THE CHROMOSOMES 193 



of these cells. In support of this hypothesis are the remarkable 

 observations, first, of M'Klung of Kansas, and, later, of E. B. 

 Wilson, of New York, that the spermatozoa of sundry insects 

 are of two orders, though there is but one type of egg. The one 

 order of spermatozoa gives rise to males, the other to females, 

 the difference between the two being in their chromosomes. In 

 the maturing spermatocytes which give origin to the sperma- 

 tozoa, either the one set of cells possess an accessory chromo- 

 some, or, in other cases, a particular chromosome in one half 

 the maturing spermatozoa is large, in the other half is minute. 

 To quote M'Klung : 



" A careful consideration will suggest that nothing but sexual 

 characters thus divide the members of one species into two 

 well-defined groups, and we are logically forced to the conclu- 

 sion that the peculiar chromosome has some bearing on the 

 arrangement." 



Here we are not discussing sex, and I do but note these 

 observations in passing. There are other cases, not as yet 

 fully worked out, in which, as in the aphides, there would appear 

 to be one type of spermatozoon and two types of ova. 



The natural conclusion to be reached from all these data is 

 that the nuclear matter conveys and determines, or controls, the 

 inherited peculiarities of the individual ; further, the conveyance 

 is through matter contained in the chromatin loops or chromosomes, 

 while it may be that these individual loops, varying among them- 

 selves, determine particular conditions. 



What we know concerning the spermatozoon points very 

 definitely to the conclusion that the groups of chromosomes 

 distributed to the spermatozoa derived from a single spermato- 

 cyte are not identical, each spermatozoon receiving only one 

 half the number of chromosomes proper to the primordial 

 germ cell, and to the cells in general of any particular species. 

 The ovum on its part exhibits a like reduction. To inquire 

 further into this remarkable reduction process would lead us 

 into the discussion of variation and the Mendelian doctrine. 

 I do but mention these matters here to call attention to the fact 

 that not merely inheritance but variation is seen to be most 

 intimately associated with the nuclear material, and that, if 

 we can trust our eyes, the one morphological constituent involved 

 in and responsible for all cases of inherited peculiarities and 



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