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 (e. g., the pig) the 

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

 Unguiculates (clawed animals hke the bat and rabbit) and Primates (e. g., 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. 70 

 A,B). Folds of the somatopleure form very early and envelop the whole embryo. 



Mesodermal \ii,nnnl 



Amnioiu i il\ ^ J 

 Upper limb I 1 1 



Posterior cardinal in 

 Dorsal a iti^ 



R. mnbili il iit^\ I i | 



Wall of yolk s.n- — '^.S 



^ pi nil ord 



"\ / ichord 

 Amnion 



Somatopleure 

 Mesonephric duct 



Mesentery 

 I "5 L. umbilical vein 

 L. vitelline vein 



Fntoderm of gul 

 Splaiitiinic mesoderm 



^ 



Fig. 72.— Transverse section through the yolk sac and stalk of a 5 mm. pig embryo showing attachment 



of amnion. 



The amnion (Fig. 72) 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 



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