6io THE WONDER OF LIFE 



In the lamprey, both the epiphysis and the parietal organ 

 show this. Above Reptiles the pineal stalk is short and 

 its terminal portion is glandular. The epiphysis is occa- 

 sionally absent in Mammals (e.g. some Cetaceans), and 

 the pineal body is absent in the dolphin and Dasypus. 

 According to some authorities, the pineal body was primi- 

 tively an unpaired median, upward-looking eye ; according 

 to others, the optic function is a secondary transformation. 



,3 



FIG. 95. Section showing developing pineal eye of Sphenodon. (After 

 Dendy.) 1, Epidermis; 2, Dermis ; 3, Lens; 4, Wall of pineal 

 eye ; 5, Choroid plexus of dorsal sac, on the roof of the third ven- 

 tricle of the brain ; 6, Nerve of pineal eye ; 7, Pineal sac. 



For it not infrequently happens that a dwindling structure, 

 tending to become vestigial, may become secondarily 

 specialized. Thus the vestigial hairs on the lips of some 

 whales have a quite extraordinarily rich innervation. 

 It must be frankly admitted that many of the examples 

 that used to be given of the re-assertion of long-lost an- 

 cestral characters were insufficiently criticized, and the 

 list of so-called reversions has been remorselessly thinned 

 by the more modern students of inheritance. Sometimes, 



