EMBRYOLOGY OF CRYPTOBRANCHUS 501 



folds, the neural groove becomes conspicuous in its anterior as 

 well as its middle portion by the broadening and deepening of 

 the former region; at the same time the posterior end becomes 

 fainter (fig. 144). 



At the time of the first faint indications of the neural folds, 

 the neural groove is both broad and deep throughout its entire 

 length but especially in its anterior portion (fig. 145). During 

 these later changes in the breadth and depth of the neural groove 

 there has been very little increase in length; at the close of the 

 period considered it has a length of about 105 degrees and extends 

 from the dorsal lip of the blastopore nearly to the upper pole of 

 the vertical axis. 



According to Griggs ('10), in Amblystoma the first groove to 

 appear in the median line of the neural plate is not the neural 

 groove, properly speaking; this appears later on the same site. 

 There first appears a 'posterior germinal depression' which does 

 not reach the blastopore; a little later an 'anterior germinal de- 

 pression' is formed, which is discontinuous with the earlier groove. 

 In a later stage both give place to the neural groove which is 

 a different structure on the same site. 



In Cryptobranchus the earliest groove to appear in the median 

 line of the neural plate extends forward without break from the 

 dorsal lip of the blastopore. The marked depression shown in 

 figure 140 probably corresponds to the 'posterior germinal depres- 

 sion' in Amblystoma; the later depression in the anterior portion 

 of the neural groove perhaps corresponds to the 'anterior germinal 

 depression,' but it is at no time sharply separated from the 

 remainder of the groove. The later history of the groove will 

 be given in the following stages and should be consulted in this 

 connection; but it may here be stated that after a careful study 

 of both surface views and serial sections I have come to the con- 

 clusion that the differences in the grooves appearing early and 

 late in the median line of the neural plate of Cryptobranchus 

 are differences in degree, not in kind, hence I have used the term 

 'neural groove' throughout. 



In living material the embryo may be viewed by transmitted 

 light. During the early part of this stage (figs'. 146 and 147) 



