It is just one of the experiments from the Salk Institute
Scientists in the US have created a human-pig hybrid - raising the prospect of being able to grow human organs for use in transplants.
In a paper published in the journal Cell, scientists at the Salk Institute report their breakthrough on multiple fronts.
They are looking to integrate stem cells from one species into the early-stage development of another.
Combining gene-editing and stem-cell technologies, the scientists were also able to grow a rat pancreas, heart and eyes in a developing mouse.
They say this provides proof-of-concept that functional organs from one species can be grown in another.
They were also able to generate human cells and tissues in early-stage pig and cattle embryos - marking the first step "toward the generation of transplantable human organs using large animals whose organ size, physiology and anatomy are similar to humans".
But they found that fully integrating cells from different species might prove more difficult than combining cells from rats and mice - which are closer evolutionary relatives.
Professor Juan Carlos Izpisua Belmonte from the Salk Institute says: "Our findings may offer hope for advancing science and medicine by providing an unprecedented ability to study early embryo development and organ formation, as well as a potential new avenue for medical therapies.
"We have shown that a precisely targeted technology can allow an organism from one species to produce a specific organ composed of cells from another species.
"This provides us with an important tool for studying species evolution, biology and disease, and may lead ultimately to the ability to grow human organs for transplant."
Scientists have long hoped to use stem cells to grow replacement organs for patients.
However this has proven challenging because of the complexity of recreating an animal's physiology from scratch.
But the new paper reports how the team expanded on earlier work, detailing the variety of experiments the lab undertook to advance the technology.
"Of course, the ultimate goal of chimeric research is to learn whether we can use stem-cell and gene-editing technologies to generate genetically-matched human tissues and organs, and we are very optimistic that continued work will lead to eventual success," Belmonte added.
The institute says future work will involve continuing to understand the limits to getting human cells to incorporate successfully into animals.