CHAPTER IV 

 HUMAN EMBRYOS AND FETAL MEMBRANES 



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 (e. g., the pig), the fetal membranes are of a primitive 

 type, resembling those of the chick. Among Unguiculates (clawed animals 

 like the bat and rabbit), including Primates (e. g., Man), the fetal mem- 

 branes 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. yo^l, B). Folds of the somatopleure form very early and envelop 

 the whole embryo. The amnion (Fig. 72) is a closed sac in embryos 



Mesodermal segment. 



Amniotic cavity 

 Upper limb bud 



Posterior cardinal vein 

 Dorsal aorta 



Glomerulus 

 R. umbilical vein 



Wall of yolk sac 



'Spinal cord 



'Notochord 

 Amnion 



1L- Somatopleure 



Mesonephric duct 



Mesentery 



L. umbilical vein 



L. vitelline vein 



Entoderm of gut 

 Splanchnic mesoderm 



FIG. 72. Transverse section through the yolk sac and stalk of a 5 mm. pig embryo, showing 



attachment of amnion. 



with only a few pairs of segments, but for some time it 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 



70 



