Amniotic Sac as Space Suit

The most fundamental innovation is the evolution of another fluid-filled sac, the amnion. in which the embryo floats. Amniotic fluid has roughly the same composition as seawater, so that in a very real sense, the amnion is the continuation of the original fish or amphibian eggs together with its microenvironment, just as a space suit contains an astronaut and a fluid that mimics the earth's atmosphere. All of the rest of the amniote egg is add-on technology that is also required for life in a...

 The Wonder of the Fertilized Egg

The opening cast members of the baby-making play are simply a sperm and an egg and a saucy Marvin Gaye song. Once these two cells are joined, they begin producing lots of cells in a small space. The human embryo soon looks like a tiny mulberry. (Indeed, one early development stage is called the morula, Latin for mulberry.) Your mulberry’s first decision is practical: It has to decide what part becomes baby’s body and what part becomes baby’s shelter. This happens quickly. Certain cel...