THE ANIMAL CELL 21 



up of a more dense colloidal portion which coagulates readily 

 with certain reagents, and the more fluid cell sap which is com- 

 posed of water with sugar, inorganic matter, and other sub- 

 stance? dissolved in it. Suspended in the protoplasm as a part 

 of its formed substance may be starch grains, fats, crystals, and 

 certain distinct, active bodies known as plastids, mitochondria, etc. 



36. 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 living 

 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 active. One or 

 more deeply stainable bodies, called nucleoli and containing 

 accumulations of chromatin, usually occur, the real significance 

 of which is difficult to estimate. 



37. Centrosomes or Centrospheres. — These bodies when 

 present lie in the cytoplasm but are closely related to the 

 nucleus, and are associated with certain phases of cell activity 

 (see "cell division," §41). The centrosphere is a somewhat 

 clear mass about the centrosomes. 



At certain times the cytoplasmic elements radiate from the 

 centrosomes in a very characteristic way (Fig. 8, c). These 

 extend even into the nucleus and are associated with a rear- 

 rangement of' the chromatic elements. Because of this the 

 centrosphere is often spoken of as an attraction sphere. The 

 origin of the centrosomes is still a matter of disagreement. 



38. Cell Wall. — A cell wall usually surrounds the proto- 

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

 sist 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. 



