490 



COMPARATIVE ANATOMY 



wall, the dorsal half is the archicortex or primordium hippocampi. The 

 medio- ventral wall forms the septum by which fibers pass to and from the 

 hippocampus. The hemispheres are interconnected by anterior, anterior 

 pallial and posterior pallial commissures located in the lamina terminalis. 



Habenular and posterior commissures persist in the roof of the dien- 

 cephalon. The epiphysis forms a pineal gland. A chorioid plexus 



Fig. 406. — Side and dorsal views of brain of young alligator, c, cerebrum; cl, 

 cerebellum; e, epiphysial structures; h, hypophysis; i, infundibulum; ol, optic lobes; 

 II-XII, cranial nerves. (From Kingsley's "Comparative Anatomy of Vertebrates," 

 after Herrick.) 



invaginates into the third ventricle. The saccus vasculosus of fishes has 

 disappeared. The thickened walls of the midbrain reduce the lumen 

 to a narrow passage, the aqueduct. The cerebellum is rudimentary like 

 that of cyclostomes. (Fig. 414) 



The ten cranial nerves of fishes persist in amphibia. Urodeles have 

 lateralis nerves, but these disappear in the anura in correlation with 

 the loss of lateral line organs. With the loss of gills, the number of 

 branches of the vagus is reduced. 



Cervical and lumbar enlargements of the spinal cord appear in cor- 

 relation with the enlargement of the appendages. 



