624 THE DIGESTIVE SYSTEM 



to the stomach. It originates thus between the foregut and midgut areas of 

 the developing digestive tube. 



a) Shark Embryo. In the 10- to 12-mm. shark embryo, Squalus acanthias, 

 the liver rudiment arises as a midventral evagination of the gut which pushes 

 downward and forward between the two parts of the ventral mesentery. It 

 soon becomes divisible into three chambers, viz., a midventral chamber, the 

 rudiment of the gallbladder, and two lateral chambers, the fundaments of the 

 right and left lobes of the liver (figs. 279B; 294A). 



b) Frog Embryo. In the frog, the liver rudiment appears as a ventro- 

 caudal prolongation of the foregut area at the early, neural fold stage (figs. 

 220B; 223B). Later, the anterior end of the hepatic rudiment diff"erentiates 

 into the liver substance in close relation to the vitelline veins as the latter enter 

 the heart, while the posterior extremity of the original hepatic rudiment dif- 

 ferentiates into the gallbladder (figs. 280; 294B, G2, G3). 



c) Chick Embryo. In the chick, two evaginations, one anterior and the 

 other posterior, arise from the anterior wall of the anterior intestinal portal, 

 beginning at about 50 to 55 hours of incubation (fig. 294C). These evagina- 

 tions project anteriorly toward the sinus venosus of the heart, where they 

 eventually come to surround the ductus venosus as it enters the sinus. (See 

 Chap. 17.) At the end of the fourth day of incubation, secondary evaginations 

 from the two primary outgrowths begin to produce a basket-like mass of 

 tubules which surround the ductus venosus (fig. 294E). The gallbladder 

 arises from the posterior hepatic outpushing toward the end of the third day 

 of incubation (fig. 294D). 



d) Pig Embryo. The liver diverticulum in the 4- to 5-mm. embryo of the 

 pig begins as a bulbous outpushing of the foregut area, immediately caudal 

 to the forming stomach (fig. 295E). This outpushing grows rapidly and sends 

 out secondary evaginations, including the vesicular gallbladder. The latter is 

 already a prominent structure in the 5. 5-mm. embryo (fig. 295A). 



Fig. 295. Development of liver and pancreatic rudiments {Continued). (A) Diagram 

 of early hepatic diverticulum in pig embryo of about 5.5 mm. (Redrawn and modified 

 greatly from Thyng, 1908, Am. J. Anat.) For early growth of liver in pig, see Figs. 261 A 

 and 262. (B) Hepatic ducts, hepatic tubules, and hepatic canaliculi in relation to blood 

 sinusoids. It is to be observed that the common bile duct ( I ) gives off branches, the hepatic 

 ducts (2), from which arise the branches of the hepatic duct (3) which are continuous 

 with the hepatic tubules or hepatic cord cells (4). Compare with Fig. 295C. (C) A 

 portion of liver lobule of human. (Redrawn and modified from Maximow and Bloom, 

 A Text-book of Histology, Saunders. Phiia.) Blood sinusoids are shown in black; liver 

 cells in stippled white; bile canaliculi shown in either white or black. (D) Section 

 showing three pancreatic diverticula in 5-day chick embryo. (Redrawn from Lillie, 1930, 

 The development of the chick. Holt, N. Y. After Choronschitsky.) (E) Pancreatic di- 

 verticula in 5.5 mm. pig embryo. (Redrawn from Thyng, 1908, Am. J. Anat. 7.) (F) 

 Pancreatic diverticula in 20 mm. pig embryo. (Redrawn from Thyng, 1908, Am. J. Anat. 

 7.) (G) Pancreatic acini and islet of Langerhans. 



