DEVELOPMENT OF THE INTESTINAL CANAL. 323 



to grow larger and more perfect. While, at first, the intes- 

 tinal tube appears only as a little appendage on one side of 

 the great intestinal germ-vesicle (Fig. 278), the remnant of 

 the latter afterwards forms only a very inconsiderable appen- 

 dage of the great intestinal canal. This appendage is the 

 yelk-sac, or navel-vesicle. It entirely loses its importance, 

 and at length disappears, while the intestinal canal is finally 

 closed at the original central opening, where it forms the 

 so-called intestinal navel (Fig. 94, vol. i. p. 312). 



It has also been said that this simple cylindrical intestinal 

 tube, in Man as in all Vertebrates, is at first entirely closed 

 at both ends (Plate V. Fig. 14), and that the two permanent 

 openings of the intestinal canal — at the anterior extremity, 

 the mouth, at the posterior, the anus — form only second- 

 arily, and from the outer skin. At the fore end, a shallow 

 mouth-furrow originates in the outer skin, and this grows 

 toward the blind, anterior end of the head intestinal cavity, 

 into which it finally breaks. In the same way a shal- 

 low furrow for the anus is formed behind in the skin, 

 and this soon grows deeper, and gi'ows toward the blind 

 posterior end of the pelvic intestinal cavity, with which it 

 finally unites. At both extremities there is, at first, a thin 

 partition between the outer skin-furrow and the blind end 

 of the intestine, and this disappears when the opening is 

 made.^^ 



Directly in front of the anus the aUantois grows out of 

 the posterior intestine ; this is the important embryonic 

 appendage which develops, in Placental Animals, and only 

 in these (thus in Man too) into the placenta (Figs. 278, 279, 1 ; 

 Plate V. Fig. 14, al). In this more developed form — repre- 

 sented in the diagi-am (Fig. 94, ^, vol. i. p. 312) — the intestinal 



