During vertebrate development, the bones in appendages are first modeled in , which is later replaced with living bone.

During vertebrate development, the bones in appendages are first modeled in cartilage, which is later replaced with living bone. This process is known as endochondral ossification.

Here are the steps:

1. Mesenchymal cells, a type of stem cell, gather together at the site where the bone will form.

2. These cells differentiate into chondrocytes, which are cells that produce cartilage.

3. The chondrocytes begin to produce a matrix of collagen and proteoglycans, forming a cartilage model of the bone.

4. As the cartilage model grows, chondrocytes in the center of the model begin to die off, leaving small cavities.

5. Blood vessels invade these cavities, bringing with them osteoblasts, which are cells that produce bone.

6. The osteoblasts begin to produce bone matrix, gradually replacing the cartilage with bone.

7. Eventually, the entire cartilage model is replaced with living bone, completing the process of endochondral ossification.