414 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



namely that, in agreement with the general principle that animals 

 with a large supply of yolk tend to show a precocious development 

 of the vessels on the surface of the yolk, the vitelline veins and their 

 derivatives make their appearance relatively earlier as compared 

 with the cardinals than they do in Zepidosiren. 



Here again the first of the main trunks to make their appearance 

 are the vitelline veins, which by their fusion anteriorly form the 

 heart (Fig. 191, A, v.v). The most conspicuous difference in the 

 next subsequent stages is that the two veins are for a considerable 

 period strongly asymmetrical (Fig. 191, B), the right becoming greatly 

 reduced, and only the left being commonly traceable back into the 

 subintestinal vein (Pristiurus — Rabl, 1892; Acanthias — Hoffmann, 

 1893). There can be no reasonable doubt that this is to be looked upon 

 as a secondary modification of a symmetrical condition of the two 

 veins as seen in Zepidosiren, and probably the impelling factor which 

 has brought about the modification is to be recognized in the fact that 

 the main channel for draining away the blood from the surface of the 

 yolk-sac is situated on the left side and opens into the left vitelline 

 vein (Fig. 191, y.s.v). 



The unpaired mesial prolongation forwards of the subintestinal 

 vein between the two vitelline veins which was so conspicuous in 

 Zepidosiren has not been noticed in Elasmobranchs. 



Both vitelline veins break up into a network as they traverse 

 the liver, the parts of the veins in front of this network persisting 

 as the hepatic veins of the adult (Fig. 191, D, h.v). The portion of 

 right vitelline vein behind this network is for a time much reduced, 

 the left aloue serving to supply the network with blood. Immediately 

 behind the liver, and in front of the yolk-sac vein, the two vitelline 

 veins are fused together for a short distance into a single vessel (Fig. 

 191, C, an), and the same is the case again from about the level of 

 the pancreas backwards where they form the subintestinal vein, 

 though it should be noticed that there still persist during early stages 

 of the development of the subintestinal vein in Elasmobranchs 

 distinct traces of the paired condition, the vein consisting of two 

 parallel components situated on each side of the mid-ventral line of 

 the intestine. These components soon become connected together 

 by numerous anastomoses and eventually fuse completely to form a 

 median vessel except where this is prevented by the presence of the 

 cloaca. 



The main vein from the yolk-sac (y.s.v) joins the left vitelline 

 vein in the region in front of the pancreas and presently the portion 

 of left -vitelline vein behind the opening of the yolk-sac vein dis- 

 appears. It thus comes about that the blood from the subintestinal 

 vein passes forwards to the liver entirely through the right vitelline 

 vein — in contrast with Zepidosiren where it did so by the left vitelline 

 vein. 



Cardinal veins and duct of Cuvier make their appearance, the 

 chief difference from Zepidosiren being the comparative shortness of 



