30 LECTURE I. 



individual processes of life. But I think that we must 

 look upon this as certain, that, however much of the 

 more delicate interchange of matter, which takes place 

 within a cell, may not concern the material structure 

 as a whole, yet the real action does proceed from the 

 structure as such, and that the living element only main- 

 tains its activity as long as it really presents itself to us 

 as an independent whole. 



In this question it is of primary importance (and you 

 will excuse my dwelling a little upon this point, as it is 

 one which is still a matter of dispute) that we should 

 determine what is really to be understood by the term 

 cell. Quite at the beginning of the latest phase of his- 

 tological development, great difficulties sprang up in 

 crowds with regard to this matter. Schwann, as you no 

 doubt recollect, following immediately in the footsteps 

 of Schleiden, interpreted his observations according to 

 botanical standards, so that all the doctrines of vegetable 

 physiology were invoked, in a greater or less degree, to 

 decide questions relating to the physiology of animal 

 bodies. Vegetable cells, however, in the light in which 

 they were at that time universally, and as they are even 

 now also frequently regarded, are structures, whose 

 identity with what we call animal cells cannot be ad- 

 mitted without reserve. 



When we speak of ordinary vegetable cellular tissue, 

 we generally understand thereby a tissue, which, in its 

 most simple and regular form is, in a transverse section, 

 seen to be composed of nothing but four- or six-sided, 

 or, if somewhat looser in texture, of roundish or poly- 

 gonal bodies, in which a tolerably thick, tough wall 

 (membrane) is always to be distinguished. If now a 

 single one of these bodies be isolated, a cavity is found 

 enclosed by this tough, angular, or round wall, in the 

 interior of which very different substances, varying ac- 



