In 2005, Senator Sam Brownback of Kansas introduced a bill imposing a $1 million fine on anyone creating and profiting from a chimera with human cells in its brain or reproductive tissues. That bill went nowhere, but in deference to public concerns the National Institutes of Health in 2015 instituted a moratorium on using public funds to insert human cells into animal embryos.

The N.I.H. proposed last August to lift the ban, but to require grant proposals involving the insertion of human stem cells into early animal embryos to be examined by an expert committee. The public comment period on the proposal drew 22,000 responses that it reviewed. .

The ban is still in place, and it’s unclear whether the Trump administration would continue to consider lifting the moratorium or whether new objections would be raised to using public funds for this line of research.

“We have no indication one way or another that they have any opinions on this,” said Carrie D. Wolinetz, the associate director for science policy at the N.I.H.

Insertion of human stem cells into the early embryos of monkeys was prohibited in 2009, and remains so because monkeys, given their evolutionary closeness to humans, might easily have their brains altered by human cells.

Biologists’ interest in chimeras has been prompted by the limited success in coaxing medically useful tissues from stem cells grown in glassware. All-purpose human stem cells were first derived from human embryos in 1998 and from ordinary adult tissue cells in 2007. After each discovery there were hopes of converting the cells into therapeutic tissues by exposing them in glassware to the sequence of natural chemicals that in the living embryo directs them into constructing the heart, brain, lungs and other organs.

Image A chimera, left, created with a Wistar rat, middle, and a mouse, right. The rat-mouse chimera was generated by injecting mouse iPS cells into a rat embryo. Credit... Tomoyuki Yamaguchi

But no one knows exactly what sequence of chemicals is required for the generation of each different tissue or organ. This may be why glassware experiments with stem cells have not yet lived up to their full promise. Some biologists believe a better approach may be to grow stem cells not in glassware but in a developing embryo, where they will be exposed to the natural sequence of chemicals required to induce each type of organ.