Cytology and Movements of the Cyanophyceoe. 297 



the poles as in the spindles of higher plants, but each seems 

 to end loosely in the cytoplasm. The spindle might, there- 

 fore, be termed "open," in the sense that for the time each 

 chromosome appears to be independent. After the forma- 

 tion of the above spindle, the chromosomes divide in the 

 middle, half of the chromatin retreating towards each end, 

 but they retain the bead-like appearance for some time, and 

 leave a linin thread connecting the two daughter chromo- 

 somes (Figs. 10 and 70). At this stage in the division 

 there is formed, around the equator of the cell, a collar-like 

 ingrowth of microsomata which gradually grows inward and 

 constricts the linin connective threads (Fig. 71), which 

 after they have been pressed to the centre, separate, but leave 

 a pore connecting the two cells. Evidently the chromosomes 

 are here divided in a transverse direction. The fact that 

 they usually retain the bead-like structure until after they 

 have separated into daughter chromosomes militates against 

 the view that the chromatin has mixed, and that in this 

 way a qualitative as well as quantitative division is effected 

 as has been supposed to be necessary in the higher plants. 

 The number of nuclear segments is not constant. There 

 may be several (Fig. 9) rather fine chromosomes, or they 

 may consolidate into two or more heavy segments (Fig. 

 11), often looking like heavy bars of chromatin without 

 any beaded appearance. These heavier segments divide in 

 the same way as the lighter ones, revealing the linin thread 

 between the divided portions (Fig. 13). The ingrowing 

 cell wall soon reaches the linin connectives between the 

 separate portions of the segments of the chromosomes, con- 

 stricting them (Fig. 13) and finally separating them en- 

 tirely. The chromatin then becomes diffuse and later forms 

 the vesicles or the network, according as the cell continues 

 to divide or enters upon a period of rest. The ingrowing 

 collar which finally forms the division wall, in the earlier 

 stages is composed of very fine microsomata of cellulose. 

 These gradually fuse to form the wall (Figs. 4, 5, 7 and 



