THE MECHANISM OF MITOSIS 



71 



carefully by Heidenhaiii in the salamander. The astral rays here 

 extend throughout nearly the whole cell (Fig. 35), and are believed 

 by Heidenhain to represent the contractile elements by means of 

 which the cell changes its form and creeps about. A similar con- 

 clusion was reached by Solger ('91) and Zimmerman ('93, 2) in the 

 case of pigment-cells (chromatophores) in fishes. These cells have, 

 in an extraordinary degree, the power of changing their form, and of 



Fig. 36. — Pigment-cells and asters from the epidermis of fishes. [ZIMMERMAN.] 

 .-/. Entire pigment-cell, fi cm Blennius. The central clear space is the central mass of the aster 

 Irom which radiate the pigment-granules; two nuclei below. B. Nucleus {?/) and aster after ex- 

 traction of the pigment, showing reticulated central mass. C. Two nuclei and aster with rod- 

 shaped central mass, fiom Sargiis. 



actively creeping about. Solger and Zimmerman have shown that 

 the pigment-cell contains an enormous aster, whose rays extend in 

 every direction through the pigment-mass, and it is almost impos- 

 sible to doubt that the aster is a contractile apparatus, like a radial 

 muscular system, by means of which the active changes of form are 

 produced (Fig. 36). 



But although these observations seem to place the theory of fibrillar 

 contractility upon a firm basis, it has since undergone various modifi- 



