mi: ORGANS or MM: I\M i. QBRM LAYER. 333 



It still remains to he nn-nt im !. in regard to the outlet of the 

 pancreas, that during development it i> continually moving nearer 

 to the duetus choledoehus. and that finally it opens in common with 

 the latter into tin- duodenum at the divert iculum of V.\ 



SUMMARY. 

 A. nrhicw <>i the Alimentary GanL 



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

 invagination of th* inner germ-layer), the primitive mouth (blast <>- 

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

 medullary riders, and furnishe> temporarily an open communica- 

 tion with the neural tube, the canalis neurentericus. 



_. The neurenteric canal likewise disappears subsequently by the 

 tu-ion of its walls. 



3. The alimentary tube acquires new openings to the outside 

 <vi>eeral 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 arise 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 iir>t t ISM ire. which is employed in the develop- 

 ment of the organ of hearing (external ear. tympanum, Eustachian 

 till*). 



C. The mouth is develop! d at the head-end of the embryo by an 

 unpaired inva^i nation of the epidermis, which, as oral sinus, grows 

 toward the blindly ending fore gut. and l.y the breaking through of 

 the primitive pharynx al membrane. (Primitive- palatal velum.) 



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

 the vintral side at 6 in front of the posterior end of the 



1'ody, so that the intestinal tube is * -out inued for a rertain distance 

 beyond the anu> to\\ard the tail. 



S. The post- anal or raudal inte-'ine. \\hirh at tir>t stretrhes from 

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

 becomes rudimentary afterward.- and wholly disap. that the 



