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The first structure of this large and elaborate gland is a very simple cone in the epidermis, which penetrates into the corium and ramifies. These gradually ramify, their ducts become hollow and larger, and rich masses of fat accumulate between the lobes.
In the harlequin fetus the whole surface of the body is thickly covered with fatty epidermic plates, about 1/16 inch in thickness, which are broken up by horizontal and vertical fissures, and arranged transversely to the surface of the body like a loosely-built stone wall. After birth these fissures may extend down into the corium, and on movement produce much pain.
From the same layer we also get the whole of the mesentery, with all the organs embedded in it the heart, the large blood-vessels of the body, etc. A three-pointed tooth rises obliquely on each of the quadrangular bony plates that lie in the corium.
While the lenticular sac is being detached and is causing the invagination of the primary optic vesicle, another invagination is taking place from below; this proceeds from the superficial part of the skin-fibre layer the corium of the head. In this way the optic vesicle acquires the form of a hood. It originates from the part of the head-plates which immediately encloses the eye.
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.
The skin is remarkably complex in its structure, and is divided into two distinct layers, which may be readily separated: the deeper layer, the true skin, dermis, or corium; and the superficial layer, or outer skin, the epidermis, cuticle, or scarf skin. The true skin consists of elastic and white fibrous tissue, the bundles of which interlace in every direction.
I need only add that in the corium of man and all the higher Vertebrates countless microscopic sense-organs develop, but the precise relation of these to the sensations of pressure or resistance, of warmth and cold, has not yet been explained.
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