46 THE MATURATION OF THE EGG OF THE MOUSE. 



This modification of Zenker's fluid, however, is the only one tried which 

 shows the finer structure of the chromosomes and does not shrink the 

 nuclei. Most of the figures by other investigators of the mouse egg 

 show imperfect preservation, and this has been, in our opinion, a potent 

 factor in causing the differences in their results. 



C. TIME RELATIONS. 



The possibility of obtaining a complete series of stages of the proc- 

 esses of maturation depends on accuracy in determining the epochs 

 of parturition and insemination. As we have seen (p. 22), there is 

 probably some individual variation in the length of the periods between 

 successive ovulations. If eggs from ovulations other than the one which 

 immediately follows parturition had been used by us, it would have been 

 extremely difficult, if not impossible, to secure a complete series, both 

 because of this variability in ovulation, and also because (see p. 17) 

 the stage of the second spindle may last for many more hours than the 

 stages which reach from the transformation of the germinative vesicle 

 to the formation of the first polar cell. It is probably a lack of precision 

 in this matter which accounts for the failure of others to get those stages 

 which pass quickly, such, for example, as the origin and metaphase of 

 the first spindle. 



As we have seen (pp. 16 and 19), the first maturation after parturi- 

 tion may occur during a period extending from about 13 hours to 29 

 hours p.p. Tafani (1889, p. 20) makes the period extend from 24 hours 

 to 48 hours, or even (18896, p. 113) 2 or 3 days p.p., a time somewhat 

 later than that indicated by our observations. 



Sobotta (1907, p. 504) says that the prophases of the first spindle 

 begin at least 24 hours before ovulation; but as he does not say when 

 ovulation occurs with respect to parturition (which is the only event 

 that can be determined directly), it is impossible to perceive how he 

 arrives at this particular number of hours as the minimum time. Appar- 

 ently Sobotta (p. 507) bases this conclusion on the parallelism which, 

 he maintains, exists between the histological changes in the wall of the 

 follicle (its ripening) and the ripening of the egg; but admitting the paral- 

 lelism, and granting that the prophase begins when the follicle is far 

 from ripe, we are unable to see any very precise ground for the estimated 

 time required for the ripening of the follicle. 



Kirkham (1907a, p. 259) states that he killed mice at various times 

 during pregnancy and at intervals from a few minutes after parturition 

 up to 30 hours after that event. In his later paper (19076, pp. 70, 71) 

 he adds: 



The ovaries of every mouse examined during the height of the breeding season 

 contained some eggs in which the first polar body had been already extruded and 

 in which the spindle for the second polar mitosis was fully formed. A majority of 

 the same ovaries revealed ovarian eggs at the end of the spireme or with the first 

 polar spindle. 



