A zygote initially develops into a hollow Sphere.

http://en.wikipedia.org/wiki/Animal

A zygote initially develops into a hollow sphere, called a blastula,which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.In most other groups, the blastula undergoes more complicated rearrangement.It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.In most cases, a mesoderm also develops between them.These germ layers then differentiate to form tissues and organs.