SYSTEMS DISPLAYING PROTEIN SYNTHESIS 117 



very little reproduction of genes. During this period the deoxy 

 nucleic acids of the oocyte nucleus are scanty in amount, rela- 

 tive to the size of the cell, whereas pentose nucleic acids are 

 plentiful in the cytoplasm both of the oocyte and of the nurse 

 cells, as has been shown by Brachet. When the oocyte has 

 reached its maximum size, the cytoplasmic nucleic acid may di- 

 minish. After fertilisation there ensues a period of rapid cell 

 division, and therefore of gene synthesis, with little change in 

 cytoplasmic protein: during this period the total nuclear deoxy- 

 nucleic acid increases rapidly. When the individual organs are 

 forming, the cytoplasm and nuclei are both rich in nucleic acid 

 during the phase of multiplication of cells and of cell growth. 

 But once the organ is established the cytoplasmic nucleic acid 

 may fall sharply and is high only in those cells which produce 

 large amounts of protein, and in these cells only at the time of 

 protein production. All these observations are consonant with 

 Caspersson's views. 



So far as investigation has gone, observations on plant growth 

 fit into the same pattern. In the growing root tip, the cells which 

 are multiplying rapidly are much richer in deoxynucleic acids 

 than are the other cells. The cells in which cytoplasmic protein 

 is being formed rapidly are rich in pentose nucleic acids, whereas 

 those cells which are not producing protein may be almost devoid 

 of the pentose acids. In yeasts and bacteria the situation is the 

 same: high protein production is associated with a high concen- 

 tration of pentose nucleic acids, whereas resting phases contain 

 little pentose nucleic acid. 



Rapidly growing tumours have cells whose cytoplasm is rich in 

 pentose nucleic acids; under the action of growth-restricting 

 treatment, the cytoplasmic nucleic acid falls rapidly. 



When the different cells composing a protein-secreting organ 

 are examined, the same pattern emerges. Those cells which are 

 vigorously secreting protein are rich in nucleic acid, and the re- 

 mainder are not. Thus the exocrine cells of the pancreas are rich, 

 the endocrine cells are poor. In the gastric mucosa the HC1- 

 secreting cells are poor in nucleic acids, and the protein-secret- 

 ing cells are rich. Amongst the richest cells of all in pentose 

 nucleic acid are the motor neurons studied by Hyden (1947). 

 When vigorously stimulated, nerve cells may lose two-thirds of 

 their protein : in a subsequent resting phase this protein is rapidly 



