By the end of the preschool period, the part of the brain has grown significantly because it becomes considerably thicker with individual fibers that coordinate brain functioning between the two hemispheres.

The answer is corpus callosum. This is the one that connects the two hemispheres of the brain. Its packed functioning is essential for the child to leap on one foot while keeping the arms in the air for balance. Younger children cannot do this exercise without holding onto something hard. in the same way, extra abilities in the academic domain which need communication between the two hemispheres cannot occur without this progress. 

the major parts of the digestive system:

salivary glands.

pharynx.

esophagus.

stomach.

small intestine.

large intestine.

rectum.

accessory digestive organs: liver, gallbladder, pancreas.

amber has allowed scientists to study dna that has been preserved over millions of years.