Humphrey, Brain of the Snapping Tiirtle. 87 



it being marked externally by a very shallow groove. The 

 floor of this groove is a thin crescent of neuroglia connected 

 with the central cinerea by a very thin neuroglia column. Caud- 

 ad this crescent and column disappear. Tracing these struc- 

 tures cephalad we find that they increase in area as the dorsal 

 furrow deepens. This increase is especially marked in the width 

 of the neuroglia column. As the furrow increases in depth the 

 column is lost in the general cinerea of the central region. As 

 the dorsal furrow deepens the pia follows it closely until it joins 

 the endyma, where the furrow widens at the formation of the 

 metaccele. 



The metaccele is continued caudad in the form of short 

 pockets beyond its union with the myeloccele and the closing of 

 the dorsal furrow. These pockets vary in number, depth and 

 form in different individuals. Fig. 8 shows a transection of 

 this region across three of these pockets. I believe two to be 

 the normal number, the mesal one not being present in all the 

 specimens examined. The fact of the continuation of the met- 

 accele caudad of the blending with the myeloccele is constant in 

 the adult specimens observed. The union with the myeloccele 

 and dorsal furrow is in nearly the same dorso-ventral plane. 



The Roof of the Metaccele. — The great cavity of the met- 

 encephal has a membranous roof — the metatela. This roof is 

 composed of a thin membrane formed, as are all the membra- 

 nous walls of the brain, of a single layer of endymal cells and a 

 layer of pia. In the embryo turtle this membrane is greatly 

 convoluted and hanging down occupies the greater part of the 

 cavity of the metaccele. At this stage the cavity is not so 

 long, owing to the flexing of the metencephal almost upon the 

 cerebellum. The metatela is attached to the dorsal and lateral 

 edges of the cerebellum and to the metencephal along the mar- 

 gin of the entire metaccele. The folding of the metatela to 

 form the metaplexus occurs just caudad of the cerebellum. In 

 the embryo and in the young turtle, i.e., one with a carapace of 

 12 cm., this plexus is composed of foldings of the thin mem- 

 brane. This membrane is at this time made up of a single layer 

 of endymal cells and a thin layer of pia (Fig. 16). In the 



