WATER 



medium the eggs gradually swell so that finally their bulk 

 greatly exceeds that of the normal q^^. With this increase 

 in volume, the yolk-spheres, so striking in the normal &2,^, 

 seem to disappear, actually they have changed to an unusual 

 degree, losing their refringency and their discrete character; 

 indeed, they may actually merge. These changes in the 

 yolk are worth noting. What happens is this: The indi- 



FiG. zoa. — Section of a fixed egg of Nereis after 45 minutes' exposure to 

 4o''per cent, sea-water. The egg is swollen, the ectoplasm thickened and opaque, 

 the yolk spheres are in process of fusion. (Figs. 20^2 to 20^ drawn by Mr. L. A. 

 Hansborough from author's preparations). 



vidual yolk-spheres increase in size with a liberation of 

 fine drops of oil, thus becoming almost transparent in 

 appearance. If exposure goes farther, the yolk-spheres 

 fuse. Yolk-spheres in fixed normal eggs are homogeneously 

 blackened by iron haematoxylin; the yolk of the swollen 

 egg also stained with iron haematoxylin is seen to be made 

 up of fine threads within a distinct membrane, each yolk- 

 sphere thus resembling a nucleus. The germinal vesicle 



131 



