STRUCTURE OF FOUR-DAY CHICKS 137 



rials eliminated through them. The watery portion of the 

 waste materials is passed off by evaporation. The remaining 

 solids are deposited in the allantoic vesicle. They accumulate 

 in the extra-embryonic portion of the allantois and there remain 

 until that portion of the allantois is discarded at the close of 

 embryonic life. 



The blood from the allantois is collected and returned to the 

 heart over the allantoic veins. From the distal portion of the 

 allantois the smaller veins converge and unite into two main 

 vessels, right and left, which enter the body of the embryo with 

 the allantoic stalk (Fig. 46, H). After their entrance into the 

 body the allantoic veins extend cephalad in the lateral body 

 walls (Figs. 47 and 46, H to D). They enter the sinus venosus 

 on either side of the entrance of the omphalomesenteric vein. 



The Intra-embryonic Circulation. The earliest vessels of 

 the intra-embryonic circulation to appear are the large vessels 

 communicating with the heart. In chicks of 33 hours the 

 ventral aorta leads off from the heart cephalically and bifur- 

 cates ventral to the pharynx giving rise to a single pair of 

 aortic arches. The aortic arches pass dorsad around the antero- 

 lateral walls of the pharynx and are continued caudally along 

 the dorsal wall of the gut as the paired dorsal aortae (Fig. 23). 



When, toward the end of the second day of incubation, vis- 

 ceral clefts and visceral arches appear, the original pair of 

 aortic arches comes to lie in the mandibular arch. In each of 

 the visceral arches posterior to the mandibular, new aortic 

 arches are formed connecting the ventral aortae with the dorsal 

 aortae. By 55 hours three pairs of aortic arches are present 

 and a fourth is beginning to form (Fig. 35). 



At about this stage extensions of the dorsal aortic roots grow 

 out anteriorly. The vessels thus derived extend cephalad in 

 close association with the brain as the internal carotid arteries. 

 In a later stage vessels arise from the ventral aortic roots and 

 grow cephalad as the external carotid arteries (Fig. 47). 



By the end of the fourth day of incubation two more pairs 

 of aortic arches have appeared posterior to the four formed in 

 55 to 6o-hour chicks. From their first appearance the fifth 

 aortic arches are very small and they soon disappear altogether. 

 The first and second pairs of aortic arches have by this time 

 suffered a great diminution in size which is indicative of their 



