The morph at the right schematically depicts formation of the grey crescent of the amphibian egg, and is adapted from a classic work by Schleip. The dense, yolky vegetal deep cytoplasm rotates with respect to the overlying cortex to produce a 30 degree relative rotation. The gray crescent is only visible in certain amphibian eggs. However, subcortical rotation occurs in many amphibian zygotes, and means have been devised to visualize the subcortical movements in these opaque species. This includes the favorite laboratory species for studies of amphibian development, Xenopus.