"gel," in which the particles have joined together to form
minute closed chambers in which liquid is confined, the whole
mass then possessing the properties of a solid. This process
occurs when a warm solution of gelatin solidifies on cooling. It
will be clear that if protoplasm contains many-formed solid
structures, as in highly differentiated cells, together with com-
paratively little free water, it may be scarcely possible to detect
its liquid nature. However difficult it may be to conceive the
way in which different chemical processes can occur at different
places in a liquid, this must be the case, and suggestions have
been made as to the temporary formation of separating mem-
branes or other less mechanical means of isolation. The notion
of protoplasm as a giant molecule in the chemical sense, with
numerous side-chains of a variety of composition capable of
reacting together, is held by but few physiologists at the present
time. A view of this kind is contained in the theory of " biogens,"
to which reference will be made later.
The cell-membrane is evidently formed by surface condensation of some constituents of protoplasm. In ordinary circumstances, it is impermeable to salts, to glucose and to amino-acids. But since these substances must obtain access to the interior as foods at certain times, it is clear that the membrane must become more permeable when required. The constituents of the membrane being deposited at the surface owing to their properties of reducing sur- face energy, the actual chemical composition of this layer must be of a complex nature. It is probably, for the most part, an intimate mixture of fatty substances with watery proteins. These may form two kinds of colloidal systems minute droplets of fat may be surrounded by a continuous solution of protein, like islands in a sea, or they may themselves run together and enclose droplets of protein, similar to the land surrounding a number of lakes or ponds. The properties of these two systems, as regards the passage across them of materials, will not be identical. Thus, rabbits cannot get through the system of islands, fish can do so; while the reverse will be the case with the other kind of system. Familiar instances of the contrast are well known in the forms of cream and butter. The former is a suspension of droplets of oil in a watery solution, the latter consists of droplets of the watery solution in a continuous medium of fat. The investigations of Clowes have shown how, by the action of electrolytes, particularly of calcium ions, one system can be changed into the other. A difficulty is sometimes felt as to how a membrane which refuses passage to sugar allows visible solid particles to pass through, as can be seen in the amoeba and leuco- cytes. The explanation lies in the manner in which a needle can be dropped through a soap nlmwithoutbreakingthefilm. A continuous film of the membrane is formed over the point as it enters and again over the eye-end as it leaves. Thus the membrane is never actually broken. A further point to be remembered in the modifications of the cell-membrane is that, being formed of materials present in the cell, it is an integral part of this and must change in correspondence with functional changes in the cell itself. Thus it appears that it becomes more permeable in states of activity, as in the contraction of muscle and in the secretory process of gland cells. If the sur- ounding medium itself contains substances that lower surface ension, these no doubt contribute to the structure of the mem- >rane. Such is usually the case in the cells making up the animal
dy, less so in the plant.
Certain substances, proteins and others, become more or less olidified in the process of concentration in surface films, as Rams- den has shown. This fact must also play a part in the formation
the cell-membrane.
As to the chemical nature of protoplasm itself, it seems very questionable whether it can be regarded as a uniform chemical ompound. Although proteins take a large share, various other ubstances, such as the complex fats known as lipoids or lipines, re important components. All the constituents are interconnected in a highly heterogeneous system, partly chemical, partly physical, of many phases and intersected by semi-permeable membranes forming and disappearing at intervals.
Attention may here be called to the special properties of the arbon atom as forming the basis of the chemical changes associated nth living matter. Compounds of carbon do not naturally react itherwise than slowly. They are, therefore, subject to ready modi- fication in this respect by the agency of the catalysts called " enzymes." Owing to the capacity of carbon atoms to unite together in long chains or rings of a relatively stable nature, along with the possession of four valencies capable of union with four different Troups, we have the possibility of the production of highly complex arge molecules. Another property of importance in the carriage of energy is the power of combining with elements of opposite charac- ter, such as hydrogen and oxygen. The hydrocarbons give up energy on oxidation, while the carbon dioxide can be reduced and built up again by addition of energy to form stores of potential energy for future use.
