rennet

encyclopedia.com:

rennet
substance containing rennin, an enzyme having the property of clotting, or curdling, milk. It is used in the making of cheese and junket. Rennet is obtained from the stomachs of young mammals living on milk, especially from the inner lining of the fourth, or true, stomach (abomasum) of milk-fed calves. The preparation of rennet was formerly a part of the domestic function of making cheese; the inner membrane was kept in salt, dried, and, when rennet was needed, soaked in water. Now extract of rennet is made and sold commercially. It is usually prepared by soaking the tissues in warm, slightly salted water and straining and preserving the resulting liquid. Heat interferes with the action of rennet.

And from the Vegetarian society:

Vegetarian cheeses are made with rennets of non-animal origin. In the past, fig leaves, melon, wild thistle and safflower have all supplied plant rennets for cheese making. However, most widely available vegetarian cheeses are made using rennet produced by fermentation of the fungus Mucor miehei. Vegetarian cheese may also be made using a rennet from the bacteria Bacillus subtilis or Bacillus prodigiosum.

In summary, rennet in regular cheese: stomachs of milk-fed calves, Mmmm.