Morphology of Fgf8 conditional mutants. a, Gross morphology of newborn and postnatal day 1 animals and forelimbs. Conditional mutants (Fgf8CR/N) display severe forelimb deformity resulting from ablation of Fgf8 function during forelimb development. b, Skeleton preparations of limbs of newborns. The radius and first digit were absent from 100% (27/27) of conditional mutants (red arrowheads). Most also lacked the humerus (blue arrowhead; 19/27, 70%) and a second digit (14/27, 52%). At low penetrance (2/18, 10%), animals generated with the hypomorphic conditional allele lacked the first hindlimb digit, indicating that formation of this structure is sensitive to decreased levels of Fgf8; in animals bearing the nonhypomorphic conditional allele, this digit was always present. r, radius; h, humerus; s, scapula; c, clavicle; u, ulna. c, Photomicrographs of E10.0 and E11.5 forelimb buds from conditional mutants and control littermates. Arrows point from anterior to posterior. Fixed, dehydrated embryos were briefly stained with eosin and haematoxylin to improve contrast. The deficiency in outgrowth and anterior mesenchyme is already apparent by E10.0, consistent with onset of Cre activity in the forelimb before E9.5.