ELASMOBRANCHTI. 



is less rapid in the immediate neighbourhood of the embryonic 

 part of the blastoderm 

 than elsewhere. As a 

 consequence of this, that 

 part of the edge, to 

 which the embryo is at- 

 tached, forms a bay in 

 the otherwise regular 

 outline of the edge of 

 the blastoderm, and by 

 the time that about two- 

 thirds of the yolk is en- 

 closed this bay is very 

 conspicuous. It is shewn 

 in fig. 30 A, where bl 

 points to the blastoderm, 

 and yk to the part of the 

 yolk not yet covered by 

 the blastoderm. The em- 

 bryo at this time is only 

 connected with the yolk- 

 sack by a narrow umbili- 

 cal cord ; but, as shewn 

 in the figure, is still at- 

 tached to the edge of the 



blastoderm. 



Shortly subsequent to 

 this the bay in the blas- 

 toderm, at the head of 

 which the embryo is at- 

 tached, becomes oblitera- 

 ted by its two sides com- 

 ing together and coales- 

 cing. The embryo then 

 ceases to be attached at 

 the edge of the blasto- 

 derm. But a linear streak 



FIG. 30. THREE VIEWS OF THE VITELLUS 

 OF AN ELASMOBRANCH, SHEWING THE EMBRYO, 

 THE BLASTODERM, AND THE VESSELS OF THE 

 YOLK-SACK. 



The shaded part (bl) is the blastoderm; the 

 white part the uncovered yolk. 



A. Young stage with the embryo still at- 

 tached at the edge of the blastoderm. 



B. Older stage with the yolk not quite en- 

 closed by the blastoderm. 



C. Stage after the complete enclosure of the 

 yolk. 



yk. yolk ; bl. blastoderm ; v. venous trunks 

 of yolk-sack; a. arterial trunks of yolk-sack; 

 y. point of closure of the yolk blastopore ; x. por- 

 tion of the blastoderm outside the arterial sinus 

 terminalis. 



formed by the coalesced 



edges of the blastoderm is left connecting the embryo with the 



