184 HAROLD H. PLOUGH 



ber because the viability of eggs laid at 31.5°C. is rather high. 

 The value in the second column of No. 4 is open to none of these 

 criticisms, and is approached by all the other values when proper 

 allowance is made for them. On the basis of the data in hand, 

 then, and taking into account the ordinary viability 3 one would 

 expect that in the ovaries of females not more than ten days 

 after mating temperature produces a measurable effect at a 

 stage so situated that 225 to 275 eggs may be laid before the eggs 

 which have passed through that stage can be laid. This rather 

 involved statement will be made clearer in connection with 

 cytological evidence which is given below. 



Before considering this evidence it becomes necessary to speak 

 of some further genetic data which has a direct bearing on the 

 statement made above. We have so far proceeded on the as- 

 sumption that high temperature becomes immediately effective 

 in changing the percentage of crossingover as soon as applied. 

 A series of fifteen pairs were made up exactly as in table 14 — 

 hatched from the same culture at 22°C. Five of these were 

 continued at 22°C. as a control, five were subjected to a tem- 

 perature of 31.5°C. for two days, and five for four days. The 

 results are given in table 16 and the percentages of crossingover 

 for the black-purple region are plotted in figure 9. The rise 

 recorded by the offspring of the series exposed for only two days 

 is less than half as high as is that reached by the series exposed 

 for four days during the same two day interval. It is not greatly 

 above the high point of the control series. The four day ex- 

 posure results in a rise in the percentage of crossingover to the 

 same point reached as a result of an exposure for eight days, 

 and there it remains for slightly more than four days' time. If 

 the temperature began to affect the developing eggs as soon as 

 applied, the two day exposure should have caused a rise to the 

 same maximum for a period of two days. Since the curve shows 

 that only a few eggs were affected, it appears that an exposure 

 of at least two consecutive days to a temperature of 31.5°C. 



3 R. C. Hyde (Jour. Exp. Zool., 1914, vol. 17, p. 369) considers a fertility of 83.6 

 per cent (total number of flies hatched divided by the number of eggs laid) as 

 unusually high for fertile stocks. 



