Not all structure can be explained in functional terms.
Size is an extremely important constraint on body design.
Differences in size explain why bones get wider and denser as organisms get bigger, why bugs can crawl up walls but we cannot, why an animal the size of a blue whale can never exist on land, etc. 2. The structural changes in design during the development of an embryo, and later a fetus can provide clues about the phylogenetic history of the organism. The outer layers of the thoracic and abdominal viscera 3. The endoderm is initially a simple tube that is surrounded by ventral mesoderm 2.