Viscosity Changes of Protoplasm 189 



all be at or near one pole, or irregularly distributed across the 

 equatorial plane. The endoplasm is rather dense in consistency, for 

 there is no Brownian movement or streaming of its contained gran- 

 ules. The superficial gel layer is relatively thin and hyaline in 

 appearance. 



As the chromosomes move to the poles in anaphase, they converge 

 to form compact daughter groups and leave behind a homogeneous 

 granule-free interchromosomal material which is, or becomes, as 

 will be seen later, less viscous than the endoplasm. It evidently 

 consists of nuclear or spindle sap left behind as the chromosomes 

 move to the poles. It is sharply distinct from the surrounding endo- 

 plasm. As the cell flattens in the equatorial region during anaphase, 

 this interchromosomal material is squeezed from a blunt spindle 

 shape to a cylindrical shape by the contractile tension of the gel 

 layer. It probably plays an entirely passive role in this. It does not 

 mix with the endoplasm at this stage. 



The flattening of the cell is probably due to the contraction of a 

 broad equatorial belt of the gel layer which has become more 

 viscous or thicker than the rest of the gel layer. A decrease in the 

 viscosity of the gel layer at the poles might also account for the 

 elongation. It is possible that both factors are at play. There is 

 scarcely any displacement of the endoplasmic granules and no 

 apparent diminution of its thickness in the equatorial region during 

 the flattening. This indicates that the endoplasm is rather viscous. 



About the time, or shortly after, the chromosomes reach the 

 poles and after polar elongation of the cell has occurred, the constric- 

 tion groove, or furrow, appears in the equatorial plane. There are 

 some indications that the gel layer is somewhat thickened in the 

 region of the furrow. As the furrow deepens, it bends the endoplasm 

 inward and the deep surface of the latter projects into the soft inter- 

 chromosomal material. The endoplasm often has a row of fat globules 

 extending across the mid-plane; they are bent inward and finally 

 separated into two groups, one for each daughter cell when the 

 endoplasm is later divided. With the deepening of the furrow, the 

 endoplasm encroaches more and more on the interchromosomal 

 material and finally divides it into two parts. 



Figs. 9-11. Cleavage of an adult rat fibroblast from a 2-day culture. X 1.100 dia. 



Fig. 9. End of anaphase, cell elongated, homogeneous exnuclear sap elongated, 

 endoplasm with many small fat globules. 



Fig. 10. Four minutes later. Mid-telophase, endoplasm with fat globules bent in- 

 ward, exnuclear sap about divided. 



Fig. 11. Three minutes later. End of telophase. Connecting stalk. 



