1905.] the Interstitial Cells of the Ovary in the Rabbit. 50 



presumably by means of the interstitial cells, since these probably furnish 

 the internal secretion of the organ, it- seems not impossible that the 

 diminution in size may be indirectly connected with the onset of labour. 



The Formation of " Primordial " Ova from the Interstitial Cells. 



In addition to the increase in size there are other changes taking place in 

 some of the interstitial cells near the peripheral parts of the ovary, during 

 the later third (approx.) of the period of gestation. It is a matter of 

 common histological knowledge that over the surface of the ovary there is 

 a layer of epithelial cells, roughly about two cells deep, although varying 

 slightly in thickness at different places. Immediately below this is a layer 

 of tissue in which are embedded the primordial ova in their early stages, 

 when they have not yet acquired a follicular epithelium or when that 

 epithelium is not very highly developed. There are in addition groups of 

 small ovarian cells which will eventually, as occasion arises, form the follicle 

 cells for the primordial ova. This whole layer together with the germinal 

 epithelium varies very considerably in thickness in different animals, the 

 variation having possibly some relation to the age of the animal under 

 investigation (cf. v. Beneden). 



In the non-pregnant animal and in the early periods of pregnancy, there is 

 a fairly sharp boundary between these outer layers and the deeper lying 

 interstitial cells. By about the twentieth day of pregnancy this state of 

 affairs is seen to be gradually changing, and some of the interstitial cells are 

 becoming surrounded by the connective tissue of the inner layer and thus 

 getting cut off from their fellows below. Whether this is brought about by 

 the passing outwards of the cells themselves or by the growth inwards of the 

 connective tissue is very difficult to decide quite satisfactorily ; but I think 

 it is reasonable to suppose that both processes are involved. It has already 

 been shown that there are two means whereby the germinal cells of the 

 embryo become embedded in the subjacent rnesoblast, namely, by an 

 ingrowth of the germinal cells and by a simultaneous upgrowth of the 

 mesoblast lying below. Here we have an analogous condition, but the 

 positions are reversed ; the germinal cells are now inside and the mesoblast 

 outside. 



This process, which is beginning to be evident about the twentieth day, 

 continues throughout the rest of pregnancy, so that as the days go on more 

 cells become cut off and press outwards, in many cases reaching almost to 

 the periphery. The number of cells thus cut off varies appreciably in 

 different animals, probably depending upon the age of the animal, but it is 

 not excessive at any time ; I have never found more than three or four rows 



