i SEGMENTATION 17 



greater richness in protoplasm. This layer shows no division into 

 cells and is therefore termed the yolk - syncytium 1 (H. Virchow : 

 Eiickert's term " merocytes " is synonymous). The marginal 

 portion round the edge of the blastoderm is sometimes termed the 

 germ-wall. 



Functionally the yolk-syncytium is apparently concerned especi- 

 ally with the digestion and assimilation of the yolk. Scattered 

 about in it are nuclei, often of enormous size and irregular form. 

 Concerning the origin and fate of these nuclei much discussion 

 has raged and the matter cannot yet be regarded as satisfac- 

 torily settled. The question is complicated by the fact that, 

 as shown by Elickert (1890), polyspermy appears to be a normal 

 occurrence in Elasmobranchs. In addition to the single micro- 

 gamete which takes part in the formation of the zygote-nucleus a 

 variable number of extra spermatozoa make their way into the egg 

 and give rise to accessory sperm-nuclei. Where such sperm-nuclei 

 are situated in the coarse yolk they apparently soon degenerate but 

 when, on the other hand, they are within the protoplasm of the 

 germ-disc they remain during the early stages of development in a 

 living and apparently healthy condition, even undergoing mitosis 

 synchronously with the nuclei derived from the zygote-nucleus up to 

 the fourth or even fifth or sixth division in the case of Torpedo. 



The importance of this fact should be noted in connexion with 

 our ideas of the reciprocal physiological relations of nucleus and 

 cytoplasm. It is fully recognized that the nucleus governs and 

 controls cell metabolism : it is not always so fully recognized that 

 conversely the cytoplasm exerts an important influence over the 

 nucleus. Clearly the fact that the accessory sperm-nuclei "keep 

 step " in their mitotic divisions with the embryonic nuclei must be 

 due to some influence exerted on the former nuclei through the cyto- 

 plasm. It should, in fact, never be forgotten that cytoplasm and 

 nucleus are merely locally specialized portions of the same common 

 living substance or protoplasm. 



At first the accessory sperm-nuclei are clearly distinguishable in 

 the germinal disc from the true embryonic nuclei by their smaller 

 size and reduced (haploid) number of chromosomes. After the 

 zygote-nucleus has undergone two mitoses however — or even before 

 the second mitosis — the accessory sperm-nuclei wander — or become 

 transported by cytoplasmic movements— outside the limits of the 

 germ-disc. They continue their mitotic rhythm for a time so that, 

 for example, at the 8-nuclear stage of the blastoderm they may be 

 seen in groups of eight lying in the yolk-syncytium. During early 

 stages of segmentation numerous such obviously accessory sperm- 

 nuclei may be seen in the syncytium but as time goes on the nuclei 



1 Although Haeckel originally defined the term syncytium (Die Kalkschwtimme, 

 Bd. I. p. 161) as a protoplasmic mass formed by the fusion of originally separate cells 

 the word has come into such general use for a multinucleate mass of protoplasm which 

 shows no subdivision into cells, whatever its origin may have been, that there seems 

 no serious objection to the use of the term yolk-syncytium as suggested by Virchow. 

 VOL. II C 



