1 86 



HKTAI, MEMBRANES. 



is gradually drawn backwards over the developing head of the 

 embryo. The fold thus covering the head is in due time 

 accompanied by similar folds of somatopleure, starting at some 



A / 



-/- pj* 



FIG. 121. 



Fig. 121 A to N forms a series of purely diagrammatic representations introduced 

 to facilitate the comprehension of the manner in which the body of the embryo is 

 formed, and of the various relations of the yolk-sack, 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 reality it is 

 in direct contact with the blastoderm or yolk, and early ceases to have a separate 

 existence. In all e indicates the embryo proper; pp the general pleuroperitoneal 

 space with its extension between the membranes; of the folds of the amnion; a the 

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

 alimentary canal ; y or 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. 





