Final answer:
The brain's first major growth spurt during prenatal development is due to the proliferation of primary vesicles. The fontanelles in the fetal skull allow for growth and expansion as the brain enlarges.
Step-by-step explanation:
The first major growth spurt of the brain occurs during the fourth and fifth months of prenatal development due to the proliferation of primary vesicles. Primary vesicles are initial enlargements of the anterior neural tube that develop into the forebrain, midbrain, and hindbrain.
In the fetal skull, the braincase bones remain separated from each other by large areas of dense connective tissue called fontanelles. These fontanelles allow for continued growth and expansion of the skull as the brain enlarges.
During weeks 9-12 of fetal development, the body elongates, and the brain continues to expand. The bone marrow takes over erythrocyte production, and the liver begins to secrete bile. At this stage, the eyes are well-developed, but the eyelids are fused shut.