22 THE MATURATION OF THE EGG OF THE MOUSE. 



V. OVULATION. 



The time when ovulation occurs in relation to parturition and the 

 maturation of the egg has already been given (p. 19). No attempt has 

 been made to determine how often ovulation occurs, nor the regularity 

 of such occurrence. It is perhaps worthy of record, however, that a 

 female kept isolated and killed 6 weeks after parturition gave eggs in 

 the oviduct in the stage of the second spindle, a fact which does not 

 conflict with Sobotta's statement that ovulation occurs at intervals of 3 

 weeks. On the other hand, careful records of the births of litters show 

 that the 3 -weeks periods are not constant, in fact, that they vary by 

 several days. As far as known to us, no careful examination into the 

 causes of these variations in mice has been made. 1 



Although the irregularity in the occurrence of ovulations which 

 may be as great as days or even weeks may possibly be caused by 

 coitus, it is certain that the first ovulation after parturition is entirely 

 independent of such external condition, because females removed from 

 males before they give birth to young always furnish eggs in the oviduct 

 if killed at the proper time. 



Just as there is a lack of perfect synchronism in the maturation 

 processes, so here some eggs pass from the ovary early enough to have 

 already reached the oviduct, while in the same individual others are in 

 the periovarial space, and still others are in the ovary. Since in each of 

 seven cases eggs were found in two, or sometimes three, of these places, 

 it is highly probable that in few or no mice do the eggs leave the follicles 

 at exactly the same time. 



In the ovaries of a mouse killed 22 J hours p.p., there occurred three 

 follicles (plate 6, figs. 38, 39, 40) showing in a rare way three stages in 

 the process of ovulation. First, the completely ripe follicle about to 

 rupture (fig. 38) ; secondly, the ruptured follicle before the escape of the 

 egg (fig. 39) ; and, thirdly, the flowing out of the contents of the follicle 

 carrying the egg with them (fig. 40) . They are all later conditions than 

 those figured by Sobotta (1907), and are an interesting supplement to 

 his observations. In fig. 38 the granulosa cells which form the sides 

 and fundus of the follicle are so numerous that they form a thick wall 

 several (four or more) cells deep, as Sobotta has pointed out; but the 

 side of the follicle next the surface of the ovary has already become 

 attenuated to such an extent that at its middle the nuclei of granulosa 

 cells are entirely wanting. The theca folliculi having also disappeared 

 in that region, the fluid contents of the follicle come into direct contact 

 with the germinal epithelium, which is stretched out into a thin mem- 



1 POSTSCRIPT. During the year 1910 Dr. J. Frank Daniel has independently 

 found the variation in the gestation of mice to be even greater than we have stated. 

 He has worked this out in considerable detail, as may be seen in his forthcoming 

 paper in the Journal of Experimental Zoology, Vol. 9, No. 4. 



