160 SOME RECENT DEVELOPMENTS 



A few words concerning the origin and the 

 earlier phases of the cell theory may fitly preface 

 my remarks. Like all great theories it is im- 

 possible to date precisely its first enunciation. 

 Commonly stated to have been founded in 1839, 

 first for plants by Schleiden, and almost directly 

 afterwards extended to animals by Schwann, the 

 cell theory in its main outlines is to be found 

 lurking in an incomplete unformed condition, 

 rather hinted at than clearly expressed, in the 

 earlier writings of Robert Brown, Dutrochet, Von 

 Mohl, and others. These earlier and less precise 

 attempts were however confined to vegetable 

 tissues. As regards animals, to which I propose 

 to limit myself this evening from no desire to 

 minimise the importance of the sister science of 

 Botany, but merely because I am unfortunately less 

 familiar with it and unable to speak from my own 

 knowledge and observation as regards animal 

 tissues, Schwann is rightly regarded as the founder 

 of the cell theory. 



By the cell theory, Schwann meant that the 

 bodies of all animals, as also of all plants, are 

 formed of cells : that it is by evolution of, and 

 changes in these cells, that the tissues of which 

 the various parts of the body consist are formed, 

 and further, " that the differences in the properties 

 of the different tissues and organs of animals and 

 plants depend on differences in the chemical and 

 physical activities of the constituent cells." Con- 

 cerning the mode of formation of cells, Schwann 

 held that they might arise independently in a 



