Vital properties--The discovery of cells--The cell doctrine--The cell--The cellular structure of organisms--The cell wall--Protoplasm--The reign of protoplasm--The decline of the reign of protoplasm--The structure of protoplasm--The nucleus--Centrosome--Function of the nucleus--Cell division or karyokinesis--Fertilization of the egg--The significance of fertilization--What is protoplasm?--Reaction against the cell doctrine--Fundamental vital activities as located in cells--Summary 54 PART II. _THE BUILDING OF THE LIVING MACHINE_. CHAPTER III. THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING MACHINE. History of the living machine--Evidence for this history--Historical--Embryological--Anatomical--Significance of these sources of history--Forces at work in the building of the living machine--Reproduction--Heredity--Variation--Inheritance of variations--Method of machine building--Migration and isolation--Direct influence of environment--Consciousness--Summary of Nature's power of building machines--The origin of the cell machine--General summary 131 LIST OF ILLUSTRATIONS. FIGURE PAGE _Amoeba Polypodia_ in six successive stages of division _Frontispiece_ 1. Figure illustrating osmosis 30 2. Figure illustrating osmosis 31 3. Diagram of the intestinal walls 32 4. Diagram of a single villus 33 5. Enlarged figure of four cells in the villus membrane 33 6. A bit of muscle showing blood-vessels 36 7. A bit of bark showing cellular structure 61 8. Successive stages in the division of the developing egg 63 9. A typical cell 65 10. Cells at a root tip 66 11. Section of a leaf showing cells of different shapes 66 12. Plant cells with thick walls, from a fern 67 13. Section of potato 67 14. Various shaped wood cells from plant tissue 68 15. A bit of cartilage 68 16. Frogs' blood 69 17. A bit of bone 69 18. Connective tissue 70 19. A piece of nerve fibre 70 20. A muscle fibre 71 21. A complex cell, vorticella 71 22. An amoeba 73 23. A cell as it appears to the modern microscope 86