EMBRYOLOGY 



227 



Liver. — The liver appears at an carlj^ stage as an evagination from 

 the lower side of the intestine just behind the stomach. In the frog, 

 the liver is present shortly after the fusion of the neural folds (see Fig. 181, 

 li). An early indication of the liver is also shown in Fig. 183, li. This 

 pouch grows in extent and soon becomes branched, as in Fig. 184, li. 

 One branch at the posterior side of the liver forms the gall bladder 

 (gb). The rest are bound together by mesodermal tissue which collects 

 about them, forming part of the body of the liver. The undivided basal 

 portion of the original pouch remains as the bile duct (bd) , through which 

 the secretions of the liver are conveyed into the intestine. During all 

 this development, of course, the liver has been covered by the layer of 

 peritoneum (mesodermal) which invests the entire digestive tract. The 



Fig. 182. — Diagrams showing the early development of some of the organs of verte- 

 brate animals, as seen in section from above. The stages here shown are not contempora- 

 neous in all cases. A, B, C, successive stages; au, auditory vesicle; br, brain; cr, crystalline 

 lens; ec, ectoderm; en, endoderm; eu, Eustachian tube; gb, gill bar; gp, gill pouch; gs, gill 

 slit; me, endodermal portion of mouth; olf, olfactory pit; opn, optic nerve; ops, optic stalk; 

 ph, pharynx; i-et, retina; sp, spinal cord; ty, tympanum or niiddle ear. Arrows in C denote 

 current of water through mouth, pharynx, and gill slits. 



adult liver is thus covered by peritoneum, and suspended b}' mesenteries 

 formed from the same layers of mesoderm. 



Pancreas. — The pancreas originates from two pouches evaginated 

 from the intestine, as in Fig. 184. One arises from the dorsal side of 

 the intestine nearly opposite the liver (dp) ; the other springs from the 

 angle between the Uver and the intestine (vp). The latter pouch turns 

 dorsal ward, to one side of the intestine, where it unites with the dorsal 

 rudiment. Their further development which in many respects is similar 

 to that of the liver need not be traced. 



Lungs. — The lungs take their origin from a protrusion from the 

 ventral side of the gut some distance in front of the stomach, as in Fig. 

 184, Ig. This pouch is at first single, as shown in Fig. 185, A, but soon 



