148 P. L. Kroiin 



that it already contains. If this view is correct then the hfe- 

 span of an ovum may a})proach that of a neurone and one 

 might expect that an ovum which was fertihsed when a woman 

 was eighteen years old would behave differently from one 

 that was fertilised when she was forty years old. Lansing's 

 (1954) demonstration in rotifers that the lifespan and repro- 

 ductive capacity of the offspring are greatly influenced by the 

 age of the mother may well turn out to be relevant in this 

 connection. It is believed that a fixed number of future 

 oocytes are incorporated into the ovaries of these animals 

 during embryological development and that no further 

 formation of oocytes takes place. Hence, the lifespan of the 

 offspring depends directly on the length of life of the individual 

 ovum before fertilization. 



The alternative view% which has become more popular 

 recently, is that there is a continuous process of destruction 

 and neoformation going on in the ovary throughout repro- 

 ductive life. It derives to some extent from the feeling that 

 the signs of active atresia are so evident in histological 

 sections of the ovary that neoformation must also occur to 

 make up, even partially, for the great loss of cells. Though a 

 possible contribution of other elements has never been ruled 

 out, the cells in the germinal epithelium covering the ovary 

 are usually thought to be the equivalent of the layer of 

 spermatogonia in the testis and to })e responsible for the 

 replacement of the oocytes that are destroyed by atresia. 

 The rates of the two processes of oogenesis and spermato- 

 genesis thus become much more similar. It has been suggested, 

 indeed, that an ovum lives no longer than a red blood cell and 

 that its lifespan is to be measured in days rather than years. 



If this second view is correct, an ovum that was released 

 when a woman was forty would possibly have suffered no 

 greater age changes than one released when she was 

 eighteen, and the effects of maternal age on the foetus might 

 depend more on changes in tlie uterine evironment than in 

 tlie ovary. It would also mean that attempts to prolong 

 reproductive life in women could be based not only on 



