304 EMBRYOLOGY. 



Hrth the two lamellie of the portion of the bursa which has grown 

 over the intestines are, as in many Mammals, separated by a narrow 

 fissure (fig. 167 B gn^) ; during the first and second years after birth 

 they ordinarily fiise into a single lamella in which fat is deposited. 



III. Development of the Separate Organs of the Alimentary Tube. 



The simple growth in length, to which is to be referred the for- 

 mation of the convolutions just described, is only one and certainly 

 not the chief means by which the inner surface of the intestine is 

 increased. The latter acquires a much greater addition from the 

 fact that the inner, originally smooth epithelial layer, which is 

 derived from the entoblast, of the germ, forms evaginations and 

 invaginations. By invaginations toward the cavity of the intestine 

 -there arise numerous folds, small papillae and villi, which give to the 

 mucous membrane at most places a velvety structure ; by evagina- 

 tions toward the outer surface of the tube there are developed 

 various kinds of larger and smaller glands. 



By this simple device, the formation of folds, — the great importance 

 of which in the determination of form in animals was particularly, 

 set forth in Chapter IV. of Part I., — ^the mucous membrane acquires 

 to a much greater extent the ability : (1) to secrete digestive fluids, 

 and (2) to absorb the nutritive substances that are mechanically and 

 chemically prepared in the iatestine, and to transfer them into , the 

 Ibody-fluids. 



I discuss the numerous organs which are produced by the process 

 of folding according to the regions into which the intestinal tube is 

 ■divided, beginning with the organs of the oral cavity. 



A. The Organs of the Oral Cavity : Tongue, Salivary Glands, and Teeth. 



(1) The Tongue arises, according to the investigations of His upon 

 human embryos, out of an anterior and a posterior fundament 



(fig. 168). 



The oMterior fwndament appears very early as an unpaired eleva- 

 tion (tuberculum impar. His) on the floor of the oral cavity in the 

 space surrounded by the mandibular ridges. It grows a good deal 

 in width, and its anterior margin projects free over the mandible, 

 thus forming the body and tip of the tongue. Even as early as the 

 beginning of the third month some papillae make their appearance 

 On it (His, KSlliker). 



The posterior fwndament produces the root of the tongue, which, 



