158 J. F. MCCLEXDON. 



ions are necessary for the rapid oxidation of the sea urchin egg 

 (Loeb), and Harvey showed that the unfertilized egg is practi- 

 cally impermeable to OH ions of low concentration. The 

 increased permeability allows hydroxyl ions in the sea water to 

 penetrate the egg, as shown by Harvey, and, since the sea is 

 always alkaline, this may explain the increased oxidation. 



Asters always develop in the egg before segmentation. In the 

 normal egg these have some relation to the division of the nucleus, 

 but even if a nucleus is not present, 1 have observed that the 

 cytoplasm constricts along a line on the surface farthest removed 

 from the centers of the asters. 



The constriction of the cytoplasm is probably due to a band of 

 increased surface tension (or to decreased surface tension at 

 the poles). This might be caused by local increase in perme- 

 ability to ions, causing decreased polarization, at the equator 

 (or increased polarization at the poles, clue to increased pro- 

 duction of the polarizing electrolyte in the asters). 



The same reasons that were given for assuming that the surface 

 of the Amceba is electrically polarized, hold good for the egg. 

 The first change is probably a general increase in surface tension, 

 indicated by rounding up of the egg. Later this may become 

 localized from internal causes and result in cleavage. 



Hyde 1 observed local changes in electric polarization of 

 Fundulus eggs during cleavage, indicating that surface tension 

 changes and cleavage are due to this cause. 



It has been objected that the segmentation of the egg is not a 

 typical case of cell division, since the egg cell is "wound up" 

 and ready for some "stimulus" to set it going, whereas tissue 

 cells must "grow" or "rest" after each division before dividing 

 again. 



1 1 may be true that growth is prerequisite to division, but 

 this cannot be formulated quantitatively. In the spore-forma- 

 tion of certain organisms, a cell may divide in a relatively short 

 time into myriads of almost ultra-microscopic cells. 



Hertwig may be right, in general, in assuming that the relative 

 growth of nucleus and cytoplasm influences division, but the 

 difficulties in proving this have been indicated, and this cannot 



1 Am. Jour. Physiol., XII.. 241. 



