MORPHOLOGICAL STUDIES. 73 
Fig. 5. — Parietal eye of Ammoccetes in transverse section. From the 
second of Herr Schwarz’s preparations. Zeiss D, oc. 2. 
Fig. 6. — Portion of the retina (/•.) and lens (/.) of the parietal eye of the 
preceding preparation under high power. Zeiss F, oc. 2. 
Figs. A, B, C. Diagrams showing three hypothetical stages in the develop- 
ment of the parietal eye. 
PLATE VII. 
Fig. 7. — Retinal elements of parietal of an adult Petromyzon planeri 
under high power. There was no black pigment in the specimen. Zeiss F, oc. 2. 
Fig. 8. — Parietal eye of adult Petromyzon planeri, showing position 
of parietal eye, the skull fossa ( s.f . ), and the distribution of the pigment in 
the skin over the eye. Longitudinal vertical section. Zeiss A, oc. 2. 
Fig. 9. — Parietal eye of an adult Petromyzon planeri, in which there 
was no pigment on the retina, showing the absence of pigment and of the 
skull fossa of the preceding figure. Longitudinal vertical section. Zeiss 
C, oc. 2. 
Fig. 10. — Retina of the parietal eye of Herr Schwarz’s third preparation. 
Transverse section. Zeiss D, oc. 2. 
Fig. 11. — Retinal elements of the parietal eye of Myxine glutinosa 
under higher power. Zeiss F, oc. 2. 
Fig. 12. — Longitudinal vertical section of brain and parietal eye of 
Myxine glutinosa. No pigment in the retina. Zeiss A, oc. 2. 
Fig. 13. — Longitudinal vertical section through the parietal eye of an 
advanced embryo of Anguis fragilis under high power, showing distinct 
boundary at B between lens and retina. Zeiss F, oc. 2. 
