The Brain of Diemydylus Viridescens 267 



scribes in Spelerpes, not in the clearly separated layers of 

 cinerea found in the frog (Osborn). 



Epenceplial. — Projecting farcephalad under the mesencephal 

 is seen the cerebellum, with bands of white matter crossing 

 the meson (Fig. 6, 26, 36, 37). The origin of the 4th nerve at 

 this point is the only indication of a vahnila. Laterally the cere- 

 bellum is larger than on the meson (Fig. 26) and projects into 

 lateral recesses which are continuous with the metacoele. The 

 origin of the 5th nerve from the floor of the recess indicates 

 that its more ceplialic portion at least may be counted as 

 belonging to the epencephalic segment and perhaps it rep- 

 resents the lateral recess (parepicoele) of the epencephal of 

 higher forms 



Meteiicephal. — All of the nerve roots belonging to this seg- 

 ment from the 6th to the 12th, were found and their ectal 

 origin is indicated on figure 5. For a further consideration 

 see p. 273. 



The metaplexus forming the roof of the segment is large, 

 and for convenience will be considered in two parts, as the 

 appearance changes. The cephalic part is wide and has a 

 mesal fold dipping into the metacoele. On either side of the 

 fold is a series of pockets lined by endyma and connected with 

 the metacoele (Fig. 27, 35). The pockets radiate from the 

 mesal fold of this portion of the plexus. A blood vessel lies 

 in the median fold, and connects with others between the 

 pockets. The spongy portion of the plexus thus formed is 

 bounded caudally by a spicule of bone which crosses the 

 meson (Fig. 6, 28). Ventrad of the bone and in the region 

 of the origin of the loth nerve, the dorsal walls of the meten- 

 cephal become approximated and the plexus narrow and sim- 

 ple (Fig. 28). This is a strong reminder of the fish-like con- 

 dition where in a corresponding region the walls touch or act- 

 ually unite across the meson (Fig. 93 of amia). The caudal 

 portion of the metaplexus extending from this point again 

 widens (Fig. 29), and retains its simple character. Just ceph- 

 alad of the closure of the my el, the endyma and pia of the 

 metaplexus are absent across the meson, thus forming a true 

 metapore (see p. 279). 



