The nuclein of the skin fibres appears to be all removed by the puer, since Abt confirms the fact that no nuclei can be seen under the microscope in a puered skin. The actual solution of the skin substance is brought about by enzymes of a tryptic character. (See Chapter V.) While the main lines of the bacteriology of the dung bate are now pretty well known and understood, it will be seen that much work still remains to be done as to details, and this principally with the anaerobic bacteria of the dung, which have been studied by few investigators.[89] I have suggested[90] that such a research might well be undertaken by the bacteriological laboratories of our Leather Industries Schools in Leeds and London. [89] See Les Anaerobies, Jungano and Distaso (Masson et Cie Paris, 1910). [90] The Bacteriology of the Leather Industry, J.S.C.I., 1910, p. 666. *Moulds and Putrefaction.*--In view of the fact that moulds are of frequent occurrence on dog dung, a brief mention of them is necessary. So far as our present knowledge goes the researches of Van Tieghem, De Bary, Rankin, Marshall Ward, V. H. Blackman and others indicate that their action on the essential bating constituents of the dung is a destructive one. They grow usually on acid media, and in so doing break down the acids present into simple inorganic bodies, such as CO_{2} and water, utilizing the carbon and nitrogen for their own growth. Although these fungi secrete almost all varieties of enzymes (Bourquelot), yet we have no evidence that any of the enzymes contained in dog dung are from this source. In the usual case of dung preserved in pits or casks, the upper surface only becomes mouldy, since moulds require a free supply of oxygen. The mycelium penetrates but a very little way into the body of the dung, and cannot therefore effect any decomposition, except of the surface layer. The dung exposed to the action of the mould is generally a bad colour, and is rejected as unsuitable for puering. The following species have been noted and classified as growing on dog dung, though probably not all of them are specific. 1. Pilaira dimidiata (Grove). 2. Mucor caninus (a variety of Mucor mucedo). 3. Circinella simplex (Van Tieghem). 4. Pilobolus crystallinus (also on cow-dung). Certain myxobacteria are found on dung, among these Chondromyces, described as long ago as 1857 by Berkeley, and at that time included among the Hyphomycetes. It was rediscovered in 1892 by Thaxton, and owing to his researches the whole class of myxomycetes is now generally considered as a division of Bacteria. Another myxomycete, Polyangium primigenum (Quehl), forming a red fructification on dog dung, is figured in the Encyclopædia Britannica, XI. edition, vol. 3, p. 163.