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Bio 32	(1)

 

At this stage in development, this seven week old human embryo is interchangeable with many other vertebrate embryos:

See these developments in motion!		(2)

This close correlation allows comparative biologists to study phylogeny through embryology: each embryo recapitulates the evolutionary process that gave rise to a vast number of species as it approaches the specific morphology and physiology that make it unlike any other species. The development of the skeleton provides one such look into evolution. Looking at the skeletons of the vertebrates, the huge flexibility in bone design becomes apparant. Even in a single skeleton, there are long bones for bearing weight, plate-like bones for protecting the brain, and hollow, marrowless bones for enclosing the nerve cord: a distinctive design for each purpose. Unlike basic muscle or skin tissue, the bone in different areas of the skeleton does not have a single embryonic origin.. Instead, it develops uniquely in the dermal, axial, and appendicular regions, reflecting the aquisition of these different structures at distant points in vertebrate evolution. In these different areas, bone arises from migrating mesenchyme, using a variety of signaling mechanisms and following pathways unique to its location: it is this diversity in development that allows a single tissue to take on the numerous shapes and functions demanded of it.

Click on a portion of the embryo above to explore skeletal development in that area.