SCHLEIDEN, SCHWANN, HENLE. 15 



more or less gelatinous. This substance possesses within itself, 

 in a greater or lesser measure, according to its chemical quali- 

 ties, and the degree of its vitality, a capacity to occasion the 

 production of cells. When this takes place, the nucleus usually 

 appears to be formed first, and then the cell around it. The 

 formation of cells bears the same relation to organic nature that 

 crystallization does to inorganic. The cell, when once formed, 

 continues to grow by its own individual powers, but is, at the 

 same time, directed by the influence of the entire organism in 

 such manner as the design of the whole requires. This is the 

 fundamental phenomenon of all animal and vegetable vegeta- 

 tion. It is alike equally consistent with those instances in 

 which young cells are formed within parent cells, as with those 

 in which the formation goes on outside of them. The genera- 

 tion of the cells takes place in a fluid, or in a structureless sub- 

 stance in both cases. We will name this substance in which 

 the cells are formed, cell- germinating material (zellenkeimstoff), 

 or cytoblastema. It may be figuratively compared to the 

 mother-lye from which crystals are deposited " (Syd. Soc., 1847, 

 p. 39). 



We perceive that Schwann added little to the conception of 

 Schleiden, but he extended it to all organisms, whether animal 

 or vegetable, and applied it with considerable success to the 

 details of the formation of animal tissues, in which process the 

 whole three cell elements were assumed to play a part, and a 

 distinctively vital one. Schwann was also more decided in ad- 

 mitting the free origin of cells in a blastema than Schleiden. 



However, in proportion as the cell theory was ap- 

 plied more extensively in the animal kingdom it 

 became more and more difficult to maintain the three- 

 fold nature of the cell. 



In giving now a general view of the development of 

 the cell theory I will not attempt to give a complete 

 history, apportioning to each observer his share of the 

 merits in the building up of the theory. I will merely 

 quote in full, or analyze, those memoirs which mark 



