CELLS. 35 



§ 14- 



Vital phenomena of the perfect cells. Growth — The cells, 

 when once completed, perform a considerable number of 

 functions, which relate as well to the form of the whole cell 

 and of its contents, as to their chemical composition, and arc 

 called growth and change of substance. 



As to growth, it occurs perhaps in all cells, though not in 

 all to the same extent. It is clearly manifested in all those 

 cells which are formed directly round a nucleus, since in this 

 case the membranes which at first closely invest the nucleus, 

 in time become more and more separated, whilst the cells which 

 arise around portions of contents or investing masses, and are 

 from the first provided with contents, often increase in size but 

 very slightly. Growth is either in surface or in thickness. The 

 former appears very usually to be general, K 10 



when cells increase without altering their 

 form, e. g. the ova, many nerve-cells, &c. ; 

 frequently however it is partial, as in all 

 cells which depart from the primitive globular 

 form, in such a manner that the cell-mem- 

 branes only add new substance and extend 

 at two or more points. Growth in thickness 

 also occurs, to a certain extent, in all cells, since all cell- 

 membranes become somewhat thicker with age ; and it pro- 

 duces in some localities a very considerable thickening of the 

 membrane, occasionally with evident lamination (as in the 

 cartilage cells), and even gives rise to certain structures which 

 have the greatest similarity to the sclerogenous cells of plants 

 (bone-cells). 



The nuclei and nucleoli also take part, to a certain extent, 

 in the growth of the cells. In the former, a general growth 

 is easily demonstrable in all growing cells ; in many, as in 

 those of the smooth muscles, of the epithelium of the vessels, 

 of the formative cells of elastic tissue, and others, there is also 

 a partial growth, in consequence of which they often assume 

 the form of long slender rods. The nucleoli also not uufre- 



Fig. 10. Cartilage cells of man, x 350. Two cells with thickened walls, from the 

 cartilage of the great cornu of the hvoid hone, containing a clear drop of fat 

 beside their nucleus. 



