io8 Journal of Comparative Neurology. 



The entire mass of the prepileum has increased so that now it is greater than 

 that of the postpileum. It has also expanded and the caudal part has extended 

 both laterad and caudad. The supra- and mediflocculus have become foliated, 

 but more folia appear at a later stage; compare Fig. 23, while no folia have as 

 yet appeared upon th6 flocculus. 



Fig. 2J. Lateral aspect of the same specimen. Shows clearly the prera- 

 mus and its vermis. 



Fig. 22. Oblique view of the same specimen intended primarily to show 

 the relation of the floccular region to the postvermis by way of the vermian 

 tract and also that of the postpileum to the supraflocculus. 



Fig. 2j. Left lateral aspect of an adult cat's cerebellum, No. 153 (x 2.3). 

 Shows the compact and pump appearance of the adult organ. It looks as 

 though the skull had been too small for it. The great size of the vermis is very 

 marked. Compare Figs. 15, 17, 20, 22. The increase in size of the prepileum 

 over that of the postpileum is astonishing. Compare Figs, il and 14, PI. I, 

 and Figs. 16, 17, 18, PI. II. Note also the comparative size of the flocculus 

 and of the medi- and supraflocculus, and its relation to them. 



The flocculus is really larger than it appears to be here, being partially 

 concealed by the mass lying dorsad ot it. In addition, note the relation of the 

 parepicoele to the auditory eminence, the flocculus, and the plexus. The heavy 

 black line represents the torn edge of pia and endyma, the ripa. A portion 

 of the tela was removed to expose the cavity. Compare Figs. 4, 5, 11, 12, 14, 

 PI. I, and Figs. 18, 19, 20, 22, PI. II. 



PLATE III. 



Figs. 24 and 24a. Sagittal sections through the mesencephal, the epen- 

 cephal, the metencephal and a part of the myel of an embryo pig 16 mm. long, 

 the same age as Figs, i, 2, 3, PI. I. The sections were cut a little obliquely 

 and the mesal aspect of Fig. 24 is reconstructed from several sections. 



Fig. 24 shows a very rudimentary condition ofithe parts, also the following 

 distinct features, viz., 



a. The comparatively thick floor of the brain tube ; it becomes thinner 

 in the floor of the diacrele. 



b. The roof is thin at the meson but thickens laterad. 



c. The thin metatela and thicker roof of the epencephal and mesencephal. 

 The change from tela to epicoelian roof is through a region of transition, 

 the kilos, which consists essentially in a decreased thickness of the parietes. 

 This is more abrupt and clearly defined in older embryos. 



d. The constriction at the isthmus. At this point there is a greater thick- 

 ening in the roof which increases laterad. At present I am unable to explain this 

 condition ; would suggest that, as a result of the dilation of the mesecephal 

 and a possible contraction at the isthmus, the difference in thickness has been 

 brought about. I shall discuss this subject as soon enough as material is available. 

 Compare Figs, i, 2, 3, PI. I, and Fig. 24a, PI. Ill, which is a sagittal section at 

 some distance from the meson. The proton of the cerebellum is marked cbl. ; 

 it does not appear in a mesal section. The other parts are self explanatory. 



