VII] OR CELL-AGGREGATES 345 



its contents or bring the whole into a spherical form ; nor will local 

 variations of surface energy be directly capable of affecting the 

 form of the system. But if the substance of our blastosphere be 

 sufficiently viscous, then osmotic forces may set up currents 

 which, reacting on the external fluid pressure, may easily cause 

 modifications of shape; and the particular case of invagination 

 itself will not be difficult to account for on this assumption of 

 non-uniform exudation and imbibition. 



