THE PINEAL BODY 235 



its position, but also because of its anatomy and connections. 

 He is of the opinion that the unpaired eye is more ancient than 

 the lateral eyes. This is the more probable since the ancestors 

 of the vertebrates were mon-ophthalmic, examples of which are 

 to be found in the pyrosomes which have but a single median 

 eye. Subsequently, the lateral eyes make their appearance in 

 tunicates and these functionate simultaneously with an unpaired 

 eye. Peytoureau 308B gives six diagrams showing the degenera- 

 tive process from the median eye of pyrosomes to the epiphysis 

 in the higher mammals, as follows: 1, In pyrosomes, a simple 

 vesicle with a lens; 2, in larval urodela there is a vesicle with 

 nerve connections and nerve centers but no lens; 3, in chamaeleon 

 there is only an epithelial vesicle which has no connections or 

 neural characteristics; 4, in batrachians the oTgan is a detached 

 epithelial cluster having no connection with the central nervous 

 system; 5, in cyclodus the organ is a gland attached to the third 

 ventricle by means of a peduncle; 6, in mammals and birds it is 

 connected with the brain by a solid pedicle but presents no vesicle. 

 Gaskell, 146 in his summary concerning the evidence of the 

 organs of vision and their bearing upon the origin of vertebrates, 

 writes as follows: 



The most important discovery of recent years which gives a direct 

 clue to the ancestry of the vertebrates is undoubtedly the discovery 

 that the pineal gland is all that remains of a pair of median eyes which 

 must have been functional in the immediate ancestor of the vertebrate, 

 seeing how perfect one of them still is in Ammocoetes. The vertebrate 

 ancestor, then, possessed two pairs of eyes, one pair situated laterally, 

 the other median. In striking confirmation of the origin of the verte- 

 brate from Palaeostracans it is universally admitted that all the Euryp- 

 terids and such-like forms resembled Limulus in the possession of a 

 pair of median eyes, as well as a pair of lateral eyes. Moreover, the 

 ancient mailed fishes, the Ostracodermata, which are the earliest 

 fishes known, are all said to show the presence of a pair of median eyes 

 as well as of a pair of lateral eyes. This evidence directly suggests that 

 the structure of both the median and lateral vertebrate eyes ought to 

 be very similar to that of the median and lateral arthropod eyes. Such 

 is, indeed, found to be the case. 



The retina of the simplest form of eye is formed from a group of 

 the superficial epidermal cells, and the rods or rhabdites are formed 

 from the cuticular covering of these cells; the optic nerve passes from 

 these cells to the deeper-lying brain. This kind of retina may be called 



