The Bearing of Cytological Research on Heredity. 339 



the view that these factors are borne by the chromosomes, and may open the 

 way to its crucial experimental test. 



The full force of the hypothesis only becomes apparent when we come to 

 ■closer quarters with the facts. I shall attempt to illustrate this by consider- 

 ing certain phenomena which now stand in the foreground of interest and 

 bring home to us the intimacy of the relation that has been established 

 between cytology and genetics. 



(3) I first ask attention to certain facts relating to the cytological basis of 

 sex, a subject with which my own researches have been especially engaged 

 during the past ten years. To the cytologist the interest of the phenomena 

 extends far beyond the special problem of sex. Nature has here performed a 

 series of experiments which gives a crucial test of many of our earlier 

 conclusions, provides a secure basis for further advances, and at the same 

 time brings vividly before us the connection of the chromosomes with 

 heredity. I will here touch only upon the main facts, especially in their 

 bearing upon the phenomena of linkage, to which, I believe, they give the 

 •cytological key. 



That the chromosomes are involved in the determination of sex was first 

 suggested, in 1902, by McGlung, who argued on a priori grounds that the 

 so-called " accessory chromosome," which enters but half the spermatozoa, is 

 •a sex-determinant. A substantial basis for this conclusion was provided in 

 1905, when the late Dr. N. M. Stevens and myself, working independently on 

 Coleoptera and Hemiptera, discovered that in some of these insects the sexes 

 differ in the composition of the diploid chromosome-groups. In the simplest 

 type, first worked out in the Hemiptera, the " accessory " — or, as I have 

 preferred to call it, the X-chromosome, or sex-chromosome — is unpaired in 

 the male, but paired in the female. Since the sexes are identical in respect 

 to the other chromosomes, the latter may be disregarded, the sexual formulas 

 being written simply as XX for the female and X (or X0) for the male. All 

 ■of the other chromosomes are paired ; hence the male possesses an odd 

 number of chromosomes, one less than that of the female. Thus is explained 

 the fact, first discovered in 1891 by Henking in Pyrrhocoris, that in the 

 reduction division of the male the X-chromosome passes undivided to one pole, 

 so that two classes of spermatozoa are formed, one with X and one without. 

 In the female, on the other hand, the two X-chromosomes conjugate to form 

 a bivalent, as usual, and then disjoin in the reduction division, so that every 

 egg receives one X. This fact, at first inferred from the other relations, was 

 soon afterwards demonstrated by direct observation, first by Morrill in 

 insects, on rather scanty evidence, later fully established by Boveri, Gulick, 

 Mulsow, and Frolowa in nematodes. It thus became clear that fertilisation 



