Casi todos los animales experimentan algún tipo de reproducción sexual. Los adultos son diploides u ocasionalmente poliploides. Tienen unas cuantas células reproductivas especializadas que mediante meiosis producen óvulos o espermatozoides. Éstos se funden para formar un cigoto que se desarrolla en nuevos individuos.
Muchos animales pueden reproducirse asexualmente. Esto tiene lugar a través de la partenogénesis por ejemplo, donde se producen huevos sin apareamiento, o en algunos casos, mediante la fragmentación.
Un cigoto se desarrolla inicialmente en una esfera hueca, llamada blástula, que experimenta un reordenamiento y una diferenciación. En las esponjas, la blástula nada a un nuevo lugar y se desarrolla en una nueva esponja. En otros muchos grupos, la blástula sufre un reordenamiento mucho más complejo. Primero se invagina para formar una gástrula con una cámara digestiva y dos hojas embrionarias separadas: un ectodermo externo y un endodermo interno. En muchos casos también se desarrolla un mesodermo entre ambos. Estas capas embrionarias se diferencian entonces para formar tejidos y órganos.
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