28 bulletin: museum of compaeative zoology. 



Various stages in the disintegration of the karyosphere and the forma- 

 tion of the tetrads are shown in Figures 21-28 (Plate 2) and 136, 

 138, 141-14-4 (Plate 8). Figure 20 shows this body at the beginning 

 of the prophase before the appearance of the chromosomes; it is from a 

 thin section, and shows excellently the true reticular nature of the 

 karyosphere. In Figures 23, 24, are shown camera-lucida drawings 

 which illustrate the origin of the chromosomes as well as could be done 

 by means of diagrams. In Figure 24 the karyosphere, much reduced in 

 size and apparently not homogeneous, has given rise to three granular 

 thread-like processes. These processes are similar in all respects to' 

 the typical chromatin filament in the spireme stages. Near the distal 

 end of each of these threads there is a filament of chromatin which 

 apparently has just become detached from the mass and has already 

 segmented longitudinally in the manner characteristic of chromatin 

 segments in the prophase. Practically the same stages are shown in 

 Figures 143, 144, and in Figures 23, 25, 27. In Figures 25, 28, 142, 

 is shown a considerably later stage, wherein the last chromosomes to 

 be formed are seen arising from the karyosphei-e and the accessory 

 chromosome, which still retains its characteristic definite outline and 

 homogeneous structure, is again plainly distinguishable. When these 

 conditions, shown both in camera-lucida drawings and in photo-micro- 

 graphs, are considered, I believe no one will hesitate to agree with 

 the conclusion that during the vesicle stage all the chromatin of the 

 cell is contained in the karyosphere and that during the subsequent 

 prophase this body gives rise to the chromosomes. 



During the vesicle stage the karyosome consists of a number of very 

 attenuated chromatin segments closely massed about the accessory chro- 

 mosome. In the early prophase following, the first change noticeable is 

 the loosening of this mass of threads (Fig. 20). Later, several ends of 

 this filamentous sphere become free, and by the simple process of uncoil- 

 ing give rise to long granular processes extending out into the linin net- 

 work (Figs. 23, 24). These protruded threads detach themselves, and 

 new projections appear in their place until sixteen segments are present 

 (Fig. 28). This is the number of segments present in the early sperma- 

 tocyte — i. e. before the vesicle stage — and the number of chromosomes 

 later seen in the metaphase, exclusive of the accessory chromosome. 

 "When all of these segments have arisen from the karyosphere, nothing 

 remains but the accessory chromosome, which has formed the core of this 

 structure (Figs. 25, 28, 142). 



As the chromatin segments arise from the karyosphere, they are long 



