The notochord is apparently of entodermal origin, though in the posterior regions, where the germ layers are continuous with each other, it is difficult to decide with certainty. The medullary folds have a curious origin, difficult to explain without the use of figures. They are continuous posteriorly with the primitive streak, so that it is impossible to tell where the medullary groove ends and the primitive groove begins, unless the dorsal opening of the blastopore be taken as the dividing point. The amnion develops rapidly, and entirely from the anterior end. The blastopore or neurenteric canal is a very distinct feature of all the earlier stages up to about the time of closure of the medullary canal. Preceding the ordinary cranial flexure there is a sort of temporary bending of the head region, due apparently to the formation of the head-fold. During the earlier stages of development the anterior end of the embryo is pushed under the surface of the blastoderm, and is hence not seen from above. Body torsion is not so definite in direction as in the chick, some embryos lying on the right side, others on the left. Of the gill clefts, three clearly open to the exterior and probably a fourth also. A probable fifth cleft was seen in sections and in one surface view. The first trace of the urinary system is seen as a dorsally projecting, solid ridge of mesoblast in the middle region of the embryo, which ridge soon becomes hollowed out to form the Wolffian duct. The origin of the hypophysis and paraphysis is clearly seen; the latter projects backward. No connection can be seen between the first rudiments of the sympathetic nerves and the central nervous system. The lumen of the œsophagus is for a time obliterated as in other forms. The choroid fissure is a very transitory but well-marked feature of the eye. LETTERING FOR ALL FIGURES ON PLATES VI.-XXVIII.