20 CELL ORGANS [CH. 



takes place. The subject will be further mentioned at a later 

 stage. 



Although it is no part of the purpose of this book to deal 

 with the problems of cell-chemistry, it may be appropriate 

 to devote a short space to the chemical constitution of the 

 various parts of the nucleus and of the cell in general. 

 Protoplasm, in the wide sense of the word, consists of a 

 mixture of proteid or albuminous substances of complex 

 chemical constitution, consisting of carbon, hydrogen, oxy- 

 gen, nitrogen and a small amount of sulphur, phosphorus 

 and usually iron. The cytoplasm is characterised especially 

 by albuminous substances containing little or no phosphorus, 

 while the nucleus contains a relatively large amount of this 

 element, especially in the chromatin. Chromatin is a com- 

 pound of a substance known as nucleinic acid, which con- 

 tains up to 10 per cent, of phosphorus, with an albumin 

 which is without phosphorus, or in some conditions it is 

 possible that chromatin may consist of uncombined nu- 

 cleinic acid. The non-chromatic granules and plasmosomes 

 of the nucleus have a larger proportion of albumin and less 

 nucleinic acid ; so that they are relatively poor in phosphorus 

 as compared with chromatin. 



These facts give some clue to the staining reactions of the 

 various parts of the cell, and also to the changes undergone 

 by the nucleus in its various stages of growth. Nucleinic 

 acid combines with basic stains, whence the affinity of these 

 stains for chromatin, while acid stains are taken up especi- 

 ally by the parts of the cell rich in albumin and poor in 

 nucleinic acid, such as the plasmosome and oxyphil granules 

 of the nucleus, and the cytoplasm. During its various 

 phases the staining reactions of the nucleus vary very 

 largely, and this is probably due to the greater or less 

 amount of combination of albumin with the nucleinic acid 



