THE FINAL ADJUSTMENTS 131 



small granules known as microsomal particles. These 

 are too small to be individually visible with the light 

 microscope, but with the electron microscope they can 

 be seen as small dense granules lying in the cytoplasm. 

 They are often arranged on the surface of a number 

 of cytoplasmic membranes, which are referred to as the 

 ergastoplasm or endoplasmic reticulum. 



It is clear that, if the genes in the nucleus are to 

 determine the sequence into which amino acids will be 

 built into proteins in the microsomal particles, there 

 must be some "messenger" which passes from gene to 

 particle, carrying, as it were, the blueprint for the pro- 

 tein. A great deal of research at the present day is 

 devoted to the problem of these messengers. There is 

 considerable evidence that the messenger is also a 

 nucleic acid, but one of a rather different type than that 

 in the chromosomes. The nucleotide building-blocks 

 of the hereditary DNA contain a sugar known as 

 deoxyribose, but cells also contain another type of 

 nucleic acid, in which the building-blocks have the 

 sugar ribose. The nucleic acid in which this is incor- 

 porated is known as RNA for short. It seems very 

 likely that the messenger passing from gene to micro- 

 somal particle is actually a kind of RNA, although it 

 also seems to be true that other types of RNA play 

 other roles in the cellular processes. 



It will be seen from this account that we already 

 know a good deal about the processes by which genes 

 exert their influence during development. There are, 

 of course, still many details to be filled in, even in the 



