20 ANIMAL BIOLOGY 



Nucleus. As already mentioned, within the cytoplasmic mass 

 there is an area of clearly differentiated material which typically 

 has a rounded form, bounded by a membrane, so that it appears 

 as a definite body of protoplasm called the nucleus. The struc- 

 tural basis of the nucleus consists of a homogeneous ground-sub- 

 stance, or karyolymph, which is permeated by a meshwork that 

 usually appears to consist of two substances, linin and chromatin, 

 which are probably chemically closely related. Chromatin is the 

 highly characteristic nuclear material which takes various forms 

 during different phases of cell activity but generally gives the ap- 

 pearance of a network of tiny granules with one or more dense 

 'knots' of chromatin. Frequently there are one or more conspicu- 

 ous, round bodies within the nucleus known as nucleoli. Later it 

 will be necessary to describe some of the important changes in 

 chromatin arrangement that occur during various phases of cell 

 activity, especially during cell division, but it is sufficient at this 

 moment to emphasize that the nucleus is a differentiated area 

 of the cell protoplasm which is the arena of the chromatin. 

 Indeed, the nucleus probably represents the highest type of or- 

 ganization in the organism. (Fig. 164.) 



2. Chemical Composition 



It is impossible to make an analysis of living matter because the 

 disturbance of its molecular organization by chemical reagents 

 kills it. Therefore our knowledge of its chemical composition has 

 of necessity been derived from a study of dead protoplasm. How- 

 ever, since in the transformation from the living to the non- 

 living state there is clearly no loss of weight, it follows that the 

 complete material basis of life is still present for examination. 

 In other words, the death of protoplasm is a result of disor- 

 ganization. 



Chemical analysis of protoplasm shows that it invariably com- 

 prises the elements oxygen, carron, hydrogen, nitrogen, 



PHOSPHORUS, SULFUR, CALCIUM, MAGNESIUM, SODIUM, POTASSIUM, 



iron, and chlorine. Probably other elements are always present; 

 certainly some others are found normally in the protoplasm of 

 certain parts of various species of animals and plants. Thus in 

 addition to the elements just mentioned which form by far the 

 larger part of the human body, there are also present traces of sev- 

 eral other elements, such as iodine, copper, manganese, and fluorine. 



