MORPHOGENESIS OF CIRCULATORY SYSTEM 759 



4) The Division of the Bulbus Cordis (Truncus Arteriosus and Conns). 

 The division of the bulbus cordis occurs synchronously with the above changes. 

 Two internal ridges opposite each other are formed during this process. These 

 ridges fuse and divide the bulbus in a spiral fashion into a dorsal aortic root 

 and a pulmonary trunk as indicated in figure 340E. The pulmonary trunk 

 opens into the right ventricle, and the aortic root opens into the left 

 ventricle. Three cup-shaped, semilunar (pocket) valves are developed from 

 internal ridges in the areas between the base of the aortic trunk and the left 

 ventricle and between the base of the pulmonary trunk and the conus portion 

 of the right ventricle (fig. 340E). 



4. Modifications of the Aortal Arches 



When the heart begins to form, its position is ventro-posteriorly to the 

 developing pharyngeal area. As the pharyngeal region enlarges, the heart 

 recedes, relatively speaking, and moves caudally. This caudal recession of the 

 primitive heart in relation to the pharyngeal area is greater in fishes than in 

 the amphibia and higher vertebrates. Therefore, the ventral aortae (and later 

 ventral aorta) are longer in fishes than in other vertebrates. Actually, in the 

 amphibia and particularly in the higher vertebrates, the primitive heart itself 

 tends to lie below the pharyngeal area. Consequently, the bulbus cordis or 

 anterior end of the primitive heart comes to lie below the midpharyngeal 

 region, and the aortal arches in amphibia and in higher vertebrates arise from 

 the anterior end of the primitive heart in bouquet fashion (figs. 34 IE, 342A, 

 E). On the other hand, in fishes, a single, elongated, ventral aorta is formed, 

 which extends the length of the pharyngeal area. The developing heart is 

 attached to its caudal end, and the aortal arches arise along its extent (fig. 

 341A). 



The aortal arches are paired vessels which run dorsally through the sub- 

 stance of the visceral arches. Six pairs of these arches are formed generally 

 in«the gnathostomous vertebrates, although some of them are transitory struc- 

 tures. The first, second, and fifth pairs of aortal arches are the most transitory 

 in all forms above the fishes. 



During development the aortal arches are modified differently in the various 

 vertebrate groups. In fishes, a permanent, branchial mechanism is inserted 

 midway along the branchial visceral arches. The aortal arch of each branchial 

 visceral arch is broken up into an afferent vessel, passing from the ventral 

 aorta to the branchial (gill) structure, and an efferent vessel, leading from 

 the gill mechanism to the dorsal aorta (fig. 341B). In the majority of amphibia, 

 the first, second, and third branchial aortal arches become involved temporarily 

 in the development of gill mechanisms, although some, such as Nectiirus, 

 retain the gills permanently. In higher vertebrates, none of the aortal arches 

 are concerned with gill formation, and are, in consequence, transformed 

 directly into the adult form. 



