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viviparous animals, monotremes, Marsupial, fertilized egg, larva

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Embryology, branch of biology dealing with the development of the animal embryo. Embryology includes within its province the development of the fertilized egg and embryo and the growth of the fetus.


A large ovum such as that of a bird or reptile contains abundant yolk, which, with the albuminous white, is sufficient to nourish the embryo until birth. The nutrients in a small ovum, however, are soon used up, and therefore the embryo must be nourished by other means. In many species the embryo is hatched at this point as a larva, a form capable of feeding itself although still lacking some of the organs of the adult form. In viviparous animals, including all mammals except the monotremes, the embryo receives nourishment from the mother by diffusion through specially developed extraembryonic membranes. Marsupial, females secrete a nutrient fluid for this purpose from uterine glands. In most mammals soluble nutrients are supplied to the embryo from the bloodstream of the mother.


Boell, Edgar J., M.A., Ph.D., D.Sc.

Professor Emeritus of Experimental Zoology and Senior Research Associate in Biology, Yale University.

Roche, Jean, M.D.

Former Professor of Biochemistry, College de France.

Article key phrases:

viviparous animals, monotremes, Marsupial, fertilized egg, larva, Embryology, diffusion, nourishment, bloodstream, reptile, organs, Nutrition, bird, point, mother, purpose, species, province, birth, females, growth, development, means

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