EMBRYOLOGY OF CRYPTOBRANCHUS 



507 



appearing and seldom clearly expressed (Stage 14), we must be 

 on our guard against a possible discontinuity or difference in 

 kind between the segmentation of the anterior and the posterior 

 regions of the cephalic plate. These points can be settled only 

 by a careful study of sections of eggs that have first been described 

 externally; but from surface views alone we are justified in claim- 

 ing that we have in the open cephalic plate transverse divisions 

 which may be homologized in different embryos, and which are 

 probably of true metameric value ; hence they may be of use in 

 solving the vexed problem of the segmentation of the vertebrate 

 head. 



Fig. 159 A. living embryo of Cryptobranchus allegheniensis in the early part 

 of Stage 13, viewed in direct sunlight, and so far as possible by transmitted light. 

 From a freehand sketch of the upper hemisphere. X 10. 



A pair of depressions just within the neural folds near the ante- 

 rior end of the cephalic plate probably indicate the anlage of the 

 optic vesicles (cf. Eycleshymer '95; Locy, '95). 



Some features of this stage are best brought out by the study 

 of living material; for -this purpose embryos have been examined 

 in direct sunlight. As shown in figure 159 a transverse opaque 

 band early appears directly in front of the neural plate in the 

 median region; in position and appearance it reminds one of the 

 ectamnion of the chick (Lillie '08, pp. 138 and 139). The neural 



