402 



BOWEN. 



definite cluster. Subsequently they become reunited to form the 

 characteristic chromatin nucleoli of the later growth period. This 

 same phenomenon occurs also in the normal cells, but the tendency 

 toward diffusion is here less marked and the fragmentation of the 

 chromosome nucleoli correspondingly less conspicuous. Indeed, it 

 was not noted by Montgomery ('11), who merely states that the 

 contour of the nucleoli at this time may be "irregular." In this con- 

 nection Wilson's ('12) figures (figs. 100 c, d,f) of the granular structure 

 of the sex chromosomes (nucleoli) of Lygaeus in the growth period are 

 very suggestive. 



Figure 4. Metaphase chromosome plates of the second maturation divi- 

 sion in Arvelius alhopunctatus (x3050). The hmits of the cytoplasm are indi- 

 cated by a simple line. (Flemming-hematoxylin.) A, from the smallest 

 generation; B, from the large generations. 



Except for these minor differences in the extent of diffusion, the 

 chromatic history in all generations of primary spermatocytes is 

 identical, and when the maturation division figures are formed, 

 the chromosome plates are likewise identical. Arvelius furnishes 

 excellent material for the study of these stages. In this bug the be- 

 havior of the chromosomes is similar to that of Etischistns (see Mont- 

 gomery '11), the first maturation spindle having six autosomes with 

 separate X and Y chromosomes, while the second has a ring of six 

 autosomes enclosing an X-Y dyad. The latter is shown in Text- 

 figure 4. Not only are the chromosomes the same in number in all 



