Thanks to 3D printing, it will soon be possible to design and build household animals to order.

GeneDupe’s Universal Pet Printer is loaded with each of the 220 cell types (grown from stem cells in the company’s histology laboratory), and is programmed with a three-dimensional map of the creature it is to create. That is devised by the firm’s scientists, based on what the nanotomographic analysis has told them about the results of arranging cells in different ways in an animal’s body.

The cells themselves are stored in suspension, in glass reservoirs, and each reservoir is connected to a computer-controlled spray gun. The hydrogels (several sorts are needed) are stored separately. One, known as osteogel, is particularly important, as this solidifies to provide the animal’s initial skeleton. Once the new creature is up and running (both literally and metaphorically), the various hydrogels are gradually replaced by natural secretions. In the case of osteogel, that secretion is bone.

Who among us is invulnerable to the puppy in the pet store window? Not everyone is a dog person, of course; some people are cat people or horse people or parakeet people or albino ferret people. But human beings are a distinctly pet-loving bunch. In no other species do adults regularly and knowingly rear the young of other species and support them into old age; in our species it is commonplace. In almost every human culture, people own pets. In the United States, there are more households with pets than with children.

On the face of it, this doesn’t make sense: Pets take up resources that we would otherwise spend on ourselves or our own progeny. Some pets, it’s true, do work for their owners, or are eventually eaten by them, but many simply live with us, eating the food we give them, interrupting our sleep, dictating our schedules, occasionally soiling the carpet, and giving nothing in return but companionship and often desultory affection.

What explains this yen to have animals in o

The world’s smallest horse was born in late April on a farm in New Hampshire. Weighing in at 6 pounds at birth, Einstein appears to have beaten the previous record holder by three whole pounds.

But Einstein probably won’t hold his place in the Guinness Book of World Records forever, because there may be no limit to how tiny we can make our horses, said equine geneticist Samantha Brooks of Cornell University. But to get teacup horses will take many generations of breeding.

“In the last 50 years, breeders have made very good progress at making a very small horse, but they periodically hit these speed bumps,” said Brooks. “It takes a while to work them out so that you end up with a horse that not only fits in the palm of your hand but is happy and healthy.”

In recent years, the genetic underpinnings of height and size in mammals have generated increasing interest from scientists. In 2007, genetics researchers made the surprising finding that a single gene plays a very large role in reg