306 bulletin: museum of comparative zoology. 



rated karyosomes and beaded segments — are often represented in its 

 different regions. 



By the time the chromatin is massed in the form of " segments," most 

 of the net strands have lost their connection with the nucleolus (Fig. 

 22), and the segments have taken a peripheral position close against the 

 membrane, leaving the central area of the nucleus vacant. This con- 

 dition of the nucleus is seen particularly well on isolation preparations, 

 when, by focussing at the equatorial level, it appears as a clear circular 

 area, surrounded by a ring of black dots, the segments in optical section. 

 It will be recalled that in the entoderm cells these become condensed to 

 form the chromosomes directly, and at first I believed the same to be 

 true of the spermatogonia as well. More attentive study has, however, 

 convinced me that this is not the case, but that here they either segment 

 or condense into numerous chromatin bodies ; the result being such a stage 

 as is represented in Figures 23 and 24. These bodies are larger than the 

 original karyosomes, stain deeper, are nearly spherical, sharply outlined, 

 and lie, like the parent segment, close against the nuclear membrane. 

 From their clear-cut appearance it is easy to make an approximate count 

 of them, and in all favorable cases I have found from 48 to 51 or 53. 

 In all pi'obability their true number is exactly twice that of the somatic 

 chromosomes, that is, forty-eight. These chromomeres — as we may call 

 them, following Downing (: 05), who finds in Hydra a condition essen- 

 tially similar — still remain connected together by delicate linin strands, 

 which never form a single continuous thread, but retain the "net" struc- 

 ture of earlier stages. This process of concentration, like the formation 

 of the beaded segments, does not always occur synchronously through- 

 out the entire nucleus, but often, as in the cell represented in Figure 

 23, some of the segments may persist ; in this case one such, x, is 

 to be seen. The nucleolus, now of course a purely plasmatic structure, 

 and still connected with some of the achromatic threads, has lost most 

 of its stainability, and after this stage either fragments or is absorbed. 

 At any rate it can be traced no further. 



In the study of this stage, cells so crushed that the membrane is 

 partly destroyed and the nuclear contents more or less isolated are of 

 great assistance in indicating the degree of independence of the different 

 structures. Such a cell, crushed by the cover glass, is represented in 

 Figure 25, and shows several distinct stages. The nucleolus still persists; 

 in the upper portion of the figure within the nucleus, where the nuclear 

 membrane still persists, are to be seen chromatic segments ; at the lower 

 side are three or four more, and a considerable number of chromomeres 



