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.