6-11 TEGUMENTARY ORGANS, 



unite to thicken the parietes of the general cell, two or more 

 nuclei are often found within the same cell, and the nuclei 

 generally retrograde in their development, or entirely dis- 

 appear, when the cells they produce have arrived at their 

 maturity. 



The successive strata of epidermic cytoblasts are most 

 accumulated, and retained, in a condensed form, on parts of 

 the skin most exposed to pressure and friction, as on the 

 palmar and plantar surfaces of the extremities, and on the 

 whole surface of thick-skinned naked animals, as rhinoce- 

 roces, hippopotami, manati, and other pachy derma and ceta- 

 cea. The difference of colours in the contained parasitic 

 pigment- cells of the epidermic cytoblasts, which are most 

 vivid and most lively in the soft, loose cytoblasts of the rete 

 mucosum, gives rise to the varied hues of all the tegumen- 

 tary parts of animals. In the interior even of these parasitic 

 pigment- cells, are sometimes seen numerous other minute 

 cells in active movement. The colour of the pigment-cells 

 often varies in different parts of the skin, giving rise to cor- 

 responding differences in the colour of the hairs, spines, and 

 other epidermic developments ; their excess produces the 

 intense colour of the rete mucosum of the negro, and other 

 deeply-coloured animals; their deficiency produces the various 

 tegumentary peculiarities of albinos ; and the ephemeral 

 existence of these coloured parasites, causes the outer strata 

 of epidermis to be shed colourless, from the most deeply 

 coloured skins of animals, as salamanders, serpents, and 

 negros. The epidermis is already a thick layer on the pal- 

 mar and plantar surfaces of the extremities in the early con- 

 dition of the embryo, and the coloured parts of the integu- 

 ments of quadrupeds are distinctly marked at an early period 

 of the foetus in utero. 



The cytoblasts of the epithelium, at the exterior open- 

 ings of mucous cavities, have mostly the same flattened form 

 and stratified arrangement as in epidermis, as seen on the 

 interior of the nostrils, the lips, the mouth, the tympanic 

 cavity and the mastoid cells, and on the surface of the con- 

 junctiva and cornea, where they were observed and described 

 by Leuwenhoek, as forming a hundred strata of superim- 

 posed scales ; but in most other parts of the mucous surfaces 

 they present a conical or cylindrical form, are compactly 



