NERVOUS SYSTEM OF THE VERTEBRATA. 



431 



at first inconspicuous prominence the rudiment of the infundi- 

 bulum (fig. 252, In}. This comes in contact with an involution 

 from the mouth, which gives rise to the pituitary body (fig. 252, 

 //), the development of which will be dealt with separately. 



In the later stages of development the infundibulum becomes 

 gradually prolonged, and forms an elongated diverticulum of 

 the third ventricle, the apex of which is in contact with the 

 pituitary body (figs. 252, 254, in, and figs. 250 and 255, inf}. 



Along the sides of the infundibulum run the commissural 

 fibres connecting the floor of the mid-brain with the cerebrum. 



In its later stages the infundibular region presents considerable variations 

 in the different vertebrate types. In Fishes it generally remains very large, 

 and permanently forms a marked diverticulum of the floor of the thalamen- 

 cephalon. In Elasmobranchii the distal end becomes divided into three 

 lobes a median and two lateral. The lateral lobes appear to become the 

 sacci vasculosi of the adult. 



In Teleostei peculiar bodies known as the lobi inferiores (hypoaria) make 

 their appearance at the sides of the a2r ,. y 



infundibulum. They appear to corre- 

 spond in position with the tuber cine- 

 reum of Mammalia 1 . In Birds, Rep- 

 tiles, and Amphibia the lower part of 

 the embryonic infundibulum becomes 

 atrophied and reduced to a mere finger- 

 like process the processus infundibuli. 



In Mammalia the posterior part of 

 the primitive infundibulum becomes the 

 corpus albicans, which is double in 

 Man and the higher Apes ; the ventral 

 part of the posterior wall forms the 

 tuber cinereum. Laterally, at the junc- 

 tion of the optic thalami and infundi- 

 bulum, there are placed the fibres of 

 the crura cerebri, which are probably 

 derived from the walls of the infundi- 

 bulum. A special process grows out 

 from the base of the infundibulum, 

 which undergoes peculiar changes, and 

 becomes intimately united with the 

 pituitary body ; in which connection it 

 will be more fully described. 



rncl 



c.in 

 'Pt 



FIG. 254. LONGITUDINAL SECTION 

 THROUGH THE BRAIN OF SCYLLIUM 

 CANICULA AT AN ADVANCED STAGE OF 

 DEVELOPMENT. 



cer. cerebral hemisphere ; pn. pi- 

 neal gland ; op. th. optic thalamus, con- 

 nected with its fellow by a commissure 

 (the middle commissure). In front of 

 it is seen a fold of the roof of the fore- 

 brain, which is the choroid plexus of 

 the third ventricle ; op. optic chiasma ; 

 ft. pituitary body ; in. infundibulum ; 

 cb. cerebellum ; au.v. passage leading 

 from the auditory vesicle to the ex- 

 terior ; mel. medulla oblongata ; c . in. 

 internal carotid artery. 



1 For the relations of these bodies, vide L. Stieda, "Stud. lib. d. centrale Nerven- 

 system d. Knochenfische." Zeit. f. wiss. Zool. Vol. xvni. 1868. 



