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 



Optic vesicle 

 Aperture of lens vesicl 



Splanclmoplcnre 

 Splanchnic mesoderm 



R. vitelline drier 



Mesodermal segment 

 Segments! son 



Neural plate 

 Entodern 



Primitive knot 



Mid-brain 



Hind-brain 

 Notochord 

 Olocyst 



iortic arches I, 2, j 

 Ant. cardinal vein 

 Atrium 



Common cardinal vein 

 Posl.cardinal vein 

 Descending aorta 



Liver anlage 

 Intestinal portal 

 Entoderm 

 Somatopleure 

 'Spinal cord 



. vitelline artery 



Edge of splanchnic mesoderm 

 Mesodermal segment 



Vascular plexus 

 Kolochord 



Hind-gut 



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

 le 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 

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



