With the further development of the alimentary canal a gradual elongation of this connecting band of mesoderm and of the contained vessels is observed, the tube itself gradually receding from the vertebral axis. The early broad attachment is replaced by a narrower stalk into which the mesoderm is drawn out. With this narrowing in the transverse and elongation in the sagittal direction the connecting tissue assumes the character of a thin membrane with two free serous surfaces, including the intestinal vessels imbedded between them. Coincident with this elongation of the enteric attachment and its narrowing in the transverse direction the primitive intestine becomes more completely invested by the serous lining membrane of the coelom cavity. In this stage we can speak of the double-layered membrane attaching the tube to the dorsal body wall and carrying the intestinal blood-vessels as the primitive dorsal mesentery. The intestinal canal itself is invested by serous membrane except along a narrow strip of its dorsal border where the mesentery is attached and where the vessels reach the intestine. We can now distinguish the serous lining membrane of the abdominal cavity, derived from the mesothelium of the splanchnic and somatic mesoderm as the _peritoneum_. The membrane presents the following topographical subdivisions: 1. _Parietal Peritoneum_, lining the inner surface of the abdominal walls. 2. _Visceral Peritoneum_, investing the external surface of the intestine and its derivatives. 3. _Mesenteric Peritoneum_, connecting these two, carrying the intestinal blood vessels and lymphatics and acting as a suspensory support to the alimentary canal. The dorsal mesentery in fishes, amphibia and reptiles contains smooth muscular fibers derived from the mesoderm. These bands of smooth muscle fibers are also encountered, though less well developed, in the mesentery of birds and mammals. The so-called "suspensory muscle of the duodenum" belongs to this category. It consists of a few strands of unstriped muscular and fibrous tissue which passes from the praeaortal tissue around the origin of the superior mesenteric artery and coeliac axis to the duodeno-jejunal angle. Fasciculi from this band may penetrate into the root of the mesentery (Gegenbaur). Similar muscular fasciculi have been observed in the peritoneal folds of the ileo-caecal junction (Luschka) and in the mesorectum--forming in the latter situation the recto-coccygeal muscles of Treitz, and in the female the recto-uterine muscles.