64 



CHORDATE ANATOMY 



accessions from the endoderm with which it is usually in close 

 relation. 



The Enteron. Gastrulation produces a two-layered embryo whose 

 endoderm 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 develop- 

 ment the enteric tube increases in length faster than the embryo with 



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 position gives rise to 

 a shallow depression or pit, the stomo- 

 deum, whose deeper wall meets the 

 forward-growing endoderm to form tem- 

 porarily a two-layered oral membrane 

 (Figs. 57, O and -720) 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 per- 

 foration and obliteration of the membrane 

 apparently result from progressive centrif- 

 ugal flow or movement of its cellular sub- 

 stance. Thus is formed the mouth. The 

 posterior enteric aperture or embryonic 

 "anus" develops usually by a similar 

 process. The blastopore 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. 57) connecting the 

 hind ends of neural tube and enteric cavity. An ectodermal pit, the 

 proctodeum, situated just below the neurenteric canal, perforates into 

 the hind end of the enteric cavity to form the definitive hind aperture, 

 either anal or cloacal (Fig. 57). 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 constitutes the digestive epi- 

 thelium, 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 



Fig. 56. — Stereogram of the 

 developing eye. The head of 

 the embryo is cut transversely 

 in the region of the fore-brain. 

 cf, choroid fissure; fb, wall of 

 fore-brain; I, ectodermal thicken- 

 ing which invaginates to form 

 lens; oc, optic cup; os, optic stalk; 

 p, outer thin wall of optic cup, 

 becoming the pigmented epithe- 

 lium which lies behind the defini- 

 tive retina; r, inner thick wall of 

 optic cup, becoming the sensory 

 retina of the eye. (From Kings- 

 ley, "Comparative Anatomy of 

 Vertebrates.") 



