No. 3] THE VERTEBRATE HEAD. 56 1 



lateral organs in the fish, where they make their first appear- 

 ance as segmental papillce in the strictest sense of these words, 

 cannot be explained on a more satisfactory hypothesis," 



III. The Pineal Sense-Organs. 



I. Growth of Knowledge regarding the Pineal Sense-Orgajis. 



The pineal outgrowth has attracted much attention since the 

 discovery, in 1886, of its eye-like structure. Since then it has 

 been accepted by morphologists as a rudimentary sense-organ. 

 The earlier literature bearing on the subject has been thor- 

 oughly reported on by Spencer ('86) and Francotte ('89). 

 Several recently published papers taken together put the 

 subject in a new light, and, as Ritter ('94) has said, " At no 

 time since the epiphysis and pineal eye have been the topic of 

 investigation has it been a more interesting or a more inviting 

 one than it is just now." 



The growth of our knowledge regarding this remarkable 

 sense-organ, scattered through the various publications of 

 Spencer, Beraneck, Francotte, Strahl and Martin, Leydig, and 

 others, may be reduced to the following summary : 



First came the demonstration of its eye-like structure 

 in Amphibia, by De Graaf, and in Lacertilia by Spencer. 

 De Graaf's publication is much briefer, and was published 

 only a few months before Spencer's extensive study. Both 

 authors investigated the structure in adult forms. Spencer 

 studied its condition in twenty-eight different species of 

 Lacertilia, and demonstrated very clearly that even in the 

 adult forms of these animals, this organ has an eye-like struc- 

 ture, with a lens, a pigmented retinal layer, and a nerve. It 

 seems doubtful, in the light of recent work, whether the 

 fibrous strand connecting the eye-capsule with the epiphysial 

 stalk, and designated the nerve by Spencer, is really nerve or 

 not. 



Following the study of the adult structure came investiga- 

 tions of the embryonic conditions. The first studies dealing 

 with the embryonic development of this organ, and with its 



