PAEIETAL EYE AND ADJACENT ORGANS IN SPHENODON. 133 



parietal eye and the stalk originate as separate outgrowths 

 from the brain. 



As one of the earliest of the first-naraed school I may cite 

 McKay (25), whose valuable work appears to have been over- 

 looked by many of the more recent writers. The figures given 

 by McKay of the early stages in the development of the 

 parietal eye in Grammatophora agree very closely on the 

 whole witii what I have observed in Sphenodon, but he 

 interprets the appearances as follows : — ''The epiphysis cerebri 

 or pineal gland arises, as is seen, as an outgrowth of the 

 thalamencephalon. At this stage the outgrowth is composed 

 of a single layer of columnar cells with well-marked nuclei 

 (fig. 1). Second stage. — In the next stage the evagination or 

 vesicle undergoes the following changes. The anterior wall 

 begins to grow forward, and this soon leads to the formation 

 of a second evagination in the wall of the primary one (fig. 

 2, Pn.). Thus we have two vesicles formed, an anterior 

 larger (Pn.), destined to become the pineal eye, and a posterior 

 smaller one [Ep.]. Since the anterior vesicle grows faster 

 than the posterior it bends forwards, and its inferior wall 

 rests on the superior surface of the columnar cells of the 

 thalamencephalon (fig. 2). The walls of both vesicles are 

 composed of a single layer of columnar cells with oval nuclei.'^ 

 Of his third stage he observes, '' The anterior of the two 

 vesicles becomes constricted off to form the pineal eye (fig. 

 4, Pn.) ; while the posterior remains as the end of the epi- 

 physis (fig. 4>,Ep.).'' 



Unfortunately I have been unable to consult Hoffmann's 

 original memoir^ on the development of Lacerta, but this 

 author has also described and illustrated his results in his 

 memoir on the Reptilia in Bronn's ' Klassen und Ordnungen 

 des Thier-reichs' (18). From this description it appears that 

 in Lacerta also the early development of the parietal eye and 

 stalk is very similar to what takes place in Sphenodon and 

 Grammatophora. A hollow outgrowth is budded off from 



' ' Weitere Untersuch. zur Eutwicklungsgesch. der Reptilien. Morpbol.,' 

 Jabrb. xi, 176 (quoted by McKay). 



