Chapter VI: Part I (3)

In several publications I have shown clearly that the external form of an egg and the arrangement of its contents, according to the different specific gravities of the component particles, determine the position of the nucleus and of the successive planes of division. Similarly, the different sizes of the cells first formed and the unequal rate of division shown at the two poles of the egg depend upon the constitution of the yolk, upon the cleavage of the yolk into a portion richer in protoplasm and a portion poorer in protoplasm, and upon the differences in the bulk of protoplasm that in this way reaches each of the first-formed cells.

In many cases it has been shown that there is a constant relation between the first three cleavage-planes of the egg and the long axis of the animal that arises from the egg. Weismann and Roux make this a proof that, in nuclear division, the nuclei that arise have different qualities; that the protoplasmic masses lying to the right and left of the median plane are set apart to build up the right and left halves of the embryo; that, similarly, the first transverse and horizontal cleavage-planes divide the protoplasm of the egg into pieces predetermined for the formation of the anterior and posterior, dorsal and ventral, parts of the embryo.

But I think I have shown beyond possibility of doubt that these events are due not to the existence of special, mysteriously working groups of determinants within the nucleus, but merely to the specific shape of the whole egg and to the segregation of the yolk. It is self-evident that, as the body of the embryo builds itself up from the actual material of the egg, the way in which the material of the egg is disposed must be of great influence upon the formation of the shape of the embryo. And so, in a recently published work, I stated that the growing embryo, especially in its early stages, must conform in many ways to the shape of the fertilised egg.

Thus, to bear out what I have been saying by actual examples, the distribution of the actual particles of the fertilised egg must correspond to the disposition of the bulk of material in the blastosphere; for, in the breaking up into cells, the spacial arrangement of the substances of different weights undergoes no change. Thus, amphibia, the eggs of which have the poles different in character, produce blastospheres the poles of which are unlike; while eggs, like those of the fowl, where the yolk does not divide, give rise to blastospheres with unsegmented yolk. In such cases the more or less complete segregation of the yolk and gravity, which causes a separation of the contents of the egg according to the weights of the particles, are agencies determining the particular kind of development. It is no case of special groups of determinants within the nucleus.

Thus, an oval and an elongate egg produce respectively an oval and an elongate blastosphere. The blastosphere determines the orientation of the gastrula, and so forth. In fact, the original distribution of mass in the material of the egg is carried directly on to the following stages of development (oval eggs of triton, insects, etc.).

So, finally, in many eggs, where, in addition to a polar differentiation, there is also a bilateral symmetry in the distribution of substances of different specific gravities and of different physiological value, the resulting blastospheres, from the reasons given above, assume a bilaterally symmetrical form.

Although, then, in eggs with polar differentiation, which have either one axis longer or are bilaterally symmetrical, under normal conditions the planes of the first two segmentations may correspond to the principal axes of the future embryo, the cause for this agreement lies in the structure of the egg, and is not to be looked for, as Roux and Weismann suppose, in differentiating processes of cleavage, undergone by the nuclei in their first divisions. It is in this way that there are to be explained the investigations made by Van Beneden and Juelin upon the eggs of ascidians, by Wilson upon the egg of _Nereis_, by Roux upon the egg of _Rana esculenta_, and by me on the egg of _Triton_.

As it fails with the process of cleavage, so Weismann's doctrine of determinants fails when we analyse the formation of the blastosphere, the gastrula, and the germinal layers.

The formation of the blastosphere seems to me to be due to the co-operation of the following processes:

(1) In the division of the egg-cell cavities arise between the four, eight, and sixteen pieces, and thus the whole contents of the egg become arranged more loosely. (2) The more the cells multiply by division and become smaller in circumference, the more closely they apply their lateral surfaces to each other, especially at the outer surface of the whole, so assuming the arrangement of cell-epithelia. (3) By the secretion of fluid, a constantly growing central cavity is formed _pari passu_ with the approximation of the superficial cells, and this probably also brings with it an increase of the internal pressure, and a wider curvature of the wall of the sphere.

Now, is there any part of these processes that has to do with the breaking of the nuclear contents into groups of determinants with different qualities? By no means. The egg divides into many pieces, because such division is a general property of cells, and it is not associated with separate, special material bearers. The appearance of spaces between the cells, resulting from division, is due to forces some of which reside within the single cells, some of which come from without. In especial, the assumption of a spherical shape--an assumption occurring also to a greater or less degree when the results of division leave each other--is caused by the yolk actively arranging itself round the two nuclei as centres of attraction. The attempt to become spherical is opposed by other forces, in accordance with which the cells resulting from division press against each other. These forces that press the cells together seem to increase, as the size of the cells diminishes, so that the cells approximate their lateral faces continually more closely. The secretion of fluid into the interior of the sphere and the resulting increase of the outer surface results from the characters of the whole wall, and cannot be explained by single, specially determined cells.

