290 THE CHICK. 



but the fact that a similar solidification of the oesophagus occurs 

 in dogfish, frogs, reptiles, and mammals, as well as in birds, 

 renders it possible that it has some further and deeper signifi- 

 cation, not yet determined. 



5. The Stomach and Intestine. 



Up to the end of the fifth day (Fig. 123), the alimentary 

 canal remains almost straight, except for a slight, ventrally 

 directed loop, GT, at the place where the yolk-stalk, YS, arises,, 

 connecting the intestine with the yolk-sac. 



The stomach is recognisable as a slight, fusiform dilatation, 

 TS, about the end of the fifth day ; during the sixth day the 

 gizzard becomes evident, as a thick-walled dilatation of the distal 

 end of the stomach, which grows rapidly, and by the twelfth day 

 has attained a great size. 



From the sixth day onwards, the intestine lengthens rapidly : 

 growth occurring most markedly at two parts of its length, and 

 giving rise to two loops, both of which are directed ventralwards. 

 Of these, the proximal or duodenal loop is formed from the part of 

 the intestine immediately beyond the gizzard. The distal, or 

 vitelline loop, which is much the longer of the two, is formed 

 by elongation of the two limbs of the V-shaped loop which is 

 already present on the fifth day, and from the angle of which 

 the yolk-stalk arises (Fig. 123, YS). 



Between the duodenal and vitelline loops there is a part of 

 the intestine which undergoes hardly any elongation at all, but 

 remains throughout life closely attached to the dorsal surface 

 of the body cavity; it corresponds to the point in Fig. 123 im- 

 mediately beyond the opening of the bile-duct, WD, where the 

 intestine bends ventralwards to form the proximal limb of the 

 vitelline loop. 



The further development of the intestine consists chiefly in 

 great elongation of the vitelline loop, which gives rise to the 

 whole length of the small intestine, beyond the duodenum. Both 

 limbs of the loop lengthen very rapidly, and become twisted 

 somewhat spirally. Up to about the seventeenth day the vitelline 

 loop lies almost entirely in the yolk-stalk, and therefore outside 

 the body of the embryo ; about the eighteenth day the greater 

 part of the loop becomes withdrawn into the body, and acquires 

 the convolutions characteristic of the adult. 



