34 LECTURE I. 



body essentially of a nitrogenous nature, and, on the 

 whole, similar in composition. The so-called membrane 

 of the vegetable cell is only met with in a few animal 

 tissues, as, for example, in cartilage ; the ordinary mem- 

 brane of the animal cell corresponds, as I showed as far 

 back as 1847, to the primordial utricle of the vegetable 

 cell. It is only when we adhere to this view of the 

 matter, when we separate from the cell all that has been 

 added to it by an after-development, that we obtain a 

 simple, homogeneous, extremely monotonous structure, 

 recurring with extraordinary constancy in living organ- 

 isms. But just this very constancy forms the best crite- 

 terion of our having before us in this structure one of 

 those really elementary bodies, to be built up of which 

 is eminently characteristic of every living thing without 

 the pre-existence of which no living forms arise, and to 

 which the continuance and the maintenance of life is 

 intimately attached. Only since our idea of a cell has 

 assumed this severe form and I am somewhat proud of 

 having always, in spite of the reproach of pedantry, 

 firmly adhered to it only since that time can it be said 

 that a simple form has been obtained which we can 

 everywhere again expect to find, and which, though 

 different in size and external shape, is yet always 

 identical in its essential constituents. 



In such a simple cell we can distinguish dissimilar 

 constituents, and it is important that we should accurately 

 define their nature also. 



In the first place, we expect to find a nucleus within 

 the cell ; and with regard to this nucleus, which has 

 usually a round or oval form, we know that, particularly 

 in the case of young cells, it offers greater resistance to 

 the action of chemical agents than do the external parts 

 of the cell, and that, in spite of the greatest variations in 

 the external form of the cell, it generally maintains its 



