628 MURIEL ROBERTSON AND E. A. .MJN(']11N. 



Finally, the chromatin masses become very definite and stain 

 very deeply, and no distinct karyosotne can be made out ; this 

 body seems to break up and to contribute by doing so to the 

 general store of chromatin. At first the chromatin masses, or 

 chromosomes, as they may now be termed, appear to be con- 

 nected together by delicate filamentous junctions (fig. 9) ; 

 this stage corresponds apparently to the spireme stage. Next, 

 the connections between the chromosome disappear, and they 

 are seen lying sepai-ately from one another as irregular 

 rounded masses, showing more or less distinctly indications of 

 division, each into two (fig. 15). In spite of much searching 

 we have not been able to find any stages other than those 

 described, and, in particular, nothing more nearly resembling 

 an ordinary spireme stage than the specimen shown in 

 fig. 9. 



These changes in the interior of the nucleus also go on quite 

 independently of the changes in the flagellum and hlepharo- 

 plast described in the previous section. Thus the flagellum 

 may have vanished, and the two daughter-blepharoplasts may 

 have taken up their definitive position when the nuclear con- 

 tents are at the beginning of their changes (fig. 14) ; or the 

 nucleus may be comparatively far advanced when the 

 blepharoplasc has only just divided (tig. 12), or before the 

 fiugelluni is absorbed (tigs. 8, 9). Finally, however, a stage 

 is reached when the nucleus has resolved itself into a mass 

 of separate chromosomes, and the two blepharoplasts, or, as 

 they may now be termed, the centrosomes, are placed on 

 opposite sides of it, indicating the two poles of the future 

 nuclear spindle (fig. 15) ; when this stage is reached the 

 nuclear membrane is absorbed and cannot be discerned. 



The formation of the nuclear spindle is seen in the two 

 stages drawn in figs. 16 and 17. After the absorption of the 

 nuclear membrane the chromosomes arrange themselves to 

 form an equatorial plate, to which delicate rays can be seen 

 to pass from the centrosomes, forming the characteristic 

 achromatic spindle. The two centrosomes appear to be pushed 

 further apart by the formation of the spindle, so that they 



