io8 Vertebrate Embryology 



hind, but is in the middle still widely open to the yolkj) 

 below. Though the axial parts of the embryo have be- 

 come thickened by growth, the body- walls are still thin; 

 in them, however, is seen the cleavage of the mesoblast, 

 and the divergence of the somatopleure and splanch- 

 nopleure. The splanchnopleure both at the head and 

 at the tail is folded in to a greater extent than the so- 

 matopleure, and forms the still wide splanchnic stalk. 

 At the end of the stalk, which is as yet short, it bends 

 outwards again and spreads over the surface of the yolk. 

 The somatopleure, folded in less than the splanch- 

 nopleure to form the wider somatic stalk, sooner bends 

 round and runs outwards again. At a little distance 

 from both the head and the tail it is raised up into a 

 fold, a/, af, that in front of the head being the highest. 

 These are the amniotic folds. Descending from either 

 fold, it speedily joins the splanchnopleure again, and the 

 two, once more united into an uncleft membrane, extend 

 some way downwards over the yolk, the limit or outer 

 margin of the opaque area not being shewn. All the 

 space between the somatopleure and the splanchnopleure, 

 //, is shaded with dots. Close to the body this space 

 may be called the pleuroperitoneal cavity; but outside 

 the body it runs up into either amniotic fold, and also 

 extends some little way over the yolk. 



"Z? represents the tail end at about the same stage on a 

 more enlarged scale, in order to illustrate the position of 

 the allantois al (which was for the sake of simplicity 

 omitted in 6'), shewn as a bud from the splanchnopleure, 

 stretching downwards into the pleuroperitoneal cavity//. 

 The dotted area representing as before the whole space 

 between the splanchnopleure and the somatopleure, it is 



