108 TEXT-BOOK OF EMBRYOLOGY. 



similar to that in the chick; that is, by a dorsal folding of the somatopleure. 

 There is, however, no head fold unless a temporary structure known as the 

 proamnion be considered as such. The entire rabbit amnion is formed by an 

 extension over the embryo of the tail amniotic fold. In other forms (bat and 

 probably man) the amnion and amniotic cavity arise in situ over the embryonic 

 disk, without any folding of the somatopleure. 



Yolk is almost entirely lacking in most Mammals, but the yolk sac is always 

 present although it soon becomes a rudimentary structure. The fact that the 

 yolk sac is always present points toward the conclusion that Mammals are 

 descended from animals which possessed large ova with abundant yolk. As a 

 matter of fact the lowest Mammals, the Monotremes, possess large ova with 

 large quantities of yolk. These are deposited by the female, are developed in a 

 parchment-like shell, and are carried about in the brood-pouch. 



The allantoic sac in many Mammals is a very rudimentary structure which, 

 as in the chick, always arises as an evagination from the caudal end of the .gut. 

 The allantoic blood vessels, however, become vastly important since they here 

 not only carry off waste products from the embryo, as in Reptiles and in Birds, 

 but also assume the function of conveying nutriment from the mother to the 

 embryo. In assuming this new function they are no longer concerned with 

 the allantoic sac proper but enter into a new relation with the chorion. 



The chorion is the most highly modified and specialized of all the mam- 

 malian fcetal membranes. In some cases (the rabbit, for example) it arises in 

 connection with the amnion, as in the chick, by a dorsal folding of the somato- 

 pleure. In other cases (bat and probably man) it arises at a very early stage, 

 partly as a differentiation of the superficial layer of the morula, partly as 

 extraembryonic parietal mesoderm which develops later. In all cases where 

 the embryo is retained in the uterus (except Marsupials) it forms a most highly 

 specialized and complex structure which, in connection with the allantoic 

 vessels, establishes the communication between the mother and the embryo. 



For the sake of clearness it seems best to describe first the earlier stages of 

 the fcetal membranes in some case where the development resembles that of the 

 chick; then later to consider the more specialized types of development, the 

 ultimate structure of the membranes, especially the chorion, and their relation 

 to the embryo and the mother. 



Amnion, Chorion, Yolk Sac, Allantois, Umbilical Cord. — Referring 

 back to the mammalian blastoderm when it consists of the three germ layers, 

 it will be remembered that the embryonic disk forms the roof, so to speak, of a 

 large cavity — the yolk cavity or cavity of the blastodermic vesicle (Fig. 82); 

 that the ectoderm of the disk is continuous with a layer of cells which extends 

 around the vesicle — the extraembryonic ectoderm; that the entoderm of. the 

 disk is continuous with the entoderm lining the cavity of the vesicle; that the 



