206 THE PERPETUATION OF ADAPTATIONS 



Wilhelm Roux, speculating on the significance of these 

 nuclear phenomena suggested that the minute and exact halving 

 of these fundamental structures of the cell and the complicated 

 processes by which this halving is brought about, must have 

 some important bearing on deeper biological problems, and 

 he concluded that karyokinesis is the means by which hereditary 

 characteristics, contained potentially in the chromosomes, are 

 distributed to all cells of the organism. This conception in 

 connection with the germ cells was taken up by Weismann and 

 worked into an elaborate theory of inheritance which was 

 published in complete form in his book on The Germ Plasm 

 in 1892. The chromosomes were regarded as aggregates of 

 different elements, each element (called a biophor) representing 

 some specific characteristic or group of characteristics of the 

 adult organism. With longitudinal division of the chromo- 

 somes each element is equally divided. 



Maturation Phenomena. In the meantime the discovery had 

 been made that the mature germ cells when ready for fertiliza- 

 tion contain only half the number of chromosomes character- 

 istic of the species, so that upon union of egg and spermatozoon 

 the normal number is restored, the resulting offspring inheriting 

 equally from the two parents. It had also been found that this 

 halving in number of chromosomes takes place in the germinal 

 endothelium during the process of ripening of eggs and sperma- 

 tozoa, and at the time of the preliminary mitoses which accom- 

 pany the formation of the germ cells. These peculiar divisions 

 became known as the maturation divisions. 



After the discovery of the reduced number of chromosomes, 

 and in accordance with his conception of the chromosomes as 

 the bearers of hereditary characteristics, Weismann in 1888 

 prophesied that in one of the maturation divisions it would be 

 found that the chromosomes do not divide longitudinally but 

 transversely so that the hereditary characteristics instead of 

 being equally partitioned between the daughter cells would be 

 divided crosswise so that the daughter cells would receive dis- 

 similar groups of biophors. The ordinary longitudinal division 

 of the chromosomes he called an equation division and the 



