28 Q 13

Describe and distinguish patterns of reproduction in monotremes, marsupials, and placental mammals. What aspects of mammalian reproduction are present in all mammals but in no other vertebrates?

Monotremes: One breeding season each year. Ovulated eggs, usually two, are fertilized in the oviduct. Embryos continue to develop in the uterus for 10-12 days where they are nourished by yolk supplies deposited prior to ovulation, and secreations from the mother. A thin, leatherery shell is secreated around the embryos prior to the eggs being laid. Young feed on milk produced by mother's mammary gland, which is located onto the belly fur of the mother.
Marsupials: Have a premetive placental called a choriovitelline, which means yolk sac. An embryo of a marsupial is at first an encapsulated by shelll membranes and floats free for several days in the uterine fluid. After hatching from the shell membranes, embryos erode shallow depressions in the uterine wall in which they lie and absorb nutrient secreations from the mucosa by way of a vascularized yolk sac. Gestation is brief, and therefore they give birth to tiny young.
Placental Mammals: Prolonged gestation. Embryos remains in the uterus, nourished by food supplied initially by a placenta and later by a chorioallantoic type of placenta.

Aspects of mammilian reproduction that is present and all mamals, but no other vertabraes is mammary glands.
