the ageing of cells in the living organism is determined by extrinsic factors in the plasma rather than by any inherent limitation in the cells. In the ordinary conditions of life senescence of the cells might be regarded as the result of increasing differentiation and, possibly from accumulation of material which cannot be utilized for rejuvenation, of diminished metabolic activity. In the physiological life of gland cells, for example, the salivary glands and pancreas, granules are manufactured and discharged, but the hypothetical material which collects in elderly cells, of which pigment may possibly be an example or the visible sign, remains in these cells and hampers their functional activity. As atrophy of the functional cells and increase of connective tissue are regarded as characteristic of senescence, and a high proportion of cells as characteristic of embryonic life, Robertson and Ray[90] suggest that the potential longevity of any individual is determined by the relative velocities of anabolism in the cells on the one hand, and in the fibrous tissue on the other hand. A low rate of cellular anabolism increases the growth of the cells and delays the increase of fibrous tissue; this they were able to effect by feeding white mice with tethelin. The relations of the functionally active cells and the connective tissues have also been investigated by Nathan[91] and his collaborators, who bring forward experimental evidence in favour of the modified view that while dense connective tissue inhibits cellular proliferation, young loose connective tissue favours the multiplication of cells in proportion as it approaches the structure of the embryonic mesenchyma. Drew’s[92] observations on the culture of tissues and tumours show that the stroma acts like embryonic tissue in favouring growth. To sum up: the cells of the complex organism depend for their duration of life not so much on an inborn store of vitality as on metabolic changes in the colloids which in their turn are modified or controlled by extrinsic factors of various kinds. RELATION OF THE ENDOCRINE GLANDS TO SENESCENCE As the metabolic activities of the cells of the body generally are markedly influenced by the secretions of the ductless glands, the relations between these endocrine glands and senescence demand some discussion.