Embryology
Embryology, branch of biology dealing with the development of the animal embryo. (For the embryology of plants, see Fertilization; Plant; Seed.) Embryology includes within its province the development of the fertilized egg and embryo and the growth of the fetus.
Comparative Embryology
Sea urchins, frogs, humans, and many other animals are remarkably similar in their early development. All begin with a single cell that divides into two cells, the first step in the process of cleavage (1a, 2a, 3a). During cleavage, cell divisions occur so rapidly that the cells do not have time to grow between divisions, and the result is smaller and smaller cells. Cleavage produces a solid ball of cells called a morula (1b, 2b, 3b). Within the morula, a fluid-filled cavity called the blastocoel develops, converting a morula into a blastula (1c, 2c, 3c). In a process called gastrulation, certain cells of the blastula migrate to different regions of the blastula to create the gastrula, a structure with three cell layers (1d, 2d, 3d). The outer cell layer of the gastrula, called the ectoderm (shown in blue), forms the outer covering of all animals, and in the frog, human, and other higher animals, it also forms the nervous system. The inner layer of the gastrula, known as the endoderm (shown in yellow), gives rise to the gut in all animals, and in higher animals, other organs including the stomach, pancreas, liver, and lungs. The mesoderm, which forms between the ectoderm and endoderm, produces the simple excretory system of the sea urchin and frogs and the kidneys of humans. In higher animals, the mesoderm also gives rise to blood, bone, muscle, and other structures. Cell specialization is followed by the development of primitive organs, which marks the larval form of sea urchins and frogs, and the embryo stage of human development (1e, 2e, 3e). Size and time of development vary widely among species. The sea urchin larva, for example, forms in 12 to 76 hours and measures 0.1 to 0.3 mm (0.004 to 0.01 in), while the human embryo takes eight weeks to fully form, and measures about 30 mm (about 1.2 in) from crown to rump.
Comparative Embryology
Sea urchins, frogs, humans, and many other animals are remarkably similar in their early development. All begin with a single cell that divides into two cells, the first step in the process of cleavage (1a, 2a, 3a). During cleavage, cell divisions occur so rapidly that the cells do not have time to grow between divisions, and the result is smaller and smaller cells. Cleavage produces a solid ball of cells called a morula (1b, 2b, 3b). Within the morula, a fluid-filled cavity called the blastocoel develops, converting a morula into a blastula (1c, 2c, 3c). In a process called gastrulation, certain cells of the blastula migrate to different regions of the blastula to create the gastrula, a structure with three cell layers (1d, 2d, 3d). The outer cell layer of the gastrula, called the ectoderm (shown in blue), forms the outer covering of all animals, and in the frog, human, and other higher animals, it also forms the nervous system. The inner layer of the gastrula, known as the endoderm (shown in yellow), gives rise to the gut in all animals, and in higher animals, other organs including the stomach, pancreas, liver, and lungs. The mesoderm, which forms between the ectoderm and endoderm, produces the simple excretory system of the sea urchin and frogs and the kidneys of humans. In higher animals, the mesoderm also gives rise to blood, bone, muscle, and other structures. Cell specialization is followed by the development of primitive organs, which marks the larval form of sea urchins and frogs, and the embryo stage of human development (1e, 2e, 3e). Size and time of development vary widely among species. The sea urchin larva, for example, forms in 12 to 76 hours and measures 0.1 to 0.3 mm (0.004 to 0.01 in), while the human embryo takes eight weeks to fully form, and measures about 30 mm (about 1.2 in) from crown to rump.
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