Finally, to take the case of the special kinds of blastospheres (_e.g._, of amphioxus, amphibia, reptiles, birds, and so forth), it has been already shown that these are produced by the shape of the egg, by the bulk of the yolk, and by the segregation of the yolk-particles under the influence of gravity; that, in fact, the shapes are determined by the general gross conditions of the structure of the egg.

Plainly, the blastosphere cannot be pre-existing as a structure of particles in the fertilised nucleus; there cannot be blastosphere determinants. The conditions for the origin of the blastosphere come into existence only by the process of segmentation, and it is only by its capacity to divide that the egg contains the conditions for blastosphere formation. Here we have epigenesis--the appearance of a new formation, not the becoming visible of pre-existing complexity.

The conditions of gastrulation and of the formation of the germinal layers are similar. The invagination of the blastosphere comes about by the co-operation of all the cells of its wall, by local differences in the rates of growth in that wall, from dissimilarities in its curvature, from many causes which have not yet been sufficiently sought out and investigated. As cell division itself depends not upon special particles, but upon changes in the entire nuclear contents, it follows that the growth of the blastosphere-wall, which is merely the sum of the growth of all the cells in it, cannot be determined by special groups of determinants.

As an attempt to explain gastrulation, the origin of the germinal layers and many other events of development, the doctrine of determinants has reversed cause and effect. Certain cells do not become invaginated into the segmentation cavity because they possess groups of determinants that impel them to the assumption of inner layer characters. The reverse is the truth. Local conditions of growth cause the invagination of a set of the cells of the blastosphere-wall. This invaginated layer of cells, brought into a new position with regard to its environment, becomes the endoderm and receives the stimulus to assume the character appropriate to the new environment. It is unlogical to speak of endoderm in the fashion of many textbooks and treatises on embryology, while the so-called endoderm cells still form part of the outer surface of the blastosphere, or even while they are still in process of formation by cleavage. For 'inner germinal layer' implies a condition of position which is created by the invagination.

In fact, it is impossible, in thinking of the gastrula as in thinking of the blastosphere, to conceive that in the egg, which is a simple cell, there can be preformed by material particles in the nucleus a condition which implies the existence of two layers of cells.

Thus analysis of a special case leads to the same conclusion as is reached by the general reasoning of the earlier part of this section.

FOOTNOTES:

[7] _The Germplasm_, pp. 68, 69.

[8] The following treatises contain criticisms of Weismann's theories: W. Haacke, _Gestaltung und Vererbung_; Leipzig, 1893; Herbert Spencer, articles in _Contemporary Review_ (1893-94); Romanes, _An Examination of Weismannism_; Longmans, 1893.

[9] Notwithstanding the objections raised by Bergh, Verworn, and Haacke, I abide by the supposition that the nucleus of reproductive cells contains the hereditary mass or germinal material. My reasons may be found in my text-book on _The Cell_ (English edit., p. 274). Briefly they are: 1. The equivalence of the male and female hereditary masses. 2. The equal distribution of the growing nuclear mass of the primary egg-cell among the daughter-cells that, arising from it, build up the organism. 3. The preservation of a constancy of bulk of the hereditary mass when fertilization occurs. 4. The isotropism of protoplasm. Following Pflueger, I mean by isotropism that the protoplasm of the egg does not contain local areas for the formation of different organs; but that, according to the conditions, any part of the protoplasm may be employed in the formation of any organ. Isotropism is merely the negation of His' doctrine of the presence of local areas for definite organs, and without losing its meaning, is compatible with the fact that many eggs have their poles different, and that others have a bilateral symmetry which determines the plane of the first division. 5. The fact that the first stages of many embryonic developments consist in the multiplication of the nuclear material and its distribution in the yolk, following which the yolk-mass cleaves into cells.

[10] English edition, p. 32.

[11] English edition, p. 34.

