The chief advance in development over the preceding stage, besides the formation of the maxillary process, is in the appearance of the appendages (_aa_ and _pa_); they have the characteristic shape of the rudimentary vertebrate appendage, though the anterior pair seem to point in an unusual direction at this stage and to be slightly more developed than the posterior. The curious, anteriorly directed heart (_ht_) is, perhaps, somewhat abnormal. The umbilical stalk (_u_) is comparatively narrow and, like the allantois, was cut off close to the body. Transverse sections of an embryo of this stage are represented in Figures 17_a_-_g_, drawn under a lower magnification than were any of the preceding figures. Figure 17_a_ is in the region of the pharynx, and passes through the forebrain (_fb_) and posterior part of the hindbrain (_hb_). In the thick walls of both of these structures histological differentiation has begun, so that even under low power an inner granular and an outer clear zone may be distinguished. Under greater magnification the presence of short fibers may be made out among the cells. The cerebral hemispheres (_ch_) are well-marked structures, their asymmetry being of course due to the obliquity of the section. Only one eye is cut by the plane of the section, and this one shows no connection with the forebrain. The outer wall of the optic cup (_oc_) is so thin that under this magnification it can scarcely be seen as a dark line surrounding the retinal wall. The lens (_ln_) is now a solid mass, of the usual type for vertebrate embryos, its front or outer wall being a scarcely discernible line. The hindbrain (_hb_) has the usual form for that region and does not differ particularly from what was noted in earlier stages except in the histological differentiation that has already been mentioned. As with the eye, it is only on the right side that the auditory vesicle (_o_) is shown. It shows some differentiation, but not so much as would be seen were it cut in another region. In the center of the section the pharynx (_ph_) forms an irregular cavity connected with the exterior on the left by a gill cleft (_g_) and by another slit which is simply the anterior margin of the stomodæum. On the right neither of these openings is in the plane of the figure, though the gill cleft (hyomandibular), which lies close to the auditory vesicle, is almost an open passage. A few small blood-vessels are scattered through the section; one of these (_bv_), lying between the notochord (_nt_) and the floor of the brain, is noticeable from its being very closely packed with corpuscles, so that at first glance, under low magnification, it looks more like a nerve than a blood-vessel.