394 The Unity of the Organism 



a speculative view based on something other than evidence 

 contained in this particular statement, that would eviscer- 

 ate this statement of its essential meaning. And such would 

 be the effect were one to speculate that the centriole is 

 "generative chromatin" and so the causal explanation of 

 the phenomena presented by the center. The "cytoplasmic 

 substance" which, Calkins says, constitutes the central 

 sphere, is a "physical basis of heredity." It is such because 

 it does that by which, and by which alone, any substance 

 can be proved to be a physical basis of heredity. It takes 

 a direct, active part in a series of structural transforma- 

 tions "to form an amphiaster, consisting of two asters with 

 connecting mantle-fibres," that closely resemble one another 

 in many individual animals genetically related and consti- 

 tuting the species Noctiluca miUaris (figure 34.) 



(j) The Centrosphere of Protozoa Generally 



We must carry a little further the examination of the 

 centrosphere, or division center, as itself a physical basis 

 of heredity. The wide occurrence among the protozoa of a 

 body, or at any rate of substance, which does not stain 

 readily and hence is not chromatin, but which plays a funda- 

 mental part in cell division, seems to be recognized by all 

 students of these animals. As we are now concerned with the 

 question of how general in the group as a whole is the ac- 

 tive participation of this achromatic substance in propa- 

 gation, a summarized account of what is known on the sub- 

 ject will meet our purposes. 



Since, as has been previously pointed out, Minchin is a 

 strong chromatinist, we shall be safe from bias in the other 

 direction if we rely chiefly on his late book for the account. 

 Speaking broadly, protozoologists recognize two types or 

 classes of achromatic substance, dependent upon its loca- 

 tion in the cell. In one class, of which Noctiluca is an ex- 



