190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



tectum (Fig. 40, ax.') and, passing laterad to the flexure in the tectum, 

 run cauilad. A tract which I believe is formed from one set of these 

 neuiites has been traced, somewhat discontinuously, through the lateral 

 part of the tectum into the cerebellum. The other set of neurites prob- 

 ably runs to the middle layer of the tectum, as traced in other forms. 



2. Lepidosteus osseus. The material of this species at my disposal 

 has been limited to youug larvae from 1 to 2 cm. long. The optic reflex 

 apparatus is in its development and arrangement in all essential respects 

 similar to that of Amia. Some differences, however, are to be noted. At 

 the time of hatching, the development of the apparatus is somewhat 

 farther advanced than in Araia, though the nervous system as a whole is 

 less developed. As in Amia, the cells lie at the anterior end of the tectum, 

 but are less numerous and smaller. In larvae 15 cm. long they are 

 about 10 micra in diameter. The cells are multipolar (Plate 6, Fig. 44), 

 giving off" usually two principal processes in addition to the axon, which 

 runs obliquely ventrad and cephalad and emerges into the ventricle in 

 the median plane in the region just posterior to where the posterior 

 commissure later develops. The axons, after passing into the ventricle, 

 are deflected obliquely backward and anastomosing form a plexus from 

 which Eeissner's fibre develops (Fig. 44, ax. cl. td. rfx.). 



Reissner's fibre is a little greater in diameter tlian in Amia of the 

 same size (1.2 micra as against 1 micron in Amia larvae 15 mm. long). 

 The branches from the fibre which later enter the walls of the cord in 

 its posterior portion are also correspondingly larger, so that Lepidosteus 

 is favorable material in which to study these structures. They run off 

 from the fibre obliquely caudad, and for the most part enter the cord at 

 the ventral sides of the canal (Plate 6, Fig. 45). These branches prob- 

 ably represent the individual axons which make up Eeissner's fibre. 

 They pass between the cells of which the wall of the cord is composed, 

 and are lost. 



The posterior canal-cells (Fig. 4G) are present as in Araia, but prob- 

 ably fewer in nvimber. I have fmnd them only in the ventriculus ter- 

 minalis. Tliey are multipolar and send off their processes to all sides 

 of the canal and into the cord. 



3. PoLYPTERUS BicHiR. My studies on this species are based on a 

 number of adult brains taken and prepared by the late Dr. N. R. Har- 

 rington while on the Senff" Zoological Expedition to the Upper Nile. 

 The marked peculiarity of this species is that the cells of the optic re- 

 flex apparatus lie not only dorsal and posterior to the posterior commis- 

 sure, as is normal in other forms, but also ventral and anterior to it, so 



