THE ACHROMATIC FIGURE, CYTOKINESIS, AND CELL WALL 189 



On the contrary, Robertson (1911, 1913) and others attribute furrowing 

 rather to a decrease in the equatorial surface tension, this decrease being 

 due to a diffusion of materials toward that region from the daughter 

 nuclei. Evidence favoring Butschli's interpretation has been afforded 

 by the studies of Spek (1918). Spek imitated furrowing and division 

 with oil and mercury droplets in water, and showed that by lowering the 

 surface tension at two poles of the droplet the relatively higher surface 

 tension at the equatorial region could be made to bring about the con- 

 striction and fission of the droplet. In both droplet and dividing egg 

 he found streamings such as Erlangen (1897) had described in the nema- 

 tode egg: an axial movement polewards to the regions of low surface 

 tension and a superficial streaming toward the equatorial region of higher 

 surface tension, the streams turning inward at the furrow (Fig. 65). 



FIG. 65. Diagram showing streaming and furrowing in the egg of Rhabditis 

 (A) and an oil droplet (B). (After Spek, 1918.) 



Although the causes of the initial changes in surface tension in the case 

 of the cell are relatively obscure, these experiments of Spek show beyond 

 question that alteration in surface tension and streaming are very im- 

 portant factors in cell-division of this type. 



The relation of periodic changes in the viscosity of the egg substance 

 to cytokinesis by furrowing has recently been discussed by Chambers 

 (1919). Immediately after the entrance of the spermatozoon into the 

 echinoderm egg the sperm aster begins to differentiate as a semi-solid 

 region near the sperm head. (See p. 279.) When the aster is most fully 

 developed the egg has its maximum viscosity (Heilbrunn 1915). As 

 the aster disappears the egg again becomes more fluid. Then a second 

 solidification begins at two centers forming the amphiaster, or bipolar 

 figure. The growth of these two semi-solid masses results in the elonga- 

 tion of the egg, and eventually in the development of a cleavage furrow 

 in the more fluid portion of the egg substance separating them. After 

 cleavage is complete the semi-solid masses (asters) revert to a more fluid 

 state. The formation of the cleavage furrow, moreover, may be pre- 

 vented by mechanical means. At the second mitosis in eggs so treated 

 (binucleate eggs) there are four centers of semi-solidification rather than 

 two, and the egg cleaves simultaneously into four blastomeres. An egg 

 cut into two pieces during the amphiaster stage will, provided it does not 



