Explain the process of early development in animals. Starting with a zygote, describe what occurs during cleavage, and explain the difference between a morula, blastula andgastrula. Explain why the size of the early embryo, from zygote to gastrula, changes very little.Describe how the blastula of frogs, chicks and sea urchin will appear very different from each other, and explain why. Finally, describe in detail the events of gastrulation in the sea urchin.