DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 317 



into smaller vessels. It is obvious that a sinusoidal circulation is purely venous or purely 

 arterial. Furthermore, development of this nature leaves comparatively little connective 

 tissue within the gland, another feature characteristic of the liver. 



All the blood carried to the liver by the omphalomesenteric veins must 

 follow the tortuous course of the sinusoids before being collected again and 

 passed on to the heart. When the umbilical veins come into connection with 

 the liver they also join in the sinusoidal circulation. Subsequently, however, a 

 more direct channel the ductus venosus is established and persists for a 



Aorta 



tnf. vena cava 

 Coelom 



Ductus 

 choledochus 



Right side 



Suprarenal glands 

 Mesonephros 



Dorsal mesogastrium 

 (greater omentum) 



Stomach 



Ventral mesogastrium 

 (lesser omentum ) 



Liver 



Left side 



FIG. 276. Tranverse section of a 14 mm. pig embryo, through the region of the stomach. 

 Photograph. The arrow points into the bursa omentalis. 



short time. This is probably due to the large volume of blood brought in by 

 the umbilical veins. Finally the ductus venosus disappears and the sinusoidal 

 circulation remains as the permanent form. (For the development of the veins 

 in the liver see p. 228.) 



The lobes of the liver develop in a general way in relation to the great 

 venous trunks which at one time or another pass into or through the gland. 

 The anlage of the organ grows into the ventral mesentery, subsequently be- 

 coming enclosed in the septum transversum. In so doing it encounters the 

 omphalomesenteric veins, and forms, in relation to the latter, two Incompletely 

 separated parts which have been called the dorso-lateral lobes. When the 

 umbilical veins enter the liver a more ventral, medial mass is formed. This 

 becomes incompletely separated into two parts which give rise to the permanent 



