24 THE MATURATION OF THE EGG OF THE MOUSE. 



VI. SIZE OF EGG. 



All measurements made to determine the size of eggs at different 

 stages of maturation have been made on eggs fixed in the same way and 

 measured with the same objective and the same eyepiece and micro- 

 meter. The diameter does not include the zona pellucida. Since the 

 egg is seldom quite spherical the longest and the shortest diameter of 

 the middle section of the series into which each egg was cut was meas- 

 ured. Half the sum of these two measurements was taken as the diam- 

 eter of the egg. 



Table 2 (p. 14) shows clearly the changes in size of the ovum as 

 it advances in maturation. Under the heading ''Diameter of eggs" 

 the first column gives the number of eggs measured; the second column, 

 the average diameter of all these eggs; the third and fourth columns, the 

 diameters of the largest and the smallest eggs of each lot measured. 



It will be seen that, with one exception, there is a steady decrease 

 in size from Stage I to Stage VII. The exception, Stage II, shows only 

 a slight deviation and is probably due to the fact that the average is 

 based on so small a number (13) of eggs. Stages IVa and TVb, hitherto 

 treated by us as Stage IV, show the same progressive decrease. There 

 is a small reduction in size at the time the first polar cell is formed 

 (Stage VI), and another in Stages VII and VIII, when the eggs have 

 left the ovary and have been in the oviduct for only a short time. Pos- 

 sibly the fact that Stage VII is not intermediate in value between Stages 

 VI and VIII may be due, as presumably in Stage II, to the small num- 

 ber (10) of eggs on which the average is based. Eggs that were observed 

 in the oviduct about 29 hours or more p.p. show a slight increase in 

 size (see foot-note to table 2). The sizes in the remaining stages can 

 have no special meaning because the eggs had been in the oviduct vary- 

 ing lengths of time. 



Aside from the change in volume, there is, as the column of maxi- 

 mum and minimum diameters shows, considerable individual variation. 



