128 FREDERICK TILNEY AND LUTHER F. WARREN 



the membrana limitans externa. The pigment in the cells is in- 

 some cases arranged in transverse bands or stripes, according 

 to Spencer 366 in Sphenodon and Ley dig ('9 1), 238 in Anguis. All 

 of the rod cells are similar. The connection of the retinal ele- 

 ments with fibers of the parietal nerve is not yet altogether 

 understood. In adults the organ is rudimentary. It is not 

 known whether the nerve fibers come from the large retinal 

 elements, from the ganglionic cells of the deep retinal layer, or 

 from the large cylindrical cells of the inner layer. The latter 

 seems most probable in view of the conditions in Petromyzon. 



The parietal nerve. This nerve was first described by Spencer 366 

 in 1886 and has been observed by many others since then. 

 Spencer believes that the parietal nerve is connected with the 

 end of the epiphysis, that is to say, a direct continuation of the 

 pineal organ. The entire course of the parietal nerve from the 

 parietal eye to the brain roof was first traced by Strahl and 

 Martin 383 in 1888 in older embryos of Lacerta vivipara and 

 A nguis fragilis. These observers showed that the nerve was 

 completely independent of the epiphysis. Beraneck 23 in 1892 

 made more exact studies and confirmed the view of Strahl and 

 Martin. Other authors are also emphatic in stating the com- 

 plete independence between the epiphysis and the parietal eye. 

 Among them are Studnicka ('93), 384 in Lacerta; Klinckowstroem 

 ('94) 209 in Iguana; Leydig ( J 96) 239 and Bendy ( 7 99) 87 in Spheno- 

 don, and Schauinsland ('03) 347A also in Sphenodon. The parietal 

 nerve begins to develop shortly after the separation from the 

 roof of the parietal eye. Of the direction of its fibers, whether 

 from the brain to the eye or, as is the case in the pineal organ 

 and the paired eyes, from the eye to the brain, there is no proof. 

 The latter course, however, is most probable. In Anguis, the 

 parietal nerve first appears at 50 mni. embryo size and reaches 

 its maximum of development between the 27 and 30 mm. size. 

 In Iguana, the nerve is well developed at fourteen days and is 

 at its maximum at twenty-four to twenty-six days. Between 

 the thirtieth and fortieth days it shows signs of reduction. 

 Strahl and Martin 383 showed that the nerve comes into relation 

 with the ganglionic cells forming a prominence with the brain 



