12 CYTOLOGICAL TECHNIQUE 



endoplasm is found especially in free-living Protozoa and in 

 certain other cells that are directly exposed to a non-cellular 

 environment. It is far from being characteristic of cells in general. 

 It is unusual in those that are bounded by other cells, and does 

 not occur in spermatogonia. Yet it is thought likely that all cells 

 contain a cytoplasmic inclusion that would appear in electron- 

 micrographs as the 'endoplasmic' reticulum. 



The 'small particles' associated particularly with the name of 

 the American cytologist Palade^*" are far too minute to be seen 

 separately by the light-microscope. There is strong reason to 

 suppose that they contain a high proportion of RNA, in the form 

 of ribonucleoprotein, and this acidic substance has an affinity 

 for basic dyes (p. 90). Cytoplasm that is seen by light-microscopy 

 to have a special affinity for these dyes can often be shown 

 by electron-microscopy to possess small particles in particularly 

 great abundance. In spermatogonia the small particles are few 

 and the cytoplasm has little affinity for such dyes. 



It is not possible to tell exactly where the small particles were 

 in life, but in electron-micrographs many of them are usually 

 situated on the outer side of the endoplasmic reticulum, which 

 looks as though it has been peppered with them (fig. 2, B, sp). 

 In some kinds of cells, however, the small particles are very 

 numerous but separate from the reticulum. It is for this reason 

 that these two cytoplasmic inclusions, so often associated, must 

 be listed separately. 



It is wise to regard with a certain amount of scepticism the 

 information about cellular constituents provided by the electron- 

 microscope. Nevertheless, one cannot fail to be struck by the 

 regularity with which the endoplasmic reticulum and small 

 particles appear in micrographs. 



It remains to consider the ground cytoplasm. This term is used 

 in two distinct senses. In light-microscopy it means the whole of 

 the substance of the cell other than the visible inclusions. In this 

 wide sense it thus includes the endoplasmic reticulum and small 

 particles. When it is stated, for instance, that the refractive index 

 of the ground cytoplasm is commonly about 1-353^^^' ^^^ or that 

 its pH is generally between 6-4 and 7-2, the figures given must be 



