Ovum, the single cell from which a multicellular animal develops, usually after fertilisation by another cell which represents the male element. In the simpler animals the ovum is one of the cells of either of the two membranes (ectoderm and endoderm), forming the wall of the body, but in higher animals it is one of the cells of a special organ called the ovary. The ovum may be composed simply of protoplasm and a nucleus, or it may have one or more envelopes; it generally contains some food material (deuteroplasm) which may be mixed up with it or be included with the ovum in an egg-shell; it is then absorbed during the subdivision and growth of the ovum. Before fertilisation the ovum undergoes the process of karyokinesis (q.v.), when half of the nucleus is expelled; the remaining half may undergo the same process. The ovum is then quiescent until it is stimulated to development by union wdth a "spermatozoon" or male element; this, however, is not necessary in some cases, which may develop directly a process known as parthenogenesis (q.v.).