Animal

:For the Muppet Show character, see Animal (Muppet). For the professional wrestler, see Joseph Laurinaitis.

Reproduction and development

Nearly all animals undergo some form of sexual reproduction. Adults are diploid or occasionally polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals.

Related Topics:
Sexual reproduction - Diploid - Polyploid - Meiosis - Spermatozoa - Ova - Zygote

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Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, or in some cases through fragmentation.

Related Topics:
Asexual reproduction - Parthenogenesis

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A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangment 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.

Related Topics:
Zygote - Blastula - Ectoderm - Endoderm - Mesoderm

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