Page:EB1922 - Volume 30.djvu/1027

Rh

incorporated in all these " segmentation " nuclei. Now it is common knowledge that the influence of the father is as potent as that of the mother in heredity and therefore there must arrive a period of development at which the nuclei again influence the cytoplasm.

An attempt to determine this period was made by the writer 1 by fertilizing the eggs of Echinocardium with the sperm of Echinus. As we have seen, the result of this cross is in most cases to produce cytolysis of the egg. but in a minority of cases a hybrid develops.

The egg of Echinocardium is oval whereas that of Echinus is spherical and the shape of the blastula of each species follows that of the egg. The blastula of the hybrid is oval, like the maternal blastula, and the gastrula is also like that of Echinocardium. But the typical larva (the four-armed echinopluteus) resembles in several points the larva of Echinus; in the vast majority of cases it is totally devoid of a large aboral club supported by a special skeleton which is characteristic of the larva of Echinocardium. It is clear therefore that at this stage the paternal nucleus is influencing the structure of the organism. When the eggs of Echinus are fertilized with sperm of a still more divergent character, such as that of the crinoid Antedon, a hybrid occasionally develops as far as the gastrula stage, but it always resembles the larva developed from the normally fertilized egg in every detail and shows no trace of paternal influence.

Nuclei and Cytoplasm. We are thus led to the conception of an intermittent action of the nuclei on the cytoplasm, and in this it seems as if we had reached the deepest point to which analysis of development will lead us. Perhaps it would be more accurate to speak of an intermittent reaction between cytoplasm and nucleus, for in some embryos there is evidence that the nuclei undergo al- teration as development proceeds. It is on cases like these that Weismann's 2 theory of development was founded. According to this theory, as growth proceeds, differential division of the nuclei takes place, some becoming specialized as ectodermal nuclei, others as endodermal nuclei and so on, whilst some retain the constitution of the original zygote nucleus; these last give rise by division to others like themselves which eventually engender the nuclei of the germ cells. The lineage or line of descent leading from these germ- cell nuclei back to their ancestors amongst the nuclei of the first blastomeres is termed the " germ-track." Now in the Nematode worm Ascaris megalocephala the zygote nucleus contains only four chromosomes, but as the egg divides into blastomeres, the nucleus of one blastomere after another undergoes the change termed diminution of the chromalin. This change involves the nipping-off of the ends of the chromosomes, and these portions are ejected into the cytoplasm and are absorbed ; the remainder of each chromosome becomes fragmented into a large number of minute granules. These granules act as chromosomes in the next nuclear division. The nucleus of one blastomere remains exempt from this change and this blastomere eventually gives rise to the genital organs.

Boveri 3 has shown that the fact that one nucleus undergoes diminution of the chromatin whilst another does not is not the consequence of a differential division of the mother nucleus of them both, but is due to the fact that one nucleus takes up its position in a region occupied by a particular cytoplasmic substance. This he proves in two ways, viz. (i) by considering the case of eggs fertilized by two spermatozoa, and (2) by the results obtained by subjecting eggs about to segment to the action of strong centrifugal force.

In doubly fertilized eggs the extra spermatozoon forms an in- dependent nucleus whilst the other fuses with the female pronucleus to form the zygote nucleus. The first division of the egg results in the formation of four nuclei and four blastomeres. In the develop- ment of the normally fertilized egg one of the two first nuclei under- goes diminution, and the cell containing it gives rise to a large part of the dorsal ectoderm; the other nucleus remains undiminished and amongst the progeny of the cell containing it are found the genital cells. Now amongst the four cells produced by the division of the doubly fertilized egg, three may contain nuclei which undergo diminution, and one may remain undiminished in such cases the egg develops into a single embryo with an unusually abundant ectoderm. In other cases only two of the nuclei undergo diminution such eggs form twin embryos of normal aspect ; whereas in still other cases one nucleus alone may undergo diminution and in these cases a monstrous triple embryo is formed. These differences are accounted for on the assumption that one region of the egg contains a substance which induces diminution and one, two or three nuclei of the doubly fertilized egg may be in it.

When eggs about to segment are exposed to the action of long- continued and intense centrifugal force the plane separating the first two blastomeres will in some cases be found to lie along a radius of the circle of rotation, and in these cases a small mass of material will be found to be ejected from the egg which then becomes divided

1 E. W. MacBride, " Studies on the Development of Echinoidea," (II) " The early larva of Echinocardium cordatum and the result of crossing this species with Echinus esenlentus," Qitarterly Journ. Micr. Science, vol. Iviii., 1912.

