No. I.] 



OPTIC VESICLES IN AMPHIBIA. 



191 



line and just within the cephalic fold, paired depressions {a) 

 which are sharply contrasted with the surrounding parts by a 

 much deeper pigmentation. A transverse section through these 

 areas shows the condition represented in Fig. III. The hypo- 

 blast (//) is a single layer forming the roof of the mesenteron; 

 in the median line a fold represents the anterior end of the 

 chorda. The mesoblast im) consists of a single layer of ir- 

 regular scattered cells, which pass insensibly into the axial 

 region, where all three layers are fused. The deeper layer of 

 the epiblast has become much thickened, resulting in the 

 formation of the broad epiblastic plates, which are united by 

 the thinner median portion. In Amblystoma and Necturus 



Fig. III. 

 Transverse section through optic pits. h. hypoblast. 7n, mesoblast, e, epiblast. 



the superficial layer of the epiblast cannot be distinguished 

 beyond the region of the neural ridge, while in Rana the two 

 are very distinct, as shown in the figure, and it is in this layer 

 that 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 the nuclei are indicative of a considerable degree 

 of histological differentiation. Between these areas the cells 

 are undifferentiated, and resemble those of the superficial epi- 

 blast in other parts of the embryo. Sections posterior show 

 the cells of this layer to be likewise uniform. 



