The Great Cell Debate

It may be the hardest decision of the Bush term: Should the U.S. fund stem-cell research? Doing so could bring on a medical revolution--but also an ethical mess. ANDREW GOLDSTEIN explains the key issu

WHAT ARE STEM CELLS?

Some call them magic seeds, for their ability to replicate indefinitely and morph into any kind of tissue. Taken from human embryos only days old, stem cells are nature's blank slates, capable of developing into any of nearly 220 cell types that make up the human body. Scientists believe they will lead to cures for diseases once thought untreatable.

1 EMBRYO An egg is fertilized or cloned to form an embryo. The embryo begins to divide.

2 1 TO 5 DAYS The embryo divides again and again and takes shape as a sphere called a blastocyst

3...

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