556 LOCV. [Vol. XI. 



eyes are formed out of neural-plate material as diverticula of 

 the fore-brain, while the other sense-organs arise outside the 

 neural plate ; they have an independent epiblast origin. 



It is questionable, whether any particular stress should be 

 put upon that argument, as the neural plate is at best only a 

 part of the modified epiblast ; moreover, the suggestion that the 

 neural plate is, undoubtedly, very much widened and expanded 

 from its ancestral condition, and that certain sensory area, 

 originally lying outside, may have been included in it, does 

 much to offset that objection. The neural plate has been a 

 prodigiously long time in forming, and the eyes have been 

 brought into closer relationship with it. The plate is broadest 

 in front, and the eyes are the most anterior sense-organs, and 

 have become included in this expansion. While there is not 

 sufficient data to give a wholly satisfactory answer to the ques- 

 tion propounded, the assumption that the eyes are closely 

 related to all the others is not without foundation, and we are 

 now in the attitude of awaiting further facts. 



II. Accessory Optic Vesicles. 



The neural plate, while still in very young stages, and while 

 the neural grooves are widely open, becomes the seat of some 

 accessory differentiations that resemble the optic vesicles. So 

 far as I know no observations upon these structures have been 

 recorded except those in my preliminary account in the 

 Journal of Morphology. I quote from that paper : But, 

 more interesting than the fact of their (the optic vesicles') very 

 early appearance in Elasmobranchs, is their apparent relation- 

 ship to other depressions that are formed upon the cephalic 

 plate, behind the already established optic vesicles. The new 

 involutions referred to, make their appearance upon the 

 cephalic plate just back of the optic vesicle. Two of them 

 (Fig. 13, ac. V and Fig. 17, ac. v^, ac. v~) take precedence of all 

 others in development, and are so distinctly formed as to afford 

 a good basis for comparison with the optic vesicles. They are 

 circular depressions formed in front of the latter, and they 

 produce upon the exterior corresponding rounded elevations. 



