88 lUMCLEATK CELLS IX TISSUE CULTURES. 



as 1855 and 1858, Remak set forth a theory to accouul for the division of the cell, 

 which may be stated in the words of Wilson ( 1900. p. 63) as follows: 



"Cell-division proceeds from the center toward the peripher> . It begins with the 

 division of the nucleolus, is continued by simple constriction and di\isioii of the nucleus, 

 and is completed by division of the cell-body and membrane." 



A type of division which bears a close resemblance to this has recently been 

 described by Howard and Schidtz (1911) in the cells of a giant-celled sarcoma from 

 the human a'sophagus. To this type of division Schultz (1915) has proposed the 

 name "promitosis," and these investigators believe it to be intermediate between 

 amitosis and mitosis. This form of cell division seems to have an interesting 

 parallel in certain protozoa, and they regard it as a reversion to a i)rimitive biological 

 condition in which the division si)hcre is permanently intranuclear — an idea analo- 

 gous to that of Wieman (1910, p. 175) for a similar form of nuclear division. 



The first ste]> in the division of the nucleus here is taken to be a separation of the 

 karyosome into tw'o or more parts, of ecjual or uiuHiual size, followed by a breaking- 

 up of the nucleus into portions corresponding in number and size with the fragments 

 of the kaiyosome, each nuclear part coming to contain a portion of the latter. This 

 function of the karyosome in initiating division of the nucleus is analogous to that 

 of the centrosome in mitosis. 



This form of nuclear division is es.sentially the same as that described by 

 C'onklin (1903) in the follicular epithelium of the common cricket, and that it is by 

 no means infreciuent is gathered from the numerous references to it which this 

 author has found in the literature. Conklin, however, has never seen actual cell 

 division following nuclear amitosis, and from the fact that the cells in which direct 

 division of the nucleus is found sjieedily degenerate after the egg is laid he belie^'(^s 

 that it is, in the material examined, "one of the last functions of these cells and that 

 it is therefore an accompaniment of cellular senescence and decay." Conklin, how- 

 ever, believes that in most cases of amitosis the nucleolus does not divide. 



The evidence from tissue-culture cells does not lend much sui>port to a h\'i)othe- 

 sis ascribing to the hssion of the nucleolus the initiation of nuclear division; true, 

 we have in ttgure 2 a nucleus which shows lateral constrictions at the eciuator, and 

 within it, lying with its long axis parallel to that of the nucleus, is an elongated 

 karyosome, which also appears to be undergoing division in the same plane as the 

 nucleus. This .somewhat resembles the nuclei described by Howard and Schultz; 

 the karyosomes of t issue-cult lu-e cells, however, are decidedly simpl(>r in structure 

 than those of the cells of the giant-cell sarcoma. Again, the fact that in the binu- 

 cleate cell each nuclear portion is usually sui)i)lied with one or more karyosomes 

 seems to point to this body having been divided before or during the division of 

 the nucleus; but against this circumstance, weighing in favor of the view that the 

 division of the karyosome acts to excite direct nuclear division, is the occurrence of 

 sueli di\ision where the karyosome has evidently not divided (fig. 6), since it is 

 present in only one of the nuclear parts. ■ Such nuclei are not uncommon in tissue 

 cultures. The peculiar condition of the nucleolus in figure 2 may thus be purely 

 accidental, since it is not at all constant. 



