Frog embryo development, animation. At stage 1, The egg has just been fertilised by a sperm. At stage 2, after around three and a half hours, the first cleavage occurs, forming 2 cells. Several subsequent cleavages occur over the next few hours. At around the seventh hour, the embryo undergoes its fifth cleavage and is composed of 32 cells (stage 7). The embryo has now reached the blastula stage, where it is a hollow ball of cells. Stages 8 to 10 (gastrulation) occur between hours 26 and 50. A crescent-shaped lip forms, and the blastula folds in on itself to form the three germs layers; the ectoderm, endoderm and mesoderm. The ectoderm forms epidermis and nervous tissue, the endoderm forms the digestive tract, glands and lungs and the mesoderm forms the connective tissue, including blood vessels and cartilage. At stage ten (neurulation) a neural tube forms that will go on to become the frog's brain and spinal cord. By stage twelve, the neural tube has folded in on itself and primitive limbs and a tail bud have begun to grow. |