Animal growth occurs through the process of mitosis, where cells reproduce more quickly than they die, leading to tissue and organ growth. New organisms are created through sexual reproduction, where male and female gametes produced through meiosis join to form a zygote. Fertilization and early development can occur inside or outside the body, with mammals being fertilized internally and giving birth to live young requiring maternal care, while many fish and amphibians are fertilized externally with no maternal care.