Xo.TxSO] 



THE EVOLUTION OF THE CELL 



21 



acquires then a greater or less degree of vagueness and 

 uncertainty of application, and it is not easy to avoid a 

 tendency to a petitio pr'uicipii in attempting to define or 

 identify it. To a large extent we are thrown back upon 

 the staining-reactions, which I have already shown to be 

 very unreliable, backed up by analogies with those forms 

 which possess definite nuclei. Since in the cells of all 

 animals and plants, and in all Protista which possess a 

 true nucleus, the chromatin is the one constituent which 

 is invariably present, as I shall point out in more detail 

 subsequently, there is at least a strong presumption, 

 though not of course amounting to absolute proof, that 

 it is present, or at least is represented by some similar 

 and genetically homologous constituents, in the forms of 

 simpler structure also. If then in Protista of primitive 

 type we find certain grains which exhibit the character- 

 istic staining-reactions of chromatin to be constantly 

 present in the organism, grains which grow and divide as 

 a preliminary to the organism multiplying by fission and 

 which are partitioned amongst the daughter-organisms 

 during the process of fission, so that each daughter-indi- 

 vidual reproduces the structure of the parent-form from 

 which it arose; then there is very strong prima facie evi- 

 dence, to say the least, for regarding such grains as 

 homologous with the chromatin-grains of ordinary cells. 



Having now defined or explained, as well as I am able, 

 the terms of which I am about to make use, I return to my 

 main theme, the cell and its evolution. To summarize the 

 points already discussed, a typical cell is a mass of proto- 

 plasm differentiated into two principal parts or regions, 

 the cytoplasm and the nucleus, or, it may be, two or more 

 nuclei. The cytoplasm may or may not contain chro- 

 matin-grains in addition to other enclosures, and may 



The cell, therefore, in its complete and typical form, is 



icleus, highly 

 variable con- 

 :>f grains and 



