236 SCIENCE PROGRESS. 



shown that pieces of cytoplasm cut off from the remainder 

 of a protozoon are incapable of maintaining life and soon 

 perish. If, on the other hand, a fragment of cytoplasm 

 similarly cut off contains nuclear matter, it is shown to con- 

 tain the attributes necessary to life, for the fragment does 

 not perish but reconstitutes itself and becomes an inde- 

 dependent living being. The converse also holds good. 

 A nucleus or a fragment of a nucleus isolated from a 

 protozoon, is incapable of life and perishes. But a nucleus 

 or a fragment of a nucleus in conjunction with a fragment 

 of cytoplasm is capable of life and constitutes an indepen- 

 dent living being. The reasonable inference is that cyto- 

 plasm plus nuclear matter is indispensable for the per- 

 formance of vital functions. 



Now cytoplasm plus nuclear matter constitutes a cell. 



I have elsewhere discussed at some length the definition 

 of a cell, 1 and I have defined it as a corpuscle of protoplasm 

 which contains nuclein. In the present state of our know- 

 ledge this definition seems the only one possible. The cell 

 then consists of two essential substances, cytoplasm and a 

 substance which is different from cytoplasm, both structurally 

 and in chemical constitution, namely, nuclein. In a great 

 majority of cells other substances are present which are 

 different from both of these. Such substances are the 

 centrosomes, that modification of cytoplasm which is called 

 archoplasm, amylum and aleurone grains and so forth. As 

 far as we know, however, these substances are not essential 

 to life, but are secondary products characteristic of dif- 

 ferentiated cells. Recent researches on the structure of 

 Bacteria and Oscillaria justify the assertion that cells exist 

 in which these substances are absent. We know next to 

 nothing about the presence or absence of centrosomes and 

 archoplasm in the Protozoa, and it may be that further 

 investigation will lead us to the conviction that these two 

 are as essential to the life of these forms as the presence of 

 cytoplasm and nuclein. Maybe not ; in any case it does 



1 Quarterly Journal of Microscopical Science, vol. xxxviii., p. 137, 

 1895. 



