330 



VERTEBRATE ZOOLOGY 



first invaginated as a continuous fold, covering the jaw from end to 

 end; this fold is known as the enamel organ. At intervals thicken- 

 ings occur at the bottom of the groove, each of which becomes bell- 



FiG. 171. — Brain of rabbit, especially to show corpus caUosum. (Nat. size.) 

 In A the left parencephalon is dissected down to the level of the corpus callosum : 

 on the right the lateral ventricle is exposed. In B the cerebral hemispheres are 

 dissected a little below the level of the anterior genu of the corpus callosum; only 

 the frontal lobe of the left hemisphere is retained; of the right a portion of the 

 temporal lobe also is left; the velum interpositum and pineal body are removed, 

 as well as a greater part of the body of the fornix, and the whole of the left pos- 

 terior pillar; the cerebellum is removed with the exception of a part of the right 

 lateral lobe. a. co, anterior commissure; a.fo, anterior pillar of fornix; a. pn, an- 

 terior peduncles of cerebellum; b.fo, body of fornix; c6', superior vermis of cer- 

 ebellum; c. 6', its lateral lobe; c. gn, corpus geniculatum; c. h, cerebral hemi- 

 sphere; c. ph, choroid plexus; cp. cl, corpus callosum; cp. s, corpus striatum; 

 c. rs, corpus restiforme; d. p, dorsal pyramid; fl, flocculus; hp. m, hippocampus; 

 m. CO, middle commissure; oi\ anterior, and ol^, posterior lobes of corpus quad- 

 rigemina; o. th, optic thalamus; o. tr, optic tract; p. co, posterior commissure; 

 p.fo, posterior pillar of fornix; pn, pineal body; pd. pn, peduncle of pineal body; 

 p. pn, posterior peduncle of cerebellum; p. va, fibres of pons Varolii forming mid- 

 dle peduncles of cerebellum; sp. hi, septum lucidum; st. I, stria longitudinalis; 

 is, tEcnia semicircularis; v. vn, valve of Vieussens; u\ third ventricle; v*, fourth 

 ventricle. (From Parker and Haswell.) 



shaped, with a dermal papilla in the hollow of the bell. The top of 

 the bell continues to grow out as the tooth and soon ruptures the 

 gum and protrudes as a naked cusp. Sometimes the enamel be- 



