THE ORGANS OF THE INXER GERM-LAYER. 333 



It still remains to be mentioned, in regard to the outlet of the 

 pancreas, that during development it is continually moving nearer 

 to the ductus choledochus, and that finally it opens in common with 

 the latter into the duodenum at the diverticulum of VATER. 



SUMMARY. 



A. Orifices of the Alimentary Canal. 



1. The original orifice of the alimentary canal (resulting from the 

 invagination of the inner germ-layer), the primitive mouth (blasto- 

 pore), becomes closed later, owing to the circumcrescence of the 

 medullary ridges, and furnishes temporarily an open communica- 

 tion with the neural tube, the canalis neurentericus. 



2. The neurenteric canal likewise disappears subsequently by the 

 fusion of its walls. 



3. The alimentary tube acquires new openings to the outside 

 (visceral clefts, mouth, anus) by the fusion of its walls with the 

 body-wall at certain places, and by the regions of fusion then 

 becoming thinner and rupturing. 



4. The visceral clefts nrise on both sides of the future neck-region 

 of the body, usually five or six pairs in the lower Vertebrates, four 

 pairs in Birds, Mammals, and Man. (Formation of outer and inner 

 throat-furrows ; breaking through of the closing plate.) 



5. In water-inhabiting Vertebrates the visceral clefts serve for 

 branchial respiration (development of branchial lamellae by the for- 

 mation of folds of the mucous membrane) ; in Reptiles, Birds, and 

 Mammals they become closed and disappear, with the exception of 

 the upper part of the first fissure, which is employed in the develop- 

 ment of the organ of hearing (external ear, tympanum, Eustachinn 

 tube). 



6. The mouth is developed at the head-end of the embryo by an 

 unpaired invagination of the epidermis, which, as oral sinus, grows 

 toward the blindly eneling fore gut, and by the breaking through of 

 the primitive pharyngeal membrane. (Primitive palatal velum.) 



7. The anus arises, in a manner similar to that of the mouth, on 

 the ventral side at some distance in front of the posterior end of the 

 body, so that the intestinal tube is continued for a certain distance 

 beyond the anus toward the tail. 



8. The post- anal or caudal intestine, which at first stretches from 

 the anus to the posterior end of the body (tail-part of the body), 

 becomes rudimentary afterwards and wholly disappears, so that the 



