. I
 
dullnry folds closed, sucker
 
VII, operculum beginning; VIII, opercnlum closing; IX, rudiments of
hind legs. (After 0. Hertw 98.)
 
s;I
 
EZIIIIIIIIIIIIIIIIIIIIIII
.A.lIIIIIIIIIIIIIIIIIIII
.. y . IIIIIIIIIIIIIIII
EIIIII7 IIIIIIIIIIIII I
 
, g:1strula.; II,
V. tail-bud
ig, 18
 
I
EEIEIWI%II%IIIIIIIIIIIIIII
IIIIIIHIIIIIIIIIIIIIIIIIII
IIIIIIIIIIHI I771
IEIIIIIIIII IIIIIHHIIIIIII
IIIIIIIIIIKIIIHIIIIIHHIMII
 
IIEIIIIIIIIIIIIIIIHIIIIII II
 
ae giv
( '3 req
inedullary plate; III,
; V, tail
 
IIIIIHWMIIMHHIIDIIIIIIII II
.I'flIMflHIHIflflIIflIIIIII
VIIIIHIIIHHIMIIHHMIII
 
1 HUIHIIIIIII
"ii
 
the effect of temperature upon the rate of
cuss e the tempera»
<11) uired to reach
 
and gills; VI,
 
metail fin ;
 
increasing rate. Lillie and Knowlton have made the same
observation for the species investigated by them.
 
III. 5
 
HEAT
 
101
102 EXTERNAL FACTORS III. 5
 
for the tadpoles of Rana viresccns and Bufo lenliginoxzw. The
same authors state that at low temperatures (below 3° in the
case of the Frog, below 6° in the case of the Toad) growth was
altogether inhibited, while at 2° there was an actual shortening
in length in the ease of the Frog tadpole, due, it is suggested,
to a diminution in the turgor of the cells.
 
The cardinal points have also been determined for the lIen’s
egg. According to Kaestner normal development occurs only
 
FIG. 53.—Efl'ect of teinperaturc upon the growth of the tadpole of the
frog (Rana fusca). A, B, developed at a temperature of 14-5"--15" (1.;
A, two days old, circular blastopore (Stage I in Fig. 52) ; B, three days
old (Stage II in Fig. 52) ; C, 1), developed at a temperature of 20° C. ;
6', three days old (Stage V in Fig. 52); D, four days old (Stage VI in
Fig. 52). (From Minot, 1907, after 0. Ilertwig, 1898.)
between 95° and 102° F. (35° and 39°C.). The maximum, the
temperature above which the embryo dies, is 43° C.; the minimum, at which development stands still, 28" C. Edwards, however, fixes the minimum or physiological zero at 20°—21” C., for,
as the annexed diagram shows (Fig. 54-),development may continue
between 20° and 29°, though it is, of course, very much retarded.
 
Edwards has further made the highly interesting observation
III. 5 HEAT 103
 
PERCENTAGE
OF NORMAL
DEVELOPMENT
 
 
FIG. 54.—~'1‘l1c index of; development (percentage of normal development) for the egg of the Hen at tenrperatures varying from 20°C. to
30-75"G. (After Edwards, 1902.)
 
AVERAG E
 
DIAMETEROF
BLASTOIXRM
IN MM.
 
_-  p , ....I
  -%-Wfifi
‘  s=....'é.‘ns 2...
 
540 .fit!m .... ,.
III: fi
 
p
 
 
 
E:
5.40 gig ii
iii,
Igga- UIIF I
as? '*“-'
4.40 " 1......
 
RELATION OF TEMPERATURE T0 GROWTH OF BLASTOOERM
 
FIG. 55.—Growth of the blastoderm of the Hen’s egg independently of
the appearance of the primitive streak, at low temperatures. (After
Edwards, 1902.)
104 EXTERNAL FACTORS III. 5
 
that at tl1e low temperatures in question growth may occur without
differentiation (Fig. 55). Thus in one series of experiments at
24-5° for six days the blastoderm increased in diameter from
4-4 mm. (the average diameter of the blastoderm in unineubated
eggs) to 6-9 mm. The primitive streak was, however, not formed.
 
A temporary exposure to low temperatures often inflicts
a permanent injury on the egg and leads to malformations.
Kaestner, by subjecting the eggs at many different stages to
temperatures of 15°—25°, 10° and 5°, has discovered that the
capacity of resistance decreases as development proceeds (though
not with absolute regularity). Thus the maximum exposure to
21° consonant with subsequent normal development was 192
hours for embryos of six hours, 96 hours for embryos of one day,
7 2.’ hours for embryos of two to six days, 4-9 hours for embryos
of eight (lays, and 24 hours for embryos of 20 days.
 
At these low temperatures development is stated to be completely arrested, though the heart never ceases beating, irregularly
and convulsively. The cooling process may be repeated over and
over again without altering the capacity for future growth and
diiferentiation, or reducing or increasing the maximum capacity
of endurance of cold.
 
Malformations, as stated above, are of frequent occurrence,
b.ut only in those cases in which the embryo has been exposed in
an early stage, during the first two or three days of incubation,
and only after long exposures.
 
The Inedullary folds may remain unformed anteriorly, the
two halves of the heart may remain widely separate, the head
amnion fold may be absent and abnormal gill slits be formed;
the heart and blood—vessels are often enormously distended, and
hacmorrhages are frequent. Kaestner attributes these monstrosities not directly to the cold but to the pressure of the
blastoderm against the shell, for in the cooled eggs, owing to
some change in the specific gravity of the albumen or yolk, the
latter rises up ; if the egg is placed with the blunt end uppermost, so that the embryo is pressed against the shcll—membrane,
no monsters are produced.
 
Mitrophanow is another observer who has utilized low temperatures to cause malformations. High temperatures also give
III. 5 HEAT 105
 
rise to abnormalities accompanied by acceleration (Féré, Mitrephanow).
 
The effects of extremes of heat and cold upon the ova and
embryos of certain Invertebrates have been studied by Drieseh,
the brothers Hertwig, Vernon, Sala, and Greeley.
 
 
FIG. .")6.—-The effect of heat upon the segmentation of the Echinoid
egg. a, b, c, (1, four successive stages in the segmentation of the same egg
of I9'¢'hinus; e, f, two successive stages in the division of the same egg of
Sphaercchinus. (After Driesch, 1893.)
 
FIG. 57.— Suppression of cell-, but not of nuclear, division by
heat (E:-hbms). (After Driesch, 1893.)
 
The first-named observed that by subjecting the fertilized ova of
Sp/zaerec/aiizus to a temperature of 30°—3l° (the normal is 19°) development was accelerated and segmentation abnormal (Fig. 56).
 
After the first furrow—though not after subsequent divisions——
the blastomeres separated and sometimes remained apart, a fact
which provided a means of watching the independent development of the first two blastomeres. After the four-celled stage
the direction of division became irregular, one spindle being
106 EXTERNAL FACTORS III. 5
 
perpendicular, instead of parallel, to the other three, or two
perpendicular to two, or all irregular; in the next phase the
formation of micromeres was partially or wholly suppressed.
Nevertheless these abnormally segmented eggs produced perfectly
normal Plutei.
 
It is also possible for nuclear division to continue while celldivision is suppressed, as a result of exposure to high temperatures (Drieseh) (Fig. 57).
 
FIG. 58.~—'l‘he effect of heat upon the development of S'pl1mu'e¢-/u'nu.s_r/ranulm'z's. a,exogu.strul-a; b, exog.1.strula.,wit.l1 tripartite gut; r, Pluteus,
with tripartite gut; (d) Anenterion, with stoinodaeum, but no gut.
(After Driesch, 1895.)
 
By exposing the blastulae to the same high temperature
Driesch brought about a very interesting malformation, an Aneuterion (Fig. 58). The arehenteron was formed and constricted
into the normal three portions, but it was evaginated instead of
invaginated. Later on it shrank up and disappeared; the rest
of the embryo, however, became a Pluteus, with a stomodaeum.
 
Vernon finds the optimum temperature for Echinoid larvae
III. 5 HEAT 107
 
to be from 17-5° to 21«5° C. Exposure to high or low temperatures after fertilization, either for longer or shorter intervals or
continuously, produced a decrease in body-length of the Plutei.
The arm-length, however, increased with increasing temperature.
 
Vernon has also made the most interesting observation that
the variability alters with the temperature. Eight-day larvae
were measured, and the mean variability (Galton’s Q) of the
body-length was found to be at 16° to 18°, 22-2, at 18° to
20°, 26-3, at 20° to 22°, 24-8, and at 22° to 24°, 24-0. Thus the
variability is greatest at the temperature most favourable for
development, and conversely.
 
It is also possible for the cell processes that occur during
fertilization itself to be seriously affected by heat and cold, as
the researches of O. and R. Hertwig have shown.
 
Moderate exposure (twenty minutes) of the eggs of Sta-ony_ylo—
cenlrotmr to a temperature of 31° C. so weakens the cytoplasm
that many spermatozoa are enabled to enter. Each sperm
forms its own aster, and these combine with one another to
form various irregular mitotic figures (triasters, tetrasters, and
so on). The segmentation of such eggs is very irregular.
With longer exposures the cones of entrance become feebly
developed and the asters are not formed, while the numerous
sperm-nuclei remain unaltered. Greater heat—over 40° C.—
prevents the entrance of the spermatozoa altogether.
 
This pathological polysperm y may also be produced by cold;
in this case also excessive exposure prevents the formation of the
vitelline membrane, the cones of entrance, and the sperm-asters,
while the spermatozoa remain in the peripheral layer of the egg.
The eifect of a low temperature on eggs which have already
been normally fertilized is seen in the reduction of the astral
rays and spindle fibres, though not of the spheres, and in the
thickening and irregular aggregation of the chromosomes. At
a normal temperature the achromatic figure reappears.
 
Very similar phenomena have been described by Sala. in
A.9carz'8. This author kept the eggs (the females, that is, containing eggs in all stages of development) at low temperatures
——-from 3° to 8° C.—for from half an hour to five hours and
longer. The eifect of a short exposure to a very low tem108 EXTERNAL FACTORS III. 5
 
perature is not so harmful as a longer exposure to a less degree
of cold. The processes of maturation and fertilization were
both abnormal. Granules of chromatin took the place of the
tetrads and were unequally distributed to the spindle-poles ; or,
if the chromosomes (tetrads) had been normally formed before
the commencement of the experiment, their division was
irregular, in extreme cases all passing to one pole and into the
first polar body. Again, the formation of the polar bodies
might be suppressed altogether, or abnormal, the second only
being formed, or both as one, or the first polar body might be
as large as the egg itself. 'l‘hc achromatic figure was also
deformed, the spindle being split at one or both poles (pseudotriaster, pseudo-tetraster), and centrosomes appeared instead of
the usual centrosomal granules. The cytoplasm became granular,
the vitelline membrane was not formed, two or more eggs
frequently fused together. Polyspermy, with consequent multiplication of asters and eentrosomes, was very noticeable, aml, in
fertilization stages, a separate pronueleus may be formed from
each female chromosome, or fragment of a chromosome.
 
Closely connected with the cytoplasmic effects brought about
by these temperature changes is the phenomenon of artificial
parthenogenesis, produced by Morgan and Greeley in Ar/mcia
and .»1stc7-ins by lowering the tempe1'at111'e of the sea—water to
the freezing-point. Greeley has also shown that a lowering of
the temperature, like the raising of the osmotic pressure, results
in a withdrawal of water, the cause to which, as is well known,
Loeb attributed the development of unfertilized ova in his
experiments.
 
Greeley has shown that by the combination cl‘ a low temperature with a chemical reagent :1. higher percentage of
swimming blastulac can be obtained.
 
LI'l‘ERATURE
 
C. B. DAVENPORT and W. E. CASTLE. Studies in lnorphogencsis;
III. On the acelilnatization of organisms to high temperatures, Arch.
Em. Mesh. ii, 1896.
 
H. DRIESCH. Entwicklungsmeeh. Stud. IV: Experimentelle Veranderung des Typus der Furehung und ihre Folgen (Wirkungen von
Warmezufuhr und Druck), Zeilschr. u-r'.vs. Zool. lv, 1893.
III. 5 HEAT 109
 
H. DRIEscH. Entwicklungsmech. Stud. VII: Exogastrula und Anonteria, Mirth. Zool. Stat. Neapel, xi, 1895.
 
C. L. EDWARDS. The physiological zero and the index of development for the egg of the domestic fowl, Gallus do;msti('u.s‘, Amer. Journ.
 
Phys. vi, 1901-2. _
 
A. W. GREELEY. On the efl‘ect of variations in the temperature upon
the process of artificial parthenogenesis, Biol. Bull. iv, 1903.
 
O. HERTWIG. Ueber den Einfluss verschiedener Temper-aturen auf
die Entwicklung der Froscheier, S.-I3. loom’;/. preuss. Al.-ml. Wis.»-. Berlin,
1896.
 
0. HER'rwI(}. Ueber den Einfluss cler Temperatur auf die Entwicklung
von Rana fusca und esculcnta, Arch. mikr. Anat. li, 1898.
 
S. KAESTNER. Ueber kiinstliche Kitlteruhe von Hiihncreiern im Verlauf
der Bebriitung, Arch. Anat. Phys. (Anna), 1895.
 
S. KAESTNER. Ueber die Unterbrechung der Bebriitung von Hiihnereiern a.1s Methode zur Erzeugung von Missbildungen, Verh. Amrt. GeseIIsr'h.,
 
1896.
F. R. LILLIE and E. P. KNOWLTON. On the effect of tempemture on
 
the development of animals, Zool. Bull. i, 1898.
L. SALA. Experimentelle Untersuchung fiber die Reifung und Befruehtung der Eier bei Ascaris megalocephala, Arch. milcr. Anat. xliv, 1895.
O. SCHULZE. Ueber die Einwirkung niederer Temperatur auf die
 
Entwickelung des Frosches, I, Anat. Anz. x, 1895.
O. SCHULZE. Ueber die Einwirkung niederer Temperatur auf die
 
Entwickelung des Frosches, II, Anat. Am. xvi, 1899.
 
6. ATMOSPHERIC PRESSURE. THE RESPIRATION
OF THE EMBRYO
 
The respiratory exchange, which is so characteristic a function
of adult organisms, is a necessity for the embryo also, and in
some cases can be detected in very early stages indeed.
 
In the case of the Chick this need of oxygen is shown by the
arrest or distortion of development, or the death of the embryo
when the egg is placed in too confined a space, or when the
shell is varnished, wholly or above only (Mitrophanow, Féré),
though a coat of varnish on the lower side has no effect according to the latter author; or again, when the egg is placed in an
atmosphere of hydrogen, or when the pressure of the ordinary
 
atmosphere is reduced (Griaeomini).
Giaeomini found that at a pressure of about 600 mm. the
110 EXTERNAL FACTORS III. 6
 
embryos were small and abnormal in respect of the medullary
tube and amnion; the optic vesicles and cranial flexure were
absent, and there were serious disturbances in the area vasculosa,
where, though the blood islands were present, the capillaries
were either not formed or failed to reach the embryo. No
haemoglobin was produced. Embryos exposed at a later stage
(four days) nearly all died in two days of asphyxia, the blood
being dark red and haemorrhages numerous. That these efieets
were due not to the reduced pressure but to the want of oxygen
was shown by the complete normality of embryos reared in an
atmosphere of pure oxygen at the same pressure (except in
certain characters always exhibited by such embryos ; see below).
 
Similar methods may be employed to demonstrate the necessity of oxygen for the Frog’s egg, a necessity which is indeed
patent to any one who has observed the inferior development,
accompanied by spina bifida and open blastopore (Morgan) of
the eggs in the middle of a mass of spawn.
 
Thus, according to Rauber, development is retarded at a
pressure of % atmosphere of ordinary air, and the mortality high,
while at pressures of 7}; or 7,1 atmosphere death very rapidly
ensues. As a result of four days’ exposure to pure hydrogen
or nitrogen (ordinary air from which the oxygen had been
removed) Samassa observed retarded segmentation, and subsequently irregularities in development of the type already referred
to. Carbon dioxide produced irregular segmentation and death
in twenty hours.
 
Godlewski’s experiments are perhaps more thorough. The
eggs subjected to ordinary air at a greatly reduced pressure
(2 .mm.), as well as those kept in thoroughly boiled water,
segmented but little, and cell-division was confined to the
animal hemisphere. In an atmosphere of pure oxygen at the
same low pressure, however, development was, in many cases at
least, neither retarded nor abnormal. Further experiments with
pure oxygen, pure hydrogen, and an atmosphere composed of
oxygen and carbon dioxide in equal parts, gave the same result,
as the subjoined table shows (Table XIII). It is also clear that
the absence of oxygen makes itself felt almost from the beginning, while pure oxygen accelerates development.
III. 6 ATMOSPHERIC PRESSURE 111
 
TABLE XIII
~__ 1- Oxygen and :
Hours Oxygen. Hydrogen. Carbon Controls.
Dioxide.
3 First furrow in No furrow M-‘ No furrow
seine
3% All but one with One - hall‘ Most with first
first furrow with first furrow
' furrow ,
4 All but one 4 cells The same, 8 All with 2 cells
2 cells ‘J3
5 All with 4 cells Most with 4 g Most with 21 cells;
ce s 4» a few Wltl 4
17% Blastomeres smaller Blastomeres a, Normal
than in controls smallerthan 3;
in controls o
22} Blastomeres very Segmenta.- 7* Normal
small tion ceases _
47 Blastopore closed White) hemisphere
visi le
73 Mcdullary folds Blastopore closed
 
This method has given similar results for the eggs of the
fishes Cte7z0la6¢'u.s and Fmzriulus (Loeb). One or two points
are, however, worthy of especial notice.
 
The former develops at the surface of the sea, and is more
sensitive to a lack of oxygen than the latter, the segmentation
of whose egg will indeed continue for twenty-four hours in pure
hydrogen, though an embryo is never formed. In Cleuolabrus,
on the other hand, segmentation never advances further than
the eight-celled stage, and the cell-boundaries already formed
subsequently disappear, though they can be restored on removal
to pure oxygen. In Fvmrlulus the capacity for enduring a lack
of oxygen decreases (or the need of oxygen increases) with the
progress of development; the fatal exposure for a newly
fertilized egg is four days, for a newly formed embryo thirtytwo hours, for an embryo with the circulation established
twenty-four hours, and for the newly hatched larva. shorter
still. Carbon dioxide is quickly fatal to both species.
 
The lack of oxygen has also a noteworthy eifect on the pigment cells which are found, especially round about the bloodvessels, on the yolk-sac of Fzmrlulus. These pigment cells are
of two kinds, black and red, and when the embryo is deprived of
oxygen the former disappear, the latter diminish only a little.
112 EXTERNAL FACTORS III. 6
 
It has been noticed elsewhere that this pigment is less abundantly
formed in darkness than in light, and Loeb has suggested that
light may promote oxidation.
 
The ova of Echinoids also require oxygen from the beginning
of their development (Loeb). Without this element segmentation is impossible, or, if segmentation has already begun before
they are deprived of it, the blastomeres swell up and fuse.
According to Lyon the eggs of Arbacia are only sensitive to
a want of oxygen for from fifteen to twenty minutes after
fertilization. Vernon has shown that water saturated with
carbon dioxide and mixed in the proportion of 20 % or more with
sea-water is fatal to the development of these forms.
 
Exact quantitative determinations of the oxygen absorbed
and the carbon dioxide excreted have been made by Godlewski
for the Frog and by Pott and Preyer for the Chick. The results
are shown in the tables annexed (Tables XIV, XV).
 
TABLE XIV
 
Showing the result of one experiment on the respiration of
the Frog's egg (Godlewski).
 
Day? anger A111ourz1llI;‘ilnpg§|;.air‘i)I(p:a‘:é;e:')f24 hrs.
femhzatlom 0 absorbed. CO, excreted.
0-03908
 
1 __
2 0-4502 0-0995
3 0-7033 0-2131
4 1-0539 0-4193
 
It thus appears to be the very general rule that the egg begins
to respire at an early age. There is a case, however, A.s-crmlv, in
which not only can the egg endure an atmosphere of nitrogen
or carbon dioxide or nitric oxide for prolonged periods and still
develop, but is actually killed by pure oxygen (at 2-} atmospheres)
(Samassa). The adult worm, of course, is an endoparasite, and
Bungc has shown that it can manage to produce carbon dioxide
though denied access to free oxygen.
 
The eifect of pure oxygen has also been tried on various embryos.
In such an atmosphere (at ordinary pressure) the development of
the chick is normal, except that skin, allantois, limbs and amniotic fluid are all very red with oxyhaemoglobin; an excessive
amount of carbon dioxide is produced. The amount of this gas
III. 6
 
ATMOSPHERIC PRESSURE
 
113
 
excreted by the undeveioped though incubated egg in pure
oxygen is, however, less than in air.
 
TABLE XV
 
Showing the oxygen a.bso1-bed and carbon dioxide excreted by the Hen’s
egg during incubation. (After Pott and Preyer, from Preyer, Spez.
 
Phys.)
Days of Amount in grammes per 24 hrs. of
iIl0l1')Mi0l1- O absorbed. CO, excreted.
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* Pulinonatry respiration begins.
 
According to Samassa. and Rauber the development of the
Frog-’s egg in pure oxygen is normal, but Godlewski states, as
 
JENKINSON
 
I
114 EXTERNAL FACTORS III. 6
 
we have seen, that it is somewhat accelerated. At a pressure of
2% atmospheres, however, segmentation is arrested and death
ensues (Samassa).
 
When the tadpoles, newly hatched, are exposed to its influence,
the hyoid becomes immensely thickened and the branchial
chamber completely closed; the internal gills a.re weak (Rauber),
and the same author states that ‘ gastrulae’ subjected to three
atmospheres of ordinary air had their development temporarily
arrested, while later embryos, in the stage of the medullary folds,
became small and immobile in air at twice the atmospheric pressure.
This result seems to be due to pressure, not to the oxygen.
 
In the foregoing the general necessity of respiration for the
life of the developing organism has alone been taken into consideration, but it should not be forgotten that oxygen may
exert a stimulus on some part, the response to wl1ich results in
a process of diiferentiation. Thus His has suggested that the
growth of the blastoderm over the yolk is oxygenotropic, and
IIerbst that the migration of the blastoderm-forming cells to
the surface in Arthropod ova, and the migration of spiculeforming cells in Echinoid larvae are cases of definite reaction
to oxygenotactic stimuli. Loeb, we may note, has found that
the regeneration of the head of ’l'ubularia will only take place
when the stem is supplied with fresh water, and the same author
has suggested that the accumulation of the pigment cells round
 
the blood-vessels on the yolk-sac of Fmztlulus is also an
oxygenotaxis.
 
LITERATURE
 
C. F1’-Jmé. Note sur l‘inllueuce des entluits particls sur l'ine-ubation dc
l’u.:ul' do poule, C. If. Soc-. Biol. (10) i, 1894.
 
C. GIACOMINI. Influence de 1‘a.ir iarifié sur le dévcloppemcnt dc
1'ueufde poule, Arch. Ital. Biol. xxii, 1895.
 
E. GODLEWSKI. Die Einwirkung des Sauerstolfes auf die Entwicklung
von Rana tempormia, Arch. Eut. Mesh. xi, 1901.
 
C. HERBST. Ueber die Bedeutung der Reizphysiologie fiir die
cauale Auffassung von Vorgangen in der thierischen Ontogenese,
Biol. Cmtralbl. xiv, xv, 1894, 1895.
 
J. LOEB. Ueber die relative Empfindlichkeit von Froschembryonen
 
gegen Sauerstoifmangel und Wasserentziehung in verschiedenen Entwicklungsstadien, Pflfigefs Arch. lv, 1894.
III. 6 ATMOSPHERIC PRESSURE 1 15
 
J. LOEB. Untersuehungen fiber die physiologischen Wirkungen des
Sauerstoffmangels, P_fliige)"s Arch. lxii, 1896.
 
P. MITROPHANOW. Einfluss der veritnderten Respirationsbedingungen
auf die erste Entwiekelung des Hfilmerembryos, Arch. Ent. Meclt. x, 1900.
 
R. Po'r'r. Versuche iibcr die Respiration dcs Hiihner-Embryo in einer
Sauerstoffatinesphiire, If/h'iger’s 47071. xxxi, 1883.
 
R. Po'r'r und W. PREYER. Ueber den Gaswechsel und die chemischen
Veri‘r_nderungen des Hiihnereies withrend der Bebrutung, Pflz'iger's Arch.
xxvii, 1882.
 
W. PREYER. Spezielle Physiologic dos Embryo, Leipzig, 1885.
 
A. RAUBER. Ucber den Einfluss dcrTen1pc1-atur, des atinospliarischen
Druckes nnd vcrschiedener Stoffc auf die Entwicklung thierischer Eier,
S.-B. Nrmuf. Gas. Leipzig, x, 1883.
 
H. SAMASSA. Ueber die étusseren Entwicklungsbedingungen der Eier
von Rana tomporaria, Verh. ])eutsch. Zeal. (v'cs. vi, 1896.
 
7. OSMOTIC PRESSURE. THE ROLE OF WATER
IN GROWTH
 
That growth seems to depend in many cases on the absorption
of water or a watery fluid—in the swelling of the Echinoderm
blastula, for example, or the enlargement of the Mammalian
blastoeyst——has been noticed by several observers; in a. few
instances experimental proof has been given of the relation
between the two.
 
Although, as is very well known, the Hen’s egg loses weight
daily throughout incubation by loss of water, this loss is due
almost entirely to the slow evaporation of the albumen, and
a humid atmosphere is necessary for development, as Pott and
Preyer have found. Féré’s experiments with eggs incubated
in desiccators demonstrated, during later stages, a slight reta.rdation accompanied by abnormalities and a high death-rate;
in earlier stages, up to about the fourth day, there was on the
contrary an acceleration of development.
 
Davenport has shown for tadpoles of various Amphibia (Amblystoma, Toads, Frogs) that increase in weight is very largely
due to increase in weight of water. Known numbers of tadpoles, from which superficial water had first been carefully
removed, were placed over sulphuric acid in a. desiccator. Re
I 2
116 EXTERNAL FACTORS III. 7
 
peated weighings were made until a constant minimum was
reached. The results are set forth in the accompanying table
and figure (Table XVI, Fig. 59). It will be seen that the
percentage of water rises very rapidly in the first fortnight,
from 56 % to 96 %, then decreases slightly, afterwards becoming
nearly constant.
 
TABLE XVI
Showing the rate of absorption of water by 'l‘u.«lpoles
(after Davenport).
Days after hatching. Percentage of water.
56
 
OOAH
 
fix!-‘H>¢:~1U1L\:>v-4
to
O‘
 
 
FIG. 59.-—Curve showing change in percentage of water in Frog tadpoles from the first to the eighty-fourth day after hatching. Abscissae,
 
days; ordinates, percentages. (After Davenport, from Korschelt and
Heider.)
 
A different, and a less satisfactory, method has been employed
by Loeb, hypertonic solutions being used to prevent the absorption of water. While the newly fertilized eggs of Fimdulus
III. 7 OSMOTIC PRESSURE 117
 
developed as normally in fresh water as in sea-water, only
a blastoderm with occasionally a dwarf embryo was formed in
a 5 % solution of sodium chloride in sea-water, and segmentation
was arrested in the thirty-two-celled stage when the concentration of the salt was raised to 10 %. Older eggs were, however,
far less sensitive, and after three or four days the embryos could
he placed directly in a. 276 % solution without arresting their
development, though the heart heat more slowly and differentiation was less rapid.
 
A I
 
 
 
 
 
FIG. 60.——A and 0, formation of ex-ovatcs in the egg of Arba('1'(( by
dilution of the sea-water; ls, nucleus; m, egg-membrane; B and 1),
hlastulae formed from A and C; If becomes constricted into two blastulae, each of which gives rise to a. Pluteus; D produces a single
Pluteus. (After Loch, from Kolschelt and lleider.)
 
The eggs can nevertheless be aeelimatized to the salt. Removed from the 10% solution after the thirty-two cells had
been formed to ordinary sea-water for eighteen hours, they were
capable, when once more replaced in the strong solution, of
giving rise to embryos which lived for a considerable time.
 
Similar experiments made on Arbacia showed that though
cell-division is suppressed in the hypertonie solution (2 Z sodium
118 EXTERNAL FACTORS III. 7
 
chloride) nuclear division continues all the same, for when returned to seaewater the eggs divided at once into as many cells
as had in the meantime been formed in tlie controls, a,result
confirmed by Morgan.
 
That the normal egg is in a condition of osmotic equilibrium
with the sea-water is further shown by its behaviour in sea»
\vater diluted to twice its volume ; in this experiment the egg
 
6‘
 
FIG. 61.—»Va.1-iations in the semnentation of Iu'¢-}u'nu.s- microtubm-culalns
produced by dilution of the sea-water. a, tetrahedral four-eel] stage;
II, eight cells, three premature niieromeres; 0, eight cells, two precocious
microineres ; (I, the same egg afterthe next division, the precociolls micromeres have divided unequally, two normal micromeres have been formed.
(After Driesch, 1895.)
 
(of /lrl/acia) absorbs water, swells and bursts its membrane and
so produces a large ex-ovate which may develop independently
of the rest of the ovum (Loeb) when replaced under ordinary
conditions (Fig. 60). Driesch has produced irregularities of
segmentation by the same means (Fig. 61).
 
Although, therefore, it seems reasonable to suppose that in
the cases just quoted the observed eifects really are due to the
III. 7 OSMOTIC PRESSURE 119
 
increased osmotic pressure of the medium and consequent withdrawal of water from, or prevention of imbibition of water by,
the eggs, the weak point of the experiment, and of all such
experiments, is our ignorance of the extent to which the ova or
embryos are permeable to the substance employed, since the
osmotic effect, or withdrawal of water, will obviously vary inversely with the permeability. The neglect of this possibly
disturbing factor has indeed led in some cases to quite unwarrantable conclusions.
 
In 1895 O. Hertwig showed that certain abnormalities could
be produced by growing the eggs of the Frog (IF. /‘urea and
 
 
FIG. 62.—Three sodium-chloride embryos of Rana fusca. df, yolkplug; hp, brain; Ici, gills; .9, margin of epidermic layer of ectoderm;
sch, tail; m-, lip of blastopore, (After 0. Hertwig, from Korschelt and
Heider.)
 
esculenta) and of the Axolotl in a solution of common salt. In
stronger solutions (1 % to 0-8 %) segmentation was confined to the
animal hemisphere, though nuclear division went on in the
yolk. Weaker solutions (0-6 %) allowed of further, but distorted, development; the yolk-cells were unable to move
beneath the lip of the blastopore, so that the latter remained
open with a persistent yolk-plug, and the mcdullary folds failed
to close in the region of the brain, a condition recalling the
abnormalities known in Human and Comparative Teratology
as Hemicrania and Aneneephaly (Figs. 62, 65). The exposed
region of the brain underwent a grey deg'enera.tion with dis—
120 EXTERNAL FACTORS III. 7
 
integration of the epithelium. Other organs were, however,
normally formed, the front end of the gut by iiivagination, the
notochord and mesoderin, protovertebrae, heart, pronephros, auditory vesicles, optic vesicles, infnndibulum, and liver, until the embryo died.
The persistence of the yolk-plug has
also been induced by Gurwitsch by
means of halogen salts (sodium bromide
and lithium chloride) and weak solutions of alkaloids (strychnine, caifein,
nicotine) (Fig. 63), by C. B. Wilson
in Hana, C/I0?'0j)/I27’IlS, and A7110/‘I/8/07ll(l
F;G,53__Me,-idiom1sec. by means of sodium chloride and
Egggruffilffiggélzff  Ringei"s solution, and by Morgan ‘with
1,, blagtocoel. (After Gm-. Various lithium salts; and Bataillon,
Witflchs from Korscllelt and who has used isotonic solutions of caneHeider.) . .
sugar, sodium chloride, and a large
number of other salts for the purpose, claims that in this case
the results produced depend upon the osmotic pressure alone, and
are therefore due to a withdrawal of water from the developing
 
embryo.
 
 
 
-B
 
' Fm. 64.—Sections of Frogs‘ eggs grown in solutions of, A, annnoniuin
iodide (1-5%), and, B, urea (2-37,). In both cases segmentation is iiieroblastie, although in A there are a few lai'::,e. divisions in the yolk. In
B the multinueleate cell masses of the animal hcniispliere protrude
above the surface. The nuclei are large, lobed, and lioiiiogeneously
clf'_Iroinal;ic in both cases. (Ammonia is probably present in the solution
0 urea.
 
Recent experiments made by the author do not, however, bear
out this conclusion. In the first place, it is to be observed that
isotonic solutions (isotonic with a O-625 % NaCl solution) do not
III. 7 OSMOTIC PRESSURE 121
 
produce the same, but markedly difierent effects. Some solutions
arrest development at an early stage (during segmentation
(Fig. 64), gastrulation, or the formation of the medullary folds);
in others development proceeds but is distorted, the medullary
folds remaining open in whole or in part, and the yolk-plug uncovered, or either of these malformations may occur without the
other; in one case (dextrose) development is quite normal in
form but very considerably retarded, while finally in urea development is normal both in form and rate (Figs. 66, 67). No
legitimate deductions can be made from these experiments, how
 
FIG. 65.—1<‘rog embryos grown in a -625% solution of sodium chloride.
A and B after five days, 0 and D after six days. In all the yolk-plug is
fully exposed. In A the medullary groove is wholly open, in B and C it
is closed behind, in D it is closed throughout.
ever, until the permeabilities of the tissues to these solutions are
ascertained. The tadpole requires water (Davenport), and the
degree of shrinkage of the tadpoles in these solutions affords a
means of determining the question ; it appears that they are perfectl y permeable to urea, more or less impermeable to cane-sugar,
dextrose, and sodium chloride, the sllrinkage being rather greater
in the first than in the other two. On the assumption that the
permeabilities of the embryo are the same as those of the tadpole,
it follows that the greater effect produced on the former by sodium
chloride than by cane-sugar, or, still more, than by dextrose,
cannot be set down to the osmotic pressure of the solution alone,
122 EXTERNAL FACTORS III. 7
 
a. result which is further corroborated by the constancy in the
relative toxicities of the bases and the acids in the case of the
 
 
FIG. 66.——F1-og embryos grown in isotonic solutions of, A, sodium
chloride (-625%); B, cane-sugar (6-6%); 0, dextrose (3-4%); and, D, urea.
(1-14%). In A the medullary folds are closed but the blasto ore open;
in B the medullary groove 18 open but the blastopore close ; in C development is normal, but retarded ; in D development is normal, both in
form and rate, though the embryos die soon after the stage shown in
the figure.
 
 
FIG. 67.—A. Longitudinal section of a Frog embryo grown in a -45%
solution of lithium chloride. The medullary groove is open, except in
front and behind. The notochord is bent in several ilaces and the gut
roof much crumpled. B. Longitudinal section of a. ‘rog embryo grown
in a. 6-6% solution of cane-sugar. The medullary groove is open, except
in front, the cells in its floor degenerating. The gut roof is incomplete in
part and there is an evident neurenteric canal.
 
monobasic salts. The observed deformities are therefore to be
attributed to some other—chcmical or physical——property of
III. 7 OSMOTIC PRESSURE 123
 
the solutions, though what this is is not known.‘ It may be
added that in Gurwitseh's experiments the concentrations of the
alkaloids employed were certainly far below those which would
be isotonic with a -625% solution of sodium chloride. It also
follows that during the closure of the blastopore the Frog’s eggdoes not need to absorb water from the outside; it may, in fact,
be exposed to a very considerable degree of desiccation at this
period without interfering in the least with the closure of the
blastopore or of the medullary folds, a result which is all the
more surprising in that the newly hatched tadpole imbibes water
at so rapid a rate.
 
The experiments which have hitherto been considered relate
to the need of water for normal development. There are,
however, certain processes for which not the absorption, but,
on the contrary, the abstraction, or at least the local abstraction,
of water appears to be essential, the phenomena, namely, of
fertilization. Cytologists have observed that the entrance cone
and funnel, the mechanism by which the spermatozoon is swept
into the interior of the egg, appear to be aggregations of
a watery substance about the aerosome or apical body, and that
the sperm sphere and aster are similarly due to the withdrawal
of water by the centrosome in the middle-piece from the cytoplasm ,- in other words, that the stimulus whereby the spermatozoon restores to the egg its lost power of cell-division is
essentially a process of local dehydration.
 
This inference is substantiated by the familiar experiments
of Loeb, who has succeeded in rearing normal larvae from the
unfertilized eggs of Echinoderms and certain worms by temporary immersion in certain solutions. In his earlier experiments
he found that a mixture in equal parts of a 2,9 a solution of
magnesium chloride and sea-water produced more Plutci than
any other solution tried, and hence believed the result to be
specific and attributable to the magnesium ion. Later, however,
this artificial parthenogeuesis was successfully brought about by
various isotonic solutions (chlorides of sodium, potassium and
calcium, potassium bromide, nitrate and sulphate, cane-sugar
 
‘ In this view Stockard, as a result of experiments on Fmululus, concurs
(Arch. Em. Mach. xxiii, 1907, and Journ. Exp. Zool. iv, 1907).
124 EXTERNAL FACTORS III. 7
 
and others). The increased osmotic pressure was, therefore,
considered to be the cause of the phenomenon, and it was
suggested that in ordinary fertilization the spermatozoon introduces a substance which has a higher osmotic pressure than,
and is therefore able to withdraw water from, the egg.
 
Hunter has also shown that sea-water concentrated to 7 0%
of its volume is sufficient to bring about the result. It must
still be remembered that the permeabilities of the ova to the
various solutions are not known; Sollmann, indeed, has proved
the secondary swelling after the primary shrinkage of many
eggs in hypertonie solutions, which must therefore enter and
cause the dissociation of the cytoplasm.
 
Further, Delage has, as a matter of fact, denied that the
increased osmotic pressure is solely responsible for the results.
The French zoologist succeeded in making the ova develop in
solutions hypertonie to sea-water, but found that isotonic
solutions of different chlorides or mixtures of chlorides did not
all give the same percentage of larvae. He holds, therefore,
that other factors are involved. Other methods, as noticed
elsewhere, are low temperatures and mechanical agitation.
 
Fischer has successfully demonstrated the phenomenon in the
Chaetopods, Nereis and A7221;/zzhile, Bullot in 01)/(elm, and
Bataillon in Vertebrates (lfamz. _/usca and I’eh-omyzon j/laueri);
but in this last case segmentation did not continue for very long
and the processes of nuclear division were highly irregular. An
attempt made by Gies to incite development (of Echinoids) by
means of extracts of spermatozoa was unsuccessful.
 
Although in brilliancy of conception and completeness of
execution Loeb’s experiments are certainly pre-eminent over
those of any other investigator, it should not be forgotten that
about the same time Morgan had succeeded in inducing asters,
and even the beginnings of segmentation, in the unfertili7.ed ova
of sea-urchins and some other forms by the use of salts and
other substances, and that the way for all recent work was
really paved by the original labours of (). and R. Hertwig, to be
described in the next section.
 
Loeb did not undertake an examination of the cytological
changes, but Wilson has shown that ordinary nuclear division
occurs with asters and centrosomes: a primary radia.tion
III. 7 OSMOTIC PRESSURE 125
 
centring in the nucleus first appears; this then fades away,
and a definite aster with a centrosome is formed just to one
side of the nucleus; this divides to form the first amphiaster
(cleavage-spindle). Asters also arise independently of the
nucleus in the cytoplasm (cytasters) ; these contain centrosomes,
and may divide, and the cytoplasm divide round them. The
part played by the cytasters in development is, however, insignificant; their activity soon comes to an end. The number
of chromosomes is one-half the normal number. This latter
statement is confirmed by Morgan, but denied by Delage, who
asserts that, as in egg fragments enucleated and subsequently
fertilized, the half number becomes doubled.
 
LITERATURE
 
E. BATAILLON. La pression osmotique et les grands problemes de la
Biologie, Ar:-71. Ent. Mach. xi, 1901.
 
E. BATAILLON. Etudes expérimentales sur l‘évo1ution des Am-'
phibiens, Arch. Em. Mecli. xii, 1901.
 
C. B. DAVENPORT. The rele of water in growth, Proc. Boston Soc.
Nat. Hist. xxviii, 1897-8.
 
C. F1’«:Iu':. Note sur l’influence de la désliydratation sur le développement de Pembryon de poulet, C’. R. Soc. Biol. (10) i, 1894.
 
A. GURWITSCH. Ueber die formative Wirkung des verandertcn chemischen Mediums auf die embryonale Entwicklung, Arch. Eur. Mach. iii, 1896.
 
O. HERTWIG. Die Entwicklung des Froscheis unter clem Einfluss
sehwiieherer und stéirkerer Kochsalzlesungen, Arch. milcr. Anat. xliv, 1895.
 
O. HERTWIG. Die experimentelle Erzeugung thierischer Missbild—
ungen, Fcstsclzr. Gegenbaur, Leipzig, 1896.
 
J. W. J ENKINSON. On the effect of certain solutions upon the development of the Frog's egg, Arch. Ent. Mach. xxi, 1906.
 
J. LOEB. Investigations in physiological morphology, Journ. Morph. vii,
1892.
 
J. LOEB. Ueber die relative Empfindlichkeit von Fischembryonen
gegen Sauerstotfmangel und Wasserentziehung in verschiedenen Entwicklungsstadien, Pflfiger’.-c Arch. 1v, 1894.
 
J. LOEB. Ueber eine einfache Methode zwei oder mehr zusammengewaehsener Embryonen aus einem Ei hervorzubringen, P_/iiige;-'3 Arch. lv,
 
1894.
LITERATURE ON ARTIFICIAL PARTHENOGENESIS
 
E. BATAILLON. Nouveaux essais do parthénogénése expérimentale
chez les Vertébrés inférieurs, Arch. Ent. Mech. xviii, 1904.
 
G. BULLOT. Artificial parthenogenesis and regular segmentation in
an Annelid (Ophdia), Arch. Ent. Mech. xviii, 1904.
 
Y. DELAGE. Etudes sur la mérogonie, Arch. Z001. E.I:p. et Gén. (3),
vii, 1899.
 
Y. DELAGE. Etudes expérimentales sur la maturation cytoplasmique
126 EXTERNAL FACTORS III. 7
 
et sur la parthénogénese artifieiclle chez les Echinodermes, Arch. Zool.
Exp. et Gén. (3) ix, 1901.
 
M. A. FISCHER. Further experiments on artificial parthenogenesis
in Annelids, Amer. Journ. Phys. vii, 1902.
 
W. J. Gms. Do spermatozoa contain an enzyme having the power of
causing the development of mature ova ? Amer. Joum. Phys. vi, 1901-2.
 
A. W. GREELEY. Artificial parthenogencsis in the star-fish produced by
lowering the temperature, Amer. Journ. Phys. vi, 1901-2.
 
A. W. GREELEY. On the analogy between the effects of loss of water
and lowering of temperature, Amer. Journ. Phys. vi, 1901-2.
 
A. W. GREELEY. On the effect of variations in the temperature upon
the process of artificial parthenogenesis, Biol. Bull. iv, 1903.
 
R. HERTWIG. Ueber die Entwicklung des unbcfruchteten Seeigeleies, Fesischr. Geyenbaur, Leipzig, 1896.
 
S. J. HUNTER. On the production of artificial parthenogenesis in
Arbacia by the use of sea-water concentrated by evaporation, AlII('I'.
Journ. Phys. vi, 1901-2.
 
