6 MANUAL OF ANIMAL BIOLOGY 



2. Nucleus 



The protoplasm of the nucleus, which we may term karyo- 

 plasm in contradistinction to the surrounding cytoplasm, is en- 

 closed in a definite nuclear membrane except when the cell is 

 undergoing reproduction by division. The structure of the karyo- 

 plasm is extraordinarily complex and varied. Typically a delicate 

 network, or linin, is found throughout the nucleus. The most 

 prominent feature of the karyoplasm, however, is the chromatin 

 which derives its name from the fact that it stains very deeply with 

 certain dyes. In a resting cell, the chromatin appears as a more or 

 less granular material which may be condensed into one or more 

 knots, the karyosomes. In a dividing cell, the chromatin forms a 

 long tangled thread, the spireme, which later breaks up into a 

 definite number of bodies, the chromosomes. It is hard to over- 

 estimate the importance of the chromatin for there is a great 

 abundance of evidence to show that it is the chief vehicle for the 

 transmission of hereditary characters from one generation to 

 another. Mention should also be made of a tiny, spherical body, 

 the nucleolus, which is frequently present in a nucleus. Its 

 function is quite obscure. (W. f. 10.) 



3. Cell Wall 



An outer limiting membrane of some type, which forms the cell 

 boundary, is always present. In some types of cells this enclos- 

 ing membrane consists only of a slightly modified region of the 

 peripheral cytoplasm, known as the plasma membrane. Usually, 

 however, there is also present a definite outer cell wall, of varying 

 thickness, which is formed as a non-living secretion of the under- 

 lying cytoplasm so that the cytoplasm of the cell is doubly enclosed. 

 In plant cells, the cell wall is particularly prominent and is largely 

 composed of cellulose, which is an extraordinarily abundant and 

 important organic compound similar to starch in its chemical con- 

 stitution ; while in animals cells, the cell wall is usually of a protein 

 nature. (W. fs. 7, 12.) 



Whatever the exact structure and the degree of development of 

 the plasma membranes and cell walls in various types of cells, the 

 really essential fact is that they are all semi-permeable membranes, 

 that is, they are of such a nature as to permit the necessary and 

 continuous interchange of certain substances between a living cell 



