1913] Holmes: Ectodennic Epithelium of Tadpoles 163 



piece. The posterior curve of these strands is due to the fact that 

 they are extending along surfaces of coagulated plasma. The 

 small amount of Ringer's solution that was introduced as the 

 tissue was transferred to the culture medium caused the area 

 around the tissue to remain fluid, while the surrounding plasma 

 was coagulated. As the posterior extension of epithelium reached 

 the coagulated plasma it began to extend along its surface in 

 both directions. The outer boundaries of the cellular extensions, 

 if continued, would describe a nearly circular fluid area within 

 the coagulated material. Subsequently the thin middle part of 

 the posterior extension was pulled apart, resulting in the forma- 

 tion of an independent island of cells which gradually became 

 more compact, while the part attached to the old piece was 

 drawn in. 



The section of the tail of Rana (pi. 7, fig. 3) developed only 

 a slight posterior extension of cells, but from the cut ends of 

 the ectoderm of one side long strands grew out laterally and 

 finally met and fused. After the fusion the strands began to 

 spread out on either side upon the cover slip. While the cells of 

 the narrow part of the strand were fusiform those of the exten- 

 sions were hexagonal and much flattened. The margins of these ex- 

 tensions were thin and hyaline and showed the fine amoeboid pro- 

 cesses previously described. While this preparation was being 

 studied under the micro.scope the thin strand showTi in the 

 lower part of the figures was seen slowly to contract owing 

 possibly to the higher temperature to which it was exposed during 

 observation. As it shortened it could be seen to become thicker 

 and the individual cells became less elongated. The next day, 

 the fourth after the preparation was made, this strand had 

 become considerably shorter, the extension of cells from the 

 opposite strand had become wider and the enclosed space between 

 the strands reduced to less than a fourth the size shown in the 

 figure. 



In a remarkable extension from the tail of the tadpole of 

 Rana (pi. 7, fig. 4) the cells grew out along a fiber of cotton, t', 

 which lay under the piece, and was covered over by the proximal 

 part of the strand so that its middle portion is not seen. Where 

 another cotton fiber, t, crossed the first the strand of cells left 



