THE DEVELOPMENT OF VERTEBRATA. 237 



11. First Blood-vessels. The first necessity for a 

 vascular system arises from the fact that all parts of the 

 embryo need the food that is stored up in the hypoblast of 

 the intestinal region. Hence the first vessels are formed in 

 the splanchnic mesoblast of this region (vitelline veins), and 

 extend forward as a median sub-intestinal vein, which in the 

 pharyngeal region becomes the heart. This is at first a 

 straight tube, giving off from its anterior end two pairs of 

 vessels which run a little way up the third and fourth 

 visceral (i.e. first and second branchial) arches : these are 

 two of the future afferent branchial arteries. In the dorsal 

 part of all the six arches the rudiments of efferent arteries 

 appear, and connect with a pair of dorsal aorta3 on either 

 side the notochord, which farther back unite into a median 

 aorta, ventral to the notochord. 



So far we have a general resemblance to the vascular 

 system of Amphioxus, allowing for thesmall number of visceral 

 arches. But in the frog-embryo the aorta gives off branches 

 to the mesoblastic somites and somatic mesoblast as well as 

 to the splanchnic, and the paired aortae in front are con- 

 tinued forwards as carotid arteries to the head. To return 

 the blood to the heart from head and somatic mesoblast 

 paired anterior and posterior cardinal veins, like those of the 

 dogfish, are formed. These meet on either side just behind 

 the last visceral arch, and a pair of iCuvierian veins run 

 transversely from their junction to the heart. Thus even 

 at this early stage the frog-embryo is nearer the dogfish 

 than Amphioxus, as regards its vascular system. 



12. The Pronephros and its Duct. In Amphioxus 

 we saw that, though at their first development all metameric 

 members had the same metamerism each gill-slit corre- 

 sponding to a myomere, and so on in the adult the 

 metamerism of the gill-slits and nephridia, on the one 

 hand, was quite independent of the metamerism of the 

 myomeres, nerves, and gonads, on the other, although these 

 were all present in the same (pharyngeal) region of the 

 body. In the embryo frog we find the metamerism of the 

 myomeres (i.e. the somites) restricted to the post-pharyngeal 

 region, while that of the gill-slits and visceral arches, 



