THE BIOLOGY OF THE CELL SURFACE 



question the experiments showing that eggs of Nereis lose 

 water in the form of drops throw some Hght. 



If we think of the water as in the external phase, the 

 effect of increasing the density of the medium — from hypo- 

 tonic to normal sea-water or from normal sea-water to 

 hypertonic — would be either to cause streams of water to 

 form rather than drops, or to free the water generally and 

 equally everywhere from the eggs, so that the water would 

 not be visible as it moves out of the egg. Since however 

 drops of water form and since water can not appear in water 

 as drops, we must assume that the drops are pressed out of 

 structures. We conclude, therefore, that the water which 

 appears in the form of drops is that which was held by solid 

 structures, i.e., was in the internal phase. The formation of 

 drops in normal viable eggs points to the same conclusion. 



As we have seen, the eg% on return from hypotonic to 

 normal sea-water regains its normal equilibrium as it 

 shrinks. In this process, inasmuch as drops of water leave 

 the yolk-spheres we conclude that the yolk-spheres possess 

 water-holding capacity and thus exercise function in the 

 distribution of water in the cell under the condition of the 

 experiment. Because it is not easy to discern yolk-free 

 areas of the endoplasm in the unfertilized &gg of Nereis, 

 one can not so readily determine to what extent water-hold- 

 ing power resides in the cytoplasm. In the various stages 

 of development — -cleavage, blastula, gastrula, larva (tro- 

 chophore) and young worm — as the yolk becomes segre- 

 gated into certain blastomeres leaving others yolk-free, it 

 becomes easy to learn that yolk-free blastomeres or those 

 in which yolk is not in visible spheres also hold water in 

 the form of drops. Therefore we conclude that the clear 

 homogeneous cytoplasm holds water within its struc- 

 ture which exists in a state of fine subdivision. Moreover, 

 the nucleus reveals water present in drops; these exist apart 

 from chromosomes. Since the water drops can be demon- 



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