CHAPTER NINE 



THE CHROMOSOMES 



Cytoplasm I. THE cytoplasm or substance of the cell incloses a 

 and nucleus re i a ti ve jy small body known as the nucleus. In some 

 cases there is more than one nucleus, and in certain 

 special kinds of cells, as the red corpuscles of the blood, 

 the bacteria, and very few Protozoa, no nucleus appears. 

 When the nucleus is absent, it is possible, at least in 

 some cases, that the nuclear matter (nucleoplasm) is 

 diffused through the cell. Ordinarily the nucleus is a 

 small, rounded body near the center of the cell, which 

 is much more deeply colored with certain stains than 

 the cytoplasm, so that microscopic preparations of ani- 

 mal tissues often appear to be minutely speckled. The 

 spermatozoon, or sperm cell, the contribution of the 

 male to the process of fertilization, carries very little 

 cytoplasm. Being motile, having to seek the egg cell, 

 it cannot afford to be burdened with anything not 

 necessary for its peculiar function. The nuclear matter 

 is present, and can be shown to consist of materials 

 similar to those in the relatively gigantic egg cell. 

 Chromatin 2 - From 'its universal presence in ordinary cells, and 

 andchromo- t h e f act t ] iat a pi ece o f cytoplasm cut from a cell, if 



somes . . J 



containing no nucleus, dies, we assume that the nucleus 

 is of special importance for life. On examining more 

 closely, we find a kind of material in the nucleus which 

 stains most readily, known as chromatin. This chro- 

 matin, when cells are dividing, is seen to collect in small 

 bodies, usually more or less rod-like or thread-like, 

 known as chromosomes. The words "chromatin" and 

 "chromosome" imply the presence of color, and are 

 misleading, since the material is colored only when 

 artificially stained. 



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