296 GENERAL PHYSIOLOGY 



Inasmuch as there exists upon the earth at present no living 

 substance that is homogeneous throughout, the absent factor, as 

 shown by the experiment, is the natural coherence and correlation 

 of the essential parts of the organism. 



This is true equally of the cell-community and the individual 

 cell. But the objection may be raised that in many cases parts 

 and even whole organs can be separated from an organism without 

 endangering its existence. This is true, but in all such cases the 

 parts are such as are not absolutely necessary to the maintenance 

 of the individual, whether it be because they are present in 

 abundance and can be replaced in function by others, or because 

 they are not closely related to the other parts, and, therefore, when 

 separated, represent complete individuals. A polyp can be cut 

 into two parts, both of which continue to live, and from a polyp- 

 stalk a single polyp can be cut off without dying. In the above 

 experiment upon Amoeba the nucleated cell-body continues living 

 even after the separation of a portion of the protoplasm, because 

 it still possesses a quantity of protoplasmic particles of the 

 same kind as were removed. But the piece of protoplasm that is 

 cut off perishes, because its connection and correlation with the 

 nuclear mass have ceased. 



The living substance that now exists upon the earth's surface 

 is recognised only in the form of cells, either alone or bound 

 together into cell-communities. The cell contains as its 

 essential constituents two different substances, the protoplasm and 

 the nucleus. 1 Wherever a little protoplasm and a little nuclear 

 substance exist in union ; there is a cell ; and only such is capable 

 of life when the external vital conditions are fulfilled. A large 

 cell can be divided into many pieces capable of life, so long as the 

 requirements are complied with that every piece shall possess 

 some protoplasm and a little nuclear substance, and that the 

 disproportion between the two masses shall not exceed a certain 

 limit. 2 With some skill it is not difficult to perform the experiment 

 upon large unicellular organisms. But, if a cell be so divided that 

 the nucleus is separated from the protoplasm, both parts invariably 

 perish. 



Since the cell is the general elementary constituent of organisms, 

 the individual of the lowest order, the association of nucleus and 

 protoplasm in the cell may be established as a general internal 

 condition of life. Only where these two are united can life 

 continue to exist. 



A physical phenomenon takes place when, on the one hand, a 



material substratum is present in which it can take place, and, 



on the other, certain external conditions are fulfilled. The same 



holds good of vital phenomena. Vital phenomena appear with 



1 Cf. p. 71. 2 Cf. Lillie ('96). 



