What marsupial gametes disclose about gamete function in eutherian mammals.

The gametes of eutherian mammals present some fundamental enigmas in regard to their structure and behaviour that have not been solved by a focus on the Eutheria alone. Although the evidence is limited still, some clues are suggested in comparison with representative American and Australian marsupials, whose gametes have undergone parallel, although sometimes quite different, evolutionary changes to those of eutherian mammals. The contrasts between them illuminate basic questions about the functions of the epididymis and sperm capacitation, about sperm numbers and sperm production, about the function of the cumulus oophorus and the configuration of the Fallopian tube and, not least, about gamete design and its bearing on the mechanisms by which spermatozoa penetrate the egg coat.