74 ELEMENTS OF BIOLOGY. 



cell-wall, irrespective of and not including the " nucleus," 

 when this is present, are usually termed the " cell-contents." 

 The nature of these materials varies much in some respects 

 of minor importance ; but it is probable that the cell-con- 

 tents are to be regarded as essentially of the nature of proto- 

 plasmic or bioplasmic matter. This, at any rate, is the case 

 in young, actively-growing cells, and in these the cell- wall bears 

 a small proportion to the cell-contents. In progress of growth, 

 however, the cell-contents seem to diminish in bulk, owing 

 to the conversion of their outermost layers into "formed" 

 material. The cell-contents are deeply reddened by a solu- 

 tion of carmine (fig. 19), and generally contain more or less 

 numerous molecules and granules. Upon the whole, the 

 cell-contents appear to be the essential and most important 

 element of the cell. They constitute the only element the 

 existence of which is constant ; and they are the main, or, 

 in some cases, the sole, agent whereby the vital actions of 

 the cell are carried on. 



3. The Nucleus. Very generally, but by no means uni- 

 versally, the cell-contents exhibit in one place a definite 

 rounded or oval body, which is termed the " nucleus " 

 (fig. 20, c). This varies much in actual structure, some- 

 times being vesicular, sometimes solid, and sometimes com- 

 posed of granules. That the nucleus plays an important 

 part in cell-life cannot be doubted ; but opinions are still 

 divided as to its exact functions, some regarding it as the 

 most important agent in cell-activity, whilst others consider 

 it of comparatively small moment. That it is composed of 

 growing and living matter is shown by the extent to which 

 it is coloured by carmine, and it seems in many cases to 

 take the initiative in the process of cell-multiplication. It 

 is not invariably present, however, and it would not, there- 

 fore, seem to be absolutely essential to cells. On the other 

 hand, " free " nuclei, which have been liberated from cells, 

 sometimes play a most important part in various vital pro- 

 cesses. 



