Eggs removed from the last two female northern white rhinos have been fertilised with sperm from the now-dead last male.
But, an Italian assisted-breeding company said, it will be about 10 days before it's known whether the eggs have become embryos.
"We expect some of them will develop into an embryo," said Prof Cesare Galli, a founder of Avantea and an expert in animal cloning.
Avantea said that only seven of 10 eggs extracted last week from the females in Kenya could be used in the fertilisation attempts on Sunday using frozen sperm that had been taken from the male, which died in March 2018.
Wildlife experts and veterinarians are hoping that the species can reproduce via a surrogate mother rhino.
Prof Galli said that to improve chances for a species' continuation, it is better not to "get to the last two individuals before you use this technology".
He added: "There are so many expectations and people don't always realise that when you are doing something for the first time, so you're a pioneer, most of the time it goes wrong before it gets right, and if you don't get killed before you get (it) right, then eventually you get there, so it's been quite stressful, but when we saw this maturation rate this morning we've been quite relieved."
The male, a 45-year-year-old named Sudan, gained fame in 2017 with his listing as "The Most Eligible Bachelor in the World" on the Tinder dating app in a fundraising effort.
Sudan was euthanised after age-related complications.
Decades of poaching decimated the northern white rhino's numbers.
The ultimate goal is to create a herd of at least five animals that could be returned to their natural habit in Africa. That could take decades.
Sudan was the last of his kind to be born in the wild, in the country he was named after.
Other rhinos - the southern white rhino and the black rhino - are also prey for poachers, who kill them for their horns to supply illegal markets in parts of Asia.