CELL-DIVISION 



they return in time to their original positions. The oil is 

 the last to return, I find, while the heavier yolk-spheres 

 return first and after them the heaviest bodies, the pigment 

 granules. After normal fertilization the inclusions shift 

 positions and come to a new arrangement which again has 

 no relation to their specific gravity. Thus, in the egg of 

 Arhacia after fertilization the heaviest inclusions, pigment 

 granules, remain close to the surface; the oil, shifting back 

 and forth fairly even in distribution except as influenced by 

 the mitotic spindle, forms clusters which mass and break up 

 again;^ the yolk is evenly distributed outside the spindle 

 area. The behavior of the pigment granules is due to the 

 fact that they are trapped at the surface; they move only 

 as the cleavage furrows are formed. In some eggs, as those 

 of Nereis, the yolk and oil come to be massed and are defi- 

 nitely distributed always to certain cells. These facts lead 

 us to assume that the flow in the cytoplasm is not every- 

 where the same and that there are currents which are cir- 

 cumscribed and limited to definite areas of the cell. If 

 there were only one current of equal velocity in a medium 

 everywhere the same, then we should expect a distribution 

 of particles according to specific gravity, as is for instance 

 the case in the blood-stream in a blood-vessel. 



The fact that cytoplasmic streaming in eggs is most 

 expressed directly following the ectoplasmic changes, which 

 result from the fertilization-reaction, gives rise to the sug- 

 gestion that the ectoplasm by its activity sets up a condi- 

 tion which increases the currents in the cytoplasm. That 

 increased fluidity of the cytoplasm might be brought about 

 by the escape of substances from the nucleus is a less likely 

 assumption for the cause of the increase in streaming. It is 

 true that streaming is not easily discerned in an e^g like that 

 of Nereis, fertilizable before maturation, until after break- 



1 Just, 1 927 a. 



26g 



