A HISTORY OF SCIENCE 



mortier, Purkinje, and Muller, all of whom Schwann 

 himself had quoted. Moreover, there were various 

 physiologists who earlier than any of these had fore- 

 shadowed the cell theory notably Kaspar Friedrich 

 Wolff, towards the close of the previous century, and 

 Treviranus about 1807. But, as we have seen in so 

 many other departments of science, it is one thing to 

 foreshadow a discovery, it is quite another to give it 

 full expression and make it germinal of other discov- 

 eries. And when Schwann put forward the explicit 

 claim that " there is one universal principle of develop- 

 ment for the elementary parts of organisms, however 

 different, and this principle is the formation of cells," 

 he enunciated a doctrine which was for all practical 

 purposes absolutely new and opened up a novel field 

 for the microscopist to enter. A most important era 

 in physiology dates from the publication of his book 

 in 1839. 



THE CELL THEORY ELABORATED 



That Schwann should have gone to embryonic tissues 

 for the establishment of his ideas was no doubt due 

 very largely to the influence of the great Russian Karl 

 Ernst von Baer, who about ten years earlier had pub- 

 lished the first part of his celebrated work on em- 

 bryology, and whose ideas were rapidly gaining ground, 

 thanks largely to the advocacy of a few men, notably 

 Johannes Muller, in Germany, and William B. Carpenter, 

 in England, and to the fact that the improved micro- 

 scope had made minute anatomy popular. Schwann 's 

 researches made it plain that the best field for the study 

 of the animal cell is here, and a host of explorers en- 



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