Embryo and Embryonic Development

An embryo is a stage directly after fertilization that signifies the early stages of growth and development of an organism. In humans, this stage ends during the third month of pregnancy, and is then called a fetus. Plants and invertebrate as well as vertebrate animals have an embryonic stage of development. For example, the embryo of the common North American leopard frog, Rana pipiens is from a species that has been studied extensively because it is common, relatively easy to induce ovulation in females, and developmental progress can be monitored in a glass dish due tot the fact that the embryo is neither within a shell (as with reptiles and birds) nor within the body of the mother (as with mammals). Much of what is known today about the stages of embryonic development and characterization of the embryo has been accomplished through research contributions related to the development of the frog. In fact, the technique of removing male and female sex cells to be used for fertilization and then replace the fertilized egg into the appropriate place in the female, also known as in vitro fertilization was first demonstrated in this species. This technique has revolutionized reproductive technologies in humans and has allowed certain reproductively challenged couples the option of having biologically-related children. Although there are many differences, development of humans is much like the frog in terms of the basic embryological terminology as well as the distinct stages of embryonic development. In the Rana pipiens, development is divided into an embryonic period which occurs prior to hatching from the jelly membranes that enclose the embryo, and larval development which is the period of the free-living feeding tadpole.