They had gonads with oogonia, spermatogonia and spermatocytes. Sexually, they reproduce with structures called antheridia, oogonia, and oospores. The antheridia, which are much smaller than the oogonia, are called dwarf male. But then many of these oogonia die and about 50,000 remain. All of the oogonia and many primary oocytes die before birth. These bridges resemble ring canals that link developing spermatogonia or oogonia in animals. Cohesin is loaded onto newly replicated chromosomes in oogonia during fetal development. In nannandrous species, filaments producing antheridia and oogonia show morphological distinction. Dwarf males are small, short, antheridium-producing filaments attached near the oogonia (female sex organ). Although filaments bearing antheridia and oogonia are morphologically similar, they differ physiologically. During oogenesis, the oogonia become primary oocytes. In some invertebrates some oogonia become nurse cells. Once they have reached the gonadal ridge they are called oogonia. After beginning the meiotic process, the oogonia (now called primary oocytes) can no longer replicate. The Oogonia multiply by dividing mitotically; this proliferation ends when the oogonia enter meiosis.