formation of the pseudopodia of an amoeba or white blood-corpuscle, or in the vibratile movements of ciliary structures, or in the powerful contractions of voluntary muscle, the underlying mechanism by which the shortening is produced is essentially the same throughout. However general the property may be, it cannot be considered as absolutely characteristic of living structure. As was mentioned before, Engelmann' has been able to show that a dead catgut string when surrounded by water of a certain temperature and exposed to a sudden additional rise of temperature will contract or shorten in a manner closely analogous to the contraction of ordinary muscular tissue, and it is not at all impossible that the molecular processes involved in the shortening of the catgut string and the muscle-fibre may be essentially the same.

That conductivity is also a fundamental property of primitive protoplasmic structure seems to be assured by the reactions which the simple motile forms of life exhibit when exposed to external stimulation. An irritation applied to one point of a protoplasmic mass may produce a reaction involving other parts, or indeed the whole extent of the organism. The phenomenon is most clearly exhibited in the more specialized animals possessing a distinct nervous system. In these forms a stimulus applied to one organ, as for instance light acting upon the eye, may be followed by a reaction involving quite distant organs, such as the muscles of the extremities, and we know that in these cases the irritation has been conducted from one organ to the other by means of the nervous tissues. But here also we have a property that is widely exhibited in inanimate nature. The conduction of heat, electricity, and other forms of energy is familiar to every one. While it is quite possible that conduction through the substance of living protoplasm is something sui generis, and does not find a strict parallel in dead structures, yet it must be admitted that it is conceivable that the molecular processes involved in nerve conduction may be essentially the same as prevail in the conduction of heat through a solid body, or in the conduction of a wave of pressure through a liquid mass. At present we know nothing definite as to the exact nature of vital conduction, and can therefore affirm nothing.

The four great properties enumerated, namely, nutrition or assimilation (including digestion, secretion, absorption, excretion, anabolism, and katabolism), reproduction, conduction, and contractility, form the important features which we may recognize in all living things and which we make use of in distinguishing between dead and living matter. A fifth property perhaps should be added, that of sensibility or sensation, but concerning this property as a general accompaniment of living structure our knowledge is extremely imperfect; something more as to the difficulties connected with this subject will be said presently. The four fundamental properties mentioned are all exhibited in some degree in the simplest forms of life, such as the protozoa. In the more highly organized animals, however, we find that specialization of function prevails. Hand in hand with the differentiation in form that is displayed in the structure of the constituent tissues there goes a specialization 1 Ueber den Ursprung der Muskelkraft, Leipzig, 1893.

1

ù emala peperies with a concomitant suppression of other properties, the virene of when that muscular tissue exhibits pre-eminently the power of evrræni ty, the nerve tissues are characterized by a highly developed power of mucusivity, and so on. While in the simple unicellular forms of animal „le the futunanuental properties are all somewhat equally exhibited within the mpse of a ge unit or cell, in the higher animals we have to deal with 2 1 udt op U uality of cells segregated into tissues each of which possesses some custardive property. This specialization of function is known technically as tur prysidogical division of labor. The beginning of this process may be rengs men in the cell itself. The typical cell is already an organism of some vons juezi ty be compared with a simple mass of undifferentiated protoplasm. 2've protopisem of the nucleus, particularly of that material in the nucleus wing is designated as chromatin, is differentiated, both histologically and poy monogez by, from the protoplasm of the rest of the cell, the so-called cytopants. The curomatin material in the resting cell is arranged usually in a we work, but during the act of division (karyokinesis) it is segmented into a mun ber of rods or filaments known as chromosomes. In the ovum there are good reasons for believing that the power of transmitting hereditary characteristics is dependent upon the structure of these chromosomes. The nucleus, suoreover, controls in some way the metabolism of the entire cell, for it has boou shown, in some cells at least, that a non-nucleated piece of the cytoplasm iwot only deprived of the power of reproduction, but has also such limited power of nutrition that it quickly undergoes disintegration. On the other band contractility and conductivity, and some of the functions connected with nutrition, such as digestion and excretion, seem often to be specialized in the cytoplasm. As a further example of differentiation in the cell itself the exbstrace of the centrosome may be referred to. The centrosome is a body of minute size that has been discovered in numerous kinds of cells. It is considered by many observers to be a permanent structure of the cell, lying ither in the cytoplasm, or possibly in some cases within the boundaries of the mule us. When present it seems to have some special function in connection with the movements of the chromosomes during the act of cell-division. In the many-celled animals the primitive properties of protoplasm become highly developed, in consequence of this subdivision of function among the various 1., uc, and in many ways the most complex animals are, from a physiological fandpoint, the simplest for purposes of study, since in them the various propatics of living matter are not only exhibited more distinctly, but each is, as it were isolated from the others and can therefore be investigated more directly. We are at liberty to suppose that the various properties so clearly recognizable in the differentiated tissues of higher animals are all actually or potentially yontained in the comparatively undifferentiated protoplasm of the simplest uniHular forms. That the lines of variation, or in other words the direction of pealization in form and function, are not infinite, but on the contrary mparatively limited, seems evident when we reflect that in spite of the urous branches of the phylogenetic stem the properties as well as the

