No. 593] THE MECHANISM OF CROSSING-OVER 303 



both just alike, we must conclude either that they were 

 both derived from the same strand, after it had already 

 crossed over— in which case crossing-over must occur at a 

 stage in synapsis before the homologous chromosomes 

 split to form tetrads— or else that the tetrads were formed 

 first, and that then crossing-over occurred at two points 

 coincidently in the case of both pairs of threads, and at 

 identical points in both. It is not probable, however, 

 that, if crossing-over occurs at the stage of four threads, 

 these two pairs of threads would both cross over at the 

 same points, for according to the observations on which 

 Jannsens bases the idea that crossing-over occurs at this 

 stage, a crossing-over of both pairs of threads at the same 

 place rarely happens. The evidence thus far gained from 

 non-disjunction is, therefore, rather in support of the 

 theory that crossing-over occurs at an early stage in 

 synapsis. 



D. A Case of Crossing-Over in an Embryonic Cell 

 It may not be out of place here to record an exceptional 

 case of crossing-over in the male, which has not been ex- 

 plained. No other case of crossing-over has hitherto been 

 found in the male Drosophila. It had been established by 

 Altenburg and the author that the factor causing truncate 

 wings is in the second chromosome, and further that the 

 truncate factor is dominant under certain conditions, but 

 it does not usually express itself unless certain intensi- 

 fying factors— one in the first chromosome and one in the 

 third— are present; even then, the character sometimes 

 fails to develop. Thus, if a hybrid truncate male is pro- 

 duced by a cross of a truncate female to a black pink male 

 (black is in chromosome II and pink in III), when this 

 hybrid is back-crossed again to black pink females, only 

 the gray flies will carry the factor for truncate, since in 

 the male truncate can not cross over with the black in the 

 homologous second chromosome. But few of the gray 

 flies from such a cross except the gray, red-eyed females 

 will show the truncate character, for the others will not 

 contain both of the intensifying factors ; and even in the 



