Resting Nucleus 5 



division. This structure, characteristic of animal cells, is also 

 found in some of the lower plants. The centrosome consists of 

 a minute granule, the centriole, surrounded by a small mass 

 of protoplasm, the centrosphere ; the protoplasm of the centro- 

 sphere is often denser than the surrounding cytoplasm. During 

 some stages of division, star-like radiations extend outward from 

 the centrosome into the cytoplasm, forming the aster. 



Another structure characteristic of animal cells is the Golgi 

 apparatus. It is found in the cytoplasm and frequently appears 

 to be a system of connecting canals, but it may sometimes have 

 a more dispersed aspect. Its function is unknown and, although 

 it is characteristic of animal cells, it may, according to some 

 botanists, also be present in some plant cells. No cellulose 

 wall is present in animal cells. 



Resting Nucleus 



For a geneticist, the most important part of a cell is the 

 nucleus, because in the nucleus are found the genes which de- 

 termine the characteristics of the organism. 



The nucleus is separated from the cytoplasm by a definite 

 membrane, the nuclear membrane. The reality of this structure 

 has been shown by microdissection studies. There is good evi- 

 dence that this membrane is differentially permeable, as is the 

 plasma membrane. If so, the substances to which it is im- 

 permeable may be very different from those which will not pass 

 through the plasma membrane. 



The structures inside the nuclear membrane are not easily 

 observed in the living condition. Living nuclei generally appear 

 clear and homogeneous, but sometimes seem to consist of many 

 fine granules. Discerning definite structures in the nucleus is 

 difficult because, while alive, most of the structures of a cell 

 are colorless and have almost the same indices of refraction. 

 Also the threads which we know to be present in the resting 

 nucleus are extremely fine and attenuated and are, therefore, 

 more difficult to see than during division stages, when they are 

 many times thicker. 



The structures of the nucleus are best observed if the cell is 

 killed, fixed, and stained. By ''fixing" is meant treating the 

 cell with certain chemicals that not only kill it but also preserve 

 the cell structures in a condition resembling a living cell. A cell 



