II.] 



THE HEAD-FOLD. 



29 



Fig. 8. 





--y 



-rKP C 



*i 



B 



D 



Fig. 8, ^4 to .2V forms a series of purely diagrammatic representations in- 

 troduced to facilitate the comprehension of the manner in which the body of 

 the embryo is formed, and of the various relations of the yolk-sac, amnion 

 and allantois. 



In all vt is the vitelline membrane, placed, for convenience sake, at some 

 distance from its contents, and represented as persisting in the later stages ; in 

 the actual egg it is in direct contact with the blastoderm (or yolk), and early 

 ceases to have a separate existence. In all e indicates the embryo, pp the 

 general pleuro-peiitoneal space, af the folds of the amnion, a the amnion proper, 

 ae or ae the cavity holding the liquor amnii, al the allantois, a the alimentary 

 canal, y or ys the yolk or yolk-sac. 



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 blastoderm 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 correspond- 

 ing to the head-fold on the right, while therefore the front limit of the embryo, 

 as distinguished from the blastoderm, is marked out by the head-fold, there 

 is at present no tail-fold, and therefore no hind limit. 



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 there- 



