THE STORY OF NINETEENTH-CENTURY SCIENCE 



firm the claims of Schwann as to the universal prev- 

 alence of the cell. The long-current idea that animal 

 tissues grow only as a sort of deposit from the blood- 

 vessels was now discarded, and the fact of so-called 

 plant-like growth of animal cells, for which Schwann 

 contended, was universally accepted. Yet the full 

 measure of the affinity between the two classes of cells 

 was not for some time generally apprehended. 



Indeed, since the substance that composes the cell 

 walls of plants is manifestly very different from the 

 limiting membrane of the animal cell, it was natural, so 

 long as the wall was considered the most essential part 

 of the structurej that the divergence between the two 

 classes of cells should seem very pronounced. And for 

 a time this was the conception of the matter that was 

 uniformly accepted. But as time went on many ob- 

 servers had their attention called to the peculiar char- 

 acteristics of the contents of the cell, and were led to 

 ask themselves whether these might not be more im- 

 portant than had been supposed. In particular Dr. 

 Hugo von Mohl, professor of botany in the university of 

 Tubingen, in the course of his exhaustive studies of the 

 vegetable cell, was impressed with the peculiar and 

 characteristic appearance of the cell contents. He ob- 

 served universally within the cell " an opaque, viscid 

 fluid, having granules intermingled in it," which made 

 up the main substance of the cell, and which particular- 

 ly impressed him because under certain conditions it 

 could be seen to be actively in motion, its parts sep- 

 arated into filamentous streams. 



Yon Mohl called attention to the fact that this mo- 

 tion of the cell contents had been observed as long ago 

 as 1774 by Bonaventura Corti, and rediscovered in 1807 



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