An animal wouldn't grow larger than an insect without specialized equipment designed to transport oxygen to and carbon dioxide from cells deep inside its body. Fortunately, thousands of different animal kinds have been outfitted with tiny machines called hemoglobins that directly and specifically interact with individual oxygen molecules, making larger body sizes possible.

Does gravity exist?

Stem cells from adults can be "coaxed" into becoming more specified tissues and used effectively for specific treatments. Stem cells from embryos, however, involve the destruction of a human life and have not yet offered any useful treatments.

For 25 years, donkeys and zebras at the Chestatee Wildlife Preserve have shared the same pasture while maintaining separate identities. When a donkey gave birth recently, zookeepers were surprised to see the baby "zedonk." Its head was donkey-like, but its legs displayed clear zebra stripes, meaning that a zebra stallion was the father.

To produce offspring, males and females contribute specialized reproductive cells called gametes--sperm cells from males and eggs from females. Most genes used to manufacture gametes differ from organism to organism. Scientists were surprised, therefore, by a recent study showing that, in spite of those differences, one gene in particular was present in all the animals surveyed.