no change of any kind whatsoever can be discovered in the dorsal root, showing that the same path can not be traversed in the reverse direction. As intraneural conduction is well known to occur in both directions this interruption is believed to be interneural, that is, synaptic; hence the conclusion that the synapse is a valve-like mechanism that permits the passage of an impulse in one direction only. The polarity of a deep-seated neurone then is determined by its synapse.

If we divest our minds of the assumed nervous significance of the cell body of the neurone and of all the misleading terminology of cellulipetal and cellulifugal conduction in relation to dendrites and neurites, we have left the simple proposition that neurones, though capable of double conduction within themselves, nevertheless conduct normally in one direction only. This is the real and sufficient basis for neuronic polarity. That it is partly dependent upon the synapse is quite obvious. For this and other reasons the synapse has been a matter of much concern to neurologists, but its extreme minuteness has been a baffling feature in its investigation. The researches of His (1886) supported by those of Harrison (1901) and others have shown that in many parts of the nervous system neuroblasts that give rise to contiguous neurones in adult neuronic chains may be in embryonic stages far separated and come together only after considerable growth. Of their initial separation there can be not the least doubt; the question that naturally arises concerns the extent of their final union as they establish synaptic relations. That this can not be complete has already been pointed out in discussing synaptic transmission, but precisely what the incompleteness consists in from a histological point of view is by no means easily determined. Bartelmez (1915) has had

the opportunity of studying the synapse under conditions in which the elements were extremely coarse (the Mauthner's cells of fishes) and he finds, as might have been expected, no continuity, but delicate. membranous separations. These membranes must be the parts concerned with synaptic activities and hence with the polarity of the neurone so far as it is dependent upon the synapse.

Thus after years of infinite pain and labor the neurologist of to-day can describe in terms of cells the nervous system of one of the higher animals as composed of an intricate association of neurones whose relations to the animal as a whole and to each other through synaptic contact have impressed upon these structures a definite form of polarity.

As this idea of the synaptic nervous system gradually unfolded itself to the more orthodox neurologists, there arose from another school of workers the diametrically opposite conception of the nervenet. This new movement received its initial impetus chiefly from the work of Apáthy (1897), who maintained on the basis of preparations of almost incredible clearness that the nervous elements of many animals were bound together by a network of neurofibrils in which there was not the slightest evidence of interruption such as is implied in the synapse. This view in a way was a revival of the idea of a continuous network as maintained in a previous generation by Gerlach. The careful reader of Apáthy's papers will find it by no means easy to separate in them fact from speculation and consequently it is difficult to state in exact terms Apáthy's real contribution to this subject, but, however this may be, it is certainly true that the appearance of his publications excited others to a further investigation of the subject with the result that nerve-nets were

eral as would be expected from the structure of their nets. Probably the only evidence of polarity that these nets exhibit is seen in the temporary condition that has been claimed for them by von Uexküll, namely, that impulses flow for the moment more freely through them into stretched regions than into unstretched regions. Aside from this momentary state they are probably quite unpolarized in their transmitting capacities.

From such a condition as this it ought to be possible to trace the transition stages that have led to the synaptic nervous system, and, in fact, examples of this kind are not difficult to find. As a first step in this direction we may examine the tentacles of sea-anemones. These organs were shown by Ránd (1909) to exhibit in their responses to stimulation a marked polarity. If a stimulus is applied to the tip of a tentacle, the whole tentacle usually shortens. If it is applied to any other point on the tentacle, this organ shortens as a rule only from the point stimulated to the base, the distal portion of the tentacle remaining unchanged. Hence it may be concluded that transmission does not proceed from any region in the tentacle freely in all directions, but only towards its base; in other words, the tentacle exhibits polarity. As this polarity disappears on treating the tentacle with chloretone or other anesthetizing agents, it is clear that it is a nervous polarity. The neuromuscular mechanism of the tentacle is well known to consist of peripheral sense cells whose deep ends are much branched constituting a nerve-net that is applied to the longitudinal muscle cells of the tentacular ectoderm. The polarity of the tentacle depends upon a peculiarity in the structure of the sense cells as pointed out by Groselj (1909), namely, that most of the fibrous prolongations from the deep ends of these cells, in

stead of spreading out in all directions, extend down the tentacle towards the base. Hence, when the sense cells are stimulated, nerve impulses are generated, which, in consequence of the direction of the cell fibers, are conducted into the proximal portion of the tentacle, where they call forth the contraction of the longitudinal muscle cells. Here then is the first evidence of permanent nervous polarity such as is so clearly shown in the neurone. occurs in a nerve-net without synapses, but so organized that its fibrous constituents, instead of being diffusely arranged, have a predominating trend in one direction.