J. W. JENKINSON. Observations on the maturation and fertilization
of the egg of the Axolotl, Quart. Jam-n. Min-. Sci. xlviii, 1904.
 
K. KOSTANECKI. Ueber die Veriinderungen im Inneren des unter
dem Einfluss von K01-Gemischen kfinstlich-parthenogenetisch sich entwickelnden Eis von Mactra, Bull. Inlrrn. Acad. Sci. Cracovie, 1904-5.
 
J. LOEB. On the nature of the process of fertilization and the artificial
production of normal larvae (Plutei) from the unfertilized eggs of the
sea-urchin (two papers), Amer. Journ. Phys. iii, 1899-1900.
 
J. LOEB. Further experiments on artificial parthenogem-sis, and the
nature of the process of fertilization, Ame)‘. Journ. Phys. iv, 1900-1.
 
J. LOEB. Experiments on artificial parthenogenesis in Annelids
(Chaetopterus), Amer. Journ. Phys. iv, 1900-1.
 
A. P. MATHEWS. Artificial parthenogenesis produced by mechanical
agitation, Amer. Journ. Phys. vi, 1901-2.
 
T. H. MORGAN. The fertilization of non-nucleated fragments of
Echinoderm eggs, Arch. Ent. Mech. ii, 1895-6.
 
'1‘. H. MORGAN. The production of artificial astrospheres, Arch. Em.
Mech. iii, 1896.
 
T. II. MORGAN. The action of salt solutions on the unfertilized and
fertilized eggs of Arbaciu, Arch. Ent. Mach. viii, 1899.
 
T. H. MORGAN. Further studies in the action of salt solutions and
other agents on the eggs of Arbacia, Arch. Em‘. Me('h. x, 1900.
 
E. B. WILSON. A cytological study of artificial parthenogcncsis in
sea-urchin eggs, Arch. Em. Mech. xii, 1901.
 
8. THE CHEMICAL COMPOSITION OF THE MEDIUM
 
By means of solutions of alkaloids and other substances the
brothers Hertwig have been able to incite very remarkable cytological changes in the eggs of sea.-urchins (Stronyylocemfrotus).
III. 8 CHEMICAL COMPOSITION 127
 
The effects of nicotine are perhaps the most striking (Fig. 68, a-e).
Various solutions—-1 % and less of a concentrated extract——were
allowed to act upon the egg for difierent lengths of time (five to
fifty minutes) before fertilization; the ova. were then replaced in
sea-water and fertilized. The cytoplasm is so paralysed by the
 
FIG. 68.~The effect of alkaloids and other poisons on the processes
of fertilization and nuclear division in the egg of the sea—urchin,
Sh'ong_:/Iocentrotus lividus. (After R. and 0. Hcrtwig, 1887.)
 
a. The egg was exposed to nicotine (one drop in 200 c.c. of seawater)
for ten minutes, and then fertilized; drawn fifteen minutes later.
 
b, c. The same for fifteen minutes; drawn after one and a half hours.
 
d. The same for ten minutes; drawn after three hours, ten minutes.
 
2. The same; drawn after three hours. Only part of the complex
figure is shown; the remainder lies in another plane.
 
f, g, h. Exposed to a 0-05% solution of quinine for twenty minutes one
and a half hours after fertilization; drawn from one to two hours later.
 
is. 1-5, male pronucleus, 6, female pronucleus. Exposed to chloral
(05%) one minute after fertilization; fixed after 150 minutes.
 
I, m. Chloral 06% one minute after fertilization; fixed after six hours.
Male and female pronuclei reconstructed and metamorphosing, in m
the ‘fan ’ form with commencing division.
 
11,0. Placed in chloral 0-5% five minutes after fertilization ; preserved
after ninety minutes.
 
n. Female pronucleus (four-rayed rosette), and male pronucleus .
 
(three-rayed rosette).
0. Fusion of pronuclei.
 
p. The same. Female pronucleus in the pseudo-tetraster forms.
128 EXTERNAL FACTORS III. 8
 
poison that the normal vitelline membrane cannot be formed and
consequently many spermatozoa enter. In such eggs segmentation does not occur in the ordinary fashion by successive binary
divisions, but many small cells are simultaneously formed. The
resulting blastulae are abnormal, the segmentation cavity being
filled with a solid granular mass (Stereoblastulac), and very few
reach the Pluteus stage. The irregularities of segmentation are
due to the complex mitotic figures and divisions which polyspermy entails. One, two, three or more of the spermatozoa
fuse with the female pronucleus ; each has its own aster, which
divides into two. Hence the most complex nuclear figures are
formed.
 
In the case where two sperm-nuclei unite with the eggnucleus a tetraster is formed, that is four asters united by
spindles in a. square or rhombus, or a triaster with an odd
aster united to one angle of the system. The chromosomes are
grouped in the equators of the four, or three, united spindles, as
the case may be, and the egg divides simultaneously into four,
or three.
 
The arrangement becomes still more involved when there are
other sperms, whether these fuse with the female pronuclcus or
not. Each amphiaster is united by one pole to the tri-, tetra-,
or polyaster developed round the combination nucleus, or to the
poles of other amphiasters; in one case there were nineteen
spindles in all, not, of course, all in one plane. Each centrosphere receives half the chromosomes of the spindle attached
to it, and each cell, when division occurs, contains one or more
nuclei.
 
Hydrochlorate of morphine will produce similar effects, but
only with longer exposures—a 0-4 % solution for from two to
five hours. Strychnine, however, is poisonous in very weak
doses (-005 % to -25 ‘Z), and quite short exposures are sufficient
to call forth marked results. Other solutions successfully tried
were chloral hydrate (from 0-2 % to 0-5 % for from one to four
and a half hours), cocaine (from 0-025 Z to 1 ‘Z for five minutes),
and sulphate of quinine (-05 % for ten minutes). In quinine
(-05 % for thirty minutes) and chloral (o5 %) the entrance
cone was small and no asters were formed, from which the
III. 8 CHEMICAL COMPOSITION 129
 
Hertwigs argue that the contractility of the cytoplasm is
impaired in these solutions. Chloroform dissolved in sea-water
has the very interesting property of stimulating—without
the addition of spermatozoa—the formation and separation of
the vitelline membrane. The male generative cells are also
sensitive to the action of these alkaloids, but not necessarily in
the same measure. They can resist, for example, the influence
of a solution of nicotine, which is ten times as strong as one
necessary to evoke pathological changes in the ova. Though
chloral hydrate (0-5 %) and quinine (0-05 %) are both temporarily
fatal to the motility of the spermatozoa, sea-water restores the
capacity for fertilization. Strychnine (0-O1 %) and morphine
(0-5 %) are without elfect.
 
In the experiments just described the abnormalities seem to
be directly due to the initial paralysis of the egg by the reagent
and consequent polyspermy.
 
Should, however, the egg have been first normally fertilized,
the irregularities produced by the subsequent action of the poison
are, though well marked, not of the same kind, for in this case
the vitelline membrane has already been formed and only one
spermatozoon has gained admittance. Chloral hydrate (Fig. 68,
/:—p) was employed for ten minutes and at varying intervals after
insemination (one, one and a half, five and fifteen minutes).
Exposure to the solution very shortly after insemination first
retards the progress of the sperm-head and the formation of its
aster, and when later on the chromosomes are formed they lie
heaped together in the centre of an achromatic figure described
as a pseudo-tri- or pseudo-tetraster. This consists of three or
four conical groups of fibres, the bases resting on, and the fibres
connected to, the chromosomes, the apices outwardly directed
and sometimes with, sometimes without, asters; in any case,
however, they are not united by spindles, as is the ease in
the complex figures observed in polyspermy. Isolated asters
are also to be seen in the cytoplasm, and, which is perhaps
more remarkable, the female chromosomes are themselves the
centre of a unipolar (fan-shaped) or multipolar apparatus of
the same kind. The reader will not fail to notice the similarity
to the phenomena occurring in artificial parthenogenesis.
 
Jmrxxuou K
130 EXTERNAL FACTORS III. 8
 
Should the pronuelei unite—-which is only possible before these
pseudasters have been developed, if the eggs have been subjected to the action of the poison immediately (one minute) after
fertilization——the conjugation nucleus itself becomes the focus
of a similar system. In eggs poisoned after a longer interval
(fifteen minutes) the male and female pseudasters may them
selves unite.
The nucleus—or nuclei—divide irregularly, the chromosomes
 
passing in unequal numbers to the poles of the figure. The
several pseudasters and isolated asters, with which nuclei may
possibly become secondarily associated, may be united by clear
streaks of protoplasm, thus giving rise to a dendritie figure.
Simultaneous and unequal division of the whole ovum follows.
 
Should the spermaster have already been developed-fifteen
minutes after insemination—it degcnerates. The subsequent
changes comprise the formation of multipolar figures and
irregular cell-division.
 
In later stages—when fertilization has been completed and
segmentation is about to beg-in—the ova are almost or quite
indiflerent to nicotine, strychnine, and morphine ; but chloral
(0-5 %) destroys the asters which are already in existence and
brings about a reconstitution of the combination nucleus with
subsequent formation of a tetraster and quadruple division. In
future mitoses, however, the spindles are bipolar. Cocaine and
quinine (-05 %) (Fig. 68,./'—/1) have the same effect.
 
The importance of these experiments does not require to be
emphasized. Not only do they throw a valuable light on the
possible causes of those pathological mitoses that occur in
malignant growths, they also contribute very greatly to the
understanding of the normal processes of fertilization and
karyokinesis.
 
Thus from the failure of the asters to appear in eggs treated
with chloral before fertilization the brothers Hertwig argue that
the contraetility of the cytoplasm is diminished by this substance,
and from the failure of the pronuelei to unite in eggs which
have been immersed in the solution shortly after fertilization
they suggest that it is the contractility of the ovum which
normally brings about the union of the pronuelei. Since,
III. 8 CHEMICAL COMPOSITION 131
 
however, both male and female nuclei are able to divide, this
division must be normally incited, not by their union with one
 
«another, but by the separate action of the cytoplasm on each,
a view which is fully borne out by the phenomena. of artificial
parthenogenesis and merogony (the development of fertilized
enueleate egg fragments), whatever interpretation may eventually
be put on the ‘ contractility ’ of the cytoplasm.‘
 
Another alkaloid which exerts an injurious influence on the
ova of Echinoderms is atropine, the sulphate of which retards
and dwarfs the development of /lxlericts and /lrbacia (Mathews).
Pilocarpine, on the contrary, has an accelerating efiect, a result
attributed by Mathews to its activity as an oxidizer, while
atropine is regarded as a reducing agent, the property to which
Loeb has also assigned the value of potassium cyanide in prolonging the life of unfertilized ova. The eggs of sea-urchins,
when once laid, are only capable of fertilization and development within a certain definite limit of time, after the expiration
of which they degenerate and die ,- after twenty-four hours,
for example, they are only able, when fertilized, to reach the
gastrula stage, and after thirty-two hours even fertilization is
hardly possible. By treatment with an appropriate solution of
potassium cyanide this limit may be considerably postponed.
In the most successful series of experiments the ova were first
 
placed in a solution of KCN 7;?) in sea-water, and then
 
1'emoved successively every twenty-four hours to
 
7!: 7!
2500’ 3000'
lengths of time, then removed to pure sea-water and fertilized.
As the table shows (Table XVII), segmentation was still possible
after 168 hours’ sojourn in the solution, but the greatest number
of Plutei was obtained after only 66 hours’ stay.
 
It was also shown that better results could be obtained with
artificial parthenogenesis if the ova were first kept in the
cyanide solution. Loeb points out that in the higher animals
 
74 7!
1400’ 2000’
In the last solution they were kept for various
 
‘ Strictly speaking, only the division of the male chromosomes can be
regarded as being stimulated by the egg cytoplasm. What exactly it is
which excites the female nucleus to divide is not at all clear.
 
K2
132 EXTERNAL FACTORS III. 8
 
the efiects of this substance are due to its inhibition of
oxidation; that this is the real cause of the prolongation of
the life of the eggs is shown by the fact that when kept in‘
an atmosphere of hydrogen for thirty-eight hours they were
still capable of being fertilized and developing intoswimming
larvae.
 
TABLE XVII
 
Showing the effect of exposures of various length of Sea-urchin
 
eggs to a solution of KCN§(%—0- (After Loeb.)
Length of exposure in hours. Result.
 
66 80 7; Plutei, vitelline membrane formed
90 30 ‘Z, Plutei, no vitelline membrane formed
99%‘ 20 Z Plutei) n 91 n n
 
112 Less than 20 Z Plutei
 
I20 Gastrulae, but no Plutei
 
139 A few blastulae
 
140 Blastulae, not swimming
 
Eight-celled stage only
 
Simultaneous lowering of the temperature to the freezing-point
enhanced the value of the cyanide treatment.
 
In later stages, however, immediately after fertilization and
subsequently, the action of potassium cyanide is by no means
beneficial ,- at this time, as we know, oxygen is a necessity (see
above, p. 112); and Lyon has shown that the moment at
which the ova are particularly sensitive to both KCN and
the lack of oxygen is the same, about fifteen minutes after
insemination.
 
Chemical agents are also able to incite irregularities of growth
and abnormalities in later stages of development.
 
In a long series of experiments Féré has shown that monstrosities can be produced by exposing the Hen’s egg to the
unfavourable influence of a large variety of substances. Vapours
of ether, alcohol, essential oils, nicotine, mercury, and phosphorus,
injections of alkaloids such as morphine, nicotine, strychnine, and
others, of bacterial toxines (those of tubercle, diphtheria), of
peptones, dextrose, glycerine, several alcohols, certain salts (KBr,
III. 8 CHEMICAL COMPOSITION 133
 
KI, S1-Brz), are all baneful, retarding and distorting the embryo
to a. greater or less extent. Ammonia, it may be noted, is fatal
at once.
 
It has already been shown (p. 123) that the malformations
induced by sodium chloride in Amphibian embryos are to be
set down to some other property than the osmotic pressure
of the solution, and it is here only necessary to advert to
 
Fm. (S9.-—Gane-sugar‘ (6-6 2,). Two stages in the forma.tion of the notochord from the whole thickness of the roof of the archcnteron in the Frog.
 
Dextrose (3-4 7,). Secondary degeneration of the gut roof and ventral
part of notochord.
 
some of the more interesting effects occurring in particular
solutions.
 
Although the more poisonous salts (e.g. LiI, CaCl2, SrBr2,
and others) inhibit altogether the formation of the blastoporic
fold, a cause which normally assists in its production—the
proliferation of small cells in the roof of the segmentation
cavity--may continue to operate, with the result that that mof
is thickened and thrown into puckers and folds.
134 EXTERNAL FACTORS III. 8
 
Again, the notochord may be formed from the whole thickness of
the arehenteric roof (cane-sugar)recalling the mode of its development in Urodela and Petromyzon (Fig. 69); the solid medullary
tube observed in potassium chloride and other salts reminds one of
the rudiment of the nervous system in Teleostei and others, while
the mode of closure of the medullary tube in, for example, some
of the magnesium salts resembles that observed in Am];/ziomas ;
the formation of notochordal tissue from the wall of the neural
 
 
FIG. 70.-—-Fornmtion of vaeuolated notochordal tissue in the medullary
tube of the Frog embryo under the influence of urea (1-6%). Underneath
the notochord is the subnotochordal rod.
 
tube and the roof of the arehenteron (Fig. 70) in strong solutions
of urea (1-17' % to 1-56 %) shows that the prospective potentialities
of these organs are not yet fixed, while the development of an
optic cup without a lens in urea, sodium chloride, and sodium
bromide demonstrates that the formation of the former is independent of that of the latter of these two parts of the eye.
 
The grey degeneration of the exposed part of the medullary
plate (due to the distribution of the pigment throughout the
cell—body), the protrusion of cells (‘framboisia’ of Roux), and
disintegration of the epithelium which is so characteristic in
III. 8 CHEMICAL COMPOSITION 135
 
many of these solutions (cane-sugar, NaCl, LiCl, MgCl2,
MgSO,), have been noticed by many observers (Roux, Hertwig,
Morgan, Bataillon). All the more violent solutions attack the
yolk-granules. In some cases the effect produced appears to be
specific; thus in lithium salts the ectoderm is often pitted and
wrinkled before any degeneration appears in the nervous system,
and in ammonia salts, which are highly poisonous, the nuclei
are much enlarged, lobed, highly chromatic, and homogeneous.
The very similar appearance of the nuclei (Fig. 64) in those
stronger solutions of urea which arrest development in an
early stage suggests that ‘the ammonia set free is the toxic
agent in this case. In solution isotonic with -625 % NaCl urea
permits of normal development up to a certain point, when the
embryos die.
 
In this connexion it is interesting to notice that Moore has
found that sodium sulphate will act as an antidote to the
poisonous effect of sodium chloride on tadpoles. Thus the
 
average length of life of tadpoles in a 3 NaCl solution was
 
four and a quarter days, but was prolonged to twenty-one days
by adding from 4% to 8 X of Na2SO4. The poisonousness of
sodium chloride, sodium nitrate, calcium nitrate, and magnesium
chloride to Fmululus embryos and the value of other salts as
antidotes has been shown by Loeb, while Lillie has noted that
sodium is fatal but magnesium and calcium beneficial to the
ciliary movement of /lrem.'c0/a larvae, a result first obtained by
Loeb for the Plutei of Ea/u'7m3; the muscular contractions of
the larva, on the other hand, are inhibited wholly by magnesium,
partly by calcium, while sodium is necessary for their continuanee. In an artificial solution which combines the three
elements in the proper proportions normal development is
possible. The nature of the part played by the ions——-whether
toxic or antitoxic— is, however, a very open question.
 
Arguing from the fact that the evil eifects of such salts as
sodium chloride and nitrate may be counteracted by calcium and
magnesium salts, Loeb has suggested that toxicity and antitoxicity are functions of valency, and also of electrical charge,
since it is further stated that toxicity increases with the valency
136 EXTERNAL FACTORS III. 8
 
of the anion, antitoxicity with that of the cation. Ions of the
same valency are not, however, necessarily equally antitoxic
(Loeb and Gies, Lillie, Mathews), and sodium sulphate, as we
have seen, may act as an antidote to the chloride (Moore).
Mathews has accordingly sought for the cause of toxicity in
another physical property, the decomposition tension of the salt,
and has certainly succeeded in showing that the poisononsness
of solutions to the eggs of Fzmrlulzw varies inversely with the
decomposition tension, and that a similar relation holds good in
certain other cases.
 
Lillie argues that a physiologically balanced solution is
necessary, one in which the electrolytes are in a state of
chemical equilibrium with the necessary ion-proteid compounds
in the tissues. Solutions which only contain some of these
substances, or solutions (for example, non-electrolytes) which
contain none, are poisonous, because they permit of the outward
diffusion of the needful ions.
 
It must be pointed out, however, that this explanation will
not fit the cases where the embryo develops perfectly well in
fresh water (lllzmtlulus) or in distilled water (the Frog), and that
some other reason must be found for the poisonous effect of
cane-sugar upon the latter. The whole question, however, is one
which belongs more properly to the province of pharmacology.
 
Poisonous although these salts are, the embryo can still be
acclimatized to them. C. B. Wilson placed the unsegmented
eggs of Amlalyertoma, Rana, and 0/102-op/zilzcs in a 0-05 % solution
of sodium chloride; after twenty-four hours they were removed
to 0-1 %, and then successively to stronger solutions by increments of 0-1% until 10% was reached, a concentration which
quickly causes death under ordinary circumstances. In this
case, however, development was normal, and the larvae hatched
out and lived for some time.
 
The distortions of development which solutions of salts and
other substances call forth in Amphibian embryos find a parallel
in the malformations which Herbst has produced in Eehinoderm
larvae (I976/dmw, Sp/zaerec/ainus) by similar means; as in the
former case, the results were at first assigned to the increased
osmotic pressure of the media.
III. 8 CHEMICAL COMPOSITION 137
 
When potassium salts are added to the sea-wa.ter—for
example, a 7 % solution in sea-water of a 3-7 % Solution of K01
in tap-water——the egg gives rise to a Pluteus in which, though
the gut is, as normally, tripartite, the skeleton is rudimentary
and the arms suppressed (Fig. 71). Herbst suggests that the
suppression of the arms is" due to the absence of a. stimulus
normally exerted by the skeletal spicules. These abnormal forms
may fuse together to form double monsters.
 
Such ‘ potassium ’ larvae are developed in sodium salts, but
lithium has a more pronounced elfect (Figs. 72, 73). In this case
 
 
FIG. 71.-—Potassium larvae of Echinoids. a. Potassium larva of Sphaerechinus (1860 c.c. sea-wate1'+ 140 c.c. 3-7% KNO3). There is no skeleton.
The rut is tripartite, and the mouth surrounded by the ciliated ring.
I), c. otassium larvae of .E(‘h’i1‘l'llS (20% of 3% K01). Note the buttonshaped apical tuft of cilia, and, in c, the secondarily evaginated archenteron. (After Herbst, 1893.)
 
the blastula becomes constricted into two portions, a thin-walled
gastrula wall provided with long cilia, and a thick-walled archenteron, which may be muscular and mobile, and is thickly covered
with short cilia. The arehenteron has, in fact, failed to invaginate, and the larva is an ‘ Exogastrula ’. Occasionally there is
an attempt at invagination at the end of the archenteric portion,
and, after temporary exposure, the invaginated part may be
divided into three, and a mouth formed. All the parts of the
gut, however, remain in the same straight line. A middle
section may be formed by further constriction of the archenteron (Ea/aimzs) or of the gastrula wall (Sp/zaerec/liuus). Double
138 EXTERNAL FACTORS III. 8
 
monsters sometimes arise by fusion of these larvae by their
archentera.
 
A skeleton is not usually developed; if present it is abnormalin position, the spicules being placed near the animal pole and
 
FIG. 72.———Lithium larvae of Splmerechin us _qrmmlw'is. a. Larva. partially
constricted into gastrula wall and archentcric portions, the former with
lon , the latter with short cilia. (980 c.c. sea-wa.ter+2O c.c. 3-7% LiCl).
b. imilar larva. to the last, but a neck or connecting piece has been
formed from the ectodermal portion. 0, (1. Progressive diminution of the
ectodermal gastrula. wall portion with increase in the quantity of Li.
 
the arms of the Pluteus formed under their influence near the
mouth instead of by the side of the anus, in the number of the
spicules, and consequently the number of arms (three, four, or
 
five, instead of two), and in the number of their radii (four or
five, instead of three).
III. 8 CHEMICAL COMPOSITION 139
 
The gastrula wall is often smaller than the archenteron, and,
as the strength of the solution is increased, becomes still further
reduced, until nothing of it is left but a small button at the
 
F10. 73.—~a. Larva with three skeletal spicules, and a. ‘ cell-rosette ’ at
the end of the archenteron. Ir. Larva with skeleton—-—more than three
spicu1es—and arms developed. The neck is invaginated into the ectodermal portion, the gut tripartite. c. Five-armed Plutcus with five
skeletal rods. The gut is normally invaginatecl and tripartite. d. Larva.
of Echirms microtuberculatus. There is a neck, and the gut is partly
invaginated. In the blastocoel are aggregations of mesenchyme and
pigment cells. (After Herbst, 1895.)
 
animal pole, which only indicates its real character by the longcilia which it carries. Such larvae Herbst terms ‘Holoentoblastia’. This nearly complete suppression of the ectodermal
140 EXTERNAL FACTORS III. 8
 
region can, however, only be realized when the salt is allowed
to act at a stage in the blastula when the difierentiation into
the two primary layers is already beginning. Should the
embryos be removed before this stage is reached, after twentyfour hours’ exposure to the solution, only ‘Exogastrulae’, not
‘ Holoentoblastia,’ can be obtained. Should, on the other hand,
older blastulae, or gastrulae, or Plutei be placed in the solutions,
they die without showing any signs of the characteristic abnormal
development. From the fact that equimolecular solutions of
monobasie lithium salts produced like effects (such solutions, it
must be observed, are also chemically equivalent), Herbst concluded at first that the osmotic pressure was responsible for the
abnormalities; but the permanent after-effects of temporary immersion just referred to subsequently convinced him that the ova
were permeable to the lithium ions to which he now attributes
the specific nature of the monstrosity. He suggests further that
they act upon the endoderm cells by increasing their absorptive
activity and their power of cell-division, while at the same time
they inhibit the functions of those mesenchyme cells which are
devoted to the formation of the skeleton.
 
As in other monstrosities, there is an alteration in the prospective potentialities of cells, elements which would normally be
ectodermal becoming converted into endoderm, and additional
mesenchyme cells being involved in the secretion of skeletal
spicules.
 
It is only by lithium salts that the typical ‘ Holoentoblastia’
can be, produced ; but Exogastrulac can be reared in others, in
sodium butyrate, for example; in this solution a stomodaeum is
formed, but is, like the arehenteron, everted. Even lithium,
however, is powerless to cause the ‘ holoentoblastic ’ reduction of
the gastrula wall in the larvae of Aaterias, although exogastrulation "may, but need not, occur. A characteristic deformity is the
absence of the pre-oral region, and the elevation of the mouth
on a sort of hypostome. In Amp/ti0.'D7(8 and Ascidians it is
impossible to obtain even exogastrulae by these methods. It
is evident, therefore, that the specific morphological reaction
depends not only on the nature of the substance employed, but
also on the constitution of the reacting organism.
III. 8 CHEMICAL COMPOSITION 14-1
 
Herbst has not omitted to point out the significance of theseand indeed of a1l—monstrosities for the theory of the origin of
those larger, discontinuous variations known as ‘sports’, or, in
more modern phraseology, ‘mutations’; and Vernon has been
able to show statistically that the degree of continuous variation
may also be altered by changes in the chemical environment.
 
In all the foregoing experiments the effect is observed of the
addition of some chemical substance to the medium in which
the embryo is placed. We have now to consider a very remarkable series of investigations, for whose planning and execution
we are indebted to the genius of Curt Herbst, investigations in
which substances which are present in the normal environment
of the larva are omitted, and an insight thus gained into the
part they play, if any, in the normal development of the
organism. Herbst has indeed succeeded in demonstrating in
the most conclusive manner the necessity to the sea-urchin egg
for the normal performance of this or that phase of developmental function of a large number of the elements present in
sea-water.
 
The sea-water at Naples, where Herbst carried out his work,
has the following c0mp0siti0n:~——
N aCl . . . . . 3 %
KCI . . . . . - 7 %
MgCl2 . . . . . ~32 %
Mg-S04 . . . . . -26 Z
CaSO4 . . . . . "-1 %
CaHl.’O4
 
Ca3P208
 
CaCO,,
 
Fe2CO
Si
 
Br
 
I
 
It may be said at once that silicon, bromine, and iodine are
unnecessary, and that, though earlier experiments led Herbst
to believe that phosphorus and iron were essential, he has since
assured himself that phosphorus is certainly, and iron probably,
 
in small quantities.
 
ll
142 EXTERNAL FACTORS III. 8
 
not. All the other elements, however, can only be omitted under
penalty of retardation, abnormality, or death (Figs. 74 A and B).
 
The method employed was a simple one. A series of artificial
sea-waters was made up, from which, one by one, each of‘ the
elements was omitted, another being substituted in its place.
Care was taken to make these artificial media approximately
 
FIG. 74.——'1‘he necessity of substances contained in sea-water for the
normal development of the larvae of sea-urchins.
 
a. Without OH. Ciliated stereoblastula of Sphaerechinus. b. KOH
has been added. c. Normal blastula of Sphaeiw-hinus. d. Blastula in
 
a K-free medium. e. Reared in K-free and replaced in sea-water (Sphm=r- '
 
echinus). f. Larva from a medium devoid of Mg (Sphaerec-himls).
g. Echinus Pluteus with tripartite gut, mouth and coelom sacs, but neither
skeleton nor arms ; reared without CaC0, or CaSO,. h. Normal Pluteus
of Echinus.
 
isotonic with sea-water, and so exclude a possibly disturbing
factor, the alteration of the osmotic pressure. The réle of each
of these necessary e1ements—or ions—will be considered separately and in some detail. Sp/2ae7'ec/Iinua and E0/timcx were the
forms principally employed.
III. 8 CHEMICAL COMPOSITION 143
 
i. S0,‘.
 
This is ordinarily provided by Mg-S04 and CaSO4 ; when the
fertilized ova are placed in a solution in which MgCl2 is substituted for it (as, for example, in 3 % NaCl+-07% KCI +
5 % MgCl2 + CaHPO4 + CaCO3) then their development is
retarded from the blastula stage onwards, the embryos are small
 
and degenerate without reaching the Pluteus stage (Fig. 74- B).
The gut is straight instead of bent, and not divided into the
 
 
FIG. 74 B.
 
a. Normal position of skeletal spicules in Sphaerechimls. b, (1. Abnormal position and number after treatment with S0,-free medium.
c. Larva of Echiuu.s- from a S-free solution. e. Pluteus of Sphaerechinus
with three fenestrated skeletal arms, instead of two. ’l‘rea.ted with
a S0,-free medium and replaced in sea-water. f. Normal Pluteus of
Sphaerechinus. (After Herbst, 1897 and 1904.)
 
usual three parts ; in S11/zaerec/u'm¢s no mouth is formed, the gut
is evaginated (Exogastrula). The endoderm is very thick, the
cells dark and dense.
 
The sulphuric acid radiele (sulph-ion) is thus necessary for the
proper development of the gut, and necessary from the very
beginning, for in embryos which have been kept in S0,,-free
water up to the mesenehyme-blastula stage and then replaced
in sea-water the alimentary tract is still abnormal.
 
Deprived of S04, in fact, the gut remains radially symmetrical,
144 EXTERNAL FACTORS III. 8
 
and the same must be said of the skeleton. Normally there are
two tri-radiate spicules, one to the right, the other to the left
of the gut and some little way from it. Without the needful
sulphate the spicules become placed near the gut, and may with
the growth of the latter be pushed towards the animal pole.
The number of spicules may also be diminished or increased
to one, three, or four, arranged in a circle round the gut. On
timely removal to sea-water, however, a secondary bilateral
symmetry may arise by two of these outgrowing the rest and
stimulating the development of the typical arms of the Pluteus.
It seems that a sulphate is present in the calcareous skeleton
of the Pluteus, as there is in that of the adult urchin.
 
A ciliated circum- oral ring is formed, but is abnormal in its
position, at right angles instead of parallel to the long axis of
the body. The pigment which should be secreted by the
secondary mesenchyme cells (separated off from the inner end
of the archenteron) remains in abeyance, and the apical tuft of
cilia is hypertrophied. Other processes, however—fertilization,
segmentation, and ciliary motion——arc independent of S0,.
 
The development of eggs which are allowed to remain in
ordinary sea-water until the blastula stage is no better, whence
Herbst concludes that no S04 is taken up during segmentation.
During the early stages of gastrulation, however, they appear to
absorb a store of it for future needs, for gastrulae reared in
sea-water develop further in the SO,-free solution than do
those embryos which have been kept in it since fertilization.
S04 is equally necessary for the continued life of the Pluteus
and of the Bipinnaria larva of Asterias, and without it the rate
of regeneration of the head of Tubularia is retarded and the
number of tentacles reduced, until eventually a completely
tentaeleless head is evolved.
 
The necessary sulphate can be, to a certain extent, replaced
by a thio-sulphate. The addition, for instance, of -35% Na2S2O,,
to the SO,-free solution renders it possible for the larvae to reach
the Pluteus stage, though the arms are short, the skeleton small,
and the pigment reduced. The larvae die.
 
Selenium and tellurium are both poisonous in an early
stage.
III. 8 CHEMICAL COMPOSITION 145
 
ii. Cl.
 
A solution was made up in which the sodium chloride was
replaced by sodium formate, the magnesium chloride by magnesium sulphate, the potassium chloride by potassium sulphate ;
thus, NaCOOH 3- 5 % + MgSO4 -26 % + MgSO4 -4- % + KZSO4
-12 Z + CaSO4 -I Z + CaHPO4+ CaCO3.
 
The eggs did not segment, and even when KCl and MgCl2
were used in their ordinary proportions, segmentation did not
progress very far. Nor did the substitution of Na2SO4 for
NaCOOH give any better results. A considerable amount of
chlorine appears therefore to be absolutely necessary for the
earliest developmental processes, its function being, Herbst suggests, to transport certain necessary cations, the tissues being
possibly more permeable to NaCl than to Na2SO4. Later stages
——blastulae, gastrulae, Plutci—all die in the Cl—free mixture.
 
Chlorine can be replaced in some measure by bromine. Plutei
are formed, though with a distorted skeleton, Tubularia regenerates its head and the eggs of the fish Labrax develop as well
as in sea-water. Iodine is, however, poisonous; so also are
chloratcs.
 
iii. Na.
 
2-96 % of MgCl2 was added to a solution containing the usual
amounts of KCl, MgSO4, CaSO4, CaHPO4, and CaCO3. In this
the ova indeed segmented, but abnormally, the blastomeres being
of unequal size. Death followed; nor was the addition of a certain
small amount (-84: %) of NaCl suflicient to save them, though
segmentation was normal and traces of an archenteron could
be detected; with more NaCl (1-34» %) the gastrula stage was
reached. The sodium which is thus necessary in the earliest
period is also required later on; without it gastrulation is impossible to eggs which have been reared in sea-water even as far
as the mesenchyme—blastula stage.
 
The part played by sodium is not clearly understood. It
is known that it counteracts the evil eifeets of calcium and is
necessary for the continuance of muscular contractions. Since
calcium is necessary for the cohesion of cells (see below) Herbst
opines that sodium may pull them apart ;. its action in that case
is capillary.
 
Jnnxmson L
146 EXTERNAL FACTORS III. 8
 
Sodium cannot possibly be replaced by lithium, potassium,
rubidium or caesium, all of which would at the necessary concentrations inevitably be poisonous.
 
iv. K.
 
In the artificial solution employed the small quantity (-07 %)
of potassium present in sea-water is simply omitted.
 
Without it scgmentation—except the first few phases-is
impossible in Ea/cinzw. Sp/zaerec/limts, however, segments, but the
blastocoel is reduced, the cells are opaque and not vacuolated, and
the ova, though ciliated, are motionless and die (Fig. 74 A, rl, e).
 
Later stages are also sensitive to the want of potassium.
Blastulae gastrulate, but are shrunken, with short archenteron,
and in gastrulae the gut does not divide into three. Plutei, like
all the others, die when deprived of it.
 
In the K-free medium spermatozoa temporarily lose their
motility, and such spermatozoa cannot effect fertilization. The
fertilization, however, of eggs which have been kept without
potassium is possible ; in fact, at this earliest stage, no potassium
is absorbed, for eggs fertilized in sea—water develop no further
in the K-free solution than do those fertilized and kept continuously in it.
 
The absence of potassium i11 segmentation leaves its effect
upon later stages. Two days’ exposure is not too long to prevent
normal development on removal to sea-water, but five days’
exposure causes abnormalities of the skeleton (asymmetrical with
several triradiate spicules round the gut) and alimentary canal
(no mouth).
 
De Vries has shown the importance of potassium for the
turgor of young plant-cells, and its function here is probably
similar, to promote growth, as the subjoined table of measurements shows. Its absence also aifects the rate of development
(Table XVIII).
 
Potassium can in a measure be replaced by rubidium and
caesium. The use of lithium either has no effect or, in larger
quantities, produces lithium larvae.
 
Other forms to which the lack of potassium was found to be
fatal were the ova of Asterias and Uotflorfiiza, and the adult
III. 8 CHEMICAL COMPOSITION 147
 
Amp/iioxus. Potassium is also necessary for the contractions of
muscles (umbrella. and tentacles of Obrlia).
 
TABLE XVIII
Showing the effect of potassium upon the growth of
Sea.-urchin blastulae. (After Herbst.)
 
‘Ratio (unit 2 Th mm.) of crossdiameter to long diameter.
 
After Without K. With K.
18 hours . 
24 hours 
45 hours . . . 
 
Showing the effect of potassium on the rate of development of
 
Sea—urchin larvae. (After Herbst.)
Artificial sea-water with
 
After -008 Z KCI. -016 Z KCl. .024 % KC].
36 hours Small gastrulae. Nearly Plutei. Plutei.
60 hours No mouth ; gut not Small Plutei. Fully formed Plutei.
constricted.
v. Mg.
 
' The fertilized ova were placed in a solution which did not
 
include the -32 % MgCl2 and the -26 Z MgSO4 present in seawater.
 
Segmentation proceeds normally, but the blastula. is slightly
smaller than when magnesium is present (the ratio of the
diameters is §§). The skeleton, however, though bilaterally laid
down, is retarded and deformed (magnesium is present in the
skeleton of the sea-urchin and possibly in that of the Pluteus)
and the gut is not properly diiferentiated, not tripartite and
without a mouth (Fig. 74 A, f).
 
When the MgSO4 is replaced by an isotonic quantity of
Na2SO4 the results are the same.
 
In the Mg-free solution cilia cease beating, development is
retarded (Table XIX), and, though the spermatozoa retain their
motility, the ova are so injured that fertilization is impossible
unless they are restored to sea-water. The ova, in fact, seem
to have a store of Mg which they lose in the Mg-free mixture.
Fertilization can, however, take place without magnesium if the
 
L 2
148 EXTERNAL FACTORS 111. 3
 
eggs have been kept in sea-water, and such eggs develop, in
Mg-free water, to precisely the same extent as those which have
been fertilized in sea-water; at this period, therefore, the egg
needs no external magnesium. The original store of this element
apparently suffices also for the first steps in the formation of the
gut; for the lack of it is felt equally in the later stages of its
difierentiation and in the first moments of its development; the
alimentary canal is as abnormal in those larvae which have been
kept without magnesium only up to the time when the mesonchyme and archenteric plate arise as in those which remain in
the solution throughout.
 
TABLE XIX
Showing the retardation of development of Sea-urchin larvae
deprived of magnesium. (After Herbst.)
Two solutions were employed, mixed in various proportions; one (1)
 
has no Mg; the other (2) contains Mg.
Solution. Result after 24 hours.
 
1 Nearly motionless; archentcron formed.
1+ 10 Z of 2 Larger.
1 +20 Z of 2 Larger still.
1 + 30 Z of 2 Larger; gut constricted ; mouth i'or1ned.
 
1 % of 2 )1 n n n 7)
1  % of 2 H n H :9 n
2 H H n 7) n
 
It is, in fact, only after the gastrula stage that magnesium is
absorbed. Eggs, blastulae and young gastrulae, reared in seawater develop far worse when placed in the Mg—free liquid
than do gastrulae.
 
The formation of pigment and the contractility of muscles
remain unaifected by the absence of Mg.
 
In Mg-free media tl1e blastomeres of .1.s-/erizw fall apart and
the adult medusae of Obeliu degenerate.
 
vi. Ca.
 
The calcium salts present in normal sea-water are C-aCO_.,,
Ca2SO4, CaHPO4, and Ca3(PO,)2.
 
When the carbonate only is absent the blastulae are crumpled
and opaque; a few gastrul-ate but are markedly abnormal, the
ciliated ring being crumpled, the gut flattened on the oral side
and the skeleton absent (Fig. 74 A, g). Should the skeleton
have already been formed when the larvae are exposed it
4 III. 8 CHEMICAL COMPOSITION 149
 
becomes dissolved. When the sulphate only is omitted (magnesium sulphate being present) development is still inferior to
the normal, inferior even to development in the presence of
CaSO, but in the absence of MgSO4 ; CaSO4 may be replaced
by CaCl2. It seems, therefore, that the sulphate is necessary as
a calcium salt.
 
FIG. 75.——a—r. Separation of the blastomeres of Echimts microtubermr
Iulus in a medium containing NaCl, 3-07%, KCI, 0-08%, MgSo,, 0-66%,
MgHPO, and FeCO3, but no Ca. Note the radially striate border, which
is the altered uniting membrane. (1. lilastuln disintegrating in the same
medium. (After Herbst, 1900.)
 
If only the phosphate (Ca3(PO4)2) is present the egg dies
during segmentation, though, if the other phosphorus compound
is substituted for it, the effect is the same as when the carbonate
alone is omitted.
 
Should, however, all calcium salts be removed the result is
 
more serious still (Fig. 75). The blastomeres are unable to
150 EXTERNAL FACTORS III. 8
 
cohere, and separate as fast as division takes place, swimming
about independently for a time, and then dying. The same
phenomenon is witnessed when later stages are placed in such
a. calcium-free mixture.
 
On removal to sea-water division continues without separation,
and should the egg membrane still be intact all the cells unite
and a whole larva is formed. Even should the egg membrane
be lost, a reunion of the cells is always possible so long as they
remain in contact with one another. The separation is due to
a. change in the surface-tension of the cells; a visible change
takes place, in fact, in the superficial layer which covers and
unites the blastomeres ; it becomes ill-defined and radially
striated.
 
The lack of calcium also aifects the rate of development, and
causes shrinkage, but leaves karyokinesis, ciliary motion, and
pigment formation unaltered.
 
Calcium is not replaceable by magnesium, strontium, or
barium.
 
vii. CO3.
 
As has just been pointed out, calcium carbonate is necessary
for the due formation of the skeleton, although a beginning
may be made without it.‘
 
Whether the crumpling of the larva, due to diminution of
internal osmotic pressure, which is observed in the absence of
calcium carbonate is attributable to the lack of CO3 or the lack
of the hydroxyl ion is difiicult to determine, since, as Herbst
points out, a carbonate necessarily introduces OH, while the
latter can convert into carbonates the CO2 of the atmosphere and
of respiration.
 
viii. OII.
 
The alkalinity of the sea-water——reckoned by the number of
free hydroxyl ions—is provided by the calcium carbonate and
calcium hydrogenphosphate. By the omission of these a solution
—-neutral to litmus—may be obtained in which the ova give rise
to thick-walled, opaque blastulae with granular contents, ciliated
 
1 CaCO,, is necessary for the formation of the skeleton of the larva of
the sponge Sgcandra selosa (0. Maas, S.-B. Ges. Mozph. Phys. Miinchen,
xx, 1905). In a medium devoid of all calcium salts the Amphiblastulae
fall to pieces.
III. 8 CHEMICAL COMPOSITION 151
 
but motionless, and doomed to eventual degeneration and death
(Fig. 74 A, a, 6). Very occasionally a gastrula with a short gut
is formed.
 
When the blastulae are immersed in the solution they give
rise to small, opaque gastrulae.
 
A certain degree of alkalinity is necessary for fertilization.
The spermatozoa are less sensitive to a want of alkalinity, more
sensitive to excessive alkalinity than the ova.
 
By the addition of a small amount of sodium hydrate to the
neutral medium development is accelerated, but an increase of
the alkalinity of ordinary sea-water is unfavourable. Loeb, on
the other hand, has found that the addition of from -006 % to
-008 % sodium hydrate to sea-water accelerates the development
of Aréacia.
 
The formation of pigment and the vibration of the cilia are
other processes which depend on the presence of the hydroxyl
ion. Plutei die without it and their skeleton is dissolved.
 
The function of the ion does not appear to be to neutralize
any acids produced by the tissues, for these give a neutral
reaction even in OH-free media.
 
Since aeration improves the development of eggs in these
media, and the more so if the air is deprived of its carbon dioxide,
Herbst has concluded that one function of the OH ion is to
neutralize the CO2 and allow of the formation of the necessary
carbonates. Another function is possibly, as Loeb suggested,
the acceleration of processes of oxidation.
 
