In lower plants and algae, the ovum is also often called oosphere.
From the en.wikipedia.org
The antheridia will then form fertilization tubes connecting the antheridial cytoplasm with each oosphere within the oogonia.
From the en.wikipedia.org
A haploid nucleus from the antheridia will then be transferred through the fertilization tube, into the oosphere, and fuse with the oosphere's haploid nucleus forming a diploid oospore.