16 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



mately about this period individual nuclei lag 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 (Buckert). 



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 Buckert. 



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 Eiickert 

 (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 

 very questionable. When a broad view is taken of the relations of 

 blastomeres and segmentation cavity in the Elasmobranchs these 

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

 Lung fishes. In these fishes, as will be shown later, the blasto- 

 meres which originally formed the floor of the segmentation 

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

 resultant change in the topographical relations and form of the 

 segmentation cavity would clearly afford no valid reason for giving 

 it a new name. 



The Yolk Syncytium.— The layer of substance immediately 

 underlying the blastoderm and segmentation- or germ-cavity is 

 distinguished from the main mass of yolk upon which it in turn 

 rests by the finer grained character of its yolk granules, and by its 



