LIVER 



277 



plasm, and (2) a mass of anastomosing cords or trabeculae, composed of 

 deeply staining cells with round nuclei and abundant granular protoplasm. 

 These two parts are so unlike in appearance that they have been thought 

 to proceed from different germ layers, the trabeculse being described as 

 formed from mesenchyma in the septum transversum. This opinion is 

 erroneous; the entire structure is entodermal, and the trabeculae grow out 

 from the diverticulum. They encounter the vitelline veins, which ramify 

 around them, producing the lacunar vessels or sinusoids already described 

 (Fig. 160, p. 167). 



In an embryo of io-i2mm. (Fig. 271, B), the hepatic diverticulum 

 has elongated and is connected with the mass of anastomosing trabeculae 

 at several points. It shows also some detached ducts and round knob-like 



.A v.v. int. B C 



FIG. 271. DIAGRAMS OF THE DEVELOPMENT OF THE LIVER. 



A, From a 4.o-mm. human embryo. B, From a i2-mm. pig. C, The ducts in the human adult, c. d., 

 Cystic duct; c. p., peritoneal cavity; d., duodenum; d. c., ductus choledochus; dia., diaphragm; div., 

 distal end of the diverticulum; f. 1., falciform ligament; g. b., gall bladder; g. o., greater omentum; 

 h. d., hepatic duct; ht., heart; int., intestine; li., liver; 1. o., lesser omentum; m., mediastinum; oe., 

 oesophagus; p. c., pericardial cavity; p. d., pancreatic duct; ph., pharynx; p. v., portal vein; s. t., septum 

 transversum; St., stomach; tr., trabecula; v. c. i., vena cava inferior; v. v., vitelline vein; y. s., yolk-sac 



swellings. The vitelline veins have given rise to the portal vein, which 

 enters the liver from below and breaks up into sinusoids among the 

 trabeculae. These reunite, and leave the liver above as the hepatic vein, 

 which was originally a part of the vitelline veins. In the lo-mm. embryo 

 the circulation of the liver is wholly venous. The trabeculae consist of 

 cells which are doubtless very active, taking up and transforming material 

 received from the blood, but it may be questioned whether bile is secreted 

 at this stage, since no complete system of ducts has been demonstrated. 



In later stages the mass of anastomosing trabeculae is drained by a 

 system of ducts lined with clear cuboidal or columnar epithelium. These 

 all empty into a single hepatic duct, which represents one of the original 

 connections between the trabeculae and the diverticulum. (In the otter 

 there are said to be as many as seven persistent ducts.) The hepatic 

 duct (Fig. 271, C) is joined by the cystic duct which comes from the taper- 

 ing pyriform gall bladder (vesica fellea). The latter is perhaps to be re- 



