218 HEREDITARY TRANSMISSION [CH. 



the heterotype division, but fuse so intimately that the 

 chromosomes after separation cannot be regarded as iden- 

 tical with those which paired in the synaptic stages 1 . Other 

 observers, again, maintain that after the chromosomes have 

 come together in pairs they never separate again, and that 

 in both the maturation divisions they split longitudinally, 

 with the result that each chromosome of the mature germ- 

 cells contains parts of the two chromosomes which paired 

 in synapsis 2 . Both these latter arguments would be fatal 

 to the hypothesis under consideration if they were sub- 

 stantiated, for each maintains that the chromosomes of the 

 mature germ-cells contain a mixture of paternal and mater- 

 nal elements, and the essence of Mendelian segregation is 

 that the characters borne by the germ-cells are pure and not 

 mixed. The evidence, however, on which these observers 

 rely is negative; in their material they cannot see the separa- 

 tion of complete chromosomes and so they deny its existence. 

 But if it is visible in material derived from other species 

 which are possibly better adapted for the study of these 

 stages, it seems more probable that the failure of some 

 observers to find it is due rather to the unsuitable material 

 on which they have worked than to the real absence of a true 

 "reduction division 3 ." 



The chief arguments that have been brought against the 

 hypothesis from the standpoint of genetics are, first, the 

 existence of segregation of characters elsewhere than in the 

 germ-cells, and, second, the fact that there are many more 

 Mendelian characters than chromosomes in at least several 

 species. The evidence for segregation of inherited characters 



1 e.g. BONNEVIE, 1908. 2 e.g. MATSCHECK. 



3 Since in some cases the chromosomes appear to fuse more or less com- 

 pletely in synapsis, it has been suggested by several writers that when this 

 happens, " blended " inheritance results, while when they pair and separate 

 without fusion the factors borne by them segregate according to MENDEL'S Law. 



