The life history of Diplodiscus temporatus Stafford. 625 



distinguishable are situated near to the mass of chromosomes which 

 are fusing to form the caryosome. In the latest telophase, after the 

 caryosome has become a sharply marked out mass and is surrounded 

 by the layer of less densely staining material, the centrosomes can 

 no longer be distinguished. The most careful study has failed to 

 show whether the ceutrosome in the resting stage is situated within 

 the darkly staining material about the caryosome, or whether it 

 lies within an indentation of the caryosome itself so that it would 

 be within the circumference of the latter body. The centrosomes 

 first become apparent in the division of a nucleus at a stage when 

 the spireme formation is well advanced. At this time there are two 

 centrosomes and a well developed spindle between them. All of the 

 earlier stages in the development of the achromatic figure are con- 

 cealed within the caryosome, or the darkly staining mass about it. 



The Formation of the Spireme. 



The nuclei in the following series of figures were drawn from 

 difterent embryos, which accouts for the differences in their sizes. The 

 resting cells, as has been previously described, have the appearence 

 shown in Fig. 14. The first stage in the prophase of mitosis shows the 

 beginning of an increase in the size of the carj^osome at the expense 

 of the nucleoplasm, (Fig. 15). As the nucleoplasm is taken up by 

 the caryosome there become apparent a number of threads, (linin?) 

 which connect the central mass with the nuclear membrane, Between 

 these strands there are left empty spaces, (Figs. 16 and 17), so that 

 the central mass appears to be suspended by threads from the 

 nuclear wall. 



In the formation of the "suspensory threads'' it can be deter- 

 mined that each one is double, having been formed by the apposition 

 of the outer walls of an area of nucleoplasm, (Fig. 15). In early 

 prophase stages the contents of the nucleus, outside of the caryosome, 

 appears to be marked off into sectors by denser areas which later 

 go to form the "suspensory strands". In the resting nucleus, however, 

 I have never been able to distinguish any ditferentation in the 

 nucleoplasm which could be interpreted as marking out definite areas. 



At a stage when the greater part of the contents of the nucleus 

 has been taken up by the caryosome, (Fig. 16), the latter body has 

 lost its intense staining reaction and now has a uniform homogeneous 

 structure, and stains a comparatively light color. 



The next stage in the formation of the spireme is marked by 



