176 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



descending colon, returns by its iliac flexure to the median plane and forms 

 the rectum. In stages between 100 and 200 mm. (CR) the lengthening of 

 the colon causes the csecum and cephalic end of the colon to descend to- 

 ward the pelvis (Fig. 181). The ascending colon thus takes the position 

 which it occupies in the adult. 



The distal end of the caecal anlage early lags in development, and, at 

 65 mm. (CR), the vermiform process is distinct from the caecum. These 

 structures make a sharp U-shaped bend with the colon at 42 mm. (CR), 

 and this flexure gives rise to the colic valve (Toldt). 



The circular muscle layer of the large intestine appears first at 23 mm., the longitudinal 

 layer at 75 mm. (C R). In 55 mm. (C R) fetuses villi are present. 



Glandular secretions and desquamated entodermal cells, together with swallowed 

 amniotic fluid, containing lanugo hairs and vernix caseosa, collect in the fetal intestir.e. 

 This mass, yellow to brown in color, is known as meconium. At birth the intestine anr 1 its 

 contents are perfectly sterile. 



Anomalies. The intestine may show atresia. This occurs most often in the duo- 

 denum as a retention of the embryonic 

 occlusion (p. 174). 



In 2 per cent of all adults there is a 

 persistence of the proximal end of the yolk 

 stalk to form a-pouch, Meckel's diverticu- 

 lum of the ileum. This varies between 3 

 and 9, or more, cm. in length and lies 

 about 80 cm. above the colic valve. It 

 is clinically important as it may cause 

 intestinal strangulation in infants. 



Congenital umbilical hernia is due 

 either to the continuance of the normally 

 V, transitory embryonic condition or to a 

 secondary protrusion of the viscera. Other 

 hernias are explained on pp. 195 and 225. 



THE LIVER 



In embryos of 2.5 mm. the 

 liver anlage is present as a median 

 ventral outgrowth from the ento- 

 FIG. 183. Model of the liver anlage of a 4 derm of thefore-gut, just cranial to 

 mm. human embryo (Bremer). X 160. In., the yolk Stalk (Fig. 167 ). Its 

 Intestine; Pa., pancreas; V., veins in contact ,-. in -j. 1 1 



with liver trabcute. thlck walls enclose a cavity which 



is continuous with that of the gut. 



This hepatic diverticulum becomes embedded at once in a mass of 

 splanchnic mesoderm, the septum transversum. Cranially, the septum 

 will contribute later to the formation of the diaphragm ; caudally, in the 

 region of the liver anlage, it becomes the ventral mesentery (Fig. 189). 

 Thus, from the first, the liver is in close relation to the septum trans- 



