54 INTERNAL YOLK-SACK. 



proximal end of the subintestinal vein becomes the portal vein, and 

 it is joined just as it enters the liver by the venous trunk from the 

 yolk-sack. The venous trunk leaves the body on the right side, and 

 the arterial on the left. 



The yolk-sack persists during the whole of embryonic life, and in 

 the majority of Elasmobranch embryos there arises within the body 

 walls an outgrowth from the umbilical canal into which a large 

 amount of the yolk passes. This outgrowth forms an internal yolk- 

 sack. In Mustelus vulgaris the internal yolk-sack is very small, and 

 in Mastelus laevis it is absent. The latter species, which is one of 

 those in which development takes place within the uterus, presents 

 a remarkable peculiarity in that the vascular surface of the yolk-sack 

 becomes raised into a number of fedds, which fit into corresponding 

 depressions in the vascular walls of the uterus. The yolk-sack 

 becomes in this way firmly attached to the walls of the uterus, and 

 the two together constitute a kind of placenta. A similar placenta 

 is found in Carcharias. 



After the embryo is hatched or born^as the case may be, the yolk- 

 sack becomes rapidly absorbed. 



Bibliography. 



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(46) Fr. Leydig. Rochen u. Haie. Leipzig, 1852» 



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