132 THE STRUCTURE OF THE FOWL 



an auditory sac, or otocyst, is formed. This is 

 the rudiment from which the labyrinth of the 

 ear is developed. 



Alimentary and Respiratory Tracts. — With 

 the formation of the head- and tail-folds of the 

 embryo, the intra-embryonic splanchnopleure 

 is constrained to take a tubular form which 

 becomes more pronounced as development 

 proceeds. Narrowing of the umbilicus pro- 

 duces a growing constriction between the 

 splanchnopleure within and the yolk-sac out- 

 side the embryo. The primitive gut thus 

 formed ends blindly at the head and the tail 

 end of the embryo, where ectodermal pits, the 

 stomodceum and proctodceum respectively, grow 

 inwards to meet and ultimately become con- 

 tinuous with the ends of the gut. 



The primitive gut may be divided into 

 three parts : — (1) the mid-gut connected with 

 the yolk-sac ; (2) the fore-gut from which is 

 formed the pharynx, oesophagus, the respiratory 

 tract, the stomach, and the duodenum, as well 

 as out -growths which result in the thyroid, 

 thymus, liver, and pancreas ; (3) the hind-gut, 

 which develops into the large intestine with 

 its caeca, and from which the allantois grows 

 in the embryo. 



