The ultrafilterable constituents of embryonic tissue extract are unable to support cell life in vitro. They stimulate cell migration and possibly multiplication, without increasing the mass of the tissue.

Embryonic tissue extract freed from amino acids by dialysis still retains a considerable part of its growth-promoting properties.

The area of growth of tissues in embryonic tissue extract free from amino acids is appreciably less than that with the whole extract, probably owing to the denaturation of part of the protein, or perhaps to the inactivation or loss of an enzyme.

The addition of either the ultrafilterable components or an artificial mixture of amino acids to this dialyzed extract increases the area of cell migration but does not restore all the activity lost on dialysis.

The observed differences in growth of tissue, due to the addition or removal of dialyzable and ultrafilterable constituents of the extract, prove that the amino acids produce a more active cell migration and possibly multiplication, but no building up of new protoplasm.

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