PROSENCEPHALON 



RHOMBENCEPHALON 



OPTIC VESICLE 

 MESENCEPHALON 



HEART 

 VITELLINE VEIN 



NEURAL TUBE 



PRIMITIVE STREAK 



Fig. 129. An embryo of 14 pairs of somites about 34 hours of incubation 

 age and about 4.5 mm. in length. The nervous system has formed three primary 

 divisions of the brain: prosencephalon, mesencephalon, and rhombencephalon. 

 Paired optic vesicles grow out laterally from the prosencephalon. The foregut 

 is outlined by two folds on either side of brain. The heart is well formed and 

 beating. Paired vitelline veins can be seen entering heart. The anterior intestinal 

 portal is located above the union of the two vitelline veins. The neural tube is 

 closed throughout most of its length. The primitive streak is much reduced in 

 size. The area vasculosa has invaded the area pellucida, and blood vessels are 

 uniting (about two-thirds of the way down from the head) to form the vitelline 

 arteries. (Courtesy Mr. Lester A. Barrer, Sloan-Kettering Institute.) 



PROSENCEPHALON 



SUBCEPHALIC POCKET 



OPTIC VESICLE 



SOMATOPLEURE 



EXTRAEMBRYONIC COELOM 

 SPLANCHNOPLEURE 



Fig. 130. The optic vesicles of a 33-hour chick embryo and the structures at 

 their level. The vesicles are lateral extensions of the prosencephalon. 



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