HISTOGENESIS OF THE LIVER 285 



be seen by comparing B and C of figure 11, become branches of 

 the hepatic veins instead of direct tributaries of the sinus venosus. 

 In older embryos an increase in the caliber of the hepatic veins 

 posterior to their entrance into the sinus venosus indicates the 

 position of the hepatic sinuses which become so prominent in 

 some selachians and which have been discussed in some detail 

 by Neuville ('01). 



In selachians as compared with the higher groups of vertebrates 

 the primitive liver veins are retained in the adult almost in their 

 entirety. The main vessels are established early before the 

 sinusoidal complex is well developed and these trunks remain 

 almost complete, although they may vary in size with changes in 

 the hepatic parenchyma. In figure 14, for example, the main 

 trunks of the hepatic vein can be seen in each section al hough 

 the size of these trunks varies greatly. The only primitive 

 hepatic vessel which is really completely broken up into sinusoids 

 by the hepatic cylinders is the small segment of the left omphalo- 

 mesenteric vein which passes over the pars hepatica medialis to 

 join the sinus venosus and even the anterior and posterior ends 

 of this vessel remain as branches of the left hepatic and the 

 hepatic-portal veins respectively. The growth in length of the 

 hepatic and hepatic-portal veins takes place at the ends of the 

 lateral lobes of the liver. Here there exist venous sinuses which 

 are joined by both veins and which are interrupted by only 

 occasional hepatic cylinders sheathed with endothelium. From 

 these sinuses the posterior ends of the afferent (hepatic-portal) 

 and efferent (hepatic) veins are differentiated by the growth of 

 a septum of hepatic cylinders which at first form a loose mesh- 

 work and later become a compact wall through which only 

 small capillary sinusoids pass from one vein to another. Some 

 of the venous branches of the second order represent remains of 

 original venous trunks as for example the two largest tributaries 

 of the hepatic veins from the median lobe. The others are the 

 representatives of the larger passageways which have pushed be- 

 tween the fairly constant tubule clusters which arise from the 

 hepatic pouches. Each of the larger branches of the hepatic-portal 

 vein in the median lobe of the liver accompanies for a short dis- 



