Chapter 6 of "Your Inner Fish"

All tetrapods have one head and four limbs arranged in two pairs (for humans, arms and legs, but for other animals, just their legs), a tail, and a variety of other features. But how and why are animals and humans so similar in the embryo stage? And where did this basic body plan come from? Chapter 6 of "Your Inner Fish" explained most of this strange phenomenon.

1. Embryology:  The study of the development of an embryo from the fertilization of the ovum to the fetus stage. The study focuses on comparing different species at their early stage (for example, shark embryos and human embryos.) By comparing these early life forms, scientists are able to find the common structures of various species, and determine how some species may be related. 

2. Germ layers:  Karl Ernst Von Baer, a Russian biologist and one of the founding fathers of the study of embryos,  discovered that there are three layers in embryos (also known as germ layers). 

a. The endoderm is one of the germ layers formed during animal embryogenesis, the process by which the embryo forms and develops. Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm. Endoderm forms many of the inner structures of our bodies, including digestive tract and numerous glands that are associated with it. 

b. The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the cells migrate inward to contribute to the mesoderm, an additional layer between the endoderm and the ectoderm. The formation of a mesoderm led to the development of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks. 

The mesoderm has several components which develop into tissues: intermediate mesoderm, paraxial mesoderm, lateral plate mesoderm, and chorda-mesoderm. The chorda-mesoderm develops into the notochord. The intermediate mesoderm develops into kidneys and gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis. The lateral plate mesoderm develops into the circulatory system (including the heart and spleen), the wall of the gut, wall of the human body, and forms the tissue in between the skeleton and the muscles. 

c. The ectoderm is the outer layer of the embryo, and it forms from the embryo's epiblast. The ectoderm develops into the surface ectoderm, neural crest, and the neural tube.

The surface ectoderm develops into many parts: epidermis, hair, nails, lens of the eye, sebaceous glands, cornea, tooth enamel, and the epithelium of the mouth and nose.
The neural crest of the ectoderm develops into: peripheral nervous system, adrenal medulla, melanocytes, facial cartilage, and dentin of teeth.
The neural tube of the ectoderm develops into: brain, spinal cord, posterior pituitary, motor neurons, and retina.