seed coat The tough outer layer of the seed, derived from the outer layers of the ovule. PICTURE

segments  Repeating units in the body parts of some animals.

segregation Separation of replicated chromosomes to opposite sides of the cell. Distribution of alleles on chromosomes into gametes during meiosis.

selective breeding  The selection of individuals with desirable traits for use in breeding. Over many generations, the practice leads to the development of strains with the desired characteristics.

selectively permeable Term describing a barrier that allows some chemicals to pass but not others. The cell membrane is such a barrier.

semen  A mixture of sperm and various glandular secretions.

semiconservative replication  Process of DNA replication in which the DNA helix is unwound and each strand serves as a template for the synthesis of a new complementary strand, which is linked to the old strand. Thus, one old strand is retained in each new molecule. PICTURE

semilunar valve  A valve between each ventricle of the heart and the artery connected to that ventricle. PICTURE

seminal vesicles  Glands that contribute fructose to sperm. The fructose serves as an energy source. The structures that add fructose and hormones to semen. PICTURE

seminiferous tubules  Tubules on the interior of the testes where sperm are produced. PICTURE

sensor  In a closed system, the element that detects change and signals the effector to initiate a response.

sensory cortex  A region of the brain associated with the parietal lobe.

sensory input  Stimuli that the nervous system receives from the external or internal environment; includes pressure, taste, sound, light, and blood pH.

sensory neurons  Neurons that carry signals from receptors and transmit information about the environment to processing centers in the brain and spinal cord. Neurons carrying messages from sensory receptors to the spinal cord. Sometimes referred to as an afferent neuron.

sensory (afferent) pathways  The portion of the peripheral nervous system that carries information from the organs and tissues of the body to the central nervous system.

sepals  Modified leaves that protect a flower's inner petals and reproductive structures. Small, leaf-like structures in flowers that enclose and protect the developing flower. These are often green, but in many monocot