EFFECTS OF AGING UPON GERM CELLS. 399 



sperm. Cleavage in physiologically good eggs is not decreased by 

 old sperm. 



From these data it will also be observed that the longevity of 

 dry sperm was greater than the eggs in the given experimental 

 conditions. In neither Toxopneustes nor in Arbacia was the 

 maximum longevity of the sperm determined. 



EFFECT OF AGE UPON RATE OF CLEA\ AGE. 



I have shown that the total number of eggs that cleaved in a 

 given time after fertilization, decreased as the eggs aged. I 

 propose now to show that there was, as F. R. Lillie, '14, indicated, 

 a corresponding retardation of the rate of early cleavage, with 

 increasing age of the egg; and in a later paper I propose to de- 

 scribe the irregular and abnormal character of cleavage in very 

 aged eggs. 



The rate of cleavage was ascertained by the total cleavage at 

 three intervals, namely, (i) 40 minutes after fertilization, when 

 the eggs first divided, (2) I hour after fertilization, when most 

 and sometimes all the eggs had divided once, and (3) 2 hours after 

 fertilization when all or very nearly all the eggs had divided at 

 least once. 



The data are brought together in Table VII. 



In experiment I, the eggs of 6 females were fertilized by one 

 male, when the germ cells were 130, 240, 350 and 470 minutes 

 old. A record was made at each interval, of the number of eggs 

 that cleaved in 40, 60 and 120 minutes after each fertilization. 

 In female no. i, 54 per cent, cleaved within 40 minutes, when the 

 germ cells were 130 minutes old; 37 per cent, cleaved in the same 

 interval when 240 minutes old; 3 per cent, when 350 minutes old, 

 5 per cent, when 470 minutes old. There was an undoubted de- 

 crease in the rate of first cleavage with age. The cleavage at the 

 same ages 60 minutes after fertilization also showed a similar re- 

 tardation with age of the germ cells, for 78, 85, 51 and 59 per cent, 

 respectively, cleaved. The total cleavage, namely 2 hours after 

 fertilization, as I have shown before, also decreased with age. 

 The corresponding figures were 82, 93, 92 and 77 per cent. 



The average for all 6 females brings out forcibly the retarda- 

 tion and decrease in cleavage. The average for the germ cells 40 



