Illustrations 



Fig. Page 



1. Amoeba polypodia in successive phases of division. 3 



2. Stages in the early growth of the peppergrass (Lepidium). 6 



3. Lengthwise sections of a sunflower root. 7 



4. Nine stages in the growth of a salamander, Amblystoma punctatum. 8 



5. Ten stages in the growth of a chick. 9 



6. A chick removed from an egg which had been incubated ten days 



and two hours. 10 



7. Nerve and muscle cells of animals. 1 1 



8. Pharaoh's serpents. 12 



9. Semi-diagrammatic representation of the division of a plant cell. 14 



10. Graph of the height of the sunflower plant, Helianthus. 20 



1 1 . Growth of an isolated corn root tip. 2 1 



12. A dwarf and normal variety of corn grown side by side. 22 



13. The effect of temperature on the growth of wheat. 23 



14. Growth of tobacco plants affected by length of day. 24 



15. The effect of long and short days on the growth of the coneflower 



(Rudheckid). 25 



16. The effect of minute amounts of organic matter upon duckweed. 26 



17. Growth of well-nourished children from 5 to 18 years. 33 



18. Age changes in the strength of grip. 35 



19. Age changes in vital (lung) capacity. 35 



20. Growth is a reversible process. 37 



21. Age changes in resistance to fatal pneumonia. 41 



22. Changes in vitality with age as measured by the breakdown of the 



nervous and the excretory systems. 42 



23. The rise and decline of basal metabolism with age. 43 



24. The rise and decline of milk production in cattle. 43 



25. The course of senescence in the domestic fowl and in man. 44 



26. Increase in percentage mortality of man with increasing age. 46 



27. Percentage mortality of the fruit fly {Drosophila) at various ages. 46 



28. Comparison of the mortality curves of man and fly. 47 



29. Decline in eye accommodation with age in man. 48 



30. Growth curve of the albino rat. 48 



31. Skeletal measurements taken of dairy cattle. 49 



