问答题 Birds evolved during the great reptilian radiation of the Mesozoic era, Amniotic eggs and scales on the legs are just two of the reptilian features we see in birds.But modem birds look quite different from modem reptiles because of their feathers and other distinctive flight equipment.
【正确答案】internal
【答案解析】文章第五段第三句提到,受精须在体内进行。因此,填入internal。