Once fertilized, the ovum is referred to as a zygote, a single diploid cell. The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo. The formation of the male and female gametes (sex cells) through meiosis makes fertilization possible. The ovum, the female gamete, is relatively large (approximately 0.1 mm across in humans) and contains a cytoplasm rich with organelles and substances to be used during development. The Development of viable offspring requires the successful union of ovum and sperm, both of which are haploid (contain half the number of somatic chromosomes), to form a diploid cell. In many mammals, the sperm is required to break through a translucent, elastic coating of the ovum, known as the zona pellucida, before fertilization can take place.
<u>Human activity</u>. The sixth mass extinction of of organisms caused by human actions potentially could go as far back as the late Pleistocene era (over 12,000 years ago.)
