STUDIES ON CHROMOSOMES 105 



a fact (Lutz, '07; Gates, '08), though it may be due in this instance 

 to a simple doubling of the whole group. Such cases led me sev- 

 eral years ago to the conclusion "that the nucleus consists of 

 many different materials that segregate in a particular pattern 

 . . . and that the particular form of segregation may readily 

 change from species to species" (Wilson, '09d, p. 2). 



Such changes must involve corresponding ones in the morpho- 

 logical and physiological value of the individual chromosomes; 

 and we must accordingly recognize the probability that these 

 individual values, though constant for the species, may change 

 from species to species as readily as does the number. Despite 

 the conformity to a general type often exhibited by particular 

 genera or even by higher groups, the individual chromosomes are 

 therefore per se of subordinate significance; and it may often be 

 practically impossible to establish exact homologies between those 

 of different species. How closely this bears on the origin of the 

 diverse conditions seen in the composition of the XY-pair is 

 obvious. 



Lastly, it is almost certain that changes of number may some- 

 times arise as a result of abnormalities in the process of karyoki- 

 nesis, such as the passage of both daughter-chromosomes, or of 

 both members of a bivalent, to one pole. In Metapodius I found 

 ('096) direct evidence of this in case of the XY-pair itself, and 

 endeavored to trace to this initial cause the remarkable variations 

 of number that occur in this genus. Many other observers have 

 recorded anomalies of this kind, in both plants and animals. It 

 seems entirely possible, as has been suggested by McClung ('05) 

 and by Gates ('08) that definite mutations may be traceable to this 

 cause; though probably such abnormalities may in general be 

 expected to lead to pathological conditions. 



CONCLUSION 



Some of the suggestions offered in the foregoing discussion are 

 admittedly of a somewhat speculative character; but they are not, 

 as I venture to think, mere a priori constructions, but are forced 

 upon our attention by the observed facts. The time has come 



JOURNAL OF MORPHOLOGY, VOL. 22, NO. 1 



