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. COUNT 5
QSN1 What is the hollow sphere that a zygote initially develops into called? ANS1 blastula QSN2 In sponges, how do blastula develop into a new sponge? ANS2 swim to a new location QSN3 What are the 2 germ layers formed by most blastula? ANS3 an external ectoderm and an internal endoderm QSN4 What may be formed between the external ectoderm and an internal endoderm layers? ANS4 mesoderm QSN5 What do the external ectoderm and an internal endoderm layers develop into? ANS5 tissues and organs