THE 72-HOUR CHICK EMBRYO 



223 



MYELENCEPHALON 



AMNION 



OTIC VESICLE 



METENCEPHALON 



MESENCEPHALON 



DIENCEPHALON 



TELENCEPHALON 



VITELLINE VEIN 



VITELLINE ARTERY 



Fig. 145. The chick embryo with about 36 pairs of somites, about 72 hours 

 of incubation. In addition to the labeled structures, note the branchial clefts 

 between the otic vesicle and the heart. (Courtesy Mr. Lester A. Barter, Sloan- 

 Kettering Institute.) 



the brain, the mesencephalon appears as a large vesicle, and a shallow con- 

 striction separates it from the diencephalon. A deep constriction marks the 

 boundary between the mesencephalon and metencephalon. Between the 

 metencephalon and the last division of the brain, the myelencephalon, no very 

 sharp boundary exists. Opposite the myelencephalon, the otic vesicle is seen 

 as a hollow, pear-shaped structure. Just below the otic vesicle are the 

 branchial clefts. 



