EMBRYOLOGY OF CRYPTOBRANCHUS 513 



the open neural plate, in this region at least, are the divisions 

 between grooves — that is, the ridges rather than the depressions, 

 for the former are in line with the body somites. If, as appears 

 likely, there is continuity between the structures of the anterior 

 and posterior regions of the cephalic plate, then the rule may 

 be extended to include the entire neural plate. As thus defined, 

 there are seven segments — ^'neuromeres' — in front of Groove I; 

 posterior to this groove an undetermined number of segments 

 also belong to the head. 



In the early stages of the formation of the neural folds trans- 

 verse grooves are sometimes found in them, continuous with 

 the transverse grooves of the neural plate (see especially figs. 154, 

 165 and 172). In such cases the neural fold is marked by an 

 outer as well as an inner notch, both in line with the transverse 

 furrow of the neural plate. This condition is only temporary 

 and apparently it is transitional to a later phase in which the 

 inner notch grows at the expense of the outer one, until an outer 

 convexity of the fold appears opposite the inner concavity (see 

 especially fig. 168). In the region of the body somites these 

 outward flexures thus lie in line with the intersomitic grooves 

 as well as with the transverse grooves of the neural plate. This 

 condition is seldom so well expressed as in the embryo shown in 

 figure 168; the convolutions of the neural folds are often irregular 

 and bear no definite relation to the segments. But it is fairly 

 certain that in all cases where the neural folds are well upraised 

 and flexures occur which are segmentally arranged, the outward 

 flexures lie opposite the transverse furrows and not opposite 

 the ridges between them. Moreover, in sagittal sections the 

 transverse grooves on the external surface of the neural plate 

 are found to correspond to ridges on the internal surface. 



Those who have described segmental structures in the neural 

 folds or closed neural tube have, as a rule, accepted Orr's ('87) 

 definition of the segmental units or neuromeres as outward flex- 

 ures of the neural folds. But' if the above considerations be well 

 founded, the true segments are to be sought rather in the seg- 

 ments between the transverse grooves of the neural plate, and 

 in the inward flexures of the neural folds. In other words neu- 



