Johnston, The Radix Mesencephahca Trigennni. 603 



cells have the same position. In the youngest embryo that I have 

 studied the cells are seen in the thin portion of the tectum behind the 

 posterior commissure and in 25 mm. specimens in which this portion 

 of the tectum bulges forward over the posterior commissure the cells 

 immediately surround the blind pouch thus formed. In one specimen 

 35 cells were counted, 28 of which were closely packed , above the 

 posterior commissure, the other 7 scattered laterally and dorsally in 



Fig. 5. Cells of origin of the radix mesencephalica in the rabbit. A, from 

 inferior colliculus ; B, caudal to n. IV ; C, seven cells from different regions 

 brought together ; D, a bipolar cell near n. IV. x l'''^- 



the near vicinity. In Catostomus (young) the cells have a similar 

 arrangement, the great majority being grouped around the anterior 

 end of the cavity of the tectum, a few being found along the mid- 

 dorsal line and an occasional cell placed farther laterally. In brief, 

 in selachians the cells are much more numerous, extend the whole 

 length of the tectum and the largest collection is in the posterior 

 region; in ganoids and teleosts the cells are fewer, are confined to 

 the anterior half of the tectum and are collected chiefly at the 



