The Fetal Membranes 71 



structures. After such infection has destroyed the placental 

 power of resistance, it is free to invade the fetus. 



The development of the placentae is a cumulative process 

 from the time of the formation of the fetal envelopes until 

 the pregnancy terminates. It has been stated that as par- 

 turition approaches, the placenta undergoes fatty degenera- 

 tion in preparation for dehiscence. That is untrue. The 

 embryonic sac at first lies free in the uterine or cornual 

 cavity without visible placental structures. Gradually the 

 placental structures become macroscopically apparent but 

 the embryonic sac can still be lifted out of the opened uterus 

 without resistance. Gradually the chorionic tufts lengthen, 

 become more branched, and sink deeper and deeper into the 

 maternal crypts. In the abattoir, it becomes necessary, in 

 separating the placenta, to use gradually increasing care 

 and force as the duration of pregnancy advances. Early in 

 pregnancy the chorionic tufts of the cow appear very much 

 the same as the short, simple structures of the soliped, but 

 they gradually become more complex as the close of preg- 

 nancy nears. This is essential to fetal development, which 

 gathers in force and rapidity from the time of fertilization 

 up to the end of pregnancy. 



The force which maintains the contact between the fetal 

 and the maternal placenta is not readily defined. It might 

 be termed physiologic magnetism. This force continues so 

 long as the fetus lives and the umbilic cord is intact. When 

 birth occurs and the navel cord ruptures, the magnetism 

 ceases. The blood in the fetal placental capillaries escapes 

 through the broken umbilic veins. The capillaries in the 

 chorionic tufts collapse, automatically fall away from the 

 crypt walls and drop out. Thus, in preparing histologic 

 sections of healthy placental structures one does not see the 

 chorionic tufts lying in contact with the crypt walls, but 

 quite detached therefrom, with the fetal and maternal epi- 

 thelial layers widely separated, as shown in Figs. 33, 34. 

 If, however, disease has invaded the placental tissues, the 

 picture promptly changes ; the damaged fetal and maternal 

 epithelia or their debris lie in contact, and placentitis with 



