CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 353 



rare cases, but it is very easy to cause the fusion of eggs by 

 other means. For example in experiments where eggs are treated 

 with carbonic acid there are inany cases of such fusion. Some- 

 times two eggs are fused as in figure 106, or three, four, or 

 many eggs may be fused into one mass. It is an interesting 

 fact that eggs rarely if ever fuse by their protoplasmic poles, but 

 almost invariably by some portion of the lower hemipshere which 

 contains yolk. There is apparently some peculiarity of the egg 

 surface over the animal hemisphere which prevents its fusing 

 with another egg at this pole. A large number of such eggs 

 which were caused to fuse together at various stages before and 

 during cleavage has been studied, and in every instance the 

 polarity of each constituent egg remains practically unchanged. 

 The ectomeres from different eggs unite in later stages of de- 

 velopment into a continuous layer, but there is no indication 

 that the polarity of one cell is changed by its fusion with an- 

 other cell, as would be true if polarity were due merely to elec- 

 tric charges on colloidal particles or to physical properties of 

 the cell membrane. On the other hand these observations in- 

 dicate that the polarity of an egg inheres in the organization of 

 its more viscid protoplasm which is but slightly miscible with 

 that of other eggs. 



2. Surface tension as a cause of polarity. It is possible that 

 the various constituents of cells are oriented and held in place, 

 or brought back to normal positions if displaced, by surface 

 tension. For example if spindles or centrospheres which are 

 attached to the surface layer are centrifuged strongly, that 

 layer may be indented as in figures 5 and 6 and the surface 

 thereby increased; in such a case surface tension would restore 

 the spherical shape of the cell after centrifuging, and if the 

 asters or spheres still remained attached to the surface they 

 would be drawn back to their normal positions. However, I 

 have never seen the egg surface indented as shown in figures 5 

 and 6 except in the stage of the first maturation spindle and even 

 at this stage it is unusual. In no other stage figured in this 

 paper is there any such indentation of the cell and yet in every 

 case all cell parts come back if possible to their normal positions 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 22, NO. 2 



