2 THE CELL [CH. 



the tissues composed of chambers with definite walls, and 

 these, chambers were not inappropriately named cells, and 

 the substance of the walls cellulose. Very little investiga- 

 tion was needed, however, to show that the essential feature 

 as far as life is concerned was not the walls, which are 

 mainly skeletal in function, but the contents, and that each 

 cell contained a certain quantity of protoplasm and a 

 nucleus. Frequently, of course, there are other structures 

 chloroplasts, vacuole, and so forth but since the almost 

 invariable and therefore presumably essential feature of the 

 cell is the presence of protoplasm containing a single nucleus, 

 and since these same features are found in animals in the 

 absence of cellulose walls, or indeed of definite walls of any 

 kind, the meaning of the word "cell," by a natural transi- 

 tion, became changed to "a mass of protoplasm containing 

 a nucleus." Such a definition is certainly better than "a unit 

 of living matter " because it involves no theoretical implica- 

 tion, but it is still far from satisfactory for various reasons. 

 In the first place, frequently in the Protozoa, and not very 

 rarely in other animals, cells, or at least definite masses 

 of protoplasm, exist, which contain two or more nuclei. 

 Secondly, bodies that can hardly be denied the name of cells 

 may either apparently lack a nucleus and nuclear matter 

 altogether, as for example the red blood-corpuscles of Mam- 

 mals, or may have nuclear matter distributed through them 

 without forming a definite nucleus, as in some Protozoa. 

 And lastly, histological investigation shows more and more 

 clearly that even when cells appear to be sharply defined 

 and distinct, there are nevertheless connections between 

 them, and that to regard the organism as built up of discrete 

 cells which co-operate physiologically but are fundamen- 

 tally independent is a false conception almost as false as 

 to regard the surface of the sea as made up of a series of 



