8 HISTOLOGY 



the entire cell is devoid of any membrane. The cell membrane, therefore, 

 is not an essential part of a cell; if present it ranges from a thin pellicle, 

 on the border line of visibility, to a well-defined wall, which may be formed 

 as a secretion of the underlying protoplasm. If the several surfaces 

 of the cell are in relation to different environments, there is often a corre- 

 sponding difference in the structure of their walls. 



In examining a group of cells, it will be important to determine whether 

 they are merely in contact, or actually continuous. Sometimes cells 

 are so completely fused that their nuclei are irregularly distributed through 

 a single mass of protoplasm. Such a formation is a syncytium in which 

 the position of the nuclei is the only means of estimating the territory of a 

 single cell. A syncytium may arise from the fusion of cells, or, as in stri- 

 ated muscle fibers, it may be due to the multiplication of nuclei in an un- 

 divided mass of protoplasm. Instead of being completely fused, cells are 

 often joined to one another by protoplasmic processes of varying length 

 and width, thus forming cellular networks. Fibrils within such a syncy- 

 tium may pass continuously from the protoplasm of one cell into that of 

 another. 



Although cell membranes are often inconspicuous in animal cells, they cannot be 

 overlooked in plants. Thus cork is a mass of dead cells from which nuclei and proto- 

 plasm have disappeared, leaving only the cell walls. In describing cork, Robert 

 Hooke introduced the name "cell," in 1664. He wrote: "I took a good clear piece of 

 Cork and with a Pen-knife sharpen'd as keen as a Razor, I cut a piece of it off, and 

 thereby left the surface of it exceeding smooth, then examining it very diligently with 

 a Microscope, me thought I could perceive it to appear 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 introduced. 



FORM AND SIZE OF CELLS. 



Cells are regarded as primarily 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 

 shape. 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 



