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To my Associate Professor of Histology and Embryology, Professor J. C. Simpson, I am indebted for the redrawing of the majority of my diagrams, and their presentation in a more artistic form. It is regretted that the cost of colour illustration prohibits the more general use of this means of simplifying the diagrams. To my wife I wish to express my indebtedness for the careful transcribing of my pencil mss.
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