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Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.

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Chapter I Salamanders and their Brains

The Salamanders

Salamanders are shy little animals, rarely seen and still more rarely heard. If it were not so, there would be no salamanders at all, for they are defenseless creatures, depending on concealment for survival. And yet the tiger salamander, to whom this book is dedicated, is appropriately named, for within the obscurity ol its contracted world it is a predaceous and voracious terror to all humbler habitants.

This salamander and closely allied species have been found to be so well adapted for a wide range of studies upon the fundamental features of growth and differentiation of animal bodies that during the last fifty years there has been more investigation of the structure, development, and general physiology of salamanders than has been devoted to any other group of animals except mankind. The reason for this is that experimental studies can be made with these amphibians that are impossible or much more difficult in the case of other animals. This is our justification for the expenditure of so much hard work and money upon the study of the nervous system of these insignificant little creatures.

The genus Ambystoma is widely distributed throughout North America and the tiger salamander, A. tigrinum, is represented by several subspecies. The individuals vary greatly in size and color, and the subspecies have different geographical distribution, with some overlap of range (Bishop, '43). The subspecies, A. tigrinum tigrinum (Green), ranges from New York southward to Florida and westward to Minnesota afid Texas. It has a dark-brown body crossed by bright-yellow stripes, as shown in the lower figure of the Frontispiece. The species probably was named for these tiger-like markings, not for its tigerish ferocity. The upper figure of the Frontispiece is an adult of a western form, with less conspicuous markings. Other subspecies range as far to the northwest as Oregon and British Columbia. Several other species of Ambystoma are found in the same areas as A. tigrinum.

Zoological names. — The approved names of the genus and larger groups to which reference is here made, as given in a recent official list (Pearse, '36), are as follows:

Salienta, to replace Anura

Caudata, to replace Urodela

Ambystoma, to replace Amblystoma Ambystoma (or Siredon) maculatum has priority over A. punctatum. The names, Anura, Urodela, and Amblystoma, are used throughout this text because they are so commonly found in current literature that they may be regarded as vernacular terms.

The Scope of this Inquiry

From the dawn of interest in the minute structure of the human brain, it was recognized that the simpler brains of lower vertebrates present the fundamental features of the human brain without the numberless complications which obscure these fundamentals in higher animals. This idea motivated much research by the pioneers in neuroanatomy, and it was pursued systematically by L. Edinger, H. Obersteiner, Ramon y Cajal, C. L. Herrick, J. B. Johnston, Ariens Kappers, and many others. In 1895, van Gehuchten wrote that he was engaged upon a monograph on the nervous system of the trout, "impressed by the idea that complete information about the internal organization of the central nervous system of a lower vertebrate would be of great assistance as our guide through the complicated structure of the central nervous system of mammals and of man." The few chapters of this monograph which appeared before his untimely death intensify our regret that he was not permitted to complete this work. Van Gehuchten's ideal has been my own inspiration.

Our primary interest in this inquiry is in the origins of the structural features and physiological capacities of the human brain and the general principles in accordance with which these have been developed in the course of vertebrate evolution. This is a large undertaking. What, then, is the most promising approach to it.'^ My original plan was, first, to review all that has been recorded about the anatomy and physiology of the nervous systems of backboned animals, to arrange these animals in the order of their probable diverse specialization from simple to complex in the evolutionary sequence, then to select from the series the most instructive types and subject them to intensive study, in the expectation that the principles underlying these morphological changes would emerge.

So ambitious a plan, however, is far too big to be encompassed within the span of one man's lifetime. The fact-finding research is extremely laborious and exacting; and, during the fifty years which have elapsed since my project was formulated, the descriptive literature has increased to enormous volume. This literature proves to be peculiarly refractory to analysis and interpretation. Until recently this vast accumulation of factual knowledge has contributed disappointingly little to the resolution of the fundamental problems of human neurology. Nevertheless, the method is sound, and this slow growth is now coming to fruition, thanks to the conjoint labors of specialists in many fields of science. What no individual can hope to do alone can be done and has been done in co-operative federation, as illustrated, for instance, by the Kappers, Huber, and Crosby team and their many collaborators.

Traditionally, comparative neurology has been regarded as a subdivision of comparative anatomy, and so it is. But it is more than this. The most refined methods of anatomical analysis cannot reveal the things that are of greatest significance for an understanding of the nervous system. Our primary interest is in the behavior of the living body, and we study brains because these organs are the chief instruments which regulate behavior. As long as anatomy was cultivated as a segregated discipline, its findings were colorless and too often meaningless.

