The inner coat of the mouth-cavity is not provided by the gut-gland layer, but by the skin-sense layer; and its muscular substratum is provided, not by the gut-fibre, but the skin-fibre, layer. It is the same with the wall of the small anus-cavity.
The chief part of all the sense-organs originates from the skin-sense layer, partly directly from the horny plate, partly from the brain, the foremost part, of the medullary tube, after it has separated from the horny plate.
I will only point out for the present that the mucous coat of the tongue and palate, in which the gustatory nerve ends, originates from a part of the outer skin. Its mucous lining is therefore formed, not from the visceral, but from the cutaneous layer, and the taste-cells at the surface of the tongue and palate are not products of the gut-fibre layer, but of the skin-sense layer.
We have, therefore, to examine the features of the psychic organ in man. The reader already knows the incontestable answer to this question. Man's psychic organ is, in structure and origin, just the same organ as in all the other Vertebrates. It originates in the shape of a simple medullary tube from the outer membrane of the embryo the skin-sense layer.
As the human embryo passes through the gastrula-form like that of all the other Metazoa, we can trace its phylogenetic origin to the Gastraea. These have just the same composition and genetic significance in man as in all the other Vertebrates. From the skin-sense layer are developed the epidermis, the central nervous system, and the chief part of the sense-organs.
It develops from the outer germinal layer, or skin-sense layer. It comes from the outermost parietal stratum of the middle germinal layer, or the skin-fibre layer. The corium is much thicker than the epidermis. These tactile or sensory particles contain the finest sensory organs of the skin, the touch corpuscles. At an early stage the simple cellular layer of this horny plate divides into two.