66 



THE STUDY OF CHICK EMBRYOS 



Caudally a fold and opacity mark the position of the tail-fold from which de- 

 velops the caudal end of the body. The curved fold embracing this is the tail- 

 fold of the amnion which will eventually meet the head-fold and completely 

 enclose the embryo. 



Mid-brain 



Optic vesicle 

 Aperture of lens vesicle 



Fore-brain 



Pharynx 

 Bulb of heart 

 Ventricle 

 R. vitelline vein 

 Fore-gut 



Splanchnopleure 

 Splanchnic mesoderm 



Dorsal aorta 



R. vitelline artery 



Mes. segment 



Segmental zone 



Neural plate 



Entoderm 

 Primitive node 



Hind-brain 

 Notochord 

 Otocyst 



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 



L. vitelline artery 



Edge of splanchnic mesoderm 

 Mes. segment 



Vascular plexus 



■Notochord 



Hind-gut 



Fig. 55. — Semi-diagrammatic reconstruction of a fifty-hour chick embryo, ventral view. X 22. 

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

 cranial third of the embryo is seen from the left side, the caudal two-thirds in ventral view owing to the 

 torsion of the embryo. 



Central Nervous System and Sense Organs (Fig. 55). — The neural tube is 

 divided by constrictions cephalad into four vesicles. The fore-brain of the 

 previous stage is now subdivided into two regions, the telencephalon and dien- 

 cephalon. The cephalic flexure has been established in the region of the mesen- 

 cephalon. The hind-brain is as yet undivided and is as long as the other three 



