Your Inner Fish Chapter 6

Embryology is the study of embryos, or eggs, before they become fully formed animals.  When eggs are fertilized, they begin to form 3 germ layers, shown above.  These layers form literally everything in the entire body, whether it be a fish body or a human body.  The endoderm develops the digestive system and lungs, the mesoderm skeletal and muscular features, as well as blood, and the ectoderm develops skin an the nervous system.    This development is controlled by the Operator works together with Hox genes to regulate the normal arrangement of organs and the creation of the correct amount, unlike flies with mutated Hox genes growing feet from their head and such.

 However, knowledge about the Operator was nor really acquired until the 90s, and it fell in and out of favor since its discovery, much like Justin Bieber.  Moved around Operators can result in organisms with two sets of organs, and switched germ layers could mean a whole host of developmental differences more the embryo.  Embryology provides insight into the arrangement of many animals that comes from our evolutionary past, and more similarities than differences between man and beast.