138 EMBRYOLOGY 



Opposite the nephrostome the dorsal aorta sends arterioles into a pocket 

 of the splanchnic mesoderm to form a vascular body, the glomus (glomeru- 

 lus). Vessels leaving the glomus enter a branch of the posterior cardinal 

 vein. The posterior cardinal vein breaks up into smaller vessels as it passes 

 around the pronephric tubules. A system such as that diagrammed in Figure 

 79 would allow excretory products from the blood to pass from the dorsal 

 aorta to the glomus, thence into the coelom to be drawn into the nephrostome 

 by ciliary action, and thus through the convoluted pronephric tubule. Possibly 

 at this point certain substances are taken back into the blood stream by the 

 postcardinal vein. In any case the excretions are collected by the pronephric 

 duct and conveyed to the cloaca and out through the cloacal aperture. Later 

 the pronephros degenerates and its function is replaced by the mesonephros, 

 which is the permanent kidney in the adult. Further development of the ver- 

 tebrate kidney is treated in Chapter 14, which deals with the mesonephros 

 and metanephros of the chick. 



In the same section (Fig. 79) the spinal cord and neural crest, which give 

 rise to the spinal ganglia of the limb nerves, are present. The somites begin 

 their differentiation into dermatome, myotome, and sclerotome, and the 

 myotome begins to form muscles, which are able to contract at hatching 

 (stage 20). The sclerotome migrates around the spinal cord and notochord 

 and later forms vertebrae. The dermatome forms the inner layers of the skin 

 (dermis). 



By this time the ventrolateral growths of the mesoderm, last shown in 

 Figure 71, have met at the midventral line and have fused. Thus the coelom 

 is continuous, and two distinct layers are seen; — the somatic layer of mesoderm 

 (sometimes called somatopleure in the amphibian embryo but not usually so 

 designated in the chick embryo) and the splanchnic layer (similarly called 

 splanchnopleure). The gut is beginning to differentiate from the yolk mass, 

 and in more anterior sections a pocket representing the primordium of the 

 liver is present ventrally. 



At this stage (20) the frog embryo presents an organization which is 

 typical of vertebrate embryos, and we do not propose to follow internal 

 changes any farther in this species. The complicated internal changes are best 

 studied by following the development of the various systems in the body, 

 and for this study we shall use the chick embryo, which has been studied very 

 thoroughly. However, at this point it is interesting to complete the story of 

 frog development by considering some of the major changes that take place 

 between stage 20 and the formation of the young adult. 



