By the Gosner staging system for exotrophic tadpoles (based upon visible morphological features), classically embryos (development in the egg) are stages 1-19, hatchlings are stages 20-25 and full blown tadpoles are stages 26-41. The gills are external through stages 19-24, the classical hatchling stages, which are also very transitional and represent a period of going from a relatively immobile embryo to a "free swimming" tadpole. If we wanted to get into a pedantic debate, the stage of attaining "free swimming" status is rather labile, as are other behaviours (some embryos for example hatch out as early as stage 16), and amongst various species (Bufo sp. included) swimming is often attained while the gills are still external to some degree. However, for the most part, John is fairly correct in that by the time most of these are swimming around in a form most people would recognise as a tadpole that they no longer have any discernable external gills.
And yes, my error for saying larvae instead of larva, it is a common mistake I make, MPSC.
