so PROTOPLASMIC CONSISTENCY AND CELL DIVISION 



time when Heilbrunn informs me he found the egg substance to be 

 of maximum viscosity. 



The increase in viscosity of the egg cytoplasm is produced by an 

 influence spreading out in all directions from the center of the aster. 

 While this occurs the central hyaline area of the aster (the hyalo- 

 plasmsphere of Wilson) increases in size, and there is strong evidence' 

 that this is due to the accumulation of a hyaHne Kquid which sepa- 

 rates out of the semisolidifying cytoplasm and flows in very fine 

 converging streams to the center of the aster. It is possible that 

 this and kindred phenomena give to the aster the appearance of 

 radiations from a common center. The consistency of the cytoplasm 

 incorporated in the aster diminishes in firmness on passing from the 

 interior of the aster to its exterior, being greatest in the region border- 

 ing on the centrosphere and least at the periphery. 



The disappearance of the sperm aster, in the opinion of the writer, 

 occurs through a process of liquefaction. During the liquefaction 

 the substance of the centrosphere collects into two areas at opposite 

 poles of the cleavage nucleus. The experiments to be described in 

 this paper indicate that shortly before cleavage each of these areas 

 becomes a center around which the cytoplasm commences again to 

 pass into a semisolid state. The radial configuration about these 

 areas constitutes the amphiaster. The comparatively firm consist- 

 ency that the egg now attains for the second time since fertihzation 

 is due to two masses, the two asters, instead of to a single aster as 

 was the case shortly after the entrance of the sperm. The importance 

 of this phenomenon in its bearing on cell division is discussed in the 

 last part of this paper. 



Experiment 1. — The consistency exhibited by the protoplasm of the 

 sea urchin egg at various periods from the moment of fertilization 

 until the completion of the first cleavage, was ascertained by careful 

 probing with the microdissection needle. 



Immediately after fertilization the cytoplasmic granules readily 

 flow by the moving needle. After the sperm has entered the egg, 

 the sperm aster constitutes a comparatively firm mass which gradually 

 increases in size as it moves to a central position in the egg. When 

 the sperm aster is at its full development the highly viscous state 

 of the cytoplasm is detected by the needle. Illustrations of this 



