The investigation of the biological action of homogeneous X-radiation of different wave-lengths was commenced because many clinical observations point to the existence of what has been termed differential action, although theoretical considerations do not support such a view. Differential action may be defined as the association of changes in living matter with certain special wave-lengths in the X-ray spectrum. The investigation was performed with the chorio-allantoic membrane of the embryo which. The X-ray spectrometer was used as a source of the different radiations. A fine pencil of X-rays impinges on a crystal, various component wave-lengths being reflected at corresponding angles connected by the formula the investigation of the biological action of bomogeneous X-radiation of different wave-lengths was commenced because m any clinical observations point to the existence of what has been termed differential action, although theoretical considerations do not support such a view. differential action may by defined as tbs association of changes in living matter with certain special wave-lengths in the X-ray spectrum. The investigation was performed with chorio-allantoic membrane of the embryo which. The X-ray spectrometer was used as a source of the different radiations. A fine pencil of X-rays impinges on a crystal, various component wave-lengths being reflected at corresponding angles connected by the formula λ = 2 d sin θ (fig. 1, p. 405.). This gives a radiation which is very nearly homogeneous, but the energy is exceedingly small, a fact which has no doubt deterred many previous workers from using this method. The present paper is an extension and confirmation of certain observations contained in a preliminary communication to the ' Medical Journal of Australia .' A selective action by different wave-lengths is described and the subject matter is arranged as follows: ( a ) The chorio-allantoio membrane and related structures are described; ( b ) the technical procedure of exposing the specimen to X-radiation is given, followed by ( c ) an account of the X-ray effects obtained, with details of their development and their relation to control experiments; ( d ) a systematic exploration of the X-ray spectrum is then described, giving microscopic results, which are expressed graphically, and details of confirmatory experiments, followed by ( e ) a brisk account of the microscopic examination. A general discussion ( f ) follows, in which the form, graphical expression, and the relationship of stimulation to destruction is considered. An apparent neutralisation between different wave-lengths in their biological action is also discussed and the technical procedure and form of tire graphs is considered in this respect. In the concluding remark ( g ) Zeleotive action is considered in relation to the atomic structure of the tissue and the main deductions are summarised.

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The term chemotaxis has been applied to a phenomenon in which the movement of certain living cells may be controlled by a concentration gradient of certain definite chemical substances in a fluid medium.

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