. Biology of the laboratory mouse. Mice as laboratory animals; Mice; Animals, Laboratory; Mice. 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, s
. Biology of the laboratory mouse. Mice as laboratory animals; Mice; Animals, Laboratory; Mice. 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 ovar
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