l6 THE MATURATION OF THE EGG OF THE MOUSE. 



drawn into the equatorial plate, (IV) the first spindle in the equatorial- 

 plate stage with or without circumpolar bodies (see p. 33), (VI) the 

 telophase of the first spindle and the first polar cell just cut off; or, 

 secondly, some of the eggs may be in one stage, some in another. If, 

 under the latter condition, some eggs show either (II) the beginning of 

 the first spindle within the germinative vesicle, or (V) the separation of 

 the daughter chromosomes of the first spindle, or (VII) the formation of 

 the second spindle, others are sure to be in one or more of the adjoining 

 stages. 



The conclusions to be drawn from these observations are, first, 

 that some stages occupy less time than others, since, owing to the some- 

 what imperfect synchronism, in some cases all the eggs fall into one 

 stage, whereas in other cases some fall into one stage, others into another 

 stage; and, secondly, that the stages passed comparatively quickly are 

 those of the formation of the first spindle (II), of the dividing of the 

 first spindle and the cutting off of the first polar cell (V), and of the 

 formation of the second spindle (VII). Furthermore, the small numbers 

 of eggs in these three stages bear out these conclusions. In a similar 

 way it can be shown that the division of the second spindle takes place 

 in a relatively very short time. 



In the foregoing considerations Stages IVa and IVb can not with 

 fairness be separated, since there is much less difference between them 

 than between any other two stages. Also, neither of them is rare. Of 

 the two, IVb is more often associated with other stages. 



There is considerable variation among mice in regard to the time 

 relation between the stage of the egg and the interval between partu- 

 rition and killing. This variation may be so great that mice killed for 

 eggs in the oviduct are found to have them still in the ovary, and vice 

 versa. Nevertheless, a detailed study of this relation shows a uniformity 

 sufficient to enable one to say about when certain stages occur, and to 

 determine approximately the time of ovulation. Moreover, in connec- 

 tion with a knowledge of the relative length of the stages, it is possible 

 to form something of an idea of the rapidity of the whole process and of 

 its parts. 



How long before ovulation the germinative vesicle presents the 

 conditions shown at parturition is not known. It may be weeks or even 

 months. But it is quite certain that for several days, perhaps weeks, 

 before ovulation it has the structure which is found during the first 

 12 hours after parturition. Usually within 15 or 16 hours after parturi- 

 tion the vesicle has given place to the first maturation spindle. More 

 seldom it persists longer, even up to 20^ hours after parturition. 



The earliest first maturation spindle that we have observed was 

 formed 13} hours p.p. 1 ; and the latest was in existence at 28 J hours p.p. 

 Since the formation of the spindle is very rapid, it is probable that the 



1 For sake of brevity we have employed for post partum the abbreviation p.p. 



