12 loUKNAI. OI Com I'A I! A ri\ K NKL;HOJJ»(i^■. 



bellum itself, while these, in turn, can be traced to the pro- 

 gressive metamorphosis of the primitive lamella, of which 

 the vertebrate cerebellum is the final product. A reference 

 to Plate I, Figs. 3-8, will place all these relations in a clear 

 light. These sections through the cerebellum and medulla 

 of a guinea-pig exhibit the same process of lateral and re- 

 trorse growth and folding which are seen in reptilia, except 

 that, instead of the flexion of the entire organ, there is a 

 comparatively thin fold of epithelium containing a delicate 

 tract at the base, but consisting caudad solely of the 

 epithelium. These sections taken through the same cere- 

 bellum at different points may fairly claim to indicate the 

 several stages which would successively appear at an}^ one 

 plane. 



Fig. 3, taken through the base of the cerebellum, shows 

 the dorsal surface to be devoid of the gray matter. From 

 either side there extends a curious upward fold containing 

 a cavity. The source of the growth is evidently the walls 

 of the cavity, as witness the numerous cells clustered at that 

 point. Fig. 4 shows the circumscribed cavity to be a pouch 

 of the recessus lateralis of ventr. I\", extending cephalad, 

 and also shows that the newly- formed cells involved in the 

 fold are drived from the ventricular epithelium. It will be 

 noticed also that the entire dorsal portion of the organ is 

 affected by this overlapping growth from behind and the 

 sides, so that a groove is formed on either side meson, dor- 

 sally. It is likewise important to observe that the develop- 

 ment of the epithelium extends the recessus lateralis ven- 

 trally as well as dorsally, and that it there also fuses with 

 the entad body (here medulla) and gives rise to proliferous 

 clusters. 



The above illustrations are derived from the guinea-pig, 

 but figures are also given derived from similar sections of 

 older stages of mouse embryos for demonstration of the last- 

 mentioned point. In these cases the dorso-lateral fold has 

 already fused with the walls of the cerebellum, and the 



