216 



EMBRYOLOGY 



METENCEPHALON 



MYELENCEPHALON 



OTIC VESICLE 



BULBUS ARTERIOSUS 



SINUS VENOSUS 



FOLD OF AMNION 



NEURAL TUBE 



MESENCEPHALON 



DIENCEPHALON 



TELENCEPHALON 

 ATRIUM 



VITELLINE ARTERY 



PRIMITIVE STREAK 



Fig. 134. The 48-hour chick embryo with some of its vascular area. In the 

 head region the five divisions of the brain have acquired definite boundaries 

 (Fig. 97). The spiral formation of the heart places the atrium anterior, the 

 ventricle posterior. A sinus venosus forms the first section of the heart to receive 

 blood from the vitelline veins. The bulbus arteriosus conducts blood to the 

 ventral aorta. The otocyst and one clear branchial groove may be seen in the 

 pharyngeal region dorsal to the heart. Continuing posteriorly, the fold of the 

 amnion has reached somite 20. A tail fold develops just posterior to the remains 

 of the primitive streak. (Courtesy Mr. Lester A. Barrer, Sloan-Kettering 

 Institute.) 



Within the embryo itself considerable development has occurred between 

 33 and 48 hours of incubation. The three divisions of the brain have in- 

 creased to five — the telencephalon, diencephalon, mesencephalon, meten- 

 cephalon, and myelencephalon. The boundaries of these divisions and their 

 derivatives were discussed in Chapter 11. The optic vesicles have induced 

 the lens of the eye and have themselves been converted into optic cups. The 



