Figure 17_b_ is also through the pharyngeal region, a short distance behind the preceding section. The growth of the cerebral hemispheres (_ch_) is better shown than in the preceding figure, as is also the general form of the optic cup (_oc_). On the left the nasal cavity (_n_) is seen as an elongated slit with thick walls; it is cut near, but not through, its opening to the exterior. The same gill cleft (_g_) that was seen in the preceding figure is seen here as a narrow, transverse cleft, open at both ends. Between the notochord (_nt_) and the spinal cord (_sc_) is the same, though now double, blood-filled vessel (_bv_) that was seen in the preceding section. The other blood-vessels are larger here than in the more anterior region. There is a faint condensation of mesoblast in the neighborhood of the notochord, and a more marked condensation (_mp_) farther toward each side is the curiously shaped muscle plate. Figure 17_c_ is through the heart region, and that organ is cut through the opening from the lower or ventricular into the upper or auricular chamber. The thickening of the wall of the ventricle, which was noticed in the preceding stage, has increased to such an extent that there is now a marked difference in the thickness of the ventricular and auricular walls. As in the preceding stage, the body wall is torn, probably in handling, so that it appears to be incomplete around the ventral side of the heart. Dorsal to the heart two small circular holes (_ent_) with thick walls are the œsophagus and trachea, cut anterior to the point of bifurcation of the latter into the bronchial or lung rudiments. On either side of these structures is an elongated blood-vessel (_ac_), the anterior cardinal vein, its elongation being due to the fact that it is cut at the place where it turns downward to empty into the heart. Dorsal to the œsophagus are the aortæ (_ao_), which are here cut just at the point where the two vessels unite to form one; the next section, posterior to the one under discussion, shows an unpaired aorta. The notochord (_nt_) and spinal cord (_sc_) need no description, except to note that the latter shows active histological differentiation, numerous mitotic figures being seen under higher magnification, especially in the cells that line the spinal canal. On the right of the cord the edge of a spinal ganglion (_sg_) is seen, in connection with which in other sections are seen the clearly defined nerve roots. The condensation of mesoblast around the notochord is quite evident, and in close contact with this medial condensation are two very characteristic, S-shaped muscle plates (_mp_), which extend from the level of the dorsal side of the spinal cord to the upper limits of the cardinal veins. In some sections the muscle plates even yet show slight remains of the myocœl at the dorsal end.