Mossman, Harland W. Department of Anatomy, University of Wisconsin, Madison, Wisconsin.
- Placental modifications
- Physiological exchange
- Types of placentation
- Related Primary Literature
- Additional Reading
The intimate association or fusion of a tissue or organ of the embryonic stage of an animal to its parent for physiological exchange to promote the growth and development of the young. Placentation enables the young, retained within the body or tissues of the mother, to respire, acquire nourishment, and eliminate wastes by bringing the bloodstreams of mother and young into close association but never into direct connection (Fig. 1). Placentation characterizes the early development of all mammals except the egg-laying duckbill platypus and spiny anteater. It occurs in some species of all other orders of vertebrates except the birds. In fact, in certain sharks and reptiles placentation is almost as well developed as in mammals. A few examples are also known among invertebrates (Peripatus, certain tunicates, and insects).
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