We may now briefly consider the genetic, histological and mechanical conditions which the above-outlined course of development imposes on the alimentary tract. The ectoderm forms the superficial covering of the embryo and in the dorsal axial line develops the medullary groove which subsequently becomes converted into the cerebro-spinal axis by closure of the medullary plates and inclusion of the neural tube within the surrounding mesoblast (Fig. 18). The entoderm forms the epithelial lining of the interior of the alimentary canal and its appendages and derivatives (Fig. 19). The mesoderm furnishes the skeletal, muscular and vascular systems. At first single, like the two remaining layers of the blastoderm, the mesoderm splits early on each side of the chorda dorsalis into two layers, including between them spaces which after coalescence form the _primitive pleuro-peritoneal_ or _body-cavity_ (Fig. 20). One of these mesodermal layers bounding this space becomes closely connected with the ectoderm, forming the _somatopleure_ or body wall, while the other joins the entoderm to complete the wall of the alimentary canal, forming the _splanchnopleure_. In the course of further development the edges of these two layers approach each other ventrally in the median line and finally fuse. The products of this fusion are two epithelial tubes, one included within the other, with walls reinforced by tissue derived from the two layers of the mesoderm. The internal or entodermal tube is of much smaller diameter than the outer or ectodermal tube, but much longer. The walls of the two tubes are placed in contact with each other by their mesodermal elements dorsally in the axial line, but elsewhere are separated from each other by the body-cavity (except in the region of the ventral mesogastrium). The splanchnopleure is not so wide as the somatopleure. As it closes in the ventral median line it includes the deepest or entodermal layer. It now forms a tube whose walls are composed superficially of mesoderm (splanchnopleure) while the lumen is lined by epithelium derived from the entoderm. This tube is the _primitive enteric_ or _alimentary canal_. The somatopleuric layers bounding the body cavity take a wider sweep and after they have united ventrally in the median line they embrace a much more extensive space, the _primitive body cavity_ or _coelom_. The walls of this space are largely made up of the skeletal and muscular elements developed from the mesoderm of the somatopleure, covered superficially by the common ectodermal investment of the body. It will be seen that the enteric tube thus becomes included within the wider and more capacious coelom cavity.