x FOWL— SECOND DAY 527 



its anterior and inner corner the network is continuous with a short 

 and wide vessel which slopes obliquely forwards and inwards and 

 disappears beneath the hind end of the foregut (shown more clearly 

 in Fig. 232, v.v). This vessel is the rudiment of the vitelline vein, 

 which drains the blood from the vascular area towards the heart. 

 Another conspicuous vessel rudiment is the terminal sinus — a 

 marginal vessel which bounds the vascular area externally. In front 

 of the head of the embryo is a somewhat rectangular area of the 

 blastoderm distinguished by its being very transparent (Fig. 232, pa). 

 This is the proamnion — its transparency being due to the fact that 



Fig. 232. — Head of Fowl embryo of same stage as that shown in Fig. 231, move 

 highly magnified and seen by transmitted light. 



f.g, foregut ; H, heart; li.f, hinder limit of head fokl of ectoderm ; inf, infundibulum ; m.s, mesoderm 

 segments ; N, notochord ; o.r, optic rudiment ; pa, proamnion ; splc, patent portion of splanchnocoele 

 containing coelomic fluid ; v.v. vitelline vein. 



the mesoderm has not yet spread into this region of the blastoderm. 

 On each side of the head of the embryo the surface of the blastoderm 

 bulges upwards into a dome-like swelling (Fig. 232, splc). This is 

 due to a precocious splitting of the mesoderm in this- region to form a 

 large coelomic space. The bulging appearance is produced by the 

 coelomic space being tensely filled with fluid. The raising up of this 

 region of somatopleure is preliminary to the formation of the head 

 fold of the amnion. 



By turning over the excised blastoderm and examining it from 

 below or by staining and then examining it in dorsal view by trans- 

 mitted light (Fig. 232) it will be seen that between the two coelomic 

 spaces there lies a A-shaped structure. The two diverging limbs of 



