276 Basic Structure of Vertebrates 



auditory passages of most other vertebrates. The pouches, except those 

 of the first pair, are merely temporary embryonic features in reptiles, 

 birds, and mammals. 



Lungs develop by outgrowth from the endoderm of the pharynx 

 (Figs. 59, 187). The entire epithelial lining, being the essential respira- 

 tory membrane of the adult lung, is endodermal and continuous, by 

 way of the lining of bronchi and trachea, with the lining of the diges- 

 tive tube. 



The air-bladders (swim-bladders) of fishes are endodermal sacs 

 which grow out from an anterior region of the embryonic enteron. They 

 are usually dorsal, rarely lateral, or ventral as in the ganoid Polypterus. 



The important endocrine glands, thyroid, parathyroid, and thy- 

 mus, and various glandlike bodies, mostly of dubious nature and func- 

 tion, arise as outgrowths of the endoderm of the pharyngeal pouches 

 or the wall of the pharynx (Fig. 189). 



More posterior regions of the enteric endoderm give rise to various 

 accessory digestive organs, most important of which are the liver and 

 pancreas. The liver develops as a midventral outgrowth, sometimes 

 more than one, from the anterior region of the prospective intestinal 

 portion of the enteron. The pancreas arises similarly and in close rela- 

 tion to the liver. Vascular and connective tissues make up a large part 

 of the bulk of the adult organs, but the essential hepatic cells and the 

 secretory tissue of the pancreas are endodermal. The position of the 

 opening of the bile duct into the intestine makes the point of origin 

 of the embryonic liver. 



The cloaca of the adult vertebrate is a superficial chamber situ- 

 ated at the hind end of the body-cavity and opening ventrally to the 

 exterior. Into it open the intestine and the ducts of the kidneys and 

 genital organs. It is commonly present in vertebrates below mammals 

 except in bony fishes (Teleostei). It is derived from the extreme hind 

 end of the embryonic enteron, although the ectodermal proctodeum 

 may contribute to it. The cloaca of the frog has been described as being 

 wholly proctodeal in origin. Mammalian embryos develop a cloaca, 

 but only those primitive mammals, the duckbill and spiny anteater, 

 retain it as such in the adult. In other mammals the embryonic cloaca 

 becomes divided into a dorsal part connected with the intestine and a 

 ventral part which receives the urinogenital ducts. In the course of fur- 

 ther development, these two divisions of the cloaca are separated and 

 carried apart and acquire independent openings to the exterior, the 

 latter being the more ventral. The urinogenital passage and urinary 

 bladder of the adult, both male and female, are a remnant of the 

 cloaca, while another remnant of it persists in the posterior region 

 of the rectum. 



