Your baby's brain is under-going rapid development. The right and left cerebral hemispheres begin to connect. Each hemisphere controls the opposite side of the body, so the right side of the brain controls muscles on the left side of the body and the left side controls muscles on the right side of the body.
Motor fibers (those that control movement) mature first, so your baby can make increasingly complex limb movements. Sensory nerves (those that control feeding) mature later and are first present on your baby's hands and in his mouth. The brain matures quickly over the next three weeks and will be complete in around 10 weeks' time, as the rest of the upper and lower limbs and trunk achieve adult levels of sensitivity to stimuli. All your baby's nerves are very immature at this stage and he doesn't have any perception of position, pain, temperature, or touch.