382 LIFE : OUTLINES OF GENERAL BIOLOGY 



of the protoplasm (autolysis). From all this, it will be seen that in 

 spite ol the ditficulties, in spite of our ignorance of the nature of 

 even artificial semi-permeable membranes, and the very much 

 greater complexity of behaviour of the protoplasmic films, the idea 

 of some kind of physical structure within the cytoplasm is not 

 easily discarded. It is possible that such structure may be diagram- 

 matically represented by a large number of minute and more or 

 less separate plasmic spheroids, each of which is a "film", with the 

 chemical, electrical, and permeability properties outlined in pre- 

 ceding sections (and possibly with the structure suggested by Clowes) ; 

 each may surround a mitochondrion or other "inclusion"; and all 

 are floating or flowing in a continuous medium of more homo- 

 geneous and more viscous protoplasm; while the whole is bounded 

 by the more substantial plasmic membrane of the surface of the cell. 



Cell Materials. — The main chemical elements present in living 

 matter are carbon, hydrogen, oxygen, nitrogen, sulphur, phos- 

 phorus, and iron; but many more may be present, often in mere 

 traces, including such unlikely things as arsenic, fluorine, and zinc. 

 The important fact is that very common elements are formed into 

 the physical basis of life; the curious fact is that rare elements 

 sometimes occur in small quantities. In illustration of these rarities 

 argon may be mentioned, the gas that Lord Rayleigh and Sir 

 William Ramsay discovered about the beginning of this century 

 in the atmosphere. In a paper published in the Comptes Rendus for 

 August 1925, it is reported by Pictet, Scherrer, and Heifer that 

 argon is present in many kinds of living cells. The investigators 

 found it in active yeast-cells, in the fresh cells of a sheep's brain, 

 and in the newly drawn blood of a bullock. They seem to have 

 satisfied themselves that the argon is present in the cells as an 

 integral part of their substance, and not simply hanging on by 

 superficial adsorption. Yet till compounds of argon are known, 

 the probability remains that it occurs as an included gas in the 

 living cells. 



But the catalogue of elements found in living matter helps very 

 little towards an understanding of the chemistry, for that depends 

 essentially on the complex organic molecules. Of these the most 

 important are the proteins, which are regarded as the most essential 

 and the most specialised of the identifiable constituents of proto- 

 plasm. The molecular weight maybe 33,000. or se\eral times this; 

 and though formuhe may be given indicating percentage composi- 

 tion, such as C55H7N,80,4S3, the construction of these giant molecules 

 remains unravelled. It is known, however, that they consist mainly 

 of large numbers of molecules of amino-acids, all linked together. 



The amino-acids, of which there are about twenty common kinds, 

 form a very heterogeneous group of substances. They all contain 



