266 THE AMERICAN MONTHLY [Aug. 



fact no two are exactly alike, but still the law of sym- 

 metry plainly dominates their structures. Cite eviden- 

 ces of this law of symmetry from animals or plants at 

 large. — Can you find anything comparable with it in 

 minerals ? 



PART II. — UNI-CELLULAR ANIMALS. 



7. Amoeba. — The properties of protoplasm and of the 

 cell can be best studied by taking up first the uni-cellu- 

 lar and simplest beings, though in many of them there 

 are specializations which must be excluded from our 

 general notion of either protoplasm or the cell. Amoeba 

 is found on the scum on the leaves of water plants, it 

 can often be found in water containing dying and de- 

 composing Spirogyra or other Algae, it must be exam- 

 ined with the high power. It is translucent, irregular 

 and changing in outline and faintly granular. A speci- 

 men should be kept under continuous observation for at 

 least an hour, the slide being moved to compensate for 

 its progression. First observe its changing outline, the 

 thrusting out of pseudopodia which are motile and some 

 of which increase while others diminish, the creature 

 flowing out into them. Make a series of drawings to 

 show the form at successive equal intervals of time. 

 Study and determine that the substance presents a 

 thinner clearer ectoplasin on the outside, and an inner 

 efidoplasm, the latter being occupied by variously shaped 

 objects, food vacuoles, some of which can perhaps be re- 

 ' cognized as microscopic plants which have been swal- 

 lowed to serve as food. You should also be able to dis- 

 tinguish in the endoplasm minute brightly shining /a* 

 droplets. Locate also the contractile vacuole, a clear 

 spherical space in the endoplasm, and watch to see that 

 it contracts and reappears in the same place at regular 

 intervals; determine the rythm. There is a nucleus in 

 the centre of the body, but it is not generally visible in 



