STUDIES ON CHROMOSOMES 347 



contradictions and to announce a definite result. Oncopeltus is 

 indeed a case in which the X- and Y -chromosomes are very often 

 sensibly equal in size, and in ivhich the sexual differences of the dip- 

 loid groups are too elusive to he certainly distinguished by the eye. 

 These differences, nevertheless, almost certainly exist. In cer- 

 tain individuals a distinct size-difference between X and Y is 

 clearly evident in a large percentage of the cells at every stage of 

 the spermatogenesis; and even in individuals where they usuall}^ 

 appear equal, inequality is unmistakably seen in a small percent- 

 age of the cells. Aside from this, the close similarity — almost 

 identity — between Oncopeltus and Lygaeus bicrucis in all other 

 features of the spermatogenesis makes it extremely probable that 

 the same essential relations of the chromosomes to sex exist in 

 both, though they are only clearly obvious in Lygaeus. 



Like many other insects of this order, Lygaeus and Oncopeltus 

 are distinctly unfavorable objects for the direct study of synapsis 

 and the reduction-division — indeed the problem of synapsis seems 

 to be practically insoluble in these particular forms. They never- 

 theless present some very interesting features for comparison with 

 other forms. In the first place, the history of the sex-chromo- 

 somes may here be traced with almost unique clearness. They 

 may be identified at a very early pre-synaptic period, and followed 

 thence as individual bodies through every later stage up to the 

 time of their final delivery to the spermatid-nuclei. Everj^ step 

 may be followed in their conjugation and subsequent disjunc- 

 tion without any intervening process of fusion. In case of these 

 particular chromosomes, therefore, I consider synapsis and dis- 

 junction to be indisputable facts. It is far otherwise with the 

 ordinary chromosomes or 'autosomes.' It is extremely difficult 

 to gain any clear idea of their behavior in the synaptic period, 

 and I fear quite impossible to trace them individually through 

 the growth-period. On the other hand, their behavior in the 

 pre-synaptic period and in the maturation-prophases exhibits 

 some ver}^ interesting features when compared with other forms 

 m which the process of synapsis and its sequel are more accessible 

 to observation. In making such a comparison I have been for- 

 tunate in the opportunity^ to make use of some remarkably fine 