forms of the differentiated tissues throughout the animal kingdom are strikingly alike. Striated muscle, with the characteristic property of sharp and powerful contraction, is everywhere found; the central nervous system in the invertebrates is built upon the same type as in the highest mammals, and the variations met with in different animals are probably but varying degrees of perfection in the development of the innate possibility contained in primitive protoplasm. It is not too much to say, perhaps, that were we acquainted with the structure and chemistry of the ultimate units of living substance, the key to the possibilities of the evolution of form and function would be in our possession.

Most interesting suggestions have been made in recent years as to the essential molecular structure of living matter. These views are necessarily very incomplete and of a highly speculative character, and their correctness or incorrectness is at present beyond the range of experimental proof; nevertheless they are sufficiently interesting to warrant a brief statement of some of them, as they seem to show at least the trend of physiological thought.

Pflüger,' in a highly interesting paper upon the nature of the vital processes, calls attention to the great instability of living matter. He supposes that living substance consists of very complex and very unstable molecules of a proteid nature which, because of the active intra-molecular movement present, are continually dissociating or falling to pieces with the formation of simpler and more stable bodies such as water, carbon dioxide and urea, the act of dissociation giving rise to a liberation of energy. "The intra-molecular heat (movement) of the cell is its life." He suggests that in this living molecule the nitrogen is contained in the form of a cyanogen compound, and that the instability of the molecule depends chiefly upon this particular grouping. Moreover the power of the molecule to assimilate dead proteid and convert it to living proteid like itself he attributes to the existence of the cyanogen group. It is known that cyanogen compounds possess the property of polymerization, that is, of combining with similar molecules to form more complex molecules, and we may suppose that the molecules of dead proteid when brought into contact with the living molecules are combined with the latter by a process analogous to polymerization or condensation. By this means the stable structure of dead proteid is converted to the labile structure of living proteid, and the molecules of the latter increase in size and instability. When living substance dies its molecules undergo alteration and become incapable of exhibiting the usual properties of life. Pflüger suggests that the change may consist essentially in an absorption of water whereby the cyanogen grouping passes over into an ammonia grouping. Loew assumes also that the difference between dead and living or active proteid lies chiefly in the fact that in the latter we have a very unstable or labile molecule in which the atoms are in active motion. The instability of the molecules he likewise attributes to

1 Archiv für die gesammte Physiologie, 1875, Bd. 10, S. 251.

2 Ibid., 1880, Bd. 22; Loew and Bokorny: Die chemische Kraftquelle in lebenden Protoplasma, München, 1882; Imperial Institute of Tokyo (College of Agriculture), 1894.

