52 



FORMATION OF THE YOLK-SACK. 



analogous to (though not genetically related with) the primitive 



streak in the Amniota, 



This stage is represented in fig. 30 B. In this figure there is 



only a small patch of yolk 

 {yk) not yet enclosed, which 

 is situated at some little 

 distance behind the embryo. 

 Throughout all this period 

 the edge of the blastoderm 

 has remained thickened : a 

 feature which persists till 

 the complete investment of 

 the yolk, which takes place 

 shortly after the stage last 

 described. In this thickened 

 edge a circular vein arises 

 which brings back the blood 

 from the yolk-sack to the 

 embryo. The opening in 

 the blastoderm, exposing the 

 portion of the yolk not yet 

 covered, may be conveni- 

 ently called the yolk blas- 

 topore. It is interesting to 

 notice that, owing to the 

 large size of the yolk in 

 Elasmobranchs, the poster! or 

 part of the primitive blas- 

 topore becomes encircled by 

 the medullary folds and tail- 

 swellings, and is so closed 

 long before the anterior and 

 more ventral part, which is 

 represented by the uncover- 

 ed portion of the yolk. It 

 is also worth remarking that, 

 owing to the embryo be- 

 coming removed from the 

 edge of the blastoderm, the 

 final closure of the yolk 

 blastopore takes place at 

 some little distance from the 

 embryo. 



The blastoderm enclos- 

 ing the yolk is formed of an 

 external layer of epiblast, 

 a layer of mesoblast below 

 in which the blood-vessels 



Fig. 30. Thkee views of the vitellus of 



AN ElASMOBBANCH, SHEWING THE EMBEYO, THE 

 BLASTODERM, AND THE VESSELS OF THE YOLK-SACK. 



The shaded part {U) 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. 



0. Stage after the complete enclosure of the 

 yolk. 



?//c.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. 



