Embryo and Embryonic Development

Specific tissue types form during embryo development. A portion of the ectoderm on the dorsal side of the embryo rolls up into a tube, which forms into the central nervous system. The anterior portion of the tube becomes the brain and the posterior portion becomes the spinal cord. Some mesoderm cells become specialized to form muscle and the muscle functions well before feeding occurs. This can be observed by muscular activity (bending and twisting of the embryo body) in the as yet unhatched embryo in its jelly membranes. Careful examination of the embryo at this time reveals a pulsation. The pulsation is the beating of the heart muscle, which begins to circulate embryonic blood cells. Embryonic gills are exposed on either side of the head. The structure of the gills is so delicate that blood cells, with a dissecting microscope, can be seen surging in synchrony with the beating heart. During embryonic development, the excretory system and the digestive system begin their formation. Within six days in the laboratory, all embryonic systems have begun developing. Hatching occurs at about this time, which marks the end of the embryonic period and the beginning of larval development. Larvae feed in about a week after fertilization. Embryonic development has been characterized by differentiation of organ systems but no increase in mass. Feeding begins and the tadpole grows enormously in size compared with its origin. Feeding and growing will continue until the larval tadpole begins its metamorphosis into a juvenile frog.