cooperate in the maintenance of a direct course towards the light.

But even if we invoke the aid of both the factors mentioned, we are not able completely to bring the phototaxis of Ranatra under the category of direct, mechanical reflexes. Consider the behavior of the Ranatra with its right eye blackened over, placed at right angles to the rays with the normal eye looking away from the light. The effect of light upon the muscular tonus of the creature would tend to make it turn towards the left side. But either at first, or after a few trials, the insect turns directly to the right and goes to the light. This is done with greater readiness after several trials are made; soon the insect turns immediately after being set free. In these experiments the specimens were turned about, first one way and then the other, before being set down on the table, but this made no difference in the directness with which they turned towards the light. In many cases the insects would struggle to turn towards the blind side to get to the light before they were liberated from my hand. So far as could be observed, there was no preliminary feeling about, no employment of the method of trial and error; the insect seemed to retain awareness of the position of the light, since it immediately made for it by the shortest route as soon as it was liberated. The non-mechanical character of the response is further evinced by the fact that habits of turning towards a certain side may be persisted in in situations in which turning would otherwise take place in the opposite direction. The fact that Ranatras and Notonectas which have one eye blackened over come, after several trials, to dispense with circus movements towards the normal side and correct deviations from their course by a direct turn in the right direction affords further evidence for the same conclusion. We have seen that Ranatras which at first will turn only a slight way towards the blinded side when the light is carried over in that direction will make sharper turns after they have become accustomed to following the light around towards that side. These features of the phototaxis of Ranatra indicate that seeking the light has an attractiveness or interest much like that which catching prey has

for a higher animal. The phototactic movements of the creature are not merely stereotyped reflexes which the insect is involuntarily forced to perform. To get to the light is an end which is arrived at if not by one method, then by another.

The phototaxis of Ranatra comes, to a considerable degree, I believe, under the pleasure-pain type of response. Why a creature should be so constituted as to derive satisfaction from so stupid a performance as wildly chasing after a strong light is a subject that need not concern us in our present quest. The behavior of Ranatra presents the essential features of the pleasure-pain reaction of higher forms, and we are justified, I believe, in classing it under this heading. The fact that Ranatra will continue to follow the light even when it is brought thereby into a situation that produces a fatal effect does not necessarily exclude its conduct from this category. While it is true that animals tend to continue reacting towards stimuli that produce a beneficial effect and away from stimuli that bring about deleterious results, the rule is by no means absolute. In a state of nature Ranatras probably are rarely, if ever, exposed to conditions that produce as strong positive phototaxis as they show under artificial conditions in the laboratory; and there seems to be no benefit that possibly can be derived from their strong propensity to seek the light. This propensity, like that of human beings for certain stimulants and narcotics, has probably not been evolved by natural selection for any useful purpose, but is an incidental result of the way the creature is constituted. Whether there is any connection between pleasure-giving acts which tend to be repeated and acts which secure some benefit to the organism closer than that which would naturally be established through selection may well be doubted. Neither an animal's direct reflexes nor its attempts to seek some source of gratification infallibly lead to securing some benefit; and the fact that a certain kind of behavior is persisted in until it brings about fatal effect does not ipso facto enable us to decide under which of these categories it falls.

It is the apparent telecity in the efforts of Ranatra to reach the light which it is difficult to understand according to a purely

reflex theory of phototaxis. This is a feature of the creature's behavior which, I am inclined to believe, we shall not be able to understand until we can explain the physiology of the process whereby certain stimuli when they have been received one or more times are sought either directly, or indirectly, by a more or less round-about method, while other stimuli when they are experienced one or more times come to be avoided. Did light afford a stimulus of an unpleasant nature, it is probable that the positive phototaxis of Ranatra would soon be inhibited. If the phototaxis of Rantra falls to a certain extent under the category of reflex action, as much in its behavior indicates, the reflexes concerned are in line with a strong instinctive interest of the animal in seeking the light. This interest may lead to successful attempts to get to the light in situations in which purely reflex responses alone would fail. It acts as a sort of regulatory agent in the conduct of the insect, bringing its actions to a successful issue, which could not be attained by a purely machine-like mode of response.

