chap, i.] CELLS. 5 



give origin to material other than protoplasm e.g., 

 collagenous, osseous, elastic, and other substances. 



o. Beginning with the ovum, and ending with the 

 protoplasmic nucleated elements found in the organs 

 and tissues of the embryo and adult, we have, then, 

 one uninterrupted series of generations of elements, 

 which with Schwann we call cells and with Briicke 

 elementary organisms. Of these it can be said that 

 not only is each of them derived from a cell (Yirchow : 

 omnis cellula a cellula), but each consists of the 

 protoplasm of Max Schultze (Sarcode of Dujardin), 



Fig. 4. Amoeboid movement of a White Blood Corpuscle of Man ; 

 various phases of movement. (Handbook.) 



is without any investing membrane, and includes 

 generally one nucleus, but may contain two or more. 

 We can further say that each of these cells shows the 

 phenomenon of growth, which presupposes nutrition, 

 and reproduction. All of them in an early stage of 

 their life history, and some of them throughout it, 

 show the phenomenon of contractility, or amoeboid 

 movement (Fig. 4.) 



Cells differ in shape according to kind, locality, 

 and function, being spherical, irregular, polygonal, 

 ^squamous, branched, spindle-shaped, cylindrical, pris- 

 'matic, or conical. These various shapes will be more 

 fully described when dealing in detail with the various 

 kinds of cells. Cells in man and mammals differ in 

 size within considerable limits : from the size of a 

 small white blood corpuscle of about g-sVo of an inch 

 to that of a large ganglion cell in the anterior horns of 

 the spinal cord of about -^^ of an inch, or to that of a 



