554 
ZOOLOGY: H. H. PLOUGH 
Further investigation has demonstrated that the increase in the per- 
centage of crossing over due to high or low temperature appHed during 
the development of the female parent is maintained for only six or seven 
days after she begins to lay. At the end of this period the percentage 
drops to the same level as the control. It has also been found that the 
high or low temperature can be applied to adult females with similar 
results. After the flies are exposed to the new temperature, an interval 
elapses during which 225 to 275 eggs are laid which do not show the 
effect of the new temperature. The percentage then jumps suddenly 
to the high point where it remains for approximately the length of time 
of the exposure. A curve illustrating this point is shown in figure 2. 
The mothers of both series were sisters hatched at room temperature. 
3 5 1 II 13 li 17 
FIG. 2 
They were mated and placed in phials which were changed regularly 
at two day intervals. From the third to the eleventh day the pairs of 
one series were exposed to a temperature of 31.5°C while the others were 
continued at room temperature. The percentage of crossing over for 
the black to purple region is shown in the curve. After an interval of 
eight days following the beginning of the treatment (i.e., beginning on 
the same day that the flies were returned to normal temperature) the 
treated series showed an increase of more than 100% in the amount of 
crossing over among their offspring. This high ratio was maintained for 
eight days, after which it returned at once to the control value. 
The facts apparently indicate that temperature affects the amount 
of crossing over at a definite stage in the oogenesis. Exposures to high 
temperatures for one, two or four day periods make it appear that a 
consecutive exposure of nearly two days is required to produce any 
effect at all. Cytological examination has shown that shortly after 
hatching about 140 eggs have usually passed the last oogonial division. 
Controls show that eggs are laid at the rate of about 50 a day or 100 
