The development of the coelom space divides the mesoderm on each side into an outer leaf, the _somatic_ or _parietal mesoderm_, and an inner leaf, the _splanchnic_ or _visceral mesoderm_ (Figs. 18 and 19). The former is closely applied to the ectoderm, forming with it the _somatopleure_ or _body-wall_. The latter, in close contact with the entoderm, forms with it the _splanchnopleure_ or wall of the alimentary canal. In the dorsal median line both somatic and splanchnic mesoderm become continuous with each other and with the axial mesoderm (Fig. 20). [Illustration: FIG. 19.--Transverse section of a seventeen and a half day sheep embryo. (Bonnet.)] [Illustration: FIG. 20.--Curves of blastodermic layers and division of mesoderm in amniote embryo. (Kollmann.)] The folds of the splanchnopleure, indenting the yolk-sac, form a gutter directly connected with the yolk, the _primitive intestinal groove_ or _furrow_, whose margins gradually approach each other (Fig. 20). In this way the primitive alimentary canal becomes separated from the yolk. At first this separation is ill-defined, and the channel of communication between the primitive intestine and the yolk is wide (Figs. 13, 16, 17 and 19). The folding of the splanchnopleure completes, at an early period, the dorsal and lateral walls of the embryonic gut, but ventrally, toward the yolk, the tube is incomplete and widely open. By union and coalescence of the splanchnopleural folds, proceeding from the caudal and cephalic ends towards the center, this primitive wide channel gradually becomes narrowed down, until the communication between the yolk-sac and the intestine is reduced to a canal, the _vitello-intestinal_ or _omphalo-mesenteric duct_. The intestinal gutter is thus converted into a closed tube except at the point of implantation of the vitelline duct during the persistence of this structure. In the meanwhile the somatopleural folds forming the body-walls grow more and more together from the sides, approaching the vitello-intestinal duct. Finally touching each other they coalesce to form the ventral body wall, in the same manner as the splanch[n]opleural folds met and united to form the alimentary tube. At the same time the vitello-intestinal duct and the remnant of the yolk-sac, to which it was attached ("umbilical vesicle"), normally become obliterated and disappear.