STRUCTURE OF FIFTY-HOUR CHICKS 105 



its division into separate chambers. As yet, however, no indi- 

 cation of the actual partitioning off of the heart is apparent. It 

 is still essentially a tubular organ through which the blood passes 

 directly without any division into separate channels or currents. 



The Aortic Arches. In 33 to 38 hour chicks the ventral 

 aortae communicate with the dorsal aortae over a single pair of 

 aortic arches which bend around the anterior end of the pharynx 

 (Figs. 23 and 24) . With the formation of the visceral arches new 

 aortic arches appear. The original pair of aortic arches comes 

 to lie in the mandibular arch, and the new aortic arches are 

 formed caudal to the first pair, one pair in each visceral arch. 

 In chicks of 50 to 55 hours, three pairs of aortic arches have been 

 established and a fourth is usually beginning to form (Figs. 

 34, 35, and 36, A and B). 



The Fusion of the Dorsal Aortae. The dorsal aortae arise as 

 vessels paired throughout their entire length (Fig. 23). As 

 development progresses they fuse in the mid-line to form the 

 unpaired dorsal aorta familiar in adult anatomy. This fusion 

 takes place first at about the level of the sinus venosus and 

 progresses thence cephalad and caudad. Cephalically it never 

 extends to the pharyngeal region. Caudally the whole length 

 of the aorta is eventually involved. At this stage the fusion 

 has progressed caudad to about the level of the i4th somite 

 (Figs. 34, 35, 36). 



The Cardinal and Omphalomesenteric Vessels. The rela- 

 tionships of the cardinal veins and the omphalomesenteric 

 vessels are little changed from the conditions in 40 to 50 hour 

 chicks. The posterior cardinals have elongated, keeping pace 

 with the caudal progress of differentiation in the mesoderm. 

 They lie just dorsal to the intermediate mesoderm in the angle 

 formed between it and the somites (Fig. 36, D). The entrance 

 of the omphalomesenteric veins into the sinus venosus, and the 

 origin of the omphalomesenteric arteries from the dorsal aortae 

 show little change from conditions familiar from the study of 

 younger embryos. 



VI. THE DIFFERENTIATION OF THE SOMITES 



When the somites are first formed they consist of a 

 nearly solid mass of cells derived from the dorsal mesoderm (Fig. 

 38, A). The cells composing them show a more or less radial 