The experiments which we have been considering are unique
of their kind, and it is impossible to exaggerate their importance.
For, whatever may be the ultimate explanation of the facts, there
can be no doubt whatever that the most complete demonstration
has been given of the absolute necessity of many of the elements
occurring in ordinary sea-water, its normal environment, for the
proper growth and differentiation of the larva of the sea-urchin.
Nor is this all. Some of the substances are necessary for one
part or phase of development, some for another, some from the
very beginning, others only later on. Thus potassium, magnesium, and a certain degree of alkalinity are essential for
152 EXTERNAL FACTORS III. 8
 
fertilization, chlorine and sodium for segmentation, calcium for
the adequate cohesion of the blastomeres, potassium, calcium and
the hydroxyl ion for securing; the internal osmotic pressure necessary for growth, while without the sulph-ion and magnesium
the due differentiation of the alimentary tract and the proper
formation of the skeleton cannot occur; the secretion of pigment
depends on the presence of some sulphate and alkalinity, the
skeleton requires calcium carbonate, cilia will only beat in an
alkaline medium containing potassium and magnesium, and
muscles will only contract when potassium and calcium are there.
 
The part played by each substance is therefore specific; for
some particular part of the morphogenetie process it is indispensable. Not, of course, independently of internal factors, but
in co-operation with them, it does, in fact, determine the production of organic form; and the relation between the embryo
and the environment in which it develops is in this case, at any
rate, of the closest and most intimate kind.
 
LITERATURE
 
C. B. DAVENPORT and H. V. NEAL. Studies in morphogenesis. V. On
the aeclimatization of organims to poisonous chemical substances, Arch.
Ent. Meek. ii, 1896.
 
C. Fr':R£:. A series of papers on the effect of various chemical bodies
on the development of the Chick, C‘. R. Soc. Biol. (9) v, 1893—1iii, 1901.
 
C. W. GREENE. On the relation of the inorganic salts of blood to the
automatic activity of a strip of ventricular muscle, Amer. Jom-n. 1’h_:/s.
ii, 1898-9.
 
C. HERBST. Experimentelle Untersuchungen fiber den Einfluss der
veranderten chemischen Zusammensetzung des umgebenden Mediums
auf die Entwickelung der Tiere: I. Versuche an Seeigcleiern, Zeitsrlz.
wise. Zool. lv, 1893.
 
C. HERBST. Experimentelle Untersuehungen fiber den Einfluss der
veriinderten chemischen Zusammensetzung des umgebenden Mediums
auf die Entwickelung der ’l‘iere: II. Weitcrns fiber die morphologischo
Wirkung der Lithiumsalzc und ihre thcorctischc Bcdcutung, Miil. Sim‘.
Zool. Neapel, xi, 1895.
 
C. HERBST. Experimentelle Untersuchungen iiber den Einfluss der
veranderten chemischen Zusammensetzung des umgebenden Mediums
auf die Entwickelung der Ticre: III-VI, Arch. Ent. Meek. ii, 1896.
 
C. HERBST. Ueber die zur Entwickelung der Seeigellarven nothwendigen
anorganichen Stoffe, ihre Rolle und ihre Vertretbarkeit: I. Die zur
Entwicklung nothwendigen anorganischen Stoffe, Arch. Ent. Mach. v, 1897.
III. 8 CHEMICAL COMPOSITION 153
 
C. HERBST. Ueber zwei Fehlerquellen beim Nachweis der Unentbehrlichkeit vom Phosphor und Eisen fiir die Entwickelung der Seeigellarven,
Arch. Em. Me¢:h. vii, 1898.
 
C. H ERBST. Ueber (las Auseinandcrgehen von Furchungs- und Gewebezellen in kalkfreiem Medium, Arch. 1911!. Ma-h. ix, 1900.
 
C. IIERBS1‘. Ueber die zur Entwickelung der Seeigellarven nothwendigen anorganischen Stoife‘, ihre Rolle und ihre Vertretbarkeit:
II. Die Vertretbarkeit der nothwendigen Stoffe dutch andere itlmlicher
chemischer Natur, Arch. Ent. Mech xi, 1901.
 
C. UERBST. Ueber die zur Entwiekelung der Seeigellarven nothwcndigen anorganischen Stoffe, ihre Rolle und ihre Vertretbarkeit: III.
Die Rolle der nothwendigen anorganisclien Stoffe, Arch. Em. Mm-h.
xvii, 1904.
 
0. and R. HERTWIG. Ueber den Bet'ruchtungs- und Teilungsvorgung
des tierischen Eies unter dem Einfluss ilusserer Agentien, Jen. Zeifsclw.
xx, 1887.
 
O. HERTWIG. Experimentelle Studien am tierisehen Ei vor, wiihrend
und naeh der Befruchtung, Jm. Zeflschr. xxiv, 1890.
 
W. H. HOWELL. On the relation of the blood to the automaticity and
sequence of the heartbeat, A mer. Joum. Pk;/s. ii, 1898-9.
 
W. II. HOWELL. An analysis of the influence of the sodium, potassium,
and calcium salts of the blood on the automatic contractions of heartmuscle, Amer. Journ. Pk;/s. vi, 1901-2.
 
R. IRVINE and G. SIMS WOODHEAD. The secretion of carbonate of
lime by animals, Proc. lfoy. Soc. Etlinbmy/h, xvi, 1889.
 
R. S. LILLIE. On differences in the effects of various salt-solutions on
ciliary and on muscular movements in Arenicolu larvae, Amer. Joum.
Phys. v, 1901.
 
R. S. LILLIE. On the effects of various solutions on ciliary and
muscular movement in the larvae of Arcml-ola. and 1’oIy_(/ordius, Amer.
Journ. Phys. vii, 1902.
 
R. S. LILLIE. The relation of ions to ciliary movements, Amer. Journ.
Ph_1/.s-. x, 19034.
 
D. J. LINGLE. The action of certa.in ions on ventricular muscle, Amer.
Journ. Plays. iv, 1900-1.
 
F. S. LOCKE. On a supposed action of distilled water as such on
certain animal organisms, Jam-n. Pity.»-. xviii, 1895.
 
.T. LOEB. Ueber den Einfluss von Alkalion und Siiuren anf die
ombryonale Entwickelung uud (las Wachsthum, Arch. Enf. Mach. vii, 1893.
 
J. LOEB. On ion-proteid compounds and their role in the mechanics
of life-phenomena: I. The poisonous character of a pure NaCl solution,
Amer. Journ. Phys. iii, 1899-1900.
 
J. LOEB. On the different effect of ions upon myogenic and neurogenic rhythmical contractions and upon embryonic and muscular tissue,
Amer. Journ. Phys. iii, 1899-1900.
154- EXTERNAL FACTORS III. 8
 
J. LOEB. The toxic and anti-toxic effects of ions as a function of their
valency and possibly their electrical charge, Amer. Journ. Phys. vi, 1901-2.
 
J. LOEB and W. H. LEWIS. On the prolongation of the life of the
unfertilized eggs of sea-urchins by potassium cyanide, Amer. Journ. Phys.
vi, 1901~2.
J. LOEB and W. J. GIES. Weitere Unteisuchungen fiber die entgiftenden Ionenwirkungen und die Rolle der Kationen bei diesen Vorgangen, Pfliiye:-‘s Arch. xciii, 1903.
 
E. P. LYON. The eifects of potassium cyanide and of lack of oxygen
upon the fertilized eggs and the embryos of the sea-urchin (Arbaeia
pzmctulata), Amer. Journ. Phys. vii, 1902.
 
A. P. MATHEWS. The action of pilocarpine and atropine on the
embryos of the star-fish and the sea-urchin, Amer. Journ. Phys. vi, 1901-2.
 
A. P. MATHEWS. The relation between solution tension, atomic
volume, and the physiological action of the elements, Amer. Journ.
Phys. x, 1903-4.
 
A. P. MATHEWS. The toxic and anti-toxic action of salts, Amer.
Joum. Phys. xii, 1904-5.
 
A. P. MATHEWS. The nature of chemical and electrical stimulation:
I. The physiological action of an ion depends upon its electrical state
and its electrical stability, Amer. Journ. Phys. xi, 1904. II. The tension
co-efficient of salts and the precipitation of colloids by electrolytes,
Amer. Journ. 1’hys. xiv, 1905.
 
S. S. MAXWELL and J. C. HILL. Note upon the effect of calcium and
of free oxygen upon rhythmic contraction, Amer. Jom-n. Phys. vii, 1902.
 
S. S. MAXWELL. The effect of salt-solutions on ciliary activity, Amer.
Journ. Phys. xiii, 1905.
 
H. MCGUIGAN. The relation between the decomposition-tension of
salts and their anti—fermentative properties, Amer. Journ. Phys. x, 1903-4.
 
A. MOORE. Further evidence of the poisonous effects of a pure NaCl
solution, Amer. Journ. Phys. iv, 1900—l.
 
A. Moons. The effect of ions on the contractions of the lymph
hearts of the Frog, Amer. Journ. Phys. v, 1901.
 
A. Moomz. On the effects of solutions of various electrolytes and
non-conductors upon rigor mortis and heat rigor, Amer. Jom-n. Phys. vii,
1902.
 
A. MOORE. On the power of Na,S0, to neutralize the ill effects of’
NaCl, Amer. Journ. Phys. vii, 1902.
 
T. H. MORGAN. The action of salt solutions on the unfertilized and
fertilized eggs of'Arbac1'a and of other animals, Arch. Ent. Mech. viii, 1899.
 
'1‘. H. MORGAN. Further studies on the action of salt-solutions and of
other agents on the eggs of Arbacia, Arch. Ent. Mech. x, 1900.
 
T. H. MORGAN. The relation between normal and abnormal development of the embryo of the Frog as determined by the effect of lithium
chloride in solution, Arch. Ent. Mech. xvi, 1903.
III. 8 CHEMICAL COMPOSITION 155
 
H. NEILSON. Further experiments on the antitoxic effects of ions,
Amer. Joum. Phys. vii, 1902.
 
S. R1NG1=.n and A. G. PH!-JAR. The influence of saline media. on the
tadpole, Jam-n. Phys. xvii, 1894-5.
 
J. RITCHIE. The relation of chemical composition to germicidal
action, Trans. Path. Soc. 1, 1899.
 
W. ROUX. Beitritge zur Entwieklungsmechanik des Embryo : I. Zur
Orientierung fiber einige Probleme der embryonalen Entwicklung,
Zeifschr. Biol. xxi, 1885.
 
D. RYWOSCH. Ueber die Bedeutung der Salze fiir das Leben des
Organismus, Biol. Centralbl. xx, 1900.
 
T. SOLLMANN. Structural changes of ova in anisotonic ‘solutions and
saponin, Amer. Journ. Phys. xii, 1904-5.
 
C. B. WILSON. Experiments on the early development of the Amphibian embryo under the influence of Ringer and salt solutions, Arch.
Ent. Mech. v, 1897.
 
W. D. ZOETI-IOUT. The effects of potassium and calcium ions on
striated muscle, Amer. Jom-n. Phys. vii, 1902.
 
9. SUMMARY
 
In the numerous experiments which we have been considering
the efiect is observed upon the development of the embryo of
certain alterations in the constitution of that embryo’s normal
environment. Either some factor which is not usually present
is added to the environment, or else some factor which is
customarily found there is altered by increase or decrease, or
removed altogether.
 
In some cases development remains undisturbed by this treatment, in others it may be merely generally retarded or accelerated,
in others again it may be altered not merely in rate but in form,
with the production of an abnormality or monstrosity, and if its
eifeet is too intolerable the death of the embryo may ensue.
 
Throwing light as they do on the causes of the formation of
natural monsters, such experiments are no doubt of the highest
interest from a general teratological point of view. The mere
possibility of the occurrence of such malformations is, however,
itself a fact of the deepest morphological significance. A monster
is an organism in which the development of some part or parts
has either exceeded or fallen short of its normal limitations, and
any such phenomenon points indubitably to a certain mutual
156 EXTERNAL FACTORS III. 9
 
independence of the parts in the growth and differentiation of
the organism; while some pursue their normal course, others
deviate from it. It follows that when such deformations are due
to changes in the external conditions the parts are not equally
sensitive to the unusual influence to which they are exposed.
Thus in the Frog embryos which exhibit persistent yolk-plugs
and open brains when grown in solutions of various kin(ls, the
yolk and the medullary folds are alone susceptible to the action
of the poison, other parts are unaffected and continue their
development as though under normal circumstances: or, again,
a sea—urchin passes through the early stages of segmentation and
gastrulation unchanged when placed in a sea-water from which
magnesium has been removed, but the subsequent differentiation
of the gut and the formation of the skeleton are abnormal ;
magnesium is necessary for these, though not for the earlier
processes. A means is thus afforded of watching the behaviour
of one or more parts independently of others, as, for example, of
the animal cells in the gastrulation of the Frog's egg when the
yolk-cellspare injured, and the most valuable information contributed, often quite unexpectedly, to our understanding of the
events of normal ontogeny.
 
Quite apart from this such experiments have already contributed, and will probably contribute still more in the future,
to the study of variation. Between conspicuous monstrosities
and those milder abnormalities which are termed ‘sports ’ or
‘ mutations ’ there is every intermediate gradation, just as there
is, on the other hand, no sharply defined limit between these
discontinuous and those far smaller continuous variations to
which the term has been often exclusively applied. The embryo
is particularly sensitive to a change in its environment and reacts
to such change by a variation in its form of greater or less degree.
And not only that ; as Vernon has shown, these changes can produce also an alteration in the variability of the species ; and so
provide greater opportunities for the operation of natural selection.
 
At the same time teratolugy is not the main inquiry with
which the experimental emhryologist is concerned. The problem that confronts him is to determine the part played by
each factor of the external environment in the processes of
III. 9 SUMMARY 157
 
normal, specific growth and differentiation, and for the solution
of this problem only those experiments, of course, are of avail in
which such factors are either altered or removed.
 
By this means, as we have seen, it has been shown that
a certain constitution of the physical environment, fixed within
certain limits, is needful for the embryo; to these conditions it is
closely adapted; those limits it can only transgress under pain
of abnormality or death.
 
Every factor, or nearly every factor, is necessary for this or
that phase or part of the process, some for the whole. Light
of a certain wave-length will accelerate development, light of
another kind, or in some instances darkness, will retard it, or stop
it altogether; a certain degree of heat is indispensable ; oxygen
is required for respiration, water for growth ; some eggs demand
constant agitation, others comparative rest; fertilization, or
segmentation, or gastrulation, or some one or other of the later
phases of development may depend absolutely on the presence
of some particular chemical element ; remove the factor in
question, whatever it may be, and that particular process will
not occur, and the specific, typical end which is reached in
normal development will not be attained. Nevertheless, the
achievement of this end does not depend wholly upon extrinsic
forces, for the ovum is no completely homogeneous ‘isotropic’
substance in which the complex circumstances of its environment conspire to produce heterogeneity and coherence. There is
no evidence that any physical factor exerts a directive influence
sufficient of itself to determine any part of the whole specific
cfiect, although this may happen under extraordinary conditions,
as when gravity impresses a bilateral symmetry upon the compulsorily upturned egg of the Frog, and so determines the
median plane of the embryo.
 
Intimately bound up though these external conditions are
with the proper conduct of the whole series of events whereby
the organism comes gradually to resemble the parents that gave
it birth, they can only operate in conjunction with internal
factors which must be sought for not only in the initial structure
and constitution of the germ-cells, but in the mutual interactions
of the developing parts.
 
==Chapter IV==
 
INTERNAL FACTORS
 
1. THE INITIAL STRUCTURE OF THE GERM AS
A CAUSE OF DIFFERENTIATION
 
§1. THE MODERN Fomu or THE PItEI<‘0RMA'[‘IONlS'1‘ DOCTRINE.
 
As has been already pointed out, it was in the hands of Roux
that the principle of germinal localization first advanced by His
assumed the rank and importance of a theory of development.
On this ‘ Mosaik-Theorie’ of self-differentiation the precise
relation observable between the several parts of the embryo and
certain definite regions of the undeveloped germ is not merely
customary or normal, but necessary and causal. The germ-cell
is endowed with a preformed structure corresponding to the
structure of the organism which is to arise from it; each part
of this structure is predetermined for the formation of some
particular member of the embryo, and out of it no other member
can, under ordinary circumstances, be made. The causes of its
development, regarded as a specific activity of the organism,
as leading to the production of a form which is like that of the
parents which gave it birth, lie wholly within this prc—cxisting
structure, and of each part within each part. Although the
influence of the environment and, in later stages, of the parts
on one another is not entirely excluded, the factors on which
the differentiation of the whole and of each part depends are
essentially internal, and all that happens is that by a continued
process of cell-division the parts are separated from one another
and the structure thus made palpable and manifest. Cell-division
in the embryo is therefore qualitatively unlike.
 
The supposed proof, by Pfliiger’s experiment with forcibly
inverted Frogs’ eggs, of the complete ‘isotropy’ or equivalence
of all parts of the cytoplasm, compelled Roux to locate the
IV. 1 INITIAL STRUCTURE OF THE GERM 159
 
selfdifferentiating substance, the Idioplasson, in the nucleus, a.
view adopted and elaborated by Weismann, while the facts of
regeneration. necessitated the assumption of a reserve Idioplasson
endowed with the potentialities of the whole, which is not qualitatively divided during development, but handed on intact to
some or all of the tissues of the body.
 
The hypothesis then assumes the existence in the nucleus of the
fertilized ovum of as many separate units as there are separately
inheritable characters in the embryo, arranged there according
to a plan which conforms to the structural arrangement of the
parts which they represent, the separation of these units by continued ‘unlike’ nuclear division, and their distribution to the
cytoplasm, where they determine the formation of the structures
to which they are beforehand assigned.
 
For Roux the theory was no mere speculation, but the
result of observation and experiment. The natural occurrence
of half or partial monsters, the correspondence of defects in the
embryo to injuries to the egg, and the coincidence of the first
furrow with the sagittal plane in a large percentage (80 Z) of,
unfortunately, only a small number of eggs, suggested the selfdiiferentiation of predetermined parts, while the last-mentioned
observation led directly to the experiment in which one of the
first two blastomeres of the Frog’s egg was killed and a halfembryo produced from the survivor.
 
By means of a hot needle Roux succeeded in at least partially
destroying one of the first two blastomeres. The other continued to segment, and passed through the ordinary stages;
first the Hemimorula, with animal and vegetative cells and a
segmentation cavity (though the last was sometimes absent),
then by continued cell-division the Hemiblastula, followed by
the Hemigastrula and Hemiembryo. In all cases the living
tissues formed an exact half——either a right half or a left halfof a normal embryo, and ended abruptly at the (sagittal) plane
of separation of the living and the dead. The segmentation
cavity, however, was observed to extend in some cases across
this plane, while in others it was confined wholly within the
living cells. ‘
 
In the Hemigastrula the greatest extent of the archenteron—160 INTERNAL FAC’1‘O1tS IV. 1
 
produced by the overgrowth of the blastoporic lip—-was parallel
to the plane of separation: the yolk-cells were pushed into the
segmentation cavity. The Hemiembryo had half a medullary
 
9
 
FIG. 76.—A and B. Meridional sections through Herniblustulae of the
Frog: kn, remains of nucleus; In’, large reticular nuclei; 1:, nuclei In
the yolk; F,b1astocoe1; 4*, vacuoles. C. Hemigastrulalatemlis. Oblique
lon itudinal section. I). Hemiembryo sinister, transverse section; .93,
me ian plane ; the right half of the egg is already completely cellulated,
post-generation of the germ-layers has begun; ch, notochord, of whole
size; mw, medullary fold; u, gut; j, two yolk-cells that have remained
embryonic. (From Korschelt and Heider, after Roux.)
 
tube, mesoderm on one side, half a gut cavity—open towards the
dead blastomere——but a whole notochord (attributed by Roux to
 
premature regeneration).
IV. I INITIAL STRUCTURE OF THE GERM 161
 
Anterior half-embryos—occurring when the second furrow by
an ‘ anachronism ’ appears first-—were also observed, but posterior
never.
Quarter and three-quarter embryos were obtained by killing
three or one of the first four cells, and Hemiblastulae superiores
by killing the four yolk-cells in the eig-l1t~celled stage.
 
The self-differentiation of the ldioplasson is thus demonstrated.
Subsequently, however, the reserve Idioplasson comes into play,
and the missing half is formed by a peculiar process, to which
 
 
FIG. 77.——A and B. Normal Frog embryos with medullary folds (m),
open (A) and closed (B). C. Hemiembryo dexter with almost complete
post-generation of the eotoderm; u,yo1k-plug. D. The same, older, but
with less post-generation. E. Hemiemhryo anterior (‘i’) with beginning
post-generation. (From Korschelt and Heider, after Roux.)
 
Roux has given the name of ‘post-generation ’ to distinguish it
from regeneration, in which lost parts are formed out of already
(liiferentiated tissue.
 
The injured blastomere first degenerates, is then reorganized,
and finally post-generated.
 
In the first of these phases the cytoplasm and yolk are seen to
be vacuolated, while the injured but not completely killed nucleus
has given rise to small and large nuclei of normal structure
 
Jmxxntson M
162 INTERNAL FACTORS IV. I
 
surrounded by masses of pigment and in addition to irregular
chromatic masses —some pale, some deeply staining. Separating
it from the uninjured half is athin layer of yolkless protoplasm.
 
Reorganization is accomplished by any one of three methods.
The first is nucleation followed by cellulation, that is to say,
masses of cytoplasm arise round normal nuclei, some of which
are derivatives of the injured nucleus, while others have migrated
across from the living blastomere. The second method involves
an abundant immigration of whole cells from the living embryo
which resuscitate or feed on the yolk and nuclei of the injured
half. In the third method overgrowths take place at various
points of the tissues of the living half-embryo, the cells dipping
into and feeding on the yolk as they pass across.
 
In post-generation, which only occurs after the first method
of reorganization, each layer of the uninjured reforms the corresponding layer of the injured, the cells of the former exerting
a directive stimulus upon the reorganized tissues of the latter,
which thus pass through a process of ‘ dependent ’ differentiation.
The ectoderm grows over from in front backwards and from
below upwards, so that a structure superficially resembling a.
yolk-plug is formed at the hinder end; as it does so it becomes
difierentiated into the usual two layers. The medullary tube is
formed in like manner. The mesoderm is formed from the
ventral side and later divided into vertebral and lateral plates.
The notochord was a whole from the beginning. The missing
half of the gut is formed directly from the half-gut of the living
embryo, not by any process of blastoporic invagination.
 
The conclusions drawn by Roux from this experiment have
already been stated. Development is conceived of as a process
governed essentially by a factor which is entirely internal, the
preformed structure of the nucleus of the germ. Into the
validity of this factor we have now to inquire by examining
the evidence oifered by the very numerous and similar experiments, performed on the eggs of animals of all kinds, which the
‘ Mosaik-Theorie’ has evoked. We shall take these experiments
in order, beginning with the form employed by Roux himself,
the common Frog 1.
 
‘ For literature see following section.
IV. I INITIAL STRUCTURE OF THE GERM 163
 
§ 2. AMPHIBIA.
 
It will be remembered that the Frog’s egg possesses, when
freshly laid, a symmetry which is radial about the axis, the line
joining the centres (animal and vegetative poles) of the pigmented
and unpigmented portions of the egg; the symmetry is marked
internally by the arrangement of yolk and protoplasm, for the
latter lies most abundantly in the smaller unpigmentcd, the
former mainly in the larger pigmented portion. Since the yolk
 
 
FIG. 78.~—Formation of the grey crescent in the Frog's egg (R. temporaria). A, B from the side; 0, D from the vegetative pole. In A, C
 
there is no crescent, in B, 1) a part of the border of the pigmented area
has become grey.
 
is heavier than the protoplasm the white pole is turned downwards, with the axis vertical, the egg being free to rotate inside
its jelly membrane. Shortly after fertilization, however, the
radial is replaced by a bilateral symmetry (in Rana flwca and
I2. temporm-z'a), a grey crescent being formed on one side of the
egg along the border of the pigmented area by the retreat of pigment into the interior (Fig. 78). The grey crescent eventually
becomes white and added to the white area. In Rama caculenta
M 2
164 INTERNAL FACTORS IV. 1
 
it is stated that the egg‘-axis becomes inclined to the vertical.
This would seem to be an error, due to a confusion between the
original white area and the white area enlarged by the addition
 
FIG. 79.—Diagrams of the closure of the blastopore in the egg of the
common Frog (R. tempo:-aria). In A~—D the egg is viewed from the vegetative pole, in E, F from below. The dorsal lip is at the top of the figures.
In D the ventral lip has just been formed and the blastopore is circular.
In E the rotation of the whole egg has begun, and in F is complete.
 
of the grey crescent (compare Fig. 44 with Fig. 78). The
egg is now bilaterally; symmetrical about the plane which
includes the egg-axis and the middle (‘point of the crescent.
IV.1 INITIAL STRUCTURE OF THE GERM 165
 
Since the crescent is formed on the side opposite to the entry
of the spermatozoon, the latter is considered by Roux to
determine the symmetry-plane of the egg. The first furrow is
also stated to generally lie in this plane, and the dorsal lip of
the blastopore, which marks, of course, the sagittal plane of the
embryo, appears in the region of the grey crescent. The dorsal
lip grows through 75°, starting 25° below the equator and
passing beyond the vegetative pole ,- with the rotation of the
whole egg and therefore also of its axis in the direction opposite
to that through which the dorsal lip has travelled, the anterior
end of the embryo, whose dorsal side is now uppermost, comes to
lie a little above and behind the animal pole, while its posterior
end is marked by the now fast-closing blastopore (Fig. 79).
 
According to Roux the first furrow and the sagittal plane
coincide and the first two blastomeres are right and left, unless,
by an anaehronism, the second furrow, which separates anterior
from posterior (this should be dorsal from ventral), occurs first.
The third furrow, therefore, separates dorsal from ventral (correctly speaking, anterior from posterior). In the small number
of eggs examined by Roux the percentage of coincidences was
found to be high (80 %) and the deviations were attributed to
experimental error.
 
Oscar Hertwig has, however, stated—-on the strength of a
small number of observations on eggs compressed between horizontal glass plates—that the angle between the two planes may
have any value; while Schulze and Kopsch think it probable
that they coincide in the majority of cases. The recent examination by the present author of a large number of cases has
shown that the angle in question may indeed have any value
from 0° to 90°, but that there is a decided tendency to small
values, that is to coincidence, as may be seen from the annexed
frequency polygon (Fig. 80), although at the same time there is
no correlation, and therefore no causal connexion, between the
two planes. Between the plane of symmetry and the sagittal
plane, however, the correlation is considerable, and the tendency
of the two to coincide much greater (Fig. 81), a result in conformity with the statements of other observers (Roux, Schulze,
Morgan). Within certain limits, therefore, the right and left
166 INTERNAL FACTORS IV. 1
 
halves, the dorsal and ventral sides (since the dorsal lip always
 
appears in the region of the grey crescent) and the anterior and
posterior ends (since the anterior end is always a little above the
 
-90-oo -10 -so -50 -40 -50 -20 -no
 
o no '20 '30 Ho 050 +60 910 +00 *9!)
 
FIG. 80.—-Frequency polygon of the angle between the first furrow of
the Frog's egg and the sagittal plane of the embryo. n = 889, M = 2-12°
 
1-914, 0- = 40-39°:-646.
 
animal pole) are predetermined in the undivided egg. But the
manner in which the material of the egg is cut up by the first
two meridional divisions makes no diiference to the result. In
IV. 1 INI'1‘IAL STRUCTURE or THE GERM 167
 
these two divisions the qualitatively distinct parts of the cyto.
plasm may be separated from one another in any assignable
manner, the nuclear material being distributed to these in any
assignable order. The symmetry of segmentation has no relation
 
H0
 
I00
 
70
 
60
 
Frequency.
 
50
‘IO
 
30
 
ID
 
-90 -80 -70 -no 50 40 '30 '20 -10 o 0-Io v20 950 +40 950 can #70 0-60 9-90
 
FIG. 81.—F1-equcncy polygon of the angle between the plane of
symmetry of the Frog's egg and the sagittal plane of the embryo.
n = 509, M = 2-23:-889, rr = 29-75:-629.
either to the symmetry of the embryo or to the symmetry of the
undivided egg, for the first furrow tends to lie either in or at
right angles to the plane of symmetry and is not correlated with
 
it to any great extent‘. The third furrow, however, being‘ always
‘ See, however, Appendix A.
168 INTERNAL FACTORS IV. I
 
at right angles to the axis, invariably separates anterior from
posterior.
 
An observation of Morgan’s has some bearing on this question ;
even when the first furrow divides the egg unequally a normal
embryo results.
 
Even when the first two blastomeres are equal the cells produced by their subsequent division may migrate across the plane
of separation (Kopsch). In Nectmws there appears to be no relation
between the first furrow and the sagittal plane (Eycleshymer).
 
The hypothesis of qualitative nuclear division has been tested
 
by O. Hertwig in another way.
 
 
FIG. 82.—'l‘he first four phases of segmentation of the F1-og’s egg.
Normally (the three upper figures) and under pressure between horizontal plates (the three lower figures) I, II, 111, 1 V, the first, second,
third, and fourth divisions. 1-16, the cells produced by successive (livisions, numbered as follows :
The egg divides into
 
1”5 3 7 2 6 4J8
 
1 95'T4:?'T77 52'1“06164?28”'fis
 
The animal cells in the normal egg and the corresponding cells in the
compressed egg are stippled. (After 0. Hertwig’s account.)
 
The eggs are aflixed to glass plates by their jelly, fertilized,
and allowed to rest until the axis has become vertical. Pressure
is then applied by a second plate, in the direction of the axis,
the plates being horizontal, or at right angles to the axis, the
plates being vertical or oblique. In such compressed eggs the
direction of elongation of the nuclear spindles is distorted, and
IV. 1 INITIAL STRUCTURE OF THE GERM 169
 
consequently the ‘qualitatively unlike’ nuclei compelled to
assume an abnormal arrangement. Between horizontal plates
the first two furrows are, as normally, meridional and vertical
(Fig. 82) ; the third, however, is parallel to the first (instead of
latitudinal), and the fourth latitudinal (instead of meridional).
 
Between vertical plates the first is meridional and at right
angles to the plates, the second latitudinal (horizontal) and near
the animal pole, instead of meridional, the third furrows are
parallel to the first, and the fourth meridional and at right
angles to the first in the four upper blastomeres.
 
Now if nuclear division is a qualitative process, if, further,
the nuclei of the compressed egg are divided in the same way
in successive mitoses as are the nuclei of the normal egg (that is
to say if the first two nuclei are right and left, each of these
then divided into dorsal and ventral, and each of these again
into an anterior and a posterior), then as a result of compression their distribution and arrangement must be altered (as
the figures show), parts which should be anterior will be lateral
or ventral, and vice versa, and a monster will be formed. Such
eggs give rise to normal embryos. Nuclear division is therefore not qualitative. From this conclusion there is only one
escape; it may be argued that the orientation of the nuclei
remains unaltered when the direction of the spindles is changed
by the pressure, and that therefore the order of sequence of the
cell-divisions may be varied ad libitum, may be as ‘ anachronistic ’
as is pleased, without affecting in the least the mode of distribution of the, qualitatively unlike, parts, a contention, surely, which
would only be urged for the sake of supporting a thesis.
 
0. Hertwig has also repeated Roux’s experiment, killing, or
at least injuring, one of the first two blastomeres by a needle or
by means of electricity. The uninjured half segments to form
a mass of cells lying on the top of the dead blastomere, as
a blastoderm lies on the yolk of a meroblastic egg, and separated
from it by a segmentation cavity, although the latter may be
wholly within the living cells. This is Roux’s Ilemiblastula,
but in Hertwig’s case the dead cell lies below, a point, as we
shall see, of considerable importance. Later on a blastopore is
formed, but always, according to Hertwig, within the bounds of
170 INTERNAL FACTORS IV. 1
 
the living portion, not at the edge; the blastopore is usually
symmetrical to the plane of separation of the two blastomeres.
Beneath the lip of the blastopore an archenteron is formed, and
notochord and mesoderm are differentiated. The closure of the
blastopore is, however, prevented by the resistance ofiered by the
dead yolk-mass, which lies ventrally and posteriorly, just as it is
retarded by the yolk in a large-yolked Fish egg; in fact, were this
dead yolk removed, the living portion, Hertwig maintains, would
develop normally. A nearly normal embryo may, in fact, be
formed, but more usually there are considerable abnormalities.
The mass of dead yolk, though partially enclosed by the growing edge of the ectoderm, is sufficiently great to impede the
ultimate closure of the blastopore, and sometimes of the
medullary folds as well; the latter frequently diverge round
the yolk-plug of dead and living tissue, the chorda being split
as well; they may be symmetrical, or one side may be much
less developed than the other, owing, it is asserted, to an
asymmetry in the resistance offered by the dead cell, and in
the more extreme cases this inequality may be so pronounced
as to give rise to a condition which does not differ in any way
from Roux’s Hemiembryo lateralis ; there is one medullary
fold, a notochord, mesoderm on one side, and a gut cavity in
the yolk-mass ; to the bare side of the yolk-cells is attached
the dead blastomere, only partially covered by extensions of the
cctoderm of the living half. Such cases are, however, very rare;
in the majority more, sometimes much more, than a half is
produced from the living blastomere, for Hertwig denies that
the missing parts are post-generated, as Roux maintains, though
he admits the overgrowth and immigration of cells, as well as
the persistence of living nuclei where the injury has been only
partial ,- all these contribute to the formation of the embryo,
which would be complete were its development not hindered by
the presence of the inert mass of yolk.
 
The diiferences of interpretation put by Roux and Hertwig
on the same phenomena appear to be radical. A reconciliation
is, however, possible, for Morgan has observed that the whole or
half development of the injured cell depends upon the position
it takes up. If the original position—with the black pole
IV.J INITIAL STRUCTURE OF THE GERM 171
 
uppermost-—is retained, then a half-embryo is formed, as is
indeed the case, according to Hertwig’s own evidence, when the
egg is prevented from turning over by compression.
 
In such half-embryos Morgan finds no traces of Roux’s postgeneration, although a whole embryo may be eventually formed
by regenerative processes, referred by the author to the retarded
development of the living parts of the injured half. It should
be noticed, however, that Roux’s statements are confirmed by
Endres and Walter.
 
Should, however, the white pole be uppermost, then a whole
embryo of half-size results. In Hertwig’s experiments, of course,
the original position was usually not retained.
 
 
FIG: 83.-.~—Doub1e embryo of Rana fusca, from an egg compressed in
the direction of the axis and inverted in the two-cell stage. (After
Schultze, from Korschelt and Hcider.)
 
 
 
A B
 
FIG. 84.—Double monsters of Rana fusca, obtained by the same method.
(After Schultzc, from Korschelt and Heidcr.)
 
This conclusion is still further strengthened by Schultze’s
observations on the development of eggs inverted in the twocelled stage and kept so. Each blastomere develops independently
and a double monster is produced (Figs. 83, 84). The result
is not due to the pressure, for controls similarly compressed
developed normally.
 
The details of development of these eggs have been carefully
worked out by Wetzel (Figs. 85, 86). In each inverted blasto172 INTERNAL FACTORS IV. I
 
mere the yolk sinks next to the plane of separation, while the
protoplasm and pigment rises to the outer side. The two cells are
now related as two whole eggs united by their vegetative poles,
their axes in one and the same (horizontal) straight line; each
has, in fact, acquired a totally new polarity of its own. When
segmentation has been completed a groove appears, in the plane
of separation, and gradually extends round the whole circumference ; the ends of the groove are forked, but the branches of
each fork unite as the groove grows round. The groove is, in
fact, a blastoporic lip common to the two, the branches the
individual lips, the material between them and finally covered
over by them a common yolk-plug, the space between the two
 
 
FIG. 86.——Section through a.
double blastula of the Frog
' (Rana fusca). Ia, blastocoels.
FIG. 8-3.—-Double embryo ob- (After Wetzel, from Korschelt
 
tained by the same method: h, and Heider.)
heads; 1», metlullary grooves;
 
c, line of union of the latter.
 
(After Wetzel, from Korschelt
 
and Heider.)
 
 
a common archenteron extending into an archenteric space in
each individual. Subsequently medullary folds and notochord
are developed in each below the groove, that is on the vegetative
or postero-dorsal side of each, but anteriorly each grows out free
of the other, and in this region medulla, notochord, and gut are
single. The result is‘, therefore, two embryos placed back to
back, and united by a common yolk-plug.
 
Experiments in which the four animal or the four vegetative
cells of the eight-celled stage are killed are not very conclusive,
as the develonment of the survivors does not go very far.
IV. I INITIAL STRUCTURE OF THE GERM 173
 
Samassa has, however, shown that a short archenteron with
dorsal lip, together with traces of notochord and mesoderm, may
be formed from the four animal cells alone, the dead yolk-cells
making a floor to the archenteric cavity and protruding as
a large yolk-plug; and Morgan has obtained a similar result
with the four vegetative cells‘ alone.
 
From all these investigations it seems reasonable to infer that
each of the first two blastomeres of the Frog’s egg may under
certain circumstances acquire the polarity of a whole, and be
capable of giving rise to an embryo whose complete development
is only prevented by the impediment offered by the presence of
the other, whether living or dead. Were it possible to completely separate the two blastomeres, we may surmise that each
would become a perfect embryo: a surmise which is raised to
a certainty by our knowledge of
what happens in the newt. For
in this form it is possible to
separate the two cells by means
of a noose of fine hair tied round a  __,,..
the egg in the plane of the first ' i‘  ’ ' _.,furrow, as Herlitzka has shown, },‘1G_ g7__Egg of the newt
 
and in  case gach half seg-- (Triton cristatus),  tW0 (E0111pletely normal embryos, obtained
 
ments as a whole and develops by tying a thread W) round the
 
into a whole larva of rather more egg  the  f'li1I‘r0iv§'é ky,ijc1Iy
- - - _ mem l'2].:n(3. er erl Z a, .l'Ol'l1
than ha’It'S1ze (F135 87)‘ H01" Korschelt and Heider.)
 
litzka has further investigated
the dimensions of the organs in these embryos ,- those of the me
dulla and notochord he finds are the same as in embryos developed
from a whole egg, those of myotomes and gut a little less. The
size of the nuclei and cells in the medulla and myotomes is the
same as in the whole (3%) embryo 1 ; the number of nuclei in the
medulla is the same, in the myotomes one-half. He concludes
that some organs need a certain minimum number of cells for their
differentiation, while the cells must attain to certain dimensions.
 
 
1 It will be convenient henceforward to designate the whole egg, or
the embryo or larva developed from it, by the symbol }, each of the first
two blastomeres and the embryo or larva, whether complete or Incomplete, formed from it by —§, and so on; thus a % embryo means one which
arises from three out of the first four blastomeres.
174 INTERNAL morons IV. 1
 
The discovery of Herlitzka has been followed up by the
researches of Spemann, who has employed the same method of
constriction, but with variations of degree, direction, and time.
These difierences have given the most interesting results.
 
The first furrow, according to this author, is usually at right
angles to the sagittal plane and separates the material for the
dorsal and ventral halves of the embryo ; only rarely do sagittal
plane and first furrow coincide. Both cases were, however,
experimentally investigated.
 
 
FIG. 88.—Three stages in the roduction of a double monster by
strong median constriction of the §Iewt’s egg. (After Spemann, 1903.)
a. Beginning of gastrulation; there is a separate lip in each half.
b. I. and 1'. Med., edullary folds of left and right embryos; *, point
where the medullary grooves separate; Bl, blastopore. c. The doubleheaded larva.
 
When the first furrow is in a horizontal plane, a slight constriction in the two-celled stage separates the dorsal lip of the
blastopore in one portion from the ventral lip in the other.
Medullary folds are developed in the first half only, the second
forms a sort of yolk-sac appendage which is later absorbed by
the single normal embryo. With tighter, but still incomplete,
constriction the dorsal half alone becomes an embryo; it contains
either the whole or only the dorsal portion of the blastopore.
The ventral half contains either no portion of the blastopore or
IV. I INITIAL STRUCTURE OF THE GERM 175
 
only the ventral lip; it develops mesoderm but undergoes no
further dilferentiation, and eventually drops ofi. Should the
constriction be delayed until after the dorsal lip has appeared
both halves may form an embryo, but the ventral embryo is
usually imperfect. These dilferences are attributed by Spemann
to differences in the time or place of constriction.
 
When a. transverse constriction is made after the appearance
of the medullary plate the anterior half develops as a whole,
with complete nervous system and optic vesicles; the posterior
forms a medullary plate, but no folds, and dies. After the
appearance of the medullary folds, however, the anterior and
posterior halves produced by transverse constriction develop as
halves, although a case is described where each had a pair of
auditory vesicles.
 
When the sagittal plane coincides with the first furrow, constriction in the two-celled stage gives rise to double-headed
monsters; if the constriction is slight the most anterior organs
only are involved (duplicitas anterior), the cerebral hemispheres,
epiphysis, hypophysis, and paraphysis being doubled; there may
be two complete pairs of eyes, or the median eyes may be more
or less fused (Diprosopus triophthalmus). Tighter constriction
brings about a reduplication of the chorda, auditory vesicles, and
fore limbs (Dicephalus tetrabrachius). Similar effects are
obtained by constriction in the blastopore stage, but not later;
halves separated in the median plane when the medullary folds
have arisen die as halves.
 
We shall now briefly consider another experiment by which
the independence of one another of the parts is demonstrated.
Schaper removed the brain, eyes, and, probably, the auditory
vesicles from newly hatched tadpoles of Rana esculenta. The
wound healed up. The mouth moved to the anterior end and
the suckers up the sides of the tadpoles, which lived for nearly
a. week, in the course of which they grew 2mm. They then
showed signs of weakness and were preserved. It was found
that the mouth had opened, that labial cartilages, pterygopalatine bar, jaw-muscles, gill-bars, gills, heart and bloodvessels, trigeminus and vagus ganglia, oesophagus, pronephros
and glomus, and dorsal muscles had all been dififerentiated.
176 INTERNAL FACTORS IV. I
 
There was, however, no operculum. The anterior end was
occupied by a mass of mesenchyme. There was no sign of a
regeneration of any of the lost organs except the anterior end
of the notochord. The nerve ganglia were normal, but the spinal
cord underwent degeneration. In spite of this the creatures
could execute spontaneous and reflex movements.
 
What this experiment shows is, of course, that the organs of
the trunk are not dependent for their development upon the
presence of the brain; fresh researches would be necessary to
determine how far in each case the parts are self-difierentiating.
In the second place the brain and eyes, when removed at this
stage, cannot be remade by the tissues, but remain behind. So
far, therefore, the body is at this stage an inequipotential system,
as Driesch would call it. We know, however, that at an earlier
stage the parts of the body are equipotential. We have, in fact,
only another instance of that loss of totipotentiality, of that increase of independence and self-diiferentiation which takes place
as development proceeds. Here, however, we are anticipating
a conclusion which can only be completely stated after a discussion of the experiments performed on eggs of other types.
 
Ll'l‘ERA'l‘URE
 
H. ENDRES and H. E. WALTER. Anstichversuche an Eiern von Rana
fusca, I'°' und lI*°' '1‘ei1, Arch. Eat. Mech. ii, 1896.
 
A. C. EYCLESHYMER. Bilateral symmetry in the egg of Nectum.9,
Anat. Anz. xxv, 1904.
 
A. HERLITZKA. Contributo allo studio della capacitia. evolutiva dei
due primi blastomeri nell’ uovo di tritone (Triton crislatus), Arch. Eur.
Mech. ii, 1896.
 
