IWhich fetal development occurs during the second trimester? Bones begin to harden. The roof of the mouth forms. The eyes can sense changes in light. The heart and circulatory system begin to develop.
The embryonic cartilage begins to harden into bone at anytime around the 2nd trimester. The embryonic cartilage undergoes endochondral ossification wherein the chondrocytes is slowly replaced by osteocytes with subsequent deposition of calcium and hydroxyapatite; therefore converting the cartilages into bones.
