512 



ker, F.R.S. — "Amraocoetes stage ofGeotria — Opoho Creek. Jany., 

 1884. Chromic and Osmic." It had evidently been preserved yviih the 

 utmost care, and proved, in spite of its age, to be in an admirable state 

 for histological investigation. 



The total length of the specimen was about 57 mm., and the great- 

 est diameter in the middle 3,5 mm. With a view especially to the 

 study of the parietal organs, the head region was cut into a series of 



transverse sections by the 

 ^Ch.pi usual paraffin method, and the 



sections were stained on the 

 slide ; for staining, carbol- 

 fuchsin proved satisfactory. 



On examination , the 

 sections thus prepared showed 

 not only a well-developed pa- 

 rietal eye and the structures 

 usually associated there-with, 

 but also a pair of conspicuous 

 ciliated grooves lying in the 

 roof of the brain inthe neigh- 

 bourhood of the posterior 

 commissure. These grooves 

 run longitudinally from the 

 recessus sub-pinealis to the 

 hinder margin of the posterior 

 commissure. They are most 

 conspicuous beneath the commissure itself (figs. 1, 2), in which region 

 they are 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 below in 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 CO . Their lining epithelium, as already pointed out, is con- 

 spicuously 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 the 

 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 is covered 



Fig. 1. Transverse section of the brain of the 



New Zealand Ammocoete through the region 



of the posterior commissure. 



