Morphological Aspects of Ageing in the Placenta 111 



species of rodents which have the shortest gestation periods 

 (hamsters, mice, rats) with that of members which have 

 periods six to seven times as long (porcupines, beavers). 



A sharp decline in the last decile of pregnancy has been 

 observed by Flexner and Gellhorn (1942) in the rate of transfer 

 of radioactive sodium across the placentas of various mammals 

 (rodents, rabbit, cat, goat), excepting the sow in which there 

 is no diminution. A similar decline in sodium transfer has 

 also been reported in the human (Flexner et al., 1948), which 

 Flexner (1955) attributes to the deposition of "fibrin over the 

 villi" and to "thromboses". This unfavourable terminal 

 change in placental transfer of sodium would seem to be valid, 

 although there are no morphological expressions of terminal 

 senescence, presently known in the animals cited, with which 

 to correlate the results. 



Of the various cytomorphic and biochemical changes which 

 occur in pregnancy, it is difficult to say specifically which 

 are progressive and which regressive. The terminal change in 

 sodium transfer found by Flexner in various mammals and 

 the changes in the mitochondria recorded by Dempsey in the 

 guinea pig's ageing yolk sac might well be regarded as regres- 

 sive. The decline in adenosine triphosphatase and of succinic 

 dehydrogenase noted in the rat's yolk sac by Padykula might 

 also be thought of as signalizing regression, especially since 

 adenosine triphosphatase plays a role in general energy 

 release. But for the majority of changes no definite clues 

 exist as to whether they exert favourable or unfavourable 

 influences upon placental exchange. 



Premature and pathological age changes have been de- 

 scribed in the human placenta, but they differ only in degree 

 from the normal terminal regressive changes alluded to. As a 

 consequence of the difficulty of measuring or assaying quan- 

 titatively the differences between normal and pathological 

 placental ageing, it has proven impractical to distinguish 

 ordinary terminal changes from premature and pathological 

 ones. 



The toxaemias of pregnancy and eclampsia have been 



