self-determined changes appear always to lead in animals to the formation of a central nervous system if they go far enough, the conclusion is reached that the nervous system is the final expression, both in arrangement and in mode of action, of the system of metabolic gradients.

A corollary of great importance can be deduced from the case of the Planarian. The degree of individuality of the daughter is a measure of the loss of control of the head-end, a not unfamiliar phenomenon. As this occurs, the daughter becomes more and more physiologically isolated and her metabolic processes proceed at a faster rate. Hence physiological isolation is a fundamental factor in asexual reproduction.

The Development of the Frog Egg as a System of Gradients.

In the light of this conception of the individual being as a reactionsystem, we may now take the unfertilised ovarian egg, say of the frog, as a primary individual. It possesses an axial gradient. One pole is the region of highest metabolic rate determined by the relations of the egg to the maternal tissues and the other external agencies. There is evidence that, from this apical pole, chemical change proceeds in waves of decreasing order of intensity through the protoplasm towards the opposite or basal pole. Though there may as yet be no visible structural change in the colloidal medium, yet the factors that produce the first visible change are there. Differentiation on this view is the expression of chemical change along the gradient. The cell or ovum is in fact a creature with a kind of a heid upon it-man could say nae mair.'

The changes that ensue during the maturation of this egg or primary individual are too involved, and too familiar, to zoologists for me to enumerate. The little sphere, still without visible differentiation, becomes a stratified power-station. The apical pole remains chemically active, the basal pole accumulates stores of potential food and energy. The whole globular microcosm becomes enclosed in a non-permeable membrane, and is shut off as a closed system from the outer world. If only one of its extruded polar bodies returned; if only something could break this too, too solid envelope; if only some messenger from the outer world, some Orpheus could visit the cold Eurydice, then development might begin. And it so happens. In the natural sequence, Orpheus-the spermatozoon -is the winged key that unlocks the imprisoned one. He casts a shadow -the grey crescent-that heralds the advent of the new gradient, the one that takes sides, and that prophetically unseams the germ from the nave to the chaps, that separates the right side from the left. As if to justify the use of emotional language, the germ at that moment of release takes an explosive breath as though the crisis were over. It will never take a deeper one. The process of development is begun.

The first trace of the embryo is the apical region or brain, formed as part of that region of greatest metabolic activity known as the dorsal lip of the blastopore, or the differentiator.' (4) This region provides the three co-ordinate lines or metabolic gradients' along which the main features of structure are elaborated-the primary gradient along which the central nervous system forms; the secondary gradient for the axial organs; and the transverse gradient along which the lateral organs are developed.

The fate of the cellular material with which the differentiator deals depends not on their pre-determined nature but on the changes they undergo in passing to their final place in the organism, and to the company they keep when they get there. So far as their fate is concerned they may say with Hamlet the readiness is all.' In the hands of the three co-ordinate gradients that radiate from the 'differentiator,' it matters nothing whether the cells they hand on to build the back or the side are those naturally presumed to fit the part. Cells that would under normal circumstances form skin cells on the outer surface, and that lie outside the reach of the differentiator, will if grafted into it become kidney-tissue, muscle-tissue, or gut-tissue. And the converse is true. Tissue of the differentiator itself, presumably destined to become kidney or muscle, may be grafted into the wound left in the skin by the previous excision, and there it will become skin. So the surface tissue that would become brain if left alone will, if grafted into the differentiator, become intricately involved, and after travelling inwards and forwards find itself transformed into the likeness of those with which it is now a companion in function. With increasing zest we may repeat Huxley's great metaphor concerning the cells of the early embryo: They are no more the producers of the vital phenomena than the shells scattered along the sea-1 a-beach are the instruments by which the gravitative force of the moon acts upon the ocean. Like these, the cells mark only where the vital tides have been and how they have acted.'

The events that I have briefly described constitute the prelude to two other phases through which the life of a multicellular animal passes. We may call them collectively the indeterminate, the determinate, and the integrated phases. During the first, the three waves of chemical activation assort the cellular material along the axis of the body and next determine irrevocably its fate as organs of the individual. This period begins in the frog with the closing of the blastopore and of the neural groove. From now onwards the evolution of the organs proceeds from determined beginnings impressed upon the constituent cells by their relation to each other and to the gradients. Remove the rudimentary organ from its normal position-the heart, the kidney, and the brain-and it will complete or at least continue its evolution even in the solitude of a moist chamber. But under normal circumstances this phase of organic determination leads insensibly to that condition of full and inter-related activity that we may call integration. The muscles may be able to develop apart from the nervous system, but without organic contact with that controlling system they cannot function. The kidney may exhibit characteristic complexities of origin and evolution without the aid of humoral or hormonic influence, but it cannot function apart from these. The primary factors of life—the metabolic gradients are supplemented by new structural factors and new chemical factors, and together constitute personality.

