DEVELOPMENT OF THE CHICK 133 



Early in development the wall of the 

 pharynx is marked by four visceral pouches 

 corresponding to each of which is an ectodermal 

 invagination (visceral furrow) on the surface 

 of the embryo. The ectoderm of the furrows 

 comes into contact with the entoderm of the 

 pouches, and for a short time there are actual 

 clefts placing the cavity of the pharynx in 

 communication with the exterior. On the 

 third day an evident constriction, correspond- 

 ing to the oesophagus, appears immediately 

 behind the pharynx, while beyond this is a 

 fusiform dilatation which will develop into 

 the stomach. By the eighth day a crop is 

 readily distinguishable as a dilatation of the 

 oesophagus. Up to the fifth day no difference 

 can be detected between the glandular stomach 

 and the gizzard, but after this time structural 

 changes occur in the embryonic stomach which 

 permit of the distinction being made. On 

 the sixth day the duodenal loop begins to form. 

 From this time onwards the small intestine 

 undergoes rapid elongation, that part of it 

 with which the yolk-sac is connected forming 

 coils which lie outside the abdomen until, 

 about the fifteenth day, they are drawn within 

 the body. The two caeca begin to grow out 



