The chief object in making the figure is to show the five gill clefts (_g¹⁻⁵_). The fifth cleft, though small and probably not open to the exterior, is quite distinct in this embryo. The writer would feel more doubt of its being a true, though rudimentary, gill cleft had not Clarke (17) found a fifth pair of clefts in his material. The nasal pit has advanced in development somewhat and shows the beginning of the groove that connects it with the mouth. In this figure the crescentic hyomandibular cleft shows its connection with the groove between the mandibular and the hyoid folds. STAGE XVI FIGURE 19 (PLATE XXIV.) This embryo is only slightly more developed than the preceding. Body flexure is so great that the forebrain and tail nearly touch. Only the anterior three gill clefts are visible. The maxillary process (_mx_) is longer and more narrow; the mandibular fold has not changed appreciably. The nasal pit (_n_) is now connected by a distinct groove with the stomodæum. The appendages have increased in size, the posterior (_pa_) being the longer. The anterior appendage (_aa_) is distinctly broadened to form the manus, while no sign of the pes is to be seen at the extremity of the posterior appendage. The heart (_ht_) is still very prominent. The stalk of the umbilicus (_u_), which is quite narrow, projects from the ventral wall in the region between the heart and the hind legs. The tail is of considerable length and is closely coiled. STAGE XVII FIGURES 20-20_j_ (PLATES XXV., XXVI.) The superficial changes noted in this stage chiefly concern the head, which has increased considerably in length (Fig. 20). The curvature of the body is slightly more marked, and the tail is more tightly coiled at the end. There are still signs of three gill clefts. The maxillary process (_mx_) is long and narrow, while the mandibular arch (_md_) is still short and broad. The fronto-nasal region has greatly increased and has the aquiline profile noted by Clarke. The nasal groove has disappeared, and there remains the small opening (_n_) at the side of the fronto-nasal region, near the end of the still separate maxillary process. The umbilicus is in about the same condition as in the preceding stage, but the heart is less prominent. The outline of the manus (_ma_) is more definite, and the extremity of the posterior appendage is distinctly flattened out to form the rudimentary pes (_pe_). The position of the elbow-joint in the anterior appendage is seen at the end of the reference line _aa_. Typical transverse sections of this stage are shown in Figures 20_a_-_j_.