166 



THE AMEBIC AX XATURALIST 



[Vol. LIV 



This is the usual result obtained with, however, the addition of 

 exceptional dark females which we are trying to explain. 



(b) If non-disjunction in the male: forms the following 

 zygotic classes : 



It should be noted that non-disjunction in the male of Droso- 

 phila does not occur. Yet in pigeons the male is presumably the 

 "homozygous" sex and this makes its chromosome condition in 

 respect to sex more closely analogous to the female Drosophila in 

 which primary non-disjunction does occur. The classes b(l) 

 and b(4) we may fairly suppose, fail to survive. The triple X 

 condition of form b(l) is fatal in Drosophila as is the absence of 

 both X and Y seen in class b (4). The two classes b (2) and b(3) 

 are however, real difficulties. The sterility of class b(2), the ex- 

 ceptional colored females has never been reported, as would un- 

 doubtedly have been the case, did it exist— just as their appear- 

 ance alone has excited comment and interest. Class b(3), more- 

 over, would certainly be white males and as we have already 

 seen there is no record of any sueh animals appearing in this 

 cross. Since these white males are the means of transmitting tin- 

 tendency for non-disjunction to further generations, their pres- 

 ence is necessary for the occurrence of secondary non-disjimetion 

 and their absence is a serious handicap to the acceptance of the 

 hypothesis in this material. 



(c) Non-disjunction in the female: 



,-lisju 



It will be seen that the exceptional colored females are neither 

 expected nor explained by this type of non-disjunction. Unless, 

 therefore, we are to assume that non-disjunction in doves is, in 

 almost all its fundamental-, different from the same process in 

 [h-n.<n/,Jnl,i. pi-uiiufin^ qualitatively different results, we must 

 agree that it fails to meet the experimental facts. If we do sup- 

 pose that it differs fundamentally, it may fairly he claimed that 

 no evidence of a conclusive nature either cytological or genetic 

 exists to lead us to say that iion-disjnnet ion is in any way involved. 



