THE DIGEl 



Fia. 981. Floor of pharynx 

 daya old. 



Arytenoid 

 swellings 



imaa embryo about thirty 

 (From model by Peter.) 





there arises a second and larger elevation, in the center of which is a median groove 

 or furrow. This elevation was named by His the furcula, and is at first separated 

 from the tuberculum impar by a depression, but later by a ridge, the copula, 

 formed by the forward growth and fusion of the ventral ends of the second and 

 third arches. The posterior or pharyngeal part of the tongue is developed from 

 the copula, which extends forward in the form of a V, so as to embrace between its 

 two limbs the buccal part of the tongue. At the apex of the V a pit-like invagination 

 occurs, to form the thyroid gland, 

 and this depression is represented in 

 the adult by the foramen cecum of 

 the tongue. In the adult the union 

 of the anterior and posterior parts 

 of the tongue is marked by the V-- 

 shaped sulcus terminalis, the apex of 

 which is at the foramen cecum, while 

 the two limbs run lateralward and 

 forward, parallel to, but a little be- 

 hind, the vallate papillae. 



The Palatine Tonsils. -The palatine 

 tonsils are developed from the dorsal 

 angles of the second branchial pouches. 

 The entoderm which lines these pouches grows in the form of a number of solid 

 buds into the surrounding mesoderm. These buds become hollowed out by the 

 degeneration and casting off of their central cells, and by this means the tonsillar 

 crypts are formed. Lymphoid cells accumulate around the crypts, and become 

 grouped to form the lymphoid follicles; the latter, however, are not well-defined 

 until after birth. 



The Further Development of the Digestive Tube. The upper part of the fore-gut 

 becomes dilated to form the pharynx (Fig. 977), in relation to which the branchial 

 arches are developed (see page 65) ; the succeeding part remains tubular, and with 

 the descent of the stomach is elongated to form the esophagus. About the fourth 

 week a fusiform dilatation, the future stomach, makes its appearance, and beyond 

 this the gut opens freely into the yolk-sac (Fig. 982, A and B). The opening is at 

 first wide, but is gradually narrowed into a tubular stalk, the yolk-stalk or vitelline 

 duct. Between the stomach and the mouth of the yolk-sac the liver diverticulum 

 appears. From the stomach to the rectum the alimentary canal is attached to the 

 notochord by a band of mesoderm, from which the common mesentery of the gut 

 is subsequently developed. The stomach has an additional attachment, viz., to 

 the ventral abdominal wall as far as the umbilicus by the septum trans versum. 

 The cephalic portion of the septum takes part in the formation of the diaphragm, 

 while the caudal portion into which the liver grows forms the ventral mesogastrium 

 (Fig. 984). The stomach undergoes a further dilatation, and its two curvatures 

 can be recognized (Figs. 983, B, and 984), the greater directed toward the vertebral 

 column and the lesser toward the anterior wall of the abdomen, while its two 

 surfaces look to the right and left respectively. Behind the stomach the gut 

 undergoes great elongation, and forms a V-shaped loop which projects downward 

 and forward; from the bend or angle of the loop the vitelline duct passes to the 

 umbilicus (Fig. 984). For a time a considerable part of the loop extends beyond 

 the abdominal cavity into the umbilical cord, but by the end of the third month 

 it is withdrawn within the cavity. With the lengthening of the tube, the mesoderm, 

 which attaches it to the future vertebral column and carries the bloodvessels for 

 the supply of the gut, is thinned and drawn out to form the posterior common 

 mesentery. The portion of this mesentery attached to the greater curvature of 

 the stomach is named the dorsal mesogastrium, and the part which suspends the 



