alimentary tract in vertebrates is as follows: Part of the outer layer of the germ becomes infolded as a pocket to form the lining of the archenteron or primitive gut. (See Embryology.) By the continued growth of the mesoderm and body cavity the archenteron comes to lie as a canal, closed at the anterior end and communicating posteriorly through the "neurenteric canal" with the neural tube. Later, an infolding of ectoderm occurs on the ventral surface of the embryo to form the proctodeum and anus. The neurenteric canal closes and the post-anal gut degenerates. Finally the ectoderm is inpocketed at the anterior end of the archenteron, forming the stomodeum, and the two cavities become confluent by the breaking away of the opposed walls. Thus, the completed alimentary tract is composed of an ectodermal anterior and posterior end and of an entodermal middle portion. It is enveloped by a mesodermal layer.
Phylogenetically, two quite distinct parts in the alimentary tract of vertebrates can be dis- tinguished, and these do not coincide with the embryological divisions. In Amphioxus more than the first half of the length of the alimentary tract is devoted to the purpose of respiration, since its walls are provided with gill-slits. This may be called the respiratory part of the ali- mentary tract in contradistinction to the remain- ing hinder portion — the digestive part. The first is also known as the prosenteron. The hinder part is often divided into two — its ento- dermal part (mesenteron) and its proctodeal portion (metenteron) . It will be convenient to treat of the alimentary tract under the three heads of prosenteron, mesenteron, and meten- teron. Before going on to this analytical treat- ment of the tract and its appendages, a few words may be said concerning the general his- tology of the entire tract. As already stated, two germ-layers are involved : entoderm { or ectoderm) and mesoderm, and to these must be added mesench;aue. The entoderm is always a single layer and forms the so-called mucosa : it gives rise to the digestive and glandular epi- thelium. Next outside lies the mesenchyraatous mass, with its blood-vessels and nerves — the sub- mucosn. Outside of this is the muscular layer derived from mesoderm and containing within circular muscle fibres and without longitudinal ones. Lastly, outside of all and continuous with the lining of the body cavity in all its parts, is the layer of flat epithelial cells, constituting part of the peritoneal membrane.
The Prosenteron. — This region is characterized, in the lower forms, by gills. In Balanoglossus and in Tunicata such a gill-bearing region is well developed, and in the Tunicata becomes extremely complex, in adaptation to their sessile habit, which requires large respiratory surface, since a change of water cannot be got by traveling. In Amphioxus the gill-slits are simple but very numerous — a hundred or more. The variability in number arises from the fact that the slits continue to increase in number as the animal grows older, new ones being formed at the posterior end of the series. Thus the prosenteron grows at the expense of the mesenteron. In embryologic history a single row of fourteen slits first arises as ventro-dextral organs: next, a second row of nine slits is formed at the right of the first, which, as it grows larger, pushes the first row to the left side, where it lies permanently. The cause of the development of the left series on the right side is that the large mouth at first occupies the left side, and its movement ventrally is accompanied by profound changes in the surrounding parts. It has been suggested that the mouth of the ancestors of vertebrates was placed in the mid-dorsal line in front of the notochord; but that, the support of the notochord being needed for the snout in the animal's journeys through the sand, it pushed forward and thrust the mouth to one side. The mouth is dorsal, or, better, neural, in young tunicates and in adult annelids; and the embryonic changes in the position of the mouth in Amphioxus apparently recapitulate the phylogenetic changes. The gill-slits of the young Amphioxus open from the gut cavity directly to the exterior, but later they open into a common atrium on the ventral side, which functions somewhat as an operculum. The details of the gill system of higher vertebrates will be discussed under Respiratory System. It is here merely necessary to say that the number of gill-slits becomes much reduced, usually to five or six pairs of slits.
The Mouth. — The beginning of the alimentary tract is enlarged to form an oral or buccal cavity, provided with teeth and glands. The glands are modified skin glands, as would be expected from their ectodermal origin. The glands de- velop by a depression of the epidermis, and come to lie imbedded deeply in the cutis of mesenchymatous origin. The function of the glands is to keep the mouth moist, consequently they are found only in land vertebrates. The poison glands of serpents are modified oral glands. Salivary glands find their highest de- velopment in mammals. They are probably im- mensely developed skin glands or groups of such. They secrete a thick, glairy fluid, whose chief function is to moisten the food and thus to assist in its mastication and deglutition. On this account these glands are most highly developed in the Herbivora and are absent in Cetacea. Saliva also acts upon starchy food, converting it into sugar.
The tongue is a mass of intertwined muscles, having various functions, as of tasting, grasp- ing, touching, and speaking. In fishes it is little developed, being represented by a thicken- ing of the mucosa covering the ventral part of the hyoid bone. In Amphibia and reptiles it shows a great advance in size and complexity, being capable of extrusion to a great extent (especially in lizards), both through the elon- gating action of its intrinsic muscles and the forward movement of the base of the hyoid bone.
The thymus gland arises in fishes by the budding off of epithelial masses from the anterior four or five gill pockets; it is thus of multiplex origin. Usually these independently arising masses fuse into a pair of spindle-shaped bodies, but in the Gymnophiona the components persist as distinct bodies. In the land vertebrates, with fewer gill-slits, the points of origin are reduced in number. Into the paired masses connective tissue and blood vessels grow, eventually constituting the greater part of the organs. The function of the thymus is still unknown. It attains its largest size in reptiles and birds. In man it reaches its maximum development in the second year and then gradually degenerates.
The thyroid gland arises directly from the alimentary tract. It has a double origin. First, it arises as an unpaired pocket of the ventral
