ORIGIN OF THE MOUTH AND ANUS. 339 



Is afterwards found, and this also grows continually deeper 

 and towards the blind anterior end of the pelvic intestinal 

 cavity ; this is the anal groove. At length the innermost 

 and deepest parts of these grooves touch the two blind ends 

 of the primitive intestinal canal, from which they are now 

 only separated by a thin membranous partition wall. 

 Finally, this thin skin is broken through, and the intestinal 

 tube now opens outward in front through the mouth- 

 opening, and in the rear through the anal opening (Fig. 

 94, 4 ; 101). At first, then, we really have before us, if we 

 look into these grooves, a partition wall separating them 

 from the cavity of the intestinal canal, and it is only later 

 that these partitions disappear. The mouth and anal 

 openings develop secondarily. 



The remnant of the intestinal germ-vesicle, which we 

 have called the navel-vesicle, or yelk-sac, grows smaller and 

 smaller as the intestine develops, and finally hangs . like a 

 small pouch from the middle of the intestine by a slender 

 stalk, by the yelk-duct (Fig. 94, G da). This yelk-duct is 

 of no permanent importance, and, like the yelk-sac itself, is 

 completely degraded and absorbed. Its contents are absorbed 

 by the intestine, and the yelk-duct itself closes. The place 

 .at which it attaches itself to the navel is the intestinal 

 oiavel. The complete closing of the intestine finally takes 

 place at this spot. (Of. Chap. XII., and Plate V. Fig. 

 14, 15.) 



Just as the intestinal tube arose from the vegetative 

 germ-layer, so from the animal germ-layer arises the outer 

 ventral wall, which surrounds the entire body-cavity 

 (coelomo), and includes the intestine. It develops from the 

 outer portions of the side-layers. As has been already 



