THE EARLY DEVELOPMENT OF THE MAMMAL 433 



veins. These empty into the posterior end of the heart after 

 penetrating the liver; later the extra-embryonic portions of 

 these veins disappear, while their embryonic portions are trans- 

 formed into the portal (hepatic portal) vein. 



In the Carnivors the yolk-sac is very large and has a complete 

 mesodermal investment which is unusually vascular. At first 

 it is closely in contact with the chorion, and thus may have 

 temporarily a placental relation, but soon it is in part separated 

 from the chorion by the extension, between the two membranes, 

 of the allantois which then assumes the definitive placental rela- 

 tion. The primarily placental character of the yolk-sac in the 

 marsupial, Dasyurus, has been mentioned. In the mole, Talpa, 

 a similar relation has been described, the vascular mesoderm 

 of the upper part of the yolk-sac, with the endodermal wall of 

 its lower part, coming into close organic relation with the 

 chorion and uterine epithelium. 



In many other Mammals the yolk-sac is from the beginning a 

 relatively small organ (Tarsius, hedgehog, Primates), or it may 

 early be reduced from an originally well-developed state (horse). 

 In man, which may be described as in a general way representa- 

 tive of this type, the yolk-sac at all stages comes far short of 

 filling the cavity (exocoelom) of the blastodermic vesicle. It 

 grows slowly out into the exocoelom during the first month or 

 more of development, its diameter about equal to the length of 

 the embryo. Its wall is richly vascular even in its very early 

 stages, the blood vessels producing a characteristic roughness 

 of its outer (mesodermal) surface (Fig. 164). After reaching 

 a size of about 11X7 mm. it begins to diminish in size. A 

 yolk stalk becomes clearly differentiated and elongates rapidly, 

 and we have already seen how it becomes enclosed proximally 

 within the umbilical stalk or cord. Finally the entire yolk stalk 

 is thus enclosed and, as the amnionic membrane expands, filling 

 the entire cavity of the "ovum," wiping out the exocoelom, the 

 yolk-sac itself disappears from view, embedded in the meso- 

 dermal tissues of the placental region (Fig. 179). 



The greater part of the yolk stalk becomes a solid cord of en- 

 doderm during the latter part of the second month, and finally 



