538 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. 



to the inner true amnion and the outer false amnion or serous 

 membrane : the former continuous at its inner edge with the 

 somatopleure of the embryo's body, the latter at its outer edge 

 with that of the blastoderm. It will be readily seen that the space 

 between true and false amnion is morphologically part of the 

 splanehnocoele. It will also be realized that both true and false 

 amnion being somatopleural in nature are' composed of ectoderm 

 and somatic mesoderm but that the relative position of these two 

 layers is reversed in the amnion as compared with the false amnion. 



Important changes have taken place in the mesoderm. The 

 mesoderm segment is no longer connected with the lateral mesoderm 

 the nephrotome having become converted into renal structures — the 

 archinephric duct and mesonephric tubules. The relations of these 

 will be understood by referring back to the general description of renal 



ent. 



N. eel 



v.v. 



Fig. 237b. — Transverse section just behind the point of union of the two vitelline veins. 



A, dorsal aorta ; am, amnion ; c, conns arteriosus ; ect, ectoderm ; ent, enteron ; /.a, false amnion or 

 serous membrane ; my, myotome ; N, note-chord ; s.c, spinal cord ; sa, sero-anmiotic isthmus ; som, 

 somatopleure ; spl, splanchnopleure ; spJc, splanehnocoele ; V, ventricle ; v.v, vitelline veins ; y, yolk. 



organs in Chapter IV. (p. 254). The inner wall of the segment has 

 lost its epithelial character and broken up into a mass of actively 

 proliferating mesenchyme cells. Many of these cells will wander 

 away in amoeboid fashion and settle down round notochord and 

 spinal cord to form the protective sheath in which eventually 

 develops the vertebral column. Collectively these amoeboid cells 

 constitute the sclerotome which is therefore much more diffuse in 

 its origin than in the lower vertebrates illustrated on p. 285. 



Certain blood-vessels are visible in the section. In the splanchnic 

 mesoderm of the yolk-sac numerous vessels of the vitelline network 

 are visible : over the mesonephros may usually be seen the posterior 

 cardinal vein, while on each side of the mesial plane ventral to the 

 notochord are the two dorsal aortae. 



As the series of sections is traced towards the head the most con- 

 spicuous change is the increasing asymmetry due to the body of the 

 embryo coming to lie over more and more upon its left side. Eig. 237b 

 represents a section just behind the posterior limit of the foregut. 



