[120] Since the above was written, Severin (Centr.-Bl. iii. [2], 628) has described seven other species of bacteria, isolated from horse dung; for the description and property of these the original memoir must be consulted. Popp and Becker, German Patent 86,335, 28, describe fourteen species which they isolated from dog and pigeon dung, of which three have considerable bating action on skins, and which form the subject of the patent. Some of these are probably identical with organisms in the above list. There are besides many unnamed species of bacteria in dung, among which are the two following isolated by me from puer-- (_a_) _Plate Cultures._--Small yellowish colonies, slightly fluorescent, liquefy the gelatin slowly. Rods resembling B. subtilis when at rest, but moving with a rapid undulatory motion. Cultures in nutrient gelatin have a considerable reducing action on skin. (_b_) _Plate Cultures._--Bluish colonies liquefying gelatin rapidly, micro-bacteria in pairs. Not so marked an action on skin as (_a_). The organism resembles Proteus vulgaris, but swarming islets not observed. Of the species mentioned above, a pure culture of B. subtilis in meat broth, maintained for one week at 35° C., had _no_ reducing action on skin. B. fluorescens liquefaciens had a moderate reducing action. The species developing on the plates vary according to the age of the dung. In horse manure, according to Severin,[121] bacilli predominate in the early stages. At the end of two to three weeks micro-bacteria, cocci and diplococci make their appearance, while the bacillar form becomes scarce. In three months cocci predominate, forming zoogloæ; strepto- and staphylo-cocci and spirilli were seldom found. Yeasts and sarcina were never observed. From observations extending over several years it is evident that a similar cycle occurs in dog dung. When quite fresh it contains comparatively few bacteria; then certain groups of species take possession, causing decompositions in the dung; these in their turn give place to other species, which decompose the products formed by the first, so that no one species produces or can produce the complex chemical and physiological changes which take place, or the bodies necessary for the bating of skin, as has been supposed by some observers. [121] Centr.-Bl. ii. [1], 97. None of the experiments, either with mixed or pure cultures of bacteria, had a perfect reducing action on skin, although the action was considerably greater than that of the chemical solutions. It was found, however, that by adding a small quantity of the amines above mentioned to the bacterial cultures, the reducing action became nearly as rapid and effective as with dung itself; and it was, therefore, clear that the principal bating effect was due to a combination of two things, viz. an active growth of bacteria in the presence of amine compounds. It was now necessary to ascertain the mode of action of the bacteria on the skins.