4 6 



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 vesicli 



Paired ventral aorta 



Ventral aorta 

 Bulbus cordis 



Ventricle 



Splanchnic mesoderm 

 Intestinal portal 



Right descending aorta u 



Vascular plexus 

 Splanchnic mesoderm 



Notochord 

 , Mesodermal segment 



Segmenlal zone 



; ore-brain 



Pharyngeal pouch 1 

 Descending aorta 



Pharyngeal pouch % 



Somatopleiire 



Left mtelline vein 

 Enlodcrm 



Section of neural tube 



Somatopleiire 

 Descending aorta 



Neural tube 

 Splanchnic mesoderm 



Capillary plexus 

 Somatopleiire 



. Neural groove 



FIG. 4 1 .Ventral reconstruction of a thirty-eight-hour chick embryo. The entoderra 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 ^differentiated. The hind-brain 

 (rhombencephalon) is elongated and gradually merges caudally with the 



