SURFACE TENSION AND CELL-DIVISION 



237 



To what extent the drop of protoplasm responds to similar 

 changes in its environment is seen from the following figures. 



It will be seen that in an alkaline medium the capacity of the 

 egg (hke that of an oil-drop), to retain a spherical form is lost. 

 The outline of the egg becomes distorted by the production 

 of numerous blunt irregular processes, just as is the surface 

 of the oil. On returning such eggs to normal sea- water the 

 spherical form is gradually reformed ; in acid sea-water the 

 spherical form is quickly regained. In some cases the recovery 



Text-fig. 2. 



d e r 



a. Egg in normal sea-water, P,, 7-9 ; 6, egg in acid sea- water, 

 P„ 6-0 ; c-e, successive stages in alkaline sea-water, Ph 9-6 ; 

 /, same egg transferred from alkaline to acid water. 



of the spherical form causes the protoplasm to divide com- 

 pletely into two or more parts. These parts are always spherical 

 or eliptical. The nucleated fragment alone divides to form 

 normal blastomeres. Saponin possesses the power of lowering 

 the surface tension at an oil/water interface, and produces 

 similar changes in the form of the egg to those produced by 

 alkalis. 



It seems reasonable to conclude that the form of the undivided 

 egg is determined, at least in part, by the surface tension at 

 the egg-surface. 



Before proceeding to consider the part played by surface 

 tension during the process of division, it is necessary to draw 

 attention to certain facts in connexion with normal cleavage. 

 The first division in the egg of Echinus miliaris takes 



