AMPHIBIA. 227 



layers of the two sides become continuous ventrally, the split 

 between them forming the body-cavity. The mesoderm on each 

 side becomes divided, posterior to the head, into two longitudinal 

 parts. The upper of these, the vertebral plate, lies alongside the 

 neural tube and notochord, and becomes divided transversely 

 into mesoderm ic somites (protovertebrne). The lower part is known 

 as the lateral plate, the somatic mesoderm of which unites with 

 the ectoderm to form the somatopleure (wall of body), while its 

 splanchnic layer unites with the endoderm to form the splanch- 

 nopleure (wall of alimentary canal). Both mesodermic somites 

 and lateral plate are made up of somatic mesoderm externally, 

 and splanchnic mesoderm internally. 



The dermis is developed from somatic mesoderm, mostly from 

 that of the lateral plates. The axial endoskeleton is formed 

 from splanchnic mesoderm, that of the mesodermic somites giving 

 rise to the vertebral column, which, with the hinder part of the 

 base of the brain-case, is moulded round the notochord. 



The visceral arches become supported by rods of cartilage, 

 similarly named. Their fate is as follows : 



I. Mandibular Suspensoria and cartilaginous basis of the jaws. 

 II. Hyoid, \ /Columellae and anterior 



III. Branchial 1, 



IV. Branchial 2, 

 V. Branchial 3, 



VI. Branchial 4, 



Fuse ventrally to 

 form hyoid ap- 

 paratus. 



cornua. 



f Anterior angles of 

 | hyoid plate. 

 Posterior angles. 

 Posterior cornua. 



The rest of the skeleton is also mesodermic. 



The alimentary canal and its outgrowths are formed (except 

 the epithelium) from the splanchnic layer of the lateral plate. 



Important changes take place in the circulatory organs in con- 

 nection with the successive appearance of gills and lungs. The 

 heart at first possesses an undivided auricle, and pumps impure 

 blood to the gills by afferent branchial arteries corresponding to 

 the last four visceral arches (branchial arches), and breaking up 

 into capillaries. After the blood has been oxygenated in these 

 gill-capillaries it passes into efferent branchial arteries, which open 

 above into a loop from which the dorsal aorta arises. These 

 vessels may be numbered 3, 4, 5, 6, like the visceral arches with 



