The Development of the Chick 107 



and early ceases to have a separate existence. In all e 

 indicates the embryo, pp the general pleuroperitoneal 

 space, af the folds of 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-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 

 the 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 later- 

 ally 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. Be- 

 neath 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 alimen- 

 tary canal is therefore closed in, both in front and be- 



