No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 40 1 



very definite statement, since if the organ is rudimentary its 

 development may be retarded, yet its ontogeny indicates that 

 it arose at a time when a neural canal was first formed. If we 

 admit the hypothesis, which I believe I have proved, namely, 

 that the lateral eyes are present as a pair of depressions in the 

 cephalic neural plate, we might anticipate that at the phylo- 

 genetic period when they become implicated by the closure of 

 the neural folds a median eye would arise and become most 

 highly functional during the period when the lateral eyes were 

 non- or least functional. This might explain the origin of the 

 epiphysis as an unpaired organ; the circumstances which give 

 rise to another sense organ, if such the paraphysis be consid- 

 ered, are extremely difficult to explain. In my preliminary 

 note I suggested the above hypothesis as one of the pos- 

 sible explanations of the origin of a median unpaired visual 

 organ. 



Concerning this hypothesis Professor B6raneck ('92) says : 



" Quel a et6 le role de I'oeil parietal .^ Pourquoi s'est-il 

 d^veloppe } Eycleshymer cherche a resoudre ce probl^me en 

 disant que, lors de la fermeture du tube medullaire, les yeux 

 pairs — auparavant directement influences par la lumiere — per- 

 dirent d'une maniere plus ou moins complete leur fonction. 

 Cette perte d'activite fonctionnelle fut compens6e par I'appari- 

 tion d'un oeil dorsal impair, qui commen^a a s'atrophier, 

 lorsque les yeux pairs eurent reconquis leur preponderance. 



"Cette explication ne me satisfait pas, je I'avoue. Tout d'a 

 bord, j'ai peine a comprendre que les yeux pairs perdent leurs 

 fonctions pour les reprendre plus tard, alors qu'un autre organe 

 visuel est apparu entretemps pour les supplier. Puis ce r61e 

 de suppliant, joue par I'oeil parietal, s'accorde peu avec la 

 persistance de cet organe dans un groupe de Vert^bres aussi 

 eieve que celui des Sauriens, groupe dont les yeux pairs sont 

 fort bien organises." 



Although laying but little stress upon this view, as expressly 

 stated in my preliminary, I still maintain that there is nothing 

 unphysiological in postulating the decrease of function in a 

 given organ for a certain length of time and its gradual revival 

 under favorable environment. Lankester ('80) and Balfour 



