pinney: organization of the chromosomes 311 



IN PHRYNOTETTIX MAGNUS. 



been able to ascertain, remain intact until an early prophase^ 

 In these stages also the accessory chromosome is readily recog- 

 nized by its position, size and apparently advanced state of 

 diffusion, which is consistent with the earlier appearance of 

 tardy condensation observed in figures 1 and 2. The diffusion 

 of chromatin within the accessory discloses to observation a 

 small, black, spherical granule lying close to the vesicular mem- 

 brane at its proximal end (stain, Heidenhain's iron-hsema- 

 toxylin). This granule is always seen in the accessory in this 

 stage and always at the polar end. The ordinary chromosomes 

 in figure 4 do not show a similar element, owing to their slight 

 degree of internal dissociation, but in the later stages, figures 

 6, 7, 9 and 10, its presence is clearly demonstrated. 



The slight shifting of position of the chromosomes, due to 

 the diffusion of the chromatin and the enlargement of the chro- 

 mosomal vesicles, presents a difficulty when it is attempted to 

 determine the location of the granules. Sections of cells may 

 be cut in all conceivable planes and many misleading figures 

 obtained. The sections in which the least complication occurs, 

 and the ones best fitted for study, are those in which the entire 

 accessory is visible in longisection lying at one side of the nu- 

 cleus. (See fig. 6.) Here there can be no doubt as to which 

 are the polar ends of the chromosomes, and their limits can be 

 ascertained, showing that the polar ends of all of the chro- 

 mosomes do not lie in the same plane but that the deeply 

 staining round granules which mark their polar ends lie near 

 the same side of the nucleus. It is also noteworthy that these 

 black bodies occur only at the polar ends. Other chromatin 

 masses which are observed occasionally at other points within 

 a chromosome are indistinct in outline, irregular in shape, 

 and cannot be shown to be constant, so are not to be confused 

 v.ith the polar granules. 



In cross-section of these diffused stages we obtain such 

 figures as 7, in which we are looking down on the 

 proximal ends of the chromosomes, and in figure 8, which 

 is a section through the central or distal portion of the 

 chromosomal group. Figure 8 shows the stage midway be- 

 tween two succeeding metaphases in which the dissociation 

 of chromomeres is at its maximum, and is present in the same 

 degree in all of the chromosomes. Here too the polar granules 

 prove constant in occurrence, staining qualities and position. 



