No.f>2!>] THE NATURE OF GRAFT-HYBRIDS 



51 



chromosomes in the germ cells was found to be twenty- 

 four, which was to be expected if these were derived 

 from cells with forty-eight chromosomes; i. e., one-half 

 the number of the twenty-four plus seventy-two chromo- 

 somes of the two parents. It is assumed by Winkler 

 that a reduction in the number of chromosomes follows 

 the fusion of the cells. He says : 



This chromosome number, i. e., forty-eight, is most readily explained 

 by the assumption that in the formation of (he graft hybrid a night- 

 cell (with twenty-four chromosomes) united. The resulting cell, from 

 which the subepidermal layer at the apex of the darwinianum hybrid 

 arose, had a nucleus with ninety-six chromosomes which then under- 

 went a reduction division resulting in forty-eight chromosomes. 



This study of the tissues of »S'. darwinianum indicates 

 that the subepidermal tissue from which the sporogen- 

 ous cells develop is of genuine hybrid nature arising 

 from a fusion of cells including the nuclei derived from 

 the two parent forms. 



In his latest paper (5) Winkler gives a brief summary 



Hybrids may be arranged in two groups, sexual and graft hybrids. 

 The latter may be divided into three classes according to the theoretical 

 possibility of* their method of origin, viz.: (1) Fusion graft-hybrids 

 arising from a fusion of two somatic cells derived from distinct species. 

 (2) "Influenced" (" Beeinflussungs Propfbastarde ") graft-hybrids 

 which arise from specific influences of one graft component upon the 

 other without cell fusion (as through chemical substances, transloca- 

 tion of cytoplasm, etc.). (3) Chimeras, in which specifically pure 

 cells from both graft components are combined to form a new individ- 

 ual. These chima>ras may be: (a) Sectorial chimeras in which the two 

 sorts of cells in the growing point are divided by a longitudinal plane. 



(b) Periclinal chimeras in which the pcridinal cell layers of the grow- 

 ing point are furnished respectively from one or the other parent form. 



(c) Hyperchima-ias in which the -rowing point is made up of a 



The first of Winkler's graft-hybrids were unmistak- 

 ably chimaeras of the first type. Of his later graft- 

 hybrids to which he gave special names, all except 8. 



