| Summary: | 1. A description of the mammalian pigmentary system is given together with an outline of the mechanism of melanin production and those factors which influence or control it. 2. The production of melanin by chemico- physical interaction between circulating 3:4 dihydroxyphenylalanine (dopa) in the blood stream and an enzyme or enzymes of intra- cellular origin is accepted,but it is emphasised that in the interaction there are numerous opportunities for physiological interference which inhibit melanin deposition. 4. It is shown that melanin once formed is very stable and that when it occurs in fibres, wool or hair, which are keratinized, it is no longer capable of being influenced by physiological conditions. 5. The current theories of the origin of melanin are discussed and evidence supporting the nuclear origin of oxidising enzymes is given. Puclear caps are recorded and described in the epidermal cells and wool fibres of the sheep. 6. It is demonstrated that in the sheep some epidermal cells are more potentially pigment - producing than others, and that a gradation exists between the cell which cannot produce pigment and that which produces it to a maximum degree . 7. A series of "in vitro" experiments planned to standarise the dopa reaction is described, and the results of staining sections of lamb and adult sheep skin with dopa are given. It is suggested that the Dopa reaction may be used as a simple chemical test in genetic analysis. 8. A study was made of the individuality of coloured and colourless mouse skin after transplantation to a different environment and it was found that pigment cells once differentiated retain their genetic characterisation. Where, however, coloured skin was grafted on to an albino mouse the mouse the production of melanin ceased owing to the lack of the necessary chromogen. On the other hand coloured skin grafted on to a white area in a piebald individual retained its pigmentation: 9. An elaborated technique for skin transplantation, which was devised in the course of these experiments, i s described in detail. 10. The Schultz reaction is recorded for piebald and coloured mice used in these experiments 11. The more important breeds of sheep are briefly reviewed with special reference to their colour characteristics, and the desirability for an understanding of the pigmentary system in sheep is emphasised. 12. A short account is given of some economic aspects of colour in the fleece. The necessity for the elimination of scattered fibres in white wool and the eradication of black lambs from pedigreed flocks is stressed. In connection with this problem special mention is made of the Merino. 13. The results of a microscopic study made of the colour transition shown by the fleece of the Suffolk from birth to maturity are given. It is shown that differences in colour tones in this wool are partly quantitative and partly due to varying: physical structure. It is demonstrated that the physical structure of the fibre cuticle, obviates the possibility of pigment inclusions. 14. It was not possible definitely to associate colour and structure. 115. An explanation for the sporadic occurrence of coloured fibres in an otherwise white fleece is attempted, and it is pointed out that however else the pigmentary system of the Suffolk may be stimulated, thresholds of irritation conditioned by temperature need not be considered as possible stimuli. 3. The morphol?gy and disposition of pigment in wool diffuse and granular, is described, and it is postulated that only one pigment exists and that all animal colours are the result of a modification of this one pigment.
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