No. 2.] THE OVARIAN STROM A OF MAMMALS. 93 



I have never found any indications of karyokinesis in any of 

 the stroma cells outside the corpora lutea, so that the presence 

 of the centrosome and sphere is probably not associated with 

 karyokinesis in this case. 



If my suggestion that the cells of Fig. i arise by hypertrophy 

 from the cells of Figs. 4 and 5 be correct, then it is probable 

 that the appearance of the centrosome and sphere is in some 

 manner connected with this hypertrophy. 



In the rat and the cat there is a somewhat similar change in 

 the structure of the stroma cells during pregnancy, but no cen- 

 trosome or sphere appears. Sobotta ('96a) also mentions noth- 

 ing of the sort in connection with the mouse. Whether there 

 is less hypertrophy in these cases I cannot as yet state. 



The results of my observations thus far are then that- in the 

 stroma cells of the ovary of the dog and the rabbit a distinct 

 centrosome and spliere appear during pregnancy, but apparently 

 are not in any way connected with karyokinesis. 



The presence of these structures under these conditions opens 

 up a number of problems, viz., Is the centrosome a permanent 

 organ of these cells ? How is the appearance and disappearance 

 of the sphere and centrosome in the cytoplasm related to the 

 other changes occurring in the cell ? How generally do these 

 structures appear among mammals, and lastly, What is the 

 relation of these changes of the ovarian stroma to pregnancy ? 



I hope to be able to throw further light on some of these 

 questions, but it seemed that a short account of the results 

 obtained thus far might be of interest. 



