Figure 13_b_, about forty sections posterior to Figure 13_a_, passes through the hindbrain in the region of the ears. Being back of the region affected by cranial flexure, this section is of course of much less area than the preceding. The ectoderm shows no unusual features; it is of uniform thickness except where it becomes continuous with the entoderm around the mandibular folds (_md_); there it is somewhat thickened. The most striking feature of the section is the presence of two large auditory vesicles (_o_). The section being not quite at right angles to this part of the embryo, the vesicles are not cut in exactly the same plane; the one on the left is cut through its opening to the exterior, while the one on the right appears as a completely enclosed cavity. In a section a short distance posterior to this one the appearance of the vesicles would be the reverse of what it is here. As may be seen in the figure, the vesicles are large, thick-walled cavities lying close to the lateral walls of the hindbrain. The hindbrain itself has the usual triangular cross-section, with thick lateral walls and a thin, wrinkled dorsal wall. Close to the ventral side of the hindbrain lies the notochord (_nt_), on each side of which, in the angle between the brain and the auditory vesicles, is a small blood-vessel (_bv_). Ventral to these structures and close to the dorsal wall of the pharynx (_ph_) are the two large dorsal aortæ (_ao_). The ventral side of the section passes through the open anterior end of the pharynx (_ph_). On the left is seen the widely open hyomandibular cleft (_g′_), between the main body of the section and the mandibular arch (_md_). On the right side the plane of the section was such that the hyomandibular cleft was not cut through its external opening. In each mandibular fold a large aortic arch (_ar_) is seen, and also a slight condensation of mesoblast, the latter probably being the forerunner of cartilage.