The mixing of species is not confined to language; it is happening apace in the laboratory. Scientists in Japan have just reported a special delivery of rat pups, fathered by rat sperm cultured in the testes of mice. Although researchers have long been able to grow the sperm of one species in the body of another species, it is the first time that such sperm have been proved viable. The rat pups appear to be healthy and fertile.

The experiment has raised the possibility that the sperm of prize cattle and even human beings could be nurtured in mice. It is also touted as a possible means of saving endangered species. Whatever the applications, the experiment falls into the troubled genre of chimeric experimentation, in which the cells of one species are implanted in another. Most troubling of all, for some, is the scant ethical debate about how far we should go in embedding human cells into other creatures. Last year five MPs on the House of Commons Science and Technology Committee dissented when the committee expressed qualified support for the laboratory production of chimeras.

A team led by Takashi Shinora, of the University of Kyoto, implanted germ line stem cells from rats into mouse testes. These stem cells produce sperm and the researchers showed that, despite being stranded in a murine environment, the cells continued to produce rat sperm. The next step was to extract the sperm, use it to fertilise a rat egg, and implant the resulting embryo in female rats. Of 339 embryos transferred, 15 resulted in successful births.

Shinora published his results last week online in the Proceedings of the National Academy of Sciences; peers regard it as a milestone. Shinora says that it would not be a good idea to manufacture human sperm in mice for the purposes of reproduction, because of the risk that animal viruses could scrawl themselves over human genetic code.

But others are determined to inject human cells into animals. The most controversial projects have been those to implant human brain cells (neurons) into the brains of other animals, such as vervet monkeys or mice. One headline dubbed the mouse-human chimera, produced by Irving Weissman, of Stanford University, as the “Stuart Little” mouse, referring to the film about a mouse who talks and has human parents. The great fear is that chimeric experimentation will lead to a human brain being trapped inside a mouse’s body.

The scientists themselves have been relatively open about addressing the natural revulsion to such an idea. Hank Greely, the Stanford law professor charged with the ethical supervision of Weissman’s project, admits: “Frankly, if we made a mouse that had a fully human brain in it, I would be concerned.”

The Stanford project involved several checks: the first mice containing human neurons were dissected to see if the human cells were causing abnormal growths; researchers were instructed to kill mice displaying unusual behaviour. But, as Professor Greely points out, it is the architecture of the brain rather than its building materials that determines its function. “If you follow an architect’s design to build a cathedral it doesn’t matter if it’s made of red bricks or grey bricks; it’s still a cathedral. Well, if you make a mouse brain with human cells, it’s still going to be a mouse brain.”

• ONE OF THE delights of this column is the mailbag (or inbox). On the downside, there are the angry capitalisers who think I get EVERYTHING wrong; the green-inkers who believe that Einstein got everything wrong; and the BNP splinter group who think that the comment pages would be very much better if I went back to where I came from (Essex).

  On the upside, I often come across thought-provoking letters. One arrived last week from Roger Voles, who sent a paper he had written for The Mathematical Gazette, entitled The Arithmetic of Cuckoldry in Family Trees. He argues that since the paternity of 10 to 15 per cent of babies is disputed, the farther back into the past the genealogist delves, the more likely the family tree is to contain errors. Auntie Maureen, whose tireless sleuthing in churchyards and local record offices results in an annual updating of my husband’s family tree, will not be amused.

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