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. 



C<~f^ 



Mid-brain 





Optic -vesicle 

 Aperture of lens vesicle 



Fore-brain 



Pharynx 

 Bulb of heart 



Ventricle 

 R. mtettine vein 

 Fore-gut 



Splanchnopleure 

 Splanchnic mesoderm 



Dorsal aorta 

 R. mtettine artery 



Mes. segment 



Segmental zone 



Neural plate 



Entoderm 

 Primitive node 



Hind-brain 

 Notochord 

 Otocyst 



Aortic arches i, 2, 

 Ant. cardinal vein 

 Atrium 



* 



Common cardinal vein tyo 

 Post cardinal vein 

 Descending aorta 

 Liver anlage 

 Fovea cardiaca 



Entoderm 

 Somatopleure 

 Spinal cord 



L. mtettine artery 



Edge of splanchnic mesoderm 

 Mes. segment 



Vascular plexus 

 Xotochord 



Hind-gut 



FlG. 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 



