252 



THE PINEAL ORGAN 



They lack the regularity of shape and size which characterizes the smooth 

 club-shaped ends of the sensory-cells of the pineal eye of Geotria (Fig. 45, 

 Chap. 3, p. 69), but they give the impression of being degenerated 

 processes of the epithelium rather than a secretion product or cilia. 



The Parietal Organ of Reptiles 



No parietal eye is developed in many of the lizards ; thus it is absent 

 in Cyclodus gigas and in the tegu, a large lizard which frequents 

 the forests of the Amazon ; there is also no parietal eye in certain geckos, 

 e.g. Hemidactylus, Gehyra, Platydactylus ; nor in some Agamida:, namely 

 Draco, Ceratophora, Lyriocephalus . A definite parietal eye is also absent 

 in the snakes, tortoises, turtles, crocodiles, and alligators. 



S.D' EpP Ep.A. 



"K r 



S.D* 



Fig. 180. — Reconstruction of the Structures on the Medial Aspect of the 

 Diencephalic Region of the Brain of Testudo geometrica ( x 40) ; 



SHOWING THE RELATIONS OF THE ANTERIOR EPIPHYSIS AND POSTERIOR EPI- 

 PHYSIS to the Dorsal Sac (Redrawn from G. W. H. Schepers.) 



CD. : dorsal commissure. Ep. A. 



C. Hb. : habenular commissure. Ep. P. 



C. Op. : optic chiasma. H.C. : 



C. Po. : posterior commissure. Hyp. : 



C.V. : ventral commissure. N. Op. 



E. Hyp. : hypothalamic eminence. S.D. 1 , 



E. Th. : thalamic eminence. S.EH. 



: anterior epiphysis. 

 : posterior epiphysis, 

 cerebral hemisphere, 

 hypophysis. 

 : optic nerve. 

 2 , 3 : dorsal sac. 

 : sulcus endohippocampi. 



In some of the above-mentioned examples in which a parietal eye 

 is stated to be absent, there is a subdivision of the primary pineal evagina- 

 tion into two parts, an upper and a lower. Thus in the tegu, there is a 

 difference of opinion as to whether " the upper division of the embryonic 

 evagination represents the rudiment of a parietal eye which at a later 



