Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body

by Neil Shubin

Eventually, as tissues move and fold, we become a tube with a folded swelling at the head end and another at the tail. If we were to cut ourselves in half right about now, we would find a tube within a tube. The outer tube would be our body wall, the inner tube our eventual digestive tract. A space, the future body cavity, separates the two tubes. This tube-within-a-tube structure stays with us our entire lives. The gut tube gets more complicated, with a big sac for a stomach and long intestinal twists and turns. The outer tube is complicated by hair, skin, ribs, and limbs that push out. But the basic plan persists. We may be more complicated than we were at twenty-one days after conception, but we are still a tube within a tube, and all of our organs derive from one of the three layers of tissue that appeared in our second week after conception.