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 cœlom cavity. Transverse section. × 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 cœlom or body-cavity, which becomes subdivided in the adult mammal into the pleural, pericardial and abdominal cavities.
As these cœlom cavities develop in the mesoderm the cells lining them become distinctly epithelial. This mesodermic epithelium lining the cœlom is called the mesothelium.
The development of the cœlom 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).
Fig. 19.—Transverse section of a seventeen and a half day sheep embryo. (Bonnet.)
