NEUROCRANIUM
Cartilaginous Neurocranium (Chondrocranium)
Medial Part
The parachordal condensation and occipital sclerotomes form the parachordal (basal) cartilage located between the hypophysis and the C-1 segment.
The mesenchymal condensation lateral to the hypophysis becomes the hypophyseal (polar) cartilage.
A narrow midline condensation in the septal region between the primitive nasal cavities represents the trabecular cartilage. It extends from the hypophysis to the nasal capsule condensation.
Lateral Part
Cartilage appears in the otic capsule surrounding the semi-circular canals and cochlear duct. The otic capsule joins with the parachordal cartilage.
A condensation called the alisphenoid develops between the parachordal cartilage and the maxillary and mandibular nerves. It is separated from the otic capsule by the internal carotid artery.
The orbitosphenoid is a well-defined cartilage in the ectomeninx around the optic stalk. It is separated from the alisphenoid by the cranial nerves III, IV and VI.
The nasal capsule surrounds the primitive nasal cavity and unites with the trabecular cartilage.
Membranous Neurocranium
The dorsal and lateral surfaces of the brain are covered by the ectomeninx, which will give rise to the flat bones of the skull.
VISCEROCRANIUM
The first and second branchial arch condensations transform into prominent bars of cartilage.
The third arch cartilage separates from the laryngeal condensation becoming a curved, fingerlike condensation at the root of the tongue. It joins the ventral end of the second arch cartilage where the two together will form the hyoid bone.
The remainder of the arch cartilages are represented as laryngeal condensations.
POSTCRANIAL PART OF AXIAL SKELETON
VERTEBRAL COLUMN
The caudal condensed part of each sclerotome forms the intervertebral disc.
The cranial, less-condensed part gives rise to the cartilaginous centrum. A cartilaginous neural arch projects dorsally from each side of the centrum. Each pair of arches is joined dorsal to the spinal cord by the ectomeninx. Projecting laterally from each centrum is a short transverse process.
RIBS AND STERNUM
The 12 pairs of rib primordia become cartilaginous and extend ventrolaterally into the body wall. The enlarged end of the rib adjacent to the centrum forms the head. The lateral, thin part is the shaft. A tubercle is present where the rib makes contact with the transverse process.
On each side the distal ends of the primordial ribs are joined together by a longitudinally arranged strip of cartilage called the sternal bar. The sternal bars are separated by the large heart and liver. Cranially the two bars join together in the midline to form the episternal cartilage.
APPENDICULAR SKELETON
There is a proximodistal sequence of differentiation in the limb buds with the pectoral girdle being slightly more advanced than the pelvic girdle. The skeletal components of both girdles with the exception of the clavicle becomes discrete pieces of cartilages. The clavicle is a dense membrane that begins ossifying very early, even before chondrification begins in other areas.
Source: Atlas of Human Embryos.