Now that this isolationism has given way to genuine collaboration among specialists in all related fields — physiology, biochemistry, biophysics, clinical practice, neuropathology, psychology, among others — we witness today a renaissance of the science of neurology. The results of the exacting analytic investigations of the specialists can now be synthesized and given meaning. The task of comparative anatomy in this integrated program of research is fundamental and essential. The experimentalist must know exactly what he has done to the living fabric before he can interpret his experiment. In the past it too often happened that a physiologist would stab into a living frog, take his kymograph records, and then throw the carcass into the waste-jar. This is no longer regarded as good physiology. Without the guidance of accurate anatomical knowledge, sound physiology is impossible; and, without skilful physiological experimentation, the anatomical facts are just facts and nothing more.

Early in my program the amphibians were selected as the most favorable animals with which to begin a survey of the comparative anatomy of the nervous system. Time has proved the wisdom of this choice, and the study of these animals has been so fruitful that by far the larger part of my research has been devoted to them.

In this work it was my good fortune to be associated with the late G. E. Coghill, whose distinguished career pointed the way to an original approach to the problems of the origins and growth of the nervous organs and their functions. The record of phylogenetic history spans millions of years and is much defaced by time; but the record of the embryonic development of the individual is measured in days and hours, and every detail of it can be watched from moment to moment. The internal operations of the growing body are not open to casual inspection, but Coghill showed us that the sequence of these changes can be followed.

He selected the salamanders for his studies for very good reasons, the same reasons that led me to take these animals as my own point of departure in a program of comparative neurology. My intimate association with Coghill lasted as long as he lived, and the profound influence which his work has had upon the course of biological and psychological events in our generation has motivated the preparation of a book devoted to his career ('48). This influence, though perhaps unrecognized at the time, was doubtless largely responsible for my persistent efforts to analyze the texture of the amphibian nervous system, for his studies of the growth of patterns of behavior and their instrumentation in young stages of salamanders brought to light some prmciples which evidently are applicable in phylogenetic development also.

While Coghill's studies on the development of salamanders were in process, we were impressed by the importance of learning just how these processes of growth eventuate in the adult body. This was my job, and so we worked hand in hand, decade after decade, for forty years. Progress was slow, but our two programs fitted together so helpfully that my original plan for a comprehensive study of the comparative anatomy of the nervous system was abandoned in favor of more intensive study of salamanders.

The Plan of this Book

The preceding details of personal biography are relevant here because they explain the motivation and plan of this book. The significant facts now known about the internal structure of the brain of the tiger salamander in late larval and adult stages are here assembled. The observation's on this and allied species previously recorded by the writer and many others are widely scattered, often in fragmentary form, and with confusing diversity in nomenclature and interpretation. As observations have accumulated, gaps in knowledge have been filled, early errors have been corrected, the nomenclature has been systematized, and now, with the addition of considerable new observation here reported, the structure may be viewed as a whole and interpreted in relation with the action system of the living animal. Many of my observations during the last fifty years confirm those of others; and, since references to these are given in the papers cited, this account is not encumbered with them except where they supplement my own experience or deal with questions still in controversy. I here describe what I myself have seen, with exceptions explicitly noted. This explains the disproportionate number of references in the text to my own papers.

Two genera of urodeles have been studied intensively to find out what is the instrumentation of their simple patterns of behavior. All observations on the brain of the more generalized mudpuppy, Necturus, were assembled in a monograph ('336) and several followmg papers. The present work is a similar report upon the brain of the somewhat more specialized tiger salamander. The original plan was to follow this with an examination of the brain of the frog, for which abundant material was assembled and preliminary surveys were made; but this research, which is urgently needed, must be done by others.

In this book the anatomical descriptions are arranged in such a way as to facilitate interpretation in terms of probable physiological operation. Though the amount of experimental evidence about the functions of the internal parts of the amphibian brain is scanty, there is, fortunately, a wealth of such observation about the brains of other animals; and where a particular structural pattern is known to be colligated with a characteristic pattern of action, the structure may be taken as an indicator of the function. The reliability of this method depends upon the adequacy of our knowledge of both the function and the structure. The present task, then, is an assembly of the anatomical evidence upon which the interpretations are based.

