276 H. M. KINGERY 



and its bearing on the origin of the definitive ova will be dis- 

 cussed later. 



Oogenesis proper, the second or post-foetal proliferation of 

 cells from the germinal epithelium, begins within two or three 

 days after birth and extends nearly to sexual maturity; that is, 

 the process lasts about thirty-five or forty days. At birth the 

 first proliferation has become so slow that the cells are differenti- 

 ating in the epitheUum instead of in the ovary underneath, and 

 pachytene and diplotene cells are seen in the epithelium itself. 

 This condition does not last, however, as the process apparently 

 stops entirely within three days after birth. That is to say, 

 the second proliferation is an indication of a renewed activity 

 on the part of the germinal epithelium, and may be considered 

 as beginning where the first* left off. Whether the two are to 

 be considered as parts of one continuous process is a question 

 which will be discussed later. 



Since the developing definitive oocytes pass through the early 

 stages of their differentiation in the germinal epithelium, it is 

 somewhat difficult to determine the correct seriation of stages. 

 Criteria employed are the relative sizes of the nuclei and of the 

 whole egg-cell in a single ovary, and the size and appearance of 

 the cells and nuclei in ovaries of different ages. 



The formation of egg-cells from the epithelium is most rapid 

 during the time from three to twenty-five days post partum, and 

 in a single ovary of this period practically all the early stages 

 may be seen. Advantage has been taken of this fact in making 

 the drawings : a number have been drawn from one ovary (eleven 

 days post partum). It has been possible thus to show relative 

 size very clearly, and the cells have all had the same fixation and 

 stain. However, cells from a number of other ovaries, differently 

 fixed and stained, have been drawn; accordingly there is no ground 

 for a possible contention that my results are due to any one 

 special method. This process goes on practically up to sexual 

 maturity although more slowly in the latter part of this period, 

 and is completed forty or forty-five days after birth. At this 

 age the majority of female mice are sexually mature, and the 

 ovaries contain all the oocytes which will be differentiated. 



