254 FISH. [Vol. X. 



epiphysis in the adult is quite small, appearing like a very 

 much depressed button or pad, lying just caudal to the supra- 

 commissure. There is usually no trace of a lumen further 

 than a small central accumulation of endymal cells. In a 

 1 7-mm. larva there was a distinct cavity, but on account of the 

 inaccessibility of earlier stages than this, I was not able to 

 determine an actual continuity of its cavity with the diacoele. 

 The postcommissure is caudal to the epiphysis, and, as has 

 been suggested by Osborn (37), may be regarded as a line of 

 demarcation between the diencephal and mesencephal. 



Mesencephal. — This segment is subspherical in outline, as 

 seen from the dorsal aspect; the cavity enlarges laterally, but 

 there are no indications of a division into the optic lobes, so 

 characteristic of the Anura. The entocinerea of the roof is 

 well developed ; in some young specimens examined, the cells 

 extended to the ectal surface at the dorsal median line. In 

 older specimens there is a considerable layer of alba between, 

 in which are scattered a few nerve cells. The cinerea, however, 

 is divisible into two nearly equal layers by the interpolation of 

 a thin stratum of alba; on the meson it is scarcely perceptible, 

 but laterally it is well developed, and extends from the post- 

 commissure nearly to the cerebellum, and is possibly significant 

 as separating the cells into an ecto- and entocinerea. Osborn 

 has found eight different layers in this region in Rana. 



Epencephal. — The cerebellum appears to be the most differ- 

 entiated part of this segment. It is not at all apparent from 

 ectal observation, but lies bent up under the caudal margin of 

 the mesencephal. There are a few cells along its cephalic 

 face ; it is constricted at the meson, is made up chiefly of 

 fibers, and is undoubtedly commissural in function as stated by 

 Osborn (38). He discriminates between fine and coarse fibers ; 

 the former he believes to be in part decussating tracts of the 

 auditory nerve, and the latter non-decussating descending tracts 

 of the trigeminus nerve. Some support is given to the view 

 of connection with the auditory nerve by the observations of 

 Koppen and Ahlborn. There is nothing in the oblongata to dif- 

 ferentiate any portion of it as belonging to this segment, unless 

 it be the origins of the fifth, sixth, seventh, and eighth nerves. 



