94 



THE ORIGIN AND EVOLUTION OF LIFE 



structure of the chromatin as developed in 

 the researches of Hertwig, Strasburger, 

 KolHker, and Weismann, who indepen- 

 dently and almost simultaneously (1884, 

 1885) were led to the conclusion that the 

 nucleus of the cell contains the physical 

 basis of inheritance and that the chroma- 

 tin is its essential constituent.^ In the 

 development from unicellular (Protozoa) 

 into multicellular (Metazoa) organisms 

 the chromatin is distributed through the 

 nuclei to all the cells of the body, but 

 Boveri has demonstrated that all the 

 body-cells lose a portion of their chroma- 

 tin and only the germ-cells retain the 

 entire ancestral heritage. 



Chemically, the most characteristic 

 peculiarity of chromatin (Fig. 13), as 



D 



t^ 



^ 



Fig. 13. 



^1^^ 



1 Wilson, E. B., 190O, p. 403. 



The Two Structural Components of 

 THE Living World. 



^t%®^ 



Protoplasm or cytoplasm represents the chicj visible fortn 



|«v , "■ •xftll'^'!^''^ or substance of the cell m the growing condition. Chro- 



^ T^ - lis ■' matin is the chief visible centre of heredity; there are 



doubtless other visible and invisible centres of energy 



concerned in heredity. 



Protoplasm (grayish dotted areas) and Chromatin (black, 



waving rods, threads, crescents, and paired spindles) in 



single cells {A-C) and in clusters of cells {D, E). 



A. Achromaliitm, bacteria-like organisms with network of 

 chromatin threads and dots. 



B, C. Single-cell eggs in the ovaries of a sea-urchin (resting 

 stage), the chromatin concentrated into a small 

 black sphere within the nucleolus (inner circle). 



D. Many cells in the root-tip of an onion. Chromatin 

 (division stage) in black, wavy lines and threads. 

 E. Many cells in the embryo of the giant redwood-tree of California. Chromatin (division 

 stage) in black, waving rods, threads, crescents, and spindles. The cell boundaries 

 in thin black lines and the dotted protoplasm are clearly shown. After Lawson. 



