No. 589] THE EVOLUTION OF THE CELL 



11 



however, and especially when animal tissues came to be 

 studied, it became apparent that the cell- wall, like the 

 fluid cell-sap, was a secondary product, and that the es- 

 sential and primary part of the cell was the viscid proto- 

 plasmic substance, in which a peculiar body, the 1 'nu- 

 cleus," or kernel, was found to be universally present. 

 Consequently the application and meaning of the term 

 cell had to undergo an entire change, and it was defined as 

 a small mass or corpuscle of the living substance, proto- 

 plasm, containing at least one nucleus. To these essen- 

 tial constituents other structures, such as a limiting mem- 

 brane or cell-wall, and internal spaces — vacuoles— filled 

 with watery fluid, might be added as products of the sec- 

 retory or formative activity of the living substance ; but 

 such structures were no longer regarded as essential to 

 the definition of the cell, since in many cases they are not 

 present. It is to be regretted in some respects that with 

 this changed point of view the term "cell," used orig- 

 inally under a misapprehension, was not replaced by some 

 other term of which the ordinary significance would have 

 been more applicable to the body denoted by it. 8 



The chief point that I wish to establish, however, is 

 that the term cell was applied originally to the protoplas- 

 mic corpuscles building up the bodies of the Metazoa and 

 M«'t:ipliyta, each such corpuscle consisting of a minute 

 individualized mass of the living substance and contain- 

 ing a nucleus. Hence a complete cell is made up of two 

 principal parts or regions, the nucleus and the remainder 

 of the protoplasmic body, termed the cytoplasm. By 

 some authors the term protoplasm is restricted to the 

 cytoplasmic portion of the cell, and protoplasm is then 

 contrasted with nucleus; but it is more convenient to con- 

 sider the whole cell as composed of protoplasm divided 

 into two regions, nucleus and cytoplasm. 



We come now to the consideration of the body termed 



yet the word has become so firmly established that every effort to replace it 

 by a better has failed, and it probably must be accepted as part of the 

 established nomenclature of science."— E. B. Wilson, "The Cell," p. 19. 



