Amniotes (reptiles, birds, and mammals) have not always existed, nor have modern amniotes existed since the first appearance of fossil amniotes. The first amniotes were preceded by amphibians that resemble them more than do any modern amniotes. Modern amniotes form a clade which shares a number of features of the skeletal system (modified skull and limb bones, strengthened vertebral column), nervous system (an organization of cerebral nuclei), cardiovascular system (reorganized blood vessels, reduction of some heart chambers, increased cardiac output, a common conduction system in the heart), respiratory system (greater internal complexity of the lungs, a trachea lengthened and divided to form bronchi, ribs attached to the sternum to function in pulmonary ventilation, intercostal and abdominal muscles used in breathing), digestive system (additional salivary glands, stronger teeth), the lymphatic system (interferons), urinary system (the metanephros became the primary kidney), reproductive system (a penis, specializations of the oviduct, a clitoris, ciliary action and muscle contraction to move the oocyte), and embryonic development (extraembryonic membranes and homologs genes which determine sexual development).