Context: 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.

Question: What is the hollow sphere that a zygote initially develops into called?
Answer: blastula

Question: In sponges, how do blastula develop into a new sponge?
Answer: swim to a new location

Question: What are the 2 germ layers formed by most blastula?
Answer: an external ectoderm and an internal endoderm

Question: What may be formed between the external ectoderm and an internal endoderm layers?
Answer: mesoderm

Question: What do the external ectoderm and an internal endoderm layers develop into?
Answer: tissues and organs