BLACKMAN. — SPERMATOGENESIS OP THE MYRIAPODS. 335 



densely massed as to present the appearance under moderate magnifica- 

 tion of a homogeneous sphere of chromatin. It is only in thiu sections 

 and with high magnification (1200 to 1800 diameters) that the spireme 

 character is shown. Later, during the prophase, the chromosomes arise 

 from this mass by a mere unwinding of the chromatin threads. When 

 this process is completed nothing remains of the karyosphere but the ac- 

 cessory chromosome. There is apparently no nucleolar material con- 

 tained in it, as no residue can be found within the nucleus and no such 

 structure can be seen at any previous stage. 



I shall now describe the changes during the same period in S. sub- 

 spinipes and compare the processes in the two species. During the telo- 

 phase of the last spermatogonium the appearance of the cells of the two 

 species is identical, but at the time of the formation of the nuclear mem- 

 brane the true nucleolus arises in S. subspinipes in close contact with 

 the accessory chromosome and grows rapidly as the cell increases in 

 size. Thus the conditions in the two species are slightly different, and 

 it is to be expected that the subsequent behavior of the cells would 

 also vary. 



In S. heros it was impossible to follow in detail the process of the 

 massing of the chromatin into the karyosphere. In S. subspinipes, how- 

 ever, this process, for reasons which I shall soon make plain, can be fol- 

 lowed much more satisfactorily. In this species the chromatin, instead of 

 becoming closely aggregated about the accessory chromosome, is deposited 

 iu a rather thin layer upon the periphery of the large nucleolus, or plas- 

 mosome. This structure, staining so differently from the chromatin, fur- 

 nishes an excellent background, by means of which the various stages in 

 the formation of the karyosphere may be studied with ease. 



In S. subspinipes, as in S. heros, the formation of the nuclear membrane 

 around the chromosomes may be taken as marking the transition from 

 spermatogonium to spermatocyte. In one species, precisely as in the 

 other, this stage ushers in a period remarkable for the concentration of 

 the chromatin. As the cells grow the chromosomes gradually become 

 much more diffuse and therefore stain less deeply. During this time 

 the nucleolus, still in close apposition to the accessory chromosome 

 (Figure 8 b), increases in size so rapidly that its volume is soon several 

 times that of the accessory chromosome (Figure 8 c, d). 



Shortly after the stage shown in Figure 8 d, the nucleolus seems also 

 to change somewhat in its staining reaction. Irregular areas upon its per- 

 iphery now show a marked affinity for chromatin stains (Figures G, 7, 8 e), 

 causing it under moderate magnification to appear of a uniform gray color. 



