16 THE PHYSICxlL BASIS OF LIFE 



them as notliiiig more than a convenient fiction or alge- 

 l)raic symbolism, a kind of ideal mental model ])y means 

 of which the genetic facts may conveniently be grouped. 

 Those, however, who prefer to take their point of depar- 

 ture in the observed cytological facts will be more likely 

 to make use of the actual model which every dividing cell 

 displays to us invisible reality — a model that is not less 

 impressive because at present the cytologist sees it only in 

 broad outline with no more than dim indications of the 

 finer complications inferred from the results of genetic 

 research. At any rate it was this actual model that gave 

 the point of departure for the foregoing conceptions con- 

 cerning the nuclear organization and thus made possible 

 some of the most fundamental of modern experimental 

 researches on heredity. Considered only as working in- 

 strimients, therefore, these conceptions have a practical 

 value almost comjjarable to that of the atomic theory as 

 employed in chemistry and physics. 



Cytology and genetics have thus combined to make real 

 to us the existence of a nuclear microcosm that is as com- 

 plex and wonderful as any pictured by the fantasies of the 

 speculative nature-philosophers. But manifestly we can 

 not halt here. The nucleus is part of a larger system, the 

 cell ; and our inquiry must now enlarge its scope in order 

 to consider the organization of that system considered as 

 a wdiole. We are thus led, first of all, to the question 

 whether an organization similar in type to that of the 

 nucleus, or anything approaching to it, may also exist in 

 the protoplasm of the cytosome (cytoplasm) or extra- 

 nuclear region of the cell-system. Conservative cytologi- 

 cal opinion has been extremely reluctant even to recognize 

 such a possibility. We have been prone to take the cyto- 

 plasmic region of the cell-system at its face value, con- 

 ceiving it as a vague and formless mass devoid of definite 



