CONCLUSIONS
Members of the three orders of the living Amphia
have converged on viviparity as a mode of reproduction.
There are some similarities in the ways
they have achieved viviparity, but also many differences.
Only two species of frog and one of salamander
are viviparous (a few others are ovoviviparous); but
viviparity is a major mode of reproduction among
caecilians, having evolved at least twice and characterizing
perhaps half or more of the 170 + species
in the order, and there are no known ovoviviparous
caecilians. Features of amphibian viviparitymaintenance
of developing young in the oviducts
of the female with maternal nutrition provided after
the yolk supply is exhausted-can be generalized
and summarized as follows:
1. All viviparous amphibians have internal fertilization;
this is a necessary but not sufficient component
of viviparity. However, the means of internal
fertilization differs among members of the three orders.
Frogs use cloaca1 apposition; salamanders use
spermatophore transfer via the substrate; caecilians
insert the male phallodeum into the vent of the
female.
2. All viviparous amphibians have corpora lutea
that appear to be involved in maintenance of the
pregnancy. The endocrinology of the system is
known only for the frog Nectophrynoides.
3. All viviparous amphibians have relatively long
gestation periods. Those of frogs apparently are rigidly
controlled by an interaction of internal and
environmental factors; the gestation period of
Salamandra atra is apparently very plastic (2-5
years), but thought to be correlated with the rigor
of the cold season; the gestation period of caecilians,
based on only a few species, seems to be strongly
tied to environmental factors, especially birth at
the inception of the rains, but is synchronous within
a population.
4. Viviparous amphibians characteristically have
small ova and small clutches. The young are born
fully metamorphosed. Ovoviviparous species, which