PHYSICS OF SEGMENTATION. IIQ 



passes therefore in the direction of the gradient from periphery to 

 center. Such inductive phenomena resulting from the increased 

 acidity of the nucleus during mitosis must in turn be due, we find, 

 to the preceding chemical changes resulting perhaps from the 

 action of ferments. Accordingly, it results that "the center of 

 the astral radiations is the region of highest positive potential, the 

 surface of negative, and hence the decrease in the surface tension 

 by the like charges present." 



But a uniform decrease in tension is no change in form ; to 

 bring this about the tension must be unequal. To explain this, 

 Lillie says, "There are indications, e. g., the elongation of the 

 spindle axis, that the primary lowering of surface tension by the 

 above agencies is at the two sides of the egg opposite the astral 

 centers. From the position of the astral centers during meta- 

 phase and telophase it is to be expected that the surface negative 

 charges are densest near regions adjoining the long axis of the 

 egg and that there surface tension is lowered to the greatest de- 

 gree. The effect in these regions will increase as the daughter 

 groups of chromosomes approach the poles, since the inductive 

 action increases as the distance decreases. The surface tension 

 at the regions adjoining the astral centers must therefore de- 

 crease as the daughter groups approach the surface, /. c., the 

 difference between the surface tension at the poles and at the 

 equatorial region progressively increases. Eventually the egg 

 is surrounded by an equatorial surface zone possessing higher 

 tension and acting like a constricting band ; a cleavage furrow 

 follows." 



" From the fact that not all of these events takes place when 

 the egg is placed in solutions of non-electrolytes, it is clear that 

 cleavage depends on the presence of ions, and that in fertilization 

 the spermatozoon carries either these necessary electrolytes into 

 the egg or ferments which initiate their formation and action." 



That the acceptance of the events either identical with or simi- 

 lar to those so ingeniously outlined by Lillie is necessary in 

 order to complete the theory of segmentation advanced in our 

 own paper, may be admitted. But that the two are not contra- 

 dictory to but rather must supplement each other seems quite as 

 necessary to admit. The constricted form of segmentation must 