Meanwhile, the inevitable price, senescence, is paid for advance. The stream of animal life, unlike its prototype, sedimenting most elaborately where it runs most strongly, is running down. Stability of construction brings the penalty of diminished dynamic activity, and the advent of puberty marks for many animals the shadow of the fell sergeant. But life has still its reserves, or at least one means of continuing the life-cycle in its descendant, if not in its undivided personality. In those lower

animals of ponds and streams, the Planarians, the act of procreation can be both naturally and artificially checked, and a return to a less highly organised state can be induced. In a similar way the act of sterilization induces fresh vigour in some of the higher animals. Finally, in many animals the body undergoes periodic retrograde evolution, renews its youth, returning to an undifferentiated state in which it passes the winter with heightened powers of resistance, and on the advent of spring redevelops its organisation.

Evidence for the Hypothesis of Metabolic Gradients.

(A) Axial susceptibility.

The evidence for these far-reaching conclusions as to the nature of the living organism is partly direct and experimental, and partly indirect and observational. The direct evidence has been drawn from experiments by Professor Child and his school on Protozoa, Coelenterata, Planarians, Liver-flukes, Annelids, Echinoderms, Fishes and Amphibia extending over about fifteen years. Recently Dr. Shearer (5) has repeated these experiments on the chick and on earthworms, with results entirely confirming the conclusions of Child and his pupils. A critical review of the evidence has recently been published by Child and Bellamy (5a).

The first class of evidence relates to axial susceptibility to the action of toxic or narcotic substances. When immersed in, for example, a weak solution (0.001 mol) of potassium cyanide in well-water, the head-end ' of the whole animal or the apical pole of the egg is the first portion of the body to undergo disintegration, and this is followed by a succession of stages during which the process slowly spreads downwards. In general, the susceptibility-curve plotted on the basis of time-ordinates against these stages as abscissae, shows a much sharper fall for young than for older animals of the same species if the solutions are above a certain degree of concentration. If very dilute solutions are used, the opposite result is obtained. Immunity is gained more rapidly by the young than by the old. These results may be explained as due to the action of the cyanide on the oxidation-process and possibly also on the physical character of the colloidal protoplasm. The important point is the definite relation of disintegration to the animal's axis. The head-region' or the apex of the egg disintegrates first and the basal region last. The evidence therefore tends to show that the susceptibility gradient is evidence of the existence of a metabolic gradient.

Estimations of this kind have been made by the use of a large number of narcotics and poisons and the results have been confirmatory. More recently, other methods of testing the presence, course, and strength of these gradients have been devised. Dr. Tashiro (6), for example, has applied to the nerves of the body an exceedingly delicate test (the Tashiro biometer) for the estimation of carbon dioxide in minimal quantities, and has shown that a gradient exists following the direction of the impulse along the nerve. Again, Child himself, and later Shearer, have demonstrated the presence of axial gradients in starfish and chick respectively, by the use of acetone and other substances, which are precipitated in the tissues of the living developing animal by oxidation, thus giving an ocular demonstration of the track of the primary gradient. Unquestionably

the development and application of biochemical methods will indefinitely increase the weight of this testimony, but the main thesis appears to be established, namely, that there is direct evidence of the presence of a primary metabolic gradient along the major axis of the body.

The indirect evidence is more easily appreciated by the general body of zoologists, and it is of the greatest interest. If the value of a hypothesis consists in the number of phenomena that are subsumed under it, then the gradient hypothesis on morphological evidence alone may take high rank. Old-established facts acquire new meaning.

The general succession of cellular events in animal development shows that the fertilised egg has a radial or bilateral symmetry before it exhibits cell division. Normal and experimental evidence point clearly to the conclusion that the first act of morphogenesis is the establishment in most animals of the head end, and in Coelenterates of an apical region. This is followed by the development of the dorsal surface in Vertebrates, and of the ventral surface in most Invertebrates, determining in each case the foundations of the nervous system. Simultaneously the lateral organs are laid down usually in the form of segments,' the outer part of which remains more embryonic and plastic, whilst the inner part, abutting on the axis of the embryo, undergoes more rapid and elaborate morphogenesis. The whole process of the gradual establishment of the primary rudiments of bodily structure in the embryo is not only consistent with the theory of gradients, but receives (perhaps for the first time) a rational explanation.'

