QO COMPARATIVE ANATOMY 



The important endocrine glands, thyroid, parathyroid and thymus, 

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

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

 of the pharynx. 



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, uniformly in all vertebrates, develops as a mid- 

 ventral outgrowth, or sometimes more than one, from the anterior region 

 of the prospective intestinal portion of the enteron. Vascular and con- 

 nective tissues make up a large part of the bulk of the adult liver but the 

 essential liver substance — the hepatic cells which carry on the specific 

 functions of the liver — are endodermal. The position of the opening of the 

 bile duct into the intestine marks the point of origin of the embryonic liver. 



The pancreas and liver arise in close relation to one another, but the 

 pancreas commonly has more than one point of outgrowth from the 

 enteron. The secretory tissue of the pancreas is endodermal. 



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. 

 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 so that eventually the anus and urinogenital aperture are far 

 removed from one another, the latter being the more ventral in position. 

 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. 



In the great majority of vertebrates the cloaca is derived from the 

 extreme hind end of the embryonic enteron. As the kidneys and gonads 

 develop, their ducts acquire connexion into this part of the enteron. 

 It becomes more or less enlarged in diameter as compared to the intestinal 

 region of the enteric tube. The proctodeal perforation provides it with 

 an exit to the exterior and it becomes the definitive cloacal chamber. 

 It has no digestive function other than serving as an avenue for passage of 

 digestive waste as well as the renal and genital products. This cloaca is 

 lined with endoderm. But the cloaca of the frog has been described as 

 developing from the proctodeum, an ectodermal invagination. Such a 

 cloaca would be lined by ectoderm. It is therefore possible that cloacal 

 chambers are not all precisely homologous. 



