THE SIMPLEST FORMS OF LIFE 



39 



of the cell is occupied by colorless protoplasm, and 

 near the outside is a bright red pigment-mass the 

 " eye-spot" (<?), near which can generally be detected 

 one or two pulsating or 

 contractile vacuoles, 

 such as frequently oc- 

 cur in the lower uni- 

 cellular animals and in 

 the slime-moulds. 



If these free-swim- 

 ming green cells are 

 placed in a glass vessel 

 full of water, and placed 

 where they are more 

 strongly illuminated 

 from one side, as for 

 example in a window, 

 it will be found that 

 very soon they collect 

 on the lighted side, and, 

 if they are present in 

 large numbers, may be 

 seen to form a green 

 line close to the side 

 where the light is 

 strongest. There is rea- 

 son to suppose that the 

 red eye-spot is in some 

 way connected with 

 this sensitiveness to 



light, as it is nearly always present in those motile 

 green cells which show sensitiveness to light, and is 



FIG. 6 (Volvocacese). A, a plant of 

 Pleodorina Californica, showing 

 the ciliated cells of which it is com- 

 posed ; the arrow shows the direction 

 in which it moves; B, one of the 

 smaller cells, much enlarged, showing 

 the two long cilia, c, the eye-spot, e, 

 the nucleus, n, the pyrenoid, p, im- 

 bedded in the cup-shaped chloroplast, 

 cl ; C, three stages in the division of 

 one of the large cells ; D, the egg ; E, 

 spermatozoid of Volvox ; 'F, two ga- 

 metes of Pandorina fusing together 

 to form the zygote, or resting-spore. 

 (Figs. B, C, after Shaw; D, E, after 

 Overtoil; F after Pringsheim.) 



