492 PROCEEDINGS OF THE AMERICAN ACADEMY. 



that with the accessory chromosome the total spermatogonia! number 

 is forty-nine. 



After the origin of the chromatin segments from the karyospheres 

 the residue consists of two sorts of material : a small, deeply staining 

 body, the accessory chromosome, and two lightly staining plasmosomes 

 (Figure 2). The plasmosomes remain in the nucleus during the pro- 

 phase, and are dissolved at the time of the breaking down of the nuclear 

 membrane. The accessory chromosome is usually closely applied to one 

 of these nucleoli, although it may be entirely free from both. 



Thus the conditions at this time are quite similar to those found in 

 Scolopendra subspinipes (Blackman, :05*), where a portion of the residue 

 of the karyosphere is nucleolar material, while the other portion is a 

 modified chromosome. In Scolopendra heros, on the contrary, all the 

 products of the karyosphere are chromatic. One point of difference, 

 however, between the spermatogonia! prophases in Lithobius on the 

 one hand, and in the two species of Scolopendra (Blackman, :03, :05'') 

 and in Scutigera forceps (Medes, :05) on the other, is the shape and 

 appearance of the chromosomes. In Lithobius these elements are 

 slender granular segments, similar to the corresponding structures in 

 insect spermatogonia, while in the other three species mentioned the 

 chromosomes are in the form of small rounded granular masses, which 

 ■ never lengthen out to form threads. The later stages of the sperma- 

 togonia! mitosis offer no points of especial interest, and will not be 

 described in this paper. 



The telophase stages of the last spermatogonia! division are similar 

 in all essentia! respects to those in Scolopendra and Scutigera. The 

 daughter chromosomes, as they move toward opposite poles of the 

 spindle, are aggregated into dense masses. At this stage the chromo- 

 somes are rounded bodies possessing clear-cut outlines. Soon, however, 

 just as in Scolopendra, they begin to lengthen out and become granular, 

 and thus in place of a mass of closely aggregated homogeneous bodies 

 there soon arises a more loosely arranged group of granular segments. 

 At first this is enclosed in no membrane, but lies within a vacuole of 

 hyaloplasm. Eventually, however, a definite nuclear membrane appears 

 and the chromatin segments become distributed throughout the space 

 enclosed by it (Figures 3, 4). It may now be readily seen that the 

 number of these chromatin segments is much smaller than the number 

 of chromosomes seen during the spermatogonia! prophase. Carefid 

 counts give the number of segments at this stage as 24, which, to- 

 gether with the accessory chromosome, produces a total of 25, the 

 number of chromosomes characteristic of the following mitosis. Thus 

 here, just as in Scolopendra, pseudo-reduction has already occurred as 



