EMBRYOLOGY OF CRYPTOBRANCHUS 521 



The embryo is still erect (i.e., with the dorsal surface upper- 

 most). In this position it has been observed, in many cases, 

 to rotate slowly on a vertical axis. To test the direction of rota- 

 tion a large number of embryos were placed separately in watch 

 glasses and individual records made. Out of sixteen embryos 

 that showed rotation, onlj^ two moved in a clockwise direction, 

 the other fourteen in an anti-clockwise direction. The rotation 

 is, of course, caused by the cilia. The direction of rotation in 

 this stage can hardly be explained as the result of a tendency 

 for the embryos to lean to one side oftener than to the other, for, 

 as will be shown in a later stage, the facts are otherwise. Possi- 

 bly more extended observations would show more equality in 

 the results; or there may be a uniform asymmetry in the distri- 

 bution, or in the rate or direction of beating, of the cilia of the 

 ventral surface of the yolk sac. 



Stage 18: (figs. 21^3 and 2J+I^). This stage is reached about 

 twenty-four hours after the beginning of Stage 17. It is charac- 

 terized by a prominent outstanding head with marked cephalic 

 flexure and distinct optic vesicles, and by the presence of the 

 pronephros and the first definite indications of the budding tail. 

 During the latter part of this stage the mandibular arch is usuall}' 

 recognizable. 



Patches of cilia are now distributed over the entire surface; 

 the beat of the cilia is in general backward and the currents are 

 much the same as figured in the next stage (fig. 190). 



During this stage the embryo topples over from its erect posi- 

 tion so as to fall to one side, on which it lies throughout several 

 succeeding stages until spontaneous movements enable it to change 

 its position. An incidental result of this position is to bring a 

 larger area of the ciliated surface into contact with the substratum; 

 as a consequence of this and of the stronger ciliation, rotation 

 of the embryo is now of more marked occurrence. The most 

 rapid motion observed was performed by an embryo that com- 

 pleted a rotation in just two minutes. 



The functional value of the ciliary motion is at least two-fold: 

 (1) it bathes the surface of the embryo with currents of water 

 which are subservient to respiration; and (2) rotation of the 



JOURNAL OF MORPHOLOOy, VOL. 23, NO. 3 



