CELLS. 47 



of the cell, as the nucleus. Generally, a nucleus contains only one 

 nucleolus, frequently there are two, rarely three, and, in solitary cases, 

 four or five may be present, which are then either eccentric or lie free 

 in the nucleus. 



A short time since, Bonders, in a very remarkable work (vide infra), 

 expressed the opinion that all cell-membranes consist of one and the 

 same, or at least of very nearly allied substances, which agree in their 

 characters with the elastic tissue. For my own part I believe that all 

 animal cell-membranes consist originally of the same substance of a 

 protein compound, in fact ; that, however, in consequence of its subse- 

 quent metamorphoses, it may acquire differences of composition and 

 of reaction. Many membranes in this manner become more resistant 

 with time and, as Donders justly states, approach elastic tissue ; others 

 change into collagenous tissue, as those of the formative cells of the 

 connective tissue,* and of the cartilage cells during ossification ; others 

 into syntonin, as in the smooth muscles ; into the so-called horn, and 

 so on. If we assume the primitive cell-membrane to be a protein com- 

 pound, and from the reaction of young cells and of embryonic paren- 

 chyma it can hardly be otherwise, we obtain a correspondence with the 

 vegetable cell, since in this case the primordial utricle, consisting of a 

 protein compound, can be considered as the analogue of the animal cell- 

 membrane, whilst the cellulose membrane appears as a secondary pro- 

 duct, as an excretion. Such may be the true condition also in those 

 animal structures of the Tunicata which are formed of cellulose, in which 

 case my assertion that here the cell-membranes are composed of woody 

 fibre, and that of Schacht (Muller's "Archiv," 1851), that they 

 are nitrogenous, would coincide. If future investigation 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 thicken- 

 ings 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 by the vesicles, and most beautifully so in embryos ; only in those 

 cells which arise directly around nuclei, are the nuclei at first more 



* [The term "connective tissue" (Bindegewebe) has been used by the translators to de- 

 signate the tissue more generally known as " white fibrous tissue.'' 



