1905.] the Interstitial Cells of the Ovary in the Rabbit. 54 



The nucleolus in the dictyate condition (fig. 6 (6)) is very definite, and the 

 chromatin is arranged more or less all over the nuclear area (which is now 

 very large), and shows a number of small nodules both at what appear to be 

 free ends and at the points of intersection. There can, in fact, be not mnch 

 doubt that the changes taking place are identical with those seen in the 

 young ovary, which lead to ovogenesis, and therefore it would appear that 

 ovogenesis also takes place in the adult animal during pregnancy. 



Previous observers on this subject appear to have all considered that 

 formation of ova must be accomplished by means of fresh invaginations of 

 germinal epithelium, and those who thought they saw invaginations con- 

 cluded at once that there was therefore a formation of ova in later life, whilst 

 those who failed to find them denied the possibility on this account. My 

 observations show that fresh invaginations of the germinal epithelium are 

 not a necessity, but that the " invagination " has taken place already in the 

 embryo. The invaginated cells of the germinal epithelium give rise to all 

 the cells of the true ovarian tissue, which are all capable of functioning in 

 any true ovarian capacity that is, they may become ova or follicle cells, or 

 interstitial cells, and most probably also lutein cells, their destiny appearing 

 to be a matter of chance. The interstitial cells, however, are still capable of 

 becoming ova, and of undergoing the changes requisite for ovogenesis should 

 the appropriate stimulus be given. This stimulus is supplied when the animal 

 becomes pregnant, and the ovarian cells enlarge in size. Towards the end 

 of the time of pregnancy some of them press towards the periphery and 

 undergo the necessary changes, becoming true ova. Thus every pregnancy 

 would seem to be a stimulus for the next, in the way of providing new ova, 

 although even of the relatively small number found probably very few ever 

 reach maturity. 



Conclusions. Summing up the conclusions reached in this paper we find 



1. That a large number of germinal cells become embedded in the subjacent 

 mesoblast. Of these the great majority undergo transformations up to a 

 certain stage. This stage having been reached, they may pass through the 

 necessary processes of ovogenesis, or they may become modified to form either 

 follicle cells or interstitial cells, this last process being the chief fate of the cells 

 near the periphery, whilst ovogenesis is that of the more centrally situated 

 ones. 



2. The interstitial cells are thus potential ova, capable of becoming ova 

 should the appropriate stimulus be given. 



3. This stimulus is provided by pregnancy, during which period the 

 interstitial cells undergo enlargement in size, exceeding that of a primordial 

 ovum. 



