In the Hydra and Hydroid Polypes gene rally, we have the essence ofthis embryonic state as a persistent condition. If', in fact, the body or almost any organ of one of these animals be examined, it will be found (see Memoir on the Structure of the Medusm, Phil. Trans. 1849) to be composed of two distinct mem branes, an inner and an outer (fig. 303. A). The junction between the two is distinctly marked by a clear line, which would elsewhere be called a basement membrane (a). External and internal to this, there is a layer of young tissue, consisting of a homogeneous periplast with minute imbedded endoplasts (" nuclei"). As we proceed tovvards the free surface, we find that a process of vacuolation and cel lulation takes place in the periplast, until the coarsely cellular appearance with which every one is acquainted is produced.
in the Hydra, then, we have the whole thickness of the body divided into two por tions by a line, on each side of which, inwards and outwards, there is an increasing histo logical metamorphosis or differentiation. There is a median plane of no differentiation, as it might be termed, external and internal to which, is a zone of indifferent tissue, while, still more remote again, is a zone of meta nrorphosed tissue. The absolute structure of the two layers thus produced is very similar*, so much so, that, as is well known, either may perform for a time the function of the other. The distinction between the integument and the mucous nieinbrane in a morphological point of view, however, is as strongly marked as in the most complex animal. The integu ment, in fact, grows from within outwards—it is endogenous, its youngest portions being internal : the mucous membrane, on the other hand, grows from without inwards — its youngest portion is external, and it is, therefore, exogenous.
We have here, I believe, the fundamental, and the only essential distinction, between true integumentary or " epidermic" structures and all others. Art integunzentary or epidermic organ fornts or has formed a part of the external surface, and grows endogenously ; 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 sec tion of the integuments of a foetal lamb 31 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 dis posed somewhat perpendicularly to the sur face. On the inner side are the less approxi
mated endoplasts of the outer youngest layer of the derma, more or less parallel to the sur face. 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 struc ture 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 integu ment—the 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 ; e. its youngest layer is ex ternal, its growth is exogenous, and the me tamorphosis of its tissue takes place from within outwards.
In fact, in all animals higher than the Hydroid Polypes (possessing therefore a vis ceral cavity) we find a complication of struc ture, corresponding with that which is pro duced in the germ, when the "membrana in termedia" divides into its parietal and intes tinal laminae. Compared with the Hydroid Polypes, the higher forms are double animals, and a section of their bodies is, morphologi cally speaking, like a section of two Hydrm, one contained within the other. Both the intestinal parietes, and those of the body, pre sent 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 ob served 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 which gave rise to the two primary layers of the germ. The previously central plane of no diPrentiation is replaced by two others, from which growth and metamorphosis proceed in the same way. The result is, of course, the division of the germ into three layers— a central and two superficial (inner and outer) planes of meta-. rnorphosed tissue— and two planes, whence growth and metamorphosis proceed.