AMITOSIS IN CELLS GROWING IN VITRO. 459 



and they were of value only in obtaining rapidly information as 

 to the obscure structure of the cell; the cell, however, was thus 

 sacrificed. 



SUMMARY. 



The observations above described and the interpretations 

 thereof may be briefly summarized as follows: 



Amitosis was found to involve only the nucleus and was not a 

 method of cell proliferation. It occurred in normal cells and 

 was characterized by a separation of the nucleus into one or more 

 parts which possessed no reproductive independence. 



The process of nuclear amitosis consisted in a unilateral or 

 bilateral constriction, manifested by a narrowing of the nucleus 

 in the region of its equator, and a streaming of its contents 

 toward the poles, with final separation of the two nuclear por- 

 tions. This phenomenon seemed to be associated with the action 

 of the mitochondria and centrosphere upon an elongated nucleus. 

 There was no amphiaster or spireme formation and no centre- 

 some fission. Division of the nucleolus was not an essential. 

 Repetition of this process leads to the formation of a giant cell. 



Not all nuclei which show elongation and constriction divide 

 by direct fission, but many return to their usual rounded or oval 

 form. When, however, the constriction has passed a critical 

 point the division goes on to completion, and this final stage is 

 rapid. 



Cells containing nuclei in process of, or the result of, amitosis 

 divide by mitosis. Mitosis in binucleate cells, which are the 

 product of nuclear amitosis, is characterized by the simultaneous 

 appearance in the nuclear parts of a spireme, from which a single 

 equatorial plate of chromosomes is formed. Furthermore, bi- 

 nucleate cells divide as frequently by mitosis as do mononucleate 

 cells, and this was the only form of division found to occur in 

 them. 



Since the parts of an amitotically divided nucleus do not 

 become separated as reproductive units but divide only by mi- 

 tosis, in which the chromatin in the parts is recombined, there is 

 nothing in nuclear amitosis opposed to the chromosome hy- 

 pothesis. 



The type of amitosis in which the nucleus is split by the growth 



