A method for more critical isolation of clones derived from three human cell strains in vitro.

technics, a number of problems soon arose which made it uncertain that all the clones were of single-cell origin. The cells which gave rise to clones could not be observed adequately in Petri dishes, so that it was not possible to see if two or more cells gave rise to a clone. As experiments in Carrel flask preparations showed, cells could come loose from the glass surface and settle in another area of the flask, especially as the clones increased in size. The cells could also come loose from the glass surface while the Petri dishes were handled and refed. Finally, the need for continuous gassing and the difficulty of manipulating Petri dishes made another type of vessel more desirable. Although the method to be described yields fewer clones (one per flask) than the Puck technics, one can be certain that the clones which arise are of single-cell origin, and this, after all, is the most important criterion for genetic studies based on such lines (2).