2O Edmund B. Wilson 



of the male. Fertilization of such an egg by a spermatozoon con- 

 taining the small idiochromosome will produce a group identical 

 with that occurring in the male; fertilization by one containing 

 the large idiochromosome will produce the characteristic female 

 group. This result is thoroughly consistent with that obtained 

 in the first type; for if the small idiochromosome be supposed to 

 disappear in the male, the phenomena become in every respect 

 identical with those occurring in the first type. The large idio- 

 chromosome is therefore undoubtedly homologous with the 

 heterotropic chromosome, and the latter owes its unpaired 

 character to the fact that its former paternal mate has vanished, 

 as I conjectured in my first paper. 



It is further evident that in synapsis, in both sexes, the members 

 of each chromosome-pair become coupled to form symmetrical 

 bivalents, except in case of the idiochromosomes of the male. 

 In this case alone do chromosomes of unequal size couple to form 

 an asymmetrical bivalent; and it is a consequence of this coupling 

 that the subsequent distribution allots the small idiochromosome 

 to one-half of the spermatozoa and the large one to the other half. 



D. Third "Type. Forms in which the Idiochromosomes are 



of Equal Size 



Of these forms I have been able to examine only a single case, 

 namely, that of Nezara hilaris; and in the course of a whole 

 summer's collecting I obtained but a single female in the proper 

 stage to show the oogonial divisions. Fortunately both ovaries 

 show a considerable number of division-figures which demonstrate 

 the facts with perfect clearness. 



A particular interest attaches to this form on account of the 

 fact, described in my first paper, that the idiochromosomes are of 

 equal size and hence give no visible differential between the two 

 classes of spermatozoa. This form gives therefore a test case con- 

 cerning my general conclusion that the differentiation of the 

 idiochromosomes has occurred only in the male; for since these 

 chromosomes are here alike in all the spermatozoa, it might with 

 some plausibility be assumed that the differentiation had in this 



