CYTOKINESIS. 95 



and wholly independent of the spindle, so that in the case mentioned one of the 

 daughter cells would contain no part of the nucleus or spindle. Furthermore, the 

 elongation of the cell usually takes place independently of the spindle in normal 

 cleavages, for here the lobing of the cell is sometimes typically present at a time 

 when the initial spindle may lie in any direction in the cell {cf. text figs. XXXII, 

 XXXIII) ; only later is the spindle turned into the longest axis of the cell. We 

 may conclude, therefore, that the elongation of the spindle is not the cause of the 

 elongation of the cytoplasm, but that the latter is quite independent of the former. 



The formation of these lobes of protoplasm in unequal cell division strongly 

 resembles the formation of pseudopodia in amoeba-like organisms. In the former, 

 as in the latter, there is an outflow of cell substance due to the diminution of sur- 

 face tension at certain points. In all unequal cleavages the reduction of surface 

 tension takes place principally at one pole of the spindle, and in most of these cases 

 this pole is directed toward the sphere substance of the preceding cell cycle. This 

 old sphere substance lies at the free surface of the cell, immediately under the cell 

 membrane. Here it is spread by the action of the aster, and its granular material 

 probably contributes to the clear plasma of the "antipodal cone" and to the sur- 

 face layer or cell membrane. A protrusion of cell substance occurs at this place, 

 and into this protrusion the pole of the spindle moves. 



The transformation of sphere substance, which is partly derived from the 

 nucleus, into the surface layer or cell membrane affords an explanation of the 

 diminution of surface tension at the poles of the spindle and the consequent elonga- 

 tion of the cell. It also affords a partial explanation of the greater diminution of 

 surface tension at one pole than at the other and the consequent inequality of the 

 resulting cleavage. It is not, however, a full and satisfactory explanation of 

 unequal cleavage, as will be shown later. 



In the formation of pseudopodia in amceba-like organisms Verworn ('92, '95) 

 considers that "it is the chemical affinity of certain parts of the protoplasm for 

 oxygen which leads to the reduction of the surface tension at definite places and so 

 to pseudopod formation." How fully my observations on the eggs of gasteropods 

 agree with these conclusions of Verworn will be apparent when it is recalled that 

 sphere substance always moves to a free surface of the cell where it undergoes 

 oxydation, and that in the end it probably takes part in the formation of the cell 

 membrane or the surface layer of the cell. 



The Equatorial Constriction. — The evidence, which I have just adduced for 

 movements of the cell substance from the equatorial to the polar regions bears also 

 upon the equatorial constriction, since the withdrawal of substance from the equator 

 and its aggregation at the poles must necessarily reduce the equatorial diameter. 



Butschli (1900) has, with characteristic insight, pointed out the fact that if the 

 cell is of a fluid or semi-fluid consistency, the equatorial constriction must be the 

 result of increased surface tension at the equator ; and he concludes that the dif- 

 fusion at the poles and the vortical movements within the plasma can have their 

 origin either in increased tension at the equator or in diminished tension at the 



