THE LIVER 



17s 



In. 



colon causes the cagcum and cephalic end of the colon to descend toward the pelvis 

 (Fig. 181). The ascending colon is thus formed and the vermiform appendix 

 takes the position which it occupies in the adult. 



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. The development 

 of the entire digestive tract has been described by Johnson (Amer. Jour. Anat., vols. 10, 

 1910; 14, 1913; 16, 1914). 



Glandular secretions and desquamated entodermal cells, together with swallowed 

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

 This mass, yeUow to brown in color, is known as meconium. At birth the intestine and its 

 contents are perfectly sterile. 



THE LIVER 



In embryos of 2.5 mm. the liver anlage is present as a median ventral out- 

 growth from the entoderm of the fore-gut just cranial to jthe yolk stalk (Fig. 

 167 B). Its thick 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 forma- 

 tion of the diaphragm; caudally, in 

 the region of the hver anlage, it be- 

 comes the ventral mesentery (Fig. 

 189). Thus, from the first the liver 

 is in close relation to the septum 

 transversum and later when the 

 septum becomes a part of the dia- 

 phragm the Hver remains attached 

 to it. 



In embryos 4 to S mm. long, 

 sohd cords of cells proKferate from 

 the ventral and cranial portion of 

 the hepatic diverticulum (Fig. 86). 

 These cords anastomose and form a 



crescentic mass with wings extending dorsad lateral to the gut (Fig. 177). This 

 mass, a network of solid trabeculae, is the glandular portion of the liver. The 

 primitive, hollow diverticulum later differentiates into the gall bladder and the 

 large biliary ducts. 



Pa: 



Fig. 183. — Model of the liver anlage of a 4 mm. 

 human embryo (Bremer). X 160. In., Intestine; 

 Pa., pancreas; V., veins in contact with liver tra- 

 beculae. 



