INTRODUCTION AND HISTORICAL BACKGROUND 9 



an egg, and attempted to revive the older fluid theory by ascribing the 

 reticular appearance to precipitations at certain places and under 

 certain conditions. If this can be taken to imply a submicroscopic 

 fibrillar organization which "condenses" into a grosser structure 

 during coagulation, then this is a close approach to the modern view. 

 Towards the close of the nineteenth century this kind of treatment was 

 much in favour though occasional workers such as Altmann remained 

 to the last firm upholders of the older granular conception, an idea 

 which was in fact still upheld by Heilbrunn in 1926, The arguments 

 against such a proposition were always that the protoplasm would then 

 lack the structural organization which was realized, on physiological 

 grounds, to be essential, just as the argument against the reticular (or 

 alveolar hypothesis, depending on whether one thought the firmer or 

 the less firm phase to be the important one) were aimed at its im- 

 probable rigidity. Both sides undoubtedly went too far; protoplasm 

 certainly cannot conceivably be regarded as rigid in any sense, nor can 

 its behaviour be harmonized with that of normal non-Newtonian 

 liquids. 



Perhaps at this time, when so little was known of the behaviour of 

 non-Newtonian liquids, the fluid nature of protoplasm was over- 

 emphasized. It had already been found by Pfejfer (1890-1) that living 

 protoplasm has tensile strength, a property which can hardly be 

 reconciled with normal fluidity; and the very varied results of viscosity 

 measurements published during the present century are clearly in better 

 harmony with the fibriflar or reticular hypothesis than with any postulat- 

 ing a fundamentally fluid nature. Once it was accepted that the physio- 

 logically important substances in protoplasm are the proteins {Leathes, 

 1925, Pauli, 1922 and many others),* then analogies with substances 

 like gelatine could be drawn and the situation became a little clearer. 

 The way was at last open for an attack on the molecular structure, 

 not of the protoplasm itself, it is true, but of the proteins upon which 

 the variability of protoplasmic behaviour so clearly depends. It is not 

 possible here to detail the remarkable advances in recent years on 

 protein structure, but one or two points should properly be made since 

 we shall need them at various times in the following pages. 



The first successful interpretations of the X-ray diagrams of proteins 



were made by Astbury working with the keratin of wool. He showed for 



the first time that proteins could be considered as long molecular chains 



of amino-acid residues, folded in some particular configuration. With 



* We now suspect that the proteins are not distributed uniformly throughout the 

 protoplasm, and it is still possible that some regions in protoplasm must still be 

 regarded as permanently fluid. 



