110 George B. Wislocki 



depress physiological transfer or influence the onset of labour. 

 In the human placenta, in which more attention has been de- 

 voted to this problem than in animals, unfavourable, terminal 

 morphological changes have been described. These include 

 the accumulation of fibrin, "fibrinoid" and calcium (Hertig, 

 1946), the " hyalinization " and loss of the syncytium covering 

 a variable number of the chorionic villi (Tenney, 1936), and 

 haemorrhagic infarcts involving the complete sclerosis of 

 placental villi (Thomsen, 1955). However, to what degree 

 and how these changes actually limit or decrease the overall 

 efficiency or individual functions of the human placenta is 

 mainly a matter of conjecture. 



With respect to animals, the epithelium of the visceral layer 

 of the yolk-sac placenta of the guinea pig, investigated with the 

 electron microscope (Dempsey, 1953), reveals that the mito- 

 chondria at full term are swollen and some are degenerating. 

 Furthermore, the cell cytoplasm is more granular than earlier 

 in gestation and some of the ergastoplasmic structures are 

 swollen. In the rat, however, which has a much shorter 

 gestation period than the guinea pig, similar degenerative age 

 changes have not been observed (Wislocki and Dempsey, 

 19556). 



These differences between the ageing of the yolk sacs of the 

 guinea pig and rat raise the interesting question as to whether 

 the definitive placentas of animals with long gestation periods 

 show more terminal age changes than those with short periods. 

 The shortest gestation periods for Eutherian mammals are 

 16 days for the golden hamster and 17 to 19 days for the 

 short-tailed shrew, whereas the longest periods are over 600 

 days for the elephant and between 420 and 500 days for the 

 giraffe and rhinoceros. Perhaps the most rewarding compari- 

 son of the degree of terminal ageing with the length of gesta- 

 tion could be carried out in rodents. There, differing from 

 most orders, the length of gestation varies greatly in different 

 species, whereas the definitive chorioallantoic and yolk-sac 

 placentas are relatively similar in their basic structure. One 

 might profitably compare the terminal placental histology of 



