Since female eggs aren't nearly as observable at male semen, it took a staggering 150 years after Anton Von Leeuwenhoek saw sperm for Prussian-Estonian embryologist Dr. Karl Ernst von Baer to follow up with the human ovum [source: CBC News]. In the meantime, erroneous theories of embryonic development and differentiation abounded; some believed a fetuses' future hinged on which testicle produced the semen and where the fluid ended up in the uterus [source: Price].
Contemporary scientists had tirelessly investigated birds' eggs, questioning whether a similar mammalian seed existed, and William Cumberland Cruikshank had claimed the honor of identifying rabbit eggs in 1779 [source: Jansen and Mortimer]. However, von Baer's observations of dog embryos led him to correctly outline the development of the human egg and explain the ovum's cross-mammalian reproductive role in his landmark 1827 paper entitled "On the Genesis of the Ovum of Mammals and of Man" [source: von Baer].
Ironically, this egg pioneer still dismissed sperm as useless in fertilization, exemplifying the simmering uncertainty over how exactly the ovum and those eel-tailed swimmers interact [source: Bernard].