THE STRUCTURE OF THE CELL 9 



cytoplasm, mostly in the vicinity of the idiozome (but never 

 within it). Nearly all cells contain lipid globules in the cytoplasm, 

 but they have been greatly overlooked, probably because they 

 are not seen in routine paraffin sections. These globules may con- 

 tain triglyceride, but usually consist mainly of more complex 

 lipids, especially phospholipids and cerebrosides. Their refractive 

 index is often low, and this suggests that they must contain water 

 associated with their phospholipid content. ^^^ Sometimes a lipid 

 cortex surrounds a non-lipid, probably aqueous core, but this 

 structure is perhaps rarer than superficial appearances would 

 suggest. ^8 



The significance of lipid globules is obscure. Adipose fat 

 accumulates mainly in particular cells set aside for the purpose, 

 but nearly every cell contains globules of more complex lipids 

 that are not used up when the animal is starved. The lipids of 

 these globules are often highly unsaturated and therefore reduce 

 osmium tetroxide and silver nitrate easily. 



Certain cytoplasmic inclusions are built on such a small 

 scale that the light-microscope cannot reveal their shape or 

 structure. We are forced to rely on the electron-microscope. 

 Since this instrument can only be applied to tissues that have been 

 completely dried, it is not possible to examine these inclusions 

 during the life of the cell. Fixed preparations must be used. A re- 

 action occurs between what was present in life on one hand, and 

 the fixative and other substances used in making the preparation 

 on the other. What is studied under the electron-microscope is the 

 reaction-product. 2" We lack the means of verification that are 

 available with all objects that are large enough to be studied 

 with the light-microscope during the life of the cell. 



An electron-micrograph is generally regarded as 'good' if it 

 shows much minute detail, or if the constituent parts appear in 

 strong contrast with one another; but we have no means of 

 knowing whether the details represent the relative positions of 

 the parts in the living cell, nor do we know whether these parts 

 were sharply separated from one another. 



The only way in which we can learn to interpret electron- 

 micrographs is by the study of known substances (especially 



