CHAPTER IV 

 THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 



The fetal membranes of mammals include the amnion, chorion, yolk-sac and 

 allantois, structures which we have seen are present in chick embryos. Most 

 important in mammals is the manner in which the embryo becomes attached to 

 the uterine wall of the mother and in this regard mammalian embryos fall into 

 two groups. Among the Ungulates or hoofed mammals (example the pig) the 

 fetal membranes are of a primitive type, resembling those of the chick. Among 

 Unguiculates (clawed animals like the bat and rabbit) and Primates (example 

 Man) the fetal membranes of the embryo show marked changes in development 

 and structure. 



FETAL MEMBRANES OF THE PIG EMBRYO 



The amnion and chorion develop very much as in the chick embryo (Fig. 

 67 A, B). A fold of the somatopleure forms very early about the whole embryo. 

 The amnion is closed in embryos with but a few pairs of segments, but for some 

 time remains attached to the chorion by a strand of tissue (Keibel) . The yolk- 

 sac develops early as in all mammals. In the pig it is small and the greater part 

 of it soon degenerates. It is important only in the early growth of the embryo, 

 its functions then being transferred to the allantois. Branches of the vitelline 

 vessels ramify in its wall, as in that of chick embryos, but soon degenerate. 

 The trunks of the vitelline vessels, however, persist within the body of the em- 

 bryo. The allantois, developing as in the chick from the ventral wall of the hind- 

 gut (Fig. 67 A-D), appears when the embryo is still flattened out on the germinal 

 area. In an embryo 3.5 mm. long it is crescent shaped and as large as the em- 

 bryo. It soon becomes larger and its convex outer surface is applied to the 

 inner surface of the chorion. As these surface layers are composed of splanchnic 

 mesoderm they fuse more or less completely. A pair of allantoic veins and ar- 

 teries branch in the splanchnic layer of the allantois. These branches are brought 

 into contact with and invade the mesodermal layer of the chorion. The outer 

 ectodermal layer of the chorion in the meantime has closely applied itself to the 

 uterine epithelium, the ends of the uterine cells fitting into depressions in the 



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