538 EMBRYOLOGY OF THE LOWER VERTEBRATES Ctt 



to the inner true amnion and the outer false ai union 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 amnioii is morphologically part of the 

 splanchnocoele. 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 niesoderm. 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 



Y 



FK;. 237fi. Transverse section just behind the point of union of the two vitelline veins. 



A, dorsal aorta ; am, amnion ; c, conns arteriosus ; ect, ectoderm ; ?nt, enterun ; /.a, false, amnion or 

 si-rons membrane; my, myotome ; N, notochord ; s.c, spinal cord; i, seni-amniotic isthmus; smu, 

 somatopleme : /</, splanchnopleure ; splc, splanchnocoele ; V, ventricle ; v.c, vitelline veins ; y, yolk. 



orpins 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 inon- dill use in 

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



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

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

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

 cardinal \rin. while on cadi side of the mesial plane ventral to the 

 notochord are the two dorsal aorl 



As the series oi'sceii.ms is traced towards the head the most, con- 

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

 embryo coming to lie OV6I more and more II]M,H its left, side. l-'i-j-. L'.' 

 action just he.hind the posterior limit of the 



