357 



points, into the follicular cells. This is brought about by the now 

 actively growing theca, which at this stage cannot be differentiated 

 into theca externa and interna, breaking through the membrana 

 propria and invading the follicle (fig. 4). These intruding points grow 

 inward towards the centre of the cavity and so, as they become longer, 

 tend to divide the young corpus luteum into lobules (fig. 2). Closely 

 associated with these ingrowths are newly formed blood vessels which 

 accompany them on their course. Ultimately the thecal irruptions 

 grow through the membrana granulosa and reach the central cavity 

 where they anastomose and form a central plug of connective tissue 

 (fig. 3). This point may be taken as the end of the formative period. 

 All this active growth has been brought about by the mitotic division 

 of the cells and throughout this period mitotic figures are frequently 

 to be met with both in the thecal ingrowths and in the endothelium 

 of the blood vessels accompanying them. 



While the proceeding changes have been taking place the cells 

 of the membrana granulosa have also altered and have gradually become 

 transformed into the characteristic luteal cells. These cells in the 

 distended follicle are small with small nuclei and are under considerable 

 pressure. After the bursting they still remain fairly small until they 

 are transformed into luteal cells. This transformation is brought about 

 by the gradual swelling of both the cell and its nucleus, an enlar- 

 gement which appears to go on rapidly until the cavity of the follicle 

 is filled (fig. 3). Such a growth however does not appear to be 

 accompanied by any increase in the number of the cells for continued 

 searching has failed to discover any trace of mitotic or amitotic 

 division in these cells. SA^'DES was unable to find evidence of cell 

 division in the pregnant animal, and recent searching through his 

 slides has yeilded no results. The time occupied by the foregoing 

 alterations cannot be accurately ascertained as the bursting of the follicle 

 does not appear to occur at any definite time after either heat or copu- 

 lation. If, however one may judge by the time taken in the pregnant 

 animal in which the changes are identical it is fairly short, perhaps 

 about three days. 



Period of growth. 



In this period the changes described above are carried still further. 

 The theca continues to irrupt and the membrana propria becomes less 

 and less distinct until it finally disappears. Within the corpus luteum 

 the connective tissue ingrowths form numerous further anastomoses 



