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 j^^octodceum 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 - gro^\i:hs 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. 



