No. 590] THE EVOLUTION OF THE CELL 115 



de nouveau constitute. ' If we delete from this sentence 

 the word "chimiques" and also the words "et du nu- 

 cleoli" and substitute for the notion of the chemical so- 

 lution of the chromatin-substance that of scattered chro- 

 matin-grains in the periplasm, we have the picture of the 

 cytodal stage of evolution such as I have imagined it. It 

 should be borne in mind that the ultimate granules of 

 chromatin are probably in many cases ultra-microscopic; 

 consequently they might appear to be dissolved in this 

 cytoplasm when really existing as discrete particles. 



In the life-cycles of Protozoa, especially of Khizopods, 

 it is not at all infrequent to find developmental phases 

 which reproduce exactly the picture of the pseudo-mon- 

 eral stage of evolution, phases in which the nucleus or 

 nuclei have disappeared, having broken up into a number 

 of chromatin-grains or chromidia scattered through the 

 cytoplasm. We do not know as yet of any Protozoa, how- 

 ever, which remain permanently in the cytodal stage, that 

 is to say, in which the chromatin-grains remain perma- 

 nently in the scattered chromidial condition, without ever 

 being concentrated and organized into true nuclei ; but it 

 is quite possible that some of the primitive organisms 

 known as Proteomyxa will be found to exhibit this condi- 

 tion and to represent persistent Pseudo-monera or 

 cytodes. 



The next stage in evolution was the organization of the 

 chromatin-grains (biococci) into a definite cell-nucleus. 

 This is a process which can be observed actually taking 

 place in many Protozoa in which "secondary" nuclei 

 arise from chromidia. It seems not unreasonable to sup- 

 pose that a detailed study of the manner in which second- 

 ary nuclei are formed in Protozoa will furnish us with a 

 picture, or rather series of pictures, of the method in 

 which the cell-nucleus arose in phylogeny. To judge from 

 the data supplied by actual observation, the evolution of 

 the nucleus, though uniform in principle, was sufficiently 

 diversified in the details of the process. As one extreme 

 we have the formation of a dense clump of small, separate 



