172 THE DEVELOPMENT OF THE’ CHICK 
From the embryological point of view the alimentary tract 
may be divided into fore-, mid-, and hind-gut. The fore-gut 
includes the anterior portion as far back as the liver diverticulum, 
the mid-gut extends from here to the ccecal appendages, and the 
hind-gut includes 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 yolk-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 embryonic layers: 
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 
