CONCRESCENCE IN EMBRYO OF CRYPTOBRANCHUS. 



255 



in Fig. 31. In the blastopore of Cryptobranchus, the point of 

 least resistance would naturally be at the center, where the 

 invagination is deepest. But in comparing Fig. 31 with the pre- 

 ceding figures such as Figs. 25 and 29 one should remember that 

 the stained areas do not represent the actual paths of moving 

 particles such paths are in reality much more oblique, as will 

 be seen by comparing the original position of the stained areas. 

 There is room for doubt if a mechanical explanation of confluence 

 is really adequate to explain the extreme obliquity of the course 

 taken by individual cells. 



FIG. 31. Diagram showing the direction of the currents in water contained in 

 a shallow vessel and allowed to escape through a slightly elliptical slit in the bottom. 



C. The Movements of the Neural Folds. We have spoken of 

 the movement of the neural folds as a phase of concrescence in its 

 widest sense; in this connection it may be worth while to examine 

 into the* character of this movement. Is it a mere wave-like 

 undulation progressing from the lateral margins of the neural 

 plate toward the median line, or is it a movement of translation 

 whereby material is actually carried toward the median line? 

 The study of transverse sections of a series of embryos inclines 

 one to the latter view, and experiments prove conclusively that 

 this view is correct. My results in general agree with and extend 

 those of Goodale ('n) on Spelerpes. 



Figs. 32 to 35 represent the history of an egg marked in the 

 early neural groove stage. The distance through which material 

 is brought from either side to the median line is remarkable, and 



