44 



THE STUDY OF CHICK EMBRYOS 



appeared as distinct trunks. The proamniotic area is reduced to a small region 

 in front of the head, which 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 rhomboidal sinus . In 



Fore-brain k- 

 Mid-brain 

 Hind-brain ■; 



Vitelline vein 



Mesodermal 

 segment S 



! Notochord 



Proamnion 



Opiii vesicle 



Free portion of head 



Heart 

 Seiiral lube 



I 



Rhomboidal sinus 



■ Primilisx^lu_d]:_ 



'1(.,k,,„., l[.K 



Fig. 40. — View of the dorsal surface of a thirty-eight-hour chick embr\ o. X 20. 



the head the neural tube is differentiated into the three brain vesicles, marked ofiE 

 from each other by constrictions. The fore-brain ( prosencephalo n") is charac- 

 terized by the outgrowing oi3tic. j>'^esides. The mid-brain (mesfi ncepha lon) is 

 undifferentiated. The hind-b rain (rhombencephalon) is elongated and gradually 

 merges caudally with the spinal cord. It shows a number of secondary constric- 

 tions, the neuromeres. The ectoderm^^s_thickened laterally over the optic ves- 



