Structure and Growth in Animals and Plants, 25 



remarked, and the contents of which form the red colouring matter. The 

 lymphatic corpuscles, of a mucous and purulent nature, likewise belong 

 to this class ; all of them are cells with a nucleus. 



The SECOND CLASS comprehends the corneous tissue, the pigment, and 

 the crystalline tissue. The cells are independent, although their walls 

 sometimes disappear. 



1. Epithelium. — This is most frequently composed of rounded cells 

 with a nucleus situated on their inner surface, and with one or two 

 nucleolules. By their union they assume the polyhedral form. In the 

 outer skin of the tadpole of the frog, Schwann has seen two nuclei in a 

 cell, and one cell of the epithelium, with a nucleus in a large cell, which, 

 according to Hcnle, does not take place among the mamraifera. The 

 cells of the epithelium may assume two other forms derived from the pri- 

 mitive globular form ; they are either flattened, with the nucleus remain- 

 ing in the centre of one of the surfaces, or else these flattened cells be- 

 come elongated, as Hcnle has observed in regard to the epithelium of the 

 vessels ; young cells arise below the old, and diminish in height in propor- 

 tion as they approach the surface (Henle), where the cells become elon- 

 gated in a cylindrical form, as has been noticed in the intestinal mucus. 



2. Cells of the pigment. — These have on their wall a cellular nucleus 

 which determines the white spot seen in their centre. The nucleus is 

 usually provided with one or two smaller nuclei (nucleolules). Some 

 pigment cells are lengthened in different directions, in the form of hollow 

 fibres, and compose stelliform cells. 



3. Nails. — The nail of a male foetus when near its birth, is composed of 

 superimposed layers, which are less obvious on the lower side of the nail 

 in proportion as we go nearer the root ; the hinder half of this portion 

 exhibits no layer, but consists of polyhedral cells, having distinct nuclei. 

 The lamella) of the nail, treated with acetic acid, divide into plates, in 

 which we rarely distinguish a nucleus. The polyhedral cells of the root 

 change by flattening into small plates. This flattening should render the 

 nail thinner in front ; but it is probable that a layer of epithelium is formed 

 beneath which equalizes the thickness. The corneous tissue of claws is 

 likewise composed, in the foetus, of cells analogous to those of plants. 



4. Feathers. — The medullary substance of feathers is composed of 

 polyhedral cells, strengthened by a nucleus in the young feather. "VVe 

 first see a finely granular mass, in which are numerous small nuclei, some 

 of them with a nucleolule : it is around these nuclei that the cells are 

 formed. The latter are not developed in the mother cells, but in the 

 neighbourhood of the organized matter of the feather which furnishes the 

 cytoblastema. The fibres of the epidermis of the shaft originate in the 

 cells of the epithelium, which aro large and flat, and have a nucleus as 

 well as nucleolules. These are long and flattened strise; from each 

 cell spring numerous fibres, then all trace of a cell disappears. The 

 barbs are feathers in miniature, the shaft secondary to the structure of 

 the principal shaft, the barbules likewise composed of epithelium cells in 

 juxta-position, and possessing a nucleus. 



