. The Journal of comparative neurology and psychology. um and the greatest numberof cells is found at the posterior part of the tectum. In Scylliumstellare there is an especially large group which extends into themedullary velum so that several cells lie beneath and behind thecrossing fibers of the IVth nerve. In Acipenser the cells are to befound only in the anterior part of the tectum, the majority of themin the immediate vicinity of the posterior commissure (Johnston,1901). In Araia embryos and young up to 25 mm. in length, the Johnston, The Radix Mesencephahca Trigennni. 603 cells have the
. The Journal of comparative neurology and psychology. um and the greatest numberof cells is found at the posterior part of the tectum. In Scylliumstellare there is an especially large group which extends into themedullary velum so that several cells lie beneath and behind thecrossing fibers of the IVth nerve. In Acipenser the cells are to befound only in the anterior part of the tectum, the majority of themin the immediate vicinity of the posterior commissure (Johnston,1901). In Araia embryos and young up to 25 mm. in length, the Johnston, The Radix Mesencephahca Trigennni. 603 cells have the same position. In the youngest embryo that I havestudied the cells are seen in the thin portion of the tectum behind theposterior commissure and in 25 mm. specimens in which this portionof the tectum bulges forward over the posterior commissure the cellsimmediately surround the blind pouch thus formed. In one specimen35 cells were counted, 28 of which were closely packed ,above theposterior commissure, the other 7 scattered laterally and dorsally in. Fig. 5. Cells of origin of the radix mesencephalica in the rabbit. A, frominferior colliculus; B, caudal to n. IV; C, seven cells from different regionsbrought together; D, a bipolar cell near n. IV. x l^- the near vicinity. In Catostomus (young) the cells have a similararrangement, the great majority being grouped around the anteriorend 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 wholelength of the tectum and the largest collection is in the posteriorregion; in ganoids and teleosts the cells are fewer, are confined tothe anterior half of the tectum and are collected chiefly at the 6o4 Journal of Comparative Neurology and Psychology. anterior border. In fishes I have seen none of the large cells alongthe course of this bundle caudal to the velum medullare Necturus the cells a
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