DEVELOPMENT OF INTESTINAL CANAL AND PERITONEUM. 1055 
notochord, and bounded towards one end by the head fold, at the other by the tail fold, 
and at the sides by the lateral folds. The anterior portion of this cavity, situated within 
the head fold, is known as the foregut, and from it are developed the back part of the 
mouth, the tongue, pharynx, cesophagus, stomach, and the greater portion of the duodenum, 
together with the organs formed as outgrowths from these. Similarly, the posterior fabulen 
portion, contained within the tail fold, constitutes the hindgut, and from it are formed 
the rectum, except its anal end, and a portion of the colon. The middle division, known 
as the midgut, gives rise to the rest of the digestive tube, and is at first widely open 
below, where it is continuous with the cavity of the yolk sac. Soon, however, it is 
eradually closed in by the approximation of the four folds which meet around the m: reins 
of the umbilical orifice, and the communication between the alimentary canal and the yolk 
sac is thus reduced to a narrow passage—the vitelline or vitello- intestinal duct. This 
duct joins that portion of the primitive tube which subsequently forms the lower part of 
the ileum, and a remnant of it is sometimes found in the adult, when it is known as 
Meckel’s diverticulum (see p. 1022). 
At this stage the primitive canal has the form of a nearly straight tube, blind at both 
extremities, and communicating only with the cavity of the yolk sac. It is lined through- 
out by the entoderm, the cells of which form the epithelial lining of the adult canal; 
outside this it is Invested by the splanchnopleure, or splanchnic lay er of the mesoderm, 
and is separated from the somatopleure or body wall by the ceelum or body cavity, w hich 
later on gives rise to the pleural, pericardial, and peritoneal cavities of theebody. At the 
back, the splanchnic mesoderm which surrounds the canal passes dorsalwards, to become 
continuous with the general mesoderm beneath the notochord, thereby forming a fold 
which connects the primitive alimentary tube to the dorsal wall of the embryo, and 
constitutes the primitive mesentery. This is at first of considerable thickness. At a 
later period it becomes reduced to a thin sheet of mesodermic tissue, covered on each side 
by a layer of flattened endothelial cells, which suspends the primitive alimentary canal 
within the body cavity of the embryo. 
At each end of the embr yo, an ingrowth of the ectoderm takes place, which meets 
and finally becomes continuous with the corresponding extremity of the primitive alimentary 
tube, giving rise to the mouth and anus respectively. The former of these ingrowths is 
known as the stomatodeum, and from it is formed the epithelial lining of the vestibule of 
the mouth, of the salivary glands, the gums, and the greater part of the nasal cavity, as 
well as the anterior lobe of the pituitary body. From the other i ingrowth—the proctodeum 
—are formed the anal aperture and probably the anal canal, below the level of the anal 
valves. An account of the formation of the mouth and the anus will be found on pages 
32 and 43 respectively. 
At an early date special outgrowths of the lining entoderm take place from the 
portion of the foregut corresponding to the future duodenum. From these diverticula 
the liver and pancreas are formed: the former grows forwards from the gut, whilst the 
latter grows backwards into the dorsal mesentery (see pages 1072 and 1078 respectively). 
The primitive alimentary canal is now a tubular cavity, suspended from the dorsal 
wall of the embryo, within the ceelom, by a simple mesentery (Fig. 711). In front, it 
communicates with the exterior through the stomatodseum or primitive mouth, and in 
the region of the future duodenum the liver appears as a bifid outgrowth, followed soon 
by outgrowths for the formation of the pancreas. Finally, at a date which has not 
been precisely determined, the anal membrane disappears, and its posterior end com- 
municates with the exterior through the proctodzum. 
Stomach.—As early as the fourth week the foregut exhibits a fusiform enlargement 
in the region of the developing heart, which is the first evidence of the differentiation of 
the stomach: this enlargement is at first symmetrical, and mesially placed. Soon, how- 
ever, as the diaphragm is being formed, the stomach descends into the abdomen, and its 
dorsal wall—the future great curvature—begins to grow more rapidly than the ventral 
wall. As a result the whole organ becomes somewhat curved, and its lower end is 
carried forwards from the posterior abdominal wall, giving rise to the curvature of the 
duodenum. The excessive growth of its posterior wall causes the stomach to turn over 
on to its right side, which now becomes posterior or dorsal. In this rotation its upper or 
cardiac portion moves to the left of the middle line, and the whole organ assumes an 
oblique direction across the abdomen. Already, at the fifth or sixth week, the adult 
form of the stomach is pretty clearly indicated. 
This rotation of the stomach around its long axis, which is accompanied by a rotation 
of the lower end of the cesophagus, explains the unsymmetrical position of the two 
pneumogastric nerves In the adult the left nerve is found on the front of the stomach, 
a 
