The Placenta 359 



any intervening fetal fluids. Mesoblast being everywhere con- 

 tiguous to mesoblast, the amnion becomes adherent to the chorion 

 over a large area, to such a degree as to make their separation 

 difficult. 



This difference in the character and extent of the allantois es- 

 tablishes a marked difference in the phenomena of parturition. 

 In the mare, where the allantois constitutes a complete envelop- 

 ing sac filled with fluid, the fetus tends to be born enclosed with- 

 in the amniotic sac, including the adherent inner or amniotic 

 layer of the allantois, while, in ruminants, the extensive area 

 of the amnion not covered by the allantoic sac causes the amnion 

 to remain adherent to the chorion and the fetus to be born 



naked. 



The Placenta. 



By the term placenta we understand those portions of the fetal 

 membranes or annexes which serve to bring about an intimate 

 contact between the circulatory system of the mother and that 

 of the fetus. The development of the mammalian embryo can 

 proceed independently for but a brief period since, having 

 virtually no nutrient reserve in the vitellus, its growth must 

 depend upon nutritive substances obtained from without, from 

 the uterus of the mother. When the morula reaches the uterine 

 cavity at the eighth to tenth day, the zona pellucida throws 

 out tufts, the primordial chorion, which enter into somewhat 

 intimate relations with the uterine mucosa, but this zone soon 

 becomes attenuated, and disappears as the blastoderm rapidly 

 enlarges within it. This primitive chorion, or prochorion, must 

 be replaced by a more permanent and efficient structure for the 

 nutrition of the fetus. 



vSoon there grow out, from the surface of the epiblast, tufts or 

 villi which extend into the mucosa of the uterus through which 

 there is an exchange of nutritive material and waste products 

 between the mother and fetus. When the amnion develops, the 

 external or false amnion is but a continuation of the somato- 

 pleuricwall of the yolk sac, which completely invests the embryo. 

 Over the entire surface of this embryonic sac, villi grow out to 

 establish relations between the embryo and mother and consti- 

 tute the amniotic chorion or placenta. These structures, like the 

 preceding, are primitive and temporary in character and soon 

 become lost as unimportant parts of the more permanent structure 

 to follow, or they atrophy and disappear. 



