MUSEUM OF COMPARATIVE ZOOLOGY. 529 



displacement of yolk granules. The assumed motion is, therefore, not 

 enough to account for the appearances ; it offers no explanation of this 

 difference of refraction. No satisfactory explanation of the cause of 

 the latter will necessarily exclude the possibility of a motion, but it 

 cannot rest on that assumption alone ; for however definite the course 

 of the flow, it could produce no such optical effect until there was a 

 differentiation into more refractive and less refractive portions. 



If the aster is only the optical expression of currents in the proto- 

 plasm, it remains to be explained why it is that such currents do not 

 uniformly produce this effect. There certainly may be a flow of sub- 

 stance without astral figures. The male pronucleus in Limax, for exam- 

 ple, grows within a short time to a comparatively large size without 

 necessitating the existence of any astral phenomena, and yet it cannot 

 be doubted that it grows at the expense of the substances of the yolk. 

 There is no reason to suppose that it has greater power than the male 

 pronuclei of other animals in rendering assimilable the substance in its 

 immediate vicinity, and that it may therefore dispense with far-reaching 

 protoplasmic currents which are necessary for their growth. If there are 

 currents in the one case, there doubtless are in the other, and their 

 magnitude and velocity, if proportionate to the rapidity of nuclear 

 growth, will not be less in Limax than in the average of other cases. 

 On the other hand, asters may possibly remain for a time unaccom- 

 panied by corresponding movements of clear protoplasm ; at least, there 

 are often great differences in the size of the areas which form the cen- 

 tres of asters having nearly the same extent.* 



It is not to be overlooked that the areas may not be exclusively due 

 to an accumulation of clear protoplasm, and thus to an indirect repul- 

 sion of the granules of the yolk. It has not yet been shown that it is 

 impossible for the granules to have been employed in the chemical 

 changes presumably taking place at the centre of the aster, — that is, 

 that their disappearance from the " area " may not be due as much to an 

 actual chemical alteration as to a mechanical dislodgment. 



The subsequent occupancy of the region of the central area by granu- 

 lar protoplasm is an argument neither for nor against the physical dis- 

 placement of granules from the area. The clear substance is at least 

 largely consumed in the growth of the nucleus. As the latter does not 

 migrate far enough to have its centre occupy the place of that of the 

 aster, this consumption of areal substance necessarily implies its re- 



* Compare Limax, Figs. 73 and 80% with Fig. 85 ; also 0. Hertwig, '75, Taf. XIII. 

 gs. 21 and 23. 

 VOL. VI. — NO. 12. 34 



