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Prof. E. B. Wilson. 



when present, is confined to the male line, and hence always passes from 

 father to son. The X-chromosome, on the other hand, always passes from 

 father to daughter (because sperms of the X class produce females), while the 

 sons receive their single X-chromosome from the mother (because the male- 

 producing sperms are of the no-X or Y class) (fig. 2). I will show a little later 

 that on this curious fact probably depends the " criss-cross " type of sex-linked 

 heredity in which the sons are like their mothers, the daughters like their 

 fathers. 



The cytological phenomena of sex-production lend strong support to the 

 theory of the genetic continuity of chromosomes. They give unquestionable 

 proof, in case of a particular chromosome pair (XY), of the conjugation and 

 subsequent disjunction of corresponding maternal and paternal chromosomes. 

 They thus substantiate the conclusions of Henking and Montgomery and 

 confirm Weismann's earlier conception of the reduction division. They are in 

 full accord with genetic studies, which prove that one sex is homozygous, the 

 other heterozygous, with respect to a sex-determining factor. They give 

 the first direct evidence of a difference of nuclear constitution between the 

 homozygous and the heterozygous conditions, and of corresponding gametic 

 differences. And finally, in the case of a particular chromosome pair, they 

 fully substantiate the general cytological explanation that has been offered 

 of Mendel's law. 



(4) The facts just considered now lead us to some of the most intricate 

 and interesting of current problems. No phenomena appealed more strongly 

 to the interest of earlier naturalists than those of correlation. A very 

 interesting light is thrown upon this problem by the phenomenon now widely 

 known as gametic coupling or linkage, and it is here, perhaps, that we may 

 best appreciate to what an extent cytology and genetics reciprocally 

 illuminate each other. 



In the second of Sutton's original papers (1903) he pointed out what 

 seemed to be an obstacle in the way of his own hypothesis, of which much 

 has been made by later critics. The number of chromosomes is probably 

 always much smaller than that of Mendelian units in any given case ; hence 

 each chromosome must bear many such units. From this it follows that if 

 the composition of the chromosomes be fixed, or even fairly constant, the 

 units should cohere in definite groups, equal in number to that of the chromo- 

 somes ; but the earlier studies on heredity gave little definite evidence that 

 such was the fact.* Sutton did not, I think, meet the difficulty, which he 



* In a general way, of course, this fact was known to earlier observers, e.g., " We appear, 

 then, to be severally built up out of a host of minute particles, of whose nature we know 

 nothing, any one of which may be derived from any one progenitor, but which are 



