532 Cannon : Studies in Plant Hybrids : 



Mottier * has described processes in the second maturation 

 division which should be considered in connection with the earlier 

 nuclear divisions as well. In the metaphase of the second division 

 the chromosomes are brought into the paired position in the 

 nuclear plate by the action of the fibers of the spindle ; that is, 

 the pairing here is probably due to the activity of the achromatic 

 elements. This seems to make unnecessary the conception of the 

 preliminary pairing of the chromosomes as a direct preparation 

 for heterokinesis, since the spindle can distribute the chromosomes 

 at its will, so to speak. It should be considered, however, that 

 the rings are bivalent, and are probably formed without the inter- 

 vention of the fibers, since the rings antedate the formation of the 

 spindle itself Also, if my observation is correct, the spindle 

 fibers are not instrumental in segregating the chromosomes in 

 the presynaptic division, because in that case, with possibly one 

 exception, the pairing was not observed until the chromosomes 

 were at the poles of the spindle and hence probably after the work 

 of the fibers of that dividing nucleus was done. So it may well 

 be that there is not only a difference in the homologies of the 

 chromosome-pairs of the metaphase of the second division and 

 those of the presynaptic one, but a difference in their affinities as 

 well ; consequently influences, whatever they may be, other than 

 those of the spindle, may cause the earlier pairing. 



The foregoing considerations are based upon the idea of the 

 individuality of the chromosomes and upon that of their being 

 qualitatively unlike.f If these conceptions are valid we should 



* Mottier, D. M. The behavior of the chromosomes in the spore mother-cells of 

 higher plants, etc. Bot. Gaz. 35 : 250. I903. 



•f-The results of Rosenberg's studies (see bibliography) on the sporogenesis 

 of the natural hybrid between Drosera longifolia and Drosera rotundifolia seem to 

 throw objections in the way of too strict a conception of the individuality of chromo- 

 somes in hybrid plants. Rosenberg finds that the reduced number of chromosomes in 

 the Drosera hybrid may be 10, as in D. rotundifolia, or 20, as in D. longifolia, or 15, 

 which is half the somatic number in the hybrid. He also finds some evidence indicat- 

 ing that part of the chromosomes may be made up of four portions of chromosomes 

 each, and others of two halves of chromosomes each; the former kind are also larger. 

 In any case it is difficult to reconcile Rosenberg's results with the idea that the chro- 

 mosomes are distinct morphological entities ; the results point rather to the preserva- 

 tion by a portion of the spores of the purity of the chromatin, a fact which may prove 

 disconcerting to recent critics in this field (Cook; see bibliography). It should be 



