THE ORGANS OF THE INNER GERM-LAYER. 335 



of the loop becomes folded to form the convolutions of the small 

 intestine. 



10. The mesentery, which is at first uniform and common to the 

 whole alimentary tube, becomes differentiated into separate regions, 

 for it adapts itself to the folds and to the elongations of the ali- 

 mentary tube. It is elongated and here and there undergoes fusion 

 with the peritoneum of the body-cavity, by means of which it eithei 

 acquires new points of attachment or in certain tracts wholly 

 disappears ; some portions of the intestine are thus deprived of their 

 mesentery. 



11. The mesentery of the duodenum, and in part also that of the 

 colon ascendens and c. descendens, fuses with the wall of the body 

 (extraperitoneal parts of the intestine). 



12. The mesentery of the colon transversum acquires a new line of 

 attachment running from right to left, and becomes differentiated 

 from the common mesentery as mesocolon. 



13. The mesogastrium of the stomach follows the torsions of the 

 latter and is converted into the greater omentum, which grows out 

 from the greater curvature of the stomach to cover over all the 

 viscera lying below. 



14. Fusions of the walls of the omentum with adjacent serous 

 membranes take place : (1) on the posterior wall of the body, in 

 consequence of which the line of origin from the vertebral column is 

 displaced to the left side of the body ; (2) with the mesocolon and 

 colon transversum; (3) on the part of the sac which has overgrown 

 the intestines, where its anterior and posterior walls come into close 

 contact and fuse into an omental plate. 



C. Development of Special Organs out of the Walls of the 

 Alimentary Tube. 



1 . The surface of the alimentary tube increases in extent inward 

 by means of folds and villi, and by glandular evaginations outward. 



2. There are developed, as organs of the oral cavity, the tongue, 

 the salivary glands, and the teeth. 



3. The teeth, which in the higher Vertebrates are found only at 

 the entrance of the mouth, are distributed in the lower Vertebrates 

 (Selachians, etc.) over the whole of the cavity of the mouth and 

 throat, and indeed as dermal teeth over the whole surface of the 

 body. 



4. The dermal teeth are dermal papillae ossified in a peculiar 



