THE BIOLOGY OF THE CELL SURFACE 



accompanying figure (Fig. 20) illustrates these changes as 

 seen in fixed eggs. First the drops are at or near the egg- 

 centre, then they move outward. As they cross the ecto- 

 plasm they become elongated. Whilst the water-drops 

 form and the egg regains normal shape, the yolk-spheres 

 return to their original condition; they again become dis- 

 tinct spheres in the living egg, appear blackened in the 

 fixed. This change runs parallel with the movement of 

 the fine oil droplets into the yolk. 



Thus the movement of water from the egg is associated 

 with a marked behavior of the yolk-spheres. As the cells 

 take up water, the yolk-spheres lose oil and increase in 

 size, even fusing to make one mass. That is, the yolk- 

 spheres take up water from the water-logged protoplasm. 

 Then as the cells lose water to the surrounding medium, 

 the yolk-spheres lose water to the cell and regain the oil 

 from the cytoplasm. In the case of the Nereis egg the 

 yolk-spheres possess a very exaggerated water-holding 

 capacity. 



It would be a mistake, however, to conclude from this 

 description that the yolk-spheres are the sole regulating 

 mechanism for the water control in the cell. Water-drops 

 also form in the nucleus, in the clear cytoplasm in cells of 

 stages of late cleavage which contain no visible yolk-spheres, 

 and likewise in the gut-cells of larvae after all yolk has dis- 

 appeared. Other animal cells than eggs similarly I find 

 show these drops of water. One factor which determines 

 the strong expression of water-drop formation is doubtless 

 the ectoplasm, because eggs and other cells that possess 

 most pronounced differentiated ectoplasm show water-drop 

 formation best. 



The rate at which the drops appear I have studied. A 

 comparison of this rate in eggs in various stages is inter- 

 esting: Drop-formation is more rapid in fertilized than in 

 unfertilized eggs. It varies in fertilized eggs depending 



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