TYPES OF VERTEBRATE AND INVERTEBRATE EYES 29 



The earlier pair of median eyes appear to have been supplanted by the 

 more highly evolved lateral eyes, and thus undergo regressive changes. 

 The vestigial median eyes are seen to arise in certain cases from a common 

 stalk. This divides into two terminal swellings, which are often of unequal 

 size. In some cases the smaller vesicle lies beneath the larger, as in 

 Ammocoetes (Fig. 22) ; in other cases one organ, which is the less highly 

 developed of the two, is shifted behind the other and the two vesicles in 

 the adult animal lie approximately in the median plane, as in Sphenodon 

 (Fig. 248, Chap. 24). These relations and their significance will be discussed 

 in detail in the sections on the eyes of fishes and of reptiles (pp. 187, 242). 

 The consideration of the geological evidence of the bilateral origin of the 

 pineal organ in vertebrates and the fusion of the two parietal organs into a 

 single median structure will also be postponed until later. It will be 

 convenient, however, at this stage to describe the condition of cyclopia, 

 which is produced by a process which is similar to that of the fusion 

 of the median eyes of invertebrates and of the pineal organs in the 

 vertebrates. 



Cyclopia 



The occurrence of one-eyed monsters has excited the curiosity not 

 only of men belonging to our own period but also of the ancients. Among 

 the latter Homer stands out pre-eminently, and his tale of the adventures 

 of Ulysses and his fellow travellers with the giant Polyphemus has 

 fascinated and stimulated the imagination of all students of mythology. 



The questions which these ancient lays suggest, are : 



1 . Is there any foundation in fact for the classical myths of a race 



of giant cyclopes living apart from the world in insular 

 seclusion ? 



2. Is it possible for human cyclopes to reach maturity and have 



normal vision ? 



Both these questions may be answered in the negative. Among the records 

 of mammalian cyclopean monsters we have not found any authentic 

 cases of such having survived their birth by more than a few hours. 

 Many cases of cyclopia are combined with a high degree of agnathia, in 

 which the jaws are absent or imperfectly developed. In many of these 

 cases the nasal cavities are completely shut off from the pharynx, and in 

 some the pharynx ends blindly. 1 In the latter, the young would quickly 

 die from suffocation, while in the former the infant would be prevented 

 from breathing during the act of suction, and the process of sucking would 



1 R. J. Gladstone, " A Cyclops and Agnathic Lamb " (1910), Brit. Med. J., 

 2, 1159. 



