THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 183 



must be admitted that there is no immediate prospect of a satisfactory 

 solution of these problems. 



Contractility. One of the simplest and most widely accepted theories 

 was that of fibrillar contractility suggested by Klein (1878) and van 

 Beneden (1883, 1887), according to which the chromosomes are simply 

 dragged apart by the contraction of two opposed groups of spindle fibers. 

 This theory and its modifications are fully reviewed by Wilson: it will 

 be sufficient here to point out that, whereas many facts were cited in its 

 favor, and elastic models made which simulated the supposed contraction 

 and its results (Heidenhain), the further evidence brought forward by 

 Hermann (1891), Driiner (1894, 1895), Calkins (1898), and others led to 

 the general restriction of the role of contractility, until it became appa- 

 rent that this factor, although it may contribute to the general result, 

 must be of minor importance. The contractility factor appeared again 

 in the more elaborate theory proposed by Rhumbler, which may be 

 briefly stated as follows: The centrosome arises as a local solidification 

 of the walls of the alveolae; the denser constituents of the protoplasm 

 collect at this point and form an attraction sphere, driving the less dense 

 constituents to the other parts of the cell where the pressure is lower; this 

 migration of fluid affects particularly those strands of the protoplasmic 

 reticulum which radiate more directly from the centrosomes; these 

 strands or rays, in giving up their fluid, shorten, and thus exert a trac- 

 tive force which draws the daughter chromosomes apart. In this theory, 

 therefore, the main factors are streaming and contractility. 



Streaming. The phenomena of streaming and surface tension have 

 been prominent factors in several attempts to explain both karyokinesis 

 and cytokinesis. The role of streaming in karyokinesis has been held to 

 be especially important since Butschli, Hertwig, and Fol showed many 

 years ago that currents exist in the protoplasm. Rhumbler (1896, 1899), 

 Morgan (1899), Wilson (1901), and Conklin (1902) all held that the 

 astral rays are due at least in part to centripetal currents. This inter- 

 pretation has recently been confirmed by Chambers (1917) in his micro- 

 dissection studies on the living cell. With regard to the aster Chambers 

 says: "The formation of the aster consists in the gelation of the hyalo- 

 plasm which comes under the influence of the astral center. A hyaline 

 liquid separates out during the gelation and flows in innumerable centri- 

 petal paths toward the center where it accumulates to form a sphere. 

 This centripetal flow brings about an arrangement of the gelled hyalo- 

 plasm containing the cell-granules into radial strands separated by the 

 hyaline-liquid paths. This produces the astral figure. The strands of 

 gelatinized cytoplasm merge peripherally into the surrounding liquid 

 cytoplasm or reach and anchor themselves in the substance of the gelled 

 surface when the aster is fully formed. The liquid rays merge centrally 

 into the substance of the sphere, the liquid of the rays and of the sphere 

 being thus identical." 



