24 STUDIES IN ADVANCED PHYSIOLOGY. 



In the meantime Robert Brown, 1831, had discovered the 

 nucleus inside the cell. Thus little by little the contents 

 were considered as the essential thing of the cell, rather 

 than the cell wall. To help this view the botanist, Unger, 

 in 1843 made a series of discoveries of a then astounding 

 character. In examining the green scum everywhere plen- 

 tiful on moist flower pots (vaucheria) , he saw the cell sap 

 l< crawl " out of the old cell wall and creep away. This is 

 the common phenomenon of rejuvenescence easily observed, 

 in which the protoplasm leaving the old cell wall becomes 

 active for a while, and then produces a new vaucheria 

 thread. But to Unger it was a plain case of the sap of a 

 plant cell turning into an animal. The misinterpretation 

 was strengthened when soon afterwards Kritzing observed 

 green amoeboid cells changing into filaments of algae. Here 

 was a case of an animal changing to a plant. But it estab- 

 lished one thing as correct that the cell contents are the 

 essential thing and the cell wall quite secondary. To the 

 mysterious contents endowed seemingly with vital proper- 

 ties, Hugo von Mohl in 1846 gave the name protoplasm . 



All these views were then moulded into a definite theory 

 by Schleiden in 1849. It will be observed that up to this 

 time all the observations had been confined to plants, but 

 in 1849 the histologist, Schwann, showed that animals, too, 

 are made up of cells, and applied the cell theory equally to 

 them. The theory was therefore called the "Schleiden and 



( __ i 



Fig. 3. CELLS FROM THE ANIMAL BODY (CORNEA). (After Schafer.) 

 c, columnar cells; p, polygonal cells; fl, flattened cells. 



Schwann cell theory." The manner in which these cells 



