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Prof. E. B. Wilson. 



The proof of the reduction division likewise remains incomplete for the 

 chromosomes generally, and is fully demonstrative only in case of certain 

 special kinds of chromosomes, in particular the sex- chromosomes and the 

 " m-chromosomes " of the coreid Hemiptera. Strong confirmatory evidence of 

 both conjugation and disjunction has, however, been afforded for the chromo- 

 somes generally by studies on the maturation-process in hybrids, especially 

 in Drosera by Eosenberg, in (Enothera by Geerts, and in Lepidoptera by 

 Federley and Doncaster. 



The promulgation of the conclusions of Boveri and Montgomery opened 

 the modern period of cytological inquiry and, as has been said, provided a 

 substantial basis for the cytological explanation of Mendel's law. This 

 explanation follows in the most simple and natural manner from the 

 observed facts. It assumes primarily that the Mendelian phenomena result 

 from the shuffling (to employ the phrase of Farmer) of chromosomes that 

 are concerned in the determination of the so-called unit characters. More 

 specifically, the main assumption is that Mendelian allelomorphs are borne 

 by corresponding pairs of chromosomes, each consisting of a maternal and a 

 paternal member. By the conjugation of the homologous members of these 

 pairs two by two, to form bivalents or gemini, as assumed by Montgomery, 

 the maternal and paternal homologues assume such a grouping that they may 

 be disjoined in the succeeding reduction division (in general accordance with 

 Weismann's early prediction) ; and from this follows the disjunction or 

 segregation of the Mendelian allelomorphs which these chromosomes bear. 

 The independent distribution or assortment of different units is explained by 

 the assumption (in favour of which definite evidence now exists) that the 

 bivalents behave independently of one another. 



The explanation as here outlined was first clearly and logically developed 

 by Sutton in 1902-3, when a student in my laboratory. Naturally enough, 

 however, several others came independently to more or less similar conclu- 

 sions nearly at the same time — in particular Guyer, Correns, Boveri, Cannon, 

 and de Vries. As will appear later, Sutton's elegant hypothesis was too 

 simply framed to account for all the facts, and has had to undergo some 

 modifications. In its main principle, however, it has received cumulative 

 substantiation by later work in many -directions. An important confirmation 

 of the fundamental assumption is given by a discovery announced by 

 B. B. Gregory before this Society only a few weeks ago. In certain plants 

 of the Chinese primrose the usual number of chromosomes is doubled in both 

 the gametes and the somatic cells. The genetic evidence obtained from such 

 plants indicates that all the Mendelian units or factors thus far examined are 

 correspondingly doubled. This result weighs strongly, I believe, in favour of 



