The work of splitting up and converting the organic compounds is primarily carried out by micro-organisms such as yeast fungi, mould fungi, algæ, protozoa and even by higher forms of life such as earthworms and insects. To what extent in addition to these other agencies take part in this bio-chemical process is not yet fully elucidated. [Sidenote: Decomposition and putrefaction most complicated processes.] Fischer in his interesting book, ‘The Structure and Functions of Bacteria,’ observes (page 99): “The decomposition of dead animal bodies, of vegetable tissues, or of substances like stable manure, is far from being a simple putrefactive process. Side by side with the disintegration of nitrogenous bodies there are going on a number of fermentative changes by which non-nitrogenous compounds are being broken up, besides nitrification and other bio-chemical processes. For this reason it is always difficult and often impossible to determine the respective parts played by the different species of bacteria.… “The phenomena of putrefaction are so complicated that we do not know all of the compounds that arise during the process.… Very careful chemical investigations on pure cultures will be necessary before the chaos of phenomena presented by putrefactive bacteria can be arranged in something like order. [Sidenote: In the decomposition of proteids five or rather six stages may be distinguished.] “Proteids are split up by putrefaction into a large number of simpler compounds both nitrogenous and non-nitrogenous. The substances thus produced are precisely similar to those resulting from the artificial decomposition of proteids by fusion with caustic potash or boiling with hydrochloric acid or barium hydrate. Five groups may be distinguished: [Sidenote: Albumoses and peptones.] “1. Albumoses and peptones: soluble diffusible bodies closely resembling albumen. They are produced by the action on albumen of bacterial enzymes, similar to the enzymes (pepsin and pancreatin) which give rise to peptones in the digestive tract of man. [Sidenote: Aromatic compounds.] “2. Aromatic compounds; among others indol and skatol, which give the characteristic odour to human excrement; also some non-nitrogenous substances such as phenol, phenylacetic acid, and phenylpropionic acid. [Sidenote: Amido compounds.] “3. Amido compounds, all nitrogenous: leucin, tyrosin, aspartic acid, glycocol. [Sidenote: Fatty and aromatic acids.] “4. Fatty and aromatic acids, all non-nitrogenous and therefore having no part in the circulation of nitrogen; acetic, butyric, succinic and valerianic acids. [Sidenote: Inorganic end-products of putrefaction.] “5. Inorganic end-products of putrefaction: free nitrogen, ammonia, free hydrogen, methane, carbonic acid, methylmercaptan, sulphuretted hydrogen. It is probable also, but not certain, that phosphuretted hydrogen is formed and is oxidised at once by the free oxygen of the atmosphere. [Sidenote: Ptomaines.]