latter cases the internal phase picks up the image forming agent e g by dissolution upon being released from the microcapsules and carries it to the developer layer or an associated developer sheet typical color precursors useful in the aforesaid embodiments include colorless electron donating type compounds representative examples of such color formers include substantially colorless compounds having in their partial skeleton a lactone a lactam a sultone a spiropyran an ester or an amido structure such as triarylmethane compounds bisphenylmethane compounds xanthene compounds fluorans thiazine compounds spiropyran compounds and the like crystal violet lactone and copikem x iv and xi are often used alone or in combination as color precursors in the present invention illustrative examples of color developers useful in conjunction with the aforesaid color precursors are clay minerals such as acid clay active clay attapulgite etc organic acids such as tannic acid gallic acid propyl gallate etc acid polymers such as phenol formaldehyde resins phenol acetylene condensation resins condensates between an organic carboxylic acid having at least one hydroxy group and formaldehyde etc metal salts or aromatic carboxylic acids such as zinc salicylate tin salicylate zinc 2 hydroxy naphthoate zinc 3 5 di tert butyl salicylate oil soluble metal salts or phenol formaldehyde novolak resins e g see u s pat nos 3 672 935.