28 METAMORPHOSES OF CELLS. [SECT. 1 6. 



cesses which we cannot hut consider as taking place in the nerve- 

 cells, deserve the name of animal functions, since these processes 

 are nothing else than what the physiologist understands as the 

 functions of the gray nervous substance. We cannot, of course, 

 enter into discussion of them here ; and the more so since these 

 functions are completely inaccessible to microscopical observation. 



Donders was the first to assert the opinion, that only the contents of cells 

 and not the cell-membranes are contractile ; and, I must confess, that the 

 longer I consider the subject, the more the idea of Donders pleases me ; yet 

 it appears to me as still rather premature, in entirely denying the power of 

 contraction to cell-membranes, there being nothing a priori to adduce against 

 this view, while a decision, founded on fact, cannot as yet be formed in many 

 cases. 



§ 1 6. Metamorphoses of Cells. — Different Kinds of Cells. — The 

 destination of the cells which occur at earlier or later periods in 

 the organism is very various. A very considerable number of them 

 remain for only a short period in their original state, and, subse- 

 quently, unite with others for the formation of the higher ele- 

 mentary parts. Others, again, do not indeed enter into any such 

 connections, but change more or less their former nature, as the 

 horny plates of the epidermis and nails. Many cells, lastly, never 

 go through any metamorphoses, but remain as cells, till they perish, 

 sooner or later, often not till the destruction of the organism itself, 

 such as epithelium-cells, the cells of glandular parenchymas, those 

 of the nervous system, etc. 



Permanent Cells may be arranged most conveniently under the 

 following heads : — 



i. True Cells, which have not altered their cellular nature in any 

 essential point, occur in the epidermis {stratum Malpighii) epithe- 

 lia, in the blood, the chyle, the lymph in gland-juices, adipose tissue, 

 gray nervous substance, the red medulla of bone, in glands (liver, 

 spleen, supra-renal capsules, closed glandular follicles) and in car- 

 tilages. These cells may be divided according to their form, into 

 round, disc-shaped, cylindrical, conical, ciliated, and stellated: 

 according to their contents, into cells containing fat, proteine, se- 

 rum, hsematine, biline, pepsine, mucus, and pigment : and, with 

 regard to their occurrence, some are isolated either in fluids or solid 

 tissues, while others are united to form simple cellular parenchy- 

 mas ; others, lastly, being connected by an intercellular substance 

 of some kind. 



2. Metamorphosed Cells, which have altered their original 

 structure more or less. To this category belong — 



