172 THE DEVELOPMENT OF THE CHICK 



From the embryological point of view the aUmentary tract 

 may be divided into fore-, mid-, and hind-gut. The fore-gut 

 inchides the anterior portion as far back as the hver diverticula, 

 the mid-gut extends from here to the coecal appendages, and the 

 hind-gut inchides the remainder. From each division there 

 arise certain outgrowths which may be termed collectively 

 appendages of the alimentary tract, and these will also be 

 considered here, so far as they arise within the period covered 

 by this chapter. Thus from the fore-gut there arise the visceral 

 pouches, the thyroid and thymus glands, the postbranchial 

 bodies, the respiratory tract, and the liver and pancreas; from 

 the mid-gut the 3^olk-sac, and from the hind-gut the ccecal 

 appendages and allantois. 



The enlargement of the body-cavity towards the middle line 

 gradually reduces the broad mesodermal septum situated between 

 its inner angles to a relatively narrow plate, which forms the dor- 

 sal mesentery of the intestine (Figs. 107, 109, 110, and 111). This 

 elongates in the course of development and forms a sheet of tissue 

 suspending the intestinal tube to the mid-dorsal line of the body- 

 cavity. It is composed of two layers of mesothelium (peritoneum) 

 continuous with the lining of the body-cavity and enclosing a 

 certain amount of mesenchyme; the dorsal mesentery extends 

 along the entire length of the intestinal canal. 



A ventral mesentery uniting gut and yolk-sac is also estab- 

 lished by the meeting of the limiting sulci in the splanchnopleure. 

 When the body-wall closes, the ventral mesentery consists of 

 two layers of mesothelium attaching the intestinal canal to the 

 mid-ventral line of the body-wall. The dorsal and ventral mesen- 

 teries, together with the alimentary canal, thus constitute a 

 complete partition between the right and left halves of the body- 

 cavity. However, the ventral mesentery is a very transient 

 structure except in the region of the fore-gut and liver, and in 

 the extreme end of the hind-gut. In these places it is persistent 

 and is the seat of formation of important organs. 



The wall of the intestine contains three embrvonic lavers: 

 viz., entoderm, mesenchyme, and mesothelium. The first forms 

 the lining epithelium of the intestine, and of all glandular attach- 

 ments, as well as of the respiratory tract and allantois; the last 

 forms the serosa; and the mesenchyme the intermediate layers. 



We shall now consider the development of each region of the 



