74 PHYSICAL BASIS OF HEREDITY 



cases. The most convincing evidence is that furnished 

 by Miss Carothers (1913) from some grasshoppers of the 

 genus Brachystola. Here, in addition to the single sex- 

 chromosome (in the male), that goes to one pole of the 

 first maturation spindle, there is also present another pair 

 of chromosomes that are unequal. In some cells the 

 smaller member of the pair goes to the same pole as the 

 sex-chromosome, in other cells it goes to the opposite 

 pole. The assortment of the unequal pair as regards the 

 sex-chromosome is therefore a random one. Thus, in 

 three hundred first spermatocytes, the smaller partner 

 went to the same pole as the sex-chromosome in 48.7 per 

 cent, of cases, and into the cell without the sex-chromo- 

 some in 51.3 per cent. Voinov ('14), Wenrich ('14) and 

 Robertson ('15) have reported similar cases. 



Other evidence of a different kind has more recently 

 ('17) been described by Miss Carothers. The evidence 

 rests on the constancy of attachment of the fibres of the 

 mitotic figure to a definite point of the chromosome, as 

 seen when the chromosomes are moving towards the poles 

 of the spindle. In one of the cases she describes there 

 are two kinds of attachments, viz., terminal, when the fibre 

 is attached at the end of the rod-shaped chromosome, and 

 subterminal when the fibre is attached some distance from 

 the end. In the latter case the end bends over, making 

 the chromosome J-shaped. There are certain individuals 

 in which one member of a pair of chromosomes may have 

 a terminally attached fibre, and its mate have a subter- 

 minally attached fibre. Throughout all the cell-divisions 

 of such an individual these two chromosomes show this 

 difference. During maturation, i.e., after conjugation of 

 the chromosomes, one member of this pair passes to the 

 pole of the spindle with a terminal attachment, and its 

 mate with a subterminal to the other pole. In the male, the 

 single sex-chromosome passes to one or to the other pole 

 at one spermatocyte division. Its relation to the two 

 members of the pair of chromosomes in question will show 



