96 .ZOOLOGY SECT. 



become thickened forming paired masses, the optic thalami (D, 

 F, L, o. th.), the roof remains for the most part in the con- 

 dition of a thin membrane (ependyme) composed of a single 

 layer of cells, but part of it gives rise to a very peculiar 

 adjunct of the brain, the pineal apparatus. This originates as 

 a narrow hollow outgrowth, the epiphysis. The epiphysis is 

 frequently double, one portion being in front of the other, 

 and the two parts may be widely separated. From this, or 

 when it is double, from one of its portions, a diverticulum is 

 developed, which becomes constricted off in the Lampreys and 

 some Reptiles to form an eye-like body, the pineal eye (pn.e.); some- 

 times a second, less fully formed parapineal eye may be formed 

 from another part of the epiphysis. In most adult Vertebrates the 

 epiphysis is represented by a gland-like structure, the pineal body 

 (pn. 5.), connected with the roof of the diencephalon by a hollow or 

 solid stalk. The term paraphysis is sometimes applied to an out- 

 growth of the roof of the fore-brain developed in front of the 

 epiphysis in the hinder region of the prosencephalon. The floor 

 of the diencephalon grows downwards into x a funnel-like pro- 

 longation, the infundibulum (inf.): with this the pituitary diver- 

 ticulum of the pharynx (p. 81) comes into relation, and there 

 is formed, partly from the dilated end of the diverticulum, partly 

 from the extremity of the infundibulum, a gland-like structure, 

 the pituitary body or hypophysis (pt.) always situated immediately 

 in front of the anterior extremity of the notochord arid between 

 the diverging posterior ends of the trabeculaB. In cases where 

 cerebral hemispheres are not developed, the roof or pallium of the 

 undivided fore-brain is reduced to a layer of epithelium (D and 

 E. pal.) : its floor is thickened so as to form large paired masses, 

 the corpora striata (c. s.). When hemispheres are developed the 

 corpora striata form the floors of the two lateral ventricles (L. c. s.), 

 and the roof (pallium) of each is formed of nervous tissue. In 

 such cases the front wall of the diencephalon remains very thin, 

 and is distinguished as the lamina terminalis (I. t.): this is the 

 actual anterior extremity of the central nervous system, the 

 cerebral hemispheres being lateral outgrowths. 



In the preceding description the brain has been described as if its 

 parts were in one horizontal plane, but, as a matter of fact, at a very 

 early period of development the anterior part becomes bent down 

 over the end of the notochord, so that the whole organ assumes a 

 retort-shape, the axis of the fore-brain being strongly inclined to 

 that of the hind-brain. The bend is known as the cerebral flexure : 

 it is really permanent, but, as the hemispheres grow forward 

 parallel to the hind-brain and the floor of the mid- and hind-brain 

 thickens, it becomes obscure, and is not noticeable in the adult. 



The brain, like the spinal cord, is composed of grey and white 

 matter, but the grey matter either forms a thin superficial layer 



