DISEASES OF THE GENERATIVE ORGANS 



83 



oped by several layers of membranes. The external envelope, the 

 chorion, is exactly adapted to the lltel1^s. The innermost enve- 

 lope, the amnion, encloses the fietns. Coverinii' the external 

 face of the amnion and lining the inner face of the chorion is 

 a donble membrane, the allantois. The envelopes mentioned 

 are not the only protection that the fd'tns has against injury. 

 It is enveloped in fluids as well. Immediately surrounding it is 



Fig. 19. — Placenta of cow. 



the liquor amnii, and within the allantois is the allantoic fluid. 

 The placenta is a highly vascular structure spread out or 

 scattered over the surface of the chorion and the mucous mem- 

 brane of the uterus, that attaches the fcetus and its envelopes to 

 the womb (Fig. 19). It is by means of this vascular apparatus 

 that the fretus is furnished with nourishment. The fetal and 

 maternal placentas are made up of vascular villi and depres- 

 sions that are separated only by the thin walls of capillaries, 



