BLACKMAN. — SPERMATOGENESIS OF THE MYRIAPODS. 515 



During the early part of the growth period all of the chromosomes 

 become massed together to form the karijosphere. The chromosomes enter 

 this body as distinct elements, and the indlcridual elements can be recog- 

 nized for a considerable time. The karyosphere also contains nucleolar 

 material. 



The karyosphere varies in structure with the species. In Lithobius 

 mordax the amount of nucleolar material is less than in the other 

 species studied. In Lithobius multidentatus the chromatin is periph- 

 eral in position, and surrounds the nucleolar material. In Lithobius 

 sp. 1 the amount of nucleolar substance is greater, and there are always 

 two or more karyospheres. The chromatin in this species is usually 

 central in position, and is surrounded on all sides by nucleolar 

 material. 



During the growth period no special aggregations of archoplasm are 

 presejit, but this substance is dispersed throughout the cytoplasm of the 

 cell. 



In the three species of Lithobius studied the chromosomes arise from 

 the karyosphere and develop into tetrads, but the manner of development 

 of the tetrads varies with the species. 



In Lithobius sp. ? the stages in the formation of the prophase 

 chromosome are similar in all essential respects to those of the 

 typical arthropod tetrad. 



In Lithobius mordax and Lithobius multidentatus the chromatin 

 arises from the karyosphere in the form of globules ivhich, as they arise, 

 become arranged iii a single row for each chromosome, and each chromo- 

 some is embedded in a separate mass of achromatic material. In the 

 formation of the tetrad each globule divides into two equal halves, giving 

 rise to two parallel rows of globules. This represents the longitudinal 

 or equational division of the chromosomes. The transverse or reduction 

 division is accomplished by a separation of the roivs of globules at their 

 middle point, the globules themselves remaining intact. Ln later stages 

 the globules forming each chromatid fuse into one, thus giving rise to 

 a tetrad composed of four sj)herules of chromati7i. 



After the origin of the chromosomes, the nucleolar portion of the 

 karyosphere remaining gradually breaks down. Portions of it may 

 often be seen at the poles of the nucleolar spindle during the meta- 

 phase and anaphase of the following division. 



In the early prophase the centrosomes separate and move through the 

 cytoplasm to the cell membrane. Along the inner face of the membrane 

 they migrate apart until they are upon opposite sides of the cell. 

 During the migration the cell outline becomes rounded off, and by the 

 time of the metaphase the cell is spherical in shape. 



