80 GENERAL EMBRYOLOGY 



divergence of the chromosomes, it was formerly believed that the fibers 

 in the amphiaster were the active elements and that different groups of 

 these fibers had different functions. Thus the mantle fibers attached 

 to the chromosomes were supposed to be contractile and by shortening 

 to draw the chromosomes toward the ends of the spindle, the central 

 spindle remaining rigid and resisting any tendency for the ends of the 

 spindle to approach as the result of the contraction of the mantle fibers, 

 and at the same time serving as a sort of track upon which the chromo- 

 somes would slide along. The asters then, either as anchors helped to 

 fix the ends of the spindle, or by contraction served to draw the fully 

 diverged chromosomes further into the daughter cells (Fol, Van Bene- 

 den, Heidenhain). This naive explanation cannot be applied in toto 

 to any known instance of division, although certain features may be 

 correct descriptions of the events in certain forms. And there are many 

 facts opposing such an account of the action of the forces of mitosis, such 

 as the absence of mantle fibers or of asters in many mitoses. Another 

 hypothesis, in greater favor at present, and apparently well founded, is 

 that the centrosomes and centrospheres rather than the achromatic 

 fibers are the active elements, and further, that their activity is primarily 

 of a chemical nature. Thus the chromosomes are believed to be chem- 

 ically attracted toward the centrosomes, the achromatic fibers being 

 passive and formed merely as the result of the rearrangement of cyto- 

 plasmic granules along the paths of the chemical transformations which 

 have their seat in the centrosomes and extend thence through the cyto- 

 plasm (Strasburger) . This hypothesis goes farther and makes it possible 

 to explain the division of the cytoplasm on the same basis. For the 

 chemical transformations centering in the centrosomes might easily 

 influence the tensions of the comparatively impermeable surface film 

 of the cell so that in the region near the centrosomes the tensions would 

 be increased while that region farthest from the centrosomes and sym- 

 metrically related to them both, namely, the plane of the equatorial 

 plate, would be a region of lower tension; the result of this would, of 

 course, be a constriction in this plane. It is quite likely too that dif- 

 ferences in electric tension accompany these chemical transformations, 

 and these might assist in the alteration of the surface tensions in such a 

 way as to contribute to the same end (R. S. Lillie). It is known that 

 there are differences in the electric potential in different regions of the 

 dividing cell, in some cases at least. 



A further development of the chemical hypothesis attempts to explain 

 the formation of the amphiaster itself. Thus, increase in the ratio of 

 the volume of the cytoplasm to volume or surface of the nucleus beyond 

 a certain point leads to a chemical alteration of the centrosomes such 

 that they become the centers of two equivalent series of chemical reac- 

 tions with the cytoplasm, the result of which is the formation of the 



