AVES. 



I8 7 



Incidentally it shews the formation of the medullary groove by the rising up 

 of the lamince dorsales. Beneath the section of the groove is seen the rudiment of 

 the notochord. On either side a line indicates the cleavage of the mesoblast just 

 commencing. 



In C, which represents a vertical longitudinal section of later date, both head-fold 

 (on the right) and tail-fold (on the left) have advanced considerably. The alimentary 

 canal is therefore closed in, both in front and behind, but is in the middle still widely 

 open to the yolk y below. Though the axial parts of the embryo have become 

 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 splanchnopleure. 

 The splanchnopleure both at the head and at the tail is folded in to a greater extent 

 than the somatopleure, 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 splanchnopleure 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, af, af, that in front of the 

 head being the highest. These are the amniotic folds. Descending from either fold, 



H 



