CROSSING OVER AND CHROMOSOMES 99 



affects crossing over. Furthermore, if we keep eggs, 

 larvffi and pupae in a high temperature, and then find out 

 how many eggs have been affected by the high tempera- 

 ture, we can find out to what stage the eggs must have 

 developed in order that crossing over may be influenced. 

 Plough has made this calculation, and finds that only the 

 eggs that have reached the stage where conjugation of 

 the chromosomes takes place are affected — all the, earlier 

 stages are not influenced. It follows that the initial effect 

 appears at about the time of conjugation of the chromo- 

 somes, but whether the crossing over occurs at this critical 

 stage or some effect only is then produced that later 

 affects the crossing over is not specifically shown. Never- 

 theless, I am inclined to think it more probable that the 

 crossing over is actually changed at the time the heat acts 

 (rather than afterwards), because in general most reac- 

 tions of living things to environmental influence take place 

 immediately rather than after a long interval. However 

 this may be, the fact of prime importance in this work 

 is that earlier than the period of conjugation of the 

 chromosomes crossing over does not take place. 



Expressed in numbers of eggs, the results show that in 

 a just-hatched virgin female there are from 125 to 175 

 eggs that will be laid before the effects of heat are shown. 

 In the females that have just hatched about 150 eggs are 

 present that have passed beyond the conjugation period. 

 This number (150) agrees with the estimated number of 

 eggs (125-175) first laid that are not affected, and estab- 

 lishes the conclusion that after conjugation of the chromo- 

 somes crossing over cannot be influenced any more than 

 it could before that period. The results clearly establish, 

 then, that crossing over cannot be affected earlier than the 

 conjugation, but can be affected at the time when the 

 conjugation isi known to occur. 



As already pointed out, the chromosomes become 

 drawn out into long threads at the synaptic period, and 

 in many animals and plants these threads have been shown 



