Development
Animal development is controlled by Hox genes, which signal the times and places to develop structures such as body segments and limbs.[25][26]
During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised into specialised tissues and organs, making the formation of complex structures possible, and allowing cells to be differentiated.[27] The extracellular matrix may be calcified, forming structures such as shells, bones, and spicules.[28] In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.[29]
Reproduction
Nearly all animals make use of some form of sexual reproduction.[30] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova.[31] These fuse to form zygotes,[32] 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.[33] In most other groups, the blastula undergoes more complicated rearrangement.[34] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.[35] In most cases, a third germ layer, the mesoderm, also develops between them.[36] These germ layers then differentiate to form tissues and organs.[37]
Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits.[38][39] Animals have evolved numerous mechanisms for avoiding close inbreeding.[40]
Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.[41][42]