This brief review summarizes our knowledge of the morphological events
that are associated with oocyte maturation and fertilization in marsu
pials in which it has been suggested that there are marked differences
from eutherians in both the developmental timetable of oocyte maturat
ion and in some of the processes associated with sperm-egg interaction
. Most of the data have been obtained from studies on four species: Mo
nodelphis domestica, Sminthopsis crassicaudata, Sminthopsis macroura,
and Trichosurus vulpecula. Differences between the species have been d
escribed for: (1) the arrangement of 'yolk' in the oocyte cytoplasm; (
2) the time of formation of cortical granules; (3) the mode of sperm p
enetration through the zona pellucida; (4) the sperm membrane involved
in sperm-egg fusion; (5) the fate of inner acrosomal and sperm plasma
membranes; and (6) the rapidity of sperm chromatin decondensation in
the ooplasm. Such differences suggest considerable variation in these
processes between different marsupial species although some of the var
iation described may be due to technical differences in the obtaining
of the data. Thus, whether there are fundamental differences between t
he two major extant infraclasses of mammals, marsupials and eutherians
, in some of the processes associated with fertilization is conjectura
l at the present time. The interspecific variation in the results obta
ined cautions one in extrapolating from observations on one or two 'mo
del' species to the infraclass as a whole; a conclusion that might not
, on reflection, be too surprising bearing in mind the long and separa
te evolutionary history of the major extant marsupial lineages.