177 



they show there, are not open to the possible interpretation of being 

 due to exaggerated drawings or to erroneous observation. 



While there can be no doubt of the existence of such structures 

 as I have described on the cephalic plate, the interpretation of them, 

 as rudimentary optic vesicles, may not meet with such ready accept- 

 ance. I should at any rate, make clear the basis for such a con- 

 clusion, and in support of the view, I repeat once more that they are 

 formed very early, along with the primary optic vesicles, before any 

 of the cranial ganglia appear, or before any "brain vesicles", in the 

 usual sense of the term are formed. They have the same histological 

 structure as the optic vesicles, and they grow like them up to a 

 certain period, and then, according to my observations, one pair fades 

 away — a very plausible fate for such a rudimentary structure — 

 and the other pair forms part of the walls of the thalamencephalon, 

 and gives origin to the pineal outgrowth. 



We have in this pineal outgrowth, or epiphysis a puzzling 

 structure that has given rise to no little perplexity. The various ex- 

 planations of the way in which it has arisen, naturally, have been 

 theoretical because its origin has up to the present been entirely 

 unknown. 



The observations recorded here will naturally form the basis for 

 some speculations regarding the nature of the pineal eye in Elasmo- 

 branchs. The latter organ, certainly, is not highly developed in these 

 animals, but whether its homologue is present or entirely lacking is 

 a question upon which there is difference of opinion. Beraneck 1 ) 

 says "il manque chez les S61aciens". But the basis of interpretation 

 is now changed, since I have followed epithelium of a visual character 

 into the epiphysis, and it will be more in harmony with this observ- 

 ation to say, that, although rudimentary the organ is undoubtedly 

 present in its essence in Elasmobranchs and, that the enlarged distal 

 end of the epiphysis probably is the equivalent of the eye-capsule in 

 those forms in which the latter structure is differentiated. This will 

 be simply returning, on a new basis, to Spencer's conclusion. 



It appears from the evidence produced by a number of reliable 

 investigators (Spencer, Beard, Beraneck and others) that there is 

 considerable diversity, in different animals, as regards the development 

 of the different parts of the epiphysial outgrowths. 



Spencer has shown the existence (in Hatteria and other forms) 

 of an eye-like capsule having connections with the epiphysis by a stalk. 

 He regards the eye-like capsule in those cases as deviations from the 



1) Anat. Anz., Vol. VII., Oct. 1893, p. 685. 



