GENERAL SENSIBILITY OF THE CELL. 15 



lutionary manner ; for their existence need not in the first in- 

 stance be conditional on the introduction of new influences, 

 since they depend on the fact that the branchiae must from the 

 first or they could not have lived, grown, and exercised their 

 functions have contained all the elements and have exercised 

 all the elementary functions which fitted them for differentiation 

 in the direction indicated by those functions, and for transfor- 

 mation into organs apparently intended for one function only. 



"We arrive at the same conclusion by a simple general con- 

 sideration. We know that the simplest and lowest animal, a 

 mere gelatinous mass, say an Amoeba, exercises, and must exercise, 

 all the vital functions as well as the ovum-cell which is so rich 

 in protoplasm, or even as the young and growing cells which 

 constitute animal tissues functions which in the higher animals 

 are apparently fulfilled exclusively by special organs. The pro- 

 toplasmic cell or the Amreba takes up nutrition, often indeed 

 of a solid nature ; it moves more or less quickly and voluntarily ; 

 it is sensible to impressions transmitted to it by the agitation or 

 chemical properties of the surrounding medium ; it assimilates 

 organic matters, and breathes, inasmuch as it expires the carbonic 

 acid formed in the process ; it is capable of more or less definite 

 sensations, for it selects the food that suits it, and it grows 

 and multiplies often by highly complex processes. All these 

 characters are to be found in each living protoplasmic cell of 

 every growing organ : but it is true that it is not every cell of 

 an organism that is in this sense living. Thus in the hair and 

 nails, for instance, there are horny cells which no longer contain 

 any fresh and unchanged protoplasm, and consequently can no 

 longer grow or multiply, their increase being effected by fresh 

 and living cells lying in the deeper skin-layers ; from these, new 

 horny cells are constantly produced and pushed forward to 

 replace the old cells as they wear off at the angles of the nails 

 or tips of the hairs. "We may apply Briicke's expression, 

 ' elementary organisms,' as a name for those deep-seated cells, 

 which, being richly supplied with protoplasm and capable of 

 multiplication, are, in the strictest sense, living cells. For as 

 the life of every organ is the sum of the individual life of the 

 living cells which compose it, it is clear that every living and 



