CELL THEORIES. 



The history of conceptions regarding cells is 

 in its general outlines exceedingly instructive. 

 The notion of a cell was first derived from vege- 

 table tissues with their easily exhibited cellulose 

 cell-walls. The vesicular form thus caught the eye 

 from the first. Then, in many instances in animal 

 tissues also a real or apparent vesicular structure 

 was easily observed ; as in adipose, epithelial, and 

 nerve-corpuscles. The contents in vesicular struc- 

 tures were seen to be various, but the frequent 

 existence of a nucleus of firmer consistence im- 

 bedded in them could be demonstrated, and within 

 this were often seen one or more nucleoli of some 

 sort or other. Thus it naturally happened that 

 the cell-wall was considered a characteristic struc- 

 ture, and was supposed to be functionally import- 

 ant ; and next to it, the nucleus was regarded 

 as the seat of vital properties, because it was 

 seen to divide preparatory to the multiplication 

 of cells, and to be distinct in young cells, however 

 it might dwindle out of sight in the old. The 

 circumstance, manifest from the outset, that cell 

 contents were various, taken in conjunction with 

 their being the part which least caught the eye, 

 led to their vital importance being long overlooked. 

 But as improved microscopic methods came into 

 use, including reagents, such as carmine, which 



