SPERMATOGENESIS IN PARATETTIX. 265 



the nucleus soon becomes irregular, and the chromosomes of 

 the primary spermatocyte is formed by a breaking up of the 

 spireme thread into segments. The compact intensely staining 

 mass which has been traced through the growth period is shown 

 in Fig. 12 as a chromosome which differs in shape from the 

 other chromosomes. It is ovoid and without a constriction in 

 the middle, while all the other chromosomes are dumb-bell 

 shaped. Not all of the chromosomes are formed at the same 

 time. The chromatin retains its loose woolly appearance, until 

 after it has broken up into parts, then it gradually becomes 

 more compact, takes the stain more readily and each part assumes 

 the characteristic dumb-bell shape. While this is taking place 

 the boundary between the nucleus and the cytoplasm becomes 

 more irregular and by the time the chromosomes are completely 

 formed the cytoplasm has formed a vesicle around each of them 

 (Figs. 13 and 14). 



The chromosomal complex of the primary spermatocyte con- 

 sists of six dumb-bell shaped chromosomes and one ovoid chro- 

 mosome. Of the six dumb-bell chromosomes two are decidedly 

 larger than the others and one of these is much larger than the 

 other one, as is shown in Figs. 13 to 16 inclusive. The ovoid or 

 accessory chromosome is never among the other chromosomes 

 but always lies near the periphery of the nucleus as it did in the 

 growth period. When the chromosomes have become arranged 

 on the spindle the dumb-bell chromosomes are well toward 

 the center of the spindle, while the accessory is always near the 

 periphery. It does not remain long in the metaphase plate 

 but soon passes toward one pole undivided much in advance of 

 the other chromosomes. For this reason many sections of 

 metaphase plates show only six chromosomes and those which 

 show seven are often cut obliquely. Not all of the chromosomes 

 in the primary spermatocyte divide synchronously. Fig. 20 

 shows five of the dumb-bell chromosomes divided while the 

 largest one shows little constriction. This division is transverse 

 as is shown in Figs. 16 and 20. There are no loops, rings, or 

 U's which would give the least indication of a longitudinal 

 division. In the metaphase or early anaphases the centrosome 

 is a small spherical body and takes the stain readily. The 



