550 EMBRYOLOGY OF THE LOWER VERTEBRATES OH. 



cephalon is still more pronounced. The operculuin (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. 

 j}The 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.U 



a.v: 



FICJ. 241. Diagram showing the main parts of the vascular system u 

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



a.a. 



in ;i Fowl 



a.a, allantoic artery; a.r.r, anterior cardinal vein; at, atrium; u.v, allantoic vein: </.r. duct of 

 t Cuvier ; d.c, dorsal carotid; iLo, iliac artery ; p.<i, pulmonary artery ; /.<./, posterior cardinal vein; 

 t>Tior vena cava ; i:A, ventral aorta ; v., vitelline ai-tery ; r., ventral carotid; v.v, vitelline 

 \ein ; 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 

 ;ireh. Dorsally the aortic root extends forwards into the head as the 

 dorsal carotid artery (d.c). A little distance behind I In- li\vr I lie \ iti-I- 

 line artery (v.a) loaves the dorsal aorta and far! her l>aek I he ;i 1 Ian t 'it- 

 artery (a.a) a branch of \\hieh, the iliae artery, ]asses t< (he hind linil-. 

 In the venous system the duct of Cuvier is seen, eonl iimous at 

 its dorsal end with the anterior and posterior cardinal \eins. Tin- 

 former (a.c.v) branchei thi-on^li the head: the latter (/).c.?') can he 



