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- 



^ Ion; 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 oKactory 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 proctodaeum and the cloa- 

 cal membrane. 

 V. The Circulatory System. 



The functional significance of the embryonic circulation; 

 the vitelHne 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. 

 VII. The Coelom 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 



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