HISTORICAL SKETCH 445 



many prefer to apply the term, if at all, to the entire protoplasmic system 

 capable of differentiation and reproduction (c/. p. 421). With regard 

 to the origin of heritable variations, geneticists are still in accord with 

 Weismann in attributing them chiefly to hybridization and alterations in 

 the inheritance units themselves. They are not, however, inclined to see 

 in any process like "germinal selection" the underlying cause of such 

 gene mutations. Weismann's prediction that there would be found a 

 reduction in the number of genetic units at gametogenesis (in animals) 

 by a special form of nuclear division has been fulfilled, though the precise 

 manner in which it is accomplished differs from that postulated 

 by him. 



Notwithstanding the abandonment of Weismann's theory of onto- 

 genesis and the changes made in his theory of heredity, his influence on 

 both cytology and genetics can never be forgotten, chiefly because of his 

 emphasis on the need for careful studies of the nuclear mechanism at the 

 critical stages of the life history, and upon the idea that this mechanism 

 is in some way bound up with the phenomena of heredity. "It has been 

 Weismann's service to place the keystone between the work of the 

 evolutionists and that of the cjrtologists " (Wilson, 1900). 



The Twentieth Century. — The year 1900 marks the beginning of a new 

 era in cytology, for reasons which may be stated in the words of Wilson 

 (1924): 



This era of cell research coincides with the new era in genetics that opened 

 in 1900 with the rediscovery of the Mendelian phenomena of heredity by de 

 Vries, Correns, and Tschermak. This discovery was the outcome of purely genetic 

 experiments on hybrids; but almost at the moment of its announcement, by a 

 remarkable coincidence, cytologists had independently arrived at a point where 

 the cytological basis of the phenomena could be clearly recognized. Riickert 

 eight years earlier (1892) had briefly suggested a conjugation and disjunction of 

 corresponding paternal and maternal chromosomes in meiosis and an exchange 

 of material between them ("amphimixis of the chromosomes"), thus to a certain 

 extent foreshadowing the modern explanation of the Mendelian segregation 

 and of recombination by "crossing-over." Montgomery (1901), without knowl- 

 edge of Mendel's fundamental law of segregation, brought together almost all 

 of the essential data for its explanation, though he did not bring them into 

 specific relation with the genetic phenomena. He pointed out the constant size 

 differences of the chromosomes, emphasized the presence in the diploid groups 

 of paternal and maternal homologues in pairs, and accepted the conjugation 

 of these homologues in synapsis and their disjunction in the reduction division. 

 Boveri, in his remarkable paper on multipolar mitosis (1902), demonstrated 

 experimentally the determinative action of the chromosomes in development 

 and proved their qualitative differences in this respect. A possible connection 

 between the Mendelian disjunction and the reduction division was suggested 

 nearly at the same time by several observers, including Strasburger, Correns, 

 Guyer, and Cannon. It was, however, Sutton (1902, 1903) who first clearly set 



