Bone development:

Ossification is the gradual transition from a soft template to bone.

• Endochondral bone - is dense and has a system of Haversian canals through which blood vessels can pass. These three stages are the way most bones in the body are formed:
  • Fibrous connective tissue develops first.
  • Cartilage forms from the fibrous tissue.
  • Finally, a bone matryx replaces the cartilage.
• Intermembranous bone - develops between two layers of fibrous connective tissue without the cartilage stage. The flat bones that form this way are not dense and do not have a system of Haversian canals.

• Visceral components:
  • Gill arches
• Somatic components:
  • Axial elements:
    • Vertebral column - Five regions of the vertebral column of birds and mammals:
      • Cervical region - neck
      • Thoracic region - chest
      • Lumbar region - middle back
      • Sacral region - lower back
      • Caudal region - tail
    • Ribs
    • Sternum
    • Skull
  • Appendicular elements:
    • Pectoral girdle - support the forelimbs
    • Pelvic girdle - support the hindlimbs
    • Limbs - adapted for aquatic or terrestrial environments
      • Tetrapod limbs contain two sets of bones:
        • Proximal elements (three bones per limb):
          • Forelimbs - Humerus, radius, and ulna
          • Hindlimbs - Femur, tibia, and fibula
        • Distal elements (numerous bones):
          • Forelimbs - Carpals, metacarpals, and phalanges (digits)
          • Hindlimbs - Tarsals, metatarsals, and phalanges (digits)
          • Pentadactyl - having five digits

The vertebrate nervous system:

• Central nervous system:
  • The brain and spinal cord.
  • All components are somatic in origin.
• Peripheral nervous system:
  • Somatic components
    • Nerves for the skeletal muscles.
    • Nerves for the integumentary sense organs.
  • Visceral components
    • Nerves for the digestive tract.
    • Nerves for the internal organs.
• Brain white matter - nerve fibers
• Brain gray matter - nerve cell bodies