14 CELL THEORY BEFORE 1860. 



This may be given up to the period of its full develop- 

 ment by Dr. Lionel Beale, and after that a review of 

 the present state of knowledge upon these, in some 

 respects, rival theories. This method will, I think, 

 conduce to clearness of understanding the subject, 

 better than the strictly chronological method in which 

 both are mingled together. 



Schleiden, who was the founder of the cell theory, 

 though by him restricted to plants, defines the vegetable 

 cell as " the elementary organ which constitutes the sole 

 essential form-element of all plants, and without which a 

 plant cannot exist ; and as consisting, when fully deve- 

 loped, of a cell wall composed of cellulose, lined with a 

 semi-fluid, nitrogenous coating." With him, therefore, 

 the cell consisted of two parts, viz., a vesicle and semi- 

 fluid contents. In plants the cell forms are distinct, and 

 easily recognized, and thus, when the conception of a 

 similar elementary organ was extended to the animal 

 kingdom by Schwann, in 1838, it is not to be wondered 

 at that the cellular form was expected to be universal. 

 Schwann added to Schleiden's two elements a third 

 the nucleus which he deemed also of essential im- 

 portance, and to be present in all cells, if not always, 

 at least in some stage of their existence. On his 

 authority this threefold doctrine of the cell became 

 universally prevalent for a time. I give here Schwann's 

 original definition of his theory, as some points in it 

 have been overlooked or forgotten in the mass of 

 controversial writing this subject has provoked : 



" The following admits of universal application to the forma- 

 tion of cells : There is, in the first instance, a structureless 

 substance present, which is sometimes quite fluid, at others 



