82 FEATHERS. 
guish the particular wall of every cell. But when the foetus 
has lain for a time in strong spirit, the horny substance of the 
hoof may be easily separated from the foot, in consequence of 
the connexion between the cells having become looser. The 
undermost layers of cells, however, remain attached to the 
foot. The interior of the layer of horny substance so sepa- 
rated, consists of a crumbling mass, somewhat resembling a 
boiled yelk. The particles cannot, however, be separated quite 
so readily from one another as those of the yelk are. With 
the aid of the microscope, this mass is found to be composed 
of irregularly angular bodies, resembling the yelk-substance 
when boiled. These bodies are the isolated cells, whose pecu- 
har walls are distinctly perceptible, and some few of them 
have a nucleus, which lies upon the inner surface of their 
wall. <A continuous firm layer of flat epithelium-scales, the 
immediate continuation of the outer lamelle of the epidermis, 
consisting of flat cells, surrounds these polyhedral cells as an 
external covering to the entire hoof. This lamella exists in 
the foetal pig at a very early age, the layer of polyhedral 
cells being at that time very slight ; in a more advanced stage 
of development, however, the latter forms the chief mass of the 
horny substance of the hoof. In the recent condition these 
cells must also have somewhat firm contents, otherwise, with 
so delicate a cell-membrane, the substance could not be so 
firm. But its elasticity was such as to prevent my crushing 
one of the cells with the compressorium, my object being to ob- 
serve whether the cell-contents would flow out, or be torn like 
a firm substance. As the cell-contents form a large portion of 
this horny substance, whilst the nails consist for the most part 
of flat cells without any discernible contents, almost entirely 
therefore of cell-walls, a chemical distinction may be presumed 
to exist between the two structures. 
5. Feathers. The feather is composed of the quill, the 
shaft, and the vane, or beard. The elementary structure of 
these parts is, however, what most interests us at present ; and 
in order to investigate it, at least in order to become acquainted 
with the relation which the different elementary formations in 
the feather bear to cells, we must take one in which a part of 
the shaft is in progress of formation. The feathers at that 
