ELASMOBRANCHII. 53 



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 dis- 

 tinct. 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 trunk {a) 

 passing forwards from the embryo and dividing into two branches. 

 No venous trunk could be detected with the simple microscope, but 

 probably venous channels were present in the thickened edge of the 

 blastoderm. 



In fig. 30 B the circulation is greatly advanced. The blastoderm 

 has now nearly completely enveloped the yolk, and there remains 

 only a small circular space {yk) not enclosed by it. The arterial 

 trunk is present as before, and divides in front of the embryo into 

 two branches which turn backwards and form a nearly complete ring 

 round the embryo. In general appearance this ring resembles the 

 sinus terminalis of the area vasculosa of the Bird, but in reality bears 

 quite a different relation to the circulation. It gives off branches on 

 its inner side only. 



A venous system of returning vessels is now fully developed, and 

 its relations are very remarkable. There is a main venous ring in 

 the thickened edge of the blastoderm, which is connected with the 

 embryo by a single stem running along the seam where the edges 

 of the blastoderm have coalesced. Since the venous trunks are only 

 developed behind the embryo, it is only the posterior part of the 

 arterial ring that gives off branches. 



The succeeding stage (fig. 30 C) is also one of considerable interest. 

 The arterial ring has greatly extended, and now embraces nearly half 

 the yolk, and sends off trunks on its inner side along its whole cir- 

 cumference. More important changes have taken place in the venous 

 system. The blastoderm has now completely enveloped the yolk, and 

 the venous ring is therefore reduced to a point. The small veins 

 which originally started from it may be observed diverging in a 

 brush-like fashion from the termination of the unpaired trunk, which 

 originally connected the venous ring with the heart. 



At a still later stage the arterial ring embraces the whole yolk, 

 and, as a result of this vanishes in its turn, as did the venous ring- 

 before it. There is then present a single arterial and a single venous 

 trunk. The arterial trunk is a branch of the dorsal aorta, and the 

 venous trunk originally falls into the heart together with the sub- 

 intestinal or splanchnic vein. On the formation of the liver the 



