130 HOW ANIMALS DEVELOP 



On the other hand, the proteins which are produced 

 under the influence of this DNA are not manufactured 

 in the nucleus, but in the cytoplasm. Evidence of this 

 has been gained largely by the use of radio isotopes. 

 One can synthesise in the laboratory an amino acid 

 which contains a radioactive atom, for instance, radio- 

 active carbon. This "labelled" amino acid can be put 

 into the medium surrounding a cell which is engaged 

 in manufacturing protein. After a time the cell can 

 be killed and sectioned and the sections covered with 

 a photographic emulsion. At any place at which the 

 labelled amino acid has been built into protein it will 

 continue to give off ionising radiations, which will pro- 

 duce a blackening in the photographic film above it. 

 In this way one can determine the sites at which the 

 protein is being produced. It turns out that only in the 

 earliest embryonic cells is the major site of protein 

 synthesis in the nucleus. In slightly later cells, which 

 are engaged in making the main proteins which will 

 build up, for instance, a muscle fibre or the substance 

 of a nerve cell, the labelled amino acids are incorpo- 

 rated into protein first in the cytoplasm. The actual 

 place at which they are formed can be located even 

 more precisely by other methods. If one grinds up a 

 cell and centrifuges it, first at low speed to remove the 

 larger debris, and then at higher and higher speeds, 

 one can isolate a series of fractions, each fraction con- 

 sisting of particles of roughly the same size. It turns 

 out that the fraction that contains the greater part of 

 the labelled amino acid consists of a group of very 



