OTHER MODES OF NUCLEAR DIVISION 



205 



network with chromatin granules at its nodes, and constricts simultane- 

 ously into a large number of daughter nuclei which pass to the conidia. 

 In Synochocystis aquatilis occurs the "primitive mitosis" type: here 

 Gardner found the only case of anything approaching mitotic behavior. 

 A spireme develops and segments into three pieces which arrange them- 

 selves parallel to the long axis of the cell and divide transversely; the 

 daughter pieces then separate and a centripetally growing cell wall 

 completes the division of the cell. Gardner thus finds in the Cyano- 

 phycese "a series of nuclear structures, beginning with a very simple 



FIG. 72. The nuclei of various members of the Chroococcaceae. 

 A, cell of Chroococcus turgidus with scattered metachromatin granules (m) and plasma- 

 tic microsomes (p) ; division beginning. X 2500. B, cell of Gloeocapsa with chromatic 

 granules. C, M erismopedia elegans, showing two stages of nuclear division. X 1500. 

 D. Chroococcus macrococcus: n, nucleus; m, metachromatin; , vacuole. X 2500. E, 

 dividing nucleus of Chroococcus macrococcus. X 2500. (After Acton, 1914.) 



form of nucleus scarcely differentiated from the surrounding cytoplasm 

 and dividing by simple direct division" and passing "by very gradual 

 steps to a highly differentiated form of nucleus which in dividing shows 

 a primitive type of mitosis, and in structure approximates the nucleus of 

 the Chlorophyceae and the higher plants." 



In a more recent investigation of the Chropcoccacese Miss Acton 

 (1914) finds that the nucleus is in general much simpler than that of the 

 higher plants. Like Gardner, however, she points out a series beginning 

 with a form in which definite organization is almost entirely lacking and 

 ending with one in which the structure of the higher plant nucleus is 

 closely approached (Fig. 72). In Chroococcus turgidus the protoplast is 



