No. 593] THE MECHANISM OF CROSSING-OVER 



305 



and whether they crossed over when long drawn out or 

 when short and thick, but at least the fact remains that 

 crossing-over may, in abnormal cases, take place in a cell 

 before the definitive growth period is reached, and even 

 in an individual (Drosophila male) in which no crossing- 

 over is the established rule. This fact is not utterly sur- 

 prising, inasmuch as even in somatic and gonial cells of 

 Diptera homologous chromosomes show a marked tend- 

 ency to lie near together (?". e., to attract each other), and 

 in Metz's preparations they may not infrequently be 

 found even twisted about each other somewhat. 



The fact that crossing-over occurs only in the female 

 Drosophila is naturally of great interest, although it is of 

 unknown significance. In the silkworms, on the other 

 hand, Tanaka has discovered that crossing-over takes 

 place in the male, but not in the female. Curiously 

 enough, although these seem at first sight to be opposite 

 cases, in both it is true that crossing-over takes place in 

 the homozygous sex, but not in the heterozygous, for in 

 Drosophila the female is homozygous for sex, the male 

 heterozygous, and in the moth these relations are re- 

 versed. Recently, however, Castle and Wright have pub- 

 lished data for the rat which, if sufficiently extensive, show 

 that crossing-over happens in both sexes. The plants in 

 which crossing-over has so far been studied have all been 

 hermaphrodites, and crossing-over takes. place m both^ 

 their spermato- and oo-genesis. There is, therefore, at 

 present no general rule which can be stated, in regard to 

 which sex crossing-over occurs in. This fact should be 

 taken into account in weighing the etiological evidence 

 in regard to crossing-over, obtained in forms in wine ) 

 the occurrence of crossing-over has not been stuc mc 

 genetically. For in such cases there is always the possi- 

 bility that the cytological studies are being conducted on 

 individuals in which crossing-over does not occur and 

 which would consequently give results quite irrelevant to 

 the subject. 



(To be continued) 



