80 M. M. RHOADES 



somes IV in Drosophila. Bridges, in Morgan, Bridges, and Sturtevant (1925), 

 established that the distribution of the chromosomes in triploid Drosophila 

 was not according to chance. Beadle (1935) reported that crossing over in 

 triploid Drosophila near the centromere region between one member of at- 

 tached -X's and a free X chromosome was correlated with autosomal dis- 

 junction. Lower crossover values were found in 1X2,4 and XX L4 combina- 

 tions than in IX 1.1 and XX 2.4 gametes. This non-random distribution 

 indicates a correlated orientation of non-homologous chromosomes on the 

 equatorial plate. 



In Sciara the paternal set of chromosomes moves away from the pole of 

 the monocentric spindle of the primary spermatocyte. The two sister .Y 

 chromosomes pass to the same pole at the second spermatocyte division 

 (Metz, 1938). Schrader (1931) observed a non-random orientation in Pro- 

 tortonia which led to selective distribution in secondary spermatocytes. 

 Catcheside (1944), in an analysis of Zickler's data on spore arrangement in 

 the Ascomycete Bombardia lunata, found that certain genes were prefer- 

 entially segregated. Not all of the above examples are strictly comparable to 

 the situations found in maize, Sciara, and Bombardia. In the latter cases a 

 specific spindle pole receives a certain chromosome or set of chromosomes, 

 while in the Drosophila cases particular chromosomes pass preferentially to- 

 gether, but presumably at random, to either pole. 



The neo-centromeres arising from chromosome ends, reported in rye by 

 Prakken and Muntzing (1942) and Ostergren and Prakken (1946), closely 

 resemble those found in maize. In both maize and rye the neo-centric 

 regions are found on arms possessing knobs (heterochromatin), and the pole- 

 ward movement of neo-centromeres is precocious in both plants. Unfortu- 

 nately, nothing is known about preferential segregation in rye, but it should 

 occur if our hypothesis is correct. 



