86 HIM (I, KA IK CKI.l.S IN TISSUE CULTURKS. 



In the process of din-ct division of the nuclens varions factors may i)lay a i)art. 

 First, we may refer the different changes in foini of the nucleus to changes in form 

 of the cell as a whole. It is of frequent occurrence that a cell, by reason of the 

 tension exerted by attached cells, or of its own amneiioid movement. In'comes 

 elongated. In conseciuence of this stretching of the cell the nucleus also becomes 

 drawn out, it being simi)ly a sac of fluid, and it is possible that it may become 

 broken into two parts much in the same manner that an oil globule, floating upon 

 water, becomes broken uj) if stn^tched. It may be asi^vnned that there is a streaming 

 of i)rotoplasm away from the ecpiator, with a constriction in this region, which 

 beconw's deejx'r and deejier until the nucleus is divided into two more or less e(iual 

 portions, these now tending to assume a more globular shajje. This view of the 

 cau.se of nuclear amitosis is somewhat similar to that of Maximow (1908), who 

 believes that amitosis in the mesenchyme cells of developing rabbits may be 

 brought about by the stretching of such cells consequent ujion rapid growth of 

 the adjacent liver. 



The jirocess of direct division of the nucleus as described is strikingly like the 

 division of the cytojilasm of ova which had been replaccnl in normal .sea-water after 

 having been treated with hypertonic sea-water (J. Loeb, 1906, ]). 06, figs. 10, 11, 

 12, and 13). It appears that here the c(>ll ftrst becomes incut from one side; the 

 ])rotoplasm thereujion streams off in opposite directions, foiniing two globules con- 

 nected by a narrow isthmus. This soon becomes reduced to a mere thread com- 

 jKised of the attentuated cell membrane, which finally disapjiears, so that there 

 remain two sacs of protoplasm, (juite without connection one with another. The 

 physical changes involved in this process seem to be very much like those seen in 

 direct division of the nucleus. Loeb's figun^s ar(> very similar to those illu.strating 

 nuclear amitosis. 



That the size of the nucleus is not a material factor in this i)roce.ss is seen by the 

 variation in size of the twin nuclei, some of which are quite small. Although a 

 twin nucleus is frequently found in a cell which is not elongated, it may be assumed 

 that such a cell has subsequently changed its form, but that it was extended when 

 the sejiaration of the nucleus occiu-red. This hy]iothesis would not, however, 

 explain the formation of giant cells, multinucleated nniscle-cells, etc., and it does 

 not provide an explanation for the evident activity of the centrosphere and mito- 

 chondria in direct division. 



A second hy])othesis to account for the sejiaration of the nucleus directly postu- 

 lates the active particii)ation of the centrosphere, or mitochondria, or both, and 

 here we may as,sume a i)urely mechanical and a purely physico-chemical activity. 

 It has been noted that the centrosphere is found commonly in the invagination of 

 the nucleus; moreover, its edge shows evidence of a curious type of movement — a 

 slow, indefinite retraction and elongation of the marginal i)rocess(\s— which seems 

 to be associated with movements of the mitochondria. It is possible that, through 

 this mechanical influence of the centrosphere upon the adjacent nuclear membrane, 

 the constriction of the latter is favored and the nucleus ultimately divided, and it 

 is easy to conceive how the mitochondria may assist in this nuclear sejiaration 