A. HERLITZKA. Sullo sviluppo di embrioni completi da blastomeri
isolati di nova di tritone, Arch. Ent. Mech. iv, 1897.
 
O. HERTWIG. Ueber den Werth der ersten Furchungszellen fiir die
Organbildung des Embryo, Arch. mikr. Anat. xlii, 1893.
 
F. KOPSCH. Ueber das Verh-liltniss der embryonalen Axen zu den
drei ersten Furchungsebenen beim Froseh, Iutemaf. Monatsc-hr. Anal.
14. Phys. xvii, 1900.
 
T. H. MORGAN. Half-embryos and who1e—embryos from one of the
first two blastomeres of the Frog’s egg, Anat. Auz. x, 1895.
 
T. H. MORGAN and E. TORELLE. The relation between normal and
abnormal development (iv) as determined by Roux’s experiment of
injuring the first-formed blastomeres of the Frog's egg, Arch. Ent. Mech.
xviii, 1904. '
IV. I INITIAL STRUCTURE OF THE GERM 177
 
T. H. MORGAN. The relation between normal and abnormal development of the embryo of the Frog, (v) as determined by the removal of the
upper blastomeres of the Frog's egg, Arch. Ent. Mech. xix, 1905.
 
W. RoUx. Ueber die Zeit der Bestimmung der Hauptrichtungen des
Froechembryo, Leipzig, 1883; also Ges. Abh. 16.
 
W. RoUx. Ueber die Bestimmung der Hauptriohtungen (lee Froschembryo im Ei und fiber die erste Theilung des Froscheies, Bresl. iirtz.
Zeitschn, 1885; also Ges. Abh. 20.
 
W. ROUX. Uebcr die kiinstliche Hervorbringung halber Embryonen,
Virchoufs A1'ch., 1888; also Ges. Abh. 22.
 
W. Roux. Ueber das entwicklungsmechanische Vermtigen jeder der
beiden ersten Furchungszellen des Eies, Verh. Anal. Ges., 1892; also Ges.
Abh. 26.
 
W. RoUx. Ueber Mosaikarbeit und neuere Entwicklungshypothesen,
Anat. Hqfte, I" Abt., ii, 1892-3; also Ges. Abh. 27.
 
W. Roux. Uebcr die Specification der Furchungszellen und iiber die
bei der Postgeneration und Regeneration anzunehmenden Vorgiinge,
Biol. Centralbl. xiii, 1893; also Ges. Abh. 28.
 
W. ROUX. Ueber die ersten Theilungen des Froscheies und ihre Beziehungen zu der Organbildung des Embryo, Anat. Anz. viii, 1893; also
Gas. Abh. 29.
 
W. ROUX. Die Methoden zur Hervorbringung halber Frosch-Embryonen, uml zum Nachweis der Beziehung der ersten Furchungeebenen des
Froscheies zur Medianebene des Embryo, Anat. Anz. ix, 1894; also Ges.
Abh. 31.
 
W. Rovx. Ueber die Ursachen der Bestimmung der Hauptrichtungen
des Embryo i1n Froschei, Anat. Anz. xxiii, 1903.
 
P. SAMASSA. Studien fiber den Einfluss des Dotters auf die Ga.strula.tion und die Bildung der primiiren Keimbliitter der Wirbelthiere :
II. Amphibien, Arch. Ent. Mech. ii, 1896.
 
A. SCIIAPER. Experimentelle Studien an Amphibienlarven, Arch.
Em‘. Mech. vi, 1898.
 
O. SCHULTZE. Die kiinstliche Erzeugung von Doppelbildungen bei
F1'oschla.rven mit Hilfe abnormer Gravitntionswirkung, Arch. Ent.
lllech. i, 1895.
 
0. SCHULTZE. Ueber das erste Auftreten der bilateralen Symmetric
im Verlauf der Entwicklung, Arch. mikr. Anat. lv, 1900.
 
H. SPEMANN. Entwickelungsphysiologische Studien am Triton-Ei,
I, II, III, Arch. Ent. Mech. xii, xv, xvi, 1901, 1903.
 
G. WE'rzEI.. Ueber die Bedeutung der cirkuléiren Furche in der Entwicklung der Schultze'schen Doppelbildungen von Roma fusca, Arch.
mikr. Anat. xlvi, 1895.
178 INTERNAL FACTORS IV. 1
 
§3. P1scEs.
 
Morgan has shown that in the fishes Ctenola/)ru.9, Serramw,
and Fmzdulus there is no definite relation between the first (or
second) furrow and the median plane of the embryo.
 
In Fumiulus when one of the first two blastomeres is removed
the other gives rise to a perfect embryo of more than half-size.
In segmentation the first furrow is in the plane of what would
be the second furrow of the whole egg, the second in that of
the third, and the third more or less in that of the fourth ,- but
as the successive furrows are at right angles to one another in
both cases it is permissible to assert that the segmentation of
the half blastomere is total.
 
Some of the protoplasm of the half that has been removed
flows in underneath the other, is nucleated by it and added to its
mesoderm. The size of the nuclei is the same in % and -} embryos.
 
Morgan also found it possible to remove two-thirds of the
yolk, but not more, without interfering with the normal development of the whole egg.
 
Injury to the germ—ring on one side of the dorsal lip resulted
in a deficiency in the mesoderm of that side posteriorly ;
anteriorly both sides were equally well formed.
 
So Kastschenko has shown for Elasmobranchs and Kopseh for
Teleostei and Elasmobranehs that injury to the lip of the blastopore will only interfere with the normal bilaterality of the embryo
when effected quite close to the middle line.
 
A series of experiments similar to the last has been carried
out by Sumner on the eggs of various fish (principally 1*'zmrlulus,
also Ewocoetzza, Salvelinus, and Batrac/ms).
 
By means of needles inserted into various parts of the blastederm the exact mode in which the material of the latter is used
in the formation of the embryo is first determined. It is
shown that the material for the embryo is brought into position
not by a conerescence of the lateral lips of the blastopore (sides
of the germ—ring), but rather by an axial concentration of the
cells originally situated in that ring at the posterior margin or
dorsal lip, the cells so concentrated being continually pushed
forwards in the middle line until the anterior end of the embryo
comes to lie near the original centre of the blastoderm.
IV. 1 INITIAL STRUCTURE OF THE GERM 179
 
Injuries to various parts of the blastoderm and its margin (the
germ-ring) prove that it possesses a certain degree of ‘isotropy’.
 
Thus the entire embryonic region of the posterior margin was
destroyed by electrocautery. The adjacent edges closed up and
completed the germ-ring ; at the posterior point of this a new
embryonic shield was formed, in which ectoderm, endoderm, and
notochord became differentiated. Or, again, a lateral piece of
the germ-ring immediately adjacent to the embryo was removed
without preventing the formation of a normal, bilaterally
symmetrical embryo, although, as Morgan fou11d, the mesoderm
might be deficient posteriorly on the injured side.
 
LITERATURE
 
N. KASTSCHENKO. Zur Entwickelungsgeschichte des Selachierembryos,
 
Anat. Anz. iii, 1888.
F. KoPscH. Experimentelle Untersuchungen iiber die Keimhaut der
 
Salmoniden, Vcrh. Aunt. Gesell., 1896.
F. KOPSCH. Experimentelle Untersuchungen am Primitivstreifen
 
des Hiihnchens und an Sc;/Ilium-Embryonen, Verh. Auat. GeseIl., 1898.
 
T. H. MORGAN. Experimental Studies on Teleost eggs, Anat. Anz.
viii, 1893.
 
T. H. MORGAN. The formation of the fish-embryo, Journ. Morph.
 
x 1895.
’F. B. SUMNER. A study of early fish-development, experimental and
 
morphological, Arch. Em‘. Mech. xvii, 1904.
 
§ 4. Amrmoxus.
 
In this form the blastomeres may be separated by shaking.
Their development has been observed by Wilson, whose account
 
‘ 8 '’GD
 
0
 
FIG. 89. A. Normal } gastrula. of Amphioams. B. Gastrula from  blas
tomere. C Gastrula from a i‘ blastomere. J). Gastrula of normal size
from an egg-fragment. (After Wilson, from Korschelt an Heider.)
has since been confirmed by Morgan. The isolated blastomeres
soon become rounded; the first division is always transverse to
the long axis.
 
N 2
180 INTERNAL FACTORS IV. 1
 
A i‘ blastomere segments like a whole ovum, except that the
second division may be unequal, and gives rise to a normal
blastula, gastmla, and embryo of half the normal size; % blastemeres behave in the same way (Fig. 89). Incomplete separation of
the cells after the first or second division leads to double embryos
in the first case, double, triple, or quadruple embryos in the
second. The double embryos may make any angle with one
another and continue to live for some time (Fig. 90).
 
A % blastomere segments like a whole egg (the second division
is, however, always unequal) and gives rise to a normal blastula
and gastrula, more rarely to an embryo of one-quarter size.
 
 
FIG. 90. Double nrastrulae of Amphioams, from incompletely separated
blastomeres. 1;‘), u’, separate blastopores; u, common blastopore.
(After Wilson, from Korschelt and Heider.)
 
A % blastomere will segment normally, or nearly so. A blastula is rarely formed, usually an open curved plate of cells,
which are ciliated and swim about but do not gastrulate.
 
Lastly 115 blastomeres divide but produce only an irregular
heap of cells.
 
It is obvious that the potentiality of a blastomere to form
a whole embryo diminishes with its germinal value, the ratio it
bears to the whole ovum; but it is not so clear whether this
diminished capacity is due to the lack of some necessary, specific,
organ-forming substance or merely to the small size and lack of
IV. I INITIAL STRUCTURE OF THE GERM 181
 
undifferentiated material. Morgan has attempted to answer
this question by counting the number of cells in the various
organs of whole and partial larvae. It appears from his estimate
that the number of cells in the archenteron, in the notochord
and in the nerve-cord of -1-, 5, and 71,- larvae is constant, and that
though the size of the whole body is less, the dimensions of the
notochord and nerve-cord are about the same in all three. It
would seem, therefore, that there is a minimum size and a
minimum number of cells necessary for the formation of these
organs. Whether the failure of the % blastomere to develop
into a larva is, however, due to more lack of material or the
absence of some specific substance needful for the development
of some particular organ is still undecided ; for the 1% and :1; cells
contain substance from both the animal and vegetative portions of
the egg, while the §- are composed of one or the other substance
exclusively, the furrows of the third phase being equatorial.
 
LITERATURE
 
T. H. MORGAN. The number of cells in larvae from isolated blastomeres of Anzphioxzts, Arch. Ent. Mech. iii, 1896.
 
E. B. WILSON. Amp7n'0.z'us and the Mosaic theory of development,
Joum. Morph. viii, 1893.
 
§ 5. COELENTERATA.
 
Many years ago Metschnikoif observed a perfectly normal
separation of the blastomeres at a certain period in the development of certain medusae (Oceania). If when the cells reunite
in a later stage the order of their rearrangement is not constant,
the egg-substance must be in some measure isotropic.‘
 
More recently the equipotentiality of the blastomeres of these
forms has been experimentally demonstrated by Zoja (Fig. 91).
 
The cells were separated by means of a needle. -3 and :1blastomeres (in the case of Lz'rz'ope, Gem/onia, Mitrocoma, Clytia,
and Laozlice) and % and 1-15 blastomeres in the case of the two
last mentioned will give rise by normal segmentation to blas
‘ It should not be forgotten that in 1869 Haeckel had cut the blastulae
of Crystallodes (a Siphonophor) in pieces and obtained from the larger
fra ments normal larvae. The development of small pieces was retar ed and abnormal (Zur En!wickelimysgeschichte der Siplconophovm,
Utrecht, 18%).
182 INTERNAL FACTORS IV. 1
 
tulae with closed blastocoel and these to normal Planula larvae.
In Olytia a hydroid was eventually reared and in Liriope a
medusa with four primary tentacles from 73 and 3%; cells. It is to
be noticed that, since the third division in these eggs is equatorial,
the ,1, and 1% blastomeres must be either animal or vegetative.
 
Zoja also finds that in Laodice and Clytia the number of cells
in partial larvae is proportional to the germinal value. In
Li;-iope the number of cells in the endoderm is equal to that
in the whole  larva.
 
35%
 
 
 
FIG. 91.
Partial development
 
in Coelenterata.
(After Zoja, 1896.)
 
a-cl. Laodice cruciata: a,§ blastomeres:
12, § blastomeres ; c,
blastula from  blastemere; d,  larva during
formation of endoderm. e—l. Liriope mucrouara. c— 2'. Development of  blastomere :
e, division of  blaste
more into  f; -§-; g,
-,"5,;; h, end ofendoderm
formation in  embryo; 1', medusa. reared
from  blastomere.
j—l. Development 01}
blastomere: j, 3; k,
,4“; Z, embryo with
endodenn.
 
7
 
LITERATURE
 
E. METSCHNIKOFF. Embryologische Studien au Medusen, Wien, 1886.
R. ZOJA. Sullo sviluppo dei blastomeri isolati dalle uova di alcune
meduse, Arch. Ent. Mach. i, ii, 1895-6.
IV. 1 INITIAL STRUCTURE OF THE GERM 183
 
§ 6. ECHINOD]*Jlh\IA'1‘A.
 
Easy to obtain in large numbers and eminently suitable for
experiment, the eggs of the sea-urchins and starfishes have provided the analytical embryologists with abundance of material;
and it is in this group that the possibility of rearing a perfect
larva from a single blastomere was first demonstrated by the
classical researches of Driesch.
 
The experiments which have been carried out, mainly by
Driesch, but also subsequently by others, are very numerous and
fall into two principal classes.
 
In the first the development of parts—wheth_er isolated blastemeres or groups of blastomeres, fragments of unsegmented eggs
fertilized or unfertilized, or pieces of blastulae and ga.strulae—has
been observed, the rate of their development recorded, and the
relation of the number of cells and dimensions of the partial
larvae to their germinal value determined. By the second series
of investigations the effect of alteration of the type of nuclear or
cell-division and consequent displacement of the blastomeres upon
the subsequent development has been discovered.
 
Before discussing these experiments, however, a word must be
said as to the normal behaviour of the whole egg.
 
The structure, segmentation and formation of the primary
germinal layers have been very completely studied by Boveri in
the sea-urchin, St1'o2z_qyZocenh'o6u8 livirlus (Fig. 92). The axis of the
ovarian egg is marked by the exeentric position of the nucleus ;
at the point on the surface nearest which it lies the polar bodies
are formed, and this point is the animal pole. At this point
there is a fine canal (micropyle) in the jelly which surrounds
the egg. After the extrusion of the polar bodies a brown pigment
which had previously been uniformly distributed through the
cytoplasm becomes aggregated in a dense but quite superficial
subequatorial zone, the upper somewhat ill-defined border of
which may, however, extend into the animal hemisphere. The
pigment-free region left about the vegetative pole occupies from
7515 to -115 the volume of the whole ovum. The spermatozoon may,
but need not, approach the egg by the mieropyle. The fertilization spindle lies in a plane parallel to the equator of the egg (the
FIG. 92.——Norma.l development of the sea.-urchin, Sh-ongyloceulrotus
l1'm'dus. (After Boveri, 1901.)
 
The animal pole is u permost in all cases, and in the first two figures
the jelly with the cana.i)(micropyle) is shown.
 
a, primary oocyte ; the pigment is uniformly eripheral.
 
b, ovum after extrusion of polar bodies: tiie pigment now forms
a. subequatoi-ial band. The nucleus is ex-axial.
 
c, (1, first division (meridional).
 
e, 8 cells, the pigment almost wholly in the vegetative bla.ston1o1-vs.
 
j, formation of mesomeres (animal cells) by meridional division: the
vegetative cells have divided into macromeres and micromeres.
 
g, blastula. h, mesenchyme blastula.
 
1', j, k, invagination of the pigmented cells to form the urchenteron of
the gastrula. In j the primary mesenchyme is separated into two
groups, in each of which, in A-, a. spicule has been secreted. In I: the
secondary, pigmented mesenchyme is being budded off from the inner
end of the archenteron.
IV. 1 INITIAL STRUCTURE OF THE GERM 185
 
‘ karyokinetic plane’) and elongates at right angles to the spermpath, which thus lies in the plane of the first, a meridional, furrow
(a similar observation had been previously made by Wilson and
Mathews on Towopuezzstzav).
 
The second furrow is likewise meridional and at right angles to
the first, the third equatorial or a little nearer to one or other pole;
the animal cells get only a very little of the pigment. In the
next phase the animal cells divide meridionally and equally to
form the eight mesomeres, while the vegetative cells divide
latitudinally and very unequally into four large pigmented
macromeres next the equator, and four small quite unpigmented
micromeres grouped round the vegetative pole.
 
In the blastula stage all the cells are of the same size, vacuolated and ciliated, and the polarity of the egg is only determinable by the position of the pigment zone. In the next stage
the clear vegetative cells derived (presumably) from the micro
 
FIG. 93.—Ec7n'nus: segmentation under pressure.
 
a, preparation for third division (radial); b, preparation for fourth
division (tangential); b’, after fourth division; c, another form of the
8-cell stage (third division parallel to first); (I, the same after removal
of the pressure. (After Drieseh, 1893.)
 
mercs wander in to form the primary mesenchyme~from which
the first triradiate spicules are developed-—and this is followed by
the invagination of the pigmented cells to form the arehenteron.
Secondary pigmented mesenchyme cells are budded off from the
inner end of the latter. The character and sequence of the
divisions in segmentation are the same in other Echinid eggs, but
the polarity is not marked by pigment, nor indeed recognizable
till the micromeres have been formed, and again when the
mesenchyme cells wander into the blastocoel.
 
By means of press11re——under a coverglass-—Driesch succeeded
(E0/12'-mw) in altering the direction of the spindles and consequently
of the cell-divisions (Fig. 93). The first and second furrows were at
186 INTERNAL FACTORS IV. I
 
right angles to one another and to the coverglass, but whether
meridional or 11ot does not appear. The third furrows were again
at right angles to the coverglass and at 45° to the first two; a flat
plate of eight cells was thus formed. The next division resulted in
the formation of sixteen equal cells still all lying in the same plane ;
the formation of micromeres is thus suppressed. Such eggs gave
rise to normal larvae.
 
In the case just quoted the egg-membrane was intact; but similar
results were obtained when it was broken. The second furrow was
sometimes at right angles to, sometimes parallel to, the first, the
third at right angles to both in the latter case, and the fourth
 
 
FIG. 94.—The effect of heat upon the segmentation of the Echinoid
egg. a, b, c, d, four successive stages in the segmentation of the same egg
ofEchinus; e, f, two successive stages in the division of the same egg of
Sphaerechinus. (After Drieseh, 1893.)
 
parallel to the third ,- but if the pressure was released the blastemeres became rounded olf and the sixteen-celled stage consisted
of two plates of eight cells each; one or two micromeres were
sometimes formed. The development of these eggs is also quite
normal. These results have been confirmed by Ziegler.
 
In this, as in the similar experiment of Hertwig on the Frog’s
egg, the normal distribution and arrangement of the nuclei is
interfered with without prejudice to the normality of subsequent
development. Nuclear division cannot therefore, Driesch contends,
be a qualitative process. Boveri has also shown in similar
fashion that the order of segmentation may be disarrang-ed and
. IV. 1 INITIAL STRUCTURE OF THE GERM 187
 
the formation of the micromeres suppressed in Stron_qylocem5rotzw and a normal larva still result. The relation, however, of the
egg-axis, as determined by the pigment-ring, to the symmetry
of the larva remains the same as in the undisturbed egg.
Increased temperature, violent shaking, dilute sea-water, and
calcium-free sea-water are all means by which Driesch has suc
ceeded in disarranging the blastomeres of E’c/tinus in various ways
(Fig. 94). Thus when the temperature was raised from 19° C. (the
 
FIG. 95.——Va.ria.tions in the segmentatioxi of Echimts mic;-otuberculntus
produced by dilution of the sea-water. a, tetrahedral four-cell stage;
I), eight cells, three premature micromeres; c, eight cells, two precocious
microineres; (I, the same egg after the next division. the precocious
niicromeres have divided unequally, two normal micromeres have been
formed. (After Driesch, 1895.)
 
normal) to 31° C. the first two (not subsequent) blastomeres
separated, though they sometimes reunited. The next division
was normal, b11t with the next phase one or two blastomeres
divided in a direction perpendicular to that of the others, or the
direction of division was different in each. In the following
stage the micromeres were wholly or partly suppressed. So also in
dilute sea-vmter (20% fresh water) the third division was unequal
188 INTERNAL FACTORS IV. I
 
in two or more blastomeres, and in the following an excessive
number of small cells was formed (Fig. 95). In aless dilute mixture (16% fresh water) the blastulac divided to form each two
Plutei. Again by shaking the eggs in the eight-celled stage the
blastomeres were disarranged and made to lie almost in one plane ;
micromeres were, however, formed as usual from the vegetative
cells, but not, of course, in the normal position (Fig. 96). In
spite of these very considerable alterations in the size and positions of their constituent cells all these ova nevertheless gave rise
to perfectly normal larvae.
 
By the use of calcium-free sea.-water either the mesomeres or
the macromeres and micromeres were separated into two groups.
In the latter case the larva had two guts, in the former a normal
larva. resulted, so that, as Driesch points out, the ectoderm at the
 
 
FIG. 96.~—-Disarrangement of the blastomeres of Echimcs by shaking,
a, eight cells; b, sixteen cells; notice that the micromeres are not close
together. (After Driesch, 1896.)
 
antiblastoporic (animal) pole must in this case have been formed
from the macromeres.
 
The conclusion to be drawn from these experiments seems
obvious; not only the nuclei, but the parts of the cytoplasm
too are equivalent; within at any rate fairly wide limits their
normal arrangement may be disturbed without affecting in the
least the normality of subsequent development; or, as Driesch
phrased it, the destiny of a nucleus or blastomere is not determined by its original situation in the preformed structure of the
egg, rather it is a. function of its position in the whole embryo
to which that egg gives rise. Its character is decided not by
its origin, but by its final position.
 
Whether there really is a limit to the rearrangement is a
question which must be reserved for future discussion. We are
4 IV. I INITIAL STRUCTURE OF THE GERM 189
A /‘
 
I
 
FIG. 97.—A and 0, formation of ex-ovates in the egg of Arbacia by
dilution of the sea.-water; K, nucleus; m, egg-membrane. B and D,
blastulae formed from A and C ; B becomes constricted into two blastulae, each of which gives rise to a. Pluteus; Dproduces a. single Pluteus.
(After Loeb,from Korachelt and Heider.)
190 INTERNAL FACTClRS IV. I
 
also indebted to Driesch for a long series of experiments on the
behaviour of isolated blastomeres and egg-fragments. In the
forms which we have hitherto considered—the Vertebrates and
Coelentera.———an isolated cell segments as a whole and gives rise
to a total larva. In these Echinoderms the isolated blastomere
also gives rise to a total larva, but its segmentation is partial;
only after segmentation has been completed does the open blastula.
close up and resume the polarity of a whole.
 
Various methods have been employed for separating the
blastomercs. Loeb showed that in dilute sea-water (50  the egg
 
 
FIG. 98.-—Development of the isolated «} blastomere of Echinus micromberculatms-. 'l‘wo1n1eromeres,twonmcromeres, four mesomeresz a, §; b 4*
 
c, Hemiblastula: on the right 1s the remaining  blastomere, dead.
Driesch, 1892.)
 
swells, bursts its membrane and protrudes an ex-ovate which
may be large and develop independently (Fig. 97). Driesch has
used heat, pressure, violent shaking with fragments of coverglasses, and the calcium-free sea-water introduced by Herbst.
The isolated 71; blastomere in Ea/ziuus becomes rounded and segments as a half, as though the other blastomere were still present.
It forms four mesomeres, two macromeres, and two micromeres,
IV. 1 INI'l‘IAL STRUCTURE OF THE GERM 191
 
and by division of these a hemispherical curved p ate, a halfblastula (Fig. 98). When, as may happen, the other astomere dies
without being separated, it is embraced by the open side of the
survivor. Only after the 32-celled stage is passed do the edges
meet and close. A blastula of half-size is the result, which
becomes later a gastrula an(l'a Pluteus of perfectly normal form
but of half the normal size.
 
©®@@
 
C :1
 
FIG. 99.——Segmentation of an isolated } blastomere of Eolmms; :1, -1-,
(2), Ag, c,h—145i o9I§e)m1cro1nere, one macromere, two mesomeres, d, 391;. (After
l‘leSC , 8 .
 
7% blastomeres behaved in the same way, segmenting as parts,
developingultimately into whole normal Plutei. Similarly -,1 blastomeres segmented partially (Fig. 99), with, in some cases, irregularities : two micromeres, for instance, were formed in the second
division. The % blastulae will gastrulate, but progress no further.
 
 
A B
 
FIG. 100.—-A, Gastrula with meaenchymc cells and trimdiate spicules
reared from a. 2; animal cell of Sphaerechinus. B, The same from a. 5- cell
of Echinus: the gut is tripartite. (After Driesch, 1900.)
 
In the next stage—-% blastomeres—a diiference becomes noticeable between the behaviour of the animal and vegetative, cells.
Both kinds.of cells will gastrulate, secrete triradiate spicules and
192 IN’I‘ERNAL FACTORS IV. 1
 
develop a normally tripartite gut (Fig. 100). But this only occurs
in a small proportion of cases, smaller for the animal than for
the vegetative cells. Further, the former frequently give rise to
blastulae provided with a row of long cilia (Fig. 101), while the
latter are delicate, and many die.
 
Boveri has pointed out that the ability of animal cells to
gastrulate depends, in Stron_r/ylocemfrotus, on whether or not they
contain a portion of the pigment-zone, and this on the position of
the third furrow. Garbowski, however, denies that the pigment
is itself an organ-forming
substance. For in the first
place there is a variety of
S51-ongyloceaztrotus livitlus,
in which the pigment
remains diffuse and never
becomes concentrated to
form a band at all ; and
secondly, even though,
when present, it is usually
in the subequatorial position described by Boveri,
this is not always so; it
may be oblique to the
egg-axis, or wholly in the
 
Era. l01.—Long-eiliatedblastula from animal» 01‘ Wh°11.Y in the
a _} animal cell of Echimcs. (After vegetative hemisphere,
D"es°h’1900‘) It would appear, therefore, either that it is merely associated with some other substance which has up to the present remained invisible, or that
in the cases described by Garbowski animal cells, if they were
pigmented, would gastrulate as readily as ordinary vegetative
blastomeres, in which case the manner in which the pre-determined material is cut up in segmentation would be a matter of
indilference.
 
The differences in the behaviour of the T15 cells are still more
marked. The mieromeres will only divide to form a heap of
about ten cells ; the mesomeres give rise to either long-eiliated
blastulae or imperfect gastrulae, with or without skeleton and
1V.1 INITIAL STRUCTURE OF THE GERM 193
 
mesenchyme, and with an undivided archenteron (Fig. 102) ; the
macromeres, on the other hand, become gastrulae provided with
mesenchyme and spicules (Fig. 103).
 
Lastly, even 51-; blastomeres will occasionally gastrulate, pro~
babl y only if derived from a macromere. This gastrulation was
 
Fm. 102.—Echinus. Larvae reared from mesomeres (animal cells) of
the 16-celled stage. a and b, mesenchyme gastrulae, a has spicules;
r, gastrula without mesenehyme; d, a long-ciliated blastula. (After
 
Driesch, 1900.)
 
not, however, observed directly, but the germinal value of the
smallest gastrulae found was calculated by a. method which will
be described below. 517; cells will not reach the gastrula stage.
 
9 O (I. b
0
 
FIG. 103,- Two gastrulae of Erhinus reared from 3;, macromeres; both
have mesenchyme cells, one has triradiate spicules. (After Driesch, 1900.)
 
 
The difierences between the developmental capacities of animal
and vegetative cells may be studied in another way. The four
micromeres were removed by Driesch in the 16-celled stage;
the remaining twelve meso— and macromeres produced a normal
Pluteus (an experiment also performed by Zoja on Stro7zy_I/Zocenlrotvue). The eight (or four in the previous stage) animal cells
alone, the eight vegetative cells alone also formed, in some cases,
a perfect larva. Segmentation was in all these cases partial.
 
JENKINSON 0
19_4« INTERNAL FACTORS IV. I
 
From the fact that the four animal cells together may form a
Pluteus while each individually fails to pass beyond the gastrula
stage, Driesch has argued that this failure depends solely on lack
of sufficient material, not on the want of any specific gut-forming
substance, and has supported his contention by other evidence.
By placing the eggs of lL'c/firms in dilute sea-water the third
division was made unequal ,- the cells were then isolated in ealeium—
free water. The large cells thus formed must contain either
more of Boveri’s pigment-ring (or rather of the substance of the
vegetative hemisphere) or else more of the animal hemisphere,
probably the former, since a larger percentage gastrulate than is
usual with animal blastomeres. According to Boveri, however,
all ought to gastrulate if all contain the specific substance for the
archenteron. Further, these large cells are able to form mesenchyme even when the gut is lacking although they possess the
middle region of the egg in any case and may lack the micromere
area.
 
Driesch also points out that a 7} blastomere and a § vegetative
blastomere l1ave both the same amount of Boveri’s pigment-ring
and mesenchyme area, the latter, however, only half as big a gut
and half as many mesenchyme cells as the former, and lastly that
»,‘;— and :—l% animal blastomeres will not only'gast1-ulate but form
mesenchyme as well.
 
The limitation of the potentialities of ectoderm and cndoderm in
later stages has been investigated also by Driesch—-by observing
the development of pieces or fragments of the blastulac and
gastrulae. These pieces are obtained by cutting or shaking.
Loeb has employed dilute sea-water to make the blastula swell,
protrude through its membrane and become constricted into two.
 
In Ea/dmes, S/1/merec/u'mw, and /18/erius, any piece of a blastula
when first cut out is crumpled, but soon becomes rounded, and
swims about and eventually gastrulates.
 
Monsters exhibiting a certain degree of duplicity have been
produced by Driesch by shaking the egg of Arte/'ia..e in the
two-celled stage (this produces apparently a partial separation
of the blastomeres) and by placing the blastulac of 152'//imzs in
diluted sea-water. In the latter case the gut is single, the
skeleton double; in the former the gut is doubled, though
the two may subsequently fuse, or even trebled, or may be
IV. I INITIAL STRUCTURE 01*‘ THE GERM 195
 
merely forked. The two guts may be similarly oriented or
turned in opposite directions, and it is interesting to observe
that Driesch, who believes the first furrow to coincide with the
plane separating the two guts, accounts for the latter case by
supposing that one blastomere had been rotated upon the other,
so that their vegetative ends—the locus of the gut-forming
substance-—faeed in opposite directions.
 
Neither the ectoderm of the early gastrula alone nor the
endoderm is capable of giving rise to a larva, though the former
can develope a stomodaeum, a statement confirmed by Morgan.
 
The vegetative half of a gastrula of AS);/zaerec/ziizzm will give
rise to a normal small Pluteus whether the cetoderm only, or
together with it the tip of the archenteron, has been removed
(Fig. 104) ; the animal half will not. VVhen the spicules, one or
 
Wfiiel/‘
 
FIG. 104. —»The potentialities of the cells of embryonic organs; (1, I).
The vegetative and aniinal portions of a gastrula of S1)/I(IeI'€(‘lIiIl1Is
grauulm-is, cut equatori-ally in two; 0. Plut-eus reared from a fragment
of a gastrula; (1. Normal tripartite pluteus gut. (After Dricsoh, 1896.)
 
J
 
both, are removed the skeleton is one-sided and the Pluteus eonsequently one-armed.
 
In As/elrias the arehenteron can form a new terminal vesicle
and mesenehyme cells afresh when these have been removed
(Fig. 105); but should the eoelom sacs be cut off ata later stage,
the (secondary) archenteron can form no more, though it divides
into the usual three portions. T
 
Parallel to the behaviour of isolated blastomeres is the segmentation‘ and development of egg-fragments, obtained by shaking,
cutting, or (Loeb) dilute sea—water. The fragments may be taken
from already fertilized eggs, or from unfertilized: in the latter
case each, whether nucleate or enueleate, must be subsequently
fecundated.
 
In segmentation there are a great many ditfereiices, which
appear to depend on the nature of the fragment, that is to say,
196
on the part of the egg from which it
 
INTERNAL FACTORS
 
IV. I
In
 
has been removed.
 
the case of the colourless egg of ]$’¢-/zz'm1.9 it is only possible to
 
FIG. 105.~-The potentialities of the cells
of embryonic organs; a. N0l'1]1i1-l Pluteus of
S'ph(wre(.'hiIms; b. Pluteus reared from a fragment of a gustrula; 0, 9. Normal Bipinnaria
of A8tP1‘iflS _qlaciali.s-; «I, f. Bipinn-aria from
the vegetative half of a gastrula; g. Larva.
of Asterias with typical tripartite gut, but
no coelom, from the vegetative half of a
gastrula. removed after development of the
eoeloni sacs. (After Dricscli, 1896.)
 
In another case wha.t appears to
 
guess at the nature of
the piece by observing
the mode of segmentation.
 
The first and second
divisions may both be
equal, as also the third
and fourth ; in this case
(1)1-ieseli supposes) a
large fragment is involved. Again, the first,
second, and third may
be normal, but the fourth
and fifth equal: no
micromeres, therefore,
are formed, and the
fragment is probably
derived from the animal
hemisphere. Or while
the first and second are
equal, one of the four
cells will divide unequally; in the next
division a large or small
number of micromeres
is formed, according,
presumably, as a larger
or smaller portion of the
mieromere region has
been included in the
fragment (Fig. 106i.
 
be a. meridional half
 
divides equally twice, and then two of the four equally (mesomeres), two unequally (macro- and micromeres) (Fig. 107), while
a supposed vegetative half is segmented to form four large and
 
four small cells.
 
(
 
Thus the type of segmentation is determined by the initial
IV. I INITIAL S'l‘RUC'l‘URl*] OF THE GER,1\[ 197
 
structure, including the physical constitution, of the ovum alone,
a conclusion in which Boveri concurs.
 
All these fragments will give rise to normal larvae, provided
they are not too small. The least egg-fragment that will gastritlate has the same germinal value as the least blastomcre, namely,
 
(1
 
 
FIG. 106.-~ Segmentation of an egg-lragxiient of E¢-Iu'nu.s- supposed to
contain the whole of the mieromere area of the egg, and some of the
animal hemisphere. u. Eight cells, three equal pairs and one unequal
pair; 1;. sixteen cells, four of the eight having formed mieromercs, the
other four divided equally. (After Driesch, 1896.)
 
31-2- (calculated from the volume of the smallest gastrnlae found).
Morgan, however, estimates the smallest egg-fiaginent capable of
giving rise to a normal larva at from 315 to -616, the least blastenicre that will gastrnlate only
 
G
 
 
FIG. 107. — Segmentation of an e;,rg-fmgnient (Ez-Irinats) supposed to
be an exact lneritlional half". (I. Eight cells, four l)e1ng im-s0Incre.~',
two n'1-.Lcro1neres and two microineres; b. sixteen cells. (After Dneseh,
1896J
 
In partial blastulae, gastrulae, or larvae the number of cells is
proportional to the germinal value ; and this is true not merely
of the whole embryo or larva, but of each of its organs, the
198 INTERNAL FACTORS IV. 1
 
eetoderm, the mescnchyme, the archenteron. This assertion Of
Driesch’s is corroborated by Morgan for the whole larvae, but not
for the organs ; according to him
the number ofeells invaginated to
form the archenteron in a partial
tends to approach that found in
 
a whole larva, about fifty.
l)riesch has also determined
the relation to their germinal
value of the dimensions of partial
blastulae, gastrulae, and Plutei
and their organs (Figs. 108
FIG. 108.— Outlines of -}, 22, 1, 112). As the table (Table XX)
 
« l bl‘th fS7 ""7' . . . .,
'(‘X;.teilf Dr?:‘S;‘11"°19"00_)” W‘ “ ‘””'S shows, the suifaees ale vex)
 
nearly as the germinal values,
the ratios of the radii of the partial a11d total larvae consequently greater, of the volumes less than these values.
 
Flu. 1O9.——Ea1'ly gastrulae of Ec-hinu.~c: a. }, b. g, c. 1, (Z. 3,.
(After Driesch, 1900.)
IV. 1 INITIAL STRUCTURE OF THE GERM 199
 
TABLE XX
 
Relation of dimensions of partial blastulae to their germinal value.
Germinal values.
 
1 1
 
1 . 1. 2+
Rzulii . . . 2 1-2 I .75
Surfaces . 4 2-2-5 1 -5625
Volumes . . 8 3375 1 -4
 
FIG. 1lU.—l’lutei of I9'rlu'nM.s-: u.  b. .1_,, 4'. 1. The skeleton is
shown. (After Drie.~xch, 1900.)
 
From the proportionality of the number of cells to the germinal
value it follows that the cells of the whole and of the partial
blastulae are of the same size.
 
If 1:’, S, and V are the radius, surl"aee, and volume of the whole
 
. . . 1 '
 
egg, and rm 3", nu the correspon(l1ng- magnitudes Ill a ; larva,
S 1:? I’
 
then 8n = ——,whenee 1'” = _, and therefore 0” = - ~—_.—. By
" 1; ~/ 713
 
this formula. the germinal value of a larva of unknown origin
may be calculated.
Finally, although segmentation takes place at the normal rate
200 INTERNAL FACTORS IV. I
 
in the isolated blastomeres, their subsequent development is progressively retarded as the germinal value decreases (Table XXI).
 
TABLE XXI
 
Showing the rate of development in partial larvae.
Germinal value.
 
   
 
5 Day ! } : 7’; 7‘: L 5
A 1 ' Gastrula IEarly Gastrulal Still earlier Blastula
f Gastrula
' 2 Pluteus Early Pluteus Gastrula with Blastula and
4 Skeleton Gastrula
3 Pluteus Pluteus Early Pluteus Gastrula
4 Pluteus Pluteus Pluteus Gastrnln
 
a 
C
FIG. 111,-SpIum'¢=chi/Hts Plutci, showing the gut. u -}, b. .1_., c. }.
(After Driesch, 1900.)
It is evident from these experiments that, in the Echinoderms,
while the isolated parts of the egg‘, whether blastomeres or egg
fragments, are only able to segment as they would have done
had they remained in connexion with the whole, they are neverIV. I INITIAL STRUCTURE 014‘ THE GERM 201
 
theless capable of total development in a very marked degree. This
capacity is not, however, unlimited, but diminishes as development proceeds, and it appears highly probable, and is indeed
often admitted by Driesch, that the chief cause of this restriction
is the lack of some specific substance and not the deficiency of
mere material, though this may be a subsidiary factor. The
progressive loss of potentialities is exhibited also by the embryonic
organs as they are formed later on.
 
(L
 
FIG. 112. --Echinu.s~, the tripartite gut of the complete l'luleus:
(1. }, b.  c. in (After Drieseh, 1900.)
 
Before concluding this section we must notice an experiment
which is the converse of those considered above, namely, the
production of one embryo from two eggs.
 
An unsuccessful attempt was made by Morgan to obtain this
result. This author only succeeded in showing that the eggs of
AS’/1//ac/‘cc/rim/.9, when (leprived of their membranes by being
shaken, tend to fuse together, and that from such pairs double
monsters may be produced with two guts and two skeletons. One
skeleton may, however, very large] y predominate over the other,
which indeed remains rudimentary, and the guts may coalesce.
202 INTERNAL FACTORS IV. 1
 
Driesch, however, who has employed the same method as
Morgan and has corroborated his results, has been able to
produce, from the fusion of two ova, a larva single in all its
parts—mesenehyme, skeleton, and gut. The organs are in
correct proportion but larger than the normal (Fig. 113). The
number of mesenehyme cells is about twice the normal.
 
From this experiment it may of course be concluded that the
egg substance possesses a degree of isotropy, but not that that isotropy is absolute. Nothing, it should be remarked, is known of
the way in \vhich the eggs which fuse to form one embryo are
oriented upon one another. Garbowski, however, has stated
 
A fl
 
Fla. 1113. A. Norlnal gastrnla of S_pIz(m'r-('h1'nMs._ B. Single gastrula
formed by the fusion of two blastulae. (After Drieseh, from Korsehelt
and Heider.)
 
that fragments of different eggs of ]3'c//iizzts (in segmentation
stages) may be grafted on one another, and that the product
of their union will give rise to’ a normal embryo, whatever the
relative positions of the fragments.
 
The fusion of blastulae to form giant Planulae has been
noticed by Metsehnikoff in the medusa, Jli/roco///a mmae, and in
0])//r_//at/'oc/Ia Korsehelt has observed a similar fusion of (llstinet
eggs in the body cavity of the parent when the latter is hermaphrodite. Lastly, Sala and Zur Strassen have observed the
fusion in twos, threes, or more of the eggs of ./1.vc¢m'.s-, a result
experimentally produced by the first author by exposure to a low
temperature, while Zur Strassen has found that such double
eggs, provided they had united before fertilization and then been
fertilized by asingle sperm, will produce perfectly normal embryos.
IV. I INI'I‘IAL STRUCTURE OF THE GERM 203
 
LITERATURE
 
T. BOVERI. Die l’o1m-itilt von Ovocytc, Ei und Larve des SIron_r/,z/Iocentrofus Iiritlus, Zool. Jahrb. (Anat), xiv, 1901.
 
T. BOVERI. Ueber die PoIa.ritii.t des Seeigelcies, Vwh. d. ph_:/s.-nwd.
Gas. cu Wiir:Inu'_q, xxxiv, 1902.
 
H. DRIESCH. Entwicklungsmechzmische Studien : I, II, Zcilschr. uv's.~:.
Zool. Iiii, 1892.
 
II. DRIESCH. Entwicklungsn1ec1mniscl1e Studieu: II|—VI, Zei!s¢'7u‘.,
1z'i.s-s. Zoo]. Iv, 1893.
 
H. Dlulascll. Entwicklungsmechanische Studien: VII-X, Mir/. Zaol.
Slut. Ncapel, xi, 1895.
 
H. Dnusscn. Zur Analysis der Potenzen embryona.lcr Orgamzcllen,
Arch. Eul. Merl). ii, 1896.
 
H. DRIESCII. Betmchtuxigen fiber die 01'gunisution des Eies und ihrc
Genese, An-h. ]v.'m. Mo:-7:. iv, 1897.
 
H. DRIESCH. Primiire und sckundi'u'e Regulationen in den‘ Entwicklung dcr Echinodermen, Arrh. Eur. Mech. iv, 1897.
 
H. DRIESCH. Die Versclnnelzung der lndivid112L]it€‘Lt bei Ec11inidenkeimen. Arch. EM. Merh. x, 1900.
 
H. DRIESCII. Die isolirten Blastolneren des Echinidenkcimes, Arch.
EM. Mech. x, 1900.
 
II. DRIESCH. Neue Ergihizungen zur Entwicklungs]physiologic dos
Eel)inidenkeimes, Arch. Em. Illech. xiv. 1902.
 
H. DRIESCII. DreiApho1'ismen 7.111‘ Entwicklungsphysiologie jiingstcr
Stndiell, Arch. 12'/1!. Jllcch. xvii, 1904.
 
II. DRIESCH. Altes und Neues zur Entwicklnngsplnysiologic dcs
 
jungun Astcridenkeiines, Ar:-h. Enl. Mc-ch. xx, 1906.
 
