This text-book is intended to cover the first or introductory semester’s work, and requires two classroom periods per week. Each student is compelled to take two laboratory periods of three hours per week along with the class work. The outline of the laboratory work is given at the end of the text. Results attained seem to justify this plan. A text-book is but one of many pedagogical mechanisms and is not intended to be an encyclopedia of the subject. The author makes no claim to originality of content, since the facts presented are well known to every bacteriologist, though the method of presentation is somewhat different from texts in general. During the preparation of this work he has made a thorough review of the literature of Bacteriology, covering the standard text-books as well as works of reference and the leading periodicals dealing with the subject. Thus the latest information has been incorporated. No attempt has been made to give detailed references in a work of this character. The photomicrographs are original except where otherwise indicated and are all of a magnification of one thousand diameters where no statement to the contrary appears. These photographs were made with a Bausch & Lomb Projection Microscope fitted with a home-made camera box. Direct current arc light was used and exposures were five to ten seconds. Photographs of cultures are also original with a few indicated exceptions. All temperatures are indicated in degrees centigrade. For use of electrotypes or for prints furnished the author is indebted to the following: A. P. Barber Creamery Supply Company, Chicago, Ill.; Bausch & Lomb Optical Company, Rochester, N. Y.; Creamery Package Manufacturing Company, Chicago, Ill.; Davis Milk Machinery Company, North Chicago, Ill.; Mr. C. B. Hoover, Superintendent of Sewage Disposal Plant, Columbus, O.; Mr. C. P. Hoover, Superintendent of Water Filtration Plant, Columbus, O.; The Hydraulic Press Manufacturing Company, Mt. Gilead, O.; Loew Manufacturing Company, Cleveland, O.; Metric Metal Works, Erie, Pa.; Sprague Canning Machine Company, Chicago, Ill.; U. S. Marine Hospital Service; Wallace and Tiernan Company, New York City, N. Y. For the preparation of many cultures and slides, for great assistance in the reading of proof and in the preparation of the index, Miss Vera M. McCoy, Instructor in Bacteriology, deserves the author’s thanks. The author trusts that the book will find a place in College and University courses in Bacteriology. C. B. M. CONTENTS Historical Introduction--Spontaneous Generation--Causation of Disease--Putrefaction and Fermentation--Study of Forms--Chronological Table 17 CHAPTER I. Position of Bacteria--Relationships to Algæ--Yeasts--Molds--Protozoa 37 PART I. MORPHOLOGY. CHAPTER II.