Age Changes in the Ovary 157 



of another strain. The host animals were necessarily spayed 

 before grafting and conditions for the snr\ i^'al of liomografts 

 should have been optimal. In '25 experiments the liomografts 

 that were recovered at autopsy, between nineteen and forty- 

 six days after grafting, had all been destroyed by a homograft 

 reaction in which round cell infiltration and replacement 

 with fibrous tissue were prominent features. In some of these 

 experiments oestrous cycles had continued after the operation. 

 The explanation for this fact, however, lay not in survival of 

 the homograft but in the presence of small masses of ovarian 

 tissue of host origin left behind after the ovariectomy. 

 Differentiation between homograft and autogenous ovarian 

 tissue was quite obvious when the material was examined 

 histologically. In further experiments the liomografts were 

 removed either six or nine days after grafting. In all a 

 reaction against the grafts was well under way; in most of 

 them the tissue organisation of the grafts had already been 

 destroyed, but a few small follicles and some luteal tissue still 

 looked reasonably healthy. The follicular apparatus does not 

 withstand the rigours of homotransplantation as well as the 

 luteal cells, scattered clumps of which may still be seen some 

 time after the rest of the ovary has been destroyed. 



The "Potential Immortality'* of the ovary 



The fact that tissue cultures of fibrol>lasts can be main- 

 tained for very long periods of time is well know^n. The same 

 sort of cellular immortality can be demonstrated for tumour 

 cells by repeated passages of the malignant tissue from one 

 host to another in such a way that the descendant cells of the 

 timiour have far outlived the animal from which the tumour 

 was originally derived. Very few attempts have been made to 

 carry out similar experiments with normal organs. Loeb 

 (1945) reports the serial transplantation of thyroid glands in 

 Strong A mice. The number of serial transplantations 

 ranged from three to seven and the transplants remained in 

 each host for three-and-a-half to six months. In some of these 

 experiments the thyroid tissue survived the process of serial 



