148 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



through the foramina interventriculares into the ventricles of the hemi- 

 spheres, thus providing for a blood supply on the interior of these 

 structures (fig. 150). 



The floor of the diencephalon remains thinner behind the optic 

 recess, a portion of it becoming funnel-shaped and pushing out from the 

 ventral surface toward the roof of the mouth. This is the infundib- 

 ulum which meets an ectodermal diverticulum, the hypophysis. 

 This arises, in the cyclostomes from the ectoderm between the nostril 

 and the mouth; in other vertebrates from the roof of the oral cavity. 

 It retains its connection with the parent epithelium, for a time, the point 

 of ingrowth being known as Rathke's pocket. Later the stalk dis- 

 appears and the infundibulum and hypophysis, closely associated, lie 

 just beneath the brain in the sella turcica on the floor of the skull 

 (p. 61). In the hypophysis (pituitary body) two parts are distin- 

 guished, rich in blood- and lymph-vessels and forming a gland of internal 

 secretion whose action is connected with the fat-storing powers of the 

 animal. The infundibulum may be a simple pit, as in most vertebrates, 

 or its lateral walls may become enlarged and folded, blood-vessels 

 lying in the folds, and the whole forming the so-called saccus vascu- 

 losus. The paired eyes are also connected with the 'twixt-brain, both 

 in origin and in the adult; they are described with the other sense 



organs. 



The cerebrum (telencephalon) consists of a pair of hemispheres, 

 separated in front by an intercerebral fissure, slight in fishes, well 

 marked in other vertebrates. Each hemisphere typically contains a 

 ventricle, the walls of which are formed by the corpus striatum below 

 and elsewhere by a thinner portion, the pallium or mantle. To the 

 roof belong the paraphysis and the inferior chorioid plexus, already men- 

 tioned. In some vertebrates, like the teleosts, the whole of the pallium 

 remains thin and epithelial throughout life; elsewhere it is invaded to a 

 greater or less extent by nervous matter. In the amphibia and reptiles, 

 where the olfactory lobes are merged in the hemispheres, the medial 

 wall of each hemisphere as far back as the interventricular foramen is 

 called the septum, while the part above the foramen, together with the 

 posterior dorsal and lateral walls, is to be regarded as homologous with 

 a region, long recognized only in mammals, the hippocampus, connected 

 with the olfactory sense. In the mammals a new element, the neopal- 

 lium, appears in the cerebrum. In the lower groups it is on the outer 

 wall, behind the olfactory tract, and, increasing in extent in the higher 



