172 C. M. CHILD. 



exceedingly rapid. In certain vertebrate embryos amitosis is 

 apparently much more frequent in the more rapidly growing than 

 in the less rapidly growing regions. In certain forms, such as 

 trematodes and polychaetes, which rapidly produce a very large 

 number of generative cells, amitosis appears to be the character- 

 istic method of division in the primitive germ-cells. 



All of these cases concern regions of extreme assimilative 

 activity and there are certain indications that, so far as the in- 

 dividual nuclei are concerned, they are regions which are far from 

 equilibrium. In all such regions, for example, the cytoplasm is 

 relatively small in amount, i. e., the nuclei are relatively much 

 more numerous than in less rapidly growing regions ; secondly 

 the nuclei in these regions are usually much smaller than in less 

 rapidly growing regions. Often there are differences in this re- 

 spect between the peripheral and central portions of such regions, 

 the central portions showing more extreme conditions than the 

 peripheral. Evidently the nuclei in these regions do not attain 

 a condition of equilibrium but are forced on in a given direction 

 by some stimulus as rapidly as the material available will permit. 

 According to the hypothesis set forth above these are exactly the 

 conditions in which amitosis may be expected to occur. 



But according to this hypothesis also amitosis is not neces- 

 sarily confined to regions of rapid growth. If little material is 

 available actual growth may be exceedingly slow. Moreover, it 

 is possible that nutritive material may be present outside the cell or. 

 even in the cytoplasm, in excess but in consequence of scarcity 

 of cytoplasm or other special conditions may become available 

 for the nucleus only very slowly or not at all. In such cases 

 amitosis might occur in the presence of an apparent excess of 

 nutritive material. Possibly various cases of amitosis observed 

 in nuclei lying in the yolk of meroblastic eggs and surrounded 

 by only a small amount of cytoplasm may be cases of this kind. 



And again it is possible that the differentiation of the cyto- 

 plasm in certain directions may bring about conditions favoring 

 amitosis ; in such cases the amount of undifferentiated cytoplasm 

 may be insufficient to maintain the nucleus in equilibrium. Thus 

 amitosis may be expected, and has often been found in highly diff- 

 erentiated cells. 



