116 



A study of curves D and E shows that the shortened length of life of 

 the Truncates reappears again, and this is true whether the grandchildren 

 have descended from the Truncate male or Truncate female. The 128 flies 

 descended from the Truncate grandmother lived an average of 29.5 days. 

 The 66 males lived 32.8 days, while 62 females lived 25.9 days. The 8!) 

 flies that descended from the Truncate grandfather lived an average of 

 29.3 days. There were 45 males which lived 31.1 days, while 44 females 

 lived 27.8 days. 



3. Discussion. 



The foregoing data brings out the fact that when the Truncate stock 

 with an average life of 21.4 is crossed to the Inbred stock with an average 

 life of 37.4 days, the hybrid that results lives 47 days. If the complex of 

 factors or whatever concerned upon which the length of life in these flies 

 depends, behaves anything like Mendelian characters in the sense that segre- 

 gation and recombination takes place, then we should expect the shortened 

 length of life of the Truncates to reappear among the grandchildren. A 

 study of the curves verities the expectation, for the grandchildren live 

 an average of only 29.5 days. 



A studj' of the curves will show in each case three modes which corre- 

 spond with three periods of the greatest mortality. The meaning of such 

 a phenomenon is obscure, and had the experiment not extended over a long 

 period of time I would be Inclined to doubt its reality. There is a possi- 

 bility, however, that these depression periods correspond with tlie outjiut 

 of the sex products. My experience in isolating eggs day by day laid by 

 over 200 females seems to indicate that the eggs are laid in cycles — that is. 

 a female begins to lay eg.^s when two or three days old. Iler egg production 

 gradually lises to a maximum, and then it declines almost to zero. In fact 

 she may cease to lay eggs for a day or two and then a new cycle begins 

 which runs tiie same course, and tbis in turn is followed by a third, in 

 the period when the female ceases to lay eggs she is most likely to die. 

 However, if a female survives such a period at the clo.se of the third cycle 

 she will as a rule live to a ripe old age, depositing a few eggs occasionally. 

 It is barely possible that these mortality iieriods correspond to the depres- 

 sion periods in the egg-laying cycles. It must be admitted however, that 

 critical evidence is h;ird to oI»tain, since the egg production seems to be 

 influenced by several fjictoi-s. Moreover it is not (nident that such an 

 explanation applies to the male. 