M. '1‘. GARBOWSKI. Ueber B1-ustomerentru.nspIa.ntationen bei Sceigeln, Bull. Iufmwal. do l'Arml. ties Sci. dc (,'r(u'orie, 1904.
 
M. T. (irARB()\‘\'Sl\' I. Ueber die l’oIzu'itiit des Scoigcleies, Hull. Internal.
((0 IA('mI. (105 Sci. dc ('rm'uvi¢', 1905.
 
M. '1‘. GARBOWSKI. Ueber Bla.sto1nerent1'ansplamtationen bci Sceigeln,
Bull. Internal. Ac. Sv. Crucoric, 1905.
 
1*}. KORSCHELT. Uebcr Kerntllcilung, I‘}ircifung und Bci'1'ucI1i.ung bci
01)/H'_IjU(I'0(']I(I1)Il(’I'i/(ls, Zeitschr. 'mf.s's. Zoo]. lx, 1895.‘
 
J. LOEB. Ueber cine eiufaclne Methode, zwei odci-1110111‘ zusu.mn1engewaclisene Fmbryoncn nus oinem Ei hervorzubringen, P_/lii_qer’s Arch.
Iv, 1894.
 
J. LOEB. Beitriige zur Entwicklungsmechanik der nus einem Ei
entstchenden Doppelbildungen, Arch. Euf. Mech. i, 1895.
 
E. ME’l‘SCIINIKOFl~‘. Embryologisclie Studien an Medusen, Wien, 1886.
 
T. H. MORGAN. EXp€l‘I1HC1IId.I. studies on Echinodcrm eggs. Anat.
Ans. ix, 1894.
 
T. H. MOREAN. Studies of the ‘ pa.rbial ' larvae of Spham'ech1'Ims, Arch.
Ent. Mech. ii, 1896.
204 INTERNAL FACTORS IV. I
 
T. H. MORGAN. A study of a. variation in cleavage, Arch. Em. Mach.
 
ii, 1896.
'1‘. H. M()RGAN. The formation of one embryo from two blastulae,
 
Arch. Euf. Mcch. ii, 1896.
L. SALA. Experimentello Untersuchungen iiber die Reifung und
Befruchtung der Eier bei Asc(u'i.~: mcgalocqrhalu, Arch. mi/cr. Anal.
 
xliv, 1895.
0. L. ZUR STRASSEN. Ueber die Riesenbildung bei Ascm-is-Eiern,
 
Jrch. Ent. Mech. vii, 1898.
H. l. ZIEGLER. Ueber l*‘urc-hung unter Pressung, Verh. Anal. (:'escll.,
 
1894.
R. ZOJA. Sullo sviluppo dei blastomeri isolati dalle uova. di alcunc
 
medusc (e di altri organisini), Arch. Eur. Mach. ii, 1896.
 
§ 7. Nimnirrinm.
 
In the behaviour of its isolated blastomeres the Nemertine
egg resembles the Echinoderm very closely; the cells segment
as parts but ultimately give rise to wholes. The limit of this
capacity is, however, sooner reached, for there is a sharper
distinction between the potentialities of animal and vegetative
blastomeres.
 
Experiments have been made on (Jcrcb/'a/zzlzw («alone by VVilson,
on Ccrcbraiulus nzaI'_I/‘iizafius by Zeleny ; Zeleuy and Yatsu have
further investigated the development of fragments removed from
various regions of the egg.
 
The axis of the egg is marked by the excentricity of the nucleus
and the point of extrusion of the polar bodies. 1\Iatura.tion
precedes fertilization. The first two furrows are meridional
and divide the egg into four equal cells- (A, 1!, C’, and 1)).
Subsequent cleavage is, however, on the spiral plan seen in Turbollarian, Annelidan, and holoblastic Molluscan eggs.
 
In the Nemertine the egg-axis becomes the axis of the Pilidium
larva, the animal being the aboral, the vegetative the oral, pole.
 
In both the § and the % blastomeres cleavage is partial and an
open blastula is formed, or sometimes a flat plate of cells merely,
sometimes a closed sphere. The 1}; later becomes a larva with an
apical organ, mescnehyme and archenteron, but no lappets; the %
larva has a solid archenteron but neither apical organ.nor lappets.
Development is slightly retarded 3 % blastomeres give rise to a
IV.1 INITIAL STRUCTURE OF THE GERM 205
 
larva. with mesenchyme and apical organ, but the archenteron is
solid. After the next phase—eight cells—-the upper quartette
 
 
FIG. 114.———Ce;-cbrafulus. Larvae from upper quartettes of the 8-cell
stage. A. Larva from whole quartette. B, C. The same, but one cell
was injured. J), E, F. Slightly older, f'rom complete quartettc. (After
Zeleny, 1904.)
 
(1 11-1 rl) produces a larva with an apical organ and meseuchyme
but devoid of an archenteron ; the larva developed from the lower
 
quartette (,-l—~—Z)) has an archenteron but no apical organ, while a
 
 
/1
FIG. 115.~—('erebralulus. Larvae from lower (vegetative) quartet-tes
of 8-cell stage. Note absence of apical organ, presence of large, hollow
or solid, archenteron. (After Zeleny, 1904.)
meridional half, comprising two macromcres and two micromeres,
develops both organs, though it is usually asymmetrical.
In the 16-celled stage the isolated apical cells (1 a 1-1 (I 1) will
 
give rise to a. ciliated blastula with mesenchyme and an apical
206 INTERNAL FACTOllS IV. I
 
organ, b11t without an archcnteron, while from the remaining
twelve is derived a ciliated embryo with a large solid arehenteron,
but destitute of an apical organ and lappets.
 
Wilson asserts that the size of cells in partial is the same as
in total larvae ; their number appears to be proportional to their
germinal value.
 
From fragments of blastulae normal, or nearly normal, dwarf
Pilidia may arise, but the degree of development again depends
on the origin of the fragment. VVhile the lower third becomes
 
 
FIG. ll6.——Cerebratulus. Larvae from portions of the 8-cell stage.
A, II. Larva from lateral 4-cell group. Note presence of both apical
organ and archenteron. C. Similar, but younger. I). Larva from upper
quartette plus two cells of lower quartette. Note that there are three
apical organs, a large bl-astocoel, a small archenterou, and two inxluggiiiations of eetoclerm at the sides of the archent-eron. (After Zeleny.
 
04.)
 
a eiliated embryo without an apical organ, a piece of the upper
two-thirds developed (in one case) into a larva with a gut, with
two apical organs, but no lappets (Zeleny). Wilson states that
in Cerebrazfulus lacteus the archenteron is small in animal fragments, the apieal organ frequently, though not apparently always,
absent in pieces of the vegetative hemisphere.
 
Animal fragments of the blastula have a greater total potentiality, a greater regulative capacity, than the animahblastom eres
IV. '1 INITIAL STRUCTURE OF THE GERM 207
 
of the eight-celled stage. This is attributed by Zeleny to the
greater time and opportunity afforded them of regaining the
polarity of the whole.
 
The behaviour of egg—fragments depends not merely on the
part of the egg from which they are taken but also on the time
at which they are removed. '
 
Pieces taken from the ovum, no matter in what way, before
the disappearance of the germinal vesicle segment as wholes and
give rise to perfect Pilidia. Only in a small percentage (15 %) of
cases is their development defective, the lappcts being absent, or
the gut abnormal, or the apical org-an absent or multiplied (Yatsu).
The proportion of perfect Pilidia obtained from fragments
removed a little later, at the time of
formation of the first polar spindle, is,
 
V
however, less, only 52  0
It is during the conjugation of the H
pronuclei that the definite localization — 4 H
of specific substances seems to begin, V '
 
for now the fate of the fragment de
. . . . 0
pends upon the direction in Wl)l(‘l1 the
cut is made. Eggs from which the animal
 
. . FIG. 117.-— Cerebmtulus.
portion——about one-third of the whole Diammn of egg with one
 
——has been taken away become perfect POW‘ b0d)'. and basal PW
- - ' t b .( I: ‘I d.‘ '
Pilidia ; when, however, by removal of t§)neS1(":)(3f-c’],(i:,.ii;(;3;i:,1 Jiffy,
 
the vegetative third, or by an oblique vertical (V), aiul oblique
cut, the fragiiient does not contain (1%4_°)“ts' (After Zelcny’
the whole of the lower two-thirds, the
 
lappets are defective and the apical organ sometimes absent as
well. If, therefore, there is present at this stage a special
material for this organ it cannot as yet be located in its ultimate
position.
 
The cleavage of these fragments is always partial (Zeleny).
After the first furrow the material for the apical organ has still
not reached the animal pole : the animal region may be removed
from both blastomcres without inhibiting its formation. bcrfeet
Pilidia may also be produced from eggs in the two-celled stage
when pa.rt of one blastoinere has been obliquely cut away.
 
Should, however, the blastomeres be cut apart before their complete
 
V
208 INTERNAL FACTORS IV. 1
 
separation, each develops into a Pilidium with one lappet but no
apical organ; it seems, therefore, that the material for this structure is at this stage placed in the bridge of plasma connecting
the two cells, and is fatally injured by the operation.
 
Taken together these experiments make it, to say the least,
highly probable that there are in the Nemertine egg definite
substances connected necessarily, that is causally, with the formation of certain organs.
 
These specific organ-forming substances may be said to be
 
preformed, but it is equally clear that they are not prelocalized,
but only reach their ultimate destination in the course of development.
 
LI'l‘ERATURl‘}
 
E. B. WILSON. Experiments on cleavage and localization in the
 
Nemertine-egg, Arch. Ent. Mech. xvi, 1903.
N. YATSU. Experiments on the development of egg-fragments in
 
Cerebratulus, Biol. Bull. vi, 1904.
C. ZELENY. Experiments on the localization of developmental factors
 
in the Nemertine egg, Joan-n. E.l'p. Zool. i, 1904.
 
§ 8. CTE.\'OPIIOR.\.
 
VVith the Ctenophora we come to a group of animals in which
the development of a total larva from a single blastomere is no
longer possible, though the missing parts may eventually be
regenerated.
 
The egg consists of a central yoll<—mass surrounded by a superficial layer of granular protoplasm. After fertilization this
protoplasm passes wholly to one side—the animal hemisphere-—and
the egg divides meridionally into equal parts. The protoplasm
then spreads itself over the outer surface of each cell, but returns
to the animal side prior to the second cleavage, which is again
meridional and at right angles to the first. Once more the
protoplasm is distributed over the whole outer surface of each
hlastomere, only to be again concentrated before the third division,
an unequal one, but nearly meridional. There are new four large
cells lying together in a square and four smaller cells lying above
them in two groups of two each, one at each end of-the plane of
IV. I INITIAL STRUCTURE OF THE GERM 209
 
the second furrow. This becomes the transverse, while the first
furrow marks the sagittal plane of the future organism. Each
of these cells contains both granular protoplasm and yolk, but at
the succeeding division nearly all of the former material passes
into the eight small cells, which are nipped off and lie in an oval
ring on the vegetative side of the egg 1 (Fig. 118). These small
cells or micromeres will give rise to the ectoderm, the large cells
or macromcres to the endoderm and mesoderm, the mesoderm
 
 
 
\
 
   
 
 
 
FIG. 11H. -Normal segmentation ofthe Ulenophorc egg. :4 u, Stomach
or sagittal plane ; I» I), funnel or transverse plane. C, E, from mieromere
pole; D, F, from the side. (After Ziegler, from Korschelt and I-leider.)
 
being separated off later on at the vegetative pole, which becomes
 
the oral pole of the embryo. In normal development, therefore,
 
the embryonic axes are marked out at quite an early stage.
Chun was the first to isolate the % and § blastomeres of
 
‘ Ziegler's description is that in this (the fourth) division the furrow
first appears near the animal pole, but yolk then streams from the
larger vegetative into the smaller animal portion, until the latter becomes
the macromere, the former the inicromere. Division is then completed
and the micronieres are at the vegetative pole. This explains the
divergences in the accounts of earlier observers (Agassiz, Kowalewsky,
Chun, F01, and Metschnikoff).
210 INTERNAL FACTORS 1v. 1
 
Euc/(aria by shaking. These gave rise to half larvae, provided
with four costae, four meridional canals communicating with two
subsagittal and two subtransverse canals in the ordinary way,
one subgastric canal, one tentacle, but a whole funnel and a whole
stomach, the latter formed by an oblique invagination ; the side
turned towards the missing blastomere was covered over by ectoderm.
 
Later on the missing half was regenerated, first the subgastrie
vessel, then the meridional canals, over these the costae, and
 
finally a tentacle.
 
 
 
FIG. 119.~Development of isolated bl-astomeres in Ctenophora. a, /2.
Segmentation of 1% blastomere of Befiio ovatu: a, two large, two small
cells; b, each gives off a mieromere; c Larva from a .1, blastomere with
four costae; (I, c. Segmentation of 1 blastomere ; f. The resulting larva
with two costae; g. Larva. with six costae from ~2~ blastomercs. (After
Driesch and Morg.m, 1896.) h. Isolated »1"‘,; blastomeres of liwfie omlu;
 
j. The resulting larva with four costae, one sense--oi'gai1, and one
 
stomodaeum ; 2'. Isolated -1'3, blastomeres; A-. The resulting larva with three
costae, one sense-organ, and one stomodacum. (After Fischcl, 1898.)
 
Such half larvae are found in the tow-net after a storm and
may become (l}olz'7za) sexually mature.
 
These experiments have been repeated and confirmed on Barbi‘
by Driesch and Morgan. The authors add that the segmentation of
these isolated blastomeres is partial. The 71; larva has two costae
only and a large and a small canal, the former passing to the
costae, the latter to the opposite side and representing apparently
IV. 1 INITIAL S'l‘RUC'l‘URE OF THE GERM 211
 
the other half of the funnel (Fig. 119 mg). Fischel has carried the
analysis of the potentialities of the blastomeres a step further
(Fig. 119 /z—zt). Though cleavage is partial, as described by
Drieseh and Morgan, there are slight irreg~ulari.ties in the position
of the blastomeres. A ,1; blastomere produces one eosta only,
-1-55 blastomeres (four macro- and four mieromeres) four eostae, {-3(five macro- and five mieromeres) five, -19%, -1%, and 14;, when
separated in the same way, respectively three, six, and two eostae.
Similar results were obtained by meridional division of the egg
in later stages. Fisehel confirms the statements of the other
 
 
_FiG. 120.——u. Beriie oz-am: egg in the 16-cell stage. The mieromeres
disarmugecl by pressure ; I». The resulting larva with two sense-organs,
and four eostae radiating from each : one stoinodacuni. (After Fischel,
1898'.) 1'. Normal segmentation of the egg of Beriie from \Vl1lCll a small
portion of the vegetative hemisphere has been removed; d. Larva produced from the same, with eight eostae, four endoderinal canals, and
one (central) stomodaeum. (After Driesch and Morgan, 1896.)
investigators as to the structure of the half larvae. In % larvae,
however, not three, but four, canals are formed, and the stomodaeum is vertical. The sense-organ is not found in embryos
developed from small fragments. Removal of the Inacromeres
left the number of eostae unaffected, but displacement of the
mieromeres by pressure led to the formation of an abnormal larva
with two sense-organs and eight eostae arranged in two groups
of four each round each sense-organ (Fig. 120 a, 6), or, if the
mieromeres were separated into two lets, a group of four round one
organ, a group of three round the other, and some scattered combs.
With greater pressure three sense-organs were produced and the
 
P 2
212 INTERNAL FACTORS IV. 1
 
costae were very irregular. In these eggs there was, however,
only one stomodaeum, and only the normal number of canals.
 
It appears, therefore, that the individual eostae are definitely
related to the meridional divisions of the segmented egg, and
further that the material which is concerned in their formation
becomes located in the micromeres of the 16-celled stage.
 
Driesch and Morgan have also shown that if the vegetative
hemisphere be taken away from the undivided egg, the animal
portion, though it segments normally, is able to produce neither
costae nor stomodaeum; it has, however, cndodermal canals. Loss
of a small portion of the vegetative hemisphere, however, does
not interfere with subsequent normal development (Fig. 120 0, II).
 
On the other hand an egg which has been deprived of a lateral
portion divides, by three unequal divisions, into eight cells, all of
which form mieromeres. Nevertheless, the larva has only four,
five, or six costae, and some of these rudimentary, only three or
four canals, and the stomodaeum, as in half larvae, is oblique.
 
As Driesch points out, the whole of the nucleus is here, and the
embryo is still defective. Difierentiation thus depends on the
cytoplasm, but is quite independent of segmentation, which may
be perfectly normal without determining the formation of a
normal embryo. l)rieseh further argues that the observed defects
are due to lack of suflicient mater-ial.merely, not to lack of any
preformed specific substance. \Vhether such substances are
already present in the unsegmented as they undoubtedly appear
to be in the segmented egg, it is perhaps impossible, on the
evidence, to decide, but it may be observed that the development of perfect larvae from those eggs from which a portion of
the vegetative hemisphere has been detached does not ncccssaril y
support the View advanced by Drieseh. '
 
LI'J.‘ERA'l‘URE
 
C. CHUN. Die Dissogonie, eine neue Form der geschlechtliehen Zeugung:
7. Zur Entwicklungsuieclianik der Ctenophoren, I"('.s'l.s'c7u'. L<'m'7.'m'!, 1892.
 
II. Dmascu and T. H. MORGAN. Zur Analysis der ersten l*lntwicklungsstadien des Ctenophoreneies, Arch. Eur. Illech. ii, 1896.
 
H. FISCHEL. Experiinentelle Untersuchungen am Ctenophorenci, 1,
.‘.lI'fh. Enl. Zlfech. vi, 1898; II, 111, IV, ibid. vii, 1398.
 
II. E. ZIEGLER. Experimentelle Studien fiber die Zelltheilung: Die
Furehungszellcn von Bertie ovata, Arch. Ent. Mech. vii, 189:4»
IV. I INITIAL STRUCTURE OF THE GERM 213
 
§ 9. Crmnrorom AND MoLLUscA.
 
In the Chaetopoda and Mollusca (except the Cephalopods) it is
possible to trace back almost every organ of the body to some particular cell, or group of cells, of the segmenting ovum, to describe,
in fact, its lineage in terms of individual cells. Specific materials
for the formation of the vaiious parts seem to be sundered from
one another by the process of cleavage, which thus presents the
characters of a ‘Mosaic’ work in an exceptional degree. 'l‘o
what extent such a preformed structure (lees in reality exist
experiment alone can decide, and the answer given by experiment,
so far, at least, as the nucleus is concerned, is in the negative.
 
By means of pressure Wilson succeeded in preventing in Nerei.s~
the normal formation of the first quartette of micromeres ; instead,
the egg divided into a flat plate of eight equal cells. The pressure
was then removed. All eight cells formed micromeres, and later
on eight instead of the usual four macromeres were found in the
Troehophore la1'va. Four of these necessarily contained nuclei
which would normally be placed in cells of the prototroch, and
yet the larva was normal. In this case, then, the causes of
differentiation cannot lie in the nucleus; they must be sought
for, if anywhere, in the cytoplasm.
 
In the eggs of these forms segmentation is holoblastic and of
the spiral type, with the cleavages alternately dexiotropic and
laeotropic, and it is easy not only to determine the lineage of
each organ, but to compare the oi-igin and the destiny of individual cells in a large series of forms. The first, second, and third
quartettes of micromeres are usually destined for ectoderm
formation, the second quartette cell in the D quadrant, 2 «I, being
the first somatoblast from which the ectodermal structures of the
trunk a1'e derived. The eiliated ring or prototroch is formed, in
most cases, from certain derivatives of the first quartettc, namely
1 a 2———I (l 5.’, the primary trochoblasts, each of which divides
into a group of four; but it may be reinforced by secondary
trochoblasts from the same or the second quartette. The second
and third quartettes may provide larval nzesoderm or mesenchyme.
The trunk mesoderm is usually derived from 4 (I. The remaining
214 INTERNAL mcwons IV.1
 
cells of this quartette (4 a—4 c) become with the residual macromeres (A—D) the endoderm (or mesendoderm).
 
A peculiar structure seen in some forms (Ilyamwsa, Dentalinm,
 
 
.41
 
FIG. 121.-—No1-nml cleavage of I(_I](m((.98(L A, B. Formation of polarlobe or yolk-lobe (til); (Y. ‘Trefoi1‘ stage; I), E, F. The polar-lobe
passes into CD; 0. The second polar-lobe protruded in H; it passes into
D. (After Cmmpton, from Korschelt and Heidcr.)
 
0/laclopteru.v, M_y:o.v/oirm) is the yolk-lobe, or polar lobe, as it should
properly be called. This is a. mass of cytoplasm p1-oi-ruded at the
IV. I INITIAL STRUCTURE OF THE GERM 215
 
vegetative pole after fertilization. It becomes attached entirely
to one of the first two blastomeres, namely to CD, into which it
is withdrawn; prior to the next division it is again protruded,
attached to D only, and then once more withdrawn : at the time
of the next division it is protruded again, for the last time
(Figs. 121, 122). '
 
Crampton has isolated the blastomeres of I/‘//amzwz and watched
their development (Figs. 123-6). —;-, §-, % (A, B, and C), 1}, §
 
FIG. 122:-Normal cleavage of II_:/mmssa. (After Crampton, 1896.)
a, 8—celled stage from above; I), 1C-celled stage from above; r, 24-celled
 
stage,  from above; :1, optical section, sliglltly oblique, of a later
 
embryo, from below. Division of mesoblast pole-cell (rI‘).
 
(a macromere and a micromere), %, -,1; (a micromere), I53, 11} and
193 all continue to segment as though the missing blastomeres were
present, except for certain small irregularities in the positions of
the cells ; the A, B, and C blastorneres, separately or together, bud
off the usual four generations of mieromeres, but D only the first
three ; the division of the mic-romeres continues in nearly normal
fashion, The 3,. mic-romerye divides twice only. All die, however,
216 INTERNAL FACTORS IV. 1
 
without giving rise to larvae. A 13- macromere, on the other hand,
 
or a. -1-13 or later blastomere, will not segment at all.
By lowering the temperature the cleavage may be made to
 
resemble that of whole ova.
 
 
FIG. ]23.—-Il_:/anassa: c-lenvzige of smaller  blastoniere (AB). a, Undivided; b,  embryo, fr_on1 sule ; (.', »j, embryo, from above; «I, -,'-‘:5 emln-yo,
gram ahdove _the clot; lindficattesil the for1ne1t'1centre of the egg; 9% .1_,'3‘«e1}J1.bryo;)
 
rom si e; , grow 1 o ec 0 erm over ie nmcromeres; 3/, orma ion 0
the nncromeres of the foulth generation, from below; It, ciliated partial
 
embryo of 46 hours. (After Cranipton.)
 
The yolk-lobe influences the character of segmentation, for if
it be removed CD divides like AB. More important ‘than this is
IV. I INITIAL STRUCTURE OF TIIE GERM 217
 
the relation of the yolk-lobe to the mesoderm. When the former
is cut olf the latter is not formed at all. There are four generations
of micromeres and four equal macromeres, and the larvae, though
eiliated and able to swim, never reach the veli_qer stage (Fig. 127).
 
1/
 
 
1/ I
 
FIG. 1'24.~Il;/«nus-.5-«I: ¢~1eavan.ge of CD. (4, Formation of yolk-lobe
(yl) ; b, ‘trel'oil'; v,  embryo from above; (1, early 1‘ e1ubr_yo, from above:
:~'econ(lary yolk-lobe (yl) ; a, resting 3, embryo from above; f, ,‘E_. embryo
from above; (1, passage to 11,- stage,  from above; h,  embryo, from
side ; i, division of n1esobla.st pole-cell (d‘), from below. (AftcrUra.1npt0n.)
 
The potentialities of the blastomeres are thus strictly limited
both in segmentation and difl’e1‘entiation. This result is amply
confirmed by Wilson's researches on I/eutulimzz and Patel/a..
218 IN'l‘l*}RN.\L FACTORS IV. 1
 
In the latter genus the development of the isolated blastomeres,
especially of those resulting from later divisions, has been minutely
followed. Each separate first quartette cell (1 a—1 (1) forms
(Fig. 128) a closed ectoblastic structure with an apical org-an at
 
 
FIG. l25.—IIgmm.~sa: cleavage of ;} blzmstomere (Al. (I, 3, froI_n side:
b, ,“:_;, from side; 0, passage to 143, fiist form, from side; (I, rest1ng_-‘g.
second form, from side; 0, pttssngo to fl, from side ; f, ;_,7,,«, from s1( e.
(After Crauupton.)
 
     
 
“ii,” ‘I
 
Fm. 126.—I/_:/auassa: aw, cleavage of Q; blnstomercs; (1, ,9, stage,
from one 1n1c_romere; 12, ~,*;; stage, from two micromeres; r, —,“,, stage,
from three nucromeres. d—-f, cleavage of  bliLSt01l1(‘1‘CS; cl, -J,-3 einbryo,
from below: P, origin of fourth qunrtefte. from below; f, embryo of
48 hours. (After Urzuupton )
IV. I INITIAL STRUCTURE OF THE GERM 219
 
/‘B co
 
 
FIG. 12"’7.—~]I_1/a12assa: cleavage of the egg after removal of the yolklobe. a, :5; b, 1.-;_ r, first quartette formed; (1, second quxutette formed,
first qu2Lrtgtt§fl1v1de(l ; a—¢l, from above; e. from below: fourth quzu-tette
formed; _/, cxllutod embryo, from side. (After Cr-.unpton.)
 
 
(I // 0
 
FIG. 128.-- Patcllu: development of isolated ;« micromeres. ((, _?;
micromere; I), c, first division; «I, 3"-‘,3 stage, with two trochoblasts
(stippled), one rosette-cell, and one p1-imary cross-cell; e, larva}. of 24 hours,
from the side, showing trocholnlasls below, apical cells above. (Aft:-1'
\Vi1Su11.) I
220 INTERNAL FACTORS IV. I
 
one end, and four cells (derived from a primary trochoblast)
provided with long cilia. at the other. When the primary trochoblasts (1 a 2-1 (Z 2) are isolated each divides into four ciliated
cells, the cilia arranged as normally in a single row, but no further
(Fig. 129 a—e). If the daughter-cells of these troehoblasts (1 a 2. 1
-1 (Z 2. 1 and 1 a 2. 2--1 (Z 2. 2) are separated they divide once
only (Fig. 129/’), while their daughter-cells do not divide at all
(Fig. 129 g, /1). The sister-cells to these (1 «L 1—1 rl 1) behave in
the same way, forming a closed larva with an apical organ at
 
'2
 
(*7 3
0 gr
5; I; 0 ll
 
 
 
FIG. 129. —-Pulellu. Development of isolated primary and secondary
troehoblasts. a, Primary troehoblast; b, 0, first division; «I, second
division ; 0, after 24 hours ; f; pair of primary t1'ocl1oblasts, the products
of division of either 1'“ or 12"’, after isolation; ,1/, h, single prilnary
trochoblasts, either 12”, 1*", 1”‘, or 1”"; i, a pair of secondary t1'oehoblasts, the products of 1"; j, Ir, single secondary trochoblasts. (After
Wilson.)
 
one end and secondary trochoblasts at the other, while the isolated
apical cells and secondary trochoblasts divide as often as they
would in the whole embryo and put out their cilia (Fig. 129 i-,{',
Fig. 130 u—e). Finally, the whole first quartette becomes an ectoblastie larva with an apical organ at one end and a prototroch at the
other, but with no archenteron (Fig. 130/; 9). The cells of the
second quartette (2 a——2 (I) likewise form each a hollow ectoblastic
 
vesicle containing larval mesenehymc, ciliated secondary trochoblasts, and a few (prcaual) ciliated cells (Fig. 131).
IV. I INITIAL STRUCTURE OF THE GERM 221
 
The cells of the third and fourth quartettes do not develop
at all.
 
A -E1, maeromere (1 A-1 D), however, segments partially, to
form a micromere of the second quartette, from which secondary
 
 
e f 9
 
Fm. 130.-—I’uIella. Development of isolated cells of first qualrtet-to
and of the whole ql1iL1'i:0tl’;C. (1, lsolzited 1‘; I», its first division; ¢', its
second division ; (I, after 12 hours ; 0, after 22 hours, showing apical cells
zmd secoinl-My troelioblasts; _f, first division of isolated first quumtette;
g, second division. (After Wilson.)
 
 
(I
 
FIG. 131. — l’uleIlu. Isolated cells of second qu-.u'tette. um, Cl0‘¢l.V1LgP,;
«I. after 24 hours, showing sec.-ond:u'y trooheblusts and ‘.’ prezuml cells.
(After Wilson.)
trochoblasts are later developed, and one of the third. Subsequently
an are-henteron is invaginated (Fig. 132). So the 116- macromere
(2 A-—-2 D) forms a micromere (3 a-~31/) and gives rise to a.
larva, with lentomesoblast (D) or entohlast only (A— C).
222 INTERNAL FACTORS IV. I
 
Isolated 71; (A—D) or is (AB or CD) larvae have an apical organ
and a prototroeh complete or semicircular; they may remain
open or close up ,- sometimes they gastrulate.
 
It is evident that each cell has exactly the same potentialities
for division and difierentiatiou when isolated as when in connexion with its fellows. Only in the shifting of the cells
and closing up of the whole cell-mass is there anything which
approaches to total development.
 
$3‘
 
.;
 
5
 
8 u
 
 
FIG. 1I)‘2.-—1’utella. Development of isolated  basal (inacromere).
a, First; 11, second division; 1-, d, -56- and 64-cell stage. In 4; the
position of the 2-group is normal, not in (I ; 0, after 24 hours, showing
two secondary troehoblasts ()roducts of 2“) and two feebly ciliated
cells (preanalcel1s‘.’). (After ilson.)
 
The dependence of this specification of the blastomercs upon
definite substances preformed——tl1ough not prclocalized—in the
unsegmented ovum is demonstrated by Wilson’s experiments on
Deizlztliuril. The newly laid egg of Ijeuializmz contains a brickred yolk—mass surrounded by a thin hyaline eetoplasm ,- the latter
is thickened a little to form an animal polar area and very considerably to form another large area. at the vegetative pole. The
nucleus is in the centre (Fig. 133). VVhen the sperm enters the
 
nucleus breaks down, and its substance becomes confluent with both
IV. I INITIAL STRUCTURE OF THE GERM 223
 
areas. This continuity is soon lost and the egg becomes divided into
three regions, a clear layer spread over the animal hemisphere,
below this the yolk-mass with the fertilization spindle, and a. large,
 
_I/ /I 1
 
FIG. 133.-V 1)mlulimu. Normal cle-.wn;,re. The red pigmented part is
represented by stippling. a, Freshly laid egg, polar view ; b, the szunc
20 minutes latter, after shedding the menibrame; c, the smile from the
side; (I, one hour after fertilization, showing polar bodies; 0, first
division and protrusion of first polar lobe; f. ‘trefoil’ stage; (1, the
polar lobe has passed into CD; h, p1‘0t1‘u~siOn of second. polztr lobe (Z2) ;
2', second cleavage. (After Wilson.)
 
nearly yolk-free area at the vegetative pole, which is 11ow protruded as the polar lobe. This becomes attached, as described
above for Ilqazzztssra, to the D blastomere. The first quartette of
224 INTERNAL FACTORS IV. I
 
micromeres is formed from the clear animal area; after this
division the polar lobe of D moves up to meet the upper white
area, and so it comes about that a large part of this lobe passes
into the second micromere, 211, which is the first somatoblast.
In the other quadrants the second mieromeres are formed from
the upper white area alone. All the cells of the third quartette
and (it is not known certainly whether this is the mesoblast) 4 (I
in the fourth are quite white. The larva is spindle-shaped with
 
FIG. 134.—l)enIalium. Cleavage after removal of the first polar lobe ;
a, first division: the isolated polar lobe is seen below; b, 4—cell
stage, from animal pole; 1', 8-cell stage, from animal pole; d, forum,tion of second quartette, from vegetative pole; 0, same stage as last,
open type; f, same stage, from animal pole. (After Wilson.)
 
a broad, triple prototroch. Tlierc is an apical organ of long cilia
and a tuft of sensory hairs at the opposite end (Fig. 135 (6).
 
When the polar lobe is removed in the ‘ trefoil ’ (2-celled) stage,
segmentation continues, but no polar lobe is formed in any stage,
the D cells are not larger than those in the other quadrants, the
embryo has no lower white area, the segmentation cavity may
be open below, and the larvae are deficient and die (Figs. 134-,
135 6). Post-troclial region and apical organ are both absent and
IV. 1 INITIAL STRUCTURE OF THE GERM 225
 
these missing parts are never regenerated ; the pretrochal region
is, however, increased and the three rows of cilia in the prototroch are usually developed.
 
The polar lobe is thus clearly connected with the formation of
the trunk and the preoral sense-organ.
 
The segmentation of theiisolated blastomeres is partial, as in
the types already examined, but their subsequent behaviour
depends on the presence or absence of the polar lobe. AB larvae
or A, B, or C larvae closely resemble the deficient trochophore
just described ; CD or D larvae are normal, though the structures
derived from the polar lobe are out of proportion to the rest
 
 
FIG. 135.——Dentalium. a, Normal trochophore of 24 hours; b, larva of
24 hours after removal of the first polar lobc. (After Wilson. 1904-)
 
(Fig. 136 a-d, 1}). The failure of the egg to develop normally is
not due to insufficiency of material, for the volumes of CD and
D are less than that of a whole but lobeless egg.
 
The micromeres 1 a, 16, 1 c, when isolated, become actively
swimming, ectoblastic embryos provided with a prototroch, but
with no apical organ, gut, or post-trochal region; the other
micromere, lzl, however, has an apical organ, though it is Still
Without the hinder region and incapable of gastrulation (Fig.
136 e,f It appears, therefore, that the material for the apical
organ is now placed in this cell, and this is proved by experiment ;
 
for if the polar lobe is cut away during the second cleavage, or
 
‘I
Jzxxuson Q,
226 INTERNAL morons IV. 1
 
from the isolated CD blastomere, then the larva has an apical
organ, but no post-trochal region. Hence between the first and
second divisions the stuff which determines the formation of this
organ must migrate out of the polar lobe into the animal hemisphere of the D cell.
 
In the unsegmented egg the specific material for both apical
organ and post-trochal region is situated in the vegetative
 
FIG. 136.—DentaIimn. Larvae from isolated blastomeres. «, b, Twin
larvae from the isolated CD (a) and AB (7)) halves of the same egg,
24 hours; 0, d, twin larvae from the isolated D (c) and C (d) quadrants
of the same egg, 24 hours; :2, f, twin larvae from '.the isolated posterior
micromere 1 d (e) and the 1 c micromere (f) of the same egg, 24 hours;
g, }larva from one of the A, B, or C quadrants, 72 hours. (After Wilson.)
 
hemisphere. The animal half of the egg, whether removed
after fertilization, or before and then fertilized, segments like an
egg from which the polar lobe has been removed (Fig. 137 6), and
develops neither apical organ 11or post-trochal region (rarely an
apical organ if the fragment is large). The vegetative hemisphere, when out off before the entry ofithe sperm and afterwards
IV. 1 INITIAL STRUCTURE OF THE GERM 227
 
fertilized, segments as a whole with the polar lobe in correct
proportion and gives rise to a dwarf larva with all its parts
complete (Fig. 137 a, c). Many, however, die.
 
A meridional half-egg is able to develop normally, or nearly so.
 
The behaviour of the enucleate vegetative fragment of the
fertilized egg is interesting. Though deprived of nucleus and
centrosome it forms its polar lobe synchronously with the
divisions of the animal portion ; three times is the lobe protruded
and three times withdrawn: the fourth time it is not taken
back; this is the moment when the first somatoblast is given off.
In this enucleatc fragment the polar lobe is not in proportion
but of the same size as in the whole egg.
 
Even the isolated polar lobe, or pieces of it, exhibits periodic
movements, amoeboid ,or of elongation, or protruding a smaller lobe.
 
 
(L
 
FIG. 137.—Dentalium. Development of egg-fragments, the plane of
section being indicated in the small figures. a, b, Twins, after horizontal
or oblique section near animal pole; c, trochophore developed from
a. fragment resembling that shown in a. (After Wilson.)
 
The existence in the egg of preformed substances specified for
the production of particular organs is thus incontrovertibly
established, and as long as an egg-fragment or blastomere
contains all these it will give rise to an embryo which is, in form,
perfect, though lack of enough material (or the shock of the
experiment) may bring its life to an untimely end.
 
At the same time these substances, though preformed, are not
necessarily preloealized; their original is not the same as their
ultimate situation, and in their redistribution we have to deal
with a process which is, as Wilson remarks, truly epigenetic.
 
Before bringing this section to a conclusion reference must
be made to another case in which it seems probable that definite
 
Q2
228 INTERNAL FACTORS IV. 1
 
organ-forming substances exist, though experimental proof is at
present lacking.
 
In the immature egg of the aberrant parasitic Polychaet
.413/zoatoma there is, according to Driesch, at the vegetative pole
a mass of greenish cytoplasm, the surface of which is convex
towards the reddish brown mass of the rest of_ the egg. When
the polar bodies are formed the red substance retreats to the
animal pole, leaving a clear equatorial zone.
 
Before segmentation a polar lobe is formed——as in Ilyauassa
 
 
FIG. 138.——Deve1opment of the egg of Jllyzosioma. The green substance on the vegetative side is shaded finely ; this passes into the polar
lobe (e), and into the D quadrant (1'). The red substance of the animal
hemisphere is coarsely dotted. (After Driesch.)
 
 
 
Fm. 139.—Development of the egg of My/zostoma. Formation of the
cells 1 d (d‘ in c) and 2 d or X (d’ in e). The latter cell takes a portion
of the substance of the polar lobe. (After Driesch.)
 
and Deutalz'um.—containing the green and part of the clear
substance: it passes first to CD and then to D. When the latter
divides it is again protruded and withdrawn. The cell 1 (3, like
1 a, 1 6, 1 c, contains the reddish protoplasm. The second micromere in the D quadrant—2 (Z——-takes part of the green substance,
and Driesch suggests that the rest of the latter would eventually
pass into the second somatoblast (4 ti = M) (Figs. ],38, 139).
IV. I INITIAL STRUCTURE OF THE GERM 229
 
LITERATURE
 
H. E. CRAMPTON. Experimental studies on Gasteropod development,
Arch. Eut. Mech. iii, 1896.
 
H. DRIESCH. Betrachtungen fiber die Organisation dcs Eies und ihre
Genese, Arch. Ent. Mech. iv, 1897.
 
E. B. WILSON. Cleavage and mosaic work. Appendix to Crampton’s
paper on Ilyanassa, Arch. Eut. Mech. iii, 1896.
 
E. B. WILSON. Experimental studies on germinal localization. I. The
germ regions in the egg of Dental imn, Jomw. Ezrp. Zool. i, 1904.
 
E. B. WILSON. Experimental studies on germinal localization.
II. Experiments on the cleavage mosaic in Palcllu and Dcnlulium,
Journ. Exp. Zool. i, 1904.
 
§ 10. ASOIDIA.
 
As has already been shown, there is in the Vertebrata and
zlmp/ziowue no constant relation between the first furrow and
the sagittal plane of the embryo, although the symmetry of the
embryo does stand in a very definite relation to that of the unsegmented ovum. Further, experiment has proved that the
isolated blastomeres, up to a certain point at any rate, a.re capable
of giving rise to normal total larvae. It is, perhaps, all the
more remarkable that in another group of the Chordata, the
Ascidians, the symmetry of the embryo is not only predetermined
in the symmetry of the egg but also by the planes of segmentation, and that the potentialities of isolated blastomeres remain
strictly what they would have been in the whole ovum.
 
The earliest experiments are due to Chabry (Figs. 140, 141).
According to this author the first furrow in the egg of /1.9ci(h'ella
asper.9a is meridional and coincides with the sagittal plane; the
second, likewise meridional, coincides with the transverse plane;
the third, equatorial, separates oral from aboral regions, and is
therefore horizontal.
 
Isolated g blastomeres or 7"} right or left blastomeres give rise
to imperfect larvae with only one fixing papilla, only one atrial
invagination and one pigment-spot, the eye; the last is absent
in the 3,: left larva. % anterior larvae have one fixing papilla, one
sense-spot, a,gut, a cerebral vesicle and a posterior notochord ;
230 INTERNAL FACTORS IV. 1
 
in %} posterior larvae the ehorda and brain are absent, though the
three germ-layers are all present.
 
In the segmentation of the isolated blastomeres the direction
of division is altered, in that a furrow (the fourth) which in the
whole egg is meridional and at an angle of 45° to the first two
becomes parallel to the plane of separation, and so resembles the
third furrow of the whole egg.
 
A right or left % blastomere or %, anterior or posterior, right
 
 
FIG. 140. -—A. Egg of Ascidiella aspersa in the two-cell stage. _One
blastomere has been killed (G). B. The survivor (D) divides to form, in (J,
a small normal gastrula. (After Chahry, from Koischelt and Heuler.)
 
 
0
 
FIG. 14l.—Larva. of Ascidiella aspersa produced from 1 blastomeres.
F, sucker; At, atrium; No, notochord; En, endoderm. (After Chabry,
from Korschelt and Heider.)
 
or left, blastomeres consist after this division of two tiers of four
cells each ; the cleavage of these cells resembles, therefore, that of
a whole egg.
 
It should be noticed that the blastomeres cannot be actually
separated; it is only possible to kill one, by a needle, and note
the development of the survivor.
 
The more recent researches of Conklin on Cyu//zz'a have confirmed and extended Chabry’s results.
 
The immature egg of Qynl/ziu, (Fig. 142) comprises a central
grey yolk surrounded by a peripheral layer of yellow pigmented
IV. 1 INITIAL STRUCTURE OF THE GERM 231
 
FIG. 142.——Norma1 development of the egg of C'_1/nthia pm-tim. Maturation and fertilization. (After Conklin.)
 
A. Unfertilized egg before the fading of the germinal vesicle (clear),
showing central mass of grey yolk (lightly dotted), peripheral layer of
yellow protoplasm (thickly dotted), test-cells and chorion.
 
B. After the entrance of the sperniatozoon the yellow protoplasm has
streamed to the vegetative pole (v); in it, excentrically, is the sperm
nucleus (spat). The clear protoplasm derived from the germinal vesicle
partly forms a. layer over the yellow protoplasm, in art remains at the
animal pole (a). The grey yolk occupies the anima part of the egg.
 
C. The yellow and clear protoplasmic substances have both streamed
to what will be the posterior side (P). A, anterior, V, ventral (animal
pole), D, dorsal (vegetative pole).
 
D. View of the same egg from behind. The two pronuclei are seen
side by side in the clear area. B, right; L, left.
232 INTERNAL FACTORS IV. I
 
protoplasm ; near one pole (the animal), and so determining the
egg-axis, is the large germinal vesicle. Prior to the extrusion of
the polar bodies the latter breaks down and provides a. third
substance, a clear cytoplasm. On the entrance of the spermatozoon near the vegetative pole a remarkable change takes place
in the arrangement of these materials. The yellow protoplasm
flows down to the vegetative pole and arranges itself symmetrically
to the axis; above it is a zone of clear protoplasm derived from the
germinal vesicle; these two areas together occupy about onethird of the egg. The remaining, the animal two-thirds, are
occupied by the grey yolk with only a small quantity of clear
cytoplasm at the animal pole in which, after the formation of the
polar bodies, the female pronucleus is embedded.
 
The egg is still radially symmetrical, but by further shiftings
of these substances it becomes bilateral.
 
