EMBRYOLOGY OF CRYPTOBRANCHUS 511 



depending upon the particular species. The position of the head of 

 the embryo seems correlated with the length of the embryo, so that 

 the longer the embryo, the higher up on the egg it develops. 



The terms 'upper' and 'lower' are evidently here used in the 

 sense of animal and vegetal, that is to say, with reference to those 

 points on the surface of the egg which were in the vertical axis 

 of the egg before it commenced to rotate; therefore the results 

 obtained with Cryptobranchus fall in line with the general state- 

 ment quoted. The results of Goqdale on Spelerpes and my own 

 work on Cryptobranchus agree closely in locating the posterior 

 end of the embryo at the vegetal pole; it is worthy of note that 

 this conclusion was reached independently and by entirely differ- 

 ent methods. 



Stage, 14: (figs. 165 to 179 and 229 to 232). This stage is reached 

 about one day later than the beginning of Stage 13. Since at- 

 this time scarcely any two embryos agree in the rate of develop- 

 ment of homologous regions of the body, it is impossible in this 

 stage to make a close classification. In marking off this stage 

 from the one following, the principal character considered is 

 the approach of the neural folds toward the median line. 



Figures 165 to 176 represent twelve embryos that illustrate 

 the principal changes in the antero-dorsal region during this 

 stage. It will be seen that there is a progressive addition of 

 transverse grooves posterior to the three that first appeared. In 

 front of Groove I the cephalic plate is traversed primarily by six 

 grooves; of these Groove 1, which was noted in the preceding 

 stage, has a very transitory existence and in most cases is lost 

 in Stage 14; likewise the median portion of Groove 2 has often 

 disappeared. Moreover in this or the following stage Groove 

 4 disappears, following a marked depression and perhaps sub- 

 mergence of the segment between it and Groove 3. 



A significant relation exists between Grooves I, II, III, etc., and 

 the intersomitic grooves which now appear just outside the neural 

 folds; by an inspection of figures 165 to 176 it will be seen that 

 in all cases these are in direct apposition. Since the mcsoblastic 

 somites are the most characteristically segmented structures of 

 the vertebrate body, it follows that the true segmental units of 



