18 GENERAL ANATOMY OF THE TISSUES. 



is unknown, their external appearance, their similarity to 

 the elementary vesicles, their disappearance in caustic al- 

 kalies, and their insolubility in acetic acid, would lead us 

 to suppose them to be fat ; the membranes may, as in the 

 elementary vesicles, be a protein compound. Nucleoli are 

 found in the great majority of nuclei, so long as these 

 are still young, and in many during their whole existence ; 

 but nuclei also exist, in which nucleoli cannot be recognised 

 with certainty, or at least become obvious only at a later 

 period ; and therefore, at present, the nucleolus cannot be so 

 unconditionally recognised to be an essential constituent 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, Donders, 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-mem- 

 branes consist originally of the same substance — of a protein 

 compound, in fact ; that, however, in consequence of its sub- 

 sequent metamorphoses, it may acquire differences of compo- 

 sition 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 compound, and from the reaction of young cells and 

 of embryonic parenchyma 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 product, 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 



