DEVELOPMENT OF MAMMALIA. 



429 



attached to the parent by means of a membrane called the 

 placenta, which more or less closely unites the fcetus to the 

 walls of the uterus in which it develops (fig. 205). The 

 placenta is formed by the allantois, the walls of the uterus, 

 and the false amnion. The embryo mammal becomes folded 

 oif in a sac, the embryonic sac, just as does the chick, from the 

 yolk-sac, called in the Mammalia the umbilical vesicle (UV). 

 The amnion grows up on each side and unites above the 

 embryo. The inner 

 fold or true amnion 

 then separates from 

 the outer or false 

 amnion. Prior to the 

 separation of the two 

 limbs of the amnion, 

 the epiblast of the 

 umbilical vesicle and 

 the false amnion form 

 a lining to the zona 

 radiata. This mem- 

 brane is the so-called 

 sub - zonal memhxme 

 (sz), and it fuses with 



-T€ 



Fig. 203. — Diagram of F<etal SlEMBRANKa 

 OF A Mammal. (After Turner.) 



J Zona with villi ; sz, sub-zonal membrane ; E, epi- 

 tVio '^n^in i-nrh'rifn I ^A blast of embryo ; am, amnion ; .4 0, amniotic cavity ; 

 me zona laaiaia \j:). j,^^ niesoblsst of embryo; H, hypoblast of embryo; 

 UV, umbilical vesicle ; al, allantois ; ALC, allantoic 

 cavity. (From Foster and Balfour.) 



The ovum is attached 

 to the uterine wall by 

 villi or processes which stick into folds of the epithelium of the 

 uterus (fig. 204). So close is this connection that it is impos- 

 sible to remove the embryo without tearing the uterine walls. 

 The allantois a little later grows out of the hind-gut as a 

 vesicle and forms a flat sac, which projects into the space be- 

 tween the amnion and the sub-zonal membrane. This allantois 

 and the sub-zonal membrane fuse and form the cliorion (fig. 

 204, ch), which more or less surrounds the ovum. Where the 

 chorion does not surround the ovum, and where the yolk-sac 



