Blackburn, D.G. (1993).
Chorioallantoic placentation in squamate reptiles:
structure, function, development, and evolution. Journal of Experimental
Zoology 266: 414-430.
Abstract:
Allantoplacentae in lizards and snakes form during the evolution of viviparity
through apposition of the chorioallantois and a vestige of the shell membrane
to the uterine lining. Generalized
squamate allantoplacentae are epitheliochorial, diffuse, adeciduate, and highly
vascular, accomplish maternal-fetal gas exchange, and possibly transfer small
quantities of organic and inorganic nutrients.
Placental gas exchange presumably is enhanced by the thinning of tissues
lying between fetal and maternal capillaries, a progressive increase in
placental vascularity, and in some species, by differences in oxygen affinity
of fetal and maternal blood. A few
saurian genera (e.g., Chalcides, Mabuya, and Pseudemoia)
include species with specialized placentae that transfer large quantities of
nutrients.
Specializations of the allantoplacentae of these lizards include
interdigitating, hypertrophied uterine and chorioallantoic tissues, and
enlarged absorptive chorionic epithelia.
South American Mabuya are further specialized by the presence of
chorionic areolae and a distinctive placentome. Weekes' 1935 classification of placental morphotypes is reviewed,
and a few minor modifications are proposed, in addition to recognition of a
fourth morphotype. Phylogenetic
analyses suggest that placental organs have originated on more than 100
occasions among squamate reptiles, and indicate that three separate lineages have converged on substantial
placentotrophy through the evolution of specialized histotrophic placentae.
Reprints: daniel.blackburn@mail.trincoll.edu