102 ZOOLOGY SECT. 



sisting of a single layer of epithelial cells, assuming the character 

 therefore of a purely non-nervous epithelial layer {cjicndrjme). In 

 the cerebellum the thickening takes place to such an extent that 

 the epicoele is usually obliterated altogether. In the mid-brain 

 the ventral wall is thickened in the form of two longitudinal 

 bands, the crura cerebri (cr. c.rh.), the dorsal wall in the form of 

 paired oval swellings, the 02itic lohes {opt. I.) : extensions of the 

 mesoccele into the latter form the o])tic ventricles or optocccles 

 (G. opt. coe.) : the median portion of the mesocjele is then called 

 the iter (I) or aqueduct of Sj/lvhcs. In the diencephalon the sides 

 become thickened forming paired masses, the 02ytic thcdami (D, 

 F, L, 0. th.), the roof remains for the most part in the con- 

 dition of a thin membrane {epenclyme) 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 diverti- 

 cula, one in front of the other, the anterior being the parietal 

 organ, the posterior the pineal organ or epiphysis : 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 

 ipn. e.) at its extremity, but is vestigial or absent in most other 

 Vertebrates. The epiphysis is eye-like (parapincal eye) only 

 in the Lampreys ; in other Vertebrates it is represented by a 

 gland-like structure, the pineal hody {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.^ The floor of the 

 diencephalon grows downwards into a funnel-like prolongation, 

 the infitndibulum {inf.) : with this the pituitar}^ diverticulum of 

 the pharynx (p. 87) 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 {pi.) always situated immediately in front of 

 the anterior extremity of the notochord and between the diverging 

 posterior ends of the trabeculse. The hypophysis in higher Craniates 

 appears to be of the nature of a blood-gland, secreting colloid 

 material and destroying blood-corpuscles. In lower Craniata it 

 consists of two distinct glandular parts, the one (saccus vasculosus) 

 situated more dorsally and formed as an outgrowth of the infun- 

 dibulum, the oih.eY {hypophysis proper) ventral and arising from the 

 pharyngeal diverticulum. The saccus vasculosus is an organ for 

 the secretion of the cerebrospinal fluid, the fluid which occupies 

 the ventricles of the brain and the central canal of the spinal 

 cord. In cases where cerebral hemispheres are not developed, 

 ^ The so-called " paraphysis " of Mammals {q.f.) is not honiologou.s with this. 



