106 George B. Wislocki 



(tree-shrew, vespertilionid bats, lemurs (Loris, Galago), 

 horse, coney (Hyrax)). Following complete inversion of the 

 yolk sac (lagomorphs, many rodents), its parietal wall, as 

 well as the chorion and the decidua capsularis associated with 

 it, degenerate completely. In the pig's placenta the un vas- 

 cularized ends of the chorionic sac wither. In the human 

 placenta, the chorion laeve and decidua capsularis are pro- 

 visional in nature and disappear completely by the fourth 

 month of gestation. The majority of these transient foetal 

 structures are functional and subserve physiological exchange 

 until regression sets in. Moreover, in most mammals, the 

 uterine endometrium undergoes various degrees of destruc- 

 tion and resorption during the course of gestation as a result 

 of being invaded by the trophoblast of the chorioallantoic 

 placenta. The degenerating endometrium forms an important 

 source of nutrient material (histotrophe) for the growing 

 placenta and conceptus. From all of this it is evident that 

 considerable portions of the placenta undergo ageing and 

 ultimate death long before pregnancy is over. These losses are 

 compensated by other parts of the foetal membranes which 

 differentiate into the definitive placental structures which 

 mediate physiological exchange until the end of gestation. 



The definitive parts of the placental membranes of Euthe- 

 rian mammals, which include a chorioallantoic placenta and 

 sometimes a yolk-sac placenta, undergo progressive histo- 

 logical and cytological changes which have been the subject 

 of numerous investigations (cf. Amoroso, 1952). Histo- 

 chemical changes in the placenta have also been investigated 

 in a variety of mammals including the sow (Wislocki and 

 Dempsey, 1946a), sheep (Wimsatt, 1950, 1951), cat (Wis- 

 locki and Dempsey, 1946&), shrews (Wislocki and Wimsatt, 

 1947), bat (Wimsatt, 1948, 1949), rat (Wislocki and Padykula, 

 1953) and man. 



The human placenta of the second and third months of 

 gestation has been compared by cytological and cytochemical 

 methods with the placenta at full term (Wislocki and Bennett, 

 1943; Dempsey and Wislocki, 1944, 1945; Wislocki and Demp- 



