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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.

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.

The original longitudinal axis becomes oblique, sinking below to the left and rising to the right, and approaches nearer and nearer to a transverse position. In the outer layer of the stomach-wall the powerful muscles that accomplish the digestive movements develop from the gut-fibre layer.

In general, it is certain that the greater part of the cells that compose the vessels and their contents come from the mesoderm in fact, from the gut-fibre layer; it was on this account that Baer gave the name of "vascular layer" to this visceral layer of the coeloma. But other important observers say that a part of these cells come from other germinal layers, especially from the gut-gland layer.

We saw that the palingenetic form of the heart is a spindle-shaped thickening of the gut-fibre layer in the ventral wall of the head-gut. The structure is then hollowed out, forms a simple tube, detaches from its place of origin, and henceforth lies freely in the cardiac cavity.

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.

The epithelium of the gut that is to say, the internal soft stratum of cells that lines the cavity of the alimentary canal and all its appendages, and is immediately occupied with the processes of nutrition is formed solely from the gut-gland layer; all other tissues and organs that belong to the alimentary canal and its appendages originate from the gut-fibre layer.

The thin wall of the allantois consists of the same two layers or membranes as the wall of the gut the gut-gland layer within and the gut-fibre layer without. This is the case also with the lowest mammals, the oviparous Monotremes and most of the Marsupials. The latter also is richly permeated with blood-vessels which bring the mother's blood to the embryo.

The inner stratum, lining the entire cavity, consists of larger and darker cells, and is the gut-gland layer. The outer stratum consists of smaller and lighter cells, and is the gut-fibre layer. The only exception is in the cavities of the mouth and anus, because these originate from the skin.