550 EMBKYOLOGY OF THE LOWEE VEKTEBEATES oh. 



cephalon is still more pronounced. The operculum (Fig. 240, Hy) 

 is conspicuous, growing back from the hyoid arch over the posterior 

 visceral clefts. The limb rudiments now project freely though their 

 form is that of simple nippers without any of the peculiarities of the 

 leg or wing of the Bird. The body of the embryo is floored in on its 

 ventral side completely but for the rounded opening (som) along 

 whose lips the somatopleure is continued into the amnion and through 

 which emerge the narrowing yolk-stalk and the stalk of the allantois. 

 !JThe study of the living embryo in situ shows the general plan of 

 the blood system to be as is shown in Fig. 241, B. The heart still 



CLC.V. 



p.c.u 



a.v: 



Fig. 241. — Diagram showing the main parts of the vascular system as seen in a Fowl 

 embryo during the third day (A) and the fifth day (B). 



a.a, allantoic artery; a.c.v, anterior cardinal vein; at, atrium; a.v, allantoic vein; d.C, duct of 

 Cuvier ; d.c, dorsal carotid ; il.a, iliac artery ; p. a, pulmonary artery ; p.c.v, posterior cardinal vein ; 

 p.v.c, posterior vena cava ; v.A,' ventral aorta ; v.a, vitelline artery ; v.c, ventral carotid; v.v, vitelline 

 vein ; I-VI, aortic arches. 



betrays its tubular origin though the chambers are clearly recogniz- 

 able as dilatations. Three aortic arches (III, IV and VI) are distinctly 

 visible and occasionally the fleeting vestige of the penultimate arch 

 as in the specimen represented in the diagram. In front of the aortic 

 arches the ventral aorta is seen extending forwards as the ventral 

 carotid (v.c) : the pulmonary artery (p.a) passes back from the sixth 

 arch. Dorsally the aortic root extends forwards into the head as the 

 dorsal carotid artery (d.c). A little distance behind the liver the vitel- 

 line artery (v. a) leaves the dorsal aorta and farther back the allantoic 

 artery (a.a) a branch of which, the iliac artery, passes to the hind limb. 

 In the venous system the duct of Cuvier is seen, continuous at 

 its dorsal end with the anterior and posterior cardinal veins. The 

 former (a.c.v) branches through the head : the latter (p.c.v) can be 



