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.
The skin-fibre layer forms the corium and the motor organs the skeleton and the muscular system. From the gut-fibre layer are developed the vascular system, the muscular wall of the gut, and the sexual glands. The manner in which these different systems of organs arise from the secondary germinal layers is essentially the same from the start in man as in all the other Vertebrates.
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 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.