6o 



BIOLOGY OF THE LABORATORY MOUSE 



number of these primordial ova is enormous. Arai (8) estimates that there 

 is a total of approximately 35,100 in the two ovaries of a new born rat. 



By birth, or shortly thereafter, another process has made its appearance; 

 namely, the degeneration of ova. This is very evident in the ovaries of rats 

 sixteen hours old and apparently reaches its height during the second and 

 third day post partum (25). Some follicles continue to grow, but degenera- 

 tion also continues, so that despite the production of new ova described 

 below, the total number of ova in both ovaries of twenty-three day old rats 

 is reduced to an average figure of about 11,000 (8). 



The proliferation of ova by the germinal epithelium continues after birth 

 (Fig. 32). There is some evidence that the process temporarily ceases or at 



Ovum in germinal 

 epithelium 



Terminal epithelium 



Tunica albuginea 



Ovum with follicle 

 cells 



Fig. 32. — The formation of ova from the germinal epithelium in a 45 day old rat. 



{Af/er Hargitt.) 



least is somewhat retarded from birth until several days thereafter (25, 70), 

 but this has also been denied (130). In any case, active proHferation is in 

 progress at six or seven days post partum. The young ova are distinguish- 

 able from the other cells in the germinal epithelium by their larger size, clear 

 cytoplasm, spherical and intensely staining nuclei, and by the fact that they 

 often occur in pairs. At eight days post partum ova may be seen separated 

 from the epithelium and in the process of passing through the thin tunica 

 albuginea toward the underlying stroma (25). At twelve to fifteen days 

 some of the follicles first acquire a small antrum (18, 44, 70). Accompany- 

 ing this (fifteenth day) the diameters of the larger folhcles show a sudden and 

 pronounced rise to a size almost equal to that of the follicle at the occurrence 

 of the first estrus (44). In rat ovaries, according to Lane (77), the percent- 



