116 



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

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

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

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

 The 66 males lived :>2.8 days, while 02 females lived 25.9 days. The 81) 

 flies that descended from the Truncate grandfather lived an average of 

 29.3 days. There were 4.5 males which lived 31.1 days, while 44 females 

 lived 27.3 days. 



3. Discussiox. 

 The foregoing data lirings 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 auytliiiig like ^lendelian characters in the si'iisc that segre- 

 gation and recombination takes place, then we should e.xpect the shortened 

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

 study of the curves verities the exiicctation. for the .rianili-liiklren live 

 an avcriige of only 29.5 days. 



.\ study of the curves will show in each case three modes which corre- 

 spond with thi-ee periods of the greatest mortality. The meaning of smh 

 a phenomenon is ol)scure, 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, tliat lliese (lei)ression jieriods corresiioiid with tlie oiilimt 

 of the sex product.s. 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 be'_nns to la\ ecx-; wben two or three days old. Her egg prodmtion 

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

 she may cease to lay eggs for a da.\' or two and then a new cycle begins 

 which I'uns the same course, and this 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 close of the third cycle 

 she will ;is a rule live t<» a ripe old age, dejiositing a few eggs occasionally. 

 It is barely jtossible thiit these morfcility periods correspond to the depres- 

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

 critical evidence is hard to obtain, since the egg production seems to be 

 influenced l)y several factors. Moreover it is not evident that such an 

 explanation ai»plies to the male. 



