In L the splanchnopleure has completely invested the yolk-sack, but at the lower pole of the yolk is still continuous with that peripheral remnant of the somatopleure now called the serous membrane. In other words, cleavage of the mesoblast has been carried all round the yolk (_ys_) except at the very lower pole. In M the cleavage has been carried through the pole itself; the peripheral portion of the splanchnopleure forms a complete investment of the yolk quite unconnected with the peripheral portion of the somatopleure, which now exists as a continuous membrane lining the interior of the shell. The yolk-sack (_ys_) is therefore quite loose in the pleuroperitoneal cavity, being connected only with the alimentary canal (_a´_) by a solid pedicle. Lastly, in N the yolk-sack (_ys_) is shewn being withdrawn into the cavity of the body of the embryo. The allantois is as before, for the sake of simplicity, omitted; its pedicle would of course lie by the side of _ys_ in the somatic stalk marked by the usual dotted shading. It may be repeated that the above are diagrams, the various spaces being shewn distended, whereas in many of them in the actual egg the walls have collapsed, and are in near juxtaposition.] [FIG. 122. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN EMBRYO. The section is supposed to be made at a time when the head-fold has commenced but the tail-fold has not yet appeared. _F.So._ fold of the somatopleure. _F.Sp._ fold of the splanchnopleure; _D._ foregut. _pp._ pleuroperitoneal cavity between somatopleure and splanchnopleure; _Am._ commencing (head) fold of the amnion. For remaining reference letters _vide_ p. 167.]