THE COLLAPSIBLE TUBE sharp edge bites into the polythene wad quite easily and gives an excellent seal. Nothing could be more simple. The compo-cork, or paper-pulp wad faced with metal foil, Ceresin waxed paper, Vinylite coated paper, etc., will continue to be used for the easy jobs, no doubt, but it must have had its day in other cases. Thus, I think it is fair to say that you should no longer suffer seepage at the nozzle. The crimped end presents a very different problem and I wish it were possible to say that there was even one method of effecting 100 per cent security, but, alas, this is not so. The one method which is giving a fair degree of success in certain applications involves the internal waxing of each tube to a depth of about « in. so that the crimp becomes wax filled and thus presents a barrier to liquids of low surface tension. Obviously, this system has little additional physical strength and its performance in hot chmates, where the wax approaches melting-point and the heat produces a rise in internal pressure, must be carefully watched. Thermoplastic coatings, such as vinyl or latex, offer increased physical strength, but are less amenable to migration under crimping pressure-- even when the jaws are hot--and thus often fail to fill that a•vkward tunnel which forms along the edges of the fold. This can only be counteracted by the use of a fairly heavy coating which, in its turn, causes production difficulties to the tube maker, although these are in the process of being overcome. The use of external clips, dipping of the crimped tube end into sealing compounds after filling, and such-like procedures are generally unsightly, troublesome, and often quite ineffective. Pressure welding was tried on tin tubes years ago, but with very limited success. In the first instance, with the optimum conditions of pressure supplemented by heat, a considerable reduction in metal thickness (50-70 per cent) is essential before a satisfactory weld ensues. When working with a tube wall only 0-005 in. thick one can hardly afford this if reasonable strength is to remain. Secondly, any trace of product between the surfaces to be joined leads to inevitable failure. Electric arc and ordinary flame welding have also been tried with varying degrees of success. You may well say that this is a rather sorry story to tell, haven't you got any ideas for solving it in the future ? We have one idea which is being investigated and, strangely enough, it concerns the aluminium tube which one might expect to be the most difficult. It is hoped to extend the use of collapsible tubes in the food industry and in many cases an hermetically sealed container, able to withstand sterilisation, is essential. Hence, we must solve it. Don't think that the answer is just around the comer or ask me to forecast how soon it will be available. If I can give you nothing more positive, at least I offer you some encouragement to go on hoping. 117

THE SKIN AS A COMMUNITY OF STRUCTURES E. W. POWELL, M.A., A.R.I.C.* The first part of a study of the sirin as a balanced complex of inter-related structures. Tissue culture research is reviewed to show the extent to which skin structure development has been found to take place in absence of the whole animal. INTRODUCTION MANY COMMONLY observable phenomena relating to the human skin and its. various component structures, the hair, sebaceous glands, sweat õlands and pigment-producing cells, give rise to interesting thoughts upon the inter- dependence of these structures. The changes in the condition of the hair which can occur with variations in general health emphasise the relation to the body as a whole, a relationship which is studied in one aspect by investigations of the effects of various natural hormones. Upon the same scalp, white and dark hairs can õrow simultaneously for many years, indicat- ing that the conditions which permit growth of hair can result in cessation of the production of pigment even when pigment-producing cells have been present. A scalp which bears but a thin growth of white hair may acquire a tan in sunlight and the ability to produce piõment is still present in the epidermis although lacking in the hair bulb. Such observations, which point to the skin and its appendages as a system of organs dependent upon the whole organism but, in addition, with each structure having some degree of individuality, prompt further enquiry into the extent of this dependence and into the differences in response of the structures to changes of conditions. The evolution of the human body, for example, is frequently represented as the development of more precise control of the environment in which the constituent cells of the body can live. Associated with this control is the specialisation of function with the cells grouped to form organs making particular contributions to the metabolism of the body as a whole. Such organs as the liver, kidneys, intestines and lungs function with a certain degree of independence, but rely upon the blood stream, the nervous system and the hormonal system for their normal activity. Similarly, the particular structures of the skin such as the epidermis, hair follicles, sebaceous glands and pigment-producing cells have their particular functions to perform which involve the production of the keratin proteins of the epidermis and the hair, the formation of the oil, sebum, and of the pigment, melanin, respectively. In the whole animal, they have their particular relations to the blood, nervous and hormonal systems. The various appendages of the skin have been described individually in a number of excellent reviews. It is the object of this paper, however, to regard the skin as a balanced and complex * County Laboratories, Ltd., Honeypot Lane, Stanmore, Middlesex. 118