The Brain of Diemydylus Viridescens 295 



cerebrum. Small as these parts are, from the exact coinci- 

 dence with the essential points in the topography of the pal- 

 lium of amia, I think they can be safely homologized there- 

 with, provided that it is admitted that the mesal wall of the 

 cerebrum in amphibia corresponds with an extreme lateral 

 point in fishes, that is, if the points at which the membrane 

 attaches to the cerebrum are homologous. 



The figures by Mihalkovics (33) of the brain of an embryo 

 rabbit show relations of the membranes much like those of 

 diemyctylus. 



CEREBRUM, 



In figures 97, loi, 105 are shown sections through the 

 brain of amia, diemyctylus and lamprey respectively in 

 regions as accurately corresponding as possible, — through the 

 cerebral commissures. In the lamprey the cavity of the pros- 

 encephal extends at right angles from the meson and the dor- 

 sal walls may easily be imagined as bent downward so that 

 the actual condition should be as in diemyctylus, or away 

 from the meson when the position would correspond with the 

 interrupted lines of figure loi. 



In the amia suppose that the recurved cerebrum be raised 

 to a nearly vertical position as shown by the interrupted lines 

 of figure 97. A strict comparison could be instituted be- 

 tween the forms, which would produce no more change than 

 occurs in nature in the position of the walls of the mesencephal 

 of different groups (p. 292). In figure 97 with the exception 

 of stretching the line between y and y' and folding the pal- 

 lium to form a paraplexus no change except raising the parts 

 is necessary. In the young amia (Fig. 102) as shown by 

 Wilder (50), young gar, shown by Wright (58), lepidosteus 

 shown by Balfour and Parker (3, PI. 24), the walls actually 

 have the position here imagined in the adult. 



In figure 97 it is noticeable that a band of alba can be seen 

 continuous with the commissure, passing ectally around the 

 sharp angle at y to the point z, where the pallium unites with 

 the solid wall. To prove that the part between x and z in this 

 figure is identical morphologically with the corresponding 



