CELL-LIFE AND CELL-MULTIPLICATION. 255 



in size from the microsomes or granules of protoplasm around 

 is not known. It can scarcely be a repelling force; since, 

 in a substance of liquid or semi-liquid kind, this could not 

 produce approximately straight lines. That it is an attrac- 

 tive force seems more probable ; and the nature of the attrac- 

 tion would be comprehensible did the centrosome augment 

 in bulk with rapidity. For if integration were in progress, 

 the drawing in of materials might well produce converging 

 lines. But this seems scarcely a tenable interpretation ; since, 

 during the so-called " resting stage," this star-like structure 

 exists exists, that is, while no active growth of the centro- 

 some is going on. 



Eespecting this small body we have further to note that, 

 like the cell as a whole, it multiplies by fission, and that the 

 bisection of it terminates the resting or growing stage and 

 initiates those complicated processes by which two cells are 

 produced out of one: the first step following the fission 

 being the movement of the halves, with their respective com- 

 pleted asters, to the opposite sides of the nucleus. 



74c. With the hypothesis, now general, that the nucleus 

 or kernel of a cell is its essential part, there has not an- 

 naturally grown up the dogma that it is always present; 

 but there is reason to think that the evidence is somewhat 

 strained to justify this dogma. 



In the first place, beyond the cases in which the nucleus, 

 though ordinarily invisible, is said to have been rendered 

 visible by a re-agent, there are cases, as in the already-named 

 Archerina, where no re-agent makes one visible. In the 

 second place, there is the admitted fact that some nuclei are 

 diffused; as in Trachelocerca and some other Infusoria. In 

 them the numerous scattered granules are supposed to con- 

 stitute a nucleus : an interpretation obviously biassed by the 

 desire to save the generalization. In the third place, the 

 nucleus is frequently multiple in cells of low types; as in 

 some families of Algae and predominantly among Fungi. 



