304 



THE CHICK. 



is formed early 011 the second day, and lies opposite the ante- 

 rior end of the fore-gut. The remaining arches are formed in 

 succession behind the first pair. 



By the end of the second day a second pair is present, and 



VJ 



vc 



vo 



w 



AA. 



TA 



FIG. 128. A diagrammatic figure showing the arrangement of the blood- 

 vessels in a Chick Embryo at the end of the fifth day of incubation. The 

 amnion has been removed, and the vitelline vessels cut short a little 

 distance from the embryo, x 12. 



A, dorsal aorta. A3, A4, A5, third, fourth, aud fifth aortic arches of the right siile, 

 lying in the first, second, and third branchial arches respectively. AA, allantoic artery. 

 AB, basilar artery. AC, carotid artery. AH, caudal artery, the terminal portion of 

 the dorsal aorta. AL, lingual artery. AP, pulmonary artery. AV, vitelline artery. 

 El, auditory vesicle. RA, right auricle. RS, sinus venosus. RT, truncus arteriosus. 

 RV, right ventricle. TA, allantois. VA, allantoic vein. VB, anterior cardinal vein. 

 VC, posterior cardinal vein. VD, Cuvierian vein. VE, meatus venosus. VH, efferent 

 hepatic vessel. VI, posterior vena cava. VJ, jugular vein. VO, afferent hepatic 

 vessel. VV, vitelline vein. 



by the commencement of the third day a third pair appears still 

 farther back (Fig. 113 AS). 



On the establishment of the visceral clefts and arches, the 



