152 THE AMERICAN MONTHLY [Sept 



measuring from this center the angular distance of the 

 object above or below the longitudinal axis of the slide ; 

 and then meaenring with the ocular micrometer the 

 linear distance from the center of revolution. Thus an 

 object at the right and 60 deg. below the axis would be 

 5 o'clock, one at the left and 30 deg. above the axis 

 would be a 10 o'clock. By this means the object can be 

 easily located within a single degree ; but that is seldom, 

 if ever, necessary, as suflBcient accuracy for the cases to 

 which this method is applicable can be gained, almost 

 automatically after a little practice, by comparison with 

 the picture of the dial " in the mind's eye." 



A Description of Cells Proper. 



The cell is the unit of structure in all animal and vege- 

 table life. The lowest plants and animals consist of 

 single cells and are called unicellular. Most cells are so 

 small as to be seen only by the microscope for they meas- 

 ure from 1-10, 000th to 1- 125th of an inch in diameter. 

 They have to b^ dissected and stained for examination. 

 Some cells consist of protoplasm with a cell-wail and 

 others without a cell-wall. It is a viscid, nitrogenous 

 substance, often granular, but usually undistinguishable 

 from albumen or the uncoagulated white of an egg. 



The Onion. — A plant is made of cells differentiated 

 into parts such as roots, stems, leaves, and flowers. The 

 cell of an onion (figure A.) consists of a cell-wall, M, 

 protoplasm, N, and nucleus with nucleolus, 0. This is 

 one of the best examples of a typical cell. 



The Am(EBA. — Cells reproduce and multiply by self- 

 division and this may be best studied in the anHsba. 

 Nearly every stagnant pool contains water which ex- 

 amined under a rather high j)Ower will reveal a small, 

 moving, jelly-like speck (fig. B). It is composed of gran- 

 ular protoplasm containing nucleus and nucleolus which 