Although such large complex molecules are of great importance, the view of the essential nature of protoplasm as a further growth
of the chemical molecule, the " biogen " hypothesis, cannot be accepted. According to this view, food material is combined chemically, as a side chain, with a giant molecule, while certain other side chains consist of oxygen. In oxidation, to give energy, combination takes place between these side chains. It follows from such a view that oxygen must be stored up in living cells in an " intra-molecular " form. Now, investigations from many points of view have failed to obtain evidence of the existence of such reserve oxygen. There is no proof that an organism possesses any store of oxygen beyond that contained in the lungs or dissolved in the fluids. It appears that the mechanism of living matter is more analogous to that of a petrol engine, in which the fuel does not form part of the structure previously to its combustion, but that this combustion takes place in intimate relation with the moving parts and that it is owing to the special arrangement of these that the chemical energy of the fuel is converted into the mechanical energy of motion. It is true that such cells as those of muscle and secreting glands have been shown to prepare during rest a store of some kind of system possessing potential energy, ready for use on stimulation to activity. But the process of conversion of this sys- tem to one of lower energy content is not associated with the con- sumption of oxygen or evolution of carbon dioxide. Thus the system is not comparable with a " biogen " and may probably be rather of a physical than a chemical nature. The energy necessary to form it is obtained by the combustion of some food material, apparently carbohydrate, with the giving off of carbon dioxide and involving some loss in the form of heat. The state of activity and the actual combustion process are therefore separate phenomena and the conception of giant molecules throws no light on their nature. It is rather as if the combustion of the fuel in our petrol engine were used, through some mechanism, to pump water to a high level, from which the energy could be obtained by allowing it to run down when required.
The Nucleus of the Cell. Although this component is present in all the more highly organized cells, it is obvious that it cannot be essential in this actual form. Bacteria, for example, do not appear to possess a nucleus, although the materials out of which it is made are probably present distributed through the cell. If, as Morley Roberts suggests, the nucleus is a store of enzymes, the tools of the cell, it might be supposed that, in the absence of the nucleus, these tools are not kept in a special receptacle. Although statements have been made that oxidative processes are especially carried out by the nucleus, the evidence is unconvincing.
The nucleus undergoes a remarkable series of changes in the course of the subdivision of one cell into two daughter cells. This process, known as " karyokinesis " or " mitosis," exhibits a complex play of directed or polar forces. The inheritance of the Mendelian characters of organisms is conveyed by the nuclear constituents. The general characters are held by some to be transmitted by the cytoplasm. But, although it is difficult to believe that the cyto- plasm plays no part, it would appear that if all the general charac- ters are thus carried, those of the male parent would be practically unrepresented, since the spermatozoa are almost devoid of cyto- plasm. Further evidence is needed.
When a nucleated cell is cut into two parts, one alone containing the nucleus, this part can continue to exist, whereas the other part degenerates and dies. Thus, the nucleus is essential to the life and growth of a cell in those cases where it has become a specially differentiated part. The fact is particularly manifest in the case of the cells called neurones, which make up the nervous system. Here there is a very long fine fibre arising from a nucleated mass. This is a " nerve-fibre," and if cut away from its nucleated origin, it degenerates and ceases to be able to conduct nerve impulses.
Nutrition. During 1910-21 much attention was given to problems of the phenomena of nutrition, necessitated in great part by the conditions brought about by the World War of 1914-8. Since the chief use of food is to supply energy and energy cannot be created, it must always be kept in mind that a given amount of food can only provide a certain definite quantity of energy. For convenience of measurement, this is expressed in terms of heat units, calories. An adequate diet must have a certain minimal calorie value, or energy value, differing according to work done, age, weight, etc. Whatever else may be necessary, and whatever may be the composition of the diet, this energy value must be provided. The fact must not be allowed to be obscured by recent work on the importance of special con- stituents, such as " vitamins " or the presence of particular chemical compounds.
Further, food is obviously required in the growing organism to make new tissue and in the adult to replace wear and tear. Although the actual quantity needed for these purposes is not great, it is clear that it must contain all the chemical elements making up the constituents of the new tissues. There are more- over certain rather complex chemical compounds that must be
