THE VISUAL APPARATUS. 153 



These relations are illustrated by Figure 76. Although degen- 

 erate in all vertebrates, yet one or the other of these organs is capa- 

 ble of light perception, at least in cyclostomes and some reptiles, 

 and perhaps in some fishes and amphibia. The structure of the 

 organ is apparently much simpler than that of the retina, but in 

 some forms the presence of rods or cones has been described. 

 There is evidence that the pineal eyes are not median structures. 

 The organ is never quite median in its adult form and in the 

 embryo paired organs begin to develop and only one .persists. 

 In selachians there have been described in early stages a series 

 of ''accessory optic vesicles" following the true optic vesicles. 

 The pineal eyes were probably originally paired organs serially 

 homologous with the lateral eyes, and the three pairs represent 

 the cutaneous sensory system in those segments of the head in 

 which the cutaneous nerves and ganglia are wanting (p. 61). 



DEMONSTRATION OR LABORATORY WORK. 



1. Study the development of the optic vesicle, lens and optic tract 

 in some vertebrate embryos. Compare the histogenesis of the retina 

 with that of the brain wall. 



2. Histology of the retina in Golgi preparations. 



3. Trace the course of the optic tracts and compare the chiasma 

 with the decussation of internal arcuate fibers in the medulla oblongata 



4. Study the structure of the tectum opticum in Golgi sections of 

 the brain of a fish and the frog. 



5. In Golgi or Weigert sections study the tracts arising in the 

 tectum and colliculus. 



LITERATURE. 



Barker, L. F. : The Nervous System and its Constituent Neurones. New 

 York. 1899. Chapter LIII especially. 



Boveri, T.: Ueber die phylogenetische Bedeutung der Sehorgan des 

 Amphioxus. Zool. Jahrb., Suppl. Bd. 7. 1904. 



Cajal, S. R.: Die Retina der Wirbelthiere. Wiesbaden. 1894. 



Eycleshymer, A. C. : The Development of the Optic Vesicles in Amphibia 

 Jour. Morph., Vol. 8. 1890. 



Froriep, A.: Die Entwickelung des Auges der Wirbelthiere. Hertwig's 

 Handbuch der Entwickelungslehre. 1905. 



Hesse, R.: Die Sehorgane des Amphioxus. Zeit. f. wiss. Zool., Bd. 63. 

 1898. 



