|
Differentiation into two sexes appears in some members of all divisions of the plant and animal kingdoms. Even in species where little or no sexual difference has occurred anatomically, an implied separation exists in forms in which conjugation occurs (e.g., among different strains in paramecia and between plus and minus strains in molds). Many lower forms reproduce within the one individual two different kinds of cell that unite to form a new individual; in others, male and female cells form in different individuals. Among the vertebrates, the sexes are usually readily distinguishable by their primary sexual characteristics, i.e., the structure of their reproductive organs. In the highest group of plants, the seed-bearing plants, the female organ is the pistil and the male organ is the stamen. The stamens and pistil may appear in the same flower, in different flowers of the same plant, or in the flowers of separate plants. Secondary sexual characteristics include the bright coloration of many male birds and fish, the antlers of male deer, the beard and deepened voice of human males, and the mammary glands of female mammals. In higher animals, hormones released by the sexual organs under stimulation from the pituitary hormones play a dominant role in the control of sexual characteristics and the sexual processes of reproduction (see pituitary gland).
Sections in this article:
|
