ILLUSTRATIONS 



1 A model of a protein molecule 16 



2 An interference photograph of chromosomes in living 



cells 17 



3 Chromosomes in the nucleus of cells from the growing 



tip of an onion root 32 



4 The giant banded chromosomes of the saUvary glands of 



the larva of the fly 33 



5 A section of a rat Uver taken with the electron microscope 48 



6 A portion of a liver cell taken with the electron micro- 



scope at a higher magnification 49 



7 Fine structure in the cell interior 64 



8 Electron micrographs of Influenza A virus adhering to a 



red fowl cell 65 



9 (a) The eff*ect of a chemical substance (a nitrogen mustard) 



on the chromosomes of a tumour cell 80 

 (b) The eff'ect of X-rays on the chromosomes of a broad 



bean cell 80 



10 (a) Turnip yellow virus 81 

 (b) Two dissimilar bacterial cells (B. coli) in conjugation 81 



1 1 Bacteriophage attacking a bacterium 96 



12 A later stage of the attack of Bacteriophage 97 



13 A cross-section of a chloroplast from Aspidistra elatior 1 12 



14 Electron micrographs of muscle fibrils 113 



15 A nerve cell of the mammalian spinal chord and the 



associated nerve fibre 128 



16 Nerve cells in the visual cortex of the brain of a cat 129 



and 28 figures in text /f^^L ^ <)\ 



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