CHAPTER XI 



THE CELL-THEORY. 



WITH the founding of the cell-theory by Schwann in 

 1839 an important step was taken in the analysis of the 

 degrees of composition of the animal body. Aristotle had 

 distinguished three the unorganised material, itself com- 

 pounded of -the four primitive elements, earth and water, air 

 and fire, the homogeneous parts or tissues and the hetero- 

 geneous parts or organs, and this conception was retained 

 with little change even to the days of Cuvier and von Baer. 

 Those of the old anatomists who speculated on the relations 

 of organic elements to one another were dominated by. 

 Aristotle's simple and profound classification, and proposed 

 schemes which differed from his only in detail. Bichat 

 enlarged and deepened the concept of tissue, but the 

 degree of composition below this was for him, as for all 

 anatomists of his time, a fibrous or pulpy "cellulosity," 

 living, indeed, but showing no uniform and elemental struc- 

 ture. It was Schwann's merit to interpose between the 

 tissue and the mere unorganised material a new element 

 of structure, the cell. And, as it happened, a few years 

 before Schwann published his cell-theory, Dujardin hinted 

 at another degree of composition which was later to take 

 its place between the cell and the chemical elements 

 sarcode or protoplasm. 



As is well known, the concept of the cell arose first 

 in botany. Robert Hooke discovered cells in cork and pith 

 in 1667, and his discovery was followed up by Grew and 

 Malpighi in 1671, and by Leeuenhoek in 1695. But they 

 did not conceive the cell as a living, independent, structural 

 unit. They were interested in the physiology of the plant 



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