1 8 THE MATURATION OF THE EGG OF THE MOUSE. 



latest epoch at which it may originate depends on the time when the 

 first polar cell is formed, which, as above stated, may be as late as 28^ 

 hours p.p. It can be found in unfertilized eggs in a normal condition up to 

 at least 40 hours p.p. 



The accompanying "curves" (page 17) are given to illustrate the 

 ratio between two classes of eggs: the "first class" embraces eggs con- 

 taining the germinative vesicle, or the first spindle in any of its stages 

 up to the recently formed first polar cell (Stages I to VI, table 2); the 

 "second class," eggs of all succeeding stages (Stages VII to XI). A con- 

 sideration of these curves reveals some facts, or at least probabilities, 

 concerning the amount of time required for certain parts of the matura- 

 tion process. In the upper diagram the unbroken line represents the 

 number of eggs in the first class, obtained at various indicated epochs 

 (hours) after parturition, and the dotted line the number of eggs in the 

 second class at corresponding epochs. The sum of the two curves, 

 shown by the dot-and-dash line, includes all the eggs obtained up to 

 30 hours p.p. In the lower diagram the unbroken and dotted lines give, 

 respectively, in percentages the ratios of the number of eggs in the first 

 and second classes to the total number of eggs; they are, of course, 

 reciprocals of each other. 



The general trend of the percentage curve shows from the 1 4-hour to 

 the 16-hour epochs p.p. a rapid decrease in the proportion of eggs of 

 the first class during the earl 5^ periods. The great fluctuations in the 

 periods between 16 and 23 hours and between 26 and 28 hours are 

 probably due to the small numbers of eggs obtained in those periods 

 (compare upper diagram), and very likely would disappear to a large 

 extent with more abundant material. 



The whole process of maturation can be conveniently divided into 

 two parts, the first embracing those stages which are included in the 

 first class of eggs and the second part those in the second class. It 

 should be borne in mind that eggs of the first class are constantly in a 

 state of activity and are steadily advancing toward the formation of the 

 first polar cell; whereas eggs of the second class, if not seminated, re- 

 main for 24 hours or more in a quiescent condition in the stage of the 

 second spindle (Stage VIII). Consequently, the length of the period 

 in which the eggs of the first class fall would be an approximate measure 

 of the time required for that part of the process; but a similar period 

 for the second class would not be a measure of the amount of time nec- 

 essary for the completion of the second part of the process. The time 

 required for the latter is calculated by other means. Since in eggs above 

 the twenty-third hour p.p. the proportion of the first class is very small, 

 it can be said that usually the first part of maturation is completed 

 within the period between 14 and 23 hours p.p. When it is noted, 

 further, that the curve representing the percentage of eggs in the first 

 class drops very rapidly from the 14th to the 18th hour, it is fair to 