[12] In this section upon heteromorphosis I rely upon the following treatises, which have appeared recently. Loeb, _Untersuchungen zur physiologischen Morphologie der Thiere. Organbildung und Wachsthum_. Heft, 1 and 2 (1891-1892). H. de Vries, _Intracellulare Pangenesis_ (1889). H. Driesch, _Entwicklungsmechamische Studien_, i.-vi.; _Zeitschrift f. wissenschaft, Zool._, vol. liii.-lv. The same, _Zur Theorie der thierischen Formbildung._ _Biol. Centralblatt_, vol. xiii., 1893. Chabry, _Contribution a l'embryologie normale et teratologique des Ascidies simples. Jour. de l'Anat. et de Physiol._ (1887). Wilson, _Amphioxus and the Mosaic Theory. Journal of Morph._ (1893). See also _Anatomischer Anzeiger_ (1892).

[13] Roux tried to give experimental evidence in favour of his mosaic theory in a treatise _On the Artificial Productions of Half-Embryos by the Destruction of one of the first two Cleavage-Cells, and on the Reconstruction of the Lost Parts_. _Virchow's Archiv._, vol. cxiv., 1888. Roux defends his mosaic theory against Driesch and myself in (1) _Ueber das entwicklungsmechanische Vermoegen jeder der beiden ersten Furchungszellen des Eies. Verhandl. der Anat. Gesellsch. der 6'ten Versamml. in Wien_, 1892. (2) _Ueber Mosaikarbeit und neuere Entwicklungshypothesen._ Anatomische Hefte von Merkel und Bonnet (1893). Also in _Biol. Centralblatt_ (1893); in the _Anatom. Anzeiger_ (1893), and in the treatise _Die Methoden zur Erzeugung halber Froschembryonen und zum Nachweis der Beziehung der ersten Furchungsebenen des Froscheies zur Medianebene des Embryo. Anatom. Anzeiger._ (1894); Nos. 8 and 9.

If, as would appear from the last treatise, Roux would avoid being reckoned with evolutionists, he must abandon his mosaic theory, and this he has not done. I think in the present essay, on theoretical and experimental grounds I have shown the untenability of Roux's mosaic theory.

[14] The terms vertical and horizontal refer to the vertical axis of the egg, which passes through the animal and vegetative poles.--_Translator's note._

[15] Further details concerning these experiments may be found in HERTWIG, _Ueber den Werth der ersten Furchungszellen fuer die Organbildung des Embryo_. Experimentelle Studien am Froschund Tritonei. _Archiv. fuer Mikrosk. Anatomie_, vol. xlii., 1893, p. 710; Plate xli.; Figs. 1, 2, 27.

[16] For the facts in this section I rely in particular upon the writings of Voechting, Bert, Ollier, Trembley, Landois, Ponfick, and others:

H. VOeCHTING: _Ueber Transplantation auf Pflanzenkoerper_. _Untersuchungen zur Physiologie und Pathologie_; Tuebingen, 1892.

VON GAeRTNER: _Versuche und Beobachtungen ueber die Bastarderzeugung im Pflanzenreich_, 1849.

LEOPOLD OLLIER: _Recherches experimentales sur la production artificielle des os au moyen de la transplantation du perioste, etc._ _Journal de la physiologie de l'homme et des animaux_, tom. ii., 1859, pp. 1, 169, 468.

LEOPOLD OLLIER: _Recherches experimentales sur les greffes osseuses_. The same, tom. iii., p. 88, 1860.

PAUL BERT: _Recherches experimentales pour servir a l'histoire de la vitalite propre des tissus animaux_. _Annales des Sciences naturelles, Ser. V., Zoologie_, tom. v., 1886.

VON RECKLINGHAUSEN: _Die Wiedererzeugung (Regeneration) und die Ueberpflanzung (Transplantation)_. _Handbuch d. Allgem. Pathologie des Kreislaufs aus Deutsche Chirurgie_, 1883.

TREMBLEY: _Memoires pour servir a l'histoire d'un genre de Polypes d'eau douce_, 1744.

LANDOIS: _Die Transfusion des Blutes_; Leipzig, 1875.

ADOLF SCHMITT: _Ueber Osteoplastik in klinischer und experimenteller Beziehung_. _Arbeiten aus der chirurgischenklinik der Koenigl. Universitaet, Berlin._

PONFICK: _Experimentelle Beatraege zur Lehre von der Transfusion_. _Virchow's Archiv._, vol. lxii.

BERESOWSEY: _Ueber die histologischen Vorgaenge bei der Transplantation von Hautstuecken auf Thiere einer anderen Species_. _Ziegler's Beitraege zur pathologischen Anatomie und zur allgemeinen Pathologie_; Jena, 1893.