EMBRYONIC DEVELOPMENT 



211 



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 the liver has been covered by the layer of peritoneum 

 (mesodermal) which invests the entire digestive tract. The adult liver 

 is thus covered by peritoneum and suspended by mesenteries formed from 

 the same layers of mesoderm. 



The pancreas originates from two pouches evaginated from the intes- 

 tine (Fig. 185). One arises from the dorsal side of the intestine nearly 

 opposite the liver (dp) ; the other springs from the angle between the liver 



Fig. 184. — 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 or cleft; me, endodermal portion of mouth; olf, olfactory pit; opn, optic nerve; ops, optic 

 stalk; ph, pharynx; ret, retina; sp, spinal cord; ty, tympanum or middle ear. Arrows in C 

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



and the intestine (vp). The two pouches intertwine their branches to 

 form the pancreas, which is likewise invested with peritoneum. 



The lungs take their origin from a protrusion from the ventral side 

 of the gut some distance in front of the stomach (Fig. 185, Ig). This 

 pouch is at first single (Fig. 186A), but soon divides into two parts 

 (B, C, D). As these grow in size they become branched. The undivided 

 stalk of the lung rudiment is the trachea, the two principal branches are 

 the bronchi, and the finer divisions are the air passages and alveoli 

 within the lungs. Mesoderm is constantly pushed before the growing 

 lung rudiments, so that the adult lungs are invested with a peritoneum. 

 Other mesodermal tissue is incorporated in the lungs among the air pas- 

 sages, where blood vessels are abundantly developed. 



It should be borne in mind that Fig. 185 is diagrammatic and does 

 not represent a condition prevailing at any one time in embryonic 

 development. For the sake of compactness, organs have been shown 

 in the same figure in stages which do not occur simultaneously. 



