WASHINGTON -- It felt like a hard-boiled egg to 17-year-old Anna Rediger, and looked a bit like cauliflower. But in her gloved hand was a piece of a sheep's brain, which she had scraped, sliced and probed to learn how the organ -- and, by extension, a human brain -- is designed.

The senior at Oakton High School in Fairfax County, Va., was performing a dissection, one of several that teacher Ann Starr's anatomy and physiology class will do this year to study the systems -- respiratory, digestive, etc. -- that make up living things. Earlier in the year, a fetal pig and a chicken were dissected; coming up are a cat and an animal heart.

"It's kind of amazing to realize that this is, well, real," Rediger said. "You can touch it. You really do learn better this way."

Dissections have been a staple of classroom science since the 1920s. Parents of today's students may well remember the venerable frogs that their teachers once forced them to slice into. But things have changed.

Once the province of high school, dissections have moved into earlier grades, science teachers say. It's common for middle school students now to dissect animals, and some elementary classes do, too.

"We do have teachers who do demonstrations in elementary school," said Odette Scovel, science supervisor for Loudoun County, Va., Public Schools. "It works if it fits their curriculum, although I don't necessarily encourage it. The skill of the teacher plays a big role in the decision."

And for those morally opposed, or simply too squeamish, modern technology has provided a ready alternative: computer-based virtual dissections, sometimes with three-dimensional technology that makes the experience far more vivid than regular photographs.

Some students prefer the computer versions because they are "tuned in electronically," Scovel said.

"That is their method of learning, and they can translate a virtual environment very easily into a real environment," she said. Other kids can't do that. "It has to do with their learning style and brain development."

A 2002 poll by the Humane Society showed that a majority of biology teachers say they believe the real thing is a better educational experience than the virtual alternative. The National Association of Biology Teachers support the practice of dissection when done for legitimate educational purposes, as does the National Science Teachers Association.

"I would use the example of driving a real car versus a driving simulation on a computer or in a game room machine. The real time dissection provides awareness to all of the senses -- touch especially -- texture, form, etc.," Kenneth R. Roy, chairman of the Science Safety Advisory Board of the science teachers association, wrote in an e-mail.

Across the country, more dissections are performed than ever before, according to advocates and critics of dissection. The nonprofit Humane Society estimates that about 6 million vertebrate animals are dissected in U.S. high schools alone; the number of dissections of invertebrate animals is probably comparable, it says. There are no estimates for elementary and middle schools.

Meanwhile, dozens of kinds of animals are used today, a broader range than ever, selected to teach different lessons. While frogs remain the most commonly dissected animal, others include dogfish sharks, rats, mice, turtles and snakes, and invertebrates including starfish, earthworms, cockroaches and grasshoppers.

The rise of the animal rights movement has sparked an increase in opposition to the practice, accompanied by a movement to allow students who want to opt out to be given alternative educational options, said Kathleen Conlee, director of program management for animal research issues at the Humane Society.

"We get calls from students who are being punished for refusing," she said. "It has happened on occasion even in states that have student choice."

Nine states have passed laws requiring schools to grant students alternatives to dissection; four have informal policies that demand the same thing.

Starr's students said they believed the learning experience was worth any discomfort they might feel in using animals.

"I think it's OK, because it's educational," Rediger said.