Development of the Third Day 207 



later (about the fifteenth day) ; so that the 

 digestive tract now ends blindly at each end. 

 The proctodaeum in the chick is very shallow, 

 and forms only the actual external opening of 

 the cloaca. 



Up to the sixth day, the digestive tract 

 remains practically straight ; but after that 

 time it begins to grow faster than the cavity 

 in which it is contained, so that it begins to 

 twist and form the loops characteristic of the 

 adult intestines. 



About the sixth day the gizzard is formed 

 as a thick-walled outgrowth from the end of 

 the stomach. 



The lungs. — The first trace of the lungs is 

 seen on the third day as two small, hollow 

 outgrowths from the ventral side of the oesoph- 

 agus near its anterior end (Fig. 67, Ig^. At 

 the point of origin of these small pouches, 

 the oesophagus becomes laterally constricted, 

 so that in cross-section it is hourglass-shaped 

 (Fig. 68). By the meeting of the lateral con- 

 strictions, the oesophagus is divided into two 

 parts: the upper part, or (esophagus proper, and 

 the lower part, into which the lung rudiments 

 open, which will be the trachea. At the an- 

 terior limits of the lateral constrictions the 



