Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 125 



diverticulum is formed in the primitive digestive tube, from the 

 hinder end of which 1 two diverticula grow out backwards into the 

 thickened mesoblastic tissue. From each diverticulum numerous 

 branches are given off, and these, in turn, branch and subdivide 

 in the thickened mesoblast, which has now assumed the form of 

 a separate lobe, and in which a rich network of capillaries make 

 their appearance. Thus are formed the lungs. Finally, at the 

 extremities of the diverticula, and some of their main branches, 

 the characteristic air-sacs of the bird become differentiated. 



In the region posterior to this the crop, proventriculus, and 

 gizzard are differentiated. 



Just behind the gizzard, in the short space between it and 

 the point of union of the splanchnic layers, a pair of diverticula 

 are formed, and grow out into the thickened mesoblast. From 

 them solid cylindrical cords of hypoblast cells push their way 

 in all directions through the mesoblastic tissue, and eventually 

 fuse into a network, between the strands of which blood capil- 

 laries are formed in abundance. Thus are formed the right and 

 left lobes of the liver > median outgrowths from which, meeting 

 and fusing in the mid-line, form the wedge-like bridge be- 

 tween them. The bile canaliculi are produced either by the 

 development of a lumen in the solid cords of hypoblast, or arise 

 primitively as interspaces between the hypoblastic cells. The 

 gall-bladder is a special pouch developed from the right primary 

 diverticulum. 



Slightly posterior to the hepatic diverticula a dorsal out- 

 growth gives rise to the posterior duct of the pancreas. The 

 numerous branches formed in connection with it do not form a 

 network as in the case of the liver, but give rise to the tubules of 

 the pancreas. The other ducts of the pancreas are subsequently 

 formed in a similar manner. 



The mid-region of the mesenteron, that which at the end of 

 the third day is not yet closed as a tube and is still widely open 

 to the yolk-sac below, gives rise to the small intestine posterior 

 to the duodenum. Lying immediately beneath the notochord 



1 The ventral diverticulum is partly formed by a pinching in of the primitive 

 tube so as to cause it to be, so to speak, double-barrelled. 



