480 THEOPHILUS S. PAINTER 



THE ASTER AND CYTOPLASMIC DIVISION 



The centrosome with its surrounding aster has been assigned 

 such a prominent role in cell division by most investigator-? 

 working on animal cells that it is very desirable to arrive at 

 some conclusion as to its function on the basis of the present 

 work. At first sight the present investigation seems to minimize 

 the importance of the aster in cell division. Indeed, I believe 

 that it has been clearly shown, especially by the narcotized eggs, 

 that in the total absence of the asters, the eggs may divide. 

 To conclude from this that the asters do not have a role, and 

 a very important one, would be a very serious mistake. For 

 the narcotized eggs and, particularly the monaster eggs, demon- 

 strate that while a force, independent of the aster, is at work 

 within the cell, this force produces little more than striking 

 changes in surface tension which do not accomplish any definite 

 regular result. A directive structure is lacking in monaster 

 and narcotized eggs, and this fact I believe, points out to us 

 the principal function of the aster in the sea urchin egg. 8 A 

 number of facts point to the correctness of this view. 



First of all, the work on enucleated blastomeres in which 

 asters are present show clearly that each aster has the power 

 of surrounding itself with a certain amount of cytoplasm. 

 When these asters are near the surface, furrows may appear 

 around or between them, a phenomenon which can only be 

 understood when we assume that the area around each aster is 

 more solid than the outlying cytoplasm. This gives us the 

 necessary mechanism for the directing of this second force 

 which causes surface tension changes. 



Again the variation in the intensity of the protoplasmic 

 movement in the first monaster cycle can best be explained 

 by assuming that the aster acts as a center of more solid proto- 

 plasm. Thus when the aster undergoes no retreat (fig. B 2 ) the 

 force which produces the movement does little more than cause 

 the ectoplasmic layer to swell, since the protoplasm is held fast 

 until after the critical period has passed. Figure D 2 shows an 



8 This applied to cytoplasmic division. 



