PINEAL OEGAN 



99 



assumes a lenticular form, its cells becoming much elongated though 

 remaining in a single layer. This lenticular thickening occasionally 

 becomes lost during development — a fact which may be taken as 

 forming a piece of evidence in favour of the view that the eye, at the 

 present time, is in a retrogressive phase of evolution. 



Those parts of the vesicle wall which do not take part in the 

 formation of the lens undergo histogenetic changes into retinal tissue. 

 The cells undergo differentiation in two directions. The one set 

 become pigment cells — tall columnar cells which traverse the whole 

 thickness of the retina and have their nuclei towards the basal or 

 outer end and which develop dark 

 melanin granules in their proto- 

 plasm. 



Interspersed with these are the 

 percipient cells, shorter in form, 

 their basal ends not reaching the 

 outer surface, and carrying at their 

 inner ends cilium-like structures 

 which project into the cavity of the 

 vesicle. The idea that these projec- 

 tions correspond physiologically to 

 rods appears to be negatived by the 

 fact that they occur also on the 

 inner ends of the cells forming the 

 lens. 



At their basal ends these cells are 

 continued into nerve-fibres, which 

 form a distinct layer internal to the 

 nuclei of the pigment cells and are 

 eventually continuous physiologi- 

 cally with the fibres of the pineal 

 nerve. Scattered amongst, and in the 

 neighbourhood of, this fibrous layer 

 ganglion -cells are present : they are 



about the first definite elements to become recognizable during the 

 histogenesis of the retina and appear first close to the point of attach- 

 ment of the optic nerve. 



The cavity of the pineal eye is kept distended by a clear substance, 

 the vitreous body, and this is colonized by a certain number of cells 

 (see Fig. 55, 0) which are most probably to be regarded as immi- 

 grant mesenchyme cells. 



In Sphenodon, the sole survivor of the. only other existing group 

 of Eeptiles in which a pineal eye is present, the development of the 

 organ according to Dendy (1899), who has worked it out in detail, 

 agrees with that of Zacerta in its main features. 



In the Lampreys, also, somewhat eye-like developments occur in 

 the pineal region. In the adult two vesicles— a dorsal ("pineal") 

 and a ventral ("parapineal")— are found overlying the roof of the 



Fig. 56. — Lacerta muralis, 25 mm., sagit- 

 tal section through roof of Thalamen- 

 cephalon. (After Novikofi*. ) 



h.c, habenular commissure ; p.c, posterior 

 commissure ; p.e, pineal eye ; p.n, pineal nerve ; 

 p.s, pineal stalk ; par, paraphysis. 



