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THE AMERICAN NATURALIST. [Vol. XXXVIII. 



should be as favorable as passible t And may we not further 

 su])p()se that in the presence of conditions lowering the vigor of 

 a nucleus, even if it were, in a case where we should ordinarily 

 expect mitosis, it would then revert to the more primitive mode 

 ot ainitosis } \Vc arc not unfamiliar with the principle of physi- 

 olo-ual ic\cisi<)ii in or-ans and tissues. Can we extend the 

 l)riiu i[)lL- to mu lci ? If so, all the cases where amitosis occurs 

 in a i)lace where mitosis is expected to occur would perhaps be 

 cajjable of being brought within its scope. Possibly both senes- 

 cence and secretion as causes of amitosis could be interpreted 

 in the same way. Thus senescence as a cause of amitosis, as it 

 undoubtedly is, would be in reality due to the lowered vitality 

 ot the cell conseciuent on age. Metabolism perhaps could be 

 sui»i)osc'(l to cause it, where there is not at the same time, as in 

 tat t ells, milk cells and others, senescence, through the presence 

 <»l the sec retion which may exercise an unfavorable influence on 

 the nucleus chemically. Pathologic tissues would also find an 

 easy ex [)la nation on this basis to account for the common occur- 

 rence ot amitosis in them. Summer by inducing optimal well- 

 bemg ot the cell would put it in condition for mitosis ; winter 

 by lowering its vitality would render it unable to divide by 

 mitosis but not by amitosis. Pfeffer's observation on the effect 

 ot ether would also fall in line, the cell tending to divide by 

 mitosis but being prevented by the interference with its powers 

 consequent on the influence of the ether. In P"asciola the 

 lai val kidney cells while young still divide by mitosis and are 

 consec|uently vigorous, notwithstanding the presence of consid- 

 erable metabolic material, but as they become old they lose this 

 P'Hvcr and divide by amitosis. The food-ova with their ten- 



their well-being that cell division does not take place at all and 

 nuclear division is only by amitosis. The deeply staining gran- 

 ular pariu les in some of them may be imperfectly formed chro- 



A careful eytr^logieai study of these food-ova would be very 

 likel\ to repay study, as it might throw light on the relation 

 between the two modes of nuclear division. The endoderm 

 <-c s are developing in the amitotic manner and this is the chief 



