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embryo(ĕm`brēō), name for the developing young of an animal or plant. In its widest definition, the embryo is the young from the moment of fertilizationfertilization,
in biology, process in the reproduction of both plants and animals, involving the union of two unlike sex cells (gametes), the sperm and the ovum, followed by the joining of their nuclei.
..... Click the link for more information. 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 germinationgermination,
in a seed, process by which the plant embryo within the seed resumes growth after a period of dormancy and the seedling emerges. The length of dormancy varies; the seed of some plants (e.g.
..... Click the link for more information. 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 differentiationdifferentiation,
in biology, series of changes that occur in cells and tissues during development, resulting in their specialization. This, in turn, permits a greater variety of organisms.
..... Click the link for more information. ), 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 BaerBaer, Karl Ernst von,
1792–1876, Estonian biologist. He was a professor at Würzburg and Königsberg and from 1834 at St. Petersburg. Considered a founder of modern embryology, he discovered the notochord as well as the mammalian ovum.
..... Click the link for more information. , who developed the biogenetic lawbiogenetic law,
in biology, a law stating that the earlier stages of embryos of species advanced in the evolutionary process, such as humans, resemble the embryos of ancestral species, such as fish.
..... Click the link for more information. , is generally regarded as the founder of embryology. E. H. Haeckel'sHaeckel, Ernst Heinrich
, 1834–1919, German biologist and philosopher. He taught (1862–1909) at the Univ. of Jena. An early exponent of Darwinism in Germany, he evolved a mechanistic form of monism based on his interpretation of Darwin's theories and set forth in his
..... Click the link for more information. "ontogeny recapitulates phylogeny" gave weight to the theory of evolution (see recapitulationrecapitulation,
theory, stated as the biogenetic law by E. H. Haeckel, that the embryological development of the individual repeats the stages in the evolutionary development of the species.
..... Click the link for more information. ). Other researchers in the field of embryology have included C. F. WolffWolff, Caspar Friedrich
, 1733–94, German biologist, a founder of observational embryology. In his Theoria generationis (1759) he reintroduced the theory of epigenesis to replace the then current theory of preformation, directing attention to the evidence of
..... Click the link for more information. , M. J. SchleidenSchleiden, Matthias Jakob
, 1804–81, German botanist. He was professor at the universities of Jena (1839–63) and Dorpat (1863–64). With Theodor Schwann, he is credited with establishing the foundations of the cell theory.
..... Click the link for more information. , and T. SchwannSchwann, Theodor
, 1810–82, German physiologist and histologist. He was a student of J. P. Müller and professor at the universities of Louvain (1838–48) and Liège (from 1848).
..... Click the link for more information. , developers of the cell theory; F. M. BalfourBalfour, Francis Maitland,
1851–82, Scottish embryologist; brother of A. J. Balfour. He was an early exponent of recapitulation. His Treatise on Comparative Embryology (2 vol., 1880–81) is a classic treatment of the evolution of the egg and embryo.
..... Click the link for more information. ; H. SpemannSpemann, Hans
, 1869–1941, German embryologist. He was professor of zoology (1919–35) at the Univ. of Freiburg. By transplanting embryonic tissue to a new location or to another embryo, he investigated the agency that governs the growth and differentiation of cells.
..... Click the link for more information. ; O. HertwigHertwig, Oscar
, 1849–1922, German embryologist. He studied medicine with Haeckel and Gegenbaur. In 1875 he established the fact that fertilization consists of the union of the nuclei of a male and a female sex cell.
..... Click the link for more information. ; F. R. LillieLillie, Frank Rattray,
1870–1947, American zoologist and educator, b. Toronto, B.A. Univ. of Toronto, 1891, Ph.D. Univ. of Chicago, 1894. He taught, conducted research, and was an administrator at the Univ. of Chicago from 1900.
..... Click the link for more information. ; and R. Levi-MontalciniLevi-Montalcini, Rita
, 1909–2012, Italian-American neurologist, b. Turin, Italy, M.D. Univ. of Turin, 1936. A dual citizen of Italy and the United States, Levi-Montalcini did her most important work with Stanley Cohen at Washington Univ.
..... Click the link for more information. .
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 fetusfetus,
term used to describe the unborn offspring in the uterus of vertebrate animals after the embryonic stage (see embryo). In humans, the fetal stage begins seven to eight weeks after fertilization of the egg, when the embryo assumes the basic shape of the newborn and all the
..... Click the link for more information. . In organisms that reproduce sexually, the union of the spermsperm
, in biology, the male gamete (sex cell), corresponding to the female ovum in organisms that reproduce sexually. In higher animals the sperm is produced in the testis of the male; it is much smaller than the ovum and consists primarily of a head,
..... Click the link for more information. with the ovumovum
, in biology, specialized plant or animal sex cell, also called the egg, or egg cell. It is the female sex cell, or female gamete; the male gamete is the sperm. The study of the ovum is included in the science of embryology.
..... Click the link for more information. results in a zygote, or fertilized egg, which begins a rapid series of cell divisions called cleavage, or segmentation (see mitosismitosis
, process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei.
..... Click the link for more information. ). 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 ectodermectoderm,
layer of cells that covers the surface of an animal embryo after the process of gastrulation has occurred. This outer layer, together with the endoderm, or inner layer, is present in all early embryos.
..... Click the link for more information. , the inner layer the endodermendoderm
, in biology, inner layer of tissue formed in the gastrula stage of the developing embryo. At the end of the blastula stage, cells of the embryo are arranged in the form of a hollow ball.
..... Click the link for more information. . 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 mesodermmesoderm,
in biology, middle layer of tissue formed in the gastrula stage of the developing embryo. At the end of the blastula stage, cells of the embryo are arranged in the form of a hollow ball.
..... Click the link for more information. , 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 coelomcoelom
, fluid-filled body cavity, found in animals, which is lined by cells derived from mesoderm tissue in the embryo, and which provides for free, lubricated motion of the viscera.
..... Click the link for more information. , 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 (cotyledonscotyledon
, in botany, a leaf of the embryo of a seed. The embryos of flowering plants, or angiosperms, usually have either one cotyledon (the monocots) or two (the dicots). Seeds of gymnosperms, such as pines, may have numerous cotyledons.
..... Click the link for more information. ) occur inside the ovule within the ovary at the base of the pistil. The matured ovule is the seedseed,
fertilized and ripened ovule, consisting of the plant embryo, varying amounts of stored food material, and a protective outer seed coat. Seeds are frequently confused with the fruit enclosing them in flowering plants, especially in grains and nuts.
..... Click the link for more information. ; the fruitfruit,
matured ovary of the pistil of a flower, containing the seed. After the egg nucleus, or ovum, has been fertilized (see fertilization) and the embryo plantlet begins to form, the surrounding ovule (see pistil) develops into a seed and the ovary wall (pericarp) around the
..... Click the link for more information. , when it is produced, is the developed ovary.
What does it mean when you dream about an embryo?
An embryo can represent the most vulnerable part of ourselves, the “inner child.” (Also see Baby, Miscarriage, Pregnant.)