180 SUMMARY OF CUKRENT llESKARCHES RELATIJJG TO 



previous conclusions : there were genetic differences present among the 

 original flies with extra bristles ; these were entirely independent of the 

 main factor that occasioned the monohybrid ratios in crosses ; selection 

 propagated the more homozygous flies, so that a race with uniform germ 

 plasm was soon secured. There is no indication that any genetic change 

 occurred during the course of the experiments. J. A. T. 



Crossing-over in Drosophila. — J. A. Detlefsen and E. Roberts 

 (Jourii. Exper. ZooL, 11)21, 32, 333-.54, 2 figs.). The question studied 

 was whether the percentage of crossing-over in Drosojjhila melanogaster 

 can be modified by selection. It was found that cross-over values are 

 very variable, and that part of this varial)ility is due to genetic causes. 

 Low selection has been effective in two entirely independent series. The 

 low cross-over stock bred true to about • 6 p.c. (almost zero) for nine 

 generations in one of the series. Tlie low cross-over stock bred true to 

 about 6 p.c for twenty-two generations in the other series. High 

 selection probably induces double crossing-over. Crossing-over in the 

 various regions of the sex chromosome (and other chromosomes ?) is 

 probably controlled bv multiple incompletelv dominant factors. 



J. A. T. 



Effect of Temperature on Crossing-over in Drosophila. — Harold 

 H. Plough {.lonm. Exper. ZooL, 1921, 32, 187-201, 3 figs.). In 

 Drosophila melanogaster a temperature of 31*5° C. has little or no 

 effect on crossing-over in any part of the sex chromosomes, nor is there 

 any significant variation with the age of the female. Crossing-over in 

 the " sepia-spineless " region of chromosome III. is increased by a 

 temperature of 31*5^ C, the etfect being most marked between 

 " dichaete " and " spineless." The same region shows a variation in 

 crossing-over according to the age of the female parent. Crossing-over 

 in the remainder of chromosome III. is influenced neither by tempera- 

 ture nor age. The chromosomal regions which are " sensitive " to 

 temperature and to age all give a very high ratio of .double to single 

 crossing-over. This is interpreted as indicating that the effects of 

 environment cause observable differences in crossiny'-over onlv where 

 crossing-over occurs least freely. It is shown that the view that 

 temperature and asfe act on crossing-over in different wavs is not 

 established. ^ ' J. A. T. 



Rectal Papillae of Panorpa. — L. Mercier (Oomptes Remlus Soc. 

 JJiol, 1920, 83, 758-60). The rectal " gills " of some aquatic larvfe are 

 represented in some adults by rectal papillaj, which may be functional 

 or vestigial. In Panorpa commtmis and P. germanica the rectal 

 papilla3 of the adult are hemispherical caps 250 to 300/x in diameter, 

 covered by polygonal epithelial cells, fibrillated externally (probably for 

 support) and enclosing the ends of trachege. .These papillae appear 

 during the pupal state as six cellular proliferations, much in the same 

 way as the rectal papillae in Calliphora ergthrorepliala and other 

 Muscidee. J. A. T. 



