206 INTRODUCTION TO CYTOLOGY 



made up of a ground substance with a reticulum bearing bodies of two 

 sorts: granules of metachromatin closely similar to chromatin in reaction, 

 and cyanophycin granules, or plasmatic microsomes. Although there is 

 no definitely delimited central region in the cell the metachromatin is 

 found mostly at the center and the cyanophycin mostly nearer the pe- 

 riphery. When the metachromatin granules become numerous division 

 sets in, a centripetally growing wall cleaving the protoplast into two 

 daughter cells. In Glceocapsa the central region is somewhat more definite 

 and may often show a spireme-like appearance such as Olive describes; 

 but this may possibly be an artifact. In Merismopedia elegans there is a 

 definitely delimited nucleus, not like that of the higher plants but merely 

 an accumulation of chromatin or chromatin-like material which divides 

 just before the cell constricts into two portions. In Chroococcus macro- 

 coccus, finally, the nucleus and cytoplasm are sharply distinct, the former 

 having a reticulum with chromatin granules at its nodes and dividing 

 by a sort of constriction at the time of cell-division. 



As a result of these observations Miss Acton advances a theory of the 

 evolution of nucleus and cytoplasm, which is briefly as follows. The 

 excess food elaborated by the protoplast with its pigments was first 

 stored as plasmatic microsomes composed of a carbohydrate, cyanophy- 

 cin. As the reserve material became more complex in nature the nucleo- 

 protein metachromatin was elaborated; this became aggregated at the 

 center of the cell, insuring its equal distribution in cell-division, as in 

 Merismopedia. There thus arose in the cell a physiological and mor- 

 phological differentiation, the nucleo-protein with its portion of the 

 supporting reticulum becoming a stable nucleus, as in Chroococcus 

 macrococcus, and the ground substance remaining as the cytoplasm. 



Summary. In the Cyanophyceae, therefore, although these forms in 

 all probability had nothing directly to do with the evolution of the higher 

 plants, we see a series of stages such as may well have occurred in the 

 evolution of the nucleus and its complicated mitotic division. In the 

 simplest forms the material concerned with those cell activities which 

 in higher organisms are associated with the nucleus, is scattered through- 

 out the cell without the morphological distinctness characteristic of an 

 organ in the strict sense. It is passively distributed to the daughter 

 cells when the cleavage wall is formed at the time of cell-division. In 

 other cases this material reacts more strongly like true chromatin and 

 may form a more or less definite aggregation separating into two masses 

 as the cell divides. This metachromatin, which is a nucleic acid com- 

 pound, has also been observed in other algse, in Protozoa, and in fungi, 

 including the yeasts. It appears to represent a reserve material, though 

 it may also have other functions. Finally, definite and well organized 

 nuclei are present in certain of the forms described in the foregoing 

 pages, and although these nuclei may lack some of the features exhibited 



