[21] In most other groups, the blastula undergoes more complicated rearrangement.[22] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.


[23] In most cases, a third germ layer, the mesoderm, also develops between them.[24] These germ layers then differentiate to form tissues and organs.
Nearly all animals make use of some form of sexual reproduction.[18] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova.[19] These fuse to form zygotes,[20] which develop via mitosis into a hollow sphere, called a blastula.
In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.

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