56 



THE STUDY OF CHICK EMBRYOS 



tube is divided by constrictions into four vesicles. The fore-brain of the previous 

 stage is now subdivided into two regions, the telencepha lon and diencepha lon. 

 The cephalic flexure has been established in the region of the mesencephalon. 

 The hind-brain, as yet undivided, equals the combined length of the other three 

 vesicles. The lens of the eye has invaginated, pushing in the wall of the optic 



Mid-brain 



Optic vesicle 

 Aperture of lens vesicle. 



Fore-hrain- 



Pharynx 

 Bulb of heart. 

 Ventricle 

 R. vitelline vein 



Fore-gut 



Splanchnopleure 

 Splanchnic mesoderm 



Dorsal aorta 



R. vitelline artery 



Hind-brain 

 Notochord 

 Otocysi 



■Aortic arches i, 2, 3 

 Ant. cardinal vein 



Atrium 



Common cardinal vein 

 Post cardinal vein 

 Descending aorta 



Liver anlage 

 Fovea cardiaca 



Entoderm 

 Somatopleure 

 Spinal cord 



vitelline artery 



Edge of splanchnic mesoderm 

 Mes. segment 



Vascular plexus 

 •Notochord 



Ilind-gut 



Mes. segment 



Segmental zone 



Neural plate 



Entoderm 

 Primitive node 



Fig. 57.— Semi-diagrammatic reconstruction of a fifty-hour chick embryo, in ventral view. X 18. 

 The entoderm has been removed save in the region of the fovea cardiaca and of the hind-gut. Owing 

 to the torsion of the embryo, the cranial third of the embryo is seen from the left side, the caudal two- 

 thirds in ventral view. 



vesicle and thus forming a double-walled structure, the optic cup. The audi- 

 tory placode has become a sac, the otocyst, which overhes the hind-brain opposite 

 the second branchial groove and is still connected with the outer ectoderm, cut 

 away in Fig. 57. The rhomboidal sinus is still open at the caudal end of the 

 neural tube. 



