THE STUDY OF CHICK EMBRYOS 



The proamniotic area is reduced to a small region in front of the head, 

 whereas the latter is now larger and. more prominent. In the posterior 

 third of the vascular area blood islands are still prominent. 



Central Nervous System and Sense Organs. -The neural tube is 

 closed save at the caudal end where the open neural folds form the rhom- 



Optic vesiclt 



Paired ventral aorta 



Ventral aorta 

 Bulbus cordis 



Ventricle 



Splanchnic mesoderm f-^ 

 Intestinal portal ^^ m 



^* 

 Right descending aorta 



i* . 



Vascular plexus -?V/ '/ 



Splanchnic mesoderm 

 Notochord 



Mesodermal segment 



Segmental zone 



'ore-brain 



'Pharyngeal pouch 1 

 Descending aorta 



K_ Pharyngeal pouch 2 



Somatopleure 



d^'j Left vitelline vein 

 &i Entoderm 



Section oj mural tube 



Somatopleure 

 Descending aorta 



Neural tube 

 Splanchnic mesoderm 



Capillary plexus 

 Somatopleure 



&m . Neural groove 



FIG. 41. Ventral reconstruction of a thirty-eight-hour chick embryo. The entoderm has been 

 removed except about the intestinal portal. X 38. 



boidal sinus. In the head the neural tube is differentated into the three 

 brain vesicles, marked off from each other by constrictions. The fore- 

 brain (prosencephalon) is characterized by the outgrowing optic vesicles. 

 The mid-brain (mesencephalon) is undifferentiated. The hind-brain 

 (rhombencephalon) is elongated and gradually merges caudally with the 



