  Second, it is not yet clear what kinds of cognitive enhancements might be possible even within a constrained environment. For example, it is possible to use genetic engineering to enhance the learning and memory of mice without modifying skull size or gestational period (Tang et al. 1999). Further, the incorporation of human glia and astrocytes into the brains of neonatal mice was found to “sharply enhance” their learning (Han et al. 2013, Cossins 2013, Cole 2013). More extremely, if large quantities of hESCs are introduced during the early stages of embryonic development, it is possible that the cells themselves could induce changes that would eliminate some of the constraints mentioned above. Although some of the experiments cited above do involve the introduction of hES and neural progenitor cells early in development, experience in this area is limited. As the NAS Committee concluded,