88 BIOLOGICAL LECTURES. 



at the same time the centrospheres of the basal cells remain 

 almost exactly in their first position, though they move slightly 

 toward the surface and sometimes a little to the left. The re- 

 versed cleavage of these cells seems not to be due to reversed 

 currents in the cells, but rather to the absence of any currents. 

 Further divisions have been followed in detail up to a late 

 stage in the cleavage, but as they illustrate merely the princi- 

 ples which have been already described, no account is given of 

 them here. 



IV. Summary and Comparisons. 



The principal facts as to the rotation of the cell contents, or 

 what might be called the cytokinesis of cleavage, are the fol- 

 lowing : I. With the escape of nuclear sap into the cell body 

 at the beginning of mitosis, vortical movements are set up in 

 the cytoplasm, the poles of the spindles being the centres of 

 such vortices. 2. The vortices in daughter-cells are in reverse 

 directions, the movement in the upper cell being in the direc- 

 tion of the cleavage by which that cell was formed. 3. Cell 

 walls form where vortices meet. 4. Successive divisions alter- 

 nate in direction when the rotations carry the nucleus to the 

 side of the cell opposite to that in which it first lay ; non- 

 alternation is due to the absence of such rotation. 5. When 

 the cell movements carry the mitotic spindle out of the middle 

 of the cell, unequal cleavage results. 6. After the first two 

 cleavages, every cell division is qualitatively differential, since 

 the sphere substance of the preceding cleavage is carried into 

 one only of the two daughter-cells. 



Such movements of the cell substance as are here described 

 have long been known in certain plant cells and among protozoa. 

 Ryder ('94) in particular has discussed the movements of 

 amoeboid organisms and finds the cause of such movement in 

 a vortical flow of protoplasm. A few observations have also 

 been made on the movements of cell substance in certain ova, 

 but in no case have these movements been followed in detail, 

 and in no case have they been connected with processes of 

 differentiation. Whitman ('87) emphasized the importance of 

 the cytoplasm in the movements of the germinal vesicle and 



