342 MARGARET R. LEWIS AND WARREN H. LEWIS 



a fine pipette, usually one at a time, with some of the solution 

 and placed each on a sterile cover-slip which was inverted onto 

 a vaseline (melting point, 46 ± °C.) ring on a hollow ground slide. 

 All instrimients and cover-slips were sterilized by passing them 

 thi'ough the flame, and aseptic precautions were observed 

 throughout. 



Great care should be taken to insure absolute cleanliness of 

 the cover-slips. The migrating and di\dding cells, as we have 

 alread}' stated, adhere to the cover-slip and utilize it as a means 

 of support, and the presence of grease seems to prevent them 

 from getting a foothold. The small drop should spread out 

 evenly and thinly over the center of the cover-slip so that the 

 surface tension keeps the explanted piece in contact with the 

 cover-slip. The stereotropic cells can thus easily creep out from 

 the piece to the cover-slip on which they migrate towards the 

 periphery of the di'op. In cases where the di'op is too deep and 

 the small explanted piece falls away from the cover-slip the con- 

 vex surface of the drop may act as a support for growth. 



Growth began within ten to twenty hours and reached a 

 maximum in extent and showed the greatest number of mitotic 

 figures on the second or third day. The cultures were incu- 

 bated at 39°C. to 40°C. in an electric incubator (with a glass 

 in the door) . The presence of the electric hght which was placed 

 in the same chamber with the cultures for the purpose of main- 

 taining the temperature of the incubator did not seem to affect 

 the growth. Cultures apparently grow as well in the light as 

 in the dark. 



Around the piece of explanted tissue the new growth forms 

 a more or less radiating reticulum, a syncytium, or a membrane- 

 like sheet of cells with varying numbers of isolated cells. The 

 growth may be several cells in thickness near the old piece, but 

 toward the periphery there is usually only a single layer of 

 flattened cells which are often scarcely 2 ^u in thickness (fig. 1). 

 The entire contents of these peripheral cells can be observed 

 with very little change in focus. The growth is so closely at- 

 tached to the cover-slip that in many cases the explanted piece 

 can be torn away without injury to the new growth. 



