10 



HISTOLOGY 



FIG. 10. Cell from root cap of calla lily, pi., plastids. X 700. 



This is the centrosome (Fig. n). It consists of a number of radiating 

 fibers, which may be absent when the centrosome is quiet; they form 

 the aster. These fibers converge about a dense area in the cytoplasm, 



the attraction sphere 

 (Fig. n). Within the 

 attraction sphere there 

 are present one or 

 more, usually two, 

 minute bodies called 

 the centrioles. 



In a living cell, 

 the cytoplasm and 

 the karyoplasm or 

 nucleus are vitally 

 related. The nucleus 

 is influenced by the 

 cytoplasm. Different 

 parts of the cyto- 

 plasm, indeed, seem 

 to affect differently 

 the dividing nucleus in such a manner as to determine the orientation 

 or position of the nucleus, as has been observed by Lillie in the dividing 

 cells of Unio eggs. This gives rise to a definite axis in the cell, and it 

 is then said to have polarity. 

 Axes of cells are also deter- 

 mined by secretory struc- 

 tures. In epithelial cells of 

 excretory or secretory func- 

 tions, fibrils and excretion 

 particles appear. These and 

 the nucleus in such cells do 

 not lie promiscuously within 

 the cell, but they have a 

 definite arrangement. The 

 fibrils run from the free sur- 

 face toward the base of the 

 cell, and in many forms con- 

 verge to a common point. 

 The elaborated products ap- 

 pear at various levels of the 

 cell and move toward the free surface either bodily as granules, or in 

 invisible solutions. Thus a longitudinal axis is established for the cell. 

 With reference to this axis, cells may be radially or bilaterally sym- 



^centr. 



FIG. ii. Spermatogonium of salamander, Spelerpes 

 ruber, containing a centrosome. centr., centrioles; cen. 

 sp., centrosphere (composed of an inner and an outer 

 zone). 



