38 LECTURE I. 



the red blood-cells no longer exhibit all the characteris- 

 tics of a cell, but have lost an important constituent in 

 their composition. But we are also all agreed upon 

 this point, that the blood is one of those changeable 

 constituents of the body, whose cellular elements possess 

 no durability, and with regard to which everybody 

 assumes that they perish, and are replaced by new ones, 

 which in their turn are doomed to annihilation, arid 

 everywhere (like the uppermost cells in the cuticle, in 

 which we also .can discover no nuclei, as soon as they 

 begin to desquamate) have already reached a stage in 

 their development, when they no longer require that 

 durability in their more intimate composition for which 

 we must regard the nucleus as the guarantee. 



On the other hand, notwithstanding the manifold inves- 

 tigations to which the tissues are at present subjected, 

 we are acquainted with no part which grows or multi- 

 plies, either in a physiological or pathological manner, in 

 which nucleated elements cannot invariably be demon- 

 strated as the starting-points of the change, and in which 

 the first decisive alterations which display themselves, do 

 not involve the nucleus itself, so that we often can deter- 

 mine from its condition what would possibly have become 

 of the elements. 



You see from this description that, at least, two differ- 

 ent things are of necessity required for the composition 

 of a cellular element ; the membrane, whether round, 

 jagged or stellate, and the nucleus, which from the out- 

 set differs in chemical constitution from the mem- 

 brane. Herewith, however, we are far from having 

 enumerated all the essential constituents of the cell, 

 for, in addition to the nucleus, it is filled with a rela- 

 tively greater or less quantity of contents, as is like- 

 wise commonly, it seems, the nucleus itself, the contents 

 of which are also wont to differ from those of the 



