These all develop through an embryo that is enclosed within a membrane called an amnion.
The amnion surrounds the embryo with a watery substance, and is probably an adaptation for breeding on land.
analogous structures: Structures in different species that look alike or perform similar functions (e.g., the wings of butterflies and the wings of birds) that have evolved convergently but do not develop from similar groups of embryological tissues, and that have not evolved from similar structures known to be shared by common ancestors. Note: The recent discovery of deep genetic homologies has brought new interest, new information, and discussion to the classical concepts of analogous and homologous structures.
anatomy: (1) The structure of an organism or one of its parts. ancestral homology: Homology that evolved before the common ancestor of a set of species, and which is present in other species outside that set of species. anthropoid: A member of the group of primates made up of monkeys, apes, and humans.
antibacterial: Having the ability to kill bacteria.
antibiotics: Substances that destroy or inhibit the growth of microorganisms, particularly disease-causing bacteria.
acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large muscles of a weightlifter).