192 CHARLES L. PARMENTER 



by the first condition, should exceed the errors of measurement 

 by a good margin. Unless the above conditions are met, the 

 errors of measurement make it possible to contend that the 

 chromosomes are arranged in a series of successively increasing 

 lengths which bear no relation to one another and therefore do 

 not represent pairs. 



In addition to the above, the form of the chromosome, which 

 is probably determined in large part by the position of the 

 spindle fiber attachment, may be used as an aid in determining 

 which chromosomes are homologues. McClung ('14, p. 674) 

 pointed out that, although the spindle fiber attachment may be 

 different on different chromosomes, nevertheless, for each chro- 

 mosome ''it is most precise and constant" in the individual. 

 Carothers ('17, p. 470) has shown that for certain tetrads (e.g., 

 figs. 32 and 63) the point of spindle fiber attachment on one 

 homologue is different from that on the other. But she also 

 finds that the point of fiber attachment is constant on a given 

 homologue for each individual. She shows (figs. 32, 32 a and 

 63, 63 a) that the point of spindle fiber attachment on these 

 homologues in the spermatogonia is preserved in the tetrads. 

 However, an exception to constancy of fiber attachment in the 

 individual has been noted by Wenrich ('16). He found in a 

 rod-shaped tetrad of another genus that the fiber attachment 

 might shift from one end of the chromosome to the other in 

 certain individuals. Therefore, according to the theory of the 

 indi\dduality, in the somatic chromosomes the homologues of 

 certain pair§ of chromosomes may be expected to be unlike in 

 form. However, indi\dduals showing such conditions are very 

 few and should be considered exceptions rather than the rule. 

 There is the possibility that these heteromorphic homologues 

 may not be confined to the Orthoptera, and certain cases in 

 Ambystoma make this appear to be so. 



Finally, it would be remarkable if any material satisfied the 

 above criterion in all points. The small difference in length 

 between some chromosomes makes it impossible to demonstrate 

 beyond doubt the presence of pairs among them. Again, the 

 possibility that the chromosomes may not condense at equal 



