Reproduction and development
Nearly all animals undergo some form of sexual reproduction.[16] They have a few specializedreproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.[17] These fuse to form zygotes, which develop into new individuals.[18]
Many animals are also capable of asexual reproduction.[19] This may take place throughparthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.[20]
A zygote initially develops into a hollow sphere, called a blastula,[21] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.[22] In most other groups, the blastula undergoes more complicated rearrangement.[23] It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.[24] In most cases, amesoderm also develops between them.[25] These germ layers then differentiate to form tissues and organs.[26]
Many animals are also capable of asexual reproduction.[19] This may take place throughparthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.[20]
A zygote initially develops into a hollow sphere, called a blastula,[21] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.[22] In most other groups, the blastula undergoes more complicated rearrangement.[23] It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.[24] In most cases, amesoderm also develops between them.[25] These germ layers then differentiate to form tissues and organs.[26]