The spermatozoon moves in the clear protoplasm to the equator
on one side, the posterior side as it will be, and the yellow protoplasm follows suit; the first is now disposed in a broad equatorial
band encircling half the circumference of the egg, the second
forms a wide crescentic mass below it and reaches nearly to the
vegetative pole. The female pronucleus, with its clear area, now
joins the male.
 
The egg is now bilaterally symmetrical about a plane which
includes the axis, and passes through the middle of the clear and
yellow substances and between the two closely apposed pronuclei.
 
It seems doubtful whether the movement of the clear and
yellow material to the posterior side is immediately due to the
agency of the sperm, for the latter does not always take the
shortest route to the equator; it may enter on what becomes
ultimately the anterior side ; and further, when two sperms enter
flley both travel to the same spot, appearing to be passively
carried along by the independent streaming of the egg cytoplasm.
 
The two pronuclei next move into the axis of the egg in the
animal hemisphere, together with their clear protoplasm. The
grey yolk is thus largely displaced and shifted into the vegetative region. The egg, therefore, now consists of. an animal
IV. 1 INITIAL STRUCTURE OF THE GERM 233
 
hemisphere composed almost entirely of the clear substance, and
a vegetative hemisphere, the anterior portion of which includes
grey yolk and nothing else, the posterior portion the yellow
protoplasm together with a little yolk near the vegetative pole.
The cleavage furrows separate these substances from one
another in a perfectly definite way (Fig. 143). The first is
 
 
 
A  '5' 1"”
 
FIG. l43.—Cleavage of the egg of Cynthia pm-tita. (After Conklin.)
 
A. 8 cells, from the left side.
 
3. 8 cells, from above. The yellow crescent (thickly dotted) is
limited to the posterior dorsal cells: the animal cells are clear, the grey
yolk (sparsely dotted) is found in the dorsal cells, both anterior and
posterior.
 
C. 16 cells, from the dorsal side.
 
D. 74 cells, dorsal view, showing the division of the 4 neural platecells (n. ), and 4 chords cells (ch.). There are 10 endoderm cells (e),
6 muscle-cells (m.s.), 8 posterior (p.m.¢-h.) and 2 anterior (u.m.ch.)
mesenchyme cells.
234 INTERNAL FACTORS IV. I
 
g /4
 
' Fm. 144.— nthia parh'tq. Succe 've stages in the development of
the same righ alf embryo, the left stomere having been iniured in
IV. 1 INITIAL STRUCTURE OF THE GERM 285
 
meridional and sagittal, the second meridional and transverse, the
third equatorial and horizontal. Since the animal pole will be
ventral, the vegetative dorsal, the right and left sides of the
future embryo are simultaneously determined.
 
The four ventral animal cells consist almost entirely of clear
protoplasm, the two anterior dorsal entirely of grey yolk, the
two posterior dorsal almost exclusively of the yellow substance.
 
The next furrows are, roughly, meridional and at angles of 45°
to the first two; they emphasize the bilateral symmetry, for in
the anterior half they meet the first furrow above, the second
below, and conversely in the posterior half.
 
The direction of the divisions in the next phase may be most
easily gathered from the figures. In the vegetative hemisphere
four anterior ehorda-neural cells are separated ofi ; the six
posterior cells include all the yellow substance and will give
rise to the posterior mcsenchyme and the muscles of the tail; six
endoderm yolk-cells remain, four anterior, two posterior. The
next division sees the separation of the ehorda from the neural
cells in front, of an outside ring of muscle-cells from an inside
ring of mcsenchyme cells behind, and of an anterior mcsenchyme
cell on each side from the anterior endoderm.
 
A few anterior animal cells become associated with the neural
cells in the formation of the nervous system; the remainder of
this hemisphere is ectodermal.
 
In gastrulation the endoderm cells first sink inside, followed
by the ehorda in front and the anterior and posterior mesenchyme
cells and the muscle-cells at the sides and behind. Overgrowth
is greatest at the anterior dorsal lip, the blastopore thus becoming
posterior.
 
In normal development, therefore, the axes of the embryo are
 
the 2-cell stage-. The yellow protoplasm is indicated by coarse
stippling. (After Conklin.)
 
a, 8-cell stage, posterior view. V, ventral; D, dorsal. b, 16-cell stage,
anterior view. ' Normally, A 5.1 and a 5.3 lie in front of A 5.2 and a 5.4.
c, 16-cell stage, posterior view. In normal eggs the cells B 5.2 and b 5.4
lie behind B 5.1 and b 5.3. d, 30-cell stage, dorsal view. e, j; 34-cell
stage; e, posterior, f, dorsal view. In normal e gs the cell B 6.4 lies
laterally to B 6.3, and the cell B 6.1 lies between 6.3 and A 6.1. g, 46
cells: posterior view. h, 48 cells: dorsal view. The cells B 7.1 and
B 7.2 should lie next the middle line.
236 INTERNAL FACTORS IV. I
 
predetermined by thearrangement of certain substances in the
egg, and segmentation exhibits the character of a ‘ Mosaic ’ work,
segregating these specific materials into the rudiments of definite
organs. The causal connexion between these materials and the
organs to which they are beforehand assigned is demonstrated by
experiment.
 
FIG. 145.— Cynthia purtita. Half and three-quarter embryos. (After
Conklin.) a, right half gastrula, dorsal view. The neural plate, chorda,
and mcsoderm cells are present only on the right side and in
their normal osition and numbers. b, left half of young tadpole,
dorsal view. he notochord is normal except for size and number of
cells: the muscle and mesenchyme cells are present only on one side:
the neural plate (71.1). is abnormal in form but not in position. c, right
half of young tadpo e, dorsal view: slightly younger than b. m’ch.,
mesenchyme, ma, muscle-cells. d, left anterior three-quarter embryo,
dorsal view. The anterior half is entirely normal with anterior mesen
chyme (m'ch.) on both sides. Posterior mcsenchyrne and muscle-cells upon
the left side only.
IV. I ‘INITIAL STRUCTURE OF THE GERM 237
 
‘The segmentation of a 4} or of 7*} right or left blastomeres is
partial except for the alteration in the direction of the furrows of
the fourth phase already described by Chabry. The blastomeres
divide as though the other half of the egg had not been killed
(Figs. 144,146, 147). Eventually a half gastrula is formed with
exactly half the normal number of neural,chorda,endoderm, mesenchyme and muscle-cells ; the gastrula cavity is open on the side of
the dead blastomere. Many are, however, abnormal in that the
endoderm and muscle- and mesenchyme cells do not invaginate (exogastrulae) or show invaginations of the ectoderm (pseudogastrulae).
 
The half gastrula may develop into a half larva, with the
notochord, muscles, and mesenchyme lying on one side of the
medullary folds. The only signs of the regeneration of the
missing half are the overgrowth of the ectoderm on the naked
side, and the passing over of a few muscle-cells ; the full number
of the latter is, however, never found on both sides (Fig. 145 a—c).
 
The % larva is especially interesting, particula.rly when a posterior blastomere has been killed, for here the anterior end is
complete with anterior mesenchyme on both sides ; the posterior
mesenchyme and muscles are, however, missing on either the right
or left (Fig.145(l). The chorda and neural plate are stated to be
smaller than the normal in these larvae, and there is no sense-vesicle.
 
An embryo developed from the two anterior blastomeres of the
4-celled stage alone possesses the full number of neural, chorda,
anterior endoderm and anterior mesenchyme cells ; its hind end is
covered by ectoderm but shows no trace of muscles or posterior
endoderm or mesenchyme (Fig. 146 0, (Z).
 
The two posterior blastomeres produce an embryo devoid of
neural plate and notochord, of anterior mesenchyme and endoderm. By overgrowth of the ectoderm a sort of solid gastrula
is formed with a central mass of endoderm flanked by musclecells and posterior mesenchyme. No tail appears (Fig. 147).
 
21,-, %, and I15 blastomeres also segment and develop partially.
An anterior % has half the proper number of neural, notoehordal,
anterior endoderm and mesenchyme cells, but the neural plate
does not become folded and the chorda cells protrude irregularly
behind. A posterior quarter may gastrulate in the way described
for the posterior half, but exogastrulae and pseudogastrulae occur.
238 INTERNAL FACTORS IV. I
 
Conklin has also divided the gastrula into anterior and posterior
halves ; neither can regenerate the missing parts, though the cut
surface of each becomes covered by ectoderm.
 
Driesch, however, states that in another form, P/zallzma
manmzillata, anterior and posterior half gastrulae will, if
 
FIG. 146.—~CyntIu'a pm-tita. Anterior half and three-quarter embryos.
 
(After Conklin.) a, b, anterior ventral three-quarter embryo, the
posterior dorsal cells (B 4.1, B 4.1) containing all the yellow pigment
 
having been killed in the 8-cell stage. a, ventral view; b, dorsal view.
 
c, d, anterior half embryo: a, dorsal view, showing the neural plate
(stip led); d, deeper focus showing two rows of chorda cells (the nuclei
are s aded), and ectoderm and endoderm.
 
separated before complete closure of the blastopore, each develop
into a. larva lacking only the otolith and the eye. In a later
stage of gastrulation this becomes impossible, although the
IV. I INITIAL STRUCTURE OF THE GERM 239
 
posterior portion of a young Ascidian will regenerate a. new head
with pharynx and eyes ; the anterior half dies.
 
Driesoh has also maintained that the isolated blastomeres
(72,, 3., %) of the same species give rise ultimately to total
gastrulae and these to larvae. The larvae are, however, unable to
hatch out and are defective—with rudimentary eyes, otolith and
suckers-or devoid of these organs altogether.
 
FIG. 147.—Cg/nthz'a parfita. Posterior half embryos. (After Conklin.)
a, 32-cell stage, dorsal view. Cleavage is quite normal. b, 76-cell
stage, dorsal view. Muscle-cells (B 8.8, B 8.7 and B 7.8) and posterior
mcsenchymc cells (B 8.6, B 8.5, B 7.7, and B 6.3) and posterior endoderm
cells (B 7.1, B 7.2) are seen. c, later stage, deep focus, showing ventral
endoderm (0 end.) with a. mesenchyme (m'ch.) on each side. d, same
stage as last, showing muscle-cells (ms.).
 
LITERATURE
L. CHABRY. Contribution a 1’embryologie normale et tératologique
des Ascidies simples, Journ. de l’Anat. et de la Phys. xxiii, 1887.
E. G. CONKLIN. The organization and cell-lineage of the Ascidian
egg, Joum. gicad. Nat. Sci. Philadelphia, xiii, 1905.
240 INTERNAL FACTORS IV. I
 
E. G. CONKLIN. Mosaic development in Ascidian eggs, Joum. Exp.
 
Zool. ii, 1905. _
H. Dnmscn. Von der Entwicklung einzelner Ascidienblastomeren,
 
Arch. Ent. Mach. i, 1895.
H. DRIESCH. Ueber Aenderungen der Regulationsflihigkeiten im
Verlauf der Entwicklung bei Ascidien, Arch. Ent. Mech. xvii, 1904.
 
§ 11. GENERAL CONSIDERATIONS AND CONCLUSIONS TO BE DRAWN
mom ma Fonaaomc Exrammnnrs.
 
From the foregoing experiments certain general conclusions
may now be drawn.
 
( 1) First, thedivision of the nucleus during segmentation at least
is not the qualitative process imagined by Roux and Weismann.
The pressure experiments of Hertwig, Driesch, and Wilson have
clearly demonstrated that the nuclei of the segmenting ovum
may be disarranged without prejudice to the subsequent normal
development of the embryo; and this result is amply confirmed
by the normal behaviour of eggs (of sea-urchins) in which the
mutual positions of the blastomeres, and therefore also of the
nuclei, are altered, as well as by the very numerous cases in which
it has been shown to be possible to rear a perfect embryo or larva
from an isolated blastomere or blastomeres.
 
It is evident that during segmentation at least the nuclei are
equipotential, and the hypothesis of self-diiferentiation in the
form originally propounded by Roux can no longer be upheld.
It has, in fact, been now abandoned by its author.
 
(2) But though in this direction its labours have ended
negatively, modern experimental research is yet able to point to a
positive achievement of the greatest value and significance. For
the same series of investigations has shown that the cytoplasm of
the undeveloped ovum is not the homogeneous and isotropic
substance which the experiments of Pfliiger led Roux to consider
it, but heterogeneous, containing various specific organ-forming
stuffs which are definitely and necessarily connected with the
production of certain parts or organs of the developing animal.
If the polar lobe of the mollusc Ilyamzssa be removed the larva
has no mesoderm; if the polar lobe of Dentalium be taken away
the larva has no trunk and no apical organ; the egg of a Ctenophor
IV. I INITIAL STRUCTURE OF THE GERM 241
 
is capable of segmenting when deprived of its vegetative hemisphere, but the larva to which it gives rise has no combs and no
sense-organ. The egg of the Ascidian Cynmia exhibits in its
cytoplasm various substances, easily distinguishable by their
colour ; in segmentation these are distributed to the various blastomeres in a perfectly definite way. Remove one or more of these
substances and the embryo will lack the organ or organs normally
produced from them. Again, a Nemertine egg from which, at
a certain stage, the vegetative portion has been cut away, gives
rise to a Pilidium without lappets and without a sense-organ.
 
In these cases, then, at least, the existence of specific organforming stuffs in the cytoplasm of the egg has been proved;
the removal of the stuffs inevitably entails the absence of the
corresponding organ later on. But though it may therefore be
justly said that such stufis are preformed, it is not invariably
true that they are preloealized, present, that is to say, at z'u2'tz'o in
what will be their eventual situation. For example, the stuff
which determines the formation of an apical organ in the larva
of Deutalium is originally in the polar lobe of the vegetative hemisphere : but between the first and the second divisions it migrates
into the animal portion of the egg ; and the same may be said of the
formation of the sense-organ in Cerebratulus and the Ctenophora.
 
It seems highly probable, though it cannot be said to have
been demonstrated, that such specific organ-forming substances
are of universal occurrence.
 
The behaviour of the isolated blastomeres of the eggs of
different forms, as well as of the blastomeres of the same form
at different stages, is markedly different. The .1, or ,1; blastomere
of an Ascidian egg can never produce more than g- or %; larva;
the isolated blastomeres of Palella produce just so much, dividing
just so many times, as they would have produced had they
remained in connexion with their fellows; a radial group of
blastomeres (one or more ,1, blastomeres, or two or four or
more 113- blastomeres) of a Ctenophore egg gives rise to a larva.
with one, two or more costae and meridional canals, as the
case may be. On the other hand, the isolated 13 or 71; blastomeres
of a Nemertine ovum will produce a. complete la.rva, while, after
the next dii./°ision, the totipotentiality is lost; for an. isolated
 
muxmsox R
24-2 INTERNAL FACTORS IV. 1
 
animal cell gives rise to a larva without an archenteron, an
isolated vegetative cell to one in which the apical organ is
lacking. So in Amp/u'o.m4s % and § blastomeres develop into
whole embryos, but g blastom eres, whether of animal or vegetative
origin, will not gastrulate. In other cases, however, the potentialities do not become restricted so soon. Either the four animal
or the four vegetative cells of a Frog’s egg will give rise to an
embryo in which the blastoporic lip and archenteron are developed, while in Echinoderms the least blastomere that will
gastrulate is 315. It is not, nevertheless, every cell of the
32—celIed stage that is capable of developing an archenteron,
but only those, as Driesch has conceded, of vegetative origin.
 
But whatever differences there may be in the potentialities of
the cells of different ova, or of the same ova at different stages
or in different regions, it is clear that sooner or later the capacity
of a part to become a whole i lost; and it seems only reasonable to conclude that this loss of totipotentiality is due to the
loss of one or more necessary cytoplamic substances, to the lack
of specific material, and not merely of material. In the egg of
Ampfiiowus, which is telolecithal, there is an obvious difference,
in the amount of yolk and size of the granules, between the
animal and vegetative hemispheres; a difierence in the capacities
of these two regions ha been experimentally demonstrated in
the Nemertine egg; and, although it may often appear homogeneous, the ovum of the sea-urchin, in all probability, possesses
that ‘stratification’ of potentialities, that is of organ—forming
stuifs, at right angles to its axis, on which Bovcri has so strongly
insisted, for Driesch has admitted that vegetative cells ‘gastrulate’
more readily than animal cells ; it would appear, then, that a gutforming substance is present, and a mesenchyme-forming substance
too, but that the concentration of these substances steadily
diminishes from the vegetative to the animal pole. Radially
about the axis the distribution of these substances may be taken
to be uniform, since meridional fractions of the egg, %, §, % or g
or g blastomeres, are invariably totipotential.
 
The same explanation may be applied to other cases. Hence,
speaking generally, the limitation of the potentialities of the
parts will depend simply on the original distribution of the
IV. I INITIAL STRUCTURE OF THE GERM 243
 
substances in the unsegmented egg. Should they be already
segregated (wholly or in part) in distinct regions, as in Ascidians,
Molluscs, and Ctenophors, then even the first blastomeres will to
that extent be unable, when isolated, to give rise to more than
they would have done in the uninjured egg; should theybe
uniformly distributed about the axis, but unequally along the
axis (Nemertines, Echinoderms, Amp/u,'oams), then a limitation of
potentialities will appear when the third division separates the
animal from the vegetative cells‘ ; should the arrangement be
 
 
FIG. 148.—Diagran1matic sections through the unsegmented egg of
Lolig/o pealii. (From Korschelt and Heider, after Watase.) A is in the
transverse, B in the sagittal plane of the future embryo. In B the
anterior side (m.) is more convex than the posterior (h.). d, dorsal (animal
pole); v., ventral (vegetative pole); 1., left; r., right. The protoplasm is
black, the yolk shaded.
 
uniform about the egg-centre, then animal and vegetative
blastomeres will be alike totipotential (Coelenterata).
 
At the same time it would appear to be true that the capacity
of total development may be lost for mere lack of material, for
Drieseh found that while all four (or all eight in the next stage)
animal cells of the sea-urchin egg might give rise to a Pluteus
larva, the isolated cells would not. The isolated cells may, how
‘ We may notice that O. Maas (Arch. Ent. Mech. xxii, 1906) has found
that the isolated anterior flagellated and posterior granular cells of the
Amphiblastula larva of S3/can are not equipotential. The latter can fix
and give rise to a. young sponge, the former cannot.
244 INTERNAL FACTORS IV. I
 
ever, gastrnlate ; when once the gastrula stage has been reached,
therefore, further development may be simply a matter of
sufficiency of substance. So again the octants of the Ctenophore
egg are alike, but each can produce one costa, and one only.
There are then certain preformed organ-producing‘ substances,
and the arrangement which they either possess before, or acquire
 
     
 
"‘l.'tnL"
FIG. l49.—Three segmentation stages in the blastoderm of Sepia aficiualis; the segmentation is of the bilateral type. l, left; r, right; I—V,
first to fifth cleavages. The top sides of the figures are anterior. (After
Vialleton, from Korschelt and Heider.)
 
during, segmentation determines the fixed relation which can be
observed in almost all cases between the structure of the egg and
the axes of the embryo. Thus in Amphibia the head of the
embryo is formed at or near the animal pole, and the plane of
egg symmetry becomes the sagittal plane. In Mollusca (not
 
Cephalopoda) and Annelida the apical sense-organ is developed
at the animal pole, while the D quadrant is posterior. A blasteTV. I I-NITIAL STRUCTURE OF THE GERM 245
 
pore is in very numerous eases formed at the vegetative pole, the
bilateral symmetry of the eggs of Cephalopoda (Figs. 148, 14-9),
Inseeta, some Crustacea, and Aseidia becomes that of the
embryo; in Ascaris the first four cells lie in one plane, which
becomes the median plane, the germ-cell is posterior and the
animal pole dorsal; and soon.
 
(3) The instances in which the segregation of the organforming substances into the blastomeres takes place at once are
especially interesting, as they appear to fulfil exactly the requirements of the ‘ Mosaik-theorie ’, if that doctiine be transferred from
the nucleus to the cytoplasm, for here, undoubtedly, segmentation
is a qualitative process, a sundering of potentialities. It must be
pointed out, however, that even in these cases the factors which
determine segmentation and those which determine difl’erentiation may be as distinct as they are elsewhere.
 
In the Frog there is a much greater correlation between the
symmetry of the egg and the median plane of the embryo than
between the former an(l the plane of the first furrow.
 
A % or % blastomere of a Nemertine or a sea-urchin segments
as a part, and yet it eventually develops into a whole larva: and
the same is true of egg-fragments. All the isolated blastomeres
of a Molluse segment as parts, yet in 1)ental2'um, that one which
contains all the necessary stuflfs, CD or D, can give rise to a
complete embryo, while its fellows cannot. The converse case
is to be found in Ctenophors ; here the egg which has been
deprived of its vegetative portion segments as a whole and yet
produces an embryo devoid of eostae and sense-organs.
 
There are also certain forms (/Imp/u'o.'ru8 and V61 tebrates,
Coelenterata) where the isolated cell segments as a whole, and
this appears to depend on the rapidity with which the part can rearrange its material or resume the polarity of the whole (Wctzel).
It is further to be noticed that the capacity of a part for total
segmentation,like its power of total differentiation, may differ in
the same egg at difi’erent times and under difierent conditions ; the
fragment of an immature egg of Care//ratulua divides Iikea whole
egg, an isolated blastomere like a part ; again, in Ilyanaaaa the
cleavage of a part may be made total by lowering the temperature.
 
When, further, it is remembered how closely similar the
246 INTERNAL FACTORS IV. I
 
geometrical pattern of segmentation may be in the eggs of
different animals, whether of the same or of diiferent groups
(spiral segmentation of the eggs of Molluscs, Chaetopods and
Turbellaria, similar segmentation of the eggs of, for example,
some Echinoderms, Polyzoa, and Vertebrates), without necessarily involving a similarity in the fate of cells which are
identical in origin, it will, I believe, be conceded that the factors
which are responsible for cleavage and those which determine
difierentiation are distinct, though the two may, as in the socalled ‘Mosaik’ segmentations, coincide: and where, as in
Ascidians, the Ctenophora, the Cephalopoda, this coincidence is
complete, the symmetry of the egg, the symmetry of segmentation, and the symmetry of the embryo are one.
 
The former factors must be sought for in the arrangement of
yolk and protoplasm, possibly in the relative strengths of the
centrosomes, in surface-tensions, in the pressure exerted by eggmembranes and so forth ; the latter in definite cytoplasmic organforming stulfs.
 
(4) Like the parts of the egg the parts of the elementary
organs of the embryo are at first equipotential, but exhibit
a gradual limitation of potentialities as their development
proceeds. As Minot puts it, there is a ‘ genetic restriction ’ of
capacities. Thus the arehenteron of Asterias can form a new
terminal vesicle when this has been removed, but only before
the outgrowth of the coelom sacs. The anterior half of the newt
embryo will develop into a whole embryo when out off before the
medullary folds have arisen, but not after. When one vitelline
vein of a chick is destroyed its fellow will form a. whole, not a
half heart; and Sumner’s experiments quoted above have shown
that the margin of the Teleostean blastoderm is isotropic.
 
(5) There still remains to be noticed a point of great importance. It certainly has not been demonstrated, and it cannot be
pretended that there are in the developed egg as many specific
substances as there are separately inheritable qualities in the
body. On the contrary, the evidence of experiment speaks
against such a supposition; for, as Driesch has urged, were
every separately inheritable quality to be represented in the
germ by a distinct morphological unit, the equipotentiality of
IV. I INITIAL STRUCTURE OF THE GERM 247'
 
the isolated parts would be inconceivable. Although the formation of the larval and elementary organs (germ~layers) may depend upon such stuflfs, yet within each such elementary organ
the parts are at first and remain for a time equipotential.
 
Diiferentiation is progressive. The origin of the primary
differentiations has been found; it remains for the experiments of the future to discover how fresh distinctions arise
until the organization of the whole is complete. Suggestions
as to the importance, in this respect, of certain other internal
factors have been made, notably of the structure of the nucleus,
and the reactions of the parts on one another. These suggestions, for they are hardly more than that at present, we shall
have to discuss. First, however, we must briefly consider the
part played by the spermatozoon in determining the structure of
the egg and the symmetry of the embryo.
 
§ 12.
 
It is of the greatest interest to observe that the definitive
distribution of these specific substances in the unsegmented egg,
and so the determination of the embryonic axes, may be brought
about, partly at least, by the spermatozoon.
 
In the Frog (Ifamz fusca, It. temporaria, and In’. 1Jalu3lrz's, probably also 1?. esculenta) the egg soon after fertilization becomes
bilaterally symmetrical. As first Roux and later Schulze have
shown, this is due to the appearance on one side of the border of
the pigmented area of a grey crescent, caused by the retreat of the
pigment into the interior. The grey crescent subsequently becomes
white and added to the white area. The side on which it appears
is opposite to that on which the spermatozoon had entered, and later
becomes the dorsal side of the embryo ; there is also a considerable
correlation between the plane of symmetry‘thus bestowed upon the
egg and the future sagittal plane, since the latter lies in a large
majority of cases in or close to the middle point of the crescent.
 
This relation between the point of entry of the sperm and
the egg- and embryonic symmetry has also been experimentally
demonstrated by Roux, who, by applying the sperm to any
arbitrarily selected meridian of the egg (by means of a fine
cannulus, _a brush, or a silk thread), was able to show that
248 INTERNAL FACTORS IV. I
 
fertilization can take_ place from any meridian, and that the
point of entry so selected becomes the ventral side later on; in
other words, the fertilization meridian becomes the sagittal plane.
This, as we have seen, is true, or approximately true. Roux,
however, believed that the plane of the first furrow also coincided
with the other two, in fact that it was the first furrow which
determined the sagittal plane, and he brought forward evidence
to show that the first furrow, if it did not pass through the
point of entry of the sperm, at least either included, or was
parallel to, the inner end of the crooked path of the spermatozoon
within the egg, the so-called ‘ copulation ’ path (Fig. 150). As we
now know there is very little correlation between the first furrow
and the sagittal plane. Nevertheless Roux’s work remains of
 
 
FIG. 150. —Roux’ diagrams to show the relation of the sperm-path
(Pig/.) to the first furrow in the Frog's egg. In A the furrow includes the
sperm-path, in B it is parallel to it, in C it is parallel to the inner portion
of the path (copulation path), in D it includes only the very last portion
of the copulation path. (From Koischelt and Heider, after Roux.)
 
the greatest significance, for it seems extremely likely that,
while it is the actual point of entrance of the sperm and the
first, radially directed, part (‘penetration path’) of its track
within the egg, which determines the position of the grey
crescent, and so of the sagittal plane, it is the second part which
determines the direction of the first furrow, since the centresome will divide at right angles to this ‘copulation’ path, or
line of junction of the two pronuclei; the axis of the fertilization
spindle is of course given by the line of separation of the two
centrosomes, its equator by the plane including the two pronuclei and the egg-axis, and this spindle-equator is the plane of
the first furrow.‘
 
In the egg of the sea-urchin Tomopzzezw/es the definitive eggaxis would appear to be fixed not by the original position of the egg
‘ See, however, Appendix A.
IV. I I°NITIAL STRUCTURE OF THE GERM 249
 
nucleus at all, but by the position of the excentric segmentation
nucleus. Both male and female pronuclei move through the egg
for a longer or shorter distance till they meet, and the combined
nucleus then takes up its definitive excentric position (Fig. 151).
The plane of the first furrow is given by the egg-axis and the
point of entrance, approximately, of the spermatozoon, which may
thus be said to determine the symmetry of segmentation, and so
of the embryo, since this plane becomes, it is said (Selenka,
 
FIG. 151.—D1ag1~ams from successive camera. drawings of the living
eggs of Toaropneustes, to show the determination of symmetry during
fertilization.
 
The original position of the eg nucleus is shown by the position of
the dotted circle marked 9; its pa.t by the succession of such circles.
 
E, the entrance-point of the spermatozoon, and cone of entrance. The
male pronucleus is rendered in black, its path marked by the line uniting
the black dots.
 
M is the meeting-point of the pronuclei. The cleavage nucleus, c, is
shaded; its successive positions are indicated. The axis of the fertilization
spindle is shown by the line with a small circle at each end passing
through the cleavage nucleus; the arrow passing through this is the
definitive egg-axis, its point the animal pole. The cleavage nucleus is
slightly excentric, and nearer the animal than the vegetative pole.
 
F is the meridian of the first furrow.
 
A and B are opposed in polarity, similar in other respects. 0 and D
differ in polarity, and otherwise as well. (After Wilson and Mathews.)
250 INTERNAL FACTORS IV. 1
 
Garbowski, Driesch), the median plane of the Pluteus. In this
case there is practically no distinction between ‘ penetration ’ and
‘copulation’ parts of the sperm-track, and, the centrosome dividing perpendicularly to this path, the point of entrance naturally
lies in, or nearly in, the first furrow. The other alternative, as
Wilson has pointed out, is to suppose that the definitive polarity
of the egg existed before fertilization, but exerted no influence
upon the egg-nucleus, although able subsequently to compel the
fertilization nucleus to take up a position in its axis.‘ There
are other Echinoderms, however (:1a5er2'a.9, Stroizyyloccietrotus), in
which the original and the definitive egg-axes coincide.
 
Another very interesting case is Uynt/zia (Fig. 142). In the
immature egg of this Ascidian the cytoplasm is concentrically
arranged, since the yellow pigmented protoplasm forms a peripheral investment for the central grey yolk: the egg-axis is determined only by the position of the germinal vesicle. Upon the
entrance of the spermatozoon the yellow substance together with
the clear plasma of the germinal vesicle, which has in the meantime broken down, streams towards the vegetative pole and becomes
radially arrayed about the axis. By a further streaming movement
these two substances are carried up with the male pronucleus——
or pronuclei in cases of dispermy—to the equator, so marking
the future posterior side of the now bilateral egg. These changes
are evidently an eifect of fertilization: it cannot, however, be
asserted that the point of entrance of the sperm determines the
posterior end, since it may enter on the opposite side and get
carried across.
 
There are a few other instances in which a11 alteration of the
egg-structure has been noticed to follow on fertilization. In
Ctenophora the peripheral layer of granular cytoplasm becomes
aggregated at the animal pole (Agassiz) ; in Polyclads (Lang) and
Cirripedes (Groom) the egg becomes telolecithal ; and so in P/rzyew
 
‘ In Diplogaster longicauda also it appears to be the point of union
of the two pronuclei which determines the definitive egg-axis and
orientation of the embryo. The polar bodies are always extruded and
the female pronucleus formed at that end of the ellipsoid egg which is
turned towards the ovary, while the sperm enters at the opposite end.
The pronuclei may meet and units at either end; but it is at the end
where their union takes place that the smaller of the first two blastemeres is found and, later, the binder end of the embryo (H. E. Ziegler,
Zeitschr. wise. Zool. lx, 1890).
IV. I INITIAL STRUCTURE OF THE GERM 251
 
(Kostanecki and Wierzejski): the ‘polar rings ’ appear in
Allololzoja/tom, Clepsine, and Rhync/aelmis (K. Foot, Whitman,
Vejdovsky), while in Teleostei the periblastic hyaline layer
becomes concentrated as the blastodisc at the animal pole. The
case of Dentalium has been already referred to (p. 223, Fig. 133).
 
Although the cases in which the role played by the spermatozoon in determining egg and embryonic structure are, as has been
observed, not very numerous, yet it is fully to be expected that
renewed investigation will show that some such rearrangement
of the materials of the egg is in many, perhaps in all, instances
one of the first results of fertilization.
 
LITERATURE
 
A. AGASSIZ. Embryology of the Ctenophorae, Mcm. Amer. Acad. Arts
and Sciences, N.S. x, 1873.
 
E. G. CONK LIN. The organization and cell-lineage of the Ascidian egg,
Jam-n. Ac. Nat. Sci. Philadelphia, xiii, 1905.
 
K. Foo'r. Preliminary note on the maturation and fertilization of the
egg of Allolobophora foetida, Journ. Morph. ix, 1894.
 
T. T. GROOM. On the early development of Cirripedia, Phil. Trans.
Roy. Soc. elxxxv, B. 1894.
 
K. VON KOSTANECKI and A. WIERZEJSKI. Ueber das Verhalten der
sogen. achromatischen Substanzen im befruchteten Ei (Physa fontinalis),
Arch. milcr. Anat. xlvii, 1896.
 
A. LANG. Fauna. und Flora des Golfes von Neapel: XI. Die Polycladen, Leipzig, 1884.
 
W. Roux. Die Bestimmung der Medianebene des Froschembryo
 
(lurch die Copulationsrichtung des Eikernes und des Spermakernes, Arch.
mikr. Anat. xxix, 1887; also, Ges. Abh. 21, Leipzig, 1885.
 
F. VEJD0vsK1'V. Entwickelungsgeschichtliche Untersuchungen, Prag,
1888-92.
 
C. 0. WIIITMAN. The embryology of Clepsine, Quart. Jouru. Micr. Sci.
xviii, 1878.
 
E. B. WILSON and A. P. MATHEWS. Maturation, fertilization and
polarity in the Echinoderm egg, Journ. Morph. x, 1895.
 
§ 13. WIIAT PART DOES THE NUGLEUS PLAY IN
DIFFERENTIATION ?1
We have already criticized and rejected the hypothesis that
the division of the nucleus during segmentation is a qualitative
process. It may still be urged, however, that the nucleus is not
 
‘insignificant in dilferentiation. In support of this contention
 
the following arguments have been put forward.
1. It is urged that the nucleus is essential for the life of the cell,
 
1 See also Appendix B.
252 INTERNAL FACTORS IV. 1
 
and not only for its metabolism (Figs. 152, 153) but for the
production of form (Fig. 154).
 
2. It is obvious that the germ-cells are the vehicles whereby
the inheritable characters of the species are transmitted from
one generation to the next; they are the material basis of inheritance. The germ-cells of the two sexes are, however, as
unlike as possible in every character except their nuclei, in
which, as the study of maturation has abundantly shown, they
are exactly alike. In fertilization, the
union of the two germ-cells, two distinct
processes are involved. The first is the
mutual stimulation whereby the lost
_ power of cell-division is restored. This
',5 ' is a process independent of the nuclei (in
Metazoa). The other is, however, the union
of the nuclei (or their chromosomes); and
since the offspring are held to inherit
equally from the two parents, it has been
- - supposed that with the union of the nuclei,
the only parts of the cells that are
 
 
FIG. 152.——Egg-cell of
 
Dytiscus mwginalis in its
follicle with two nurse
cells. From the nurse
cells nutritive material
passes into the egg-cell,
towards which the nucleus
 
exactly alike, the paternal and maternal
contributions are intermingled, and that
therefore it is in the chromatin of the
nuclei that the vehicle of transmission
of hereditary qualities must be looked for.
 
sends out pointed pseudopodia. (After Korschelt,
from Korschelt and
Heider.)
 
3. Thirdly, it has been pointed out
(originally by Roux) that karyokinesis
appears to be a mechanism expressly
adapted to the simultaneous division of a large number of qualitatively different units. From the existence of karyokinesis it is
argued that the nucleus is not homogeneous, and that in development the various units of which it is composed are directly
concerned in the process of dilferentiation.
 
The doctrine of the individuality of the chromosomes has been
brought forward in support of this belief, as well as the
 
constancy of their number.
4. The diminution of the chromosomes. Boveri has observed
 
in Aacmia, in the nuclei of purely somatic cells, apeguliar process
IV. I INITIAL STRUCTURE OF THE GERM 253
 
to which he has given the name of the diminution of the chromatin.
The middle parts of the chromosomes in somatic cells become
divided transversely into small granules, the ends remain rodshaped. The granules of the middle parts are alone divided and
 
pass to the daughter nuclei,
while the ends are cast out
into the cytoplasm and there
degenerate. The chromosomes
of the germ-cells (or of the
parent-cells of the germ-cells)
do not undergo this change,
but remain intact (Fig. 155).
 
It might be argued that in
such a. case as this the determination of certain characters
—-somatic-—is brought about
by the expulsion of the chromatin into the cytoplasm.
 
5. A great deal of stress
has been laid on the importance of the experiment, due to
Boveri, in which the enucleate
fragment of the egg of one
species of sea-urchin gives rise,
when fertilized with the sperm
of another species, to a larva _ _ _
which exhibits the charams i..F’1»‘TI..i§’.3 3?‘.-:...‘3.‘£T"""q~i‘.’.f ‘.”...’l.‘i‘.§f.'.i'“iL'
 
of the male parent alone. placed at the point of origin. B, the
 
- same in Cmurbita pepo. C, root-hair
6' Boven has brought for of Cannabis .-atim: the nucleus is at
 
ward evidence to show that the growing point. (After Haberlandt,
the abnormal development of from Korschelt and Ileider.)
Echinoderm eggs which follows on polyspermy is in reality due
to the irregular distribution of the chromosomes to the daughtercells. Hence it is argued that the chromosomes are qualitatively diflferent.
 
These are the reasons which are, or have been, brought
forward in support of the belief that the nucleus is not insignificant in difierentiation. We may now discuss them in order,
 
A B C
254 INTERNAL FACTORS IV. 1
 
premising only one thing, namely, that both maternal and
paternal nuclei are certainly not necessary for the production of a
new individual, however beneficial it may be for the elements of
two parents to be commingled in the body of one ofispring. In
artificial parthenogenesis an egg is stimulated to development by
a solution, and produces a normal larva; this creature possesses
nuclei of maternal origin. The
 
_ e converse case is seen in the
W "K "- i ‘ phenomenon of ‘ merogony’.
 
M An enueleate egg-fragment,
fertilized with sperm of the
 
!!lll|
  1
same species, gives rise to a
new organism, provided only
with paternal nuclei.
 
The older view that the
union of the pronuclei was
the essential act in fertilization ean no longer be held,
for, certainly, both nuclei are
not necessary. The most,
therefore, that can be said
will be that a complete set of
the chromosomes (or smaller
 
chromatin units) of the species
FIG. 154.——0n the left the protozoan
 
Stentor, cut into three pieces, each con- ls necessary for d1fierentla't10n'
 
tainingha piecle of thfi nucleus. On Whether this is so or not we
the rig t, eac piece as regenerated - - ,
the missing parts and become a com— must now mqune'
 
lete Stentor. (After Gruber, from 1. It will of course be al°“°h°1t “ml Heme") lowed thatthe nucleus is necessary for the life of the cell and to a certain extent for the assumption of form. The numerous and familiar experiments on Protozoa
have sufficiently established this point} A nucleate piece will live
and regenerate lost parts, an enucleate piece will not (Fig. 154-).
2. It will also be admitted that the maturation processes in
the germ-cells of the two sexes are extraordinarily alike and
end in the reception by each germ-cell of one-half the normal
‘ See especially M. Verworn, Die physiologisehe Bedeutung des
 
Zellkerns, 12fl1'1ger's Arch. li, 1892 ; in this paper an account of the work
of Gruber and others will also be found. ‘
IV. 1 INITIAL STRUCTURE OF THE GERM 255
 
FIG. l55.——The process of chromatin diminution as seen in the
somatic cells of Ascaris megalocephala. (After Boveri, 1899.?
 
1. Mitosis in the 2-celled stage. In the first somatic cel (S, or AB),
the primary ectoderm, the chromatin undergoes diminution, not in the
germ-cell (Pl). la chromosomes being ‘diminished’.
 
2. 4-cell stage, T-shape. In A and B the discarded masses of chromatini are seen. S, (EM St), second somatoblast or endomesostomodaeal
ru iment.
 
3. 4-cell stage, lozenge-shape. In A and B the next mitosis is beginning, in P, and E M St the nuclei are in the resting stage. A is anterior,
2:] and g are dorsal, all four cells lie in one plane, the sagittal plane of
t c em ryo.
 
4. In EM St the chromatin is being diminished. Division of P, into
P3 and S3 (C), the secondary ectoderm. a, b, primary ectoderm of right,
a, 8, primary ectoderm of left side.
 
5. The endoderm (E,, E,) has now been separated from mesoderm and
stomodaeum. P3 has just divided into P4 and SԤd)' tertiary ectoderm.
 
6. Diminution of chromatin in S, (D). The our endoderm cells (E)
beginning to be invaginated: on each side two mesoderm cells (M) in
which franular chromosomes may be seen, and two stomoclaeal cells (St).
Ventra. view.
256 INTERNAL FACTORS IV. 1
 
number of chromosomes of precisely the same shape and size.
In fertilization the two sets of chromosomes—each in half the
normal number-are united. It is of course difficult to believe
that this extraordinary resemblance of the nuclei, while all the
other characters of the cells are unlike, is without significance.
 
3. With regard to the third point a distinction must be made
between two hypotheses. The first is the individuality of the
chromatin, the second the individuality of the chromosomes.
 
For the first, as we shall see, independent evidence exists, and
mitosis is certainly a mechanism admirably adapted to the
simultaneous division (or separation of already divided halves) of
a large number of qualitatively distinct bodies.
 
But the second hypothesis in no way follows from the first,
for the grouping of the unlike units may obviously be different
at each successive division without in the least impairing their
individuality. There is, indeed, evidence for the persistent individuality of the chromosomes in only one or two cases.
Boveri has noticed, in the segmenting egg of A.<:carz'.s-, a constancy
in the position of the pockets of the nuclear membranes which
lodge the ends of the chromosomes, and a similarity in the
arrangement of the chromosomes in the nucleus in successive
nuclear divisions. Sutton again has observed that in Bracfi_g/stola each chromosome forms a separate reticulum, situate in a
separate pocket of the nucleus. There are other cases in which
each chromosome forms its own vesicle (Echinoderm eggs). These
instances are, however, few and far between. There are so many
nuclei in which nothing can be observed of the chromosomes in
the resting stage, often there is nothing but a fine granular
mass, and a study of the early and end stages of mitosis seems
to show that the chromosomes are precipitated from solution in
the nuclear sap and redissolved when the division is over.
 
In such a solution the chromatin elements would retain their
individuality, their qualitative difierences, just as each one of
a number of crystals retains its distinctive properties in a mixed
solution and exhibits them when recrystallization occur. In
such a sense, and such only, is it possible to speak in general of
the individuality of the chromatin. '
 
4. With regard to the diminution of the chromosomes in
IV. I INITIAL STRUCTURE OF THE GERM 257
 
Ascm-is, Boveri has pointed out that the extruded outer ends of
the chromosomes are very irregularly distributed to the daughtercells, whereas they should surely, if they are determinants, be
equally divided between the two. More than that, however,
Boveri has found evidence which shows that not only does the
diminished chromatin not determine the characters assumed by
the cytoplasm, but that on the contrary it is the cytoplasm
which decides which chromosomes shall be diminished, and
which remain intact.
 
Boveri has observed certain cases of dispermy in Ascaria,
which are followed by simultaneous division of the ovum into
four, consequent, presumably, on a quadripolar mitosis which is
due, in turn, to the presence of an extra pair of centrosomes.
The total number of chromosomes present in such eggs is 3 n,
where n is the reduced number, since each spermatozoon introduces n. This number becomes doubled by division, and the
number is then 6n or 12, since 1» in Ascm-2'3 megalocep/zala v.
bivalens = 2. These twelve chromosomes have to be distributed
over the four cells into which the ovum divides: their distribution is irregular. The next division is described as being
tangential in two on? the cells, at angles of 45° to the first
divisions in the other two: it leads to the formation of two
groups of four cells each, in each of which the cells are arranged
in the T shape characteristic of the normal four-celled stage.
The normal four-celled stage is reached by (1) an equatorial
division,  followed by a meridional division in the animal cell
(/1 B), and a latitudinal division in the vegetative cell, which
separates a cell S2, which like A and B is somatic, from a cell P,
at the vegetative pole. The chromosomes in P2 are intact, in
A, B, and 82 they are diminished (Fig. 155).
 
