AMITOSIS IN CELLS GROWING IN VITRO. 455 



segregation into short rodlike masses and finally into a spireme) 

 can take place in a nucleus which is undergoing constriction. 

 Thus it appears that mitosis can proceed as usual in nuclei 

 partially divided or wholly divided by the amitotic process. 



DISCUSSION. 



The direct division of the nucleus is associated with certain 

 changes of the cell as a whole. Elongation of the nucleus seems 

 to be a prerequisite, and this apparently is secondary to a 

 lengthening and narrowing of the cell body occasioned by the 

 pull of its processes. It has been pointed out that the centro- 

 sphere is situated characteristically in a concavity at one side 

 of the nucleus, and, when the nucleus lengthens, this body sinks 

 deeper into its side; at the same time, judging from fixed prep- 

 arations, and also from the appearance of the living and dividing 

 nucleus, mitochondria come to lie across this narrow nuclear 

 isthmus. These bodies, the centrosphere and associated mito- 

 chondria, seem to play a part in the fission of the nucleus. The 

 exact manner of their action is not clear, but it may be that 

 streaming of the nucleoplasm away from the equator of the 

 nucleus follows upon the mechanical irritation of the nuclear 

 membrane by their movements, or possibly upon local alteration 

 of surface tension from their chemical change. Certainly the 

 position which these bodies take with reference to the constricted 

 nucleus points to their participation in the process of fission. 



The centrosphere does not divide, nor does it encircle the 

 nucleus as in the form of division described by Meves ('91). 

 The nuclear membrane remains intact and nothing resembling 

 an amphiaster is formed. 



Some theories of amitosis have attempted to place the re- 

 sponsibility for the initiation of the divisional stimulus upon the 

 nucleolus, and it has been found by some investigators that in 

 certain nuclei the nucleolus is the first body to become divided. 

 Such a function of the nucleolus does not obtain, however, in the 

 nuclei studied, for, although in some cases there was a division 

 of the nucleolus into two parts, one of which became alloted to 

 each separate nuclear part, yet, sometimes the nucleolus did not 

 divide, and one of the nuclear fragments was without visible 



