NERVOUS SYSTEM. 



53 



A plausible suggestion is that the infundibulum represents the 

 invertebrate mouth, the ectodermal portion of the hypophysis a 

 modified pair of sense organs. The optic nerves are outlined 

 as hollow outgrowths from the sides of the twixt brain, while 

 on its ventral surface 

 may be developed ac- 

 cessory structures, — 

 the lobi inferiores, 

 sacculi vasculosi, cor- 

 pus albicans (corp. 

 mammilare) tuber 

 cinereum, etc. 



A topographic 

 point is to be kept in 

 view, — the cerebral 

 hemispheres and the 

 diencephalon are in 

 front of the anterior 

 end of the notochord 

 — are prechordaL 



The mesencepha- 

 lon has its walls 

 thickened so that its 

 contained ventricle, in the higher groups, is reduced to the nar- 

 row aqueduct already mentioned. Its dorsal surface is divided 

 by a longitudinal groove into right and left lobes (corpora bi- 

 gemina), and these in turn may each be subdivided transversely 

 into two (corpora quadrigemina).^ Leading ventrally and for- 

 wards from these lobes in all except the cyclostomes are the 

 optic tracts connecting with the optic nerves. The floor of the 

 mid brain is formed by a pair of fibre tracts — crura cerebri — 

 separated by a longitudinal fissure. 



The cerebellum or metencephalon is a thickening of ner- 

 vous matter on the dorsal anterior end of the hind brain. It 

 may exist as a small transverse fold, or it may be greatly 

 enlarged, extending forwards over part of the mid brain, and 

 backwards over the anterior end of the medulla. It may be 



1 In older works the anterior of these lobes were called nates ; the posterior, testes. 



Fig. 56. Two stages in the development of the 

 hypophysis in the pig ; A in an embryo 10 mm. long, 

 Bin 15.5 mm. long. E, epithelium of roof of mouth; 

 H, hypophysis connected with the mouth cavity in A 

 by the hypophysial duct, in B by the solid hypo- 

 physial stalk NS; I, infundibulum. 



