466 DEVELOPMENT OF ELASMOBRANCH FISHES. 



I shall follow the figures on this plate in my descriptions. 



Fig. i represents my earliest stage of the circulation of the 

 yolk-sack. At this stage there is visible a single aortic trunk 

 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. 2 the circulation was greatly advanced 1 . 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 nearly 

 form a complete ring round the embryo. In general appearance 

 it resembles the sinus terminalis of the area vasculosa of the 

 Bird, but in reality bears quite a different relation to the circula- 

 tion. It gives off branches only on its inner side. 



A venous system of returning vessels is now fully developed, 

 and its relations are very remarkable. There is a main venous 

 ring round the thickened edge of the blastoderm, which is 

 connected with the embryo by a single stem which runs 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 which gives off 

 branches. 



The succeeding stage, fig. 3, 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 as a result of this, the venous ring no longer exists, 

 but at the point where it vanished there may be observed a 

 number of smaller veins diverging in a brush-like fashion from 

 the termination of the unpaired trunk which originally connected 

 the venous ring with the heart. This point is indicated in the 

 figure by the letter y. The brush-like divergence of the veins is 



1 My figure may be compared with that of Leydig, Rochen und Haie, Plate in. 

 fig. 6. Leydig calls the arterial ring the sinus terminalis, and appears to regard it as 

 venous, but his description is so short that this point is not quite clear. 



