268 Cell as Basis of Organic Activity 



vidiial chromonema (sing, of chromonemata) appear to be connected 

 by even finer filaments known as linin. These particular structures 

 are of extreme interest due to the fact that at the time of cell division, 

 they form the chromosomes upon which the hereditary factors are 

 carried. The hereditary factors are in the chromatin. The chromo- 

 somes are probably the most studied of all parts of the cells due to 

 this importance. The individual chromosome may, in general, be de- 

 scribed as a rod-shaped, highly staining structure, which is capable 

 of reproducing itself. 



The nucleolus is a small refractive body which is found in most 

 animal cells ; at times, there are even several nucleoli within a single 

 nucleus. Their exact function in the cell has been very difficult to 

 determine. The confusion arises from the fact that the nucleolus dis- 

 appears soon after cell division starts and the chromosomes form ; it 

 reappears as the chromosomes disappear. From this relationship, it has 

 been suggested that the nucleolus is a reserve of material which is 

 added to the chromonemata to form the chromosomes. 



The protoplasm of the nucleus which is termed nucleoplasm is 

 separated from the cytoplasm of the remainder of the cell by the nuclear 

 membrane. While this membrane is a definite structure, its exact 

 nature is not clearly understood. 



The Cyloplasm. — All the protoplasm of the cell outside the 

 nucleus is termed cytoplasm. It appears granular in structure and 

 usually is less dense than the nucleus. Within the cytoplasm are many 

 structures which can be demonstrated by proper fixation and staining. 



In all cells there is a varying amount of material which is collec- 

 tively known as cellular inclusions. These are made up of fat drop- 

 lets, crystalline salts, protein bodies, and other materials. In egg 

 cells these inclusions of stored food assume an important role for the 

 nourishment of the embryo. The single-celled bird's egg has the 

 major part of its bulk formed of these stored materials. 



Within the cytoplasm there are often small cavities, vacuoles, which 

 are filled with a less dense material. They are separated from the cyto- 

 plasm by a membrane. They are very large in plant cells, and often 

 completely lacking in animal cells. Scattered throughout the cytoplasm 

 are many small dark-staining bodies which are either filamentous, granu- 

 lar, or rod shaped. These are the mitochondria or chondriosomes. 

 Their exact function is not clearly understood, but it has been shown 

 that they carry certain enzymes and may act to aid in the concentration 



