70 THE PINEAL ORGAN 



absent over the pineal organ, and the pigment cells of the epidermis are 

 much reduced here both in size and number (Fig. 134, Chap. 17, p. 188). 

 The whole area thus appears pale as compared with the surrounding skin. 

 In the middle of the parietal cornea is a circular white spot (" weissen 

 Scheitelfleckes ") produced by the retina of the parietal organ which is 



P.St 



L.Mri 



N C.T.N 

 Fig. 46. — Parietal Sense-organ of Geotria, as in Fig. 45. 



Diagram showing the minute structure of the retina ; on the right side the pigment 



is represented, on the left the cells as seen when the pigment has been removed. 

 Lettering as in Fig. 45, in addition : 

 C.T.C. : connective tissue cell. 

 I.S.P.C. : inner segment of pigment 



cell. 

 L.M. : limiting membrane. 

 N.F.N. : network of nerve-fibres. 

 N.P.C. : nucleus of pigment cell. 



N.S.C. : nucleus of sense-cell. 

 O.S.P.C.: outer segment of pigment 



cell. 

 R.S.C. : retinal sense-cell. 

 S.C.K. : terminal knobs of sensory 



cells of retina. 



seen shining through the cornea. Dendy describes the appearance of this 

 minute central area in the New Zealand lamprey, when seen from above 

 (Fig. 47), as a white rim enclosing a central spot, the latter lying directly over 

 the " pellucida " or " lens." The white rim is due to the white pigment of 

 the retina (? phosphate of lime) shining through the transparent cornea. 



