Before maternal-zygotic transition ... There was morphogenetic function of nuclei.
The idea of a major developmental transition that includes the activation of the embryonic genome has a long history. In the 1950-1960s this concept was developed to a large extent due to the efforts of Alexander Neyfakh, who described a specific type of deleterious effect resulting from X-ray irradiation of fish eggs. He interpreted the radiation-sensitive target as the nucleus and established the onset of the function of the zygotic genome, naming it the morphogenetic function of nuclei, what we call now the midblastula transition. Most of his studies were performed using the loach (Misgurnus fossilis), a European teleost. Neyfakh's efforts paved the way to understanding the whole phenomenon of the maternal-zygotic transition.