PARIETAL EYE AND ADJACENT ORGANS IN SPHENODON. 151 



Dermis. Epd. Epidermis. Ei/e. Ordinary paired eye. F. B. Fore-brain. 

 F. M. Foramen of Monro. F. N. P. Fronto-nasal process. Gang. Hub. R. 

 and L. Right and left ganglia liabenulse. H. B. Hind brain. H. C. Head 

 cavity. H. M. C. Hyomandibular cleft, hif. Infundibulum. Int. Integu- 

 ment. In. W. Inner layer of wall of parietal optic cup and of parietal stalk. 

 Iter. Iter a tertio ad quartum ventriculum. L. Lens of parietal eye. L. P. 

 Lens of paired eye. L. V. Lateral ventricle. Mand. Mandibular arch. 

 M. B. Mid-brain. Mes. Mesoblast. Mo. Mouth. Mol. Molecular layer in 

 retina of parietal eye. Not. Notochord. Nu. Nucleus. 0. L. Optic lobes. 

 0. Par. Origin of paraphysis between the choroid plexuses of the lateral 

 ventricles. Op. ch. Optic chiasma. Out. IF. Outer layer of wall of parietal 

 optic cup and of parietal stalk. 0. V. Optic vesicle of paired eye. Pa. E. 

 Parietal eye. Pall. Pallium of cerebral hemisphere. Pa.N. Nerve of parietal 

 eye. Par. Paraphysis. Pur. T. Tubules of paraphysis. Pa. S. Parietal 

 stalk. P. B. Pituitary body. p. m. Pia mater. P. V. Primary parietal 

 vesicle. Pig. Pigment. B. T. Roof of tiialamencepbalon. Tlial. Thalameu- 

 cephalon. Thai. Op. Optic thalamus. F. 3. Third ventricle. X. (Figs. 16 

 and 17.) Space due to separation of outer and inner layers of wall of parietal 

 optic cup. X. (Fig. 18.) Inner surface of retina of parietal eye. Z. Evagina- 

 tion of the roof of the third ventricle forming the proximal part of the 

 " epiphysis " of the adult, and sometimes known as the " Zirbeli)olstcr." 



Figs. I — 27. Sphenodon punctatus. 



Fig. 1. — Stage J. Embryo 4i. Transverse section of head, showing over- 

 lap of the left side of the roof of the fore-brain on the apparent ventral 

 aspect. (Reversed as compared with Fig. 5.) Zeiss A, oc. 1, camera outline. 



Fig. 2. — Stage K. Embryo 39. Transverse section of head, showing first 

 appearance of the primary parietal vesicle to the left of the middle line on the 

 apparent ventral aspect. (Reversed as compared with Fig. 5.) Zeiss A, 

 oc. 1, camera outline. 



Fig. 3. — Stage L. Embryo 50. Longitudinal vertical section of head 

 (nearly median), showing origin of primary ])arietal vesicle as an evagination 

 from the roof of the fore-brain. Zeiss A, oc. 1, camera outline. 



Fig. 4. — Stage L. Embryo 50. Portion of the same section, more en- 

 larged, to show the histological structure of the primary parietal vesicle. 

 Zeiss D^ oc. 1, camera outline. 



Fig. 5. — Stage M. Embryo 81. Transverse section of head, showing 

 origin of primary parietal vesicle on the left side of the middle line, and 

 apparently ventral owing to cerebral flexure. Zeiss A, oc. 1, camera outline. 



Fig. 6. — Stage N. Embryo 96. Median longitudinal vertical section 

 through the paraphysis at its first appearance, showing its origin as a backward. 

 pointing outgrowth of the roof of the fore-brain. Zeiss D, oc. 1, camera 

 outline. 



