THE CELL DOCTRINE. 43 



" A mode of formation of new cells, different from 

 the above described, is exhibited in the multiplica- 

 tion of cells by division of the existing ones ; in this 

 case, partition vs-alls grow across the old cell, if, as 

 Sehleiden supposes, this be not an illusion, inasmuch 

 as the young cells might escape observation in con- 

 sequence of their transparency, and at a later stage, 

 their line of contact would be regarded as the parti- 

 tion wall of the parent cell." 



Schwann believed that the cell-wall was the most 

 active constituent of the cell, that it possessed the 

 power not only of producing physical and chemical 

 changes in its own substance and the cell contents, 

 but of secreting materials from the surrounding sub- 

 stance, and depositing them in its interior, explain- 

 ing in this manner the secretions of glands, the for- 

 mation of fat in some cells, pigment in others, etc. 



It would be easy to point out other defects in the 

 theory of Sehleiden and Schwann, when it is tested 

 by comparison with the more accurate observation 

 of the last twenty-five years, none of which should 

 be permitted to detract from the credit which at- 

 taches to the originators of this conception. It must 

 not be forgotten, that it is no less true of science than 

 of art, that great and important truths in their en- 

 tirety are gradually developed, and that no single 

 mind is capable of elaborating them from their in- 

 cipiency to their complete expression. And as many 

 clever people had daily noticed the rising of steam 

 from the boiling kettle without thinking of utilizing 

 its principles of expansion, so also, many careful ob- 

 servers had time and again witnessed the cellular or 



