1 64 THE HUMAN SPECIES 



The position of the parietal eye in the Petromyzon species 

 is indicated by a white parietal spot between the permanent 

 eyes, and by the epidermis and corium being destitute of pig- 

 ment. 1 



In the Selache the vesicle has neither retina nor pellucida 

 nor a true cornea, and only in the Spinax is a parietal spot 

 without pigment to be recognised. - 



The parietal organ has the same form in the Ganoidea, but 

 in the Teleostei it is a somewhat reduced organ, very de- 

 fective in structure, without cornea and parietal spot, connected 

 with the roof of the skull either by the distal extremity of the 

 terminal vesicle, or by the entire upper surface of the same, 

 whereby the roof of the skull is rarely perforated. 3 Of still 

 simpler organisation is the parietal organ of the Urodelian 

 amphibians, being merely a sac with slight folds, lying in close 

 proximity to the roof of the mesencephalon, but, on the other 

 hand, the conditions prevailing in the Anura prove beyond a 

 doubt that their progenitors possessed parietal eyes, for the 

 white parietal spot between the existing eyes is, with very few 

 exceptions, plainly visible. 4 In reptiles the parietal organ varies 

 greatly. Of the two parietal organs of the Saurians and 

 Prosaurians only the anterior organ can claim to be a true 

 parietal eye, but this claim has far more justice than in the case 

 of any other vertebrate, for, where it does occur, all the attri- 

 butes of the true eye are present, namely, the cornea above the 

 foramen orbitale, lens, retina and pigment, and a corpus vitreum. 

 and the nerves connected with the brain are clearly recognisable 

 (see Fig. 83). 5 



Contrasted with this third eye of the Saurians the parietal 

 organ of the Ophidians is only a solid structure abundantly 

 furnished with blood vessels with a strand of nerve fibres in its 

 peduncle, and is probably to be considered as a secreting gland. 

 Of a foramen parietale and a parietal spot there is no trace. 6 

 Nor can the Chelonia and crocodiles be said to possess a true 

 parietal eye, only rudimentary sac-like appendages to the 

 cerebral cortex being present without peduncle. Rudimentary 



1 Studnicka, loc. cit., pp. 42-44. z lbid., p. 48. 



3 Ibid., pp. 82-85. 4 Ibid., pp. 110-120. 



5 Ibid., pp. 134-198. 6 Ibid.. p. 199. 



