494 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In Scolopendra heros (Blackmail, :03, :05) their changes are as fol- 

 lows : " The chromatin segments begin slowly to break down, and their 

 substance becomes aggregated about the accessory chromosome, thus 

 apparently increasing the size of this body enormously. This process 

 continues until finally all of the chromatin of the cell is aggregated in 

 one large intensely staining body." (Blackmail, :05, p. 14.) This body 

 I have called the karyosphere. It is not a simple homogeneous body of 

 chromatin, but a very complex mass of chromatin fibres closely en- 

 veloping a solid chromatin body, the accessory chromosome. It is 

 made up of chromatin, linin, and karyolymph, but contains no nucleolar 

 material. 



In Scolopendra subspinipes (Blackman, :05^) the conditions are 

 slightly different, owing to the presence of a true nucleolus. Here the 

 karyosphere is formed by the massing together of all of the chromatin 

 of the cell, but it also contains a nucleolus in addition to the material 

 derived from the chromosomes. The chromosomic material is deposited 

 in a dense granular mass over the outside of the nucleolus, forming a 

 mantle around it. 



In Scutigera forceps (Modes, :05) the origin of the karyosphere is 

 quite similar to that in Scolopendra heros. The material composing 

 it in these early stages is derived entirely from the chromosomes. 

 Later, however, during the growth period, the karyosphere acquires in 

 addition a large quantity of nucleolar material, as is shown both by 

 its staining reaction in the vesicle stage and by the nucleolar residue 

 remaining after the origin of the chromosomes in the prophase fol- 

 lowing. Thus, while the component materials of the karyosphere in 

 Scutigera are identical with those of the same body in Scolopendra 

 subspinipes, there is in the former no such definite localization of 

 chromatic and nucleolar material as is found in the latter. 



In the three species of Lithobius which I have now studied, two con- 

 ditions exist as to the origin of the karyosphere. In Lithobius mordax 

 the origin of the karyosphere is more like that in Scolopendra heros 

 and Scutigera forceps, while in Lithobius multidentatus and Lithobius 

 sp. the stages observed resemble more closely those found in Scolopendra 

 subspinipes. 



Stages in the formation of the karyosphere in Lithobius mordax are 

 shown in Figures 3, 5, 6, 7, and 8. In Figure 3 the outlines of the 

 accessory chromosome can still be distinguished. Closely apposed to 

 one side of this element are several masses, which are more granular. 

 These represent chromatin segments which have become shortened and 

 thickened by a rearrangement of their granules, and have taken up a 

 position upon one side of the accessory chromosome. Other chromo- 



