Development

In all living mammals, milk from the mother's mammary glands nourishes newborns. However, the pattern of development of the offspring differs from group to group.

Monotremes

A female monotreme typically lays one or two large eggs encased in thin, leathery shells and then incubates them. Her body heat keeps the eggs warm. The yolk nourishes the developing embryo within the egg. At hatching, a monotreme is very small and only partially developed. Its mother protects it and feeds it milk from her mammary glands until it is ready to survive on its own.

Marsupials

In marsupials, such as opossums and kangaroos, embryos develop for just a short period within the mother's uterus and then emerge from the uterus and crawl into the mother's pouch, a skin-lined pocket on her abdomen. The newborn offspring of a kangaroo at this stage is only 2 to 3 cm (1 in.) long. In the mother's pouch, the newborn attaches to a nipple to feed. The newborn's development and growth then continue inside the pouch for several months.

Placental Mammals

Placental mammals, such as the horse in Figure 43-9, give birth to well-developed young after a long period of development inside the uterus. During this period, the placenta provides nourishment and oxygen to the developing offspring. The placenta begins to form shortly after fertilization, when the fertilized egg attaches to the lining of the uterus. Extensions from the chorion, the outer membrane of the embryo, grow into the lining of the uterus. Blood vessels from the uterus surround these extensions. Nutrients and oxygen diffuse from the mother's blood into the blood of the offspring, and carbon dioxide and other wastes diffuse from the offspring into the mother's blood. After birth, infants feed on milk for several weeks or months.

figure 43-9

Placental mammals, such as horses, carry their developing fetuses for a long time and give birth to infants that are relatively large and well developed, but still need parental care.

figure 43-9

Placental mammals, such as horses, carry their developing fetuses for a long time and give birth to infants that are relatively large and well developed, but still need parental care.

Word Roots and Origins placenta

from the Greek plakos, meaning "flat object" or "flat cake"
Pregnancy And Childbirth

Pregnancy And Childbirth

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