28 Journal of Comparative Neurology. 



of the body stands at the outer surface of the tectum, and from 

 the inner end arises a dendrite which immediately divides into 

 a most compHcated interwoven series of branches. The den- 

 drites are short and their profuse division into small branches 

 gives them a very characteristic appearance. It would be im- 

 possible to represent the more complicated of these cells in a 

 drawing, that shown in Fig. 25 being extraordinarily simple. 

 The neurite arises from some of the larger branches of the den- 

 drite and breaks up in the near vicinity of the cell, but deeper 

 than the dendrites. These cells probably can not be directly 

 compared with any in the tectum of Acipenser. 



The typical structure of the tectum as made out from 

 GoLGi sections extends ventrally somewhat into the central 

 grey of the lateral wall of the mid brain. This area may be 

 compared wuth the torus semicircularis Halleri of the brain of 

 Acipenser, but its extent is not indicated in Petromyzon by 

 anything in the gross anatomy as in Acipenser. The whole 

 lateral wall of the mid brain forms a projection into the cavity, 

 and this would be called the torus from the standpoint of gross 

 anatomy. Only a small dorsal portion of it, however, belongs 

 to the tectum and corresponds to the torus in Acipenser. 



A large fiber tract (Figs. 13, 14, 15, 16), embracing the 

 whole fiber zone, descends from the lateral border of the tectum 

 through the wall of the mid brain. Before reaching the base 

 of the brain the larger part of the tract turns backward and 

 enters the medulla. The greater part ot the remainder enters 

 into the formation of the ansulate commissure and then joins 

 the uncrossed tract to the medulla. A small cephalic portion 

 of the tract from the tectum turns forward above the ansulate 

 commissure and goes to the lobi infcriores. Whether there is 

 any crossed. tract to these lobes could not be made out. 



The inner portion of the common tract just described con- 

 sists of coarser fibers and these are especially evident in the 

 ansulate commissure where a few of the large fibers from the 

 spindle cells of the acusticum are impregnated among them. 

 This inner coarse-fibered portion is doubtless made up of the 

 internal arcuate fibers from the medulla as in Acipenser. The 



