"Hydra" has two main body layers, which makes it "diploblastic". The layers are separated by mesoglea, a gel-like substance. The outer layer is the epidermis, and the inner layer is called the gastrodermis, because it lines the stomach. The cells making up these two body layers are relatively simple. Hydramacin is a bactericide recently discovered in "Hydra"; it protects the outer layer against infection. A single "Hydra" is composed of 50,000 to 100,000 cells which consist of three specific stem cell populations that will create many different cell types. These stem cells will continually renew themselves in the body column"." "Hydras" have two significant structures on their body: the "head" and the "foot". When a "Hydra" is cut in half, each half will regenerate and form into a small "Hydra"; the "head" will regenerate a "foot" and the "foot" will regenerate a "head". If the "Hydra" is sliced into many segments then the middle slices will form both a "head" and a "foot".