What is the physiology of reproduction in mammals?

Answer 1

There are three major divisions of mammals, monotremes (egg laying mammals), marsupials, and placentals. In each of these groups individuals are either male or female. Unlike other species (various fish, amphibians, and a few varieties of births) sexual "identity" is fixed at birth. There are occasional instances of hermaphrodism, where an individual may display characteristics or even organs of both sexes, but these tend to be fairly rare.

The male reproductive organ, the penis, becomes tumescent (engorges with blood or has an actual bit of bone slide into it as with the Ursidae). This organ is pushed inside the walls of the female reproductive organ, or vagina, in an action often referred to as "mounting." After a brief period of time seminal fluid from the male is ejected or "ejaculated" into the female. This fluid may be stored for hous, in some cases even days, before conception occurs. Conception is when a male sperm cell within the seminal fluid (which contains millions of sperm cells) manages to penetrate the wall of the much larger female egg cell. This forms a zygote, a special cell containing DNA (genetic material) from both parents. The zygote divides, forming a blastocyst, which is either implanted in the wall of the female uterus where gestation occurs, or passes out of the body and conception fails. Gestation is the development of the embryo through into a fetal stage, followed by birth.

After the baby mammal is born the placenta sloughs off the uterine wall and is expelled. This is known as the after birth, in placental mammals. For egg laying mammals fertilized eggs are expelled from the mother's body before birth, and yet a slightly different process occurs for marsupials.