It is through instinctive interests in certain things rather than by simple or even complex reflex acts that the conduct of higher animals is mainly guided. The play activities of higher animals, for instance, are performed, not because they are reflex responses to particular things in the environment, but because the animal is so constituted that it derives satisfaction from their performance. An animal interest may be chained, by virtue of its organization, to certain ends, such as the capture of a certain kind of prey, or the construction of a particular kind of habitation; but at the same time its conduct may show considerable plasticity as regards the methods by which these ends are attained. In instinctive behavior, as in the morphogenic processes which lead to the establishment and maintenance of the normal form of the body there is an apparent effort to reach a certain end result, despite obstacles and unusual conditions. The explanation of this peculiarity of animal behavior is a problem of fundamental interest. If we attempt to resolve highly complex modes of behavior into simple direct reflexes, we soon find ourselves at the end of our tether. Even in so apparently

mechanical procedure as the phototaxis of Ranatra we encounter peculiarities which indicate that we have not struck bottom in our analysis of the phenomenon.

In many ways the phototaxis of the Ranatra seems to be intermediate between purely reflex conduct on the one hand and conduct of the pleasure-pain type on the other. These two kinds of behavior seem to be harmoniously combined in many instincts, if not in most of the more highly involved modes of instinctive action. Just what the steps are which lead from the one to the other we do not know. We are still in want of a satisfactory explanation of the pleasure-pain type of response. When we are able to supply one we shall be in a position to give a more adequate interpretation of the phototaxis of higher forms than can be supplied at the present time.

LITERARY NOTICES.

Verhandlungen der Anatomischen Gesellschaft auf der Achtzehnten Versammlung in Jena. Anat. Anz. Erganzungsheft zum 25 B.

1904.

Schultze, O. Ueber die Entwickelung des peripheren Nervensysteme. 2-7. The growing nerves in the embryo are syncytial in structure. The sheath nuclei are derived from the nuclei of this nervous syncytium. The plexus nervosus profundus of amphibian larvae (CZERMAK) is a sensory syncytium which is, and has arisen, in continuo with the developing nerve, and is not derived by a fusion of independent units. SCHULTZE'S interpretation is in opposition to the neurone theory. Koelliker, A. Ueber die Entwickelung der Nervenfasern. 7-12.

This paper is directly opposed to SCHULTZE'S and in favor of the neurone theory. Each axone grows out as a process of a single nerve cell and the sheath nuclei are mesodermal in origin. This mode of development, the author asserts, is followed in Vertebrata, Arthropoda and Mollusca. The process may be simpler in lower forms.

In the discussion which follows these two papers, FRORIEP argues for the ectodermal origin of the sheath nuclei by a migration from the central system along the nerve roots. RETZIUS, BENDA, BALLOWITZ, HARRISON and DISSE cite various instances and observations in favor of the neurone theory. The necessity of experimental work upon the regeneration of the peripheral end of the severed nerve fiber is emphasized by Roux and BARFURTH.

Joseph, H. Ueber eigentümliche Zellstrukturen im Zentralnervensystem von Amphioxus. 16-26.

The cells in question are the large cells in the anterior region of the nerve cord which v. KUPFFER called "dorsale Ganglienplatte.” The author finds similar cells in the corresponding position in the caudal region of the cord. All of these dorsal cells, contrary to other authors who differ among themselves, are unipolar, and, in structure, correspond exactly with the "Sehzelle" of HESSE. They are not, however, capped with the pigment cell which is characteristic of the typical "Sehzelle." Their axones pass towards the periphery of the cord and probably enter the posterior roots. In the caudal region of the cord "Sehzelle" occur without the pigment cap, in which case they are identical in structure with the dorsal plate cells of the same region.

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