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embryo

embryo

The Columbia Encyclopedia, 6th ed. | 2008 | Copyright

embryo , name for the developing young of an animal or plant. In its widest definition, the embryo is the young from the moment of fertilization until it has become structurally complete and able to survive as a separate organism. Embryology, the scientific study of embryonic development, deals with the period from fertilization until the hatching or birth of an animal or the germination of a plant. However, since the young animal may undergo metamorphosis or may remain wholly dependent on the mother for some time after birth, and since the seedling derives nourishment from food stored in its fleshy cotyledons even after it has sprouted, the exact limit of the time during which an organism is an embryo has not generally been well defined.

Modern embryology, using the techniques of molecular biology, genetics, and other disciplines, has focused on the question of what makes the embryo differentiate (see differentiation ), what genetically directed molecular signals tell a single cell to divide and follow the specific pattern of growth and specialization that results in a complex multicellular organism with species-specific and individual characteristics.

Karl Ernst von Baer , who developed the biogenetic law , is generally regarded as the founder of embryology. E. H. Haeckel's "ontogeny recapitulates phylogeny" gave weight to the theory of evolution (see recapitulation ). Other researchers in the field of embryology have included C. F. Wolff , M. J. Schleiden , and T. Schwann , developers of the cell theory; F. M. Balfour ; H. Spemann ; O. Hertwig ; F. R. Lillie ; and R. Levi-Montalcini .

Developmental Stages

Among humans, the developing young is known as an embryo until eight weeks following conception, after which time it is described, until birth, as a fetus . In organisms that reproduce sexually, the union of the sperm with the ovum results in a zygote, or fertilized egg, which begins a rapid series of cell divisions called cleavage, or segmentation (see mitosis ). Each kind of organism has its own characteristic sequence of development, and related species usually have similar developmental patterns.

In a typical animal, cleavage proceeds in the following pattern. Early divisions produce a hollow ball one cell thick, called a blastula, which encloses the blastocoel, or cleavage cavity. The cells divide more rapidly in the area where the nucleus of the ovum was located; this results in an invagination (inpushing) of these cells to form a ball two cells thick (the gastrula). The new cavity thus formed is the gastrocoel, also known as the primitive gut or archenteron, and its opening is the blastopore. The outer layer of cells is called the ectoderm , the inner layer the endoderm . Among the coelenterates (e.g., sponges and jellyfish), these two layers become the chief functional tissues of the adult.

In higher forms of life, a third layer of cells, the mesoderm , develops from one or both of the first two layers and fills the blastocoel, and invagination forms a digestive tract with only a single opening at this early stage. The flatworms (e.g., the tapeworm and the fluke) stop developing at this time. In most organisms, however, a later invagination of the ectoderm results in a gut that is open at both ends. The mesoderm then divides into two layers, the space between them being called the coelom , or body cavity. The embryo now roughly resembles a tube within a tube.

From the three primary germ layers, the organs and tissues develop. In general the ectoderm gives rise to the skin, or integument, the skin appendages (e.g., scales, feathers, hair, and nails), and the nervous system. The endoderm forms the digestive glands, as well as the lining of the alimentary tract and lungs. From the mesoderm develop the major internal organs: the skeletal, muscular, and connective tissue and the circulatory, excretory, and reproductive systems. Sense organs and endocrine glands arise from combinations of all three layers.

Nourishment of the Embryo

In lower animals, which lay their eggs in water, the developing embryo is nourished by yolk, absorbing oxygen from and discharging wastes directly into the water. In terrestrial oviparous forms, the egg contains the yolk and also a surrounding fluid (e.g., the albumen of bird eggs). In mammals, accessory membranes, comprising both embryonic and uterine tissue, develop around the embryo—the amnion, filled with liquid, and the chorion and allantois, which help to form the placenta, through which nourishment and oxygen in the blood of the mother diffuse into the fetus and wastes diffuse back. In the higher plants, the divisions of the fertilized ovum and the differentiation of the tissues to form the embryonic root (hypocotyl), stem (epicotyl), and leaves ( cotyledons ) occur inside the ovule within the ovary at the base of the pistil. The matured ovule is the seed ; the fruit , when it is produced, is the developed ovary.

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embryo

A Dictionary of Biology | 2004 | Copyright

embryo
1. An animal in the earliest stages of its development, from the time when the fertilized ovum starts to divide (see cleavage), while it is contained within the egg or reproductive organs of the mother, until hatching or birth. A human embryo (see illustration) is called a fetus after the first eight weeks of pregnancy.

