Bennet Mills Allen 133 



■i. Interstitial Cells and Stroma. — The first generation of interstitial 

 cells in the pig ovary appears in the 2.5 cm. emhryo. They divide by 

 mitosis, but are on the whole short-lived, disappearing in the stage of 

 4 cm. The stroma consists of fibrous connective tissue filling in all the 

 space between the remaining structures, and forming a very important 

 element of the ovary. No interstitial cells are found in the rabbit ovary 

 until the stage of 45 days after birth, when a few cells are to be found, 

 which can unmistakably be assigned to this class. Their presence is 

 associated with the degeneration of certain follicles in which a theca 

 interna has developed from the stroma investment. Such a theca interna 

 is not formed until the follicle has acquired about three layers of granu- 

 losa cells and the rudiment of a follicular cavity. Their development 

 can be best understood in the ovary of the 85-day rabbit. Fully-formed 

 follicles at this stage are seen to be surrounded by a connective tissue 

 investment which consists of an inner layer of modified cells — theca 

 interna — and an outer layer of ordinary connective tissue cells. All 

 transition forms between these two kinds exist (Plate VII, Fig. 26), 

 showing that the cells of the theca interna have originated from the 

 general stroma by a process of transformation in which the cell body 

 becomes cylindrical instead of fibrillar, the cytoplasm becomes clearer, 

 the nucleus larger and spherical, and a centrosome makes its appearance. 

 In realit}^, the theca interna is separated from the follicle by a thin 

 layer (follicular capsule) of attenuated connective tissue cells which 

 send fibres in a diagonal direction through the theca interna to the 

 theca externa. This arrangement has been previously described l)y a 

 number of authors, Paladino, 87, Clarke, 98, and Rabl, 98. 



Very many of these follicles are degenerating at this stage. As soon 

 as the granulosa cells have begun to degenerate by chromatolysis, those 

 of the theca interna begin to enlarge slightly. A few cells from the inner- 

 most fibrous layer (follicular capsule) and from the theca interna break 

 through the basement membrane and enter the cavity of the follicle. 

 The theca interna derivatives undergo fatty degeneration, eventually dis- 

 appearing together with the granulosa cells. The only elements that 

 ultimately persist in the mass of degenerating cells enclosed by the fol- 

 licular capsule are the thin connective tissue cells that have become 

 separated from the capsule. These remain unaltered after all the cells 

 that have migrated from the theca interna have disappeared by fatty 

 degeneration and the granulosa cells by chromatolysis. 



During the degeneration of the cells enclosed by. the follicular cap- 

 sule, the cells forming the latter join to form a thick densely-staining 

 membrane which contracts and thiq]vens as degeneration proceeds. It 



