lined by a sharply defined epithelium of very long columnar cells, totally different in appearance from the epithelium which lines the remainder of the brain-cavity. The inner margins of the two grooves in this region touch one another in the middle line. Their lumina are deeply concave and open widely into the brain-cavity, which is here represented by a rather narrow vertical slit, terminating l>elow
FIG. 2.
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ill the infundibulum (fig. 1, Inf.). Thus the cross-section of the two ciliated grooves lying beneath the posterior commissure has the form of the figure o . Their lining epithelium, as already pointed out, is conspicuously different from the lining epithelium of the brain-cavity elsewhere. It is composed of narrow columnar cells with conspicuous nuclei (fig. 2). While very short at the margins of the grooves, these cells gradually increase in length towards the middle, so that lining epithelium is very much thicker in the middle of each groove than it is at the two edges. The inner surface of each groove i covered by a thick coating of very short cilia. The transition from
