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bulletin: museum of comparative zoology. 



tional during the first day of larval life. As might be expected, the 

 young fry are active and respond readily to optical stimuli. 



The morphology of the anterior portion of the mesencephalic roof in 

 this family calls for some description. The tectum opticum attains a 

 relatively enormous development both in area and thickness (Fig. F). 

 This is probably to be directly correlated with the activities of these 

 animals, and the consequent high value placed upon them as game fishes. 

 As the result of its increased development, relative to other parts of the 

 brain, the part which is cephalad and dorsad to the posterior commissure 



Fig. E. 



Fig. F. 



Figure E. Salvelinus fontinalis, larva, 25 mm. long. Transverse section through 

 the anterior extremity of the mesencephalon, showing the torus longitudinalis 

 enclosed between the halves of the tectum. 



Figure F. Salvelinus fontinalis, larva, 25 mm. long. Transverse section, caudad 

 of the preceding, through the posterior commissure. For meaning of abbrevi- 

 ations, see Explanation of Plates, p. 257. 



bulges forward (Plate 7, Fig, 51. See also Sargent, :03 *, Fig. 20). The 

 mesocoele is prolonged into an elongated conical recess extending dorsad 

 of the commissure (rec. ms'coel.'). Tlie tectum has so great a thickness 

 that it extends below the torus longitudinalis (Plate 8, Fig. 60), and in 

 Salmo and Salvelinus meets the mesal edge of the tectum of the opposite 

 side, so that the torus is enclosed between them (Figs. E, F). The two 

 mesal edges of the tectum are more closely applied in their anterior por- 



