The outer layer of the optic cup undergoes none of the com plicated histogenesis seen in the retina. It persists as a single layer of polygonal cells which show their sensitiveness to light by the deposition of granules of dark melanin pigment in their cytoplasm, and in the fully developed condition by characteristic reactions to light stimulus. They constitute what is termed the pigment-layer of the retina.
The ectodermal lens and retina, which constitute the essential optical part of the eye, become ensheathed during development by a thick coating of connective tissue or mesenchyme. This, in its outer layers, becomes condensed into the tough protective wall of the eyeball, the part between the lens and the surface of the head becoming clear and transparent (cornea) while the remainder becomes white and opaque (sclerotic). Between the sclerotic and the pigment layer of the retina there is a rich development of blood-spaces and of dark melanin, which gives its characteristic ap pearance to the choroid.
The peripheral nerves, i.e., the bundles of nerve fibres which serve as pathways for the nerve impulses, have provided one of the most contentious chapters in vertebrate embryology. A com mon observation in studying sections of early embryos is that of nerve-trunks springing from the central nervous system and apparently terminating in a free end. Such appearances naturally suggest the view—associated especially with the name of the German anatomist His—that the nerve-fibre actually develops in the embryo as an outgrowth from a nerve-cell, at first ending freely and growing gradually through the intervening connective tissue till it establishes secondarily continuity with its end-organ. This outgrowth view of the mode of development of nerve fibres has in its favour by far the greater volume of observations, in cluding, in recent years, very interesting observations by Har rison and others of an experimental kind. (See NERVOUS SYSTEM.) Alimentary Canal.—The alimentary canal of the vertebrate is seen in its simplest form in the archenteron of the Amphioxus embryo. Its wall consists of endoderm, and it possesses a single opening to the exterior—the blastopore. With further develop ment there comes to exist the tubular alimentary canal, in which the layer of endoderm cells forms merely a lining ensheathed by a much thicker mesoderm coat. The wall of the alimentary canal
in this more complex definitive state is spoken of as the splanch nopleure or gut-wall, in contradistinction to the somatopleure or external wall of the body. The common error should be avoided of restricting these terms to the mesodermal constituent of the gut and body-wall. The alimentary canal of the vertebrate is in its early stages closed anteriorly, the mouth being a secondary perforation. As in many other animals, the portion of external surface in the region of the mouth becomes involuted to form the lining of the buccal cavity, so that the protective and glandular functions of the skin extend inwards into the first portion of the alimentary canal. The buccal cavity of the vertebrate is therefore morphologically a stomodaeum. For a time, in many vertebrate embryos, the stomodaeal involution remains isolated from the rest of the alimentary canal by a thin partition, covered on its buccal face by ectoderm, on its enteric face by endoderm. This partition eventually ruptures and disappears, although in Amphioxus it remains distinctly visible as the velum, perforated in its centre by a circular opening.
The most important features to note in the buccal cavity of the vertebrate are the organs in its lining which have been brought in from their original position on the outer surface of the head. Amongst these are glands which become of special importance in terrestrial vertebrates, where their watery secretion serves to keep moist the buccal lining and in certain cases play a preliminary part in the digestive process, as the salivary glands. Still more conspicuous however are the placoid scales, which, in the primi tive elasmobranch scattered all over the outer surface of the body, are also recognizable in the buccal lining. Around the mar gin of the jaw a series of these placoid scales become specially enlarged, forming the teeth of the adult. The embryology of the elasmobranch then demonstrates that the teeth of vertebrates are vestiges of the placoid scales on the surface of the body.