198 ON THE INTERNAL FOEM AND [ch. 



from the nucleus; but the interaction of the two is necessary, 

 that other and more important properties or functions may be 

 manifested. We know, for instance, that portions of an Infusorian 

 are incapable of regenerating lost parts in the absence of a nucleus, 

 while nucleated pieces soon regain the specific form of the organism : 

 and we are told that reproduction by fission cannot be initiated, 

 though apparently all its later steps can be carried on, indepen- 

 dently of nuclear action. Nor, as Verworn pointed out, can the 

 nucleus possibly be regarded as the "sole vehicle of inheritance," 

 since only in the conjunction of cell and nucleus do we find the 

 essentials of cell-Hfe. "Kern und Protoplasma sind nur vereint 

 lebensfahig," as Nussbaum said. Indeed we may, with E. B. 

 Wilson, go further, and say that "the terms 'nucleus' and 'cell- 

 body ' should probably be regarded as only topographical expres- 

 sions denoting two difEerentiated areas in a common structural 

 basis." 



Endless discussion has taken place regarding the centrosome, 

 some holding that it is a specific and essential structure, a per- 

 manent corpuscle derived from a similar pre-existing corpuscle, a 

 "fertihsing element" in the spermatozoon, a special "organ of 

 cell-division," a material "dynamic centre" of the cell (as Van 

 Beneden and Boveri call it) ; while on the other hand, it is pointed 

 out that many cells live and multiply without any visible centro- 

 somes, that a centrosome inaj disappear and be created anew, 

 and even that under artificial conditions abnormal chemical 

 stimuli may lead to the formation of new centrosomes. We may 

 safely take it that the centrosome, or the "attraction sphere," 

 is essentially a "centre of force," and that this dynamic centre 

 may or may not be constituted by (but will be very apt to produce) 

 a concrete and visible concentration of matter. 



It is far from correct to say, as is often done, that the cell-wall, 

 or cell-membrane, belongs " to the passive products of protoplasm 

 rather than to the living cell itself" ; or to say that in the animal 

 cell, the cell-wall, because it is "shghtly developed," is relatively 

 unimportant compared with the important role which it assumes 

 in plants. On the contrary, it is quite certain that, whether 

 visibly differentiated into a semi-permeable membrane, or merely 

 constituted by a hquid film, the surface of the cell is the seat of 



