90 bulletin: museum of comparative, zoology. 



walls of the vesicles. The method of formation for these vesicles 

 parallels very closely that described for the accessory, the chief 

 difference being that in the case of the accessory the process is much 

 more rapid. In figures 10 and 11 we find the chromatin much diffused 

 and occupying most of the space within the original vesicles. The 

 vesicular walls are no longer visible, however, except on the periphery 

 as an undulating nuclear membrane, and around the accessory. In 

 spite of this fact, the chromatic masses or blocs, each of which has 

 arisen from a single chromosome, are still recognizable as distinct from 

 one another. This is especially well shown in the optical section 

 drawn in figure 11. There are only eighteen masses shown in this 

 section, but the apparent reduction in number need cause no appre- 

 hension as to the fate of the other members of the complex. It fre- 

 quently happens that the chromosomal vesicles do not all lie parallel 

 to each other, so as to be represented in a single transverse section, 

 and some may even assume a position at right angles to the axis of the 

 majority. Such a case is shown in the upper left-hand corner of figure 

 10. If, now, we examine the stages shown in figures 12 and 13, which 

 are of the period of greatest diffusion that I have been able to find, we 

 may still see, both in optical section (fig. 13) and in side view (fig. 12), 

 the positions of the individual chromosomes represented by a more 

 condensed band or core. In the case of the optical section, nearly 

 the complete number of chromosomes, as represented by these denser 

 masses or cores, can be counted. It is true that there seems to be an 

 anastomosing system of fibrils connecting the adjacent masses, but 

 this need not mean that there has been a loss of chromosome-iden- 

 tity in a common nuclear mass. 



An early prophase is represented by figures 14a and 14b (Plate 2), 

 which show the two sections of a single cell. We see at this stage the 

 beginning of the process of chromatin concentration which results, 

 finally, in the formation of the condensed chromosomes ready for the 

 next division. The chromatic material of each chromosome first con- 

 centrates near the middle of the region that it occupied in the nucleus in 

 the diffuse condition. There is thus formed a loosely reticulated core 

 (fig. 14a and 14b), out of which there develops a spirally coiled thread, 

 as shown in figures 15a and 15b. The two stages represented in figures 

 14 and 15 are very close together in time, for they occurred side-by- 

 side in the same cyst. These coiled threads are at first rather small 

 in diameter, but they rapidly thicken and shorten, as indicated in 

 figures 16-20 (Plate 2). During the process of shortening and thick- 

 ening the outlines of the vesicular walls become more distinct. This is 



