174 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



tinal loop rotates. As a result, its caudal limb lies at the left and cranial 

 to its cephalic limb (Fig. 179). 



The small intestine soon lengthens rapidly, and, at 17 mm. (Fig. 180), 

 forms loops within the umbilical cord. This constitutes a normal umbili- 

 cal hernia. Six primary loops occur and these may be recognized in the 

 arrangement of the adult intestine. In embryos of 42 mm. the intestine 

 has returned from the umbilical cord into the abdominal cavity through a 

 rather small aperture; the ccelom of the cord is soon after obliterated . 



Brain 



Hypophysis 



Foramen cacum 

 -Root of tongue 



Esophagus 



Trachea 

 -Notochord 

 Spinal cord 



Urogenital sinus 

 Anal membrane 



FIG." 1 80. Diagrammatic median sagittal section of a 17 mm. human embryo, showing the 

 digestive canal (modified after Mall). X 5- 



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

 epithelial 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 23 mm. (Johnson). They begin to form at the cephalic end of the jejunum, 

 and, at 130 mm. (CR), 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. (CR). The duodenum glands (of Brunner) are said to 

 appear during the fourth month (Brand). In embryos of 10 to 12.5 mm. the circular 

 muscle layer of the intestine first forms. The longitudinal muscle layer is not distinct 

 until 75 mm. (CR). 



