INDEPENDENT CELLS, ETC. 73 
CLASS IT. 
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 thei 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 layer 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.m—l. 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.—lt 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 esculenta, 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 difference consisting in 
