290 S. J. HOLMES 



entiation into these different types before their iniphintation or 

 whether their differentiation was entirely iniated after their 

 transfer is uncertain. 



A few experiments were tried with pieces of skin from the adult 

 frog. Many of these pieces failed to show any epithelial out- 

 growth, although they were kept apparently in a healthy condi- 

 tion for weeks. In some instances large extensions of epidermis 

 were observed. The cells at the margin of the outgrowths showed 

 the same thin hyaline border with its minute changing pseudo- 

 pods, as is found in the preparations of amphibian larvae. The 

 general character of the epithelial outgrowth, the method of its 

 formation and its behavior in relation to solid surfaces and to 

 various stimuli are practically the same as in the extensions of 

 larval epithelium. 



Since my previously cited paper on the movements of epithe- 

 lium in tadpoles was published, two papers have appeared 

 which deal with the same subject. One of these is an article 

 by OppeP which is devoted entirely to the epithelium of the 

 tadpoles of Rana; the other, written by Osowski,-^ includes a 

 study of the movements of epithelial cells in tadpoles and in 

 the embryos of fishes and of birds. The methods employed by 

 both authors in their work on tadpoles are much the same. Tails 

 of young tadpoles were cut off near the base, placed in diluted 

 Ringer's solution for different periods, and then fixed and studied 

 in serial sections so as to follow the way in which the cut surface 

 came to be covered with epithelium. The covering of the cut 

 surface was also followed by Osowski by making observations 

 from time to time on the living tissue. No evidence of mitotic 

 division was observed by Osowski in the epithelial cells that were 

 extending over the cut surface. Both authors conclude that the 

 movements of epithelial cells are responsible for covering the 

 wound and that the movements in question are not the result of 

 pressure or tension to which the cells may be subjected, but a con- 



* Oppel, A. Demonstration der Epithelbewogung im Explantat von Frosch- 

 larven. Anat. Anz. Bd. 45, 173-185, 1913. 



^ Osowski, H. E. Ueber aktive Zellenbewegung im Explantat von Wirbel- 

 tierembryonen. Arch. f. Entw.-mech., Bd. 38, 547-583, 1914. 



