130 AUTHOR DENDY. 



(2) a double or triple row of spherical nucleated elements, 

 apparently formed from the deeper cells of the inner embryonic 

 layer. 



The outer of the two primary layers, consisting in the 

 embryo of a single layer of short columnar cells, appears to 

 have given rise to spherical elements, cone-shaped bodies and 

 spindle-shaped elements according to Spencer, but Hoffmann 

 (18) only recognises one layer in place of these three. 



The Lens. — In the adult Sphenodon the lens of the 

 parietal eye has become much more strongly convex on its 

 inner aspect (by elongation of the columnar cells occupying 

 its centre) than it is in the latest embryo examined. In 

 advanced embryos the lens (figs. 1-i — 16) is much more 

 sharply marked off from the retina than is indicated in Spencer's 

 figure, and I have noticed the same fact in the development 

 of the parietal eye in an Australian Hinulia. l^eard (4) has 

 also called special attention to the sharp delimitation of the 

 lens from the retina in Anguis, in support of de Graaf's 

 original description (16), 



Supposed Arthropod Characters. — The 'Mmraour," 

 so frequently noticed as forming a coagulum in the cavity of 

 the parietal eye vesicle, apparently persists in the adult, as 

 indicated in Spencer's fig. 3. As a result of his observations 

 on Ammocoetes, Gaskell (12) has put forward what I cannot 

 help regarding as a mistaken view as to the nature of this 

 humour, at any rate so far as the types examined by me are 

 concerned. He says, "Everything seems to me to point to 

 the conclusion that the appearance of a large central cavity 

 [in the parietal eye] is brought about by the partial degenera- 

 tion of elements which originally filled it, and that their 

 remains have given rise to the impression held by Spencer and 

 Beard, that a large cavity exists which is filled by a coagulated 

 albuminous fluid." Gaskell accordingly claims an Arthropod 

 structure for the parietal eye of vertebrates.^ 



It appears to me, however, that the observations of Beard 



^ I learn from a paper by Prenant (26) that Leydig has also compared the 

 parietal orgaus with Arthropod eyes. 