It

Judging from the nature of the responses, polarized nerve-nets occur in many other places. Thus the stalk of the giant hydrozoan, Corymorpha, has recently been shown to transmit nervous impulses more freely on its length than transversely, a condition that immediately suggests locomotor waves that pass over the foot of a creeping snail are believed with good reason to depend upon the presence of a nerve-net, in which case the net must be strongly polarized, for these waves are limited in almost every instance to a single direction. In a similar way the peristalsis of the vertebrate digestive tube implies a polarized net in the wall of that structure. Thus the primitive, diffuse, or apolar, nerve-net may be imagined to undergo the first change toward a synaptic system by becoming polarized, a process that may be described as due to a lengthening of its fibers in one direction, whereby transmission in that direction predominates over transmission in any other. The cells whose processes exhibit this change are the ordinary sense cells and nerve cells of the nerve-net. They may be looked upon as the forerunners of neurones, protoneurones so to speak, and from them have arisen by further differentiation the highly special

ows the final stage of this process as seen in vertebrates where the cell bodies are almost invariably at the receptive ends of centrally situated neurones (Fig. 4, C). Thus, in the evolution of the protoneurone of the nerve-net into the neurone of the specialized central organ, the cell body migrates from a central position to a polar one at the receptive end of the neurone.

The second type of migration is quite the reverse of that just described. It is seen in the sense cells of the nerve-nets and in those cells that are derived from them and that are associated with the more differentiated sensory surfaces of the higher animals. This type of migration was long ago pointed out by Retzius (1892) and his account needs only to be supplemented by what is now known of the cœlenterates in order to bring it thoroughly up to date. In the cœlenterates the sense cell, or receptive protoneurone, has its cell body at its receptive end whence its fibrous prolongation reaches into the nerve-net (Fig. 5, A). Much the same con

A

B

C

FIG. 5. Sensory nerve-cells; A, sensory nervecell from a cœlenterate; B, sensory neurone from a mollusk; C, sensory neurone from a vertebrate. In each example the peripheral end of the cell is toward the left.

dition is found in the earthworm, though in many other worms the cell body has moved to a deeper position, leaving only a process of the cell in connection with the

sensory surface. In mollusks this inward migration of the cell body is still more pronounced (Fig. 5, B.) And finally in vertebrates (Fig. 5, C), the cell body of what has now become the primary receptive neurone has migrated so far inward as to come to lie much nearer to the central nervous organ than to the peripheral receptive surface from which it started. Thus in the two types of cells, peripheral and central, the directions of migration are opposite, for while in the primary sensory neurone the cell body has moved away from the receptive pole, in the central neurone it has moved to that pole.

These migrations, in my opinion, are not to be interpreted as direct expressions of nervous changes in the neurone, as would probably have been surmised by the older school of neurologists. They are the migrations of the trophic center of the cell and they probably find their explanation in the changed metabolic needs of the evolving neurone rather than in its immediate nervous changes. Something of what these relations are may be gathered from the conditions presented by the receptive neurones of the chemical senses of vertebrates. Of these the most primitive are the olfactory neurones in which the trophic center is at the receptive end of the cell (Fig. 6, A) reproducing in this respect the conditions found in the integument of seaanemones and of earthworms. Next in sequence are the receptive neurones of the common chemical sense (Fig. 6, B) in which the trophic center has migrated far inward toward the central organ, a strictly vertebrate condition. The last members of the series are the receptive neurones of the sense of taste (Fig. 6, C), which are like those of the common chemical sense, except that they have appropriated distally certain integumentary cells, often called sec