 The young embryo is oval in shape, like a rugby ball.A small number of genes produce messenger RNAs that set up concentration gradients along the long axis of the embryo.In the early embryo, the "bicoid" and "hunchback" genes are at high concentration near the anterior end, and give pattern to the future head and thorax; the "caudal" and "nanos" genes are at high concentration near the posterior end, and give pattern to the hindmost abdominal segments.