150 bulletin: museum of compaeative zoology. 



sagittal section a large number of axons, whose cells lie laterally, and 

 therefore fall in other sections, may be seen entering the ventricle. 

 Frequently, however, the axon may be seen passing directly from the 

 cell, and occasionally the tapering end of the cell projects into the 

 ventricle (Figs. I, 3, ax). The axons are directed ventrad and caudad, 

 and within tlie ventricle run together to form numerous trunks, which 

 gradually coalesce as they continue into the fourth ventricle. 



It is at this stage, about the 30th day, that the developing tissue 

 of the cerebellum begins to grow around and enclose Pieissner's fibre. 

 In some individuals Eeissner's fibre may be seen in sagittal section 

 passing under the cerebellum into the fourth ventricle, quite free. In 

 more advanced specimens of this age the fibre is already partially 

 surrounded by cerebellar tissue. Studuicka also has found this free 

 condition of Reissner's fibre in young Ammocoetes. 



b. Posterior Canal-Cells. In the fourth ventricle at this stage Reiss- 

 ner's fibre, though very fine, is sharply outlined, and may be traced for 

 a short distance into the central canal, where it fades away. In the 

 middle portion of the canal I have been unable to find any trace of it. 

 Studnicka records having failed to find it in the canal of larvae 1 cm. 

 long. In the posterior end of the canal and in the ventriculus termi- 

 nalis I find in this 30-day stage the posterior canal-cells already well 

 developed. I have found from two to four in an individual at this 

 stage. As in Amia, the nuclei are prominent, and possess a sharply 

 staining nucleolus. The cyptoplasra is diffuse, staining lightly. The 

 cells are connected with the cord by dendrites. The axons run forward 

 through the canal, and can be followed for some distance till they fade 

 out. It is evident that at this stage of development the anteriorly 

 running axons of the posterior canal-cells have not yet met and united 

 with the posteriorly running axons of the tectal cells. 



2. Adult, a. Morjjhology of the Mesencephalon. The best works on 

 the brain of Petromyzon (Ahlborn, '83, Johnston, :02) are incomplete on 

 even the gross morphology of the brain. Moreover, there are considerable 

 differences in the brain of Petromyzon planeri, described by Ahlborn, 

 and Petromyzon marinus which I have studied. It is necessary, there- 

 fore, to give some preliminary account of the morphology of the middle 

 region of the brain before the optic reflex apparatus can be intelligibly 

 described. 



Viewed from the dorsal side, the bi'ain of Petromyzon marinus is an 

 elongated structure having a length four to five times its greatest width. 

 The medulla is the largest and most conspicuous structure, and in the 



