1913] Holmes: Ectodcrmic Epithelium of Tadpoles 165 



slow advance of the cells. After the strand had advanced about 

 twice the diameter of the terminal part of the extension the tip 

 apparently lost its grip upon the cover glass and was suddenly 

 pulled back so as to lie entirely behind the mark. At the same 

 time it became turned somewhat toward one side (pi. 8, fig. 9). 

 Soon the terminal portion began to advance again, the kink 

 between it and the rest of the strand became straightened out, 

 the part immediately behind the clear extremity became longer 

 and narrower as if it were being pulled out (figs. 10, 11). The 

 lateral lobe near the tip gradually became less prominent, its 

 hyaline border narrow and the pseudopods smaller. Finally 

 with the further extension of the tip, to which it was apparently 

 acting more or less in opposition, it was drawn in entirely. 



The behavior of the hyaline tip of the strand whose pseudo- 

 pods acted much like those of an advancing rhizopod, the 

 straightening out of the bend, and the form changes of the region 

 behind the tip, indicate that the strand was pulled out by the 

 amoeboid activity of the terminal protoplasm, rather than pushed 

 out from behind. And the fact that the tip of the strand was 

 suddenly pulled backward indicates that its advance was effected 

 against a certain tension of the proximal part of the stand. 



Actual growth of cells probably plays but a small part in the 

 production of the extensions described. The cells composing 

 them are no larger than in places where there is practically no 

 change; and the structures are produced too rapidity for either 

 the growth or the multiplication of cells to be factors of prime 

 importance. 



The ectoderm cells possess to a remarkable degree the prop- 

 erty of thigmotaxis which is so prominent a trait of amoeboid 

 protozoans. Apparently they are indil¥erent as to the kind of 

 solid object to which they attach themselves, since they readily 

 extend upon the surface of glass, cotton fiber, clots of plasma as 

 well as their normal substratum of connective tissue. They also 

 readily extend upon one another, exercising a sort of mutual 

 attraction which tends to keep them in continuous masses. They 

 are also very free to move past one another and to shift about 

 their relative positions. Those protopla.smic strands or bridges 

 which may have .joined them together in their normal situations 



