16 



GENERAL ZOOLOGY 



Yi G . 2. — Cells. A, two pigment cells from the skin of the salamander. B. 

 Paramecium, a ciliated protozoan, a single-celled animal, c, contractile vacuole, 

 expanding; e, contractile vacuole, expanded; f, food vacuoles; g, gullet, a primi- 

 tive alimentary tract; M, macronucleus; n, micronucleus. X 300. The con- 

 tractile vacuoles are filled with cell fluid which is emptied on the outside when 

 the vacuole contracts, thus maintaining an intracellular circulation. Food 

 vacuoles are simple organs of digestion. 



Fig. 3. — Endothelial cell from a tissue culture of the chick embryo, grown in 

 a culture medium on a glass cover-slip inverted over a hollow-ground slide; 

 stained to show mitochondria, m. X 1080. {From a photo by W. H. Lewis.) 



