gone so far that in known vertebrates few or no segments contain, even in the form of embryonic vestiges, all of the structures which were typically present in the anterior segments of the ancestral vertebrates. On this account it has been extremely difficult to reconstruct complete segments, even where our information as to existing structures is apparently complete. Not only have certain organs with their nerves and nerve centers undergone reduction, or disappeared, but new organs have arisen and old ones have been modified so that new nerves and centers have replaced or overshadowed old ones. Furthermore it appears that as a result of these changes various organs and nerves have been displaced from their proper segments by the crowding of lately formed highly developed structures. We are met by a present condition which is the result of processes of reduction and disappearance, modification and growth, and shifting of position of organs whose real segmental relations are to be discovered only by tracing them back to their primitive condition. Thus the deciphering of the segmental relations, depending, as it does, on the proper interpretation of the homology and phylogenetic history of all the organs, will carry with it the solution of the major problems of head morphology. It must not be hoped that the last word upon any of these problems can be said at once. The data are yet too meager for the solution of this greatest problem of the evolution of animal structure, in spite of the efforts of a large number of workers directed to it during several decades. What the writer hopes to do in the present paper is: first to apply a new method for the interpretation of head segmentation which recent work on the nervous system has made possible; and second to point the way to some profitable lines of investigation.

b. Nerve components.

The function of the nervous system to coördinate and direct all the organs of the body requires a definite and constant relation of its constituent parts to the several tissues and organs. The structural and functional relationships within the organism become impressed upon the nervous system and the

arrangement of its cells and fibers serves as a guide to these relationships. Further, by a study of its residual or vestigeal structures and its ontogeny in various classes of animals we gain hints of many of the past relationships between the other organs. On account of its function as a go-between, the nervous system becomes for us an interpreter. The truth of this has long been realized but it has become of practical value only since the work of OSBORN, ALLIS and EWART led the way to the development of the theory of nerve components by STRONG, HERRICK and others. According to this theory, those fibers in the cranial nerves which supply the same kind of end organs enter the same or comparable regions in the brain. For example, all general cutaneous fibers (free nerve endings in the skin) enter the spinal V tract in the brain and the dorsal tracts in the cord. It happens that a nerve trunk is commonly composed of two or more sets or kinds of fibers. All those fibers in the several nerves which have the same central and peripheral connections are said to belong to one and the same system of nerve components.

Although this theory concerns itself directly with the analysis of peripheral nerve trunks, its value rests upon the existence of functionally distinct types of end organs on the one hand and distinct nerve centers in the brain on the other hand. The analysis of nerve trunks into nerve components necessarily implies the analysis of the nervous system as a whole from the same point of view, that is, on the basis of function. In this new way of looking at the nervous system the brain, which from the standpoint of structure has always been regarded as the most complex and obscure portion, becomes the most illuminating-and this just because of its complex relations. Three considerations are important in this connection. First, the reasoning on which the theory of nerve components rested could not be made good unless it was shown that the fibers of two components were independent in their central relations. For example, unless it were clearly shown that the centers in which the general cutaneous fibers end are structurally and hence functionally distinct from those into which sensory fibers

from the mucosae enter, we could not assume that the observed distinct peripheral course of these two sets of fibers had any functional significance. If it had proved that the fibers from the skin and those from the mucosae entered the same centers and had the same primary and secondary connections within the brain, we should have been compelled to conclude that sensory impulses from the skin and those from the mucosae would bring about the same reactions. As a matter of fact it has proved not only that the primary centers of these two sets of fibers are distinct, but that their secondary and tertiary fiber tracts remain separate and distinct. Hence it may be concluded that any fiber bringing impulses into the cutaneous center must set up reactions which characteristically follow cutaneous stimulation. The same reasoning may and must be applied to each system of nerve components. Furthermore, the central relations of any system of nerve components when certainly known serve as a starting point from which one may reason back with perfect validity to the homology and probable function of the organs innervated by that system. To illustrate by a case which is still under some dispute, it was by this method that the writer concluded that the end buds belonged to the visceral sensory system before their gustatory function had been proved by extended experiment (67, 49).

