EMBRYONIC DEVELOPMENT AND FOETAL MEMBRANES
In kangaroos, fertilisation occurs before the outer keratinous shell membrane is formed around the egg. The egg is shed into the fibria of the oviduct covered only with the thin secondary shell membrane, the zona pellucida. Spermatozoa attach to the zona pellucida and proceed to dissolve it. Once one succeeds and effects fertilisation there is an inhibition of further sperm penetration, possibly involving the initial secretion of a mucoid coat.
Development within the mother requires mechanism for nutrient and gas exchange. In marsupials and placentals the yolk sac and allantois have been variously adapted to provide the embryo's needs. When the foetal membranes, such as the embryonic yolk sac or allantois, are applied to the uterine wall the structure formed is referred to as the placenta.
In kangaroos and many other marsupials the yolk sac is primarily involved and a 'yolk sac' placenta is formed. This facilitates respiratory gas exchange and provides the developing embryo in the uterus with nutrients which are additional to those supplied by the egg yolk. The yolk sac placenta adheres to the uterine wall after the breakdown of the outer shell, but fusion of tissues does not occur. Such fusion does occur in placentals and bandicoots, where the allantois fuses with and breaks down the uterine wall to form a chorio-allantoic placenta. In kangaroos the allantois expands rapidly near full term, but it does not form a placenta because it is prevented from doing so by the expanded vascularised yolk sac. There are indications that in kangaroos' yolk sac placentae there is a division of function. The vascular part of the yolk sac placenta seems to be largely involved in respiratory gas exchange and it expands quickly in the later stages of gestation when the metabolic rate of the foetus also rises. The non-vascular part of the yolk sac is concerned with the transfer of nutrients and the building blocks for growth, such as proteins.