THE CELL. • 31 



Let us now compare the amoeba with some other varieties of cell, 

 in order to learn what they all have in common. 



The amoeba has an outer, soft, transparent layer of cytoplasm, 

 the ectoplasm. This is not present in all cells. In many the 

 granular cytoplasm has no envelope, but appears to be quite naked. 

 In other varieties it is enclosed in a distinct membrane. 



In the great majority of cells the active streaming of the cyto- 

 plasm and the pseudopodial protrusions described in the amoeba are 

 wanting, but the Brownian movement of the granules is more con- 

 stantly present. The cells have fixed positions and their food is 

 brought to them, usually in solution, so that the more active move- 

 ments so essential to the welfare of the amoeba would be superfluous. 

 For a similar reason, as already stated, they can dispense with the 

 contractile vacuole. 



We learn, then, that when we reduce the cell to its simplest 

 terms, it consists of a mass of cytoplasm enclosing a nucleus. To 

 these we must probably add a third essential constituent, the centro- 

 some, which is a minute granule situated in the cytoplasm. It is so 

 small that its presence has not been established in all cells, its detec- 

 tion in many cells being extremely difficult because of the general 

 granular appearance of the cytoplasm in which it lies. It plays such 

 an important part, however, in the division of those cells in which 

 it has been studied, that the inference that it is an essential part of 

 all cells appears justified. 



These three constituents, the cytoplasm, nucleus, and centrosome, 

 appear to be the essential organs of a cell among which its activities 

 are distributed (Fig. 7). We do not know how they do their work, 

 but we have a general conception of the distribution of the work 

 performed by the whole cell among these three organs. 



1. The cytoplasm, which usually makes up the chief bulk of the 

 cell, especially in those varieties which have active metabolic functions, 

 appears to be the part of the cell in which the assimilated food is utilized 

 in the production of chemical substances, either fresh cytoplasm or 

 some other product, or in the execution of movements or the libe- 

 ration of energy in other forms. Most of the active processes that 

 are obvious seem to be carried on in the cytoplasm during the greater 

 part of the life-history of the cell. Many of these processes involve 

 highly complex chemical transformations. Some of these are syn- 

 theses, or the building up of more complex molecules from those of 

 simpler constitution, and are called anabolic processes or anabolism. 



