TEGUMENTARY ORGANS. 643 



ture, and are contained in a soft gelatinous connecting sub- 

 stance, a cytoblastema, which enables them to grow, and to de- 

 tach concentric layers or enveloping cells from their surface. 

 The exterior cells grow more rapidly than the contained nuclei 

 which first developed them, and there are commonly minuter 

 pigment-cells, like internal parasites, free in the contained fluid 

 of these cytoblasts. The soft, round, loosely aggregated, newly 

 produced, growing cytoblasts, forming the lower strata of epi- 

 dermic cells, compose the rete mucosum of Malpighi, where the 

 various hues of the contained pigment-cells, in all deeply- co- 

 loured animals, are most fresh and intense, and where the cu- 

 ticular cells are still most agglutinated to the surface of the 

 cutis. As the epidermic cells, by their own independent vitality, 

 enlarge, and thicken in their parietes, the connecting gelatinous 

 matter, or cytoblastema, disappears, they become contiguous, 

 compressed, and polyhedral, and the nuclei are still perceptible 

 towards the centre of the thus flattened cells, attached to their 

 interior surface. In the outer strata of the epidermis, the cells 

 are thin, empty, flattened disks, bleached, deprived of their 

 colouring matter, compressed into a continuous layer, and 

 they at length fall from the surface as dried, isolated scales, 

 with their opposite parietes coherent, and with single persist- 

 ent nuclei. The black pigment-cells of the cuticle of the 

 tadpole, undergo remarkable changes of form, like a poly- 

 gastric proteus, and they contain numerous, minute, parasitic, 

 spontaneously moving cells, in their interior. 



The epithelial cytoblasts of internal parts present similar 

 phenomena of growth, development, and metamorphosis, to 

 those of the exterior epidermis ; they are seen on the lining 

 membrane of the heart, in veins, on the chorion, the amnion, 

 and on all mucous and serous surfaces ; their form is some- 

 times lamellar, sometimes conical or cylindrical, and they 

 often exhibit distinct vibratile cilia at their free extremity on 

 mucous membranes. Cytoblasts abound in all secretions, 

 they constitute the first rudiment of the ovum, and the 

 globules of blood, milk, and other animal fluids ; they give 

 origin to capillary vessels, to cartilage, to the fibres of the lens, 

 of the teeth, of cellular tissue, of nerves, muscles, and most 

 other tissues of animal bodies. The primitive germinative 

 nuclei often develope two or more concentric enveloping cells 

 around each, these concentric spheres often coalesce and 



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