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 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.