Explanation of Plates. 



All drawings were made witii the aid of a camera lucida. A Lcitz -j^.T-inch homo- 

 geneous immersion objective and No. 4 ocular were used, producing, at the pro- 

 jection distance employed, a magnification of 1230 diameters. All figures were 

 reduced ^ in reproduction, making a final magnification of 820 diameters. 



PLATE 1. 



Figure 1. Spermatogonium of Lithobius mordax in the " resting stage." Each 

 nucleus possesses two karyospheres, which contain all of the chromatin of the cell. 



Figure 2. Prophase of spermatogonium of L. mordax. The chromosomes 

 have arisen from the karyosphere, and are distributed throughout the nucleus in 

 the form of fine granular threads. The nucleoli and the accessory chromosome 

 are all that remain of the karyosphere. 



Figure 3. Early spermatocyte of L, mordax. The chromosomes have under- 

 gone pseudo-reduction. The massing of the chromosomes about the accessory 

 chromosome to form the karyosphere has already begun. 



Figure 4. Lithobius sp. ? at about the same stage as is shown in Figure 3. 

 Here the nucleoli serve as the basis of the karyosphere. Two nucleoli are seen, 

 one of which is already acquiring chromatin. 



Figures 5, 6, 7. First spermatocytes of L, mordax in the early growth period, 

 showing massing of the chromosomes to form the karyosphere. The outlines of 

 many of the chromosomes can still be seen. 



Figure 8. Later stage of growing Jirst spermatocyte of L. mordax. The 

 karyosphere has increased in size, is becoming more regular in outline, and 

 stains less deeply, owing to the acquisition of nucleolar material. The nucleus 

 also contains diffuse deposits of achromatic material. The outlines of some of 

 the chromosomes are still to be seen. 



Figure 9. Nucleus of Jirst spermatocyte of L. mordax in the vesicle stage, 

 showing the presence of both chromatin and nucleolar substance. 



Figure 10. The vesicle stage in L. mordax, showing characteristic shape of 

 cell, dense structures of the cytoplasm, and characteristic appearance of the 

 nucleus. The karyosphere appears entirely black, and is surrounded by a mantle 

 of non-chromatic material. 



Figures 11, 12. Nuclei ot Jirst spermatocytes of L. multidentatus in the vesicle 

 stage. Tlie amount of chroTnatin is here less than in L. mordax. It is spread 

 out over the surface of the nucleolus, forming a karyosphere similar to that of 

 Scolopendra subspinipes. The achromatic deposits are confined to several dense 

 masses. 



Figure 13. Very early prophase of first spermatocyte of L. multidentatus. The 

 chromatin in the karyosphere is becoming more loosely arranged. The deposits 

 of achromatic material have broken up into a number of smaller masses. 



Figures 14-17. Nuclei of spermatocytes of Lithobius sp. ? in the vesicle stage. 

 Two or more karyospheres are usually present, each containing considerable 

 nucleolar material. 



