To Professor E. B. Wilson, for his well-known and all but indispensable figures of the cell (figs. 42–51, 53); to M. A. Prenant, for other figures (41, 48) in the same chapter; to Sir Donald MacAlister and Mr Edwin Arnold for certain figures (335–7), and to Sir Edward Schäfer and Messrs Longmans for another (334), illustrating the minute trabecular structure of bone. To Mr Gerhard Heilmann, of Copenhagen, for his beautiful diagrams (figs. 388–93, 401, 402) included in my last chapter. To Professor Claxton Fidler and to Messrs Griffin, for letting me use, with more or less modification or simplification, a number of illustrations (figs. 339–346) from Professor Fidler’s _Textbook of Bridge Construction_. To Messrs Blackwood and Sons, for several cuts (figs. 127–9, 131, 173) from Professor Alleyne Nicholson’s _Palaeontology_; to Mr Heinemann, for certain figures (57, 122, 123, 205) from Dr Stéphane Leduc’s _Mechanism of Life_; to Mr A. M. Worthington and to Messrs Longmans, for figures (71, 75) from _A Study of Splashes_, and to Mr C. R. Darling and to Messrs E. and S. Spon for those (fig. 85) from Mr Darling’s _Liquid Drops and Globules_. To Messrs Macmillan and Co. for two figures (304, 305) from Zittel’s _Palaeontology_, to the Oxford University Press for a diagram (fig. 28) from Mr J. W. Jenkinson’s _Experimental Embryology_; and to the Cambridge University Press for a number of figures from Professor Henry Woods’s _Invertebrate Palaeontology_, for one (fig. 210) from Dr Willey’s _Zoological Results_, and for another (fig. 321) from “Thomson and Tait.” Many more, and by much the greater part of my diagrams, I owe to the untiring help of Dr Doris L. Mackinnon, D.Sc., and of Miss Helen Ogilvie, M.A., B.Sc., of this College. D’ARCY WENTWORTH THOMPSON. UNIVERSITY COLLEGE, DUNDEE. _December, 1916._ CONTENTS CHAP. PAGE I. INTRODUCTORY 1 II. ON MAGNITUDE 16 III. THE RATE OF GROWTH 50 IV. ON THE INTERNAL FORM AND STRUCTURE OF THE CELL 156 V. THE FORMS OF CELLS 201 VI. A NOTE ON ADSORPTION 277 VII. THE FORMS OF TISSUES, OR CELL-AGGREGATES 293 VIII. THE SAME (_continued_) 346 IX. ON CONCRETIONS, SPICULES, AND SPICULAR SKELETONS 411 X. A PARENTHETIC NOTE ON GEODETICS 488 XI. THE LOGARITHMIC SPIRAL 493 XII. THE SPIRAL SHELLS OF THE FORAMINIFERA 587 XIII. THE SHAPES OF HORNS, AND OF TEETH OR TUSKS: WITH A NOTE ON TORSION 612 XIV. ON LEAF-ARRANGEMENT, OR PHYLLOTAXIS 635