The central nervous system (brain and spinal cord) is derived from the ectoderm by the development of a groove in the long axis of the embryonic area (Figs. 13, 14, 16 and 17), and by the subsequent union in the dorsal midline of the ridges bounding the groove to form a closed tube (Fig. 18). (Medullary groove, plates and canal.) [Illustration: FIG. 13.--Transverse section of embryonic area of ovum of sheep of fourteen and a half days. (Heisler, after Bonnet.)] [Illustration: FIG. 14.--Germinal area of rabbit's ovum. (Kollmann.)] [Illustration: FIG. 15.--Surface-view of area pellucida of an eighteen-hour chick-embryo. (Balfour.)] [Illustration: FIG. 16.--Transverse section of human embryo before development of protovertebrae or chorda dorsalis. (Keibel.)] [Illustration: FIG. 17.--Transverse section of a sixteen and a half day sheep embryo. (Heisler, after Bonnet.)] [Illustration: FIG. 18.--Embryo of bird, at beginning of third day, with four blastodermic layers, resulting from the division of the mesoderm into parietal and visceral layers, separated by the coelom cavity. Transverse section. x 170. (Kollmann.)] The following changes in the ventral aspect lead to the formation of the alimentary canal and body-cavity: The developing embryo at first lies flat on the subjacent yolk-mass, and subsequently becomes gradually separated more and more from the rest of the blastoderm by grooves or furrows which develop along the sides and at the cephalic and caudal extremity of the embryo. The folds resulting from these furrows indent the yolk more and more as development proceeds and tend to approach each other at a central point, the future _umbilicus_. In the meanwhile changes in the region of the mesoderm have led to conditions which produce a differentiation of the ventral portion of the embryo into two tubes or cylinders, the _alimentary_ or _intestinal canal_ and the _general body-cavity_, the former being included within the latter. Early in the course of development a number of spaces appear in the mesoderm on each side of the axial line of the embryo. These spaces soon unite to form two large cavities, one on each side. Taken together these cavities constitute the _coelom_ or _body-cavity_, which becomes subdivided in the adult mammal into the pleural, pericardial and abdominal cavities. As these coelom cavities develop in the mesoderm the cells lining them become distinctly epithelial. This mesodermic epithelium lining the coelom is called the _mesothelium_.