xm PHYLUM CHORDATA 99 



paired oval swellings, the optic lobes (opt. I.) : extensions of the 

 mesocoele into the latter form the optic ventricles or optocceles 

 (G. opt. cos.) : the median portion of the mesocoele is then called 

 the iter (I) or aqueduct of Sylvius. In the diencephalon the sides 

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

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

 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 an outgrowth which consists 

 typically of two narrow diverticula, one in front of the other, the 

 anterior being the parietal organ, the posterior the pineal organ or 

 epipliysis : these two parts may be developed independently, or 

 the latter may originate by outgrowth from the former. The 

 parietal organ in the Lampreys and some Reptiles develops an 

 eye-like organ, the pineal eye (pn. e.) at its extremity, but is vestigial 

 or absent in most other Vertebrates. The epiphysis is eye-like 

 (parapineal eye) only in the Lampreys ; in other Vertebrates it is 

 represented by a gland-like structure, the pineal body (pn, b.), 

 connected by a hollow or solid stalk with the roof of the diencephalon. 

 The term parapliysis is applied to a non-nervous outgrowth of 

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

 the hinder region of the prosencephalon. 1 The floor of the dien- 

 cephalon grows downwards into a funnel-like prolongation, the 

 infundibulum (inf.) : with this the pituitary diverticulum of the 

 pharynx (p. 84) 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 and between the diverging 

 posterior ends of the trabeculae. The hypophysis in higher Craniates 

 appears to be of the nature of a ductless, internally secreting gland. 

 In lower Craniata it consists of two distinct glandular parts, the 

 one (saccus vasculosus) situated more dorsally and formed as an 

 outgrowth of the infundibulum, the other (hypophysis proper) 

 ventral and arising from the pharyngeal diverticulum. 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 

 1 The so-called " parapliysis " of Mammals is not homologous with this. 



