Oviductal retention of developing embryos has
evolved independently several times in members of
the Class Amphibia-twice in Anura (frogs and
toads), once in Urodela (salamanders and newts),
and at least twice, perhaps three times, in Gymnophiona
(caecilians). Each group has effected retention
in rather different ways, and none have developed
placentas (with the possible exception of a pseudoplacenta
in one group of caecilians). In all species
of amphibian oviductal embryo retainers for which
there are data, either little to no nutrition is supplied
in addition to yolk, though there may be gaseous
exchange across vascularized membranes
(ovoviviparity), or cells of the oviductal epithelium
secrete a highly nutritious material that is orally
ingested by the developing fetuses after yolk is resorbed
(viviparity) (Amoroso, ’52, ’68). Amphibians
may present a special case in that viviparity and
ovoviviparity can be rather clearly delineated in
terms of presence or absence of significant maternal
nutrition (a protein, carbohydrate and lipid-rich
material, depending on the stage of development)
following yolk resorption. I therefore will use the
restrictive definitions mentioned above for viviparity
and ovoviviparity in this discussion, since the
“grey areas” of nutrient uptake found in placental
reptiles, and in fishes, seem not to obtain in
amphibians.