INDEPENDENT CELLS, ETC. 73 



CLASS II. 



Independent Cells united 'into continuous Tissues. 



This class presents us with the greatest similarity between 

 animal and vegetable structure, and, indeed, in so high a 

 degree, that even an experienced botanist cannot distinguish 

 some of the objects which belong to it from vegetable tissue. 

 Most animal cells may be distinguished from the mature vege- 

 table cells by their greater softness and delicacy ; but those 

 characteristics are in some measure wanting in this class, and it 

 would be very difficult to distinguish microscopically between a 

 thin laver cut off from the interior of the shaft of a feather and 

 a portion of vegetable tissue. We shall, therefore, take the 

 feather as our example, and endeavour to trace these cells, 

 which correspond in so striking a manner with vegetable tissue, 

 backwards to their primitive condition, explaining this transi- 

 tion by delineations, and in this way convince ourselves that, 

 in their early stage, they also accord with the primitive cells 

 of all "other tissues. The tissues comprised under the term 

 horny belong to this class, and the crystalline lens may also 

 be included in it. The cells of these tissues generally remain 

 independent, but more or less intimate blendings of the cell-walls 

 with one another also occur in this class. Horny tissue may be 

 reduced to two unessential subdivisions, viz. — 1. Its mem- 

 branous expansions, to which belong the Epithelium, in the 

 extended sense of the term (including the Epidermis), and the 

 Pigmentum nigrum, which must be enumerated here, in con- 

 sequence of its intimate alliance with the epithelium. 2. The 

 compact horny formations, including the Nails, Claws, Hair, 

 Feathers, &c. 



1. Epithelium. — It is very difficult to determine what this 

 term ought to comprise. The cortical substance of the chorda 

 dorsalis, which is composed of flattened hexagonal cells (in the 

 larva of Rana esculcnta, for example), cannot be regarded as 

 epithelium, since it is made up of the same cells as those of the 

 interior of the chorda dorsalis : the sole dinerencc consisting in 



