Figure 20_a_ is a section through the middle region of the head, cutting the hindbrain on one side and the forebrain on the other. The walls of the brain show rather more histological differentiation than was seen in the preceding sections, though this cannot be shown under the low magnification used. The hindbrain (_hb_), which is cut near its anterior border, exhibits the usual membranous dorsal and thick ventral walls. The forebrain is here seen as three distinct cavities--a median third ventricle (_tv_), with a thick ventral wall, and a thin dorsal wall extended to form a large paraphysis (_epi_), and two lateral ventricles (_ch_), the cavities of the cerebral hemispheres, whose walls are quite thick except on the side next the third ventricle. The sections of this series being slightly oblique, the eye is here cut on the right side only, where it is seen as a large, semicircular cavity (_e_) with thick, dense walls. The mesoblast, in which several blood-vessels (_bv_) are seen, exhibits three distinct areas--a median, lighter zone, with a more dense area on either side. The significance of this variation in the density of the mesoblast is not apparent. Figure 20_b_ is only a few sections posterior to the section just described. It is drawn chiefly to show the appearance of the forebrain, the other structures being about as in the preceding figure, except that both eyes (_e_) are here represented. The section passes through the wide opening between the third (_tv_) and the lateral ventricles (_ch_) and cuts the anterior edge of the pineal body[8] (_epi_). The paraphysis is very large and is directed backward instead of forward, as is usually the case among the lower vertebrates (if the alligator may be so classed). It is shown in Figure 17_a_ of a preceding stage and will be again shown in a sagittal section to be described later. The same areas of more dense and less dense mesoblast noted in the preceding figure are seen here. [8] Subsequent investigation showed that the structure here described as the pineal body is, in reality, the paraphysis; the pineal body is absent in _A. mississippiensis_.