DEVELOPMENT OF GALLUS 217 



and pass just in front of the anterior intestinal portal where 

 they fuse in the middle line. This vessel, lying immediately 

 beneath the floor of the fore gut, represents the subintestinal 

 vein of the frog, and in this region it becomes differentiated 

 into the heart. It is suspended from the floor of the fore gut 

 by a mesentery, the dorsal mesocardium, and that part of the 

 coelomic cavity into which it hangs will become the pericardium. 

 At early stages, the heart is still outside the embryo, and it is 

 not drawn into it until the base of the amnion has grown in 



Fig. 103. — Gallus : view of the face of a chick after four days' incubation. 

 cf, choroid fissure of the optic cup ; e, optic cup ; fnp, fronto-nasal 

 process ; ha, hyoid arch ; /, lens ; Ing, lachrymo-nasal groove ; Inp, lateral 

 nasal process ; m, mouth ; ma, mandibular arch ; mp, maxillary process ; 

 ns, nasal sac. 



beneath it. The anterior vitelline veins from the yolk-sac 

 soon become replaced by the larger pair of posterior vitelline 

 veins. 



Running forwards from the heart, the aortic arches run up 

 round the fore gut (pharynx) on each side passing between the 

 gill-slits in the visceral arches. The hyomandibular and three 

 pairs of branchial pouches are developed as outgrowths from 

 the endoderm to the ectoderm. Of these, the hyomandibular 

 and the first two branchials actually become perforated for a 



