190 EMBRYOLOGY 



they differentiate and grow. The results show that the liver rarely, if ever, 

 differentiates unless a heart also differentiates. The two structures are there- 

 fore closely correlated. Since a heart may develop in the absence of a liver, 

 however, it is reasonable to suppose that the heart induces the liver rather 

 than that the liver induces the heart. A certain caution must be exercised in 

 this interpretation, since it is possible that these two structures, heart and 

 liver, are so close together spatially that in the early stages we are not able 

 to separate their rudiments by dissection. 



Whatever the causal factor, the more ventral outgrowth of the liver very 

 soon forms a secondary branch, which becomes the gallbladder. This bladder 

 communicates with all parts of the liver by means of very fine tubules and 

 retains its connection with the duodenum. It acts as a storage reservoir for 

 bile and discharges this secretion through the bile duct into the duodenum. 

 During later development mesoderm cells migrate into the liver tissue and 

 form support for the lobes, which grow tremendously in the adult. 



The pancreas also has a multiple origin. Three endodermal outgrowths — 

 one dorsal and two ventral — fuse to form a single pancreas, which remains 

 connected to the duodenum by three small pancreatic ducts. The factors 

 stimulating the development of this very vital organ are completely unknown. 



The midgut, as can be seen from Figure 113, remains open to the yolk 

 during the early development of the liver and pancreas. With time, the 

 opening of the midgut into the yolk becomes constricted, and at hatching the 

 much reduced yolk sac is withdrawn into the body of the chick. 



The hindgut 



In the posterior region of the early embryo the hindgut forms as a pocket 

 of endoderm by a folding process. The posterior part of the hindgut differ- 

 entiates into the cloaca, which receives the mesonephric duct of the early 

 embryo (Fig. 114). The ureter of the older embryo is also connected with 

 the cloaca. Urea and uric acid from the mesonephros and metanephros, 

 therefore, are discharged into the cloaca. A ventral outgrowth of the endo- 

 derm forms the allantois, which provides a storage for these excretory 

 products. This very important structure has two functions. The cavity lined 

 with endoderm serves to keep the toxic waste products, urea and uric acid, 

 isolated from the embryo. The mesoderm which covers the surface of the 

 allantois carries the umbilical (allantoic) veins and arteries. These take 

 carbon dioxide from the embryo and carry oxygen back to the embryo. The 



