442 PROCEEDINGS OF THE AMERICAN ACADEMY. 



not necessarily incompatible, factors, that the present work has been 

 undertaken. In this work the chemical relations in living organisms, 

 especially with reference to their power to divide, have been studied, and 

 the method has of necessity been experimental. 



The work wss done under the continuous supervision of Professor 

 E. L. Mark, and I take this opportunity to thank him for his valuable 

 and kindly criticism. I am also under much obligation to him for meet- 

 ing the numerous and varied material needs of these experiments. 



The influence on cell-divisiou of substances having a relation to it can 

 be best studied experimentally upon free-living cells. Their media afford 

 the important requirement of the experimental method, namely, the 

 ability of the experimenter to vary the conditions. Since there is an 

 active interchange between medium and cell, even the internal conditions 

 of an organism may thus be varied. However, the distinction between 

 external and internal conditions is at best one of not much lojiical force. 

 The necessity of performing experiments upon the living object by means 

 of a liquid medium made the analysis of physico-chemical relations a 

 primary object. No adequate interpretation of the changes of cells in 

 media is possible without this analysis. Wliether it is only a preliminary 

 necessity, or whether, if complete, such analysis would constitute the 

 whole interpretation, must be left for the progress of investigation to 

 determine. In this research the object was to push physico-chemical 

 interpretation as far as the present development of physico-chemical 

 methods permits. This part of the work constitutes one of the two 

 essential modes of explanation here attempted. But it soon became evi- 

 dent that in the present state of knowledge this method of interpretation 

 when applied alone accounts only incompletely for the ascertained facts. 

 Hence the adjustment of the organism to a particular combination of 

 conditions was adopted as a complementary, but in itself also incomplete, 

 principle of explanation. This latter principle is elaborated in the con- 

 crete case of Stentor in a subsequent section. It is true that the concep- 

 tion of adjustment is an unanalyzed complex, and we do not know whether 

 it expresses more than, or only as much as, the physico-chemical inter- 

 pretation could if developed to its limit. At present it is a useful means 

 of description, and the two contrasted modes of interpretation are here 

 regarded as of equal practical importance. The media of free-living 

 cells always contain salts in solution. Likewise it is shown by chemical 

 analysis that cells contain salts, although their condition in the living 

 organism is not so precisely known. Salts that are normally present in 

 the cell or its medium I shall for convenience term the physiological salts. 