Boveri argues that if the diminution of the chromosomes were
an intrinsic property of the chromatin, there should always be
6 (3 7;) chromosomes intact and the rest diminished, just as there
are always 4« (2 n) intact in the normal egg, no matter how the
chromosomes had become distributed over the first four cells and
their descendants. This, however, is not the case. Whole
chromosomes are found only in those cells——one in each group
of four—which correspond in position to the P2 cell of the
 
JENKINSON S
258 INTERNAL FACTORS . IV. 1
 
normal egg; the chromosomes in the other cells are all
diminished, and no cell contains chromosomes of both kinds.
Further, the number of whole chromosomes may be 5 in one
cell and 3 in the other, or 4 and 3, or 3 and 3, or 4 and 2, or
3 and 2.
 
Boveri concludes, it seems with perfect justice, that it is the
(vegetative) cytoplasm which has determined that the chromosomes it contains shall remain whole, while, contrariwise, the
animal cytoplasm of the other cells brings about the process of
diminution.
 
In Ascaria lumbricoizles there is a diminution of the chromosomes
very similar to that seen in A. megalocep/mla, and, to judge from
 
 
FIG. 156.—Pluteus of_Sphaerechz'nus granularis from in front and from
the side. (After Boveri, 1896.)
 
certain abnormalities similar to those just mentioned, brought
about by the same means (Bonnevie). In Dytiacus there is something like a diminution of the chromatin in the nurse-cells, while
the oocyte retains all the chromatic material intact (Giardina).
The distinction seen here may, however, only be that observed in
all ova between the nutritive and reproductive functions of the
nucleus (trophochrornatin and idiochromatin).
 
5. Boveri’s hybridization experiment, on which rests the
assertion that the nucleus alone conveys the inheritable characters
of the species, was as follows.
 
The Plutei of Sp/laerec/ainua _qranuZarz's and of Ecfiinus microtuberculatue were found (at Naples) to be markedly difierent-.
IV. I INITIAL STRUCTURE OF THE GERM 259
 
The larva of the former (Fig. 156,) is short and squat, the oral
lappet not divided into lobes, and the skeleton provided with a
fenestrated anal arm—produced by three long parallel bars united
by numerous cross-bars--an apical branch to the oral arm, and,
at the apex, a square ‘frame ’ formed by the union of twigs
from the last mentioned and from the apical arms.
 
The larva of Ea/limzs, on the other hand, is long and lank, the
oral lappet is deeply cleft, and in the skeleton the anal arm is not
fenestrated ; there is no apical branch of the oral arm, and the ex
 
 
FIG. 157.-—Pluteus of Echinus mhromberculatus from in front and
from the side. (After Boveri, 1896.)
 
tremity of the apical arm is thickened and club-shaped (Fig. 157).
Boveri first fertilized the ova of Spfiaerecfiimw with the sperm
of Ea/zz'mw, and so produced a hybrid larva whose characters
were intermediate between those of the two parents in the
following respects—in form, being shorter and broader than
that of Ea/ainus, longer and narrower than that of Spfiaerec/zinus,
in having the oral lappet slightly divided, and in the skeleton,
the extremity of the apical arm being swollen and branched,
the anal arms being double but not fenestrated, and the oral arms
being (sometimes) provided Vvith an apical branch (Fig. 158).
s 2
260 INTERNAL FACTORS IV. I
 
A mass culture of egg-fragments of Sp/Eaerec/2imw—containing
presumably whole eggs, and nucleate and enucleate fragmentswas then made, and this was fertilized with the sperm of Ea/tinua.
Amongst the larvae developed in such a culture Boveri found
a small number (twelve) of dwarf individuals which possessed the
paternal (Ec/zimw) characters alone (Fig. 159 6), and he suggested
that these had come from enucleate fragments of eggs. He was
further strengthened in this opinion by the fact that the nuclei
were small, which he attributed to their containing only one-half
the normal number of chromosomes.
 
This conclusion has been adversely criticized by Seeliger.
Seeliger, working at Trieste and subsequently at Naples, has
 
 
FIG. 158.—Pluteus of the_ cross Sphaerechinus granularis 9 x Echinus
microtuberculatus c7’, from in front and from the side. (After Boveri,
 
1896)
pointed out that the diiferences between the Plutei of these two
sea-urchins are not as great as Boveri asserted them to be—-the
extremity of the apical arm in I]c/ainus is, for instance, not always
club-shaped—and that therefore it cannot be so confidently asserted
that the hybrid is intermediate. Secondly, he asserts that in an
ordinary hybrid culture (whole eggs of Sp/zaerecfiz'nus fertilized by
sperm of Ea/timw) there are to be found together with variable
larvae of more or less an intermediate type, individuals with purely
paternal characters, though the pure Sp/merecfiz'nu.s type never
occurs, and that the nuclei are variable in size. This contention
has been upheld by Morgan, and, after a good deal of controversy,
has finally been admitted by Boveri himself. No conclusion can
therefore be drawn from the original experiment.
IV. I INITIAL STRUCTURE OF THE GERM 261
 
An explanation of the discrepancies in the results of the difierent
observers has been offered by Vernon. Vernon has found that
Stranyylocentrotus livirlus (which has a Pluteus almost exactly like
that of Ea/ainzw) is at a minimum of sexual maturity in the summer
months, but that from that time onwards the power of the male to
transmit its characters to the hybrid larva, when crossed with
Sp/aaerec/mms 9 , increases, and that it is
possible to obtain a culture consisting entirely of larvae of the paternal type, in
respect, that is to say, of the skeleton.
Vernon was inclined to look upon this
difference in maturity as a seasonal variation, but Doneaster has since attributed it
to the effect of temperature alone.
 
The transmission of the characters of the
skeleton may therefore be the function of
the nucleus, though, as just pointed out,
there is no stringent proof of this. There are
other larval characters, however, which,
as Driesch has justly urged, must depend
on the cytoplasm of the ovum, the colour,
for example, if the egg is pigmented, and
the size of the larva, so far as this depends
on the size of the egg. From a series
of hybridization experiments between Fm. 159"”, Fully
 
E0/ainus, :5):/uwrec/zi7ms, Stroizgz/loce7zt1'otu3, fonned platens of E-ch,-_
 
and /lrbacia, Driesch indeed concludes gut; m1'c(21'otubefr1culatusf.‘
that the rate of cell-division up to the tile °1i,1,:l,§, E’},i,-,,,:,“;:;e
formation of the primary mesenchyme, r<leared(?from fan enu
- _ - e eatc )c<rg— raoment
the vacuolation of the ectodeim cells (in of Sphamf,=,h,.,m;* ,.er_
 
S};/merec/zines), the number of the primary tilized with the sperm
mesenchyme cells, are all characters which five (After
depend on the ovum, and the ovum alone, i i
 
and therefore on its cytoplasm. With regard to the skeleton
Driesch admits that this, and hence, indirectly, the shape of the
larva, may be influenced by the sperm, at least when the egg
of Sp/taercc/¢z'nu8 is fertilized either by Ea/Linus or Strongyl0centrotus. .Fischel has made similar experiments and arrived
 
at a like conclusion.
262 INTERNAL FACTORS IV. I
 
Boveri has traversed some of these statements of Driesch’s. The
number of primary mesenchyme cells is, he maintains, a mean
between the paternal and maternal number, and the form of the
larva may be intermediate before the skeleton is developed.
Between these contradictory opinions it is not easy to decide,
but it may be mentioned that the number of mesenchyme cells is
peculiarly liable to variation, as Driesch has himself found.
 
Before concluding this section we should mention some most
interesting experiments on heterogeneous cross-fertilization.
 
Loeb was the first to show that by the addition of a small
quantity of calcium chloride and sodium hydrate to sea-water, it
was possible to fertilize the eggs of Stron_9gylocent1-otu.s- [)mp1tmine with the sperm of Arte)-ias ockracea. The matter has since
been more completely investigated by Godlewski, who has
succeeded in rearing Plutei from the eggs of sea-urchins fertilized
with the spermatozoa of the Crinoid Amfedon. The necessary
condition of success is the prior treatment of both eggs and
sperm with an abnormally alkaline sea-water (about 1'75 c.c.
 
T73 NaOH per 100 c.c. of sea-water).
 
The sea-urchins employed were Sp/merec/«mus, 8trou_qylocentrotua, and Ecfiimzs. The eggs of the first reached the
gastrula stage, of the two last the Pluteus. Fertilization was
found to be perfectly normal, with production of a vitelline
membrane, rotation of the sperm-head, and so on. Cleavage
was of the Echinoid type (maternal) with formation of micromeres at the fourth division; no such micromeres occur in
Antetlon, the fourth division being meridional in both hemispheres. Primary mesenchyme was formed (this again is absent
in Anterlou), but the number of cells was slightly in excess of
the normal: and after gastrulation the skeleton of the Pluteus
was developed (except in 15'];/(ae7'ec/tinzts); the larva of Antedon
of course has no skeleton.
 
The Antedon chromosomes persisted and the nuclei of the hybrids
were intermediate in size between those of the parent forms.
 
When an enucleate fragment of the egg of Ecfiimw was fertilized by Antezlon all the processes mentioned above were normal,
except that segmentation was irregular, and that development
ceased after gastrulation. The death-rate of these larvae was high.
IV. I INITIAL STRUCTURE OF THE GERM 263
 
6. It remains for us now to consider the conclusions which
Boveri has drawn from the behaviour of eggs in which owing
to pathological mitosis the distribution of the chromatic elements
was irregular.
 
The investigations of O. and R. Hertwig had previously shown
that if the eggs of sea-urchins were weakened by treatment
with poisons (strychnine, quinine, and others), then the vitelline
membrane which normally prevents the entrance of more than
one spermatozoon was not formed, that consequently several
 
 
FIG. 160.——Diagram of one case of irregular chromosome distribution
in adoubly fertilized egg; an b. , c,, and d, ; a,, be, c,, and d2,and as, b,,
c3; and d, are the three complete, specific sets of unlike chromosomes.
(After Boveri, 1904.) '
 
spermatozoa entered, with the resulting formation of multipolar
mitoses and irregular cleavage.
 
Boveri has succeeded in inducing dispermy in the eggs of
sea-urchins without the use of the poisonous reagent, by simply
adding an excess of sperm to the eggs. The number of abnormal larvae found in such a culture is greater than that
observed when the quantity of spermatozoa used is less, and in
proportion to the number of dispermic fertilizations.
 
In dispermic eggs there is an extra pair of centrosomes, and
these two usually form with the other two a quadripolar figure
with four spindles occupying the four sides of the square(Fig. 160).
In the equators of these four spindles are placed the chromosomes,
264 INTERNAL FACTORS IV. 1
 
the number of which is 39: x 2, where n is the reduced number
(in E'c/limzs 9), since there are two male and one female
pronuclei and all the chromosomes have divided. Cell-division
occurs across each spindle and the whole breaks up simultaneously into four. To these four cells the 37: x 2 chromosomes are distributed, and, as Boveri points out, the chance of
each cell obtaining a complete set of the n chromosomes of the
species (supposing these to be diflferent from one another) is
 
C (1
 
FIG. 16l.——La.rvae from doubly fertilized eggs; (I, of SphaerechimI.s_4/r_anu?qris; b—d, of Echimts micrombcrt-ulatus; a, r, and (Z are from
tripartite, b from a quadripartite egg. (Aft-er Boveri, 1904.)
small. Such eggs, as a matter of fact, practically always give
rise to abnormal larvae. Out of 1,170 examined not a single
one was normal.
 
The chance, however, that one cell of the four will receive a complete set of chromosomes is far greater, and when the =5 blastomeres
were isolated it was found that 25 % gastrulated, though none
gave rise to Plutei. Moreover, all four developed differently.
IV. I INITIAL STRUCTURE OF THE GERM 265
 
Again, by shaking the eggs it is possible to prevent the
division of one centre. A tripolar mitosis is the result and
a simultaneous division into three. Out of 695 such eggs 58
became normal Plutei. This is also in accordance with the
theoretical expectation, for the probability of each of three
receiving a complete set of chromosomes when there are 3 u x 2
to be distributed is much greater than when these chromosomes
have to be distributed amongst four cells.
 
The abnormal larvae are of various types, some showing no
trace of the Pluteus organization, some a small trace, such as
a skeleton on one side. Others have no gut, or some of the
pigment cells are missing (Fig. 161).
 
The distortion of development cannot be attributed to the
 
deficiency in the number of chromosomes. Each of the cells
31; x 2 3 n
 
4 = 72- chromosomes,
for the larvae reared from them may have all their nuclei of
the same size (containing the same number of chromosomes), and
it is known (artificial parthenogtnesis and merogony) that a
larva will develop quite well with only 11. Nor is it needful that all the parts should have the same number of chromosomes, for normal larvae may be experimentally produced, with
71 chromosomes in the nuclei of one region of the body and 2 n in
the rest (see below). At the same time dispermy does not necessarily involve abnormality : in such eggs the two pairs of centres
may remain apart and no quadripolar figure be produced: these
develop into ordinary Plutei (Fig. 162 B). Conversely, quadripolar mitoses may be found in monosperm eggs, but these produce
pathological blastulae. No explanation of the abnormality,
therefore, is possible, so Boveri concludes, except that it is due
to the irregularity in the distribution of the chromosomes. For
normal development it is essential that each cell should receive
not a definite number but a definite combination of chromosomes,
a complete set of the chromosomes of the species. It is the
lack of this complete combination in one or more cells that is
the direct cause of the derangement of development.
 
Hence the chromosomes are not alike, but qualitatively
difierent. _
 
Their division is, however, never (except in reduction) a
 
 
of a quadripartite ovum may have
266 INTERNAL FACTORS IV. I
 
qualitative but always a quantitative process. The combination
is handed on in its entirety to every cell of the body. The
chromatin has in this sense an individuality, for however much
the chromosomes may disappear while the nucleus is at rest, the
set of qualitatively distinct units always re-emerges (though not
necessarily in the same order) to be quantitatively divided between
the two daughter—cells. We reach, therefore, a conception of the
chromatin akin to Roux’s idea of the ‘ Reserve-idioplasson ’,
the germ-plasm which is passed on entire to all the tissues of
the body.
 
And now Boveri proceeds to develop a theory already suggested
by Driesch of the part played by the chromatin in differentiation. The cytoplasm is heterogeneous, anisotropic; and these
difierenees in the cytoplasm are sufficient to account for the
earliest diiferentiations, the pattern of segmentation, the determination of the embryonic axes and possibly some few others.
But then the chromatic elements come into play—not the
chromosomes, which are too large and too few to be regarded
as determinants—but far smaller subdivisions of these. Under
the influence of the cytoplasm, diiferent in the various cells,
some of the units become patent and active, while others
remain latent, and by the reaction of the former upon the
cytoplasm new difierentiations arise which will in their turn
call forth into activity new nuclear elements.
 
Whatever value may be attached to these speculations, there
can be no doubt of the importance of the experiments on which
they are founded. Provisionally we have good ground for believing
in the dissimilarity of the chromatic elements and of their
necessity for differentiation. It only remains to be seen whether
further investigations on other forms will bear out the interpre—
tation which Boveri has placed upon his observations.
 
There is one more point worthy of notice. An ordinary
sexually produced individual possesses in its nuclei 2n chromosomes, that is, two complete sets. In maturation one division of
the chromosomes is transverse, and Boveri suggests that in this
transverse division homologous units are separated from one
another, or, to use the expression made familiar by current
Mendelian theory, members of pairs of allelomorphs. c
 
Before bringing this section to a close a brief reference must
IV. I INITIAL STRUCTURE OF THE GERM 267
 
be made to the relation established by Boveri, as a result of the
examination of sea-urchin larvae (Ecfiimw), between the number
of chromosomes on the one hand and the size of the nucleus
and the size of the cell on the other.
 
The number of chromosomes in a larva, or part of a larva, may
be varied in the following ways.
 
1. In merogony the number is half the normal number, i.e.
 
'71. (The larva is termed by Boveri hemikaryotic arrhenokaryotie.)
 
2. In artificially parthenogenetic larvae it is also 12. (Hemikaryotic thelykaryotie.)
 
3. In ‘ monaster’ eggs the number is twice the normal, i. e.
4 oz. (Diplokai-yotic.)
 
This condition is brought about by shaking the eggs, and so
preventing the division of the centrosome. The pronuelei unite
and enter a resting stage. From this the chromosomes emerge
in the full number and split, but since the centrosome remains
single, there is no cell-division, and the chromosomes once
more form a resting nucleus, but now in twice the normal
number.
 
Later the centrosome divides and the egg develops normally.
 
4. If the eggs are kept for twenty-four hours in sea-water and
then fertilized with sperm which has been weakened in dilute
potash the male pronucleus lags behind its centre. The latter
divides about the female pronucleus, and thus in the two-celled
stage one blavstomere has a male and a female nucleus, and therefore 21; chromosomes (amphikaryotie), while the other has only
a female (thelykaryotic). In this stage, or subsequently, the
male unites with the female pronucleus, and so one-half, or onequarter, possibly only one—eighth, of the larva has nuclei with
2 72, the remainder nuclei with n chromosom_es.
 
5. It sometimes happens in dispermy that the two pairs of
centrosomes remain apart without forming a quadripolar figure
(Fig. 162). Such eggs, as we have seen, develop normally.
Sooner or later the cells become divided into two sets, one with
nuclei containing 2 71 chromosomes (amphikaryotie), derived from
the female and one of the male pronuelei, the other having n
chromosomes from the remaining male (arrhenokaryotic).
 
A superficial examination shows that the nuclei of hemikaryotic
embryos and larvae are smaller but more numerous than in
268 INTERNAL FACTORS IV. I
 
normal (amphikaryotic) embryos or larvae of the same stage.
In diplokaryotie larvae, on the other hand, they are larger and
fewer; while in larvae in which one portion is amphikaryotic,
the other hemikaryotic, the two regions are distinctly marked ofl
by the difierence in the size and number of their nuclei (Fig. 163).
 
The exact relation between the number of chromosomes originally entering into the composition of the nucleus and the
dimensions of the nucleus and the cell has been ascertained by
Boveri to be as follows :—
 
I. The surface-area of the nuclei is directly proportional to
the number of chromosomes. The chromatin, it is worth
noticing, is often placed beneath the nuclear membrane.
 
 
FIG. l62.—Dispermic eggs of Strongylocentrotus Iividus. A, The four
centres have united to form a tetraster. B, The two pairs of centres
have remained apart to form two independent spindles, one of which
contains the female and one of the male nuclei, the other the remaining
male nucleus. (After Boveri, 1905.)
 
II. The number of nuclei is inversely proportional to the
number of chromosomes ,- but
III. Cell-volume is inversely proportional to the number of
 
cells in larvae of the same stage developed from whole eggs _
 
of the same size; hence
 
IV. Cell-volume varies directly with the number of chromosomes and with the surface area of the nucleus. This Boveri terms
the fixed relation between nucleus and cytoplasm.
 
V. It follows from I and IV that nuclear-volume increases
more rapidly thon cell-volume.
 
VI. Conversely, the law holds good of egg-fragments of
different sizes but containing the same number of chromosomes.
 
Since, in the larvae developed from such fragments, the
IV. I INITIAL STRUCTURE OF THE GERM 269
 
thickness of the cell-layers is the same, the surface may be taken
as an index of the volume and is proportional to the original
volume of the fragments. Since the number of chromosomes is
the same, the cell-volumes should be the same, and the number
of cells ought to be proportional to the surface. This was, as
a matter of fact, found to,be true in the few instances examined.
 
This result is merely a restatement of Driesch’s rule, that in
larvae developed from isolated blastomeres the surface of the
larvae and the number of cells are both proportional to the
germinal value.
 
It appears, therefore, that the moment at which segmentation
shall end may be determined by the moment at which this
constant relation between nucleus and cytoplasm is reached.
 
{.'S‘°7°i‘o\
 
J”  .6.
 
 
 
6
 
FIG. 163.—— a, Gastrula reared from an eggof Slrougylocenfrotus lim'du.s-,
in which the sperm nucleus had passed entirely into one of the first two
blastomeres. Optical section of a stained preparation. (After Boveri,
1905.)
 
b, Ectoderm in the neighbourhood of the blastopore of J. dispcrmic
gastrula of .S'trong_r/locentrotus lividus. (After Boveri, 1905.)
 
Driesch has, however, recently reported certain exceptions to his
rule, amongst the mesenchyme cells in particular. These may
be more or less numerous than they should be, may differ in size
and in the size of their nuclei. It is possible, as suggested,
that the division of the centrosome has been suppressed (monaster)
with consequent reduplication of the chromosomes. Such an
explanation may account for the variability in the size of the
nuclei observed by Seeliger in his hybrids.
 
In the Alga /5:piro_qym Gerassimow has observed a close relation
between the size of the nucleus and that of the cell; here, however, it is the nuclear volume which is directly proportional to
the cell-surface.
270 INTERNAL FACTORS IV. 1
 
LITERATURE
 
K. BONNEVIE. Ueber Chromatindiminution bei Nematoden, Jen.
Zeitschr. xxxvi, 1902. J
 
K. BONNEVIE. Abnormitiiten in der Furchung von Ascaris lumbricoides, Jen. Zeitschr. xxxvii, 1903.
 
M. Bovmu. Ueber Mitosen bei einaeifiiger Chromosomenbildung,
Jen. Zeitschr. xxxvii, 1903.
 
T. BOVERI. Ueber die Befruchtungs- und Entwicklungsfiihigkeit
kernloser Seeigeleier und fiber die Moglichkeit ihrer Bastardirung, Arch.
Ent. Mech. ii, 1896.
 
T. Bovmu. Zur Physiologie der Kern- und Zelltheilung, S’.-B. ph_1/s.med. Gee. W1212bz¢2'g, 1896.
 
T. BOVERI. Die Entwicklung von Ascaris megalocephala, mit besonderer Riicksicht auf die Kernverhiiltnisse, Festschr. Kuppfer. J ena, 1899.
 
T. Bovmu. Ueber mehrpolige Mitosen als Mittel zur Analyse des
Zellkerns, Verh. phys.-med. Ges. Wzlnzburg, xxxv, 1903. '
 
T. Bovnm. Ueber da.s Verhalten des Protopla.sma.s bei monocentrischen
Mitosen, S’.-B. phys.-med. Ges. Wdrzburg, 1903.
 
T. Bovmu. Ueber den Einfluss der Samenzelle auf die La.rvencha.raktere der Echiniden, Arch. Ent. Mach. xvi, 1903.
 
T. BOVERI. Noch ein Wort iiber Seeigelbastarde, Arch. Ent. Mech.
xvii, 1904.
 
T. BOVERI. Protoplasmadifferenzierung als ausléisender Faktor fiir
Kernverschiedenheit, S.—B. phys.-med. Ges. Wfirzbu;-g, 1904.
 
T. BOVERI. Ergebniase fiber die Konstitution der chromatischen
Substanz des Zellkerns, Jena, 1904.
 
T. Bovmu. Ueber die Abhfingigkeit der Kerngrosse und Zellenzahl
der Seeigel-Larven von der Chromosomenzahl der Ausgangszellen, Jena,
1905.
 
L. DONCASTER. Experiments in hybridization with especial reference
to the effect of conditions on dominance, Phil. Trans. Roy. Soc. cxcvi,
B. 1904.
 
H. Dnmscn. Ueber rein-miitterliche Charaktere an Bastardlai-ven
von Echiniden, Arch. Ent. Mech. vii, 1898.
 
H. Dnmscn. Ueber Seeigelbastarde, Arch. Eut. Mech. xvi, 1903.
 
H. DRIESCH. Zur Cytologie parthenogenetischer Larven von Strongylocentrotus, Amh. Ent. Mech. xix, 1905.
 
A. FISCHEL. Ueber Bastardirungsversuche bei Echinodermen, Arch.
Ent. Mech. xxii, 1906.
 
J. J. GERASSIMOW. Ueber den Einfluss des Kerns auf das Wa.chsthum der Zelle, Bull. Soc. Imp. Nat. Moscou, 1901.
 
J. J. GERASSIMOW. Die Abhfingigkeit der Grosse der Zelle von der
Menge ihrer Kernmasse, Zeitschr. allg. Phys. i, 1902.
 
A. GIARDINA. Origine dell'oocite e delle cellule nutrici nel Dytiscus,
Internat. Monatachr. Anat. u. Phys. xviii, 1901. '
IV. I INTERACTIONS OF THE PARTS 271
 
E. GODLEWSKI. Untersuchungen fiber die Bastardierung der Echiniden- und Crinoidenfamilie, Arch. Ent. Mech. xx, 1906.
 
M. KRAHELSKA. Sur le développement mérogoniquc des ueufs de
Psammechinus, Bull. Internai. Acarl. Sci. Cmcovie, 1905.
 
J. LOEB. Ueber die Befruchtung von Seeigeleiern durch Seestem
samen, P_flflger's Arch. xcix, 1903.
J. LOEB. Ueber die R.eaction‘des Seewassers und die Rollo der Hydro
xylionen bei der Befruchtung der Seeigeleier, Pfliiger-’s Arch. xcix, 1903.
J. LOEB. Weitere Versuche fiber heterogene Hybridisation bei Echino
dermen, Ifflfiger-‘s Arch. civ, 1904.
A. P. MATHEWS. The so-called cross-fertilization of Asterias by
 
Ar-bacia, Amer. Journ. Phys. vi, 1901-2.
T. H. MORGAN. The fertilization of non-nucleated fragments of
 
Echinoderm eggs, Arch. Eut. Mech. ii, 1896.
 
W. ROUX. Ueber die Bedeutung der Kerntheilungsfiguren, Leipzig,
1883, Ges. Abh. 17. Leipzig, 1895.
O. SEELIGER. Giebt es gesehlechtlich erzeugte Organismen ohne
miitterliche Eigenschaften ? Arch. Em. Mech. i, 1895.
 
O. SEELIGER. Bemerkungen fiber Baatardlarven der Seeigel, Arch.
 
Ent. Mech. iii, 1896.
 
E. TEICHMANN. Ueber Furchung befruchteter Seeigeleier ohne Beteiligung des Spermakerns, Jen. Zeitschr. xxxvii, 1903.
 
H. M. VERNON. The relations between the hybrid and parent forms
 
of Echinoid larvae, Phil. Trans. Roy. Soc. cxc, B. 1898.
H. M. VERNON. Cross-fertilization among Echinoids, Arch. Eut.
 
Mech. ix, 1900.
 
2. ON THE ACTIONS OF THE PARTS OF THE
DEVELOPING ORGANISM ON ONE ANOTHER
 
Up to the present we have discussed only the original structure
of the fertilized ovum as an internal factor in difierentiation.
We have now to consider to what extent the development of each
part may be due to influences exerted upon it by other portions
of the embryo.
 
One of the first suggestions of such an interaction was put
forward by Oscar Hertwig. ‘We recognize’, he says in his
Zez't- mul Streiyfrage, ‘apart from the nature of the egg-substance
itself, the conditions of development, first in those perpetually
changing mutual relations in which the cells of an organism are
placed to one another, and secondly in the influences exerted
upon the organism by the external world ’ ; and again, ‘physiologically expressed, the dissimilar diiferentiation of cells consists
in the reaction of organic substance to dissimilar stimuli,’ and
272 INTERNAL FACTORS IV. 2
 
he proceeds to illustrate his meaning by referring to the influence
of light in determining whether the sexual organs shall be
formed on the upper or under side of a fern prothallus, to the
dependence of the fate of indifierent buds of plants upon external
conditions, to the formation of stems on the upper, roots on the
lower side of a horizontal stolon of Amfennularia, as demonstrated
by Loeb, and to certain alterations in the character of epithelia
and other tissues which are brought about by pathological
conditions.
 
For a more detailed elaboration of this very fruitful idea we
are indebted to Herbst. Herbst classifies organic reactions to
stimuli as either directive or formative. The former are of two
kinds, tactic when the response is some locomotion of a freer
body, tropic when the movement exhibited in response is a
movement of growth. The reactions may be further classified
according to the nature of the stimulus, gravity, light, heat,
electricity, chemical substances, currents of water, the contact
of a solid surface. Thus we have positive and negative heliotropism, galvanotaxis, geotropism, galvanotropism, thigmotaxis,
and so on, and many examples of all of these are cited. These
are mostly familiar facts, but some, for their special interest,
may be quoted here. Thus the hydroid Sertularellw in one-sided
illumination produces stolons instead of hydranths, the tubes of
the Polychaet Sergmla grow upwards towards the light (or
against gravity), an inverted stock of Sertularia produces a polyp
sympodium at the upper—originally lower——end, the stems of
Aulemuluria are negatively, the stolons positively geotropic,
spermatozoa swim to solid bodies, and stolons of Hydroids attach
themselves to some foreign object.
 
Speaking generally, a specific reaction depends not merely on
the nature of the stimulus, but on the structure of the reacting
body itself. Further, the degree of sensitiveness to the stimulus
is not constant. It varies at different periods of life, under the
influence of external conditions such as temperature, and with
the strength of an already existing stimulus (Weber’s Law).
 
Now many of the events of ontogeny, as we have seen, resolve
themselves into movements of the parts of the organismmovements of free parts, locomotion, and growth movementsIV.2 INTERACTIONS OF THE PARTS 273
 
and the suggestion is made by Herbst that these movements are
to be interpreted as so many responses to stimuli, which are then
not merely directive but formative, the stimuli being either
external or exerted by other parts of the body. Thus the
spreading of the blastoderm over the yolk may be a case of
oxygenotaxis, the movement of cells of the Frog’s blastula
towards one another described by Rouxl as Cytotropismus may
be due to a mutual chemotactie stimulus, the migration of cells
to the surface of the Arthropod ovum is perhaps oxygenotactic,
the vitellophags are ehemotactic, the application of mesoderm
cells to the outgrowing axis cylinders of nerve fibres, of connective
tissue and muscle-cells to blood-vessels, of the choroidal cells to
the optic cup (they are absent ventrally when the choroid fissure
fails to close), the reunion of separated blastomeres in Triclads
and others, may all be tactic phenomena of a positive kind, while
the dispersal of the elements of a complex may be negative.
Similarly, the outgrowth and anastomoses of nerves, glands,
ducts, the concrescence of layers may be tropisms of various sorts.
 
The suggested stimuli in some cases proceed from the external
environment, in others they are exerted by one part on another.
The former have already been discussed, but it may be pointed
out again that the effect called forth depends both on the stimulus
and on the reacting organism or organ. Neither in these cases
nor in any others is the production of specific form due to the
action of an external agency upon a homogeneous material. For
even in those instances where the fate of some apparently indifferent tissue seems to be decided by the action of such an
agency-—as in the production of stems from whichever side of an
Anlemmlaria stock happens to be uppermost (Loeb)—it is evident
that all that the external agent can do is to decide between
a limited number of definite alternatives in respect of each of
which the parts of the organism are equipotential. Responses
to stimuli of the second kind possibly play an important part
in ontogeny. Unfortunately, the experimental evidence for this
is at present somewhat meagre.
 
In the fish Funrlulus the yolk-sac is deeply pigmented, the
pigment-cells being especially closely aggregated round the
 
‘ See above, Ch. II. 1, p. 55.
 
unumrson T
274 INTERNAL FACTORS ' IV. 2
 
blood-vessels. When the creature is deprived of oxygen the
pigment disappears, and it is supposed that under normal conditions the pigment cells are chemotactically attracted by the
oxygen in the blood (Loeb).
 
Two other cases come from Echinoid larvae. Driesch has been
able to displace, by shaking, the two groups of primary mesenchyme cells, those destined to secrete the skeletal spicules of the
 
FIG. 164.-Diagrams of Driesch's experiment into the responsiveness
to stimuli of mesenchyme cells in Echinus. A, Normal arrangement of
mesenchyme cells. B, The cells deranged by shaking, returning in C to
their original position. (From Korschelt and Hcider, after Driesch.)
 
larva. The cells, displaced towards the animal pole, return to
their original position after six hours, and subsequent development is normal  164). Driesch has attributed this movement of the cells to a stimulus exerted by the endoderm.
 
Again Herbst has found that when the larvae of sea—urchins
are placed in solutions of lithium salts or in sea-water deprived
of the sulph-ion, or devoid of the carbon—ion, the skeleton of the
IV. 2 INTERACTIONS OF THE PARTS 275
 
Pluteus is either distorted, in the first two cases, or absent, as in
the last. The distortion takes the form of a multiplication of
the triradiate spicules, and the arms are correspondingly multiplied. In water devoid of CO2 there are no spicules and no arms}
Herbst has therefore urged that normally the outgrowth of the
ciliated ring into the arms is due to a stimulus—thigmotropic,
perhaps——exe1-ted by the tip of the spicule. The converse of this
is seen later on when the arms diminish in length as the calcium
carbonate of the Pluteus skeleton is made use of by the developing
urchin.
 
The development of the lens of the vertebrate eye has also
been asserted to be due to a contact_stimulus exerted by the
optic cup upon the overlying ectoderm. Spemann was the first
to show that when the medullary plate of the Frog (Rana fusca)
was injured in front of the optic vesicle, the optic cup (if
developed at all) may or may not come in contact with the
ectoderm, the latter being, at this point, uninjured. In the first
case a lens is formed, in the second not. Similar experiments
have been carried out by Lewis on other species of Frog and on
Amblystoma, with similar results. Lewis has also shown that
a lens will be formed from any patch of ectoderm taken from
some other part of the body and grafted over the optic cup.
Schaper’s evidence in support of this conclusion is doubtful.
This author removed the whole of the nervous system from a 4 mm.
tadpole by a horizontal cut with the exception alone of the
downwardly growing fore-brain and optic vesicles. The wound
healed and development continued. The optic cup was solid,
being filled with a mass of degenerating cells. A lens was
formed opposite the upper edge of the optic cup. It did not,
however, separate from the ectoderm, but difierentiated in silu,
with development of lens fibres. Hence Schaper argues that the
formation of the lens is a process of self-differentiation. The
contact of the optic cup is, however, as he admits, in this case
not excluded.
 
A more serious objection is raised by Miss King, who has found
(in the Frog) that even when, owing to a prior injury, the optic
cup is far away from the skin, or in some cases absent altogether,
 
‘ See above, Ch. III. 8, Figs. 73, 74.
T 2
276 INTERNAL FACTORS IV. 2
 
the lens may still be formed. It may also be mentioned that
Mencl has described a double-headed monster of Salmo salar in
which one head has two well-formed lenses, though the other
parts of the eye are absent. The lenses, however, lie quite close
to the fore-brain, which may have exerted the stimulus, supposing
a stimulus to be necessary.
 
It will be seen that the evidence is extremely contradictory,
and forbids any definite conclusion.‘
 
It is curious that in tadpoles grown in certain solutions
(sodium chloride, bromide, and nitrate) the optic cup is often at
some distance from the ectoderm, and that in these cases the lens
is absent, though it may happen that the upper edge of the
optic cup is sharply bent down in front of the aperture, as
though to form a lens from the edge of the iris, as occurs in the
regeneration of the lens, after extirpation, in the Newt.”
 
Spemann and Lewis have also found that in the absence of
contact between the optic cup and the ectoderm the cornea is
not developed, that is to say, the overlying ectoderm does not
‘clear’ (lose its pigment), it does not thin out, and Descemet’s
membrane is not formed. The more extensive series of experiments is due to Lewis, who turned back a flap of skin and
wholly or partially removed the optic vesicle, the skin being then
returned to its place. In the first case there was no eye, no lens,
no cornea, though on the other side of the body all were present.
In the second case the lens and cornea were both formed,
provided that enough was left of the vesicle for the optic cup
to come into contact with the ectoderm. As a control experiment the flap of skin was merely removed and replaced; the
development of the eye was normal. If at a later stage, when
corneal changes had already begun, both optic cup and lens were
taken out, the cornea was not completed. The pigment indeed
disappeared, but the ectoderm did not diminish in thickness and
Descemet’s membrane was not formed.
 
Removal of the lens alone did not interfere with the production
 
1 Stockard has recently shown that when the egg of Fzmdulus is placed
in MgCl, the two optic cu s wholly or partly fuse while a single or
double median lens is deve oped (Arch. Ent. Mech. xxiii, 1907). This
would seem to support the view advanced by Lewis.
 
2 See above, Ch. III. 8, p. 134.
 
C
IV.2 INTERACTIONS OF THE PARTS 277
 
of the cornea provided that the optic cup could touch the ectoderm, and if this same condition is fulfilled, the cornea will
be differentiated in the ectoderm that grows in after the layer
that originally lay over the eye had been destroyed, but if the
cup and lens are taken away no new cornea is formed even
after four weeks. The size of the cornea is in proportion to the
area of contact. Finally, if, at a later stage still, the optic cup
and lens are taken away, the already formed cornea degenerates.
Should the results of these extremely interesting investigations be confirmed, the development of some parts at least of the
vertebrate eye would be processes of ‘ dependent diiferentiation ’ 1,
dependent, that is, on causes outside themselves, on stimuli
exerted by other parts, though not, of course, wholly dependent,
since the structure of the reacting, aswell as that of the stimulating,
organ is contributory to the quality of the eifect. How far such
an action of the parts upon one another is of general occurrence
as a factor in differentiation future research alone will show. It
seems probable, however, that it will in any case be found to be
limited to the first moments in the formation of the organs 2, for
we know from other evidence that as development proceeds the
system which was ‘ equipotential ’, to use Driesch’s phrase,
becomes ‘ inequipotential ’, that the parts lose their totipotence,
that they gain independence and the power of self-differentiation,
that the value of the correlations between them diminishes. This
is what Minot has called the law of genetic restriction. But
in early stages there are correlations between the‘ organs, the
differentiation of one does depend upon that of another, and it is
this mutual stimulation of the parts that figures so prominently
in the theory of development worked out by Driesch. This
theory we shall have to examine in the following chapter.
 
' 1 Although the expression ‘dependent differentiation‘ is due to Roux,
and although Roux expressed the opinion that certain developmental
processes—the beginning of parthenogenetic development, the determination of the position of the grey crescent and first furrow by the
entrance-point of the spermatozoon, the ‘post-generation’ of the dead
blastomere by the living one in the Frog's egg, for instance—-were of the
nature of responses to stimuli, still the role assigned by the ‘Mosa.ikTheorie’ to the influences of the parts upon one another in differentiation
was subordinate, and restricted to the later stages of development.
 
2 Except, presumably, in cases of ‘composition’ where elements of
different origin unite in the formation of a. single organ.
278 INTERNAL FACTORS IV. 2
 
LITERATURE
 
H. Dnmscn. Die taktische Reizbarkeit der Mesenchymzellen von
Echinus microtuberculatus, Arch. Ent. Mech. iii, 1896.
 
C. HERBST. Ueber die Bedeutung der Reizphysiologie fiir die kausale
Auffassung von Vorgfingen in der tierischen Ontogenese, Biol. Centralbl.
xiv, xv, 1894, 1895.
 
C. HERBST. Experimentelle Untersuchungen iiber den Einfluss der
 
veriindert-en chemischen Zusammensetzung des umgebenden Mediums
 
ztuf die Entwickelung der Thiere: II. Mitt. Zool. Stat. Neapel, xi, 1895.
 
C. HERBST. Ueber die zur Entwickelung der Seeigellarven nothwendigen anorganischen Stofi‘e, ihre Rolle und ihre Vertretbarkeit. I. Die
zur Entwiclrelung nothwendigen anorganischen Stoffe, Arch. Ent. Mach.
v, 1897. 11. Die Rolle der nothwendigen anorganischen Stofi'e, ibid.
xvii, 1904.
 
0. HERTWIG. Zeit- und Streitfragen der Biologie, Jena, 1894.
 
H. D. KING. Experimental studies on the eye of the Frog embryo,
Arch. Ent. Meek. xix, 1905.
 
W. H. LEWIS. Experiinental studies on the development of the eye
in Amphibia. I. On the origin of the lens, Amer. Journ. Anal. iii, 1904.
 
W. H. LEWIS. Experimental studies on the development of the eye
in Amphibia. II. On the cornea, Journ. Exp. Zool. ii, 1905.
 
J. LOEB. Untersuchungen zur physiologischen Morphologie der
Thiere: II. Wfirzburg, 1892.
 
J. LOEB. A contribution to the physiology of coloration in a.ni1na.ls,
Journ. Mmph. viii, 1893.
 
E. MENCL. Ein Fall von beiderseitigen Augenlinsenbildung wiihrend
der Abwesenheit von Augenblasen, Arch. Ent. Mech. xvi, 1903.
 
W. Roux. Ueber den Antheil von ‘Ausltssungcn ’ an der individuellen Entwicklung, Arch. Ent. Mech. iv, 1897.
 
A. SCHAPER. Ueber einige Fiille atypischer Linsenentwicklung unter
abnormen Bedingungen, Auat. Am. xxiv, 1904.
 
H. SPEMANN. Ueber Linsenbildung bei defekter Augcnblase, Anat.
 
Anz. xxiii, 1903.
 
 
==Chapter V==
DRIESCH’S THEORIES OF DEVELOPMENT
 
GENERAL REFLECTIONS AND CONCLUSIONS
 
To the inquirer into the causes of development the central
difiiculty must always be the problem of difierentiation. Growth
and division of the nucleus and the cell, processes which always
accompany differentiation, are, as we have seen, side issues ; but
the increase of structure, the production of form out of the
relatively formless germ, and the gradual passing of this into
a new individual which is like the parents that gave it birth,
this is the marvel which has always excited the wonder of the
observer, and demands all his wit to understand and to explain.
 
Experimental investigation, as far as experiment has at present
gone, has shown, first, that a certain complexity of the physical
and chemical environment is a necessary condition of normal
development ; that complexity may, it is true, vary within
certain limits, but those limits can only be transgressed under
pain of abnormality or death. In the second place it has been
demonstrated that the initial structure of the germ, and the
mutual interactions of its parts as they develop are both indispensable internal factors. It now only remains for us to discuss
the value of those theories which are not only attempts to
explain, to give the most general account of, the phenomena in
causal terms, but also serve to provide a light to guide the
investigations of the future. One such hypothesis we have
already examined in some detail, Roux’s hypothesis of selfdiffercntiation. According to this belief not only is the development of each part determined by causes which reside in itself
alone, but the parts—or rather their determinant representatives,
which are all ea: /1._1/pot/Eesi necessarily present ab 2'mYz'o in the
undeveloped germ—are located in the nucleus. The qualitative
division of the nucleus sunders these units from one another,
which then determine the characters assumed by the cytoplasm,
and so the whole process happens.
 
As we have seen, the hypothesis, in this its original form, is
280 DRIESCH’S THEORIES OF DEVELOPMENT V
 
untenable. Quite apart from the fact that the complex architecture of the nucleus still demands an explanation—an explanation of the same kind, perhaps, which would at once involve us
in an infinite regress—the facts which experiment has brought
out show conclusively that nuclear division is never a qualitative
process. In the other direction the hypothesis errs in attributing
a homogeneity, an isotropy to the cytoplasm, for the same
experiments have proved the existence in the ovum of definite
substances, necessarily concerned in the production of the
primary organs of the embryo.
 
There is, however, no evidence to show that--as imagined in
Weismann’s, and to a certain extent in Roux’s hypothesis—there
 
is a separate morphological unit for each separately inheritable‘
 
character of the species; such an idea would indeed seem to be
precluded by the ease with which, in some cases at least, the
germ may be divided into parts, each of which is endowed with
the potentialities of the whole.
 
