FORM AND SIZE OF CELLS. 7 



Although cell membranes are usually lacking, or if present are often in- 

 conspicuous in animal cells, they are highly developed in plants. Thus cork 

 is a mass of dead cells from which nuclei and protoplasm have disappeared, 

 leaving only the cell walls. In describing cork, Robert Hooke introduced the 

 name "cell," in 1667. He wrote: "I took a good clear piece of Cork and with a 

 Pen-knife sharpened as keen as a razor, I cut a piece of it off and thereby left 

 the surface of it exceedingly smooth, then examining it very diligently with a 

 microscope, me thought I could perceive it to be a little porous. . . . These 



pores or cells were not very deep but consisted of a great many little Boxes ." 



In this way one of the briefest and most important of scientific terms was intro- 

 duced. 



FORM AND SIZE OF CELLS. 



Cells are regarded as typically spherical in form. Spherical cells 

 are comparatively numerous in the embryo, and in the adult the resting 

 white blood corpuscles which float freely in the body fluids assume this 

 form. Such cells are circular in cross section. When spherical cells 

 are subjected to the pressure of similar neighboring cells they become 

 polyhedral and usually appear six-sided in cross section. Such cells, as 

 a whole, may be cuboidal, columnar, or flat. Certain cells become fusi- 

 form (spindle-shaped) or are further elongated so as to form fibers; others 

 send out radiating processes and are called stellate. Thus the form of 

 cells is extremely varied. The shape of the nucleus tends to correspond 

 with that of its cell. It is usually an elliptical body in elongated cells, 

 and spherical in round or cuboidal cells. In stellate cells it is either 

 spherical or somewhat elongated. Crescentic nuclei and others more deeply 

 and irregularly lobed are found in some of the white blood corpuscles 

 and in giant cells. 



The size of cells ranges from that of the yolks of birds' eggs 

 which are single cells at least shortly before being laid down to micro- 

 scopic structures four thousandths of a millimeter in diameter. The 

 thousandth of a millimeter is the unit employed in microscopic measure- 

 ments. It is called a micron, and its symbol is the Greek letter ( . The 

 small cells referred to are therefore four microns, 4 jn, in diameter. The 

 size of any structure in a section of human tissue may be roughly estimated 

 by comparing its dimensions with the diameter of a red blood corpuscle 

 found in the same section. These red corpuscles are quite uniformly 

 7.5 >t in diameter. 



VITAL PHENOMENA. 



The vital properties of cells are more fully treated in text-books of 

 physiology. They include the phenomena of irritability, metabolism, 

 contractility, conductivity, and reproduction. Under irritability may 



