Multiple cameras tracked a mouse as it wandered around a small arena. A computer charted its position and speed, leaving Pong-like traces on a monitor. The reason to watch this rodent so carefully? Nearly half its brain volume had been replaced with human cells. The effort to mix the brain tissues of distant species is...

This is today's edition of The Download, our weekday newsletter that provides a daily dose of what's going on in the world of technology. Meet a mouse whose brain cortex is made up of human cells Multiple cameras tracked a mouse as it wandered around a small arena. A computer charted its position and speed,...