Figure 12_g_ is through the neurenteric canal (_nc_), a distinct opening through the floor of the spinal canal. The section is of course just back of the posterior end of the notochord. The entoderm (_en_) along the margin of the neurenteric canal is naturally continuous with the wall of the spinal cord (_sc_). The ectoderm (_ec_) is thicker than ever, except in the median plane, where it passes over the spinal cord. The mesoblast is more abundant than in the preceding figure, and shows on the left what appears to be a distinct myocœl (_myc_), though in surface view the mesoblastic somites do not extend this far toward the tail. STAGE X FIGURES 13-13_g_ (PLATES XIX., XX., XXI.) This embryo (Fig. 13) is about 5 mm. in length, and hence is slightly smaller than the preceding stage, though somewhat more advanced in development. The medullary canal is still _apparently_ unclosed for a short distance at the extreme posterior end; this appearance is probably due to the neurenteric canal (_nc_) and to the thinness of the roof of the medullary canal rather than to any lack of fusion of the medullary folds. The optic vesicle is more distinct than in the preceding stage; a somewhat similar, though smaller, opacity (_o_) marks the position of the ear. There are now about twenty pairs of somites, though it is difficult to determine their exact number on account of the torsion of the body. The amnion is at about the same stage of development as in Stage IX. The heart (_ht_) is a large double mass, whose outlines may be dimly seen when the embryo is viewed by transmitted light. The vitelline vessels (_vv_) are still but faintly outlined in the vascular area; the veins and arteries cannot yet be distinguished from each other. The gill clefts, though not visible externally in the embryo drawn, may be seen in sections of this stage as evaginations of the wall of the pharynx. The transverse sections of this stage are slightly more advanced in development than was the embryo that has just been described in surface view. Only those sections have been figured which show a decided advance in the development of some special structures over their condition in the preceding stage. The sections of the preceding stages were drawn under a magnification of eighty-seven diameters; those of this and the following stage were drawn under a magnification of only forty-one diameters. All of the figures have been reduced one half in reproduction.