16 



BOYS AND GIRLS IN BIOLOGY. 



something like a ball-cover, if it were white and clear, 

 or transparent. This bag, or ball-cover, is tough and 

 solid, and, instead of being tilled with yarn or India- 

 rubber, it is full of a soft, jelly-like substance, which is 

 thick and brownish next the wall of the bag, but thin- 

 ner and more transparent toward the centre (Fig, 8). 

 This jelly is called protoplasm, which means first form, 

 or mould, and the thin space in the centre is an air-cell, 

 or vacuole (Fig. 9). If you color the yeast-cells, you 



Fig. 11. 



Yeast-cell, or Torula. Jelly stained, and the Broken Cell. 



«, Jelly, or Protoplasm; Sac clear. Sac clear, Jelly stained. 



Thin Space, or Vacuole. 



can see the different parts much better, a drop of ma- 

 genta will pass right through the sac without staining 

 it at all ; the cell-jelly, or protoplasm, will be quite red, 

 and the thin spot, or vacuole, will not be colored, 

 though it may look pinkish because you look at it 

 through a layer of red jelly, or protoplasm (Figs. 10, 

 11). Now, if the cell were all made of the same mate- 

 rial, it would probably all be colored by the magenta. 

 Chemists have found that this sac or ball-cover is made 

 of the same materials as the cells of wood. It is called 



