12 



CYANOPHYCEAE 



chromatin granules only just differentiated. In C. macrococcus, a 

 more complex type, there is a central body which, according to 

 Acton (1914), contains a fine reticulum with chromatin at the nodal 

 points, but a reinvestigation of this species is perhaps desirable and 

 might well lead to a different interpretation (cf. fig. 3). At cell 

 division this "nucleus" divides by simple constriction, but there is 

 no evidence of a mitosis. In Gloeocapsa a similar condition is 

 observed, but in this case with evidence of a rudimentary mitosis. 



Chroococcaceae : Merismopedia {merismo, division; pedia, plain). 

 Fig. 4. 



The free-floating colonies form regular plates one cell in thickness 

 at first, but with increasing age they become irregularly square or 



Fig. 4. Merismopedia elegans. A, portion of colony ( x 345). B, portion of 

 colony (X1125). C, structure in cells about to divide (X1875). (A, after 

 Geitler; B, C, after Acton.) 



rectangular and are often curved or twisted. The cells are spherical 

 or ellipsoidal and their individual sheaths are confluent with the 

 colonial envelope. There is every transition from compact (M. aeru- 

 ginosa) to extremely loose colonies (M. icthyolabe), the number 

 of cells enclosed in one envelope depending on the rate of division 

 which only takes place in two planes. In M. elegans, prior to cell 

 division, an accumulation of chromatin occurs in the centre of the 

 cells to form a central body or so-called "nucleus" which divides 

 by constriction immediately preceding cell division. The "nucleus" 

 then disappears until the next division. 



