368 TEGUMENTARY ORGANS 



all others. An integumentary or epidermic organ forms or has formed 

 a part of the external surface, and grows endogenoitsly ; its youngest 

 portion and plane of no differentiation being directed inwards. 



If, for instance, we compare the young skin of a mammal with 

 the body of the Hydra, we shall find precisely the same planes 

 and zones. 



Fig. 303. B, represents a perpendicular section of the integuments 

 of a fcetal lamb 3^ inches long. (A) marks the position of the line of 

 no differentiation separating the epidermis from the derma ; on the^ 

 outer side of that line lie the close-set endoplasts of the deepest 

 layer {rete) of the epidermis, which are disposed somewhat per- 

 pendicularly to the surface. On the inner side are the less, 

 approximated endoplasts of the outer youngest layer of the derma, 

 more or less parallel to the surface. From a to b, lies the epidermic 

 area of metamorphosis, the indifferent tissue becoming gradually 

 converted into flattened horny cells. From a to c, on the other hand,, 

 is the dermic area of metamorphosis, the indifferent tissue gradually 

 changing into connective tissue. 



It will be observed here, that as the whole serous layer of the germ 

 corresponds in structure with the epidermis only, of the fully formed 

 animal, so the whole integument of the Hydra corresponds with what 

 is usually considered as only a portion of the integument — the epidermis 

 — of the mammal. The derma, or true skin of the latter, would not 

 come at all under our present definition of integument, since it has all 

 the morphological characters of the mucous layer of the Hydra, or of 

 the germ ; i.e. its youngest layer is external, its growth is exogenous, 

 and the metamorphosis of its tissue takes place from within outwards.. 



In fact, in all animals higher than the Hydroid Polypes (possessing 

 therefore a visceral cavity) we find a complication of structure,, 

 corresponding with that which is produced in the germ, when the 

 '^ inembrana intermedia" divides into its parietal and intestinal 

 laminae. Compared with the Hydroid Polypes, the higher forms are 

 double animals, and a section of their bodies is, morphologically 

 speaking, like a section of two Hydrje, one contained within the other. 

 Both the intestinal parietes, and those of the body, present the same 

 distinction into a central plane of no differentiation, from which growth 

 and metamorphosis proceed inward and outward on the two respective 

 surfaces, as that observed in the parietes of the Hydra. 



The formation of this so-called membrana intermedia, in fact,, 

 appears to result from a repetition of the process with gave rise to the 

 two primary layers of the germ. The previously central plane of no- 

 dflierentiation is replaced by two others, from which growth and 



