64 FORMATION OF THE YOLK -SACK. 



edge of the blastoderm. This streak is probably 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. Through- 

 out 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 conveniently called the yolk blastopore. 

 It is interesting to notice that, owing to the large size of the 

 yolk in Elasmobranchs, the posterior part of the primitive 

 blastopore 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 uncovered portion of 

 the yolk. It is also worth remarking that, owing to the embryo 

 becoming 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 enclosing the yolk is formed of an external 

 layer of epiblast, a layer of mesoblast below in which the blood- 

 vessels are developed, and within this a layer of hypoblast, 

 which is especially well marked and ciliated (Leydig, No. 46) in 

 the umbilical stalk, where it lines the canal leading from the 

 yolk-sack to the intestine. In the region of the yolk-sack 

 proper the blastoderm is so thin that it is not easy to be quite 

 sure that a layer of hypoblast is throughout distinct. Both the 

 hypoblast and mesoblast of the yolk-sack are formed by a 

 differentiation of the primitive lower layer cells. 



Nutriment from the yolk-sack is brought to the embryo 

 partly through the umbilical canal and so into the intestine, and 

 partly by means of blood-vessels in the mesoblast of the sack. 

 The blood-vessels arise before the blastoderm has completely 

 covered the yolk. 



Fig. 30 A represents the earliest stage of the circulation of 

 the yolk-sack. At this stage there is visible a single arterial 



