14 PHYSIOLOGY explosive, exhibits little power of contractility or move ment. In another specimen, on the other hand, a very large portion of the energy similarly set free may be spent in producing visible changes of form, the protoplasm in this instance being exquisitely mobile. Such differences must be due to different internal arrangements of the proto plasm, though, since no vision, however well assisted, can detect these arrangements, they must be of a molecular nature rather than of that grosser kind which we generally speak of as structural. It is true that, as the differences in properties become more and more prominent, as the protoplasm becomes more and more specialized, features which we can recognize as structural intervene ; but even these appear to be subsidiary, to accompany and to be the result of the differences in property, or to be concerned in giving special directions to the activities developed, and not to be the real cause of the differences in action. We are thus led to the conception of protoplasm as existing in various differentiated conditions while still retaining its general protoplasmic nature, a difference of constitution making itself felt in the different character of the work done, in a variation of the results of the protoplasmic life. We have a division of physiological labour going hand in hand with a differentiation of material, accompanied ulti mately by morphological results which may fairly be spoken of as constituting a differentiation of structure. Some of the simpler and earlier features of such a divi sion and differentiation may be brought out by comparing with the life of such a being as the amoeba that of a more complex and yet simple organism as the hydra or fresh water polyp. Leaving out certain details of structure, which need not concern us now, we may say that the hydra consists of a large number of units or cells firmly attached to each other, each cell being composed of proto plasm, and in its broad features resembling an amoeba. The polyp is in fact a group or crowd of amoeba-like cells so associated together that, not only may the material of each cell, within limits, be interchanged with that of neighbouring cells, but also the dynamic events taking place in one cell, and leading to exhibitions of energy, may be similarly communicated to neighbouring cells, also within limits. These cells are arranged in a particular way to form the walls of a tube, of which the body of the Endo- hydra practically consists. They form two layers in appo- dermandsition, one an internal layer called the endoderm, lining the 111 tube, the other an external layer called the ectoderm, form ing the outside of the tube. And, putting aside minor details, the differences in structure and function observ able in the organism are confined to differences between the ectoderm on the one hand, all the constituent cells of which are practically alike, and the endoderm on the other, all the cells of which are in turn similarly alike. The protoplasm of the ectoderm cells is so constituted as to exhibit in a marked degree the phenomena of which we spoke above as irritability and contractility, whereas in the endoderm these phenomena are in abeyance, those of assimilation being prominent. The movements of the hydra are chiefly brought about by changes of form of the ectoderm cells, especially of tail-like processes of these cells, which, arranged as a longitudinal wrapping of the tubular body, draw it together when they shorten, and lengthen it out when they elongate, and it is by the alternate lengthening and shortening of its body, and of the several parts of its body, that the hydra changes its form and moves from place to place. Inaugurating these changes of form, the products of contractility, are the more hidden changes of irritability ; these also are especially developed in the ectoderm cells, and travel readily from cell to cell, so that a disturbance originating in one cell, either from some extrinsic cause, such as contact with a foreign body, or from intrinsic events, may sweep from cell to cell over the surface of the whole body. The animal feels as well as moves by means of its ectoderm cells. In the endoderm cells the above phenomena, though not wholly absent, are far less striking, for these cells are almost wholly taken up in the chemical work of digesting and assimilating the food received into the cavity, the lining of which they form. Thus the total labour of the organism is divided between these two membranes. The endoderm cells receive food, transmute it, and prepare it in such a way that it only needs a few final touches to become living material, these same cells getting rid at the same time of useless ingredi ents and waste matter. Of the food thus prepared the endoderm cells, however, themselves use but little ; the waste of substance involved in the explosions which carry out movement and feeling is reduced in them to a mini mum ; they are able to pass on the greater part of the elaborated nourishment to their brethren the ectoderm cells. And these, thus amply supplied with material which it needs but little expenditure of energy on their part to convert into their living selves, thus relieved of the greater part of nutritive labour, are able to devote nearly the whole of their energies to movement and to feeling. Microscopic examination further shows that these two kinds of cells differ from each other to some extent in visible characters ; and, though, as we have seen, the differ ences in activity appear to be dependent on differences in invisible molecular arrangement rather than on gross visible differences such as may be called structural, still the invisible differences involve or entail, or are accom panied by, visible differences, and such differences as can be recognized between endoderm and ectoderm, even with our present knowledge, may be correlated to differences in their work ; future inquiry will probably render the correlation still more distinct. The ectoderm cells together constitute what we have spoken of above as a tissue, whose function in the modern sense of the word is movement and feeling, and the endo derm cells constitute a second tissue, whose function is assimilation ; and the phenomena of the whole being result from the concurrent working of these two functions. Of organs, in the old sense of the word, of mechanical contrivances, there is hardly a trace. 1 The performances of the being are, it is true, conditioned by its being moulded in the form of a long tubular sac with a crown of like tubular arms, but beyond this the explanation of every act of the hydra s life is first to be sought in the characters of the endoderm and ectoderm. The physiology of the hydra is, for the most part, a series of problems, dealing on the one hand with the intimate nature of the ectodermic protoplasm and the changes in that protoplasm which give rise to movement and feeling, as well as with the laws whereby those changes are so regulated that movement and feeling come and go as the needs of the organism may require, and on the other hand with the intimate nature of the endodermic protoplasm and the changes in that protoplasm whereby the dead food is, also according to the needs of the economy, transformed into living substance. Whereas the older physiology dealt almost exclusively with mechanical problems, the physio logy of to-day is chiefly busied with what may be called molecular problems. The physiology of the higher animals, including man, is merely a development of the simpler physiology of the hydra, which has been rendered more complex by a greater division of physiological labour, entailing greater differen- 1 The existence of certain minute mechanisms called urticating organs lodged in the ectoderm cells does not affect the present
argument.