Later on, the female and male pronuclei fuse, the cortex of the egg relaxes and the fertilization membrane is raised up from the surface of the egg. The middle point of the grey crescent has been shown to be equivalent to the future middorsal point of the body. Read C . The pigmented cortex is apparent on some of the slides of the blastula, and not so easily identifiable on other slides. Cleavage Early cleavage divisions occur without overall increase in mass.
First cleavage begins at the animal pole and travels to the vegetal pole.
It is meridional and bisects the grey crescent, occurring approximately 2.5 hours post-fertilization. It is also meridional and occurs at approximately 3.5 hours post-fertilization. It occurs at approximately 4 hours post-fertilization. At its completion, there are now eight cells total, four micromeres and four macromeres. Cleavages follow synchronously and rapidly.