Polar bodies serve as a landmark for anteroposterior axis formation in spiders

The early embryogenesis of many spiders involves the formation of a radially symmetric germ disc. While the cells of the rim of this germ disc develops into anterior structures, the center of the disc will form the posteriorly located segment addition zone of the embryo. Therefore, germ disc formation sets the anterior-posterior (AP) body axis of spider embryos. The early spider egg is a spherical structure with no apparent asymmetry, and it is believed that germ disc formation is a stochastic process. For this study, we have re-analyzed early spider embryogenesis and found a strong correlation of the position of the polar bodies and the formation of the germ disc. Our results suggest that germ disc formation in the common house spider Parasteatoda tepidariorum is not a stochastic but a pre-determined process. Furthermore, we provide evidence that this correlation might be conserved between araneomorph and mygalomorph spider species.