1880-1881, Francis Maitland Balfour, A Treatise on Comparative Embryology It is clear that the ovum after fertilization is an entirely different body to the ovum prior to that act , and unless the use of the same term for the two conditions of the ovum had become very familiar , a special term , such as oosperm , for the ovum after the fusion with the spermatozoom, would be very convenient.
Source: wiktionary