Hekhick, Morjy/iolooy of Nervous Syslon. I'i^ 



cavity has nearly disappeared. Fig. 7 well exhibits the ex- 

 tent to which the gray matter of the dorso-lateral aspects 

 of the medulla is derived from the epithelium of the prim- 

 itive recessus lateralis. Fig. S illustrates the method of 

 plexus formation, the origin of the cornucopia, and condi- 

 tions prevailing at the tip of the cerebellum at this stage. 



The origin of Purkinje's cells cannot be fully made out 

 in this series, yet the great similarity to the cells of the den- 

 tate nidulus, at a stage shortly preceding birth, is such as to 

 suggest community of origin. 



Additional evidence that the principle formulated above 

 is the prevailing one has been derived from a study of 

 embryos of the black-snake (see plate X). While in this 

 case the organ does not itself undergo retroflection, but re- 

 mains a leaf-like organ, longitudinal sections indicate that 

 proliferation is most rapid near the tip, and a dense cluster 

 of cells is pushed dorsad and then cephalad upon dorsal 

 surface. This does not exclude direct migration from the 

 ventricular aspect of the cerebellum. There is every reason 

 to conclude from these sections that the cells of Purkinje 

 likewise spring from near the ventricular border. At the 

 tip and lateral margins, the layer of these cells in the em- 

 bryo comes in contact with the epithelium, and the cells, 

 which are obviously multiplying rapidly, either spring from 

 the epithelium by subdivision of its undifferentiated cells or 

 the multiplication of special germination cells. The neuro- 

 blasts at first become spindle-shaped and give rise to a pro- 

 cess which passes cephalad (Plate X, Fig. 11). Nearer the 

 base the layer of Purkinge cells is represented by a thick 

 nucleary zone from the epithelium. At the very base, 

 however, there is a superficial (dorsad) cell aggregate 

 which likewise seems to have its origin in the epithelium of 

 the ventricle. 



It seems unnecessary to say more in illustration of the 

 complete homology between the method of cerebellum 

 formation in the several groups of vertebrates. That this 



