ABSTRACT
The cleavage of the echinoderm egg is total, and radially symmetrical. The first two cleavages are vertical, through the animal pole. The third is horizontal, running slightly above the equator, so that the upper four cells are rather smaller than the lower four. From this point onwards, the cleavages in the animal and vegetative halves take different courses. At the fourth cleavage, the animal cells divide into a flat ring of eight, while the vegetative ones cleave very unequally into two rings, four large 'macromeres' above and four tiny 'micromeres' below; the cleavage planes are thus nearly at right angles in the animal and vegetative halves. The same is true at the next cleavage, but here it is in the animal half that the division is horizontal, while in the vegetative half it is more or less vertical. At the sixth cleavage, all the division planes are horizontal, and thus we come to a stage with four rings of animal cells, two rings of macromeres, and two of micromeres, there being eight cells in each ring.
