i] PROTOPLASM 5 



may thus be divided up into masses, sometimes distinct 

 and sometimes continuous, each of which is in relation with 

 one nucleus, and such a nucleated portion of protoplasm 

 constitutes a cell. The word protoplasm is commonly used 

 as a general designation for the various substances of which 

 both nucleus and cell-body consist, and which together con- 

 stitute "living matter," while the cell-body or extra-nuclear 

 protoplasm is distinguished as cytoplasm. Probably the 

 nuclear substance and certainly the cytoplasm undergo very 

 various modifications in different kinds of tissues, and the 

 cytoplasm frequently contains inclusions of various sorts 

 produced by its own activity, such as the yolk granules in 

 an egg, the fat globules in adipose cells, the secretion in 

 gland cells, etc. These various inclusions produced by the 

 cytoplasm, but not actually part of it, are often designated 

 collectively as " deutoplasmic " ; the word " metaplastic " 

 has also been used. Some cells also contain larger or smaller 

 cavities filled with watery fluid, known as vacuoles, which 

 play an important part in plant cells and in certain skeletal 

 tissues of animals. In a relatively unmodified embryonic 

 cell, however, the cytoplasm is a clear, semifluid, colourless 

 substance resembling egg-albumen, and it is to this sub- 

 stance that the word protoplasm in its stricter sense is 

 applied. 



Although the protoplasm of an unmodified cell may 

 appear homogeneous, it undoubtedly consists of a complex 

 mixture, and the question of its structure has given rise to 

 much controversy. In the practical study of cytology it is 

 generally necessary to examine structures after they have 

 been "fixed" and stained, that is to say, after the tissues 

 have been killed and coagulated by reagents, and stained 

 with dyes in order to differentiate the various parts which 

 in the living cell are barely distinguishable from one 



