14 EMBRYOLOGY OF THE LOWER VEETEBRATES oh. 



that the first latitudinal furrow cannot be identified in the Elasmo- 

 branch and further that the study of sections shows that the furrows 

 sometimes cut into the germinal disc obliquely instead of being 

 perpendicular to the egg surface. 



The nuclei of the blastoderm divide synchronously and after four 

 divisions have taken place, when there are sixteen nuclei in place of 

 the original single zygote nucleus, the segmentation furrows (Fig. 8, 

 C) form a network dividing up the blastoderm into smaller central 

 and larger peripheral blastomeres. These blastouieres are, however, 

 not completely isolated from one another but are still in continuity 



B 



B 



Pig. 8. — Surface views of the blastoderm of Elasmobranchs illustrating the process 

 of segmentation. (After Buckert, 1899.) 



A, B, C, Torpedo ; B*, Pristiurus ; D, E, Scijllium. Fig. B* shows an abortive segmentation which 

 may often be observed transitorily round the margin of the germinal disc [Merocyte segments of 

 Riickert]. 



at their bases, and, in the case of the peripheral blastomeres, at their 

 outer ends. 



Up to the fifth mitotic division the axes of the mitotic spindles 

 have been approximately parallel to the surface but now blastomeres 

 begin to divide with their spindle axes perpendicular to the surface 

 so that a set of superficial segments becomes separated off. Beneath 

 these fluid accumulates intercellularly and a segmentation cavity 

 arises (Fig. 9, B). 



During the sixth division some of the blastomeres forming the 

 floor of the segmentation cavity become separated off from the 



