[Morphogenesis of the chick embryo foot as studied by Janus green B-induced malformations].

Janus green was injected into the amniotic sac of 6-5-day-old chick embryos at a single dose of 8-5 or 15 mug; the dye causes respectively 55 and 82% malformations of the feet. Toe are affected by partial or total soft tissue syndactyly, hypophalangy and infrequently by hyperphalangy. Lateral toes are more severely and more frequently affected by syndactyly and hypophalangy than medium ones. Hyperphalangy is found exclusively in toes I and II and causes the formation of one excess phalanx at most. Syndactylous toes are joined by an overdeveloped digital membrane. Hypophalangic toes have a truncated (non-pointed) distal tip. Two phalanges are lacking at most. These malformations are due to two distinct phases of the Janus green action. The first one, which is early and fast, inhibits interdigital programmed cell death, causing in term the non-regression of the interdigital membranes and thus syndactyly. The second one, which is late and slow, blocks the apical growth; this leads to hypophalangy.