CROSSINGOVER IN DROSOPHILA 191 



net eyes, and forked bristles — v-s-g-f — -a series of tests was 

 made which in every way resembled those made with second 

 chromosome stock. These characters were used in two different 

 combinations, as shown in the table. The results were as above. 



The percentages of crossingover show no significant differences 

 at any of the four different' temperatures used. It is also ap- 

 parent that no decrease of any significance is observable in the 

 second brood values. This fact was noted by Bridges at the 

 time when he recorded the decrease in the second brood values 

 for the second chromosome. 



The third chromosome data is too limited to record at present, 

 but it so far has shown no increase in the amount of crossing-over 

 due to temperature. This failure of temperature to cause a 

 change in the percentage of crossingover is interesting, though 

 it in no way invalidates the conclusions drawn from the second 

 chromosome data. Taken in conjunction with the fact that no 

 alteration in the percentages for second broods is found, it may 

 mean that the units in the three groups as we are accustomed to 

 calculate them, do not have the same significance, or that the 

 sections of the real chromosomes, which we know as genetic 

 chromosomes, do not correspond. Whatever is the meaning of 

 this difference in reaction to temperature, it gives one added 

 reason for believing that the 'genetic chromosomes' are discrete 

 elements which differ among themselves, and retain this indi- 

 viduality from generation to generation. 



VIII. ACKNOWLEDGMENTS 



In addition to those already mentioned, the author takes this 

 opportunity to express his indebtedness to Prof. T. H. Morgan 

 and Dr. A. H. Sturtevant for valuable advice and assistance in 

 the preparation of this research, to Dr. C. W. Metz for facilities 

 for work at Cold Spring Harbor and to Dr. J. W. Gowen for 

 suggestions as to statistical methods. 



IX. SUMMARY 



The foregoing paper has brought out the following important 

 facts : 



