EMBRYOLOGY OF CRYPTOBRANCHUS 525 



movement of the blood has been observed in this stage. The 

 embryos are ciliated, and undergo rotation, much as in the pre- 

 ceding stage. Muscular movements do not ordinarily occur, 

 but have been observed when the embryos were placed in fixing 

 fluids. 



Stage 21: (figs. 191 and 252 to 255). This stage is reached 

 about four days later than the beginning of Stage 20. It is char- 

 acterized by the presence of three budding external gills, and 

 small front limb rudiments. The tail is longer, and has a more 

 decided ventral flexure, than in the preceding stage. 



The dorsal surface of the embryo is now for the first time 

 slightly pigmented. The pigmentation begins in that portion 

 of the ectoderm overlying the nervous system, and gradually 

 extends downwards over the sides of the body. During this 

 and the following stages (McGregor '97), the pigment cells in 

 the region of the mesoblastic somites are grouped metamerically. 



Numerous veins now branch off dorsally from the lateral vas- 

 cular bands. In the bands themselves, the two main trunks 

 of the omphalo-mesenteric or vitelline veins, one on each side 

 of the yolk sac, are being differentiated, but they rarely become 

 complete before the next stage. In general the differentiation 

 of the vitelline veins has gone further on the side of the body that 

 happens to be uppermost. During the latter part of this stage 

 the heart is pulsating regularly, with about twenty-five to forty 

 beats per minute; the blood sometimes surges back and forth 

 in the principal veins overlying the yolk sac, with a slight excess 

 in the forward movement, but the vitelline circulation is not 

 yet completely established. 



The distribution of cilia remains much the same as in the pre- 

 ceding two stages, but the ciliary currents move more uniformly 

 toward the posterior end of the body (fig. 191). 



Spontaneous muscular movements, consisting chiefly of a 

 bending of the body laterally into a U shape, now occur, but the 

 embryo is as yet unable to turn over. 



Stage 22: (figs. 193, 194 cind 256 to 259). This stage begins 

 about five days later than Stage 21; its most distinctive charac- 

 teristic is that the external gills have rudimentary branches and 



