CVTOLOGIC'AL ASPFXTS OF HYBRIDS. 



91 



Guyer had, I believe, like results from his study of hybrid cannas, except 

 in the canna he found all gradations between direct and indirect divisions. 



The writer has studied the maturation divisions in the male nucleus of 

 hybrid cotton, which was supplied him by Dr. Webber. The results of this 

 study may be summarized as follows : Two sorts of maturation divisions 

 were distinguished, one clearly normal, the other abnormal. Concerning the 

 former it need only be said that the ring-formed chromosomes of the first 

 division were of uniform size and of the reduced number. The spindle gave 

 no indication of being double, and was entirely analogous with that in the 

 pure race. The abnormal divisions were all direct, but varied in a manner, of 

 which it is unnecessary here to speak further. Transitions between direct and 

 indirect divisions were not observed. 



From my study I concluded that the normal divisions lead to fertility, 

 the abnormal to sterility. 



The remaining purely cj-tological study is that by Juel on the sperma- 

 togeneses of Syringa rothoinagensis. Juel found abnormal mitoses only. These 

 were all stages between direct nuclear divisions and indirect, save only that 

 the chromosomes did not split, in the metaphase. 



What is the significance of Juel's results ? It is of interest iiere to note 

 that the plant may be wholly sterile, since Focke observes that he has seen no 

 fruit on it — that is, the abnormalities in spermatogenesis seen by Juel in 

 the hybrid Syringa lead to sterility. 



Thus Juel's results cause one to ask whether some, if not all, of the ab- 

 normal sex nuclei observed by Guyer in pigeon and canna were not func- 

 tionless, and whether the normal divisions alone do not lead to fertility? 

 Such is apparently the case in the hybrid cotton. If only sex nuclei that 

 undergo typical maturation mitoses form functional reproduction cells, it is 

 clear that the variation, in some hybrids at any rate, must be occasioned by 

 other causes than those of the irregularities in the splitting, distribution and 

 union of the chromatin of the sex cells. 



Here it maj' be suggested that an experimental stud3' of such a hybrid 

 as shows a tendency toward maintaining the individuality of its chromosomes 

 might be carried on with good results. Such a study should show the rela- 

 tive sterility of the sex cells, and thus it would supplement the results derived 

 solely from the cytological investigations. This has not yet been done, 

 however. 



While the cytological data just given do not harmonize completely, fur- 

 ther study may show that they are really not antagonistic, but that the differ- 

 ent hybrids studied should either be placed in different hybrid classes, or that 

 the "splitting" may not occur in the first, but later generations. 



Thus the leading results which have been obtained by the C}'toIogical 

 study of hybrids may be summarized as follows : 



(i) The establishment of one cause of sterility among hybrids. 



(2) The variation of the hybrid may or may not be associated with 

 abnormalities in spermatogeneses ; and 



('3) That in certain cases the chromosomes derived from the original 

 parent tend to preserve their individuality. 



Before closing this brief account of the cytological studies in hybrids I 



