16 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



inately about this period individual nuclei lair behind others and the 

 regular rhythm becomes lost. 



This rhythm of nuclear division is of interest in relation to the 

 size of the individual blastomeres. It is often noticeable in an 

 Elasmobranch blastoderm that the blastomeres are somewhat smaller 

 in what is shown by later development to be its posterior half i.e. 

 the half next the side on which the embryonic rudiment makes its 

 appearance later. It would be natural to suppose that the smaller 

 size of the blastomeres is due to their having gone through a greater 

 number of divisions but this explanation is rendered less satisfac- 

 tory by the synchronism of the mitotic divisions. Apparently the 

 inequality is at least to some extent due to the zygote nucleus, and, 

 later on, the first segmentation furrows, being not quite central in 

 position in the germinal disc but situated slightly towards its 

 posterior edge (Riickert). 



The stage up to which mitosis remains synchronous varies 

 amongst individuals of one species and a fortiori amongst those of 

 different species and genera. Thus in Pristiurus it is, .commonly, 

 regular only up to the fifth mitosis according to Riickert. 



While segmentation has been proceeding, important changes have 

 been taking place in the segmentation cavity. About the time of 

 the seventh division the rounded inner blastomeres fill up most of 

 the cavity so that it becomes reduced to chinks between the individual 

 blastomeres. These chinks are filled with fluid secreted by the egg 

 substance, and in the yolk beneath the blastoderm the activity 

 of this process of secretion is indicated by the appearance of fluid 

 vacuoles. 



As development goes on the amount of fluid increases greatly and 

 about the tenth division it begins to collect especially between the 

 blastoderm and the yolk, forming the "germ cavity" of Riickert 

 (Fig. 9, D, E, F). This cavity is best marked towards the posterior 

 side of the blastoderm and in ground-plan is crescentic in shape. 

 It varies greatly in its degree of development in different 

 individuals. 



Whether it is advisable to use a separate name for this cavity is 



\. -i v questionable. When a broad view is taken of the relations of 



blastomeres and segnientat inn cavity in the Elasmobranchs these 



seem t< be similar in kind to those which hold in the case of the 



LIIIILT fishes. In these fishes, as will he shown later, the blasto- 



whieh originally formed the floor of the segmentation 



cavity become later on shifted in position towards its roof hut the 



leMillant change in the I opo-raphical relat ions and form of the 



it y would clearly alf<.rd no valid reason for ^ivin- 



new name. 



The Yolk Syncytium. The layer of substance immediately 



underlying tin- blastoderm and segmentation- ,,r ^erm-ca vily is 



dMiirjui-lied from the main : yolk upon \\hich it in turn 



i.y the !im-r -jiMim-d character of its yolk granules, and by its 



