CELL DIVISION OR MITOSIS 383 



of single cells or combinations of cells; (2) that all organisms 

 begin life as a single cell, giving rise in metazoans to a body of 

 more or less complexity, and (3) that the function of this com- 

 plex (multicellular) organism may be expressed in terms of the 

 activities of the individual cells of which it is composed. The 

 third proposition may need some modification. "It was 

 through the cell theory that Kolliker, Remak, Nageli, and 

 Hofmeister opened the way to an understanding of the nature of 

 embryologic development, and the law of genetic continuity 

 lying at the basis of inheritance. It was the cell theory again 

 which, in the hands of Goodsir, Virchow, and Max Schultze, 

 inaugurated a new era in the history of physiology and pathol- 

 ogy, by showing that all the various functions of the body, in 

 health and in disease, are but the outward expressions of cell 

 activities. And at a still later day it was through the cell 

 theory that Hertwig, Fol, Van Beneden, and Strasburger solved 

 the long-standing riddle of the fertilization of the egg and the 

 mechanism of hereditary transmission. No other biologic 

 generalization, save only the theory of organic evolution, has 

 brought so many apparently diverse phenomena under a com- 

 mon point of view, or has accomplished more for the unification 

 of knowledge. The cell theory must, therefore, be placed be- 

 side the evolution theory as one of the foundation stones of 

 modern biology."^ 



Cell Structure. — In the typical cell are found the following 

 parts: (1) the cell wall; (2) the cytoplasm, or cell substance, 

 which includes the plasma (that is, the living protoplasm around 

 the nucleus) and the chylema, or "cell-sap"; (3) a nucleus which 

 is usually inclosed by a delicate membrane, and which contains 

 chromatin and achromatin fibers and one or more nucleoli, and 

 (4) one or two centrosomes or attraction spheres. 



Cell Division or Mitosis. — The centrosome divides into two 

 parts, which gradually separate, and each of which becomes the 

 center of a system of fine achromatin fibers radiating about it. 

 A spindle of achromatin fibers is also extended from one centro- 

 some to the other. At the same time the chromatin granules 

 scattered throughout the nucleus are arranged into a continu- 

 ous thread or skein of closely contorted filaments. The nuclear 

 •Wilson, "The Cell." 



