ILLUSTRATIONS 



These large sequoias in the foreground as well as the 



several pines in the background are organisms Frontispiece 



FACING PAGE 



Photomicrograph of the plant Bacillus tuberculosis, an or- 

 ganism that keeps its individuality under ordinary 

 growing conditions for about thirty minutes .... 20 

 A restoration of the herbivorous dinosauer, Diplodocus 



carnegii Hatcher 22 



A portion of common corn stem 28 



A portion of the trunk of the sequoia tree 28 



Photomicrograph of the red blood cells of the fish Amia 



calva 28 



Photomicrograph of section of pine leaf (needle) ... 32 

 Photomicrograph of the spinal ganglion of the turtle , . 32 

 Photomicrograph of section of young stem of Aristolochia, 



a climbing vine 34 



Photomicrograph of section of mature stem of Aristolochia 34 

 Young egg-cell of one of the common earth-worms . . 74 

 A photomicrograph of the same egg after it has become 



full size 74 



Greatly enlarged photomicrograph of the egg-nucleus only 



of the egg shown in Figure 19 76 



The Chromosomes lying free in the cytoplasm and be- 

 ginning to separate 76 



Various stages of the embryo of a salamander .... 80 

 The salamander embryo has become a free-swimming ani- 

 mal with well-developed gills and sense organs . . 84 



A diagram showing in outline a few of the more important 



changes through which the sperm-head passes ... 90 



A four millimeter human embryo 92 



An older human embryo 92 



xiii 



