No. 3.] 



THE VERTEBRATE HEAD. 



573 



eye in this animal to come normally from the right ganglion 

 hebenula, while in Petromyzon it comes from the ganglion of 

 the left side. But, his discovery of the occasional presence of 

 two nerves in Iguana, the left one being smaller, affords an 

 interesting transition between the two. It also enables us to 

 account for the marked asymmetry in the ganglia hebenulse 

 in Petromyzon, on the hypothesis that the right ganglion, in 

 Cyclostomes, has, for some reason, ceased to have any nerve 

 connection with the pineal organ, while the left ganglion has 

 retained its connection. 



As Beraneck has shown, a nerve leading to the pineal eye 

 arises in front of the epiphysis, and has but a transitory existence. 



Putting the facts together and basing relationships on the nerve- 

 supply, and comparative study of the vesicles, we may express 

 the probable homologies in the following diagrams and table : 



.<VV:.T; -.5- 



?,sX 





>• 



•;ii!UJSi 



^^ 



Petromyzon. 



■■yp^~- 



Teleosts. 



Cut II. 



.-.■^.r 

 -/ 



'^^ '. 





%v 



■■!l\^ 



Lacertilia. 



I. Upper vesicle in Petromyzon, epiphysis in Teleost and Lacertilia. 

 II. Lower vesicle in Petromyzon, Hill's anterior vesicle in Teleosts, pineal eye in Lacertilia. 

 N. Nerve to posterior commissure. N^. Nerve from lower vesicle in Petromyzon to Gan- 

 glion hebenula, embryonic nerve of transitory existence in Lacertilia. 

 Z.P. "Zirbelpolster." 



Petromyzon. 



Teleosts. 



Posterior epiphysial 



vesicle of Hill. 

 Pineal organ of Stud- \\. 



nicka. 

 Epiphysis of others. 



Is the superior 

 vesicle. Nerve- 

 supply from 

 posterior com- 

 missure. 



Is the epiphysis 

 with nerve con- 

 necting it to 

 posterior com- 

 missure. 



Lacertilia. 



Is the epiphysis 

 nerve-supply 

 from posterior 

 commissure. 



