1 68 M. LOUISE NICHOLS. 



Besides the idiochromosomes, Wilson discovered in the He- 

 miptera a pair of chromosomes equal in size but noticeably 

 smaller than the others, which he designated as w-chromosomes. 

 According to the researches of Stevens these are occasionally 

 present in the Coleoptera, i. e., in Trirhabda virgata and T. 

 canadense and in an unidentified buprestid. They likewise are 

 represented in Eiichroma (Figs. 19, 23, 24). In addition there 

 are, in the spermatocytes, eleven chromosomes of more nearly 

 equal size, making the total reduced number thirteen. 



In most forms heretofore studied, the idiochromosomes are 

 evident not only at the time of mitosis but also in the resting 

 stage and prophases, for while the other chromosomes become 

 resolved into the nuclear network, the idiochromosomes remain 

 compact. It is in the manner of formation of the chromosomes 

 during the prophases of the first maturation division and in the 

 fact that neither at that time nor in the previous stages are the 

 idiochromosomes distinctly different in behavior from the other 

 chromosomes that the chief interest of the spermatogenesis of 

 this beetle lies. 



The nuclear network of the last generation of spermatogonia 

 is of delicate texture. Chromatin masses occur at intervals, 

 at first few in number and without constancy of position or shape 

 (Fig. 5). The masses gradually become more distinct and form 

 elongated threads near the center of the nucleus (Figs. 6, 7). 

 The network breaks away from the nuclear wall and the synapsis 

 is inaugurated (Figs. 8, 9). During this time there is no evidence 

 of the idiochromosomes being isolated from the synaptic threads 

 or failing to take part in their formation, nor, in the resting 

 spermatocyte, do the idiochromosomes differ from the others. 

 Stevens (1906) has reported a somewhat similar condition in 

 the beetle Tenebrio molitor. 



The nuclear network of the resting spermatocyte is more 

 clearly defined than that of the spermatogonia and bears chro- 

 matin masses distributed with a fair degree of regularity (Fig. 

 10). This condition, however, does not continue. Instead of 

 the usual spireme formation, the chromatin granules commence 

 to migrate towards a specialized area within the nucleus (Figs. 

 11-14). The final result of this process is the formation of a 



