274 Basic Structure of Vertebrates 



median cord of cells which form the notochord. It may, however, 

 receive accessions from the endoderm, with which it is usually in close 

 relation. 



Enteron. Gastrulation produces a two-layer embryo whose endo- 

 derm surrounds a cavity opening to the exterior by the blastopore. 

 This archenteric cavity is the prospective digestive cavity. As the 

 embryo elongates, the cavity is correspondingly elongated, and in later 

 development the enteric tube increases in length faster than the em- 

 bryo, with the result that the tube becomes bent or even coiled to 

 adapt itself to the coelomic space. % 



In the early embryo the ectoderm at a median anteroventral posi- 

 tion gives rise to a shallow depression or pit, the stomodeum, whose 

 deeper wall meets the forward-growing endoderm to form temporarily 

 a two-layer oral membrane (Fig- 228) separating the external 

 stomodeal cavity from the enteric cavity. Soon a perforation appears 

 at the center of the membrane and its peripheral remnant is rapidly 

 obliterated. The perforation and obliteration of the membrane appar- 

 ently result from progressive centrifugal flow or movement of its cellu- 

 lar substance. Thus is formed the mouth. The posterior enteric aperture 

 or embryonic "anus" usually develops by a similar process. The blasto- 

 pore rarely persists as a definitive posterior aperture, although it does 

 so in cyclostomes and possibly in some urodele amphibians. Otherwise, 

 exactly as in Amphioxus, it becomes roofed over by the neural folds 

 and thus converted temporarily into a neurenteric canal (Fig. 228) 

 connecting the hind ends of neural tube and enteric cavity — in reptiles 

 and birds, more or less obscured by the massive yolk. An ectodermal 

 pit, the proctodeum, situated just below the neurenteric canal, per- 

 forates into the hind end of the enteric cavity to form the definitive 

 hind aperture, either anal or cloacal (Fig. 228). As result of the mode 

 of development of the enteric apertures, the lining of more or less of 

 the mouth cavity is derived from stomodeal ectoderm and that of the 

 posterior region from proctodeal ectoderm. The remaining and by far 

 greater part of the adult enteric tube is lined by endoderm which con- 

 stitutes the digestive epithelium, the essential secreting and absorbing 

 layer of the tube. 



It is a noteworthy fact that various organs which have nothing 

 directly to do with digestion have their origin in the enteric endoderm. 

 The anterior region of the embryonic enteron — the part becoming the 

 pharynx of the adult — is concerned particularly with the organs of 

 respiration. Gills of fishes and amphibians develop in relation to paired 

 apertures, the pharyngeal or visceral clefts, which pierce the lateral 

 walls of the enteron and the ectoderm and open to the exterior. A 

 pharyngeal cleft is developed as follows: A deep lateral pouch or 



