The Brain of Diemydylus Viridescens 293 



the parts unite and a distinct commissural band is present 

 while there is a great depression on the dorsimeson separat- 

 ing the parts into two lateral lobes (geminums Fig. looj. In 

 larval diemyctylus, though the walls approximate each other, 

 they unite across the meson merely by a layer of endymal 

 cells (Fig. 81). In the lamprey the solid parietes show a 

 bend, or sulcus, similar to that of the amia but the flexure is 

 not as great, nor do the parts unite across the meson except by 

 a wide membrane (Fig. 108). The membrane is plexiform 

 with a mesal fold, while at the union of the membrane with 

 the solid wall the endyma is reflected over a ridge or lophius 

 comparable to those of the metencephal. These different 

 forms arise from a common embryonic one where the walls are 

 thin and uniform. 



This recapitulation and comparison of figures is introduced 

 to recall the fact, that in parts which are homologized without 

 hesitation, the mere condition of a more or less upright posi- 

 tion and the closer or more remote union by a well organized 

 commissural band or a mere membrane are not considered 

 bars to such homologizing. 



PAI,I,IDM. 



Since Rabl-Riickhard's (40) memorable work on the brain 

 of fishes, the pallium has been known as a membrane which 

 represents and takes the place morphologically of the dorsal 

 and mesal walls of the cerebrum of other groups. A recent 

 work of Herrick upon the ganoids (24. p. 153), shows that he 

 considers the portas of other forms to be represented by the space 

 between the proplexus [auliplexus] dividing the hemicere- 

 brums and the floor of the prosencephal. They are elongated 

 slits. " These changes and the backward revolu- 

 tion of the mantle portion of the cerebrums make all the diffi- 

 culties disappear, and we seek the commissures of the mantle 

 far cephalad in front of the thin membranous portion, which 

 seems to be homologous, in part at least, with the velum cere- 

 bri supporting the proplexus." Here is a hint that he has 

 seen a new interpretation of the pallium though in other parts 

 of the article he seems to accept in full the idea of Rabl- 

 Riickhard. 



