88 Journal of Comparative Neurology 



adult turtle the appearance of this membrane changes. To the 

 unaided eye it has the appearance of being ridged. These 

 ridges radiate from a common center on the dorsimeson. Cau~ 

 dad of these ridges the membrane becomes smooth and very- 

 delicate (Fig. 5). The appearance of this membrane in the 

 great sea-turtle is shown in (Fig. 7). This appearance is sub- 

 stantially the same as in Chelydra. A section through this 

 ridged formation shows it to be formed of the foldings of the 

 metaplexus in which the pia has become enormously thickened, 

 until each fold is a solid mass of pia between the two layers of 

 endyma. The endyma retains its one celled form. Fig. 17 

 shows a section of a single fold of adult plexus. Numerous 

 blood vessels are found in this fold and among the fibrous pia 

 layers connective tissue cells are numerous. 



A sagittal section through the entire length of the meten- 

 cephal near the meson (Fig. 16) shows the attachment of the 

 metatela along the caudo-dorsal margin of the cerebellum. The 

 endymal cells pass around the caudal portion of the metence- 

 phal and join the pia on the dorsal surface. The metaplexus 

 is formed at once just caudad of the cerebellum, then the mem- 

 brane is continued caudad in a direct course until near the cau- 

 dal portion of the metaccele where it is folded and extends cau- 

 dad of the closure of the metaccele as a distinct pocket. This 

 pocket is formed of a very delicate membrane. In the embryo 

 turtle (Fig. 19) I could detect no appreciable layer of pia cross- 

 ing the meson but in the older turtles a thin layer was notice- 

 able. In the large sea-turtle the same structure was found, but 

 the membrane of the pocket was extremely thin. In the em- 

 bryos of Chelydra the pocket was very small. This pocket 

 seems to have considerable lateral extent. It is evident that it 

 does not necessarily have the form shown in these sections, for 

 being made of a very thin membrane it takes whatever form 

 pressure and gravitation may dictate. That the form figured is 

 most common results from the position in hardening and the 

 presence of considerable quantity of fluid in the brain cavities. 

 This pocket of extremely thin membrane is especially interest- 

 ing as Wilder (92) has shown the presence of a metapore at this 



