in the Brain of the Ainmnccde. 



4.S7 



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. 



%*V?oc% c c 



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 



