396 EYCLESHYMER. [Vol. X. 



of the wall until the optic vesicle is reduced to a single layer; 

 meantime the external layer of the epiblast in this region has 

 divided. In a few cases paired depressions have been found 

 in earlier stages (Fig. 9). Sections, however, reveal nothing 

 in the way of histological differentiation, and one might justly 

 consider them as artifacts. The only thing which would dispel 

 all doubt as to their meaning, would be to find some form in 

 which they are so well marked that they may be traced, step 

 by step, through the phases of involution of the neural plate, 

 to the future optic vesicles. This condition is perfectly ful- 

 filled by Rana palustris. In an early embryonic stage (Fig. 5), 

 when the neural ridges are just forming, and are widely sepa- 

 rated, paired depressions appear just within the cephalic folds, 

 on either side of the median line. These areas are sharply 

 contrasted with the surrounding parts by a much deeper 

 pigmentation. A transverse section through them is shown in 

 Fig. 12. The hypoblast (/y/.) is a single layer forming the roof 

 of the mesenteron. In the median line there is a fold which 

 is the end of the chorda. In many cases this extends beyond 

 the region of the neural fold, and is a point of theoretical 

 interest. The mesoblast (mes.) consists of a single layer of 

 cells, which passes insensibly into the axial region, where all 

 three layers are fused. The sensory layer of the epiblast 

 {s.e/>.) exists as two lateral thickenings, united by the thinner 

 median portion. 



In Amblystoma and Necturus the superficial layer of the 

 epiblast cannot be distinguished beyond the region of the 

 neural folds, while in Rana it extends over the entire plate, as 

 shown in the figure. In this layer the optic pits are formed. 

 In addition to the fact that these areas are sharply defined by 

 the presence of pigment to which more or less importance may 

 be attached, they are further remarkable in that the elongation of 

 the cells and the position of nuclei are indicative of a considerable 

 degree of histological differentiation. Between these areas the 

 cells are undifferentiated and resemble those of the superficial 

 epiblast in other parts of the embryo. Sections further poste- 

 rior show the cells of this layer to be uniform. Fig. 13 repre- 

 sents a section through a later stage, when the folds are still 



