522 



BERTRAM G. SMITH 



embryo, when it occurs, serves to prevent adhesion of the embryo 

 to the envelope with consequent abnormahties. 



The cihation and rotation of the frog embryo have been 

 described by various writers, notably Assheton ('96). Piersol 

 ('09) has described rotation in the embryo of Plethodon. 



Stage 19: {jigs. 190 and 21^5 to 248). This stage begins about 

 twenty-four hours later than Stage 18. It is characterized by 

 from two to three distinct gill invaginations, a budding tail, a 

 very marked outward expression of the pronephros (see especially 

 figs. 245 and 246), and beginning lateral vascular bands, the 

 anlage of the vitelline veins (see especially fig. 245) . In addition 



190 



191 



Fig. 100 Diagram of an embryo of Cryptobranchus allegheniensis in Stage 

 19, showing the direction of the water currents produced by cilia. 

 Fig. 191 Same as figure 190, for Stage 21. 



to the cephalic flexure there is a slight cervical flexure which 

 reaches its maximum in this stage. About sixteen to twenty 

 mesoblastic somites are apparent in surface views. 



In the living embryo, the lateral vascular bands are conspicuous 

 structures, for they are pink with blood; but they are not yet 

 differentiated into true veins. Overlying the upper j^art of the 

 yolk sac, they extend from the heart region longitudinally on 

 each side of the body and meet posteriorly a little below the tail. 

 During this stage and the stages immediately following, they 

 shift slowly toward the ventral surface. There is considerable 

 variation in the position of this band in embryos that are otherwise 

 in the same stage. A similar vascular area has been figured for 

 Cryptobranchus japonicus by de Lange ('07). 



