CELLS. 19 



of woody fibre, and that of Schacht (Miiller's ' Archiv/ 1851), 

 that they are nitrogenous, would coincide. If future investi- 

 gation justify this comparison of the animal cell with the 

 primordial utricle of plants, the further question would arise in 

 animals, whether perhaps all the so-called metamorphoses of 

 the cell-membrane are not to be laid to the account of deposits 

 which are thrown down upon the outer side of it, similarly to the 

 cellulose in plants, so that, perhaps, together with the original 

 protein membrane, other secondary collagenous or elastic 

 membranes, &c., might be distinguished, and even the most 

 considerable thickenings of the animal cell be produced, in a 

 manner analogous to that which occurs in the ligneous tissues 

 of plants on the outer side of the protein membrane ; so that, 

 for example, within ossified cartilage-cells the original cell- 

 membrane might perhaps still exist. 



In all normal cells of the higher animals, the nuclei can be 

 readily shown to be vesicles, and most beautifully so in em- 

 bryos ; only in those cells which arise directly round nuclei, are 

 the nuclei at first more homogeneous, and subsequently exhibit 

 a distinct membrane. In pathological formations, this cha- 

 racter of the nucleus, which may be called an undeveloped 

 form, is very frequent, and the nucleus-like structures in the 

 Protozoa are also for the most part homogeneous bodies.] 



§ 9. 



Development of Cells. — With regard to the development of 

 cells, we have to distinguish between their free origin and their 

 production by the intermediation of other cells. In the former 

 case the cells are developed, independently of others, in a 

 plastic fluid, the cytoblastema of Schleiden, containing chiefly 

 fat, protein, and salts in solution ; in the other, or in cell- 

 multiplication, the existent cells either produce the so-called 

 daughter or secondary cells within themselves, or multiply 

 by division ; endogenous cell-formation and fissiparous cell- 

 formation. Both kinds of cell-formation agree in this, that 

 the cell-nuclei play a very important part, and appear to be 

 the proper centres of development (bildungs-punkte) of the 

 voung cells. 



