54 THE STUDY OF CHICK EMBRYOS 



Transverse Section through the Fore-brain and Optic Vesicle (Fig. 38). — 

 The neural tube is open here and constitutes the first brain vesicle or fore-brain. 

 The opening is the anterior neuropore. The ectoderm is composed of two or three 

 layers of nuclei and is continuous with the much thicker wall of the fore-brain. 

 The lateral expansions of the fore-brain are the optic vesicles, which eventually 

 give rise to the retina of the eye. The two ectodermal layers are in contact 



Ecioderm 

 Optic Vesicle 



roammon 



Fig. 38. — Transverse section through the fore-brain and optic vesicles of a twenty-five-hour chick. X 90. 



with each other except in the mid-ventral region, where the mesenchyma is 

 beginning to penetrate between and separate them. The proamnion consists 

 of a layer of ectoderm and of entoderm. 



CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS (THIRTY-SBC HOURS) 



The long axis of this embryo is nearly straight (Fig. 39), the area pellucida 

 is dumb-bell shaped and the vascular network is well differentiated throughout 

 the area opaca. The tubular heart is bent to the right, and opposite its posterior 

 end the vascular network converges and becomes continuous with the trunks of 

 the vitelline veins. Connections have also been formed between the descending 

 aortas and the vascular area, but as yet the vitelline arteries have not 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 

 the head the neural tube is differentiated into the three brain vesicles marked off 

 from each other by constrictions. The fore-brain (prosencephalon) is charac- 

 terized by the outgrowing optic vesicles. The mid-brain (mesencephalon) is 

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

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

 tions, the neuromeres. The ectoderm is thickened laterally over the optic ves- 



