CHAPTER XIII 



THE DEVELOPMENT OF THE CHICK DURING THE 

 THIRD AND FOURTH DAYS OF INCUBATION 



1. External Features. 



Torsion; flexion; the visceral arches and clefts; the oral 



region; the appendage buds; the allantois. 

 II. The Nervous System. 



Summary of development prior to the third day; the 

 formation of the telencephalic vesicles ; the diencepha- 

 lon; the mesencephalon; the metencephalon; the 

 myelencephalon; the ganglia of the cranial nerves; 

 the spinal cord; the spinal nerve roots. 



III. The Sense Organs. 



The eye; the ear; the olfactory organs. 



IV. The Digestive and Respiratory Systems. 



Summary of development prior to the third day; the 

 establishment of the oral opening; the pharyngeal 

 derivatives; the trachea; the lung-buds; the oesopha- 

 gus and stomach; the liver; the pancreas; the mid- 

 gut region; the cloaca; the proctodasum and the cloa- 

 cal membrane. 

 V. The Circulatory System. 



The functional significance of the embryonic circulation; 

 the vitelline circulation; the allantoic circulation; the 

 intra-embryonic circulation; the heart. 

 VI. The Urinary System. 



The general relationships of pronephros, mesonephros, 

 and metanephros; the pronephric tubules of the chick; 

 the mesonephric tubules. 

 Vll. The Ccelom and Mesenteries. 



I. EXTERNAL FEATURES 



Torsion. Chicks of three days incubation (Fig. 39) have 

 been affected by torsion throughout their entire length. Tor- 

 sion is complete well posterior to the level of the heart but 



109 