And yet development is specific. How, then, is this mechanism
of inheritance to be conceived of? It is to Hans Driesch that
we are indebted for an exhaustive attempt to think out the
whole problem. In his Aualytiec/Le T/teorie tier organise/len Entwic/clzmg Driesch starts with the facts with which we are
already acquainted, the similarity of the nuclei, the dissimilarity
of the cytoplasm in the several regions of the developing germ.‘
 
The arrangement of these dissimilar substances determines
first of all an axis in the egg, an axis with unlike poles ; around
this axis the cytoplasm is radially symmetrical or isotropic, but
in the direction of the axis it is not ; or, as Boveri puts it, there
is a ‘stratification ’ of the substances of the egg at right angles
to the axis, the concentration of the animal substance decreasing
towards the vegetative pole, that of the vegetative substance
in the contrary direction. There are cases (Coelenterates,
Sponges, Ctenophora) in which this radial is the only symmetry; but in other types (Bilatcralia) a third point may
be established by the disposition of some special substance (the
grey crescent in the Frog"s egg or the yellow pigment in the
 
1 It may be mentioned that in 1887 Platner had already denied the
existence in development of any qualitative nuclear division.
V GENERAL REFLECTIONS AND CONCLUSIONS 281
 
Ascidian Cyntfiia, for example), or by the arrangement of the
blastomeres (as by the large posterior cell of Annelids and
Mollusca), and this point, together with the axis, determines
a plane about which the ovum is bilaterally symmetrical. The
axis and the plane of symmetry of the egg are definitely related
to the axis and symmetry of the embryo, the substances to its
primary organs.
 
Further, in very many, though not in all, instances the parts
of the ovum—-blastomeres or egg-fragments——are totipotent ;
and the same is true of the parts of elementary organs like the
archenteron of Echinoderms or the optic vesicle of Amphibia.
The totipotence is, however, sooner or later lost, and this limita
 
FIG. 165.——Diagram to illustrate Driesch’s conception of the minute
structure of the (Echinid) ovum. It is supposed to be composed of
particles all similarly polarized and oriented to the whole. In A onemeridional—half is shown. In B this has become spherical and the parts
have been disturbed. In 0 they have regained the original orientation.
(From Korschelt and Heider.)
 
tion is apparently due to the way in which the substances are
distributed in the ovum, an explanation which seems to be
accepted by Driesch for most cases. But in accounting for the
phenomena in the Echinoderm egg, the form with which he
himself has chiefly experimented, he urges a different hypothesis.
Here he conceives of the egg as composed of like particles, each of
which is polarized and oriented in the same manner as the egg
itself (Fig. 165), and consequently the only limitation to totipotence is due to size ; any isolated part that is not too small can
develop into a whole as soon as its polarized particles have reassumed a similar orientation. Again, it is stated that the
blastomeres can be disarranged to any extent without interfering
282 DRIESCH’S THEORIES or “DEVELOPMENT v
 
with the normal development of the larva; they are all equivalent, without limit, at least until the ectoderm and endoderm have
been difierentiated ; any part can contribute to the formation of
any organ: ‘ jeder Theil kann jedes.’
 
This conception, of the absolute equipotentiality of the parts,
as we have already had occasion to remark, is erroneous; but
for Driesch it is of the first importance, for it dominates, as we
shall see, all his theoretical speculations.
 
The position of the embryonic axes and primary org-ans being
thus determined in the whole egg (or its isolated parts), it is still
left to inquire into the causes which decide the destinies of the
remainder. Driesch’s answer to this question is twofold: there
 
FIG. 166.—Diagram to illustrate the possible part played by stimuli
(‘inductions’) in ontogeny, and by ‘position '. A, B, and C’ are three
larval organs (ectoderm, endoderm, stomodaeum). C may exert a stimulus
on that part of B nearest it; this part, reacting to the stimulus,
becomes (3. Were the position of C’ altered the position of B in the
cquipotential system B might be altered too, so that the fate of any
part of B would be a function of its position relative to 0. So, under
the influence of B, part of A may become a. (After Driesch, 1894.)
 
are two possible factors, one is ‘ position ’, the other ‘induction ’.
In the case of the first, the destiny of a part is imagined to be
determined by its distance from the system of points already
established, ‘ its fate,’ so runs the famous formula, ‘ is a function
of its position in the whole.’ It would, however, be absurd to
suppose that the behaviour of any one of a number of precisely
similar bodies could depend upon its mere geometrical position.
The points already diifcrentiated——the animal pole, for instance—
must be supposed to exert an influence with a force which is
some function of the distance upon the parts which are at present
equivalent, and so to excite their differentiation 1 Fig. 166).
V GENERAL REFLECTIONS AND CONCLUSIONS 283
 
Properly speaking, therefore, this factor of ‘ position ’ belongs to
the second category of ‘ induction ’.
 
An ‘induction’ is simply an effect produced upon the parts
that are developing by other parts, or possibly by some factor in
the external environment. These inductions are, according to
Driesch, of the nature of those events which are brought about
by the addition of a single antecedent condition, an ‘occasion’,
to an assemblage of antecedent conditions, as a spark fires
a rocket or the movement of a lever sets a piece of machinery
in operation; or, in the language of physiology, the causes of
these inductions are stimuli, the efiects, reactions or responses,
 
50
 
/
 
~\&_
 
2
 
FIG. 167.—Diagram to illustrate Driesch's hypothesis of the part
played by the nucleus in development. A is the cytoplasm, '1‘ its totipotent nucleus, ml, ac, are stimuli.
 
A is sensitive to a:, and becomes A,, T is sensitive to A, and is set in
activity (Ta). A‘ being sensitive to Ta becomes A1, and A, is sensitive to
.L‘._., and so on. (After Driesch, 1894.)
 
the quality and the quantity of which are determined not only by
the stimulus but also by the nature of the reacting body itself.
The analysis of these ‘ inductions ‘is not, however, yet complete.
Occurrences of this kind imply the stimulus, the reception of
the stimulus and the response. The first of these is some other
organ or some external agent; the second and third are functions
of the organ which is to be produced; and Driesch imagines
that while it is the cytoplasm which is receptive of the stimulus,
it is the nucleus which makes the response. Each totipotent
nucleus is supposed to contain ferment-like bodies; under the
influence of. some stimulus received through the cytoplasm some
284 DRIESCH’S THEORIES OF DEVELOPMENT V
 
one of these is called into action; this works on and alters the
cytoplasm; this then becomes receptive to a new and dilferent
stimulus, and so on, as shown in the accompanying diagram (Fig.
167). The cytoplasm thus becomes perpetually altered, while the
nucleus retains the entirety of the potentialities of the organism,
a supposition which is held in reserve to account for the phenomena. of budding and regeneration.
 
The progressive limitation of potentialities, therefore, which
we observe in ontogeny is a limitation of sensibilities to stimuli,
equality of potentialities an equality of such sensibilities. In an
‘ equipotential system’ (the blastula or the archenteron of
Ec/linus) all (meridional) parts are equally susceptible, and what
stimulus each will as a matter of fact receive is a function of its
position in the whole, that is to say of its distance from the
centres of force situate in the dissimilar points fixed ab initio in
the system of symmetry of the egg (Fig. 166).
 
Development then proceeds from the comparatively simple
organization given in the structure of the fertilized ovum by the
creation of ever-increasing complexity by the action and reaction
of the parts on one another. Each ontogenetic eifect produced
becomes in turn the cause of further eifects, the possibility of
fresh specific action, for it becomes the seat of a new specific
stimulus and response, and so on until the complexity of the
‘ ultimate organs ’ of the adult is achieved.
 
In order, however, that differentiation may be normal it is clear
that these stimuli and these responses must be accurately coordinated: the right stimulus must be ready at the right time
and at the right place for the right organ to respond to. This
indispensable temporal and spatial co-ordination is the ‘ Causal
Harmony ’ of development; and it is given by the initial
structure of the egg and the constitution of the external environment. ’
 
This is, however, not the only kind of harmony involved in
development. Potentialities become restricted as differentiation
progresses, and the development of the primary, secondary,
tertiary and subsequent systems of organs successively produced
is a process of ‘ self-diiferentiation ’, depending on factors residingin each system itself. Nevertheless these independent systems
V GENERAL REFLECTIONS AND CONCLUSIONS 285
 
frequently unite later on to form complex organs, and the
necessary co-ordination of these is a ‘ harmony of composition ’.
 
Such, in brief, is the idea worked out in the Analytiec/ze
T/leorie. On this hypothesis development is an epigenesis ;
it involves a super-additio partium, an increase of structure,
a creation of fresh form out of the simple cytoplasmic organization of the egg, and the causes which operate in this process of
morphogeny are just that initial structure and the stimuli which
the parts respond to, stimuli which may proceed from the parts
themselves or from the world outside.
 
These responses are primarily physico-chemical and only
secondarily structural, and they happen in accordance with
a pre-established harmony. Only in this sense can development
be described as evolution; but this is not the evolution of the
preformationists of the eighteenth century, nor even that of the
school of Roux and Weismann, less gross, more subtle, but still
morphological; rather it is the realization of a form which is
physically and chemically predetermined, not structurally preformed, in the simple organization of the germ.
 
The hypothesis is worthy of the attention of every serious
embryologist. It is scientific in the strict sense of the word, for
in employing Herbst’s suggestion to bring the events of ontogeny into the category of physiological responses to stimuli it
gives meaning and precision to Hertwig’s somewhat vague idea
of the ‘ mutual relations of the parts’, and makes thereby
a genuine effort to think the particular under the universal, to
bring the facts of embryology under wide general laws of
causation.
 
Its chief Weakness is at present lack of evidence; the development of every organ requires to be examined by the touchstone
of experiment before the theory can rise beyond the rank of
a working hypothesis. That, however, is the common lot of all
working hypotheses, and this one has certainly some counterbalancing advantages. Its conception of the réle of the nucleus
has been to some extent borne out by Boveri’s recent researches
and adopted by their author, a conception which, as we have
observed before, is not irreconcilable with the facts of budding
and re2'eneration. Secondly, only the very simplest original
286 DRIESCH’S THEORIES OF DEVELOPMENT V
 
organization of the germ is demanded. This organization is not,
however, to be looked for in the similar orientation of similarly
polarized particles. On the contrary, it is to be found in the
existence of definite cytoplasmic organ-‘forming substances whose
arrangement must be simple enough to allow of the divisibility
of the whole into totipotent parts, complex enough to account for
that limitation of the potentialities of these parts which, sooner
or later, inevitably ensues. And lastly, when modified in this
important respect, Driesch’s views are no longer hopelessly at
variance with Roux’s position, provided that we discard the
vicious fallacy of qualitative nuclear and cell—division, and
substitute for the idea of complete morphological preformae
tion at origine the successive preformation by the action of the
parts on one another in the organs of each stage of the structures
that are to be developed out of them in the next. To use Roux’s
own terms, development, in its early stages at least, is both a process of ‘ self- ’ and of ‘ dependent ’ differentiation ; or, as Nag-eli
expressed it, it is by the continued combination and permutation
of a few original elements that inheritance is brought about.
 
Driesch, however, is not merely a scientific thinker who
is fully alive to the prime importance of pushing the causal
analysis to its extreme limit. He is also a philosopher; and as
a philosopher he realizes that when science has said all it has to
say the account may still need to be completed from a new and
distinct point of view. In the case of living organisms this
new standpoint is the teleological. The harmony—causal
harmony and harmony of composition—deduced from develop:
ment, the functional harmony exhibited by the organs of the
adult, all appear to be directed to an end, which is the reproduction or the preservation of the specific form, and it is only
when this end is understood that the mere reference to beginnings
which a knowledge of the mechanism gives acquires a genuine
significance. Purposiveness, in a word, is a characteristic of all
organic functions and cannot be ignored.
 
This principle is borrowed avowedly from Kant’s Kritik of Me
Teleological Judgement. Like the scientists of to-day, Kant lays
it down as a rule that the mechanical method, by which natural
phenomena are brought under general laws of causation and
V GENERAL REFLECTIONS AND CONCLUSIONS 287
 
so explained, and without which ‘ there can be no proper knowledge of Nature at all,’ should in all cases be pushed as far as it
will go,‘ for this is the principle of the determinant judgement.“
 
There are cases, however, in which this alone does not
sufficef‘ The possibility of the growth and nutrition, above all
of the reproduction and regeneration of organisms is only fully
intelligible to human reason through another quite distinct kind of
causality, their purposiveness. Organisms are not mere machines,
for those have merely moving power. Organisms possess in themselves formative power of a self-propagating kind which they
communicate to their materials. They are, in fact, natural
purposes, both cause and eifect of themselves, in which the parts
so combine that they are reciprocally both end and means,
existing not only by means of one another but for the sake of
one another and the whole. The whole is thus an end which
determines the process, a final cause which ‘ brings together the
required matter, modifies it, forms it, and puts it in its appropriate
 
' §§ 70, 78, 79, 80. _
 
’ It should be mentioned, perhaps, that Kant employs the terms
‘determinant’ and ‘reflective’ judgement in two senses. In the Introduction to the Kritik of Judgement judgement is defined as that faculty
which thinks the particular as contained under the universal, and
is stated to be of two kinds. It is ‘determinant’ when the universal
is given (as in the mathematical sciences). When, however, only the
particular is given, for which the universal has to be found (as in the
inductive sciences), it is ‘ reflective ’, and Kant insists that the ‘ reflective’
judgement requires a principle which it cannot borrow from experience,
and this principle is, in brief, the ultimate intelligibility of Nature by us.
Only so far as this holds good can we hope to gain a knowledge of
general, empirical laws of causation.
 
The ‘reflective’ judgement is again of two kinds, for which Kant,
however unfortunately, employs the same two terms, ‘ determinant‘ and
‘reflective’. The duty of the former is, assuming that all phenomena are
explicable in mechanical terms (‘ causal’ terms in the usual sense of the
word), to push the analysis of these efficient causes (nexus e;fi'eclit'us) to
its extreme limit. The latter, on the other hand, is concerned with
a kind of causality after the analogy of our own causality according to
purposes, in order that it may have before it a rule according to which
certain products of nature, namely organisms, must be investigated.
The whole argument of the Kritilc is directed to proving that these two
uses of the judgement are mutually supplementary and both indispensable,
though in the last resort the first has to be subordinated to the second.
 
In the text the terms ‘ determinant’ and ‘reflective’ are used in the
second sense. The ‘determinant judgement‘ of the deductive sciences
 
does not of course come into this discussion at all.
 
 
‘ place ’.1 Such purposiveness is internal, for the organism is at
once its own cause and an end to itself, not merely a means to
other ends, like a machine whose purposiveness is relative and
whose cause is external.
 
Such is the principle of the teleological judgement. It is a
‘heuristic principle’ 2 rightly brought to hear, at least problematically, upon the investigation of organic nature, ‘ by a distant
‘ analogy with our own causality according to purposes generally,’ 3
and indispensable to us, as anatomists, ‘ as a guiding thread if we
‘ wish to learn how to cognize the constitution of organisms
‘ without aspiring to an investigation into their first origin.’ 4
 
For ‘ we cannot adequately cognize, much less explain,
‘ organized beings and their internal possibility, according to
‘ mere mechanical principles of nature ’, and it is therefore absurd
‘ to hope that another Newton will arise in the future who shall
‘make comprehensible by us the production of a blade of grass
‘ according to natural laws which no design has ordered ’.5
 
Could our cognitive faculties rest content in this maxim of
the reflective judgement it would be impossible for them to
conceive of the production of these things in any other fashion
than by attributing them to a cause working by design, to
a Being which would be ‘productive in a way analogous to the
‘ causality of an intelligence ’.°
 
Natural science, however, needs not merely reflective but
determinant principles which alone can inform us of the possibility of finding the ultimate explanation of the world of
organisms in a causal combination for which an Understanding
is not explicitly assumed, since the principle of purposes ‘ does
‘ not make the mode of origination ’ of organic beings ‘ any more
‘comprehensible ’.7 And then, in a passage“ remarkable for
its prophetic insight, Kant proceeds to show how this might
be. ‘ The agreement of so many genera of animals in a common
‘ scheme . . . allows a ray of hope, however faint, to penetrate into
‘ our minds, that here something may be accomplished by the aid
‘ of the principle of the mechanism of nature (without which
‘there can be no natural science in general). This analogy of
 
1;8s6. ‘@978. 3§e5. 4§72. v§7.5. °§-15.
V GENERAL REFLECTIONS AND CONCLUSIONS 289
 
‘forms, he says, ‘which with all their differences seem
‘ to have been produced according to a common original type,
‘strengthens our suspicions of an actual relationship between
 
.‘ them in their production from a common parent, through the
 
‘ gradual approximation of one animal-genus to another—from
‘ those in which the principle of purposes seems to be best authen‘ ticated, that is from man, down to the polype, and again from
‘this down to mosses and lichens, and finally to the lowest
‘ stage of nature noticeable by us, namely, to crude matter.
‘And so the whole Technic of nature, which is so incompre‘hensible to us in organized beings that we believe ourselves
‘ compelled to think a different principle for it, seems to be
‘derived from matter and its powers according to mechanical
‘ laws (like those by which it works in the formation of crystals).’
 
A purposiveness, however, must be attributed even to the
crude matter, otherwise it would not be possible to think the
purposive form of animals and plants.
 
Although there are doubtless in the Kritil: many obscurities and apparent inconsistencies, to which we cannot allude now,
the general meaning of Kant’s reflections upon organisms is
perfectly clear. He who would ‘complete the perfect round’
of his knowledge must think not only in beginnings but
in ends. The end in the case of a living being is plain-—it
is the maintenance and reproduction of its form; the end in
the case of the cosmic process, though perhaps not so plain,
is to be sought in the ethical, or, in Kantian phraseology, the
‘practical’ concept of the freedom of the moral consciousness
of man.
 
Such a position is quite intelligible, philosophically; and it can
only be a matter of surprise that Driesch has not been able to abide
by it. In his later writings he has indeed ‘executed acomplete
change of front and repudiated the philosophical doctrine laid
down in the earlier treatise; and the principal reason for this
voZte- ace is that there are cases in which the localization of
 
ontogenetic effects cannot be explained by any theory of formative stimuli. In the theory we have already considered the
causal harmony which secures the due co-ordination in space and
 
in time of the stimuli and responses into which the process of differentiation is resolved is held to result from the initial
structure of the germ and to be maintained by the constitution
of the environment. Now, however, it is urged that no materia
factor can possibly account either for this harmony or for the
secondary harmony of composition or the functional harmony
seen in the activities of the adult. When, for example, the
gastrula of a sea-urchin is transversely divided into two, each
develops into a diminished whole larva in which the gut
becomes divided into the characteristic three regions, and all the
other organs are formed in correct proportion. For each of these
acts in the whole uninjured larva an explanation may conceivably
be given in terms of stimuli or forces emanating from the
originally distinct parts of the egg and producing effects which
vary with the distance upon other parts, as suggested before.
A mechanism may be thought of which, when set in motion,
will achieve a certain end in accordance with its own pre-established harmony, but a mechanism which can be subdivided ad
libitrm, or almost arl libitzcm, and the parts of which will still
achieve the same end, will still behave as wholes with their parts
co-ordinated in the same ratio, temporally and spatially! Such
a mechanism is an inconceivability, for to ensure the result
which does happen the working distance of the forces imagined
must be altered in each case according to the size of the fragment
removed. Something is therefore required to superintend, to
co-ordinate, to harmonize the causes of development in the case
not only of the part but of the whole egg as well; and this
something is not material. A corroborative proof of the inadequacy of the purely material explanation——-the causal explanation
in the ordinary sense of the word—may be derived from a consideration of certain other vital processes. The facts of acclimatization and immunity betray an extraordinary adaptability of
the organism to a change in its environment; an organ will
adapt itself structurally to an alteration, quantitative or qualitative, of function (Roux’s ‘ functional adaptation ’) ; lost parts can
be regenerated; and then there is the physiology of the nervous
system!
 
In all these cases of ‘ regulation ’—and indeed'in all other
responses to stimuli—-the same element, inexplicable in chemical
‘ V GENERAL REFLECTIONS AND CONCLUSIONS 291
 
and physical terms, exists as must exist in development. This
entity is not a form of energy but a vital constant, analogous
to the constants or ultimate conceptions of mechanics and physics
and chemistry and crystallography, but not reducible to these,
just as these cannot be translated into one another. Driesch
describes it as rudimentary feeling and willing, as a ‘ psychoid ’,
as ‘ morphaesthetic ’, or perceptive of that form which is the
desired end towards which it controls and directs all the material
elements of differentiation. Its activities are thus verae causae—
unconditional and invariable antecedents—psychical factors which
can intervene in the purely physical series of causes and efiects,
and for it he revives the Aristotelian term ‘Entelechy ’. Such
is the ‘vitalism’ introduced by Hans Driesch, a teleological
theory clearly, but not the ‘static ’ teleology of the Analytiac/ze
T/zeorie ; rather it is a ‘ dynamic’ teleology which not only sees
an end in every organic process but postulates an immaterial
entity to guide the merely mechanical forces towards the realization of that end.
 
This theory would seem to be open to serious criticism, and
from two sides, the scientific and the philosophical.
 
In the first place, we must remind Driesch that on his own
showing a comparatively simple structure is all that is necessary
to form the starting-point of a developmental process, however
complex that may be, and that there is no reason why such a
structure should not be divisible into portions, each of which will
possess all the parts of the structure in correct proportions, and
be therefore totipotent. But such division cannot continue
indefinitely, for as we know, and as Driesch knows too, there is
always, sooner or later, a restriction of potentialities, and this is
due to the manner of distribution at or2'gz'ne of the constituent parts
of the whole. When Driesch asserts that this restriction is due
to size alone, to mere lack of material, and not lack of specific
material, when he tells us that the blastomeres can be dislocated
indefinitely without prejudice to a normal development, when he
exclaims that ‘ J edes jedes kann ’, he is manifestly led away by
the reaction against the theory of the preformation of as many
units as there are inheritable characters on the one hand, and
on the othe; by his own erroneous presuppositions as to the construction of the egg out of like particles all similarly polarized
and all oriented in the same way.‘
 
This being so, the first argument based on the ‘ causal harmony ’
will fall to the ground; for this will be given in the initial structure
of the egg, and if that may be divided, then the ‘ causal harmony ’
may be divided too. The correct proportionality of the organs
of partial larvae, then, offers no peculiar difficulty. The corroborative argument is founded on a consideration of responses to stimuli.
This is not a question for the embryologist, but it may be
pointed out that there are still physiologists who maintain that
even the complex phenomena presented to us in the activities of the
nervous system are susceptible of a purely mechanical explanation.
 
The second series of objections to the new ‘vitalism’ is
philosophical. Driesch has quoted the authority of Kant and
Aristotle in support of his doctrine. The former is, however,
rather a difficult witness, as Driesch is well aware. He complains, indeed, that Kant’s teleology is descriptive or ‘static ’
rather than ‘dynamic’, as is perfectly true, except in the case
of man, a point of which Driesch naturally makes the most.
There are no doubt passages where Kant speaks of ‘a cause
‘ which brings together the required matter, modifies it, forms it
‘and puts it into its appropriate place ’,2 but against these must
be set the explicit statement that if the body has an alien
principle (the soul) in communion with it, ‘the body must either
‘ be the instrument of the soul—which does not make the soul
‘a whit more comprehensible’3— or be made by the soul, in which
case it would not be corporeal at all. ‘ Vitalism ’ can glean small
comfort from this.
 
Let us turn, then, to the second authority.
 
Aristotle’s matured reflections on the soul (xpvxfi), its nature,
functions, and development, are to be found in the treatises
De Anima and De Generatione Animalium.
 
 
‘ C. M. Child (Biol. Centralbl. xxviii, 1908) has recently published
a similar criticism of Driesch’s absolutely equipotential systems. Child
points out in particular that in the _regeneration of the head of Tubularia
the pro ortionality of the parts d1fi'ers in different regions of the stem,
and un er different conditions, and cannot therefore always be exact.
This §eq1éipotentiality osf gh6e5system is therefore not absolute.
 
 
Soul is defined in the most general way as an activity of a natural organic living body, life being autonomous nutrition
and growth and decay. The activity (évrekéxaa) may, however,
be latent or patent, passive or active, sleeping or waking, without
losing its peculiar characters. This activity is substance (ofmia),
but substance as ‘ form ’, as opposed to the material substance
of the body; the living body is therefore also a substance, in
a double sense.
 
Soul is not, however, identical with the body, but as form,
proportion ()\o'yos‘), activity (e’ve’p~/eta), essence (16 ref 1312 Juan), it
is related to the body, mere matter (z'5)\n) and potentiality (bzfuams),
in just the same way as the seal is related to the wax, and the
body is the instrument whereby it effects its purposes; though
subsequent in time it is prior in thought to the body, as all
activities are to the materials with which they operate.
 
At the same time neither it nor its parts are separable from
the body, with the exception, possibly, of mind (vofis); it is indeed
the actual or possible functioning of the body, like the seeing
of the eye or the cutting of the axe, and with the disappearance
of the capacity of this functioning the soul itself also perishes.‘
Lastly, it is a cause (dpxip xal atria) in a triple sense: first, as
the source of motion, secondly, as that for the sake of which
the body exists, and thirdly, as its essence (ofiafa), or formal
cause.”
 
The soul is of several kinds, which form together an ascending
series, each member of which is necessarily involved in those
above it.“
 
The lowest is the nutritive soul (fipsvrrmf), found in all
living things, and the only soul possessed by plants. It is
defined as motion in respect of nutrition, decay and growth,
processes which involve alteration (&Mo[aoa-is‘) in the body, and
its functions (épya) are to utilize the food’ for the maintenance
and reproduction of the form of the body, and to control and
limit growth.
 
The second is the perceptive soul (aicrflm-uni), the possession of
which distinguihes animals from plants. Perception is a kind of
alteration (dkkoiwcts 119),‘ and consists in being moved and affected.
The fundamental and indispensable perception is touch (écjni),
 
1 De Au. II. 1. ’ Ibid. II.4. 3 Ibid. II. 3,4. ‘ Ibid. II. 5.
 
for it is concerned in the acquisition of the food. It is invariably
present; the others may or may not, some or all, be present.
 
Some animals are also possessed of a capacity of locomotion,
and the performance of this function requires again a special
kind of soul.
 
Lastly, there is the reasoning soul (ecauonnmi), or mind (vofis).
This is found in man alone, unless there be other beings similar
to him, or even nobler than he. Mind alone is eternal and
separable from the body.
 
In all reproduction (except in generafio eguivoca) the startingpoint of a new individual is what Aristotle calls a a--rréppa. In
plants, in which he does not recognize the sexes, this is the
seed; in sexually produced animals it is the result (mfimua) of
the mingling of the male (yomi, or anépua in a narrower sense)
and female elements ; the latter is an egg or, in Mammals, the
catamenia.‘
 
This avréppa, he holds, does not come from all the organs of
the body by a kind of pangenesis, but is a tissue, homogeneous
like bone and flesh, and separated out from the food in its final
stage of digestion, when it is in the form of blood and ready for
assimilatiou, and hereditary resemblance is explained by the fact
that the food which is about to be assimilated by the organs is
naturally like that set aside to form the <r1re'pp.a."
 
In the matter of the o-n-éppza all the parts of the organism that
is to be formed are indeed present potentially, but this means
no more than that the material is there.“ Actually (évepyetq),
they cannot be present until the soul has been developed, and in
particular the soul that is characteristic of animals, the perceptive.‘
Out of this matter the organs are differentiated successively, the
heart first, not only as a matter of observation (on the chick),
but as a. theoretical necessity, since in it is the principle of growth,
then the blood, the blood-vessels, the tissues gathering about
these by a process of condensation and coagulation, the foreparts of the body first, and then the hinder.5 The anéppa, then,
or xiinua, is a material cause of development, but it is also
a cause in other senses ,- it is the eflicient cause, since it must
 
‘ De Gen. I. 18-20. ’ Ibid. I. 18, 19; IV. 1. 39. I 3 Ibid. II. 4.
‘ Ibid. 1. 19; II. 3, 5. ‘ Ibid. II. 6. ,
 
 
contain the source of motion, and it is further the final and the
formal cause. These several causes are not, however, all contributed by both parents. The teaching of Aristotle is that the
matter is provided by the female, and the female alone.‘ The
egg (or catamenia) is described as being matter (z'5)\n), body (a63p.a),
potentiality (bfiuams), passive'(1ra0nru<6v), and merely quantitative,
although it is true that a sort of soul, the nutritive, is somewhat
grudgingly conceded to it, since unfertilized eggs appear in
some sense to be alive.“ The male element, on the other hand,
provides the principle of motion (dpxi; 1-fir xwfio-ems) and the
form (eZ6os) ,- it is qualitative, it is activity, it produces the
perceptive soul, if it is not itself that soul, and it is responsible
for the ‘ correct proportionality ’ (Myos) of the organization.3
 
The male element contributes only motion; it acts upon the
female element as rennet acts when it coagulates milk, except
that the analogy is incomplete, since the yomi brings about
a qualitative, and not merely a quantitative, change in the
material on which it operates.‘ To this it imparts the same
kind of motion which itself possesses, the motion which was
present in the particles of the food in its final form from which
it was itself derived.5
 
The communication of this motion is enough to set going the
machinery (az’;ro';ua1-ov) ; the rest then follows of itself in proper
order.“ To impart this necessary motion is the function of the
nutritive soul, which is primarily associated with the male, only
somewhat doubtfully with the female, element; the perceptive
soul which is, and therefore presumably also imparts, motion of
a kind (dmofwms) is found in the former alone.7 As to the third
kind of soul, mind, Aristotle says little, but it is not introduced
in the male element: it is separable and comes in from outside.
 
Lastly, the sperm of the male acts like a cunning workman
who makes a work of art, using heat and cold as its implements
as the workman uses his tools; 3 for this heat and this cold could
never of theme1ves—by coagulations and condensations——produce
the form of the body, as the older naturalists had supposed,
 
‘ De Gen. I. 20, 21; II. 1, 4. 3 Ibid. II. 5. 3 Ibid. I. 20, 21; II.
1, 4. ‘ Ibid. I. 20; IV. 4. “ Ibid. II. 3. “ Ibid. II. I, 5.
7 Ibid. II. E’); De An. II. 5. ‘ De Gen. II. 4.
 
regarding only the material and eflicient, and ignoring the
formal and the final cause ; for the organic body is not what it
is because it is produced in such and such a fashion, rather it is
because it is to be such and such that it must be developed as it is.‘
And here lies the kernel of the whole matter. For while
Aristotle has made it perfectly plain that according to his idea,
the soul, at least its nutritive and perceptive faculties, is to be
regarded as a function of matter and that this function may be
ultimately expressed in terms of movement, and further that
development is a mechanism which is set going by the communication of motion proceeding from the ‘soul’ of the male element,
and derivable in the last resort from the ‘ motions’ into which
the ‘ functions ’ or ‘ soul ’ of the parent can be resolved, to the mere
matter which the female provides, it is equally evident that he does
not regard this mechanical explanation—in terms of material and
eflicient causes—as satisfactory or complete. But when we
inquire why, he gives us no certain and consistent answer. On
the one hand there are passages in which he tells us that there
must be something which controls the material forces and
imposes upon them a limit and proportionality of growth,” that
the soul makes use of these forces as the artist makes use of
his implements,“ and such passages are naturally interpreted by
Driesch in the sense of a ‘ dynamic ’ teleology ; it is the xpvxrf (not,
of course, voiis, but the two lower kinds) which superintends and
controls, and the \[tux1f is ‘ Entelechy ’. Elsewhere, however,
we are informed that even the proportionality of the developing
parts is simply the outcome of the motion imparted by the male,
which is actu what the female material only is 12otent2'a”.4
Moreover it may be questioned whether Aristotle ever intended
to imply more than an ‘ analogy with the causality of purpose’
when he uses the figure of the workman and his implements to
illustrate his meaning of the formal cause. The formal cause
of a work of art is an intelligible ‘ vera causa ’, it is the idea in
the mind of the artist antecedent to the execution of the work,
but the formal or final cause of an organism, the end which it
apparently strives to attain, is only metaphorically prior in time
to the existence of the organism itself. Prior in thought, how
‘ De Gen. V. 1. 9 De An. II. 4. ’ De Gen. II. 4. ‘ Ibid. II. 1. 44V GENERAL REFLECTIONS AND CONCLUSION S_ 297
 
ever, it certainly is, for it is only the performance of its functions
(iv-rehéxeca) by the organism complete in all its parts that makes
the mere mechanism of development comprehensible to us; the
process, therefore, exists for the sake of the end. Only as eflicient
cause is the soul prior in time ; only so far as it is prior in thought
can it be said to be a final cause.‘
 
Such a teleology is, it is obvious, indistinguishable in principle
from the position in which Kant leaves us. It is the position
adopted by Driesch, as we have seen, in the Analytisc/le T/zeorie, but
abandoned in the Vitalismue in favour of a theory of ‘pyschoids ’.
 
Now, quite apart from the meaning which Aristotle may or
may not have intended to convey, there appear to be grave
objections to this belief.
 
This ‘ psychoid ’, to which the name ‘ Entelechy’ is surely
misapplied, this rudimentary feeling and- willing, which is aware
of the form it desires to produce, must be, psychically, at least
as complex as the phenomena it is designed to account for, and
stand, therefore, as much in need of explanation as they, which
will involve us at once in an infinite series of such entities. In
fact, to borrow the epithet which Driesch himself has bestowed
on the nuclear architecture imagined in the Roux-Weismann
hypothesis, it is only a photograph of the problem, and not
a solution at all. Again, when we ask what the modus opcramli
of this cause is we get no reply either from Driesch or from any
other neo-vitalist, though this is just the knowledge that we so
urgently stand in need of. The objection that the intervention
of a psychical cause in a physical process is unintelligible, an
objection which would probably appeal to many, may be waived,
for in the last resort the connexion between any——even simply
mechanical—-causes and efiects is equally hard to understand.
It may, however, be seriously doubted whether these entities are
not being ‘ multiplied beyond necessity ’, and whether the progress
of science would not be better served by an adherence to a simpler
philosophy.
 
‘Vere scire est per causas scire.’ The maxim of the great
founder of modern inductive science is the watchword of
embryologists to-day. By exact observation and crucial experi
‘ De Part. II. 1. 7.
 
ment, utilizing every canon of induction, the facts of development
are to be brought under wide general laws of causation, which
will be in the first instance physiological laws—of response to
stimuli, of metabolism, and of growth: by means of these laws
we can predict, and our predictions can be verified. The
thought process cannot, however, rest here. Ultimately—-—as we
believe—it may be possible, no more than that can yet be said,
but it may be possible to state the widest generalizations of
biology in chemical and physical, and these again in purely
mechanical, terms. Thus evolution of form in the individual as
well as the larger evolution of form in the race, become but the
final terms in a far vaster cosmic progress, from ‘ homogeneity
to heterogeneity ’.
 
The idea, of course, is perfectly familiar ; it is the analysis of
purely physical causes carried to its extremest limit. Phenomena
are thought out in terms not of origins merely but of one origin,
and that one origin is the only mystery that remains. This
unification of the sciences always has been, and must still remain,
the dream and the faith and the inspiration of the scientific man,
and could such an edifice of the intellect ever be realized the
task of science would have been completed.
 
But where science leaves off there philosophy begins, and it
is for philosophy to attempt the solution of this last mystery
of all.
 
Philosophy cannot rest content in an endless regress of cause
and efiect, and a first supreme cause, first in time that is, is
metaphysically out of the question. An original homogeneity
is equally unthinkable, for out of a system all whose parts are
absolutely alike, by no imaginable process could any heterogeneity ever be evolved.
 
That first simplicity must have contained potentici all that has
since developed out of it, it must have possessed a structure, an
arrangement of parts such that the end which it has realized, or is
to realize, would be what it is or will be, and to regard the end as
well as the beginning is the duty of philosophy, a duty which
Aristotle and Kant have both impressed on us. The outlook of
pure science, an outlook to which, gm? science, it cannot too
rigidly confine itself, is thus supplemented and enlarged.
V GENERAL REFLECTIONS AND CONCLUSIONS 299
 
Knowledge through material and eflicient causes is rounded
into a whole through a knowledge of the final cause, which, in
the last analysis, is just as much a rem cauea as they are; for in
our total ignorance of what constitutes the invariable connexion
we observe between antecedent and consequent it is as true that
the second causes the first as the converse. Only because of the
inherent desire of the human mind to predict from the past,
which is known, to the future, which is not, have we come in
ordinary usage to restrict the term to the antecedent.
 
The ultimate end of the human or any other race we cannot
tell, the ultimate end of the universe we cannot tell, any more
than we can imagine its absolute beginning, unless we find these
ends in the freedom of the moral consciousness of man. But
the end of an organism, the production of specific form and
the maintenance of that form which itself has produced, does
seem self-evident and plain, though we must never forget that
form is variable and subject to change, that species are not
immortal any more than individuals, and that the effort to achieve
that form does not invariably succeed.
 
Here, however, we touch on the fringe of a problem—the
problem of evil—too large to be discussed in this place.
Putting this aside, a purposiveness is an unmistakable characteristic of the functions of living things, of the production and
preservation of form, a characteristic which still remains when
those functions have been expressed in terms of the chemistry
of the proteids. It is only, however, by a remote analogy with
our own ‘causality according to purposes’ that we can speak
of organic functions as purposive ,- it is only as  they were
guided and controlled by an intelligence; their purposiveness is
indeed only the expression of our inability to comprehend their
beginnings except in terms of their ends; it is relative to us,
though not, therefore, any the less real.
 
Biology, then, although built upon the ultimate conceptions
of chemistry and physics, has yet peculiar features of its own.
Its relation, indeed, to these lower sciences is just what their
relation is to one another. A survey of the whole hierarchy
displays to our view a series in ascending order of complexity;
each member of this series has its own ultimate conceptions, the
300 DRIESCH’S THEORIES OF DEVELOPMENT V
 
most general expression for the facts with which it deals, but
the ultimate conceptions of each, as in Aristotle’s series of ‘ souls ’,
are necessarily involved in the one next above, while conversely
each endeavours to translate its own ultimates into those of the
science below: a translation, however, which, be it never
forgotten, leaves the reality of the original undestroyed.
 
Thus, mechanics expresses molar motions in terms of pure
numbers, physics explains forms of energy-—heat and light and
electricity—as the motions of molecules; chemical affinity is to
be reduced to the mutual attractions of intramolecular atoms; purposive responses to stimuli may be stated in terms of chemical
reaction, and the psychical phenomena of mental and moral
science—understanding and feeling and will—are a form of these.
 
The cosmic process thus takes place in a succession of stages,
and the peculiar features which mark each individual stage
are simply the outcome of an increase in complexity of the
peculiarities characteristic of the stage below. To establish this
is the final achievement of science.
 
Nevertheless, the facts with which each science starts, the
facts which come first in the order in which knowledge is
acquired, do not become wholly merged in those simpler facts
into which they are at each stage translated ,- when the translation
has been accomplished the original still remains.
 
The ‘secondary qualities’, as well as the other properties of
those bodies whose behaviour the physicist investigates, are
as real as, no more and no less relative to our intelligence than,
those ‘primary qualities ’ of impenetrability and extensionexhibited by those bodies whose behaviour forms the subjectmatter of mechanical science—into which it is his endeavour to
translate them, just as the primary qualities themselves obstinately refuse to be reduced to mere number. Chemical affinity
remains as a phenomenon sui generis after it has been reduced to
the operations of intramolecular forces, and the purposiveness
of responses to stimuli is something over and above the chemical
reactions to which they are rightly referred. Last of all, the
final term in the series, "the mental and moral consciousness,
the ‘other side’ of certain purely physical functionings of the
organism, is as real as any’ of those qualities of matter which
V GENERAL REFLECTIONS AND CONCLUSIONS 301
 
have been step by step involved in its evolution. In a word, the
increase in the complexity- of the phenomena which marks the
transition from each of these stages to the next is itself a new
phenomenon and cannot be ignored.
 
And herein we may perhaps discover the essence of the
relation between ‘mind’ and ‘ matter’, whether in ordinary
function or in development. The mind is not matter, not even
living matter; rather it is the new quality constituted by an
increase in the complexity of living matter, immaterial and as
distinct from that matter as is ‘ blueness’ from vibration of
a certain wave-length. Dependent on and inseparable from
matter, however, it is; when that matter, whether in the
individual or in the race, attains a certain degree of complexity,
then and then only does mind appear; and with the disappearance of that complexity it perishes.
 
While, therefore, we have no reason for supposing that the
mind ‘ comes in from outside ’, we are at the same time saved from
that somewhat extravagant ‘ psycho-physical parallelism ’ which,
to explain the evolution of consciousness, postulates a complete
psychical, accompanying the complete physical series of causes
and eifects, and credits, of necessity, the merest matter with the
rudiments of feeling, thought and will. On the other hand,
if the mind cannot be said to intervene in the physical series,
there seems to be no alternative but to suppose that the operations
of the one, when they exist, are parallel to those of the other.
 
Such a system of philosophy as that which we have here
ventured to suggest can give no countenance to a Vitalism
which interpolates an unnecessary psychical element into the
complete causal chain of physical events. But it is not for that
reason to be condemned as materialistic ; for the mind, developed
out of and conditioned by matter, the last term and final cause
of the whole process, is not itself matter but an accompaniment
of certain material complexes, and still remains when they have
been resolved into simple mechanical expressions. And in this
mind, last in time but first in thought, a larger philosophy will
perceive not only the end towards which, in time and space,
matter strives, but the Undertanding which, itself eternal, imposes
the forms pf space and time upon that Nature which it makes.
 
LITERATURE
 
ARISTOTLE. De Generations Animalium, ed. Bekker, Oxford, 1837.
ARISTOTLE. De Partibus Animalium, ed. Bekker, Oxford, 1837.
Anrsrorm. De Anima, ed. Trendelenburg, Berlin, 1877.
 
H. Dnnzscn. Analytiache Theorie der organischen Entwicklung,
Leipzig, 1894.
 
H. Dawson. Entwicklungsmechaniache Studien. X. Ueber einige
allgemeine entwicklungsmechanische Ergebnisse, Mitt. Zool. Stat. Neapel,
xi, 1895.
 
H. DRIESCH. Resultate und Probleme der Entwicklungsphysiologie
der Tiere, Anat. Hefte, 2“ Abt., viii, 1898.
 
H. DRIESCH. Die Lokalisation morphogenetischer Vorgiinge: ein
Beweia vitalistischen Geschehens, Arch. Ent. Mech. viii, 1899.
 
H. DBIESCH. Der Vitalismus a.ls Geschichte und ale Lehre, Leipzig,
1905.
 
I. KANT. Ktitik der Urteilskmft, Eng. Trans. by J. H. Bernard,
London, 1892.
 
C. VON NAGELI. Mechanisch-physio]ogische Theorie der Abst:uumungslehre, Mfinchen und Leipzig, 1884.
 
Gr. PLATNER. Kern und Protopla.sma., Breslau, 1887.
 
Review in Jahresber. yes. Medicin, xxii, 1888.
 
W. Roux. Zu H. D1-iesch‘s ‘Analytischer Theorie der orgzmischen
Entwicklung’, Arch. Ent. Mech. iv, 1897.
 
 
==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==

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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.


Cite this page: Hill, M.A. (2024, June 26) Embryology Book - Experimental Embryology (1909). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Experimental_Embryology_(1909)

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