From the moment that we enter on a dynamical conception of the cell, we perceive that the old debates were in vain as to what visible portions of the cell were active or passive, living or non-living. For the manifestations of force can only be due to the _interaction_ of the various parts, to the transference of energy from one to another. Certain properties may be manifested, certain functions may be carried on, by the protoplasm apart {198} 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, independently 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-life. “Kern und Protoplasma sind nur _vereint_ lebensfähig,” 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 expressions denoting two differentiated 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 permanent corpuscle derived from a similar pre-existing corpuscle, a “fertilising 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 centrosomes, that a centrosome may 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.