THE CELL AND CELL DIVISION 41 



cytoplasm outside the definite nucleus. These are usually 

 small granules and bits of chromatin of varied form and 

 significance in different cells. Collectively they are termed 

 chromidia; for the most part they are not formed in situ, but 

 are derived directly from the nucleus (Fig. 18). They are most 

 frequent in very active cells such as gland cells, the rapidly 

 growing germ cells, and many others. Many forms of chro- 

 midia, functionally as well as structurally distinct, have been 

 given special names such as chondriosomes, " Chondromiten," 

 idiochromidia, mitochondria, pseudochromosomes } " Nebenkern," 

 "Trophospongien," etc. 



It has been said already that no single cell shows typically 

 all the structures described above and illustrated in Fig. 16. 

 Without stopping for further description many of the details 

 of structure truly characteristic of a few varied types of cells 

 are shown for comparison in Figs. 14, 15. (For descriptions and 

 further details the student should consult the standard texts 

 of Cytology and Histology, e.g., Dahlgren and Kepner's 

 "Principles of Animal Histology," New York, 1908.) 



One further aspect of cell structure remains to be mentioned. 

 This is the important fact that there is in most tissues a fairly 

 definite cell form combined with a nearly constant arrangement 

 of the cell organs and contents (Van Beneden, Rabl, Heidenhain). 

 In any epithelial cell the different physiological conditions at 

 the free and the attached surfaces lead to this definite relation 

 of cell structures which is called polarity. In such cells the 

 nucleus lies toward the basal end of the cell, the centrosome 

 either toward the free end or on that side of the nucleus. Thus 

 an imaginary axis may be passed through the centrosome and 

 nucleus perpendicular to the free surface, about which the 

 cell organs are arranged symmetrically, either bilaterally or 

 radially (rotatorially). This polarity extends also to other 

 less constant structures and even occasionally to non-epithelial 

 cells. It may be seen for example in the arrangement of cilia/ 

 cuticle, conductile and contractile fibers, granules or drops of 

 substances being excreted or secreted, and yolk or other stored 

 materials (Figs. 14, 15, 16, etc.). In the germ cells, as we shall 



