ELASMOBRANCHII. 65 



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 blasto- 

 derm 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 con- 

 nected 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 circumference. 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 subintestinal or splanchnic vein. On 

 the formation of the liver the proximal end of the subintestinal 

 vein becomes the portal vein, and it is joined just as it enters 



B. III. 5 



