I '.\NCKE\S 



175 



trabeculae are hollow from the first, and in Cyclost' raata the arrangement is exactly like a 

 tubular gland in respect that the branches are separate and not joined into a network. 



The tabulated arrangement of the parenchyma is not completed until after 

 birth. In the younger stages the primitive lobules are larger than those of the 

 adult, and show an irregular arrangement of the liver trabeculte. In each lobule 

 there are several anastomosing branches ol the hepatic vein. Later, owing to 

 extension of branches of the portal vein with a certain amount of connective 

 tissue into the mass, it is broken up into as many secondary lobules as there are 

 branches of the hepatic vein. These branches become the central veins of the 

 lobules ; the trabeculse assume a radial disposition and the intralobular venous 

 network takes form. 



The primary lobing of the liver is determined by the vessels in the septum transversum 

 along which the cellular strands extend. Accordingly there are four primary groups of liver- 

 tissue, two right and two left (Brachet). When the septum transversum has become fullv 

 occupied by liver-substance there is a large ventral mass and an upper lateral expansion on each 

 side. The primary lobing is largely lost in later stages owing to the formation of secondary 

 lobes and fissures. The organ begins to be asymmetrical by the eighth week, owing to the 

 presence of the stomach on the left side and to a greater expansion of liver trabeculse on the 



neural canal itolochord aorta dorsal pancreas 



liepatic duct 



ventral pancreas cystic duct gall-bladder 



FIG. 221. RECONSTRUCTION TO SHOW THE DEVELOPMENT OF THE PANCREAS OF A HUMAN 

 EMBRYO OF 6'8 MM. (After Piper.) 



right side along the vena portjp. The lobus Spigelii appears as a swelling on the inner face of 

 the right lobe, the surface which forms the outer wall of the epiploic (hepato-enteric) sac 

 (see Ccelom). It intervenes between the ventral mesentery (gastro-hepatic omentum) and the 

 vena-caval fold (see Ccelom), and projects into the epiploic sac. 



The pancreas is developed from two rudiments, a dorsal and a ventral. The 

 dorsal pancreas arises as a diverticulum from the duodenum opposite the bile-duct ; 

 the ventral pancreas springs in the form of two lateral diverticula from the base 

 of the bile-duct (fig. 221). According to some authorities, these two outgrowths 

 both give origin to pancreatic tissue, but according to others only the right persists 

 (Helly). L The dorsal diverticulum becomes the duct of Santorini, the persistent 

 ventral outgrowth the duct of Wirsung. Each diverticulum gives off lateral offshoots 

 which form the ducts and alveoli, as in other compound acinous glands (fig. 170, 

 p. 126, and fig. 222). Owing to the rotation of the duodenum, the two rudiments 

 are united with one another on the ventral aspect of the portal vein to form a 

 single body, which extends into the mesoduodenum and mesogastrium. The 



1 Helly, Arch. f. mikr. Anat. Ixvii. 1905. Other papers dealing with the mammalian pancreas which 

 have appeared since Maurer's list (Hertwig, Entwickelungslehre II. Teil. i. and ii., p. 250) are Y">lki-r, 

 Arch. f. mikr. Anat. lix. ; Low, Proc. Anat. and Anthro. Soc. University of Aberdeen, 1900-1902 ; Piper, 

 Arch. f. Anat. 1900 ; Ingalls, Arch. f. mikr. Anat. Ixx. 



