82 FEATHERS. 



guisli the particular wall of every cell. But when the foetus 

 has lain for a time in strong spirit, the horny substance of the 

 hoof mav 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- 

 liar 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 lamellse 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 



