The theory of embryonic development we follow today was proposed 2000 years ago by Aristotle. This idea, called epigenesis, the form of an animal arises gradually from a generally formless egg. The organisms development is largely determined by the genome of the zygote and the organization of the cytoplasm of the egg cell. The first part of the development of a new animal is fertilization. The gametes, the sperm and the egg, are both highly specialized …showed first 75 words of 1925 total…

…showed last 75 words of 1925 total…posterior side of the bud is attached to the body. The ZPA is necessary for proper pattern formation along the anterior-posterior axis of the limb. Cells nearest to the ZPA give rise to the posterior structure and those farthest away form the anterior structure. ZPA secretes morphogens, the most important being a protein growth factor called Sonic Hedgehog. Production of this protein in the wrong part of the body sometimes results in an extra limb.