Two transparent polar bodies are formed, but no polar lobes. The first noticeable change occurs shortly before first cleavage when there is a slight flattening of the egg at the animal pole. The first cleavage furrow usually divides the egg into two equal blastomeres, but in a small percentage of eggs one blastomere is perceptibly larger. The second cleavage is at right angles to the first, and again in some cases the D cell is slightly larger than the others. The cells of the first quartet of micromeres, given off by the dexiotropic third cleavage, are distinguishable from the larger macromeres.
Further divisions follow the regular pattern of spiral cleavage.
Four quartets of micromeres are produced. The following schedule, based on a batch of eggs developing at 23 to 24 C., is offered as an approximate outline of developmental rate. The times are recorded from insemination. The larva rotates on its longitudinal axis, following a spiral path. Other regions of the digestive tract are obscured by the yolk mass.