66 CHORDATE ANATOMY 



embryonic features except as those of the first pair are, in a modified way, 

 represented in the auditory passages. 



Lungs develop by outgrowth from the endoderm of the pharynx 

 (Figs. 235, 238). The entire epithelial lining, being the essential respi- 

 ratory membrane, of the adult lung, is endodermal and continuous, byway 

 of the lining of bronchi and trachea, with the lining of the digestive 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 th3mius, 

 and various gland-like bodies mostly of dubious nature and function, 

 arise as outgrowths of the endoderm of the pharyngeal pouches or the 

 wall of the pharynx. (Fig. 235) 



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 mid-ventral outgrowth, sometimes 

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

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

 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 marks the point of origin of the embryonic 

 liver. 



The cloaca of the adult vertebrate is a superficial chamber situated 

 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 Teleostei. It is derived from the extreme hind end of the embryonic 

 enteron. Mammalian embryos develop a cloaca but only those primitive 

 mammals, Ornithorhynchus and Echidna, retain it in the adult. In 

 other mammals the embryonic cloaca becomes subdivided into a dorsal 

 part connected with the intestine and a ventral part which receives the 

 urinogenital ducts. In course of further 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. Therefore 

 the more distal portion of the urinogenital passage of the adult, both male 

 and female, is a remnant of the cloaca while another remnant of it persists 

 in the posterior region of the rectum. 



The Mesoderm. The vertebrate mesoderm is at first devoid of seg- 

 mentation and ordinarily contains no definite cavity (Fig. 44). At an 

 early embryonic stage the mesoderm upon either side splits into two layers; 

 an outer, lying against the ectoderm, and an inner lying against the endo- 

 derm. The two layers remain connected, however, at the upper edge 



