436 AN INTRODUCTION TO ZOOLOGY 



becomes very large, but the extra-embryonic ccelom or exocoel dilates 

 until it practically fills the cavity of the blastodermic vesicle. 



In man the formation of the amnion is not accompanied by the 

 production of folds, but, as pointed out in the last chapter, the 

 primitive amniotic cavity appears as a space between Rauber's 

 layer and the embryonal shield. The manner of its formation is 

 such that no proamnion appears. The final relations of the folds 

 are the same in the two cases, however, in spite of their different 

 origin. The human amniotic cavity enlarges very rapidly, and 

 finally fills the whole of the inside of the vesicle save for the part 

 occupied by the yolk sac and allantoic stalks, which therefore come 

 to be, as it were, bound together to form a combined structure, the 

 umbilical cord. The dilatation of the amnion is so great that it 

 completely obliterates the exoccel, and the external mesoderm layer 

 of the amnion comes to lie close to the inner mesoderm of the chorion 

 and the two fuse. 



In the rabbit a vascular area is established in a normal way in 

 the splanchnopleure, and it is bounded by the sinus terminalis. 

 The rapid extension of the exocoel, which soon reaches the sinus 

 terminalis, limits the extension of the vascularisation to the upper 

 side of the yolk sac. Beyond the vascular area the mesoderm never 

 extends, so that the remaining hemisphere of the vesicle, termed the 

 omphalopleure, is simply bilaminar. Thus the chorion is limited 

 to the upper regions. 



Yolk sac, 



At an early stage the yolk sac in the rabbit occupies the main 

 part of the blastodermic vesicle, and its splanchnopleuric portion is 

 separated from the chorion by the exoccel. In this species the 

 entoderm of the sac develops but slowly, so that for a considerable 

 time it is incomplete on the ventral side. Finally, it is completed, 

 and this last portion lies in contact with the chorionic ectoderm and 

 the mesoderm but slowly pushes its way in between them. Thus 

 it is that in this form the yolk sac is only splanchnopleuric in its 

 upper portion, while below its entoderm is in contact with the 

 blastodermic ectoderm. The extra-embryonic ccelom expands 

 markedly in the rabbit, and so compresses the cavity of the yolk 

 sac until this structure finally takes on a sort of umbrella shape. 

 The long narrow yolk sac stalk represents the handle, and the 

 flattened expanded sac itself the cover. The splanchnopleure 

 becomes richly vascularised and has a well-developed sinus terminalis, 

 and its main vessels are termed the vitelline, or, perhaps more 

 frequently, the omphalo-mesenteric arteries and veins respectively. 

 The veins penetrate the liver and enter the hinder portion of the 



