Chapter 6: The Best-laid (Body) Plans

This chapter focused greatly on embryos. Shubin stated early on that embryos generally all look alike; stating that we were seeing common architecture. During the 1800s, a couple of scientists found that all organs in chicken can be traced to one of three layers of tissue in a developing embryo; these tissues became known as germ layers. Scientists then went on to discover that with every animal, the same organ came from the same germ tissue. These three layers: outer layer (ectoderm), inner layer (endoderm), and middle layer (mesoderm) all behave the same in each type of embryo. We know that the ectoderm forms the skin and nervous system, the endoderm forms many structures of the inner body (e.g. digestive tract), and the mesoderm forms tissue in between the guts and skin (including much of muscles and bones). Many discoveries were made that include the idea that there are two features in development: the features shared by every species (developed early on) and then the features that vary from species to species (developed later in development). The common idea of "ontogeny recaptitulates phylogeny" states that species track their evolutionary history as they proceed through development. However, I found it more interesting when Shubin stated that this common idea was not as good as the idea developed in the 1800s because it does not compare species at the same time. Once Shubin started talking about more modern discoveries, I found it pretty cool how Spemann's experiment resulted in identical twins (more than one offspring can be made from one egg), and then one of Spemann's students, Mangold, had such nimble hands that she discovered a small patch of tissue able to direct other cells to form an entire body plan; this patch of cells became known as the Organizer (pretty easy name to remember...since it organizes other cells). It surprised me that scientists used flies to help explain how DNA works and to help us understand where and how mutations occur within species. Scientists also discovered how genes are organized: their position along DNA strand parallels the structure of body from front to back. On top of all of this, they also discovered that versions of the Hox genes appear in every animal with a body (these organize the body and establish proportions). During the Hox frenzy, scientists stumbled upon genes (such as Noggin) that interact to organize the body plan. However, this statement became less impressive once Shubin stated that genes interact with others at all stages of development (sometime to turn another on or off; off-on interactions). The last thing that Shubin stated that stuck with me was that the recipe that builds us is a more intricate version of one that builds species like jellyfish and sea anemones (invertebrates!)....so apparently I have an inner sea anemone too!