176 RALPH S. LILLIE. 



egg, and that there the surface-tension will accordingly be low- 

 ered to the greatest degree. It is also clear that the influence in 

 these regions will increase as the daughter groups of chromo- 

 somes approach the poles since the inductive action of the chro- 

 matin on the cell-surface must increase as the interval between 

 the two diminishes. The surface-tension at the regions adjoining 

 the astral centers must therefore decrease still further as the 

 daughter groups of chromosomes approach the surface ; in other 

 words, the difference between the surface-tension at the polar and 

 at the equatorial regions of the egg (speaking with reference to 

 the spindle axis) progressively increases as the groups of chro- 

 mosomes diverge. Eventually the egg is surrounded by an equa- 

 torial surface-zone possessing a distinctly higher tension--/, e., a 

 stronger tendency to contract --than the polar surface-areas. 

 The effect is naturally the same as would be produced by the 

 presence of a constricting band surrounding the equator ; a 

 "cleavage-furrow" appears which progressively deepens until 

 complete bipartition is effected. 1 



Cleavage, on this theory, is the result of an inequality of sur- 

 face-tension between polar and equatorial regions of the dividing 

 cell, due to a greater lowering of surface-tension at the poles 

 than at the equator. This diminution of surface-tension is de- 

 pendent on the ions present in the protoplasm, hence withdrawal 

 of these, as by the use of non-electrolyte-solutions, prevents 

 cleavage, and, by heightening the normal surface-tension, tends 

 to favor fusion of adjacent blastomeres. 



Why similar effects should follow withdrawal of water and 

 compression is not clear. It might be suggested that the effect 

 of compression is an instance of the second of Lippmann's laws 

 cited by Ostwald 2 in his " Lehrbuch ": if by mechanical means the 

 surface of the fluid (with tension lowered by the electrical double 



1 So early as 1876 Biitschli {AbJiandl. d. Senkenbergscheii iVafurf. Gts., Bd. 10) 

 referred division to changes of surface-tension due to influences emanating from the 

 astral centers. In his recent paper (Archiv fiir Entwicklungsinechanik, Id, 1900) 

 the equatorial constriction is ascribed to a relative increase in the surface-tension of the 

 equatorial zone of the egg. Conklin (Journal of the Academy of Natural Sciences of 

 Philadelphia^ Second Series, Vol. XII., Part I., pp. 95 et seq.) is in essential agree- 

 ment with Biitschli so far as regards the immediate origin of the cleavage furrow. 



2 Ostwald, /. c., p. 923 ; see also ibid., p. 929. 



