DEVELOPMENT OF THE DIGESTIVE TUBE 627 



tery, it tends to project forward below the forming stomach and the caudal 

 limits of the heart (figs. 295A; 362H). Within the ventral mesentery, secondary 

 evaginations or epithelial cords of entodermal cells sprout from the primary 

 entodermal evagination of the entodermal lining of the gut (fig. 295A). These 

 epithelial or liver cords grow in between the paired vitelline veins, and the 

 veins become changed into a mass of capillary-like sinusoids. The liver cords 

 come to lie in the interstices between the vitelline sinusoids (fig. 295B). 



As the liver cords grow within the ventral mesentery, mesenchymal cells, 

 given off from the medial surfaces of the mesentery, come to surround the 

 liver cords and give origin to the connective-tissue substance of the liver. The 

 outer surface of the ventral mesentery retains its integrity and functions as the 

 peritoneal covering of the growing liver. 



It is apparent that the growth of the epithelial (liver) cords progresses 

 dichotomously, branching into a tree-like system of branches from the original 

 hepatic diverticulum of the gut tube, thus forming the parenchyma of the 

 liver (Bloom, '26). The proximal portion of the original hepatic diverticulum 

 forms the common bile duct, or ductus choledochus, whereas the larger 

 branches of the hepatic cords develop lumina and form the duct system. The 

 gallbladder represents an original diverticulum from the common-bile-duct 

 rudiment. The liver cords appear to be hollow from the beginning. The bile 

 capillaries thus apparently develop directly within the liver cords. The liver- 

 cord cells probably assume their typical cuboidal shape under the influence 

 of the surrounding young connective tissue and branches from the portal vein 

 (Bloom, '26). The ultimate relationship between hepatic cell cords, liver 

 sinusoids, and bile ductules is shown in figure 295C. 



In the majority of vertebrates, as the liver substance increases within the 

 ventral mesentery below the stomach area, it expands the ventral mesentery 

 enormously until the liver, with its coating of ventral mesentery, fills the 

 coelomic space below the gut tube and posterior to the heart. The developing 

 liver thus comes in contact with the ventral and lateral body walls and becomes 

 fused to these walls. The anterior face of the liver, eventually, forms a par- 

 tition across the coelomic cavity just caudal to the heart (figs. 261; 295A). 

 The anterior face of the liver substance gradually separates and forms a 

 primitive partition across the body cavity. This partition is the primary septum 

 transversum (fig. 295A). (See also Chap. 20.) 



As the liver rudiment develops in the pig embryo, the septum transversum 

 forms essentially as described above, i.e., it develops as a modification of the 

 ventral mesentery covering the anterior face of the liver. However, in the 

 human embryo, the primary septum transversum develops precociously, form- 

 ing a partition across the ventral area of the coelomic cavity between the 

 developing heart and liver (fig. 362F-H ) . When the hepatic cords in the human 

 embryo grow forward within the ventral mesentery, they secondarily become 

 related to the previously formed, primitive septum transversum along the 