the existence of certain groupings of the atoms. Influenced in part by the power of living material to reduce alkaline silver solutions, he supposes that the specially important labile group in the molecule is the aldehyde radical -C The nitrogen exists also in a labile amido- combination, -NH2, and the active or living form of these two groups may be expressed by the

formula

H'

verted to the grouping

-CH-NH
C-CHOH'

it would form a comparatively inert compound such as we have in dead proteid. Latham1 proposes a theory which combines the ideas of Pflüger and of Loew. He suggests that the living molecule may be composed of a chain of cyan-alcohols united to a benzene nucleus. The cyan-alcohols are obtained by the union of an aldehyde with hydrocyanic acid; they contain, therefore, the labile-aldehyde grouping as well as the cyanogen nucleus to which Pflüger attributes such importance. Actual investigation of the chemical structure of living matter can hardly be said to have made a beginning. The first step in this direction has been made in the study of the chemical structure of the group of proteids which have usually been considered as forming the most characteristic constituent of protoplasm. Proteids as we obtain them from the dead tissues and liquids of the body have proved to be very varied in properties and structure, so much so in fact that it is impossible to give a satisfactory definition of the group. Many of them can be obtained in a pure, even in a crystalline form, and their percentage composition can therefore be determined with ease. But the fundamental chemical structure that may be supposed to characterize the proteid group, and the changes in this structure producing the different varieties of proteids are matters as yet undetermined. Several promising efforts have been made to construct proteids synthetically, but the results obtained are at present incomplete. On the other hand, Kossel has isolated from the spermatozoa of certain fishes a comparatively simple nitrogenous body of basic properties (protamine), which he regards as the simplest form of proteid and the essential core or nucleus characterizing the structure of the whole group. It is an interesting thought that in the heads of the spermatozoa with their complex possibilities of development and hereditary transmission, dependent as these properties must be upon the chemical structure of the germ protoplasm, there may be found the simplest form of proteid. Kossel's work, it may be noted, has not gone so far as to indicate the possible molecular structure of the protamines.

It has been assumed by many observers that the properties of living matter, as we recognize them, are not solely an outcome of the inner structure of the hypothetical living molecules. They believe that these latter units are

1 British Medical Journal, 1886, p. 629.

2 Zeitschrift für physiol. Chem., 1898; xxv. 1899, xxvi.

fashioned into larger secondary units each of which is a definite aggregate of chemical molecules and possesses certain properties or reactions that depend upon the mode of arrangement. The idea is similar to that advanced by mineralogists to explain the structure of crystals. They suppose that the chemical molecules are arranged in larger or smaller groups to which the name "physical molecules" has been given. So in living protoplasm it may be that the smallest particles capable of exhibiting the essential properties of life are groups of ultimate molecules, in the chemical sense, having a definite arrangement and definite physical properties. These secondary units of structure have been designated by various names such as "physiological molecules," " somacules," micella, etc. Many facts, especially from the side of plant physiology, teach us that the physical constitution of protoplasm is probably of great importance in understanding its reaction to its environment. Microscopic analysis is insufficient to reveal the existence or character of these "physiological molecules," but it has abundantly shown that protoplasm has always a certain physical construction and is not merely a structureless fluid or semi-fluid mass.

3

What has been said above may serve at least to indicate the prevalent physiological belief that the phenomena shown by living matter are in the main the result of the action of the known forms of energy through a substance of a complex and unstable structure which possesses, moreover, a physical organization responsible for some of the peculiarities exhibited. In other words, the phenomena of life are referred to the physical and chemical structure of protoplasm and may be explained under the general physical and chemical laws which control the processes of inanimate nature. Just as in the case of dead organic or inorganic substances we attempt to explain the differences in properties between two substances by reference to the difference in chemical and physical structure between the two, so with regard to living matter the peculiar differences in properties that separate them from dead matter, or for that matter the differences that distinguish one form of living matter from another, must eventually depend upon the nature of the underlying physical and chemical structure.

In the early part of this century many prominent physiologists were still so overwhelmed with the mysteriousness of life that they took refuge in the hypothesis of a vital force or principle of life. By this term was meant a something of an unknown nature that controlled all the phenomena exhibited by living things. Even ordinary chemical compounds of a so-called organic nature were supposed to be formed under the influence of this force, and it was thought could not be produced otherwise. The error of this latter view has been demonstrated conclusively within recent years: many of the substances formed by the processes of plant and animal life are now easily produced within the laboratory by comparatively simple synthetic methods. 1 Meltzer: "Ueber die fundamentale Bedeutung der Erschütterung für die lebende Materie," Zeitschrift für Biologie, Bd. xxx., 1894.

2 Foster: Physiology (Introduction). 3 Nägeli: Theorie der Gährung, München, 1879.