THE CELL. 



31 



For a similar reason, as already intimated, 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, 



FIG. 7. 



Schematic diagram of a cell : a. ectoplasm composed of hyaloplasm ; b, spongioplasm ; c, 

 chromosome, composed of "chromatin," and forming a part of the intranuclear reticu- 

 lum ; between these chromatic fibres is the achromatin ; d, hyaloplasm in the meshes of 

 the spongioplasm ; e, one of the two nucleoli represented in the diagram ; /, one of eight 

 bodies constituting the metaplasm represented ; g, centrosome, with radiate arrangement 

 of the surrounding spongioplasm ; h, nuclear membrane. 



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

 performed by the whole cell among these three organs. 



