54 STUDIES IN SPERMATOGENESIS. 



Comparison of the two types in Coleoptera, especially where, as in 

 the Carabidas, both occur in one family, has suggested to me that here 

 it is possible that the small chromosome represents not a degenerate 

 female sex chromosome, as suggested by Wilson, but some character 

 or characters which are correlated with the sex character in some spe- 

 cies and not in others. Assuming this to be the case, a pair of small 

 chromosomes might be subtracted from the unequal pair, leaving an 

 odd chromosome. The two types would then be reduced to one. It 

 may be possible to determine the validity of this suggestion for partic- 

 ular cases by observation or experiment. 



Since the first of this series of papers was published, there have 

 appeared three important papers by Prof. E.iB. Wilson, bearing on the 

 problem of sex determination in insects. These papers are based on 

 a study of many species of the Hetniptera heteroptera. These insects 

 fall into two classes — one in which a pair of " idiochromosomes," 

 usually of different size, remain separate and divide quantitatively in 

 the first spermatocyte, conjugate and then separate in the second 

 maturation mitosis; and another class in which an odd chromosome — 

 the ' ' heterotropic ' ' chromosome — divides in one of the maturation 

 mitoses, but not in the other. Wilson regards the odd chromosome as 

 the equivalent of the larger of the "idiochromosomes," its smaller 

 mate having disappeared. In the somatic cells of the former class he 

 finds in the male the unequal pair, in the female an equal pair, the 

 smaller chromosome being replaced by an equivalent of the larger 

 " idiochromosome." , In the latter class the male somatic cells con- 

 tain the odd number, the female somatic cells and obgonia an even 

 number, the homologue of the odd chromosome of the male being 

 present and giving to the female one more chromosome than are found 

 in the male. 



In his latest paper Wilson C'o6) makes a variety of suggestions as 

 to sex determination. He shows that if the "idiochromosomes " and 

 the heterotropic chromosome be regarded as sex chromosomes in the 

 double sense that they both bear sex characters and determine sex, the 

 following scheme accounts for the observed facts in all cases where 

 an unequal pair or an odd heterochromosome have been found : 



Sperm. Egg. 



( Large (^ ' ' idiochromosome ' ' I 

 I J or >-\- Large J sex chromosome=a J 



i Odd chromosome. J 



f Small " idiochromosome "1 

 II J or f + Large <^ sex chromosome=a ^ 



\ No sex chromosome I 



