222 BULLETIN OF THE 



of the belief. From the morphologist's point of view, the evidence 

 that would remove all doubt as to the correctness of this opinion would 

 be that the vesicle regarded as the eyeball should be composed of ele- 

 ments essentially similar to elements found somewhere in organs known 

 to perform the mechanical part in the act of vision ; and, isecond, that 

 this vesicle should be connected with the brain by a nerve comparable 

 with the optic nerve of some known functional eye. I think no one 

 familiar with the structure of the vesicle as it exists in many Lacertilia 

 and in Petromyzon, will refuse to accept as satisfactory the evidence 

 on the first point. The evidence on the second point is less conclusive. 

 In many cases where the vesicle is well developed, as in Phrynosoma, it 

 is certain that nothing which can be justly compared to an optic nerve 

 exists. Spencer ('86 and '87) and several succeeding writers have held 

 it as beyond doubt that in several species, notably of the genera La- 

 certa, Ilatteria, and Varanus, there is a nervous connection between the 

 brain and vesicle. Leydig ('89), however, in his preliminary, based on 

 his study of Lacerta ocellata, Varanus elegans, and other forms, says 

 " der von Spencer beschriebene Nerv ist kein Nerv sondern das stning- 

 artig ausgehende Ende der Zirbcl." Lacerta ocellata is one of the 

 forms in which Spencer ascribes, with least question, a nervous nature 

 to the structure under consideration ; but apparently Leydig has not 

 examined either of the species of Varanus, viz. gigantea and Bengalen- 

 sis, which Spencer studied ; while, on the other hand, V. elegans, Leydig's 

 species, is not mentioned by Spencer as having been studied by him. 

 This denial in toto of the existence of the nerve as described by Spen- 

 cer, Leydig practically repeats in his most recent contribution to the 

 subject (Leydig, '90), and adGs, as further confirmation of his opinion, 

 that he has studied Hatteria (he does not tell us what species) and finds 

 that here also the so-called nerve is of the nature of connective tissue. 

 He also comes to the conclusion in this communication, that, while from 

 the structure of the vesicle alone the organ must at least be put among 

 the sense organs, it is yet "as good as itripossible to do so while it is 

 recognized that in the parietal structure of all the animals investigated 

 by me not one contains a nerve, for we must hold fast to the proposition 

 that for the equipment of a sense organ the peripheral end of a nerve is 

 necessary." It appears to me, however, that we are not compelled to 

 relinquish the belief that the organ was originally an eye, even though 

 we accept Leydig's statement, as against Spencer's and others, regarding 

 the nature of the supposed nerve in the cases which both have exam- 

 ined ; or even should it appear that in no case does the nervous con- 

 nection noiv exist. 



