EPIDERMIS WHEN CULTIVATED OUTSIDE BODY 293 



may be thrown on various morphogenetic processes in this way, 

 but it is perhaps premature to attempt to speculate as to the 

 value or promise of this method of analysis. 



SUMMARY 



The epidermis of amphibian larvae grows well in a mixture of 

 equal parts of a two per cent gelatin solution and blood serum 

 from the adult animal. Epidermis shows a marked thigmotaxis, 

 and in hanging drop cultures extends away from the implanted 

 tissue along the lower side of the cover slip and the lower surface 

 of the drop of culture medium. 



The extending mass of epithelium usually takes the form of a 

 broad, thin sheet whose outer border is formed of very thin, 

 clear protoplasm furnished with fine pseudopodia. 



Sheets of extending epidermis are sensitive to slight stimula- 

 tions, and show a remarkable degree of contractility, often draw- 

 ing in to a small fraction of their previous superficial area. 



Strong contractions of epidermis are evoked by thermal, 

 chemical, osmotic or contact stimuli, but strong light has no 

 appreciable influence on the epithelial cells. After contraction 

 the epithelial mass may spread out as widely as before. 



Epidermis from amphibian larvae may be kept alive for sev- 

 eral months if it is transferred occasionally into fresh culture 

 medium. 



Mitotic cell divisions were observed in the living cells fifty 

 days after implantation into the culture medium. 



Amitotic nuclear divisions were found, especially in prepara- 

 tions that were kept for several weeks without a fresh supply of 

 nutrient fluid. 



Epidermal cells from embryos of Diemyctylus were seen to 

 undergo differentiation in vitro into the different types of cells 

 found in the epidermis of late larval stages. 



Pieces of epidermis from the adult frog form extensions much 

 like those from the larva, but growth is less rapid. 



The extensions of the epidermis in both larval and adult 

 forms are due to the amoeboid activity of the hyaline protoplasm 

 along the margin of the extending mass. 



