The development of sperm cells from spermatocytes to mature sperm, including meiosis. PICTURE

spicules  Needle-shaped skeletal elements in sponges that occur in the matrix between the epidermal and collar cells.

spinal cord  A cylinder of nerve tissue extending from the brain stem; receives sensory information and sends output motor signals; with the brain, forms the central nervous system. Nerve cell collections extending from the base of the brain to just below the last rib vertebrae.

spindle apparatus Microtubule construction that aligns and segregates chromosomes during eukaryotic cell division. PICTURE

spleen An organ that produces lymphocytes and stores erythrocytes.

spongy bone  The inner layer of bone; found at the ends of long bones and is less dense than compact bone. Some spongy bone contains red marrow.

spongy mesophyll Parenchyma cells found in plant leaves that are irregularly shaped and have large intracellular spaces. PICTURE 1 | PICTURE 2

sporangia  The structures in which spores are produced (sing.: sporangium).

spores  Impervious structures formed by some cells that encapsulate the cells and protect them from the environment; haploid cells that can survive unfavorable conditions and germinate into new haploid individuals or act as gametes in fertilization.

sporophyte  The diploid stage of a plant exhibiting alternation of generations. The diploid, spore producing phase of the plant life cycle. PICTURE

Sporozoans Members of the protists that are referred to as slime molds; may include organisms resembling the ancestors of fungi.

stability  One of the phases of a population's life cycle. The population's size remains roughly constant, þuctuating around some average density. Also, the ability of a community to persist unchanged.

stabilizing selection  A process of natural selection that tends to favor genotypic combinations that produce an intermediate phenotype; selection against the extremes in variation. PICTURE

stalk  A leaf's petiole; the slender stem that supports the blade of a leaf and attaches it to a larger stem of the plant.

stamens  The male reproductive structures of a þower; usually consist of slender, thread-like filaments topped by anthers. The male reproductive structures in the flower, composed of a filament and anther. PICTURE

stapes One of the three bones that function in hearing.

start codon  T