If you think it's complicated in placental mammals, it's REALLY fun in zebrafish... all embryos start off building an ovary and dozens of loci all over the genome on autosomes rather than sex chromosomes alter the probability of the ovary spontaneously regressing then transforming into a testis. Immature egg cells are vital to both the process by which it becomes an ovary and by which it becomes a testis. Every breeding pair of zebrafish will produce a unique sex ratio of offspring depending on their genotypes at many loci and what they pass on to their offspring.