6 JouRXAi. OF Comparative Neurology. 



Like all other organs arising from the walls of the em- 

 bryonic vesicles, the cerebellum can be traced primarily to 

 a more or less distinctly circumscribed area of this wall; and 

 its subsequent modifications consist, in part, of irregularly 

 distributed increase in size accompanied by rapid multipli- 

 cation of cells, and, in part, of folds and contortions in the 

 resulting structure. 



The several processes concerned in the development are 

 briefly outlined by Mihalkovics,(') rendering it unnecessary 

 to do more than briefly recapitulate the processes. 



Fig. 2 of Plate II is a camera drawing of an exactly 

 median section through the brain of a guinea-pig before the 

 cerebellum has at all diflerentiated from the cephalic portion 

 of the dorsal wall of the nerve-tube. The transition cau- 

 dad into the velum medulare posterior (v. p.) is gradual, and 

 the latter shows very little tendency to fold upon itself, as it 

 will soon do in the formation of the metaplexus. Fig. 3 is 

 a similar section, though not strictly median, of a later stage 

 in the mouse embryo. Here the velum has sharply folded 

 and the plexus is very rapidly forming by a succession of 

 inward folds, as a result of which the original epithelium 

 of the roof of the fourth ventricle is converted into the 

 epithelium of the plexus. The cerebellum itself exhibits a 

 decided tendency to a forward (cephalad) fold, and the 

 whole organ may be readily compared with that of amphibia 

 or lower reptilia. 



It will be specially observed that the dorsal surface is 

 devoid ot cells and contains a rudimentary superficial tract. 

 This condition is the permanent one in reptiles like the tur- 

 tles. Compare Plate III, Figs. 4, 5 and 6, with the above 

 mentioned. In the case of the turtle, Aspidonectes spinijer, 

 as there figured, the basal portion of the cerebellum is per- 

 pendicular to the axis, the dorsal portion being twice flexed. 

 with a slight tendency to recurve at the tips. The specimen 



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 hoheren Wirhelthieren." 



