Studies on Chromosomes 199 



As Montgomery and Sutton have shown, the ordinary paired 

 chromosomes of the spermatogonia give rise to bivalents of corre- 

 sponding size at the time of general synapsis. The actual coupling 

 of the ordinary chromosomes at this time is still a matter of 

 dispute;*^ but no doubt can exist in regard to the couplings 

 that occur at a later period in case of the m-chromosomes, the 

 idiochromosomes, and the supernumeraries. These characteristic 

 coupHngs are not determined merely by the size of the chromo- 

 somes. The union of the unequal idiochromosomes after the 

 second division takes place with the same regularity as that of the 

 equal m-chromosomes in the prophases of the first. A small 

 supernumerary that is indistinguishable from the m-chromosomes 

 in the spermatogonia never couples with the latter in either divi- 

 sion, but with the much larger idiochromosomes. The coupHngs 

 are equally independent of the original positions of these chromo- 

 somes, either in the spermatogonia or in the growth-period, as is 

 seen with especial clearness in case of the m-chromosomes. These 

 phenomena naturally suggest the conclusion that the coupHngs 

 result from definite affinities among the chromosomes. Thepossi- 

 biHty no doubt exists that the coupHngs are produced by extrinsic 

 causes (such as the achromatic structures) but the evidence seems 

 on the whole opposed to such a conclusion. I consider it more 

 probable that they are due to intrinsic qualities of the chromosomes 

 and that the diflPerences of behavior shown by different forms may 

 probably be regarded as due to corresponding physico-chemical 

 differences. This conclusion is in harmony with Boveri's experi- 

 mental results, though based on wholly different data. While 

 it does not seem worth while to attempt its wider development 

 here, I may express the opinion that all the chromosomes may con- 

 sist in the main of the same material basis, differing only in respect 

 to certain constituents; and further that the degree of quaHtative 

 difference may vary widely in different species. 



Zoological Laboratory 



Columbia University 



August 10, 1908 



'^ See for example, Meves ('07, pp. 453-468) who, like O. Hertwig, Fick and others, rejects the theory 

 of " individuality." 



