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; _af_ 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. Incidentally it shews the formation of the medullary groove by the rising up of the laminæ 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.