412 Mr. William Bate Banhj [April 6, 



characters, say a black and a white. [In the first case, no matter 

 what its parents or their pedigrees may have been, the zygote breeds 

 true indefinitely, unless some fresh variation occurs.] 



" If, however, the zygote be gametically cross-bred, its gametes 

 [or germ cells] in their formation separate the pair of characters 

 again, so that each gamete contains only one character of each pair. 

 At least one cell division in the process of gametogenesis is therefore 

 a differentiating or segregating division, out of which each gamete 

 comes sensibly pure in respect of the unit characters it carries, 

 exactly as if it had not been formed by a cross-bred zygote at all." 



For our purposes this may be reduced to three propositions : (1) 

 that inheritance consists in the transmission of independent characters, 

 of which each race or species possesses a definite number ; (2) that 

 these characters form pairs of opposites or alternatives ; (3) that in 

 the formation of the germ cells these characters are sorted out and 

 distributed so that no germ cell carries both members of a pair. 

 Can any material basis be found for these ? To this we will now 

 turn. 



Five years ago it is doubtful whether there existed in the whole 

 domain of science such a charnel-house of dead facts as in that of 

 the science of cell structure. Thirty years of active study of animal 

 and plant cells prepared for microscopical examination in various 

 ways had resulted in the accumulation of a multitude of details 

 respecting the structure of the cell nucleus and of the extraordinary 

 way in which it behaves in cell division, and especially in those cell 

 divisions which produce the germ cell. It was known that from the 

 characteristic substance of the nucleus — which stains very deeply 

 with aniline dyes, and hence is called chromatin — a continuous 

 thread is spun as the first step in cell division, and that this thread 

 of chromatin splits across into rods called chromosomes, each of 

 which again splits, this time not across, but lengthwise, so as to form 

 two " daughter " chromosomes, which, under the influence of a 

 peculiar field of force formed in the substance of the cell, move away 

 from one another and gather at the opi)osite poles of the spindle- 

 shaped field, there to fuse and form the two nuclei of the " daughter " 

 cells. 



A further very significant and curious fact was known — namely, 

 that the number of chromosomes formed in the process is not a 

 chance one, but, in the first place, it is always an even luunber, and, 

 in the second place, each species of animal or plant has a character- 

 istic number. In the division of the cells of the human body, for 

 instance, there are formed thirty-two chromosomes. But to these 

 and many other similar facts no significance could be attached, 

 beyond the obvious one that the nuclear substance is not divided 

 grossly to form a new cell generation, but disti'ibuted by a comi)lex 

 and minutely detailed process of sul)division and segregation. 



Not only were the facts of nuclear division without significance, 



