Sea Urchin Embryology (click on thumbnail photos to enlarge)
Unfertilized Egg	Fertilized Egg	Zygote cleavage
2 Cell Stage	2 Cell Stage Late	4 Cell Stage
Eight-cell Stage	Sixteen Cell Stage	Thirty-two Cell Stage
Blastula	Blastula -late	Early Gastrula
Gastrula	Gastrula - late	Zona emphasized