2 A. Weismann, The Germ-Plasm. A Theory of Heredity (1893). 8 Th. Boveri, " Die Potenzen der Ascaris-Blastomeren bei

abgeanderter Furchung," Festschrift zum 60 ten Geburtstag Richard Hertwigs, vol. iii., No. 8, 1910.

into two appreciably equal and similar blastomeres, the nucleus of neither of which undergoes reduction. This suppression of reduction must be attributed to the even distribution of the cytoplasmic materials under the stress of the centrifugal force, so that no region of the egg contains more of the peculiar substance than any other. Diminution of the chromatin apparently results from the action of an excess of this substance on any nucleus contained in it.

Regeneration. In the phenomena of regeneration and of bud- ding we meet with evidence of the renewed influence of the nuclei in causing the formation of cytoplasmic substances.

We have already learnt that when one of the first two blastomeres into which a frog's egg divides is killed the survivor frequently develops into a half gastrula which may even grow into a half tad- pole. Roux, 4 however, has shown that if this half tadpole survives it becomes a whole tadpole by what he calls the " post-generation " of the missing half. This is effected by the multiplication of the cells lying at the- edges of the half embryo. The nuclei increase in number and confer on the cytoplasm in their neighbourhood new powers. In this case it might be objected that each kind of tissue in the old half gives origin only to the same kind of tissue in the new half. But Morgan 6 has shown that if the head (including the pharynx) of the annelid Nereis be cut off, a new head with pharynx will be regenerated from the stump ; whereas, however, the original pharynx was formed by an intucking of ectoderm, the new pharynx is formed by an outgrowth from the endodermal tube in the stump. The new powers thus conferred on the cytoplasm of the endodermal gut can only be explained as the result of the calling-forth of new potentialities in the nuclei lying in the cut edge. More remarkable evidence still has cropped up in connexion with the regeneration of the lens of the eye of the newt. In the embryo the lens is formed as a thickening of the ectoderm on the side of the head. But if the original lens be torn out, a new lens is developed either from the edge of the iris or of the retina tissues that have no connexion with the skin of the head. Some try to meet this difficulty by the phrase that in these cases the organism acts as a whole, independently of the germ- layers into which we analyse it. But what meaning can be attached to this phrase, except that the organism under different circumstances uses different means in order to effect a restoration of its integrity, it would be difficult to say. In fact we approach very closely to the celebrated " entelechy " 6 of Driesch; that is an indwelling " some- thing " in an organism which strives to realize a purpose.

Vitalism, and the Theory of an Entelechy. It may be argued that such an idea is unscientific, because it introduces " vital force " and similar mystical ideas amongst our biological conceptions. It may be answered that in the last resort all explanation is comparison, and that those who reject vitalism seek to compare all the activities of living beings to phenomena which go on outside the body in test- tubes. But this is equivalent to referring all the phenomena of life to structure, in other words the juxtaposition of definite chem- ical substances in a definite spatial arrangement; in regeneration, however, we encounter phenomena where structure appears to be irrelevant. If we are to do justice to such phenomena we must have some working hypothesis similar to that of Driesch. Whether the assumption of an " entelechy " is better or worse than the statement that all the nuclei in the body are totipotent and that varying potentialities are called forth seems to be a matter of taste.

Budding. Regeneration is in many respects akin to budding, since buds in many cases may be regarded as portions of the mother organism restored after natural amputations.

In the growth of buds we often meet with a wide divergence between the materials used to build up certain organs, and those used to construct similar organs in the embryo. To give an example the bud of the ascidian Botryllus begins its existence as a little two-layered vesicle very similar to the gastrula of the same species. But in the embryo the central nervous system is developed from the outer layer as it is in all other Vertebrata. In the bud, on the contrary, it is formed as an outgrowth from the inner layer. Hjort, 7 who described this phenomenon, suggested as the explanation for it the fact that the outer layer of the bud is an outgrowth of the adult maternal ectoderm, which is specialized for the secretion of the cellulose " mantle " and not sufficiently plastic to be turned into nervous tissue. This is only another way of saying that the forma- tive nuclei act differently in different cases and distribute the organ- forming cytoplasmic substances in a different manner in the bud from their arrangement in the egg.

One or Two Embryos. The primary organs, i.e. the germ- layers, are the material out of which the higher organs are built up, and one of the most remarkable of recent discoveries in embryology is the fact that the question of whether this material

4 Roux, loc. cit.

6 Morgan, Regeneration (1901).

6 H. Driesch, Zwei Vortrdge zur Naturphilosophie (1910) ; see also Gifford lectures for 1907 and 1908.

7 Hiort, " Germ-layer Studies based on the Development of Ascidians," Zoo/. Results Norwegian N. Atlantic Exped. (1896).