90 Journal of Comparative Neurology 



A part of such a section is shown in Fig. 20. The internal 

 structure is easily differentiated into three parts or layers. The 

 ectal layer A is composed of neuroglia with a few scattered 

 cells, — the typical structure of the barren cortical layer of the 

 brain. As shown in the figure this layer has a slight fibrous 

 appearance. This is probably due to remnants of epithelial 

 structures. In young turtles there is a layer of small, deeply 

 staining cells lying just entad of the pia. This layer was not 

 found in the adults. These cells are probably the remnants of 

 the embryonic cells from which the barren layer was developed. 

 Entad of the barren layer is a layer of large Purkinje cells send- 

 ing their processes ectad into the barren layer. These cells lie 

 in a compact layer only one cell deep, they are oval in outline 

 with large nuclei and deeply staining nucleoli (Fig. 20 B). The 

 ental layer of the cerebellum is composed of densely packed, 

 deeply staining cells lying in a thick stroma. These cells are 

 quite similar to those lying scattered in the barren layer. This 

 layer is bounded entad by the endyma. 



At the base of the cerebellum on either side is found a 

 small nidus — the dentate nidus of authors. These nidi lie at 

 the level of the decussation of the 4th nerve and are closely 

 enveloped by fiber tracts. They are composed of large cells of 

 the oval type. Sagittal sections near the lateral margin of the 

 cerebellum show, coming from that segment, a large fiber tract; 

 this tract divides at the level of the base of the cerebellum and 

 one part turns sharply cephalad into the mesencephal. The 

 caudal part extends ventrad and subdividing sends one part to 

 the region of the origin of the trigeminus, while a second 

 branch curves caudad into the region of the origin of the acous- 

 tic nerve. In the cerebellum these tracts seem to be in close 

 relation with the Purkinje cells. 



Mesencephal. 



Structurally the mesencephal is limited cephalad by the 

 post-commissure and the cephalic limit of the nidus of the 3d 

 nerve. Its caudal limit in the dorsal wall is the decussation of 

 the 4th nerve, while the ventral wall shows no clear point of 



