The nucleic acids which were principally studied from the early 1900s were those derived from the thymus gland, and from the spermatozoa of salmon, herring, and other fish ; they were thought to be probably all the same.
Levene (1910) recognizes three sorts of nucleic acid, of which the most complex is termed thymonucleic acid. This consists of two purine bases, guanine and adenine ; two pyrimidine bases, thymine and cytosine ; a hextose (carbohydrate) ; and phosphoric acid.
Its formula, according to Schmiedeberg, is C 4 oH 56 NuOie . 2 PaOs, and according to Steudel, C 43 H57 Ni 5 Oi 2 . 2 P2O 5 . Considerable progress has been made, especially by Emil Fischer and his students, in the synthesis of protein-like bodies. Many complex polypeptides have been built up which resemble peptones in many of their reactions and when injected into living organisms appear to be utilized in metabolism in much the same way as are native proteins.
They were still, however, very far from an adequate understanding of the nature of chromatin. Delia Valle (1912), for example, after an exhaustive study of the physico-chemical properties of chromatin both in the resting nucleus and in the dividing cell, has concluded that this substance resembles that of fluid crystals. "Consequently all of the phenomena presented by the chromosomes ; their mode of origin, differences in size, state of aggregation, form, structure, colorability, optical characteristics, variations in form, longitudinal division and the phenomena which follow this mode of scattering, demonstrated that the chromosomes were crystalloids."
Two other primary constituents of protoplasm may be mentioned briefly. The phosphatide, lecithin, belongs with cholesterin to a group of compounds called lipoids. It consisted of glycerophosphoric acid plus certain fatty acid radicles, such as stearic acid, oleic acid, etc., and a nitrogenous base (cholin). It
probably plays some part in cell metabolism, may furnish material for building up nucleins, and ac cording to Faure-Fremiet is concerned in the formation of mitochondria. Cholesterin is considered a waste product of cell life, although it is known to inhibit haemolysis produced by certain bodies and is thus a protective against toxins, and may have other functions. We should look forward with great interest to the results of investigations that are now being carried on by biochemists, since we depend upon them for an explanation of many of the phenomena of life, cellular differentiation, and heredity. We even hope that they may be able to create compounds in the laboratory that we may consider living organisms. However, the task does not seem to be so simple to the biochemist, who should know, as it does to the biologist. Nevertheless, as Jacques Loeb has said, we should "either succeed in producing living matter artificially, or find the reasons why this should be impossible."