182 



THE ENTODERMAL CANAL AND ITS DERIVATIVES 



cord into the abdominal cavity through a rather small aperture and the coelom 

 of the cord is soon after obliterated. 



In embryos between 10 and 30 mm. vacuoles appear in the wall of the duodenum and epi- 

 thelial septa completely block the lumen. The remainder of the small intestine remains open, 

 although vacuoles form in its epithelium. Villi appear as rounded elevations of the epithelium 

 at 22.8 mm. (Johnson). They begin to form at the cephalic end of the jejunum, and at 130 

 mm. they are found throughout the small intestine (Berry). Intestinal glands appear as 

 ingrowths of the epithelium about the bases of the villi. They develop first in the duodenum 

 at 91 mm. and in the jejunum at 130 mm. The duodenal glands (of Brunner) are said to appear 

 during the fourth month (Brand). From 10 to 12.5 mm. the circular muscle layer is formed. 

 The longitudinal muscle layer is not distinct until 75 mm. 



The impervious duodenum of the embryo may persist as a congenital anomaly, and we 

 have already alluded to the persistence of the yolk-stalk as Meckel's diverticulum. 



FIG. 174. Three successive stages showing the development of the digestive tube and the mesen- 

 teries in the human fetus (modified from Tourneux) : i, stomach; 2, duodenum; 3, small intestine; 4, 

 colon; 5, vitelline duct; 6, caecum; 7, great omentum; 8, mesoduodenum; 9, mesentery; 10, mesocolon. 

 The arrow points to the orifice of the omental bursa. The ventral mesentery is not shown (Heisler). 



The large intestine, as we have seen in 9 mm. embryos, forms a tube ex- 

 tending from the ccecum to the cloaca. It does not lengthen so rapidly as the 

 small intestine and, when the intestine is withdrawn from the umbilical cord (at 

 42 mm.), its cranial or caecal end lies on the right side and dorsal to the small 

 intestine -(Fig. 174). It extends transversely to the left side as the transverse 

 colon, then bending abruptly caudad as the descending colon, returns by its iliac 

 flexure to the median plane and forms the rectum. 



The caecum (Fig. 175) is differentiated from the vermiform process at 65 

 mm. (Tarenetzky). The caecum and vermiform process make a U-shaped bend 

 with the colon at 42 mm., and this flexure gives rise to the ileo-cacal valve (Toldt). 

 In stages between 100 and 220 mm. the lengthening of the colon causes the caecum 



