This uncertainty provided the background for the famous papers of Oscar Hertwig. Claiming in 1876 and 1877 that the "cleavage nucleus [i.e., the zygote] arises from the conjugation of two different sexual nuclei, a female nucleus which is derived from the germinal vesicle and a male nucleus which is derived from the body of an entering spermatozoon," Hertwig posited a morphological theory of fecundation that challenged the generally held physicochemical interpretation.