64 



BIOLOGY OF THE LABORATORY MOUSE 



these conditions, survival of primitive germ cells for at least 115 days has 

 been recorded (91). 



The high mortality among ova under normal conditions can be appreci- 

 ated from figures presented by Allen (4). An average of 800 to 1000 ova 

 are differentiated in the two ovaries of a female mouse at each estrous period, 

 while only about 9 ova of an earlier generation mature. The percentage of 

 survival is thus about 1%. 



Brambell (19) has made an interesting study of the growth of oocyte and 

 follicle, finding the relation shown in Fig. 33. It will be seen that at first 

 both oocyte and follicle increase in size, the relation between the two being 



70 



50 



so- 



lo 



600 



200 300 400 500 



DIAMETER OF FOLLICLE 

 Fig. ^^. — Graph showing the relation between oocyte size and follicle size. {From 



Brambell 1928.) 



linear. When the oocyte attains a diameter of approximately 70^1, and the 

 follicle a diameter of 125 yu, the former stops growing while the growth 

 of the follicle continues, so that the average follicle diameter at ovulation is 

 550 /z. The antrum first appears as an irregular fluid-filled cleft in the 

 middle of the follicular cells on one side of the oocyte in follicles about 200 /jl 

 in diameter. 



Ovarian regeneration. — As a result of reports in the medical literature of 

 conception following complete double ovariectomy, there have been a 

 number of experiments with mice and rats to determine whether or not there 

 is any ovarian regeneration following removal of both ovaries. While the 

 results have been conflicting, the most recent experiments seem to indicate 

 that regeneration does not occur if the removal is complete (66, 107). If 

 regeneration does occur it is probably the result of a small piece of ovarian 

 tissue having been left. Regenerated ovaries contain follicles and may be 

 fully functional (37, 105). 



