Age Changes in the Ovary 155 



mice are -Aso acceptable to the liybrid produced from crossing 

 the two strains. In this way it becomes possible to study 

 what happens not only when an old ovary is placed in a young 

 environment and vice versa, but also when ovaries of two 

 strams, which may difPer in the rate in which they become old 

 (see above), are put into a common environment. (Cf. Huseby 

 and Bittner [1950] who were interested in the influence on 

 the incidence of mammary cancer of ovaries from strains A 

 and Z when grafted into AxZ hybrids.) 



Many difPerent sites for transplants have been chosen and 

 the functional activity of the grafts has usually been assessed 

 by the reappearance of oestrous cycles and by histological 

 methods. Most sites, such as the subcutaneous tissues, the 

 spleen or the anterior chamber of the eye, necessarily preclude 

 the most valuable and, for our purpose, the essential criterion 

 of function. The ovary must be transplanted so that the ova 

 can reach the fallopian tubes, become fertilized and implant 

 in the uterus. Such orthotopic grafting of ovaries was first 

 carried out in rabbits and guinea pigs, which do not have a 

 complete ovarian capsule, some fifty years ago. Foa (1901) 

 and Castle and Phillips (1913) believed that pregnancies 

 which took place after grafting were derived from the grafted 

 ovary and not from regenerating host ovary. Much more 

 recently Whitney (1946) believed that he had successfully 

 exchanged orthotopically the ovaries from a bloodhound to 

 a foxhound, although the animals failed to become pregnant 

 afterwards. 



The fact that its ovary is surrounded by a capsule makes 

 it easier to achieve this purpose in the mouse than in the other 

 two species that have been mentioned. It is relatively simple 

 to open the capsule, remove the^ animal's own ovary and 

 replace it with another. This method of intracapsular grafting 

 of the ovary has been employed mainly by Robertson and by 

 Russell and Hurst (1945). Robertson (1942) has been able to 

 differentiate, for example, between the genetic and environ- 

 mental factors which are responsible for the intra-uterine 

 death of homozygous yellow mice. The Bar Harbor workers, 



