32 EARLY AMPHIBIAN DEVELOPMENT 



while the Hmbs arise (early in Urodela, much later in Anura) as 

 little thickenings which rapidly become conical and continue to 

 elongate by growth. 



As regards the internal development, the dorsal portions of the 

 mesoderm of the trunk and the mesoderm of the tail become 

 metamerically segmented, and give rise to the myotomes or muscle 

 plates. These myotomes are at the outset connected with the meso- 

 dermal lining of the general coelomic cavity by short stalks, called 

 intermediate cell-masses or nephrotomes, which, like the myotomes, 

 are segmental in arrangement. From some of these stalks, out- 

 growths are formed, ultimately giving rise to the tubules of the 

 kidney, and from these tubules a duct (the pronephric duct) grows 

 back on each side into the proctodaeum, which from now on can 

 be styled the cloaca. 



The heart arises beneath the anterior part of the gut in the mid- 

 ventral line, but the rudiments which form its muscular wall (parts 

 of the splanchnic layer of coelomic epithehum) are at first paired. 

 When they have fused together in the middle line, these rudiments 

 roll up along the longitudinal axis of the embryo to form a tube, 

 suspended by a mesentery (strictly, mesocardium) from the dorsal 

 wall of the coelomic cavity, w^hich in this region takes the name of 

 pericardial cavity. Within the tube thus formed, some cells are en- 

 closed which will give rise to the lining or endothelium of the heart. 

 Originally these cells lay scattered irregularly between the floor of 

 the gut and the splanchnic layer of coelomic epithelium, whence 

 they arose. 



The gut-cavity still contains a considerable quantity of yolk-cells, 

 and these are heaped up and occupy most of the central and hinder 

 parts of the gut, being piled up high on the floor, and reducing the 

 actual free cavity to modest dimensions. Just behind the region of 

 the heart, and in front of this mass of yolk-cells, a downgrowth is 

 formed from the floor of the gut. This is the rudiment of the liver : 

 its cavity will eventually develop into the lumina of the liver 

 tubules and gall-bladder, while the connexion with the gut persists 

 as the bile-duct. A lengthening of the gut takes place in the region 

 immediately in front of the. cloaca, and this gives rise to the in- 

 testine, which later becomes coiled on itself like a watch spring. 



By these processes of stretching, displacement, folding, and 



