THE STRUCTURE OF THE CELL 13 



taken to represent a summation of the refractive index and pH 

 of the substance, taken as a whole, that lies between the micro- 

 scopically-visible inclusions. In the strict sense, however, the 

 term 'ground cytoplasm' must apply only to the material in which 

 the submicroscopical inclusions are embedded (fig. 2, A). In this 

 sense, then, it excludes the endoplasmic reticulum and small 

 particles. Even when the reticulum is bulky and the particles 

 numerous, a large proportion of the cell is occupied by the 

 ground cytoplasm in the strict sense. It is regrettable that we 

 should know so little of this cellular component. In electron- 

 micrographs that are considered 'good', the space in question is 

 often criss-crossed by very fine fibres. These are thought to be 

 formed of protein. We can scarcely be far wrong in supposing 

 that the ground cytoplasm in the strict sense consists mainly of 

 water and protein, associated with one another through the 

 hydrophil groups of the protein (p. 20). The latter may be 

 supposed to exist partly as long, extended polypeptide chains 

 (p. 19), linked to one another here and there to form a weak gel, 

 partly as globules consisting of such chains wound up into 

 skeins. ^° The water must hold some of the cytoplasmic ions in 

 solution (principally potassium and phosphate, with some mag- 

 nesium and bicarbonate, and other ions in lesser amount). 



Since this book is concerned mainly with light-microscopy, the 

 expression 'ground cytoplasm' is to be understood in the wide 

 sense, except where the contrary is distinctly indicated. 



