190 



E YCLESHYMER. 



[Vol. VOL 



find that at a stage (Fig. I) corresponding closely to that 

 described, there is considerable differentiation, in that mitoses 

 are more numerous in these regions, as well as a marked mi- 

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

 indication I have been able to make out satisfactorily. In a 

 few cases paired depressions have been found in earlier stages, 

 even before the neural ridges have become prominent; but 

 sections through these depressions show nothing in the way of 

 histological differentiation, and one might justly consider them 

 as nothing more than artifacts. A like condition occurs in 

 Amblystoma, yet the cases are by far too rare to be of any 

 special significance. 





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Fig. I. 



Surface view of an embryo of N'ecturus. 

 a. optic vesicles. 



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Fig. II. 



Surface view of an early embryo of 

 Ra7ta palustris. a. optic pits. 



The only thing which would dispel all doubt as to the meaning 

 of these areas would be to find some form in which they are so 

 well marked that they may be traced through the phases of in- 

 volution of the neural plate to the future optic vesicles; this 

 condition is perfectly fulfilled by Rana palustris (.'') a form 

 hitherto unstudied. For an abundant supply of material I am 

 indebted to Dr. William M. Wheeler. 



In an early embryonic stage (Fig. II) when the neural ridges 

 are just forming and are widely separated, there appear in the 

 anterior portion of the neural plate, on either side of the median 