2. The structure in plants that develops from the zygote prior to germination. In seed plants the zygote is situated in the embryo sac of the ovule. It divides by mitosis to form the embryonic cell and a structure called the suspensor, which embeds the embryo in the surrounding nutritive tissue. The embryonic cell divides continuously and eventually gives rise to the radicle (young root), plumule (young shoot), and one or two cotyledons (seed leaves). Changes also take place in the surrounding tissues of the ovule, which becomes the seed enclosing the embryo plant.
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embryo

World Encyclopedia | 2005 | Copyright

embryo Early developing stage of an animal or plant. In animals, the embryo stage starts at fertilization. In mammals, an embryo is sustained through blood supplied by the mother via the placenta. In humans, the embryo is called a fetus after the first eight weeks of pregnancy. In invertebrate animals the embryo is usually called a larva. In plants, the embryo is found in the seed, and the embryo stage ends on germination. An embryo results when the nuclei of a gamete, an egg (ovum) and a sperm or male sex cell fuse to form a single cell, called a zygote. The zygote then divides into a ball of cells called a blastula. In 2004, thirty human embryos were cloned in South Korea. See also meiosis; mitosis
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embryo

The Oxford Pocket Dictionary of Current English | 2009 | Copyright

em·bry·o / ˈembrēˌō/ • n. (pl. -os) an unborn or unhatched offspring in the process of development. ∎  an unborn human baby, esp. in the first eight weeks from conception, after implantation but before all the organs are developed. Compare with fetus. ∎  Bot. the part of a seed that develops into a plant, consisting (in the mature embryo of a higher plant) of a plumule, a radicle, and one or two cotyledons. ∎ fig. a thing at a rudimentary stage that shows potential for development: a simple commodity economy is merely the embryo of a capitalist economy.

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embryo

The Oxford Companion to the Body | 2001 | | Copyright

embryo An organism in its early stages of development. The developing human is known as an embryo for about its first two months in the womb. Conventions differ in defining when the name first applies — whether it is immediately after fertilization or after implantation of the fertilized ovum in the uterus a week or so later.

Stuart Judge


See antental development.
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COLIN BLAKEMORE and SHELIA JENNETT. "embryo." The Oxford Companion to the Body. 2001. Encyclopedia.com. 11 Feb. 2012 <http://www.encyclopedia.com>.

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COLIN BLAKEMORE and SHELIA JENNETT. "embryo." The Oxford Companion to the Body. 2001. Retrieved February 11, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O128-embryo.html

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embryo

A Dictionary of Nursing | 2008 | Copyright

embryo (em-bri-oh) n. an animal at an early stage of development, before birth (see illustration). In humans the term refers to the products of conception within the uterus up to the eighth week of development, during which time all the main organs are formed. Compare fetus.
—embryonic (em-bri-on-ik) adj.
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embryo

A Dictionary of Zoology | 1999 | | Copyright

embryo A young animal that is developing from a sexually fertilized or parthenogenetically activated ovum and that is contained within egg membranes or within the maternal body. The embryonic stage ends at the hatching or birth of the young animal.
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embryo

The Concise Oxford Dictionary of English Etymology | 1996 | | Copyright

embryo XVI (also †embryon, -ion XVI–XVIII). — late L. embryō, alt. of embryon — Gr. émbruon new-born animal, foetus, f. EM-2 + brúein swell, grow.
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T. F. HOAD. "embryo." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 11 Feb. 2012 <http://www.encyclopedia.com>.

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embryo

A Dictionary of Plant Sciences | 1998 | | Copyright

embryo A young plant developed from an ovum sexually or asexually and, in seed plants, contained within the seed.
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embryo

Oxford Dictionary of Rhymes | 2007 | Copyright

embryo •Cleo • Carpaccio • Boccaccio •capriccio • braggadocio • Palladio •cardio • radio • video • audio • rodeo •studio •Caravaggio, DiMaggio •adagio •arpeggio, Correggio •Sergio • radicchio • Tokyo • intaglio •seraglio •billy-o, punctilio •folio, imbroglio, olio, polio, portfolio •cameo • Romeo •Borneo, Tornio •Antonio • Scipio • Scorpio •barrio, Mario •impresario, Lothario, Polisario, Rosario, scenario •stereo • embryo •Blériot, Ontario •vireo • Florio •oratorio, Oreo •curio • Ajaccio • Lazio • nuncio •pistachio •fellatio, Horatio, ratio •ab initio, ex officio •patio • Subbuteo • physio

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Free newspaper and magazine articles

Frozen embryos more likely to develop into bigger babies.(NEWS)
Magazine article from: OB GYN News; 8/1/2011
Embryos preserved in stem-cell creation; Firm hails end to moral debate.(PAGE...
Newspaper article from: The Washington Times (Washington, DC); 8/24/2006
Embryo Experimentation.
Magazine article from: The Hastings Center Report; 9/1/1991

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