168 University of California Publications in Zoology [Vol. 11 



these changes the cells of the ectoderm would gradually use up 

 their yolk and undergo modifications closely similar to those 

 followed in normal development. In the same cultures i.solated 

 ectodermic elements were usually present, but they remained, 

 except for some changes of form, nearly the same as when first 

 mounted. The masses of cells that remained, so to speak, out 

 of the organization failed for .some reason to keep pace with the 

 cells from which they had been separated. So far as conditions 

 for nutrition, respiration and excretion were concerned, the 

 advantages would seem to be with the isolated cells. Their fail- 

 ure to differentiate so well as the others may be due to the absence 

 of certain conditions of stimulation that are supplied by their 

 normal association with other cells, rather than the untoward 

 influences of the surrounding medium. What these conditions 

 are is at present obscure. Whether tissue culture will make it 

 possible to apply methods of analysis which will disentangle any 

 of them remains to be seen. 



SUMMARY 



1. Pieces of the body of late embryos or young tadpoles of 

 various amphibians cultivated in lymph or plasma frequently 

 send out extensive strands or sheets of ectodermic cells into the 

 surrounding medium. 



2. The cell masses tend to extend upon some solid surface 

 such as the cover slip, fibers of cotton, or the surface of a coagu- 

 lated ma,ss of plasma. 



3. The isolated cells of ectodermic epithelium show a tendency 

 to .spread out widely and to creep along a solid surface. Scattered 

 cells which come into contact in the course of their spreading 

 may form a perfeetlj^ continuous secondary membrane. 



4. Active epithelial cells have a thin clear margin of proto- 

 plasm which puts forth fine pseudopods and undergoes marked 

 changes in outline. It is probably by means of this amoeboid 

 movement that the spreading and migration of individual cells 

 are accomplished. 



5. Evidence from several facts was obtained to show that the 

 masses of cell that were put forth were not pushed out, and did 

 not grow out, but were drawn out by the amoeboid activity of 

 the cells at the free surface of the mass. 



