THE DEVELOPMENT OF THE OPTIC VESICLES 



IN AMPHIBIA. 



ALBERT C. EYCLESHYMER. 



Notwithstanding the searching observations made during 

 the past score of years, but little has been found which would 

 essentially modify the statements of the pioneer embryologists : 

 that the eyes first appear as a pair of diverticula budding out 

 from the sides of the anterior cerebral vesicle. 



It should not be said, however, that we are wholly without 

 observations pointing toward an earlier differentiation. Bis- 

 choff, Kolliker, His, Van Beneden and others have noted that 

 in mammals the optic vesicles appear extremely early. Heape^ 

 finds in an early stage of the Mole, where the neural folds are 

 closed along the center of the embryo, that " at the anterior 

 end the floor of the neural groove, on either side, is swollen, 

 and on the outer and anterior edge of the two masses a deep 

 narrow groove indicates the commencement of the formation 

 of the optic organs." Keibel ^ describes a like condition in 

 the embryo of the guinea-pig. This apparently precocious 

 development of the eyes in mammalia, showing no differen- 

 tiation beyond the fact that depressions are present, is prob- 

 ably due to the retarded closure of the cephalic portion of 

 the neural groove, and can scarcely be considered as a fact of 

 phylogenetic significance. Whitman ^ discovered that in Nec- 

 turus there is a very early appearance of the eye, " its basis 

 being discernible as a circular area — after treatment with 

 osmic acid, followed by Merkel's fluid — long before the closure 

 of the neural folds of the brain." 



Through the kindness of Professor Whitman I have been 

 able to study the development of the eyes in Necturus, and 



1 Quart. Journ. Micr. ScL, 18S3, p. 106. 



2 Arch. f. Anat. u. Physiol., 1889, p. 372. 



3 Journal of Morphology, 1S89, p. 593. 



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