328 



INTRODUCTION TO CYTOLOGY 



turn gave rise to (1) their corresponding primary type, (2) normals, and 

 (3) plants like themselves. 



In studying the chromosomes of these 25-chromosome ("simple 

 trisomic") mutants, Belling observed that the three members of the 

 trisome tended to form characteristically different trivalent configurations 

 at diakinesis and metaphase / in the primary and two secondaries 

 (Fig. 186). In the primary, Rolled, they most frequently formed either 

 a V-shaped chain of three or a ring of two with the third one projecting 



Normal 

 ^ gamete 



Z Z 



Normal 



0) a 



Z Z- 



2 



Rolled 



1 /■ 9 ■() 



2 Z 



Rolled 



Sugarloaf 



Gamete with 

 extra chromosome 



01 



Pojycarpic 



A 



a^ /a 



ao> 





Fig. 186. — Diagram showing chromosome constitutions of the primary Datura mutant 

 "Rolled" and its two secondaries, "Sugarloaf" and "Polycarpic," as postulated by 

 Belling. See text. 



from one point of union; and when the third chromosome was free from 

 the others it was straight. In the corresponding secondaries, on the 

 contrary, the commonest configuration was a closed ring of three, usually 

 with one of the members sharply bent; and when the third chromosome 

 was free from the others it formed a small ring by itself. To account for 

 these configurations and for the characteristic outward differences 

 between the primary plant and its corresponding secondaries. Belling 

 and Blakeslee (19246) announced the hypothesis that in some manner 

 an interchange of unlike terminal segments had occurred between two 

 chromosomes I, giving a chromosome with two "right" ends and another 

 with two "left" ends. When the extra chromosome in a 25-chromosome 



