Abstract
Among the many substances which contribute to the effectiveness of embryonic tissue extracts as growth-promoting media for cells in vitro, some have been found to be associated with the proteins of the extracts. The ease of inactivation of the extracts by heat and by proteolytic enzymes were early indications that proteins played some role in the effects which result in increasing the area of explanted tissue relative to the area of sister control-cultures.The lability of the high molecular substances in the embryonic extracts makes it necessary to use only very mild methods in attempts to extract or purify active fractions. It has been found by Fischer and Astrup1 that a rather high activity was associated with a fraction separated by the method used by Hammarsten2 for preparation of nucleoproteins. Experiments have now confirmed that such fractions contain the relevant activity. It is, however, not yert certain that the activity necessarily resides in a protein, as distince from a protein-complex contai...
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