Herrick, Morphology of Nervous System. 7 



figured being very young, it cannot be supposed that these 

 folds have been completed. The pons flexture has been 

 considerable, bringing the extremity of the cerebellum cau- 

 dad into proximity to the obex and taenia? fossre rhomboid- 

 ealis. The velum medullare posterior is therefore almost 

 wholly converted into the epithelium of the metaplexus. 

 The entire configuration of the cerebellum is hood-like(Plate 

 III, Figs. 9, 10 and 11). 



In the black-snake the cerebellum is a leaf-like expan- 

 sion, having on the caudad surface (really the ventral or 

 ventricular aspect) the granular substance and the white 

 fibrous zone cephalad; but this leaf is folded ventro-caudad, 

 so that the white layer is dorsad and uppermost and the 

 gray matter faces toward tne ventricle in which lies the 

 large plexus. 



In the case of the lizard we have apparently a com- 

 pletely dissimilar plan of structure. Here the gray matter 

 is dorsal and the white ventral (Plate IV, Figs. 4 and 5). 

 This reversal of the two layers is explained, upon a more 

 careful examination, as the product of a complete forward 

 and median fold of the caudad and lateral margins of the 

 cerebellum. (Compare Plate IV, Figs. 6—9). This is but 

 the completion of the process indicated by the incipient 

 retroflexion seen in the turtle. The result of this fold is the 

 formation of an actual cavity surrounded caudad and laterad 

 by the white (morphologically dorsal) zone of the cerebel- 

 lum. Thus the fusion of the lateral margins, or, more 

 accurately, the union of the whole latero-caudad reflected 

 margins due to a general cephalo-median increase, produces 

 the hollow organ just described. 



In the alligator the cerebellum is formed in the same 

 way, but the organ is larger and the internal secondary 

 cavity is much greater, the whole forming a cone with its 

 apex protruding caudad. In this case the folded portion has 

 undergone less flattening dorso-ventrally, and more clearly 



