av:es. 



155 



amnion proper ; ae or ac the cavity holding the liquor amnii ; al the allantois ; a' the 

 alimentary canal ; y ox ys the yolk or yolk-sack, 



A, which may be considered as a vertical section taken longitudinally along the 

 axis of the embryo, represents the relations of the parts of the egg at the time 

 of the first appearance of the head-fold, seen on the right-hand side of the embryo e. 

 The blastoderm is spreading both behind (to the left hand in the figure), and in front 

 (to right hand) of the head fold, its limits being indicated by the shading and 

 thickening for a certain distance of the margin of the yolk y. As yet there is no fold 

 on the left side of e corresponding to the head- fold on the right. 



B is a vertical transverse section of the same period drawn for convenience sake on 

 a larger scale (it should have been made flatter and less curved). It shews that the 

 blastoderm (vertically shaded) is extending laterally as well as fore and aft, in fact 

 in all directions ; but there are no lateral folds, and therefore no lateral limits to the 

 body of the embryo as distinguished from the blastoderm. 



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

 laminae dorsales. Beneath the section of the groove is seen the rudiment of the noto- 

 chord. 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 thedivergence 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 



H 



---m 



