642 EMBRYOLOGY : FROG 



venosus and carry to the heart the food matter derived from the 

 yolk. The venous system is at first arranged on the same plan 

 as in the dogfish, with two ductus Cuvieri and anterior and 

 posterior cardinal veins. Subsequently the posterior cardinal 

 veins disappear and are replaced by the inferior vena cava, the 

 ductus Cuvieri becoming the superior venae cavae, and the anterior 

 cardinals the internal jugulars (Fig. 468). The vitelline veins 

 are transformed into the hepatic portal system and hepatic 

 veins. The arterial system of the tadpole closely resembles that 

 of a fish. The conus arteriosus leads into a long ventral aorta, 

 from the end of which arise four vessels to the branchial arches 

 and traces of similar vessels in the hyoid and mandibular arches. 

 From the gill capillaries there arises in each arch an efferent 

 vessel which discharges into a longitudinal lateral dorsal aorta. 

 The two of these, which are the earliest arteries to appear, join 

 behind to form the median dorsal aorta. In front they are con- 

 tinued as the internal carotids. In the presence of a single efferent 

 vessel in each arch and of the two suprabranchial arteries, the 

 tadpole, while it differs from the dogfish, resembles certain other 

 fishes (p. 600). When the lungs are formed, a vessel to supply 

 each of them arises from the fourth efferent branchial vessel of 

 the same side. Before the gills are lost, direct communication 

 is established between the afferent and efferent vessels, so that 

 when the gill capillaries disappear blood can pass direct from 

 ventral to dorsal aorta through four continuous aortic arches. 

 After the loss of the gill capillaries certain parts of the four arches 

 disappear, while other parts persist and become the great arteries 

 of the adult. The general fate of these in vertebrates is discussed 

 in Chapter 27. 



EXCRETORY AND GENERATIVE ORGANS 



The rudiment of the excretory system arises before hatching, 

 as a longitudinal thickening of the somatic mesoderm on each 

 side at the front end of the coelom, immediately below the 

 myotomes. The mesoderm here is the intermediate cell mass. In 

 many animals it is composed of nephrotomes, one for each myo- 

 tome. Its front part gives rise to the pronephros (p. 615), which 

 consists of three twisted tubules each opening into the coelom 

 (nephrocoele) of the intermediate cell mass. Opposite the openings 

 (nephrocoelomostomes) a sacculated outgrowth of the splanchnic 



