5. Mixed culture from dog dung one month old, as used in the bate. The action of this in a certain medium has already been described (see p. 160). In the special medium it was distinctly better and almost equal to that of the dung bate. 6. Mixed culture from fresh fæces gave a similar result to (3). A microscopic examination showed pairs of small bacteria and micrococci, but no bacillar forms. 7. Mixed culture from fresh horse dung had a moderate but distinct action on skin, about equal to (4). 8. Bacillus _d_, isolated from wool infusion. Very slight action. 9. Bacillus _e_, isolated from wool infusion. Similar result to (8). 10. Mixed culture of bacilli _d_ and _e_ only. Very powerful bating action, skin bated more rapidly than with dung. These experiments tend to confirm the conclusion I previously arrived at, viz. that no single species of bacteria produces the complex chemical and physiological changes which take place in the dung, and which result in the production of the bodies necessary for the proper bating of skin. It is a well-known fact that dog dung requires keeping for at least a month before it gives the best result. During this time it undergoes a kind of fermentation and continues to improve up to two months, after which it deteriorates. The rate of fermentation depends upon the season. It is evident from these facts that the bacteria present in the dung when it leaves the animal’s body do not produce the required enzymes and chemical compounds, and that these are produced by bacteria which obtain access from the air. It is also evident that the production of these enzymes depends upon the composition of the nutrient medium, since this exerts a selective influence on the species of bacteria obtaining access to it. In the spontaneous souring of milk numerous bacteria in the air have free access to the milk, yet the lactic ferment is generally so pure that it may be and is used as a pure culture on a large scale in the manufacture of lactic acid. Dog dung is a favourable medium for the bacteria secreting the bating enzymes; but if dextrose or other carbohydrate be added to fresh dung an acid fermentation is set up which effectually prevents the development of the bating organisms.