Professor Bonner has rewritten more than half of this standard treatise to take account of the great amount of recent research on the cellular slime molds. He has included a larger selection of material, more figures and new plates. The bibliography has been greatly enlarged. Originally published in 1967. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.