SECT. 1 3.] PROCESSES IN INTERIOR OF CELLS. 21 



selves or to deposition upon the outer surface of the cell-mem- 

 brane. 



The Nuclei and Nucleoli also participate in the growth of the 

 cells to a certain degree; and the former, more especially, present, 

 together with an omnilatcral, an unilateral growth, in consequence 

 of which they become elongated. 



§ 13. Processes taking place in the Interior of Cells. — In order 

 to obtain a clear insight into the processes going on in the inte- 

 rior of cells, it would be especially necessary to have a more accu- 

 rate knowledge of the chemical properties of their contents than 

 we now possess. Only two kinds of cells, the ovum and the blood- 

 cells, have been more carefully investigated; but these are so 

 peculiarly conditioned, that they can scarcely serve as types of 

 cells in general. Nevertheless, from their analysis, some conclu- 

 sion may be drawn as to other cells, and from such analysis com- 

 bined with what micro-chemical investigation furnishes, we may, 

 perhaps, be warranted in regarding cell-contents in general as a 

 moderately concentrated solution of proteine, with alkaline and 

 earthy salts and dissolved or suspended fat-particles. Many cells, 

 however, differ very considerably from this, which is undoubt- 

 edly the ordinary condition of all cells, at least in the young state, 

 in so far as in them some of the above-mentioned constituents 

 preponderate very much, or entirely new substances are super- 

 added. Thus, there are cells with much proteine, as the ganglionic 

 globules; and with much fat, as the fat-cells, the cells of sebaceous 

 glands, and the mammary glands, etc. ; further, cells with hsema- 

 tine, pigment, constituents of the gall and urine, mucus (epithelial 

 cells), serum (pathological fat-cells, corpuscles of connective tissue, 

 lacunse of bone), etc. 



The phenomena manifested by these so variously constituted 

 contents, during life, may be best designated as — absorption, 

 assimilation, and excretion. 



These are chiefly owing to chemical and physical causes, and may 

 for the most part even be followed with the microscope, seeing that 

 changes in the form and contents of the cell very frecmently go 

 hand-in-hand with them. With regard to absorption, it presents 

 itself in all cells, and the primary cause of the entrance of 

 material is simply to be sought in the capability of imbibition of 

 the cell-membrane and contents. This imbibition is, however, not 

 to be understood as if the cells admitted indiscriminatelv all sub- 

 stances approaching them ; on the contrary, they present, according 