Second, the brain sometimes gives clues to primitive functional relationships of which the peripheral nerves no longer bear traces. In certain segments of the head this or that nerve component is wholly lacking owing to the disappearance of the structure which it originally innervated. Yet in these same segments the brain retains in some cases the center or column which this component should enter. A simple example of this is the presence of the somatic motor column throughout the medulla oblongata even in those forms in which several segments are without any other vestige of ventral motor nerves. The cases of the cerebellum and tectum opticum have been considered at some length in previous papers (67, 68, 69, 70), and these and other cases of the same sort will be treated further in the present paper. Wherever the brain contains a

representative of one of the primary columns there is presumptive evidence that in the primitive vertebrate the corresponding peripheral structures were present in the same segment.

Third, in cases of extreme modification of other systems of organs and of the peripheral nervous system the brain retains structural features which serve to point the way to the interpretation of new structures. The changes of structure and function, so far as yet observed, are less fundamental than the primitive functional divisions of the nervous system. "New structures" arise as modifications within one division, not by structures belonging to one functional division taking on functions belonging to another division. It does not happen, for example, that cutaneous structures (sense organs, components, centers, or fiber tracts) are so modified as to serve visceral functions, or vice versa. This conservatism of the brain aids in the interpretation of structures which otherwise would be very difficult to understand. By this method of reasoning the evidence that the ear is a part of the lateral line system has been completed, and it has been shown that both ear and lateral line system have arisen by modification within the primitive general cutaneous division of the nervous system. The principle will be applied in this paper for the interpretation of the eye and other organs. It is perhaps needless to say that this principle has been only slightly tested and it is not yet known whether it is universally true. While it promises to be of great value, it must be used with extreme caution.

C. Functional divisions of the nervous system.

Such considerations as the above led the writer some time since to offer a scheme of the primitive functional divisions of the nervous system. This theory is not only in accord with the theory of nerve components but is an extension of that theory so as to give due recognition to the central organ. It is unnecessary here to enter into explanations or arguments in support of this theory of functional divisions beyond the matter contained in previous papers (69, 70). Here a concise outline of the functional divisions is given, with their subdivisions and

constituent elements.

This outline differs from the one previously published chiefly in that the visual and olfactory organs and the sympathetic system are assigned to definite places in the scheme, a thing which was not attempted before.

general cutaneous system of components,

dorsal tracts of the cord, spinal V tract in the medulla oblongata, together with their accompanying nuclei: the dorsal horn, nucleus funiculi, nucleus trigemini spinalis, acusticum and cerebellum, secondary tracts and centers: internal and external arcuate fibers forming the tractus spino- and bulbo-tectalis (fillet), tectum opticum, colliculus and other nuclei,

tertiary tracts to motor nuclei and coördinating centers.

2. Special cutaneous subdivision.

Consists of:

neuromasts (acustico-lateral sense organs),

neuromast components,

spinal VIII tract and nucleus, nucleus funiculi, acusticum, cerebellum, secondary tracts and centers and tertiary tracts as in I,

the cochlea, its nerve and centers in higher vertebrates.

3. Special sense organs belonging to the somatic sensory division. lateral eyes. Consist of:

retina, which includes the equivalent of sensory ganglion, nerve component, and primary brain center,

optic tract and tectum opticum corresponding to the secondary tracts and centers of the cutaneous subdivision.

pineal eyes. (Compare Sec. 13 below).

B. Splanchnic sensory division.

1. General visceral subdivision. Consists of:

free nerve endings in the mucosae,

fasciculus communis system of components,

CLARKE'S column or its equivalent, nucleus commissuralis CAJAL, lobus vagi, and lobus facialis,

secondary vagus tract and its continuation in the cord, = the direct cerebellar tract in higher forms,

secondary vagus nucleus, end nucleus of direct cerebellar tract in the vermis of higher forms,

tertiary tracts not well known; part probably run to the thalamus or hypothalamus in lower vertebrates.

2. Special visceral subdivision. Consists of:

end buds (taste buds),

components and central nuclei and tracts not yet distinguished from those of the general visceral subdivision.

3. Special sense organ belonging to the splanchnic sensory division. Consists of olfactory epithelium and nerve, bulbus olfactorius, tractus