70 CLASS PISCES. 



higher brains to the choroid plexuses of the lateral ventricles. 

 It is called the velum transversum. 



This folded-in part of the roof is generally regarded as belong- 

 ing to the cerebrum : in all Vertebrata it retains throughout 

 life its epithelial condition. In front of it, the roof of the brain 

 (pallium) behaves in a different way in different animals. In 

 Elasmobranchs, Marsipobranchs, Dipnoi, and in all Vertebrata 

 above fishes, it loses its simple epithelial condition and develops 

 nervous tissue, forming the dorso -lateral part of the cerebral 

 hemispheres above the lateral ventricles. In most other fishes, 

 Teleosteans, and Ganoids, the pallium retains its epithelial 

 condition throughout life, so that in these groups the roof of 

 the lateral ventricle remains permanently thin and epithelial, 

 as does the roof of the third ventricle and that of the posterior 

 part of the fourth ventricle, and in Lampreys of the aqueductus 

 sylvii as well. 



Curiously enough for what reason it is difficult to under- 

 stand the condition in which the cerebral pallium consists of a 

 thin epithelial layer is regarded as secondary. By all the or- 

 dinary tests which are applied in speculations of this kind viz., 

 embryonic development and general diffusion of the character 

 in the lower Vertebrata and absence in the higher, it should surely 

 be regarded as a primitive character. Indeed, if we may be 

 allowed to indulge in a little speculation of this kind, it would 

 appear from development that the whole medullary canal at 

 one time had purely epithelial walls, and there appears to be a 

 tendency to the retention of this character along the middle 

 dorsal line throughout life in all Vertebrata. 



To return to the pineal body. It is developed as a divert- 

 iculum of the hinder part of the roof of the thalamencephalon. 

 Its terminal portion becomes the pineal body or epiphysis ; its 

 proximal part is the pineal stalk. The terminal part sometimes 

 gives rise on its anterior wall to an outgrowth which is called 

 the parietal organ. The parietal organ may be developed 

 separately from the brain roof just in front of or by the side 

 of the epiphysis.* It is not always formed, and usually van- 

 ishes with later growth ; but in Lampreys and Lizards it 



* On account of this fact it has been suggested that the epiphysis is 

 really a paired organ, one of the pair becoming the actual epiphysis 

 (pineal body) of the adult, and the other either degenerating or becoming 



