THE STUDY OF CHICK EMBRYOS 



formed by the invaginated ectoderm. The median, ectodermal pouch 

 next the brain wall is known as Rathke's pouch and is the anlage of the 

 anterior lobe of the hypophysis. The pharynx shows laterally three out- 

 pocketings, of which the first is wing-like and is the largest. These phar- 

 yngeal pouches occur opposite the three branchial grooves and here entoderm 



Mid-brain 



Optic -vesicle 

 Aperture of lens iiesicl 



Fore-brai, 



Splanchnopleure 

 Splanchnic mesoderm 



R. vitelline arter 



Mesodermal segment 



Segmental zone ' 



Neural plate 



Entoderm 

 Primitive knot 



Notochord 

 Otocyst 



Aortic arches i, 2, j 

 nt. cardinal vein 



Atrium 



Common cardinal vein 



Post.cardinal vein 



Descending aorta 



'Liver anlage 

 Intestinal portal 

 ^Entoderm 

 Somatopleure 

 'Spinal cord 



vitelline artery 



Edge of splanchnic mesoderm 

 Mesodermal segment 



Vascular plexus 



Hind-gut 



FIG. 57. Semi-diagrammatic reconstruction of a fifty-hour chick embryo, in ventral view. 

 X 1 8. The entoderm has been removed save in the region of the intestinal"portal and 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. 



and ectoderm are in contact, forming the closing plates. At about this 

 stage the first closing plate ruptures, thereby forming a free opening, or 

 branchial cleft, into the pharynx. Between the pouches are developed the 

 branchial arches, in which course the paired aortic arches. Toward the 

 intestinal portal the fore-gut is flattened laterally, and before it opens out 



