embryo

/em"bree oh'/, n., pl. embryos, adj.

n.

1. the young of a viviparous animal, esp. of a mammal, in the early stages of development within the womb, in humans up to the end of the second month. Cf. fetus.

2. Bot. the rudimentary plant usually contained in the seed.

3. any multicellular animal in a developmental stage preceding birth or hatching.

4. the beginning or rudimentary stage of anything: He charged that the party policy was socialism in embryo.

adj.

5. embryonic.

[1580-90; < ML embryon-, embryo < Gk émbryon, n. use of neut. of émbryos ingrowing, equiv. to em- EM-² + bry- (s. of brýein to swell) + -os adj. suffix]

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Early stage of development of an organism in the egg or the uterus, during which its essential form and its organs and tissues develop.

In humans, the organism is called an embryo for the first seven or eight weeks after conception, after which it is called a fetus. In mammals, the fertilized egg or zygote undergoes cleavage (cell division without cell growth) to form a hollow ball or blastocyst. During the second week following fertilization, gastrulation (cell differentiation and migration) results in the formation of three tissue types. These three types of tissue develop into different organ systems: the ectoderm develops into the skin and nervous system; the mesoderm develops into connective tissues, the circulatory system, muscles, and bones; and the endoderm develops into the lining of the digestive system, lungs, and urinary system. In humans, by about the fourth week, the head and trunk can be distinguished and the brain, spinal cord, and internal organs begin to develop. By the fifth week, limbs begin to appear and the embryo is about .33 in. (.8 cm) long. By the end of eight weeks, the embryo has grown to about 1 in. (2.5 cm) long and all subsequent change is limited primarily to growth and specialization of existing structures. Any congenital disorders begin in this stage. See also pregnancy.

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▪ human and animal

 the early developmental stage of an animal while it is in the egg or within the uterus of the mother. In humans the term is applied to the unborn child until the end of the seventh week following conception; from the eighth week the unborn child is called a fetus.

      A brief treatment of embryonic development follows. For full treatment, see morphology: Embryology (morphology).

      In organisms that reproduce sexually, the union of an ovum with a sperm results in a zygote, or fertilized egg, which undergoes a series of divisions called cleavages as it passes down the fallopian tube. After several cleavages have taken place, the cells form a hollow ball called a blastula. In most mammals the blastula attaches itself to the uterine lining, thus stimulating the formation of a placenta, which will transfer nutrients from the mother to the growing embryo. In lower animals the embryo is nourished by the yolk.

      By the process of gastrulation, the embryo differentiates into three types of tissue: the ectoderm, producing the skin and nervous system; the mesoderm, from which develop connective tissues, the circulatory system, muscles, and bones; and the endoderm, which forms the digestive system, lungs, and urinary system. Mesodermal cells migrate from the surface of the embryo to fill the space between the other two tissues through an elongated depression known as the primitive streak. As the embryo develops, the cell layers fold over so that the endoderm forms a long tube surrounded by mesoderm, with an ectodermal layer around the whole.

      Nutrients pass from the placenta through the umbilical cord, and the amnion, a fluid-filled membrane, surrounds and protects the embryo. The division of the body into head and trunk becomes apparent, and the brain, spinal cord, and internal organs begin to develop. All of these changes are completed early in embryonic development, by about the fourth week, in humans.

      Between the head and the heart, a series of branchial arches (branchial arch), cartilaginous structures that support the gills of fishes and larval amphibians, begin to form. In higher vertebrates these structures form part of the jaw and ear. Limb buds also appear, and by the end of the embryonic stage, the embryo is distinguishable as a representative of its species.

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