94 Dr Allen Thomson 07i the Vascular System 



which traverses the surface of the yolk to the vena cava, and 

 thence to the auricle of the heart (a). 



The liver, which in the early stages of development of all 

 vertebrated animals appears to be intimately connected with the 

 veins of the yolk, is formed in osseous fishes from a mass of 

 granular substance (c) deposited in the hollow between the 

 yolk and back part of the intestine. This mass, as it increases 

 in size, is collected round the trunk of the vein distributed on 

 the posterior surface of the yolk, and is gradually supplied 

 with vessels formed by the subdivision of the main trunk of the 

 vein. As the liver becomes larger, and receives arterial vessels 

 from the coeliac trunk, the sac of the yolk is diminished in bulk, 

 and the blood is not so uniformly distributed over its surface 

 as before ; the general capillary network being less supplied 

 with blood, two or three of its vessels become dilatedg and con- 

 vey the greater part of the blood, which previously was spread 

 over the surface of the yolk, directly from the liver to the heart. 

 At last, when the yolk is much diminished in size, only one of 

 these vessels remains, becoming the hepatic vein. 



The envelope of the ovum of osseous fishes is generally so 

 thin, that, from the first formation of the blood, the changes 

 induced by the surrounding water are not materially impeded. 

 When the foetus, or little fish, bursts its covering, and escapes 

 from the ovum, it swims about in the water, with the yolk, 

 proportionally very large, hanging from its abdomen, and the 

 blood is then more directly exposed to the current of water *. 

 Towards the latter end of foetal life, the yolk, while still of a 

 considerable size, is in some fishes enclosed in the abdomen, and 

 probably serves for some time to nourish the animal, while in 

 others its substance is almost entirely absorbed before its enclosure. 



In some of the larger cartilaginous fishes, on the other hand, 

 as the Rays and Sharks, the sac of the yolk is connected with the 

 posterior part of the intestine, and arterial as well as venous 

 bloodvessels are distributed on the vascular area covering its 

 surface (Fig. 6. of the Skate). The vessel ramified on the yolk 

 appears to be a branch of the coeliac artery. The vein formed 



• See account of the Spawning of Salmon, &c. by Daniel Ellis, Esq. in 



vol. iv. of this Journal. 



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