(B) Regeneration.

Perhaps even more suggestive than the facts of individual development are the conclusions of experiment, both natural and artificial, upon the regeneration of animal organs and tissues. The main facts as to the extent and occurrence of the faculty for renewal of lost parts by animal tissues are well known, and need not be traversed here, but there are some special cases that are little known, and that form a test of the validity of the gradient hypothesis. Moreover, as this view grew out of the consideration of data given by the regeneration of animals, it is appropriate that this large body of analytical work should receive mention.

Child's work, and that of his pupils, has shown that in certain freshwater Planarians, only experimental difficulties set a limit to the minimal quantity of the body that will regenerate the whole. If and when these difficulties are overcome, it is probable that a single isolated cell of many of the lower animals may be induced to regenerate the whole, as is the casein many plants. We are only at the threshold of these inquiries, and the progress of tissueculture, which is now being actively pursued, will undoubtedly open up new ranges of control over the technique of physiological isolation. It will be remembered that H. V. Wilson and J. S. Huxley (7) have shown that from the artificial fragmentation of a sponge or hydroid, new individuals arise. From a few of those fragments-sheddings composed of cellgroups, and even a few isolated cells placed in suitable conditions-there arises by cellular conjunction a small amorphous mass, which acquires polarity and differentiation, and forms a new sponge or hydroid, recalling the reconstitution of an exceeding great army' in Ezekiel's vision of the valley of dry bones. We seem driven to the conclusion that every cell of these animals only develops a portion of its potentiality when actively

functioning as a part of the whole, and that each cell has in addition the opposite faculty of dedifferentiation-of becoming young and resistant at the same time. When this rejuvenated cell develops either singly or in company with other dedifferentiated cells, the resultant in either case exhibits a new metabolism and a new orientation, giving rise to an organism with typical arrangements of dominance and subservience of parts, such as characterise all normal animals.

The morphology of fixed colonial animals such as corals acquires fresh interest when considered in the light of this principle. Wood-Jones (8), as Child has pointed out, has found from a study of living Madrepora under natural conditions, that there is an apical radially symmetrical zooid at the top of the stem which give rise by budding to bilaterally symmetrical lateral zooids. These, however, do not bud off others so long as the apical zooid is present and active, until by growth of the whole shoot' they become separated by a certain distance from the dominant apex. When that occurs, one of them becomes transformed into a radial member, puts out lateral zooids and becomes a new apex. If the apical shoot and stem are removed, several branches may arise by transformation of bilateral into radial reproducing zooids. The whole process so strikingly recalls the fundamental relations of dominance and isolation leading to organic reproduction in animals and also in plants that Child does not doubt the general applicability of the principle to organisms in general.

(C) Independence of the Apical Region.

One of the most striking pieces of evidence on the subject of regeneration is the work of Ivanov on certain sea-worms, Spionids and Serpulids. Unfortunately, the greater part of the work (1912) is in an inaccessible Russian dissertation (9), but the first part of it appeared in 1908 as a continuation of his earlier researches on Lumbriculus, a fresh-water worm. In order to make the results clear, reference must be made to Ivanov's division of the Annelid body. By reason of certain peculiarities of the mesoderm of the anterior segments, he accounts as cephalic, or, as he later calls them, ' larval' segments, not only the prostomium and peristomium of zoological nomenclature (i.e. the apical and sub-apical segments), but those which follow, so long as they possess certain mesodermal characteristics. The rest of the body he calls post-larval.' This postlarval body is specialised in Serpulids into a thoracic and an abdominal portion. If now the head' or three larval segments of Spirographis be removed, the process of regeneration is no simple or direct operation, but resembles, to a remarkable degree, the embryonic development of these segments; whilst the regeneration of the body-segments proceeds in a different way, but also along the lines of the embryonic development of that region. What, however, chiefly concerns my argument is the establishment of a new head, not by morphollaxis (dedifferentiation followed by reconstruction on a new type), but by the appearance of an apical plate typical of the trochosphere stage, of pre-oral antennae (which have disappeared from the Serpulid trochosphere), and of the cerebral ganglia by thickenings that correspond to the ciliated pre-oral bands of the trochosphere. The interior of this dedifferentiated thoracic end of the decapitated body is now filled by immigration of ectoderm cells that assemble in three