“…In-vitro studies demonstrate that corpora lutea of oviparous turtles (Klicka & Mahmoud, 1972) and viviparous lizards (Xavier, 1982;Girling & Jones, 2003) can convert pregnenolone to P 4 , and the rate of P 4 secretion by non-luteal ovarian tissues is insignificant compared with its rate of production by corpora lutea (Xavier, 1987;Bennett & Jones, 2002). However, at variance with this pattern, the viviparous skink Niveoscincus microlepidotus (Girling, Jones, & Swain, 2002) and viviparous snake Natrix sipedon pictiventris (Chan, Ziegel, & Callard, 1973) exhibit, in addition to peaks during gestation, significant preovulatory rises in plasma P 4 concentrations, which may reflect P 4 synthesis by the preovulatory follicles. The intensity of 3b-HSD activity in the corpus luteum has been positively correlated with plasma P 4 concentrations in both oviparous and viviparous species (Shanbhag, Radder, & Saidpur, 2001;Martínez-Torres, Hernández-Caballero, Alvarez-Rodriguez, Luis-Díaz, & Ortíz-López, 2003), although in some viviparous species the placentae may take over the primary role in P 4 production during later gestation (Guarino et al, 1998).…”