PROTOPLASM \ 7 



deeply ("Nissl bodies"), but which afterwards disappears almost entirely. Whether this 

 something, which appears and disappears, was originally present in the cells as aggregated 

 masses, or uniformly diffused through the cell substance, we cannot tell. From the usual 

 coagulating action of fixatives, especially from the separation of albumose and serum albumin 

 in A. Fischer's experiments described above (page 14), the latter view is the more probable. 

 Mott (1912) and Marinesco (1912, see page 470 below), in fact, have shown, by observations on 

 living nerve cells under dark ground illumination, that there are neither Nissl bodies 

 nor neurofibrils in the living state. Fine colloidal particles of a special nature are 

 to be seen, but the protoplasm appears to have the uniform general nature of an "organised 

 hydrosol. " 



We may also justifiably assume that, when we find structures in the same 

 organ or cell, stained in different colours by a method of double staining, there 

 is some difference between them, although not necessarily of a chemical nature, 

 and the respective structures were probably of quite a different appeal ance 

 during life. 



DEHYDRATION AT LOW TEMPERATURES 



A method has been introduced by Altmann (1894, pp. 27-29) which seems 

 to offer possibilities for the investigation of the structure of cells without the 

 use of fixing reagents. If a piece of tissue be allowed to dry at ordinary 

 temperature, it is well known that it becomes so hard and horny that it is 

 impossible to cut thin sections from it. And, even if this were possible, the 

 structures would be altogether distorted. On the other hand, if dried over 

 phosphorus pentoxide in vacuo, at a temperature so low that the salts of the 

 tissue freeze out together with the water (forming a "eutectic " mixture see the 

 book by Nernst, 1911, p. 121), the cells are never exposed to the action of 

 saturated salt solutions, which are formed when the tissue is dried at ordinary 

 temperatures. A temperature of - 40 to - 30 C. is found to be low enough. The 

 tension of water vapour at this temperature, although not absent, is very small, 

 so that, even when accelerated by the use of a vacuum, the drying, even if 

 very small pieces are taken, lasts for four days or so. Tissues so dried may 

 be directly impregnated with toluene and paraffin at a temperature not exceeding 

 40 C. in vacua. They cut as well as the best fixed and hardened preparations. 



This fact I am able to confirm. My experiments were made by the use of the calcium 

 chloride tank of a carbon dioxide freezing machine ; the solution of calcium chloride was made 

 of such a concentration that its freezing point \vas about - 35 C. , so that by working the 

 compressor all day the solution froze, and, being well insulated from heat, the temperature was 

 maintained sufficiently low until the next morning. The object aimed at by Altmann was to 

 compare the action of different fixatives on sections of the same piece of tissue. The sections 

 were therefore exposed to these reagents at once, and no further difficulty was met with. 

 My object, on the other hand, was to replace the water lost in the dehydration process, in 

 order to examine the structure when unfixed, but a great difficulty was experienced owing to 

 the immediate disintegration of the sections when brought into contact with water. It may 

 be found necessary to allow water to be gradually taken up from ice at the same temperature 

 as that at which the dehydration took place, allowing the temperature to rise very slowly. In 

 any case, the method seems deserving of more attention than it has as yet received. Most 

 laboratories are now provided with ice-making machinery, so that opportunities should not 

 be wanting. 



CHEMICAL NATURE 



It is obvious that, by ordinary methods of analysis, it is impossible to decide 

 satisfactorily the much debated question as to whether protoplasm is a "giant 

 molecule " in the chemical sense of the word. Even if this were so, the reagents 

 used would certainly split it up into smaller ones. A great variety of bodies 

 have been obtained from cells by chemical treatment, and much light has been 

 thrown in this way on many of the activities to be described in later chapters. 

 All the chemical elements usually found in organic compounds are present, 

 together with salts, inorganic and organic, and water in large amount, as much 

 as 85 per cent, to 90 per cent, or more. It is certain that the complex nitro- 

 genous bodies known as proteins play a great part in the chemical reactions 

 or metabolism of the cell, and it appears also that bodies of a fatty nature, 



