General Discussion 255 



I pass now to a consideration of Tuchmann-Duplessis' account of 

 the effects of growth hormone and of cortisone in inhibiting foetal 

 growth and in prolonging pregnancy. Of particular interest was his 

 demonstration that growth hormone inhibited foetal growth only 

 during the period of the normal gestation time and that there- 

 after the embryos grew at an accelerated rate. We may infer, 

 therefore, that the rabbit placenta which is commonly regarded as 

 having reached the limits of its life-span on the thirty-first day of 

 pregnancy is, thereafter, still capable of functioning efficiently and 

 with even greater expediency. As judged by these criteria the 

 placenta can scarcely be regarded as senescent. 



Zuckerman, Williams and Rowlands dealt with different aspects 

 of ageing in the transient tissues of the ovary. Zuckerman boldly 

 and appropriately opened with a discussion of the germinal epi- 

 thelium. He argued that the regenerative capacity of the germ cells 

 was poorly developed, and the ten points which he advanced in 

 support of his thesis, were to me at any rate satisfying evidence that 

 oogenesis does not occur to any remarkable extent in the life of 

 mammals and probably not in birds either. 



Williams' and Rowlands' contributions dealt with the fate of two 

 ovarian components — follicles and corpora lutea. Such research is 

 laborious and inevitably slow, but it is as direct an attack on the 

 problem of ageing of transient tissues as can at present be made. 

 Moreover, Rowlands' observation of the inability of induced corpora 

 lutea to prolong pregnancy in the guinea pig merits further investi- 

 gation, since induced corpora lutea have long been regarded as a 

 reliable means of extending the duration of pregnancy in some 

 animals. It is possible that the time of induction may be the 

 deciding factor. 



A group of contributions including those of Wislocki, Harrison, 

 Villee and Huggett dealt with the morphological, biochemical and 

 physiological aspects of ageing in the placenta. Hitherto it was more 

 or less taken for granted that age changes in the placenta were 

 associated with profound changes in morphology and biochemical 

 stability. However, the results that have been presented can hardly 

 be said to support this view, for as judged by its biochemical ac- 

 tivities and by its structural characteristics at term, the placenta 

 shows but slight evidence of senescence and, as we have already 

 remarked, may continue to function efficiently as an organ of ex- 

 change for much longer than its normal life-span. 



Fawcett and Dempsey dealt with different aspects of cytomorph- 

 osis. In each case the electron microscope has been used to provide 

 the most elegant series of electron micrographs that it has been my 

 privilege to see. Their studies are as valid a contribution to the study 



