No. 2.] EARLY DEVELOPMENT OF AMBLVSTOMA. 395 



nary paper ('93), the substance of which is here repeated, that 

 this was precisely the case in Rana palustris. Soon after- 

 ward Locy ('93) found the same in the Shark. 



It should not be said, however, that we were wholly with- 

 out observations pointing toward an earlier differentiation. 

 Bischoff, Kolliker, His, Van Beneden, and others have figured 

 the early appearance of the optic vesicles in mammals. Heape 

 (-84) finds in an early stage of the Mole, where the neural folds 

 are closed along the center of the embryo, " 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 nar- 

 row groove indicates the commencement of the formation of 

 the optic organs." 



Keibel ('89) describes a like condition in the embryo of the 

 Guinea-pig. This apparently precocious development of the 

 eyes in mammalia, showing no differentiation beyond the fact 

 that depressions are present, is probably 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 ('89) discovered that in Necturus 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 this form. In the 

 stage (Fig. 10) corresponding closely to that described, the 

 embryo is well defined, the closure of the neural folds having 

 taken place posteriorly, and closely approximated well up 

 toward the region which later forms the head. At the anterior 

 end of the embryo, on either side, slight evaginations are visi- 

 ble. A section through these vesicles (Fig. 1 5), shows more 

 numerous mitoses in these regions, as well as a marked migra- 

 tion of the nuclei toward the periphery. This is the earliest 

 indication I have been able to make out satisfactorily. In 

 Fig. II a later stage is represented, in which the optic vesicles 

 are much better defined. Sections (Fig. 16) at this time show 

 a continued migration of the nuclei, together with a thinning 



