Final answer:
The oogonium that produced an ovum with 18 chromosomes would have had 36 chromosomes, as it is a diploid cell which goes through meiosis to produce a haploid ovum with half the number of chromosomes.
Step-by-step explanation:
The oogonium that produced an ovum (egg) with 18 chromosomes would have originally had 36 chromosomes. This is because oogonia are diploid cells, meaning they contain two sets of chromosomes. After completing the process of meiosis, which includes two rounds of cell division, the resulting ovum is haploid, containing only one set of chromosomes, which is half the number present in the oogonium. Therefore, if an ovum has 18 chromosomes, the original oogonium had to have 36 chromosomes (18 pairs).