Final answer:
The indentations between the bones of the fetal skull are called fontanelles, which are large areas filled with fibrous connective tissue that allow the skull to change shape during birth and grow after birth as the brain enlarges. These soft spots close up by the age of two.
Step-by-step explanation:
The indentations between the bones of the fetal skull are known as fontanelles. These expanded areas of fibrous connective tissue separate the braincase bones prior to birth and in the initial years after birth. The fontanelles are often referred to as 'soft spots' on an infant's head because of the lack of bone in these regions. They play a critical role during birth, allowing the fetal skull to change shape as it passes through the birth canal and providing space for the rapid growth of the brain after birth. The largest fontanelle is situated anteriorly at the junction of the frontal and parietal bones. By age 2, the fontanelles decrease in size and no longer exist as they ossify and close up. However, the adjacent skull bones remain connected by sutures, which are dense fibrous connective tissue that allow for continued growth of the skull as the brain enlarges during childhood.
During the development of a fetus, the bones of the skull form through a process called intramembranous ossification, where local mesenchymal cells differentiate directly into bone-producing cells. Conversely, the vertebrae of the spine develop through a process known as endochondral ossification, where mesenchyme cells form a hyaline cartilage template that eventually ossifies into bone.