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Prof. E. B. Wilson. 



genetics, not because the chromosomes are the only elements concerned in 

 heredity, but because they offer the most available point of attack and have 

 in fact yielded the most definite results. The limitations of time compel me 

 to take a good deal for granted, and to pass over, for the most part, the 

 historical and controversial aspects of the subject. I must be content, in the 

 main, to state briefly what I believe to be established or indicated by the 

 evidence. My task is much lightened by Prof. Farmer's earlier presentation 

 of many important aspects of the subject in his Croonian Lecture of seven 

 years ago. Permit me, nevertheless, for the sake of present clearness, to 

 indicate briefly some of the essential facts determined prior to the re-discovery 

 of Mendel's law in 1900. 



(1) The work of cytology in its period of foundation laid a broad and 

 substantial basis for our more general conceptions of heredity and its physical 

 substratum. It demonstrated the basic fact that heredity is a consequence 

 of the genetic continuity of cells by division, and that the germ-cells are the 

 vehicle of transmission from one generation to another. It accumulated 

 strong evidence that the cell-nucleus plays an important role in heredity. 

 It made known the significant fact that in all the ordinary forms of cell- 

 division the nucleus does not divide en masse but first resolves itself into 

 a definite number of chromosomes ; that these bodies, originally formed as 

 long threads, split lengthwise so as to effect a meristic division of the 

 entire nuclear substance. It proved that fertilisation of the egg every- 

 where involves the union or close association of two nuclei, one of maternal 

 and one of paternal origin. It established the fact, sometimes designated as 

 " Van Beneden's law " in honour of its discoverer, that these primary germ- 

 nuclei give rise to similar groups of chromosomes, each containing half the 

 number found in the body-cells. It demonstrated that when new germ- 

 cells are formed each again receives only half the number characteristic 

 of the body-cells. It steadily accumulated evidence, especially through the 

 admirable studies of Boveri, that the chromosomes of successive generations 

 of cells, though commonly lost to view in the resting nucleus, do not really 

 lose their individuality, or that in some less obvious way they conform to the 

 principle of genetic continuity. From these facts followed the far-reaching 

 conclusion that the nuclei of the body-cells are diploid or duplex structures, 

 descended equally from the original maternal and paternal chromosome- 

 groups of the fertilised egg. Continually receiving confirmation by the 

 labours of later years, this result gradually took a central place in cytology ; 

 and about it all more specific discoveries relating to the chromosomes 

 naturally group themselves. 



All this had been made known at a time when the experimental study of 



