There was an interesting exchange in the stem cell debate earlier this week between Harvard biologist George Daley and Senator Sam Brownback on when a human embryo acquires moral rights. According to Wired News, Brownback “persistently” asked Daley at what age he would decline to use an embryo for medical research. Wisely, Daley said he could not define when an embryo becomes a human being. “I think there would be consensus among scientists that it would be impossible to define that time,” Daley said. “But I don’t think it’s at the age of the blastocyst.”

Wired writes:

Brownback pressed further, asking him to envision his two children and determine at which point in their development it would be OK for scientists to perform research on them. “I can’t hug an embryo,” Daley said. “I think (scientists) are comfortable with using the earliest microscopic ball of cells.”

Of course I don’t have an answer either–or at least one that will convince the Brownbacks of the world–but it’s always seemed to me that the development of a nervous system ought to be involved. The “primitive streak,” from which the central nervous system eventually develops, begins to appear at about 14 days after fertilization when the embryo consists of about 2000 cells–significantly later than the 100-cell blastocysts Daley is talking about.