Chromatin-reduction and Tetrad-formation in Pteridophytes. 



By Gary N. Calkins. 

 (Plates 295, 296.) 



From the time when Van Beneden, in 1883, found that mature 

 reproductive cells have only half as many chromosomes as the 

 ordinary somatic cells, until the present time, cytologists have 

 endeavored to explain the meaning of this reduction and to show 

 how it takes place. These efforts have not as yet been attended 

 with complete success. 



One of the most widely known theories as to the meaning of 

 reduction is that of Weismann whQ accepted the earlier concep- 

 tion of Roux (1883) as to the significance of mitosis and built 

 upon it an elaborate theory of development. In this he predicted 

 that a form of mitosis would be found in the maturation of the 

 reproductive elements " in which the primary equatorial loops 

 are not split longitudinally" (p. 371) and "by means of which 

 each daughter nucleus receives only half the number of ancestral 

 germ-plasms possessed by the mother nucleus" (p. 375). 



Weismann's prediction has been confirmed by recent observa- 

 tions on the copepod Crustacea, and it is now known that in this 

 group of animals at least, the chromosomes of a maturing cell un- 

 dergo a transverse division, giving reduction in the Weismann 

 sense. This process of reduction, wherever definitely made out, 

 is invariably preceded by an arrangement of the chromatin into 

 four-parted chromosomes, to which the name " Vierergruppen' or 

 " tetrads " has been given. These tetrads are always half as nu- 



