144 P. L. Krohn 



At puberty the material which has been locked up in the 

 pituitary is set free to act on ovaries which have also become 

 progressively more responsive. The process which triggers 

 the initial release is, however, altogether unknown. If Kallas 

 (1929) is correct, the immature gonads already hold the 

 pituitary in check before puberty. He found that a 15-20 g. 

 rat in parabiosis with a castrated immature rat of the same 

 weight rapidly came into oestrus. Kallas believed that the 

 absence of the gonads in one parabiont allowed its pituitary 

 to release gonadotrophin which stimulated growth in the 

 gonads of the other. We now know, however, that the 

 non-specific stress of surgical operations may be sufficient to 

 induce precocious puberty (Mandl and Zuckerman, 1951). 



(c) During maturity 



The reproductive processes continue to mature after the 

 first pregnancy and maximum efficiency, as determined by 

 the number, size and development of the individual foetuses, 

 is not reached until several pregnancies have occurred. This 

 is true both of laboratory and agricultural animals. Not only 

 are the number of ovulations increased (either because more 

 gonadotrophin is being secreted or because the ovaries are 

 more sensitive) but the intra-uterine conditions evidently 

 become more satisfactory since the individual foetuses are 

 larger as well as more numerous. 



{d) The end of reproductive life 



The maximum reproductive capacity is maintained for 

 several pregnancies before it begins to decline. The most 

 dramatic and rapid change is seen in women when the onset 

 of the menopause, with which is associated a decreased 

 production of sex hormones and the disappearance of oocytes 

 from the ovaries, indicates the end of the child-bearing epoch. 

 Whether the increased amounts of gonadotrophin circulating 

 in the blood and urine at this time represent extra production 

 by the pituitary or decreased utilisation by the gonads is not 



