20 ZOOLOGY. 



the living protoplasm (plasma) outside the nucleus, and the 

 more fluid cell-sap (chylema) which includes such non-living 

 materials as starch, fats, and inorganic matter dissolved in 

 water. 



35. The Nucleus. The usually single nucleus lies im- 

 bedded in the cytoplasm and is ordinarily separated from it 

 by a thin membrane. Nuclei vary greatly in shape, size, and 

 degree of differentiation. While it is not always possible to 

 find definite nuclei in all cells, it seems probable that all cells 

 have nuclear material in one form or another at some stage 

 of their history. The internal structure of the nucleus is 

 equally as complex as that of the cytoplasm, having both liv- 

 ing and non-living portions. It usually consists of a network 

 of threads (chromatin) readily stained by certain dyes. In 

 the meshes of this a less easily stainable material occurs 

 (achromatin) , a portion at least of which is living. One or 

 more deeply stainable bodies, called nucleoli, usually occur, 

 the real character of which is difficult to estimate. 



36. Centrosomes or Centrospheres. These bodies lie in 

 the cytoplasm but are closely related to the nucleus, and ap- 

 pear to have an important place in certain phases of cell act- 

 ivity (see "cell division," 40). 



At such times the cytoplasmic elements radiate from the 

 centrosomes in a very characteristic way (Fig. 7, c). The 

 influence extends into the nucleus and is accompanied by a 

 rearrangement of the chromatic elements. The origin of the 

 centrosomes is still a matter of disagreement. They are often 

 spoken of as attraction spheres from the fact that they ap- 

 pear to exert an attractive influence upon certain portions of 

 the protoplasm. 



37. Cell-wall. A cell membrane usually surrounds the 

 protoplasm. It may be a non-living organic secretion, or may 

 consist of metamorphosed or altered protoplasm in connection 

 with such secretion. The wall is protective and supportive 

 in function, and varies much in thickness, resistance, etc. 