The first part of this work is written to give biologists and psychologists an outline of the plan of organization of a generalized vertebrate brain and some insight into the physiological principles exemplified in its action. These eight chapters, with the accompanying illustrations, can be read independently of the rest of the book.

Part II is written for specialists in comparative neurology. It covers the same ground as the first part, reviewing each of the conventional subdivisions of the brain, giving details of the evidence upon which conclusions are based, with references to sources and much new material. This involves some repetition, which is unavoidable because all these structures are interconnected and in action they co-operate in various ways. Many structures must be described in several contexts and, accordingly, the Index has been prepared with care so as to enable the reader to assemble all references to every topic.

The most important new observations reported in Part II relate to the structure and connections of the isthmus (chap, xiii), interpeduncular nucleus (chap, xiv), and habenula, including analysis of the stria medullaris thalami and fasciculus retroflexus (chap, xviii). In chapter xx a few of the more important systems of fibers are described, including further analysis of the tegmental fascicles as enumerated in the paper of 1936 and references to other lists in the literature. The composition of the commissures of the brain is summarized in chapter xxi. The lemniscus systems are assembled in chapter xi, and other tracts are described in connection with the structures with which they are related.

Since most neurologists are not expert in the comparative field, where the nomenclature is technical and frequently unintelligible except to specialists, the attempt is made in Part I to present the salient features with a minimum of confusing detail and, so far as practicable, in terms of familiar mammalian structure. This is not an easy thing to do, and no clear and simple picture can be drawn, for the texture of even so lowly organized a brain is bafflingly complicated and many of these structures have no counterparts in the human body. Homologies implied by similar names are rarely exact, and in many of these cases the amphibian structure is regarded as the undifferentiated primordium from which the mammalian has been derived. This is emphasized here because homologies are usually defined in structural terms and because organs which are phylogenetically related are regarded as more or less exactly homologous, regardless of radical changes in their functions. Thus the "dorsal island" in the acousticolateral area of the medulla oblongata of Necturus is regarded as the primordium of the dorsal cochlear nucleus of mammals, despite the fact that Necturus has no recognizable rudiment of a cochlea or cochlear nerve. This is because, when the cochlear rudiment and its nerve appear in the frog, the tissue of the "dorsal island" receives the cochlear nerve with radical change in the functions performed (p. 138).

The histological texture of these brains is so different from that of mammalian brains that the development of an intelligible nomenclature presents almost insuperable difficulty — a difficulty exacerbated by the fact that in the early stages of the inquiry it was necessary to apply descriptive terms to visible structures before their relationships were known. With increase of knowledge, errors were corrected, and unsuitable names were discarded; but terms already in use are still employed so far as possible, even though they are in some cases cumbersome and now known to be inappropriate. In all these descriptions I have consistently used the word "fissure" to designate visible furrows on the external surface of the brain and "sulcus" for those on ventricular surfaces. Attention is called to the list of abbreviations (p. 391) and to previous lists there cited where synonyms are given. In all my published figures of brains of urodeles the intent has been to use the same abbreviations for comparable structures. This intention has been approximately realized, but there are some inconsistencies, in most of which the differences express a change in emphasis rather than a correction of errors of observation.

Many well-defined tracts of fibers seen in fishes and higher animals are here represented in mixed collections of fibers of diverse sorts, here termed "fasciculi," or they may be dispersed within a mixed neuropil. The practice here is to define as a "tract" all fibers of like origin or termination, whether or not they are segregated in separate bundles. The customary self-explanatory binomial terminology is used wherever practicable — a compound word with origin and termination separated by a hyphen. But, since a single fiber of a tract may have collateral connections along its entire length, the fully descriptive name may become unduly cumbersome ('41a, p. 491). Thus, in accordance with strict application of the binomial terminology, tractus strio-tegmentalis would become tractus striothalamicus et peduncularis et tegmentalis dorsalis, isthmi et trigemini. The chemists seem to be able to manipulate similar enormities even without benefit of hyphens or spaces, but not many anatomists are so hardy. Few of the named tracts are sharply delimited, and all of them are mixtures of fibers with different connections. Any analysis is necessarily somewhat arbitrary.

Simple action systems of total-pattern type, wherever found (cyclostomes, primitive ganoid fishes, urodeles), are correlated with a histological texture of the brain which is characteristic and probably primitive (chap. iii).

The external configuration of the urodele brain also is generalized, much as in a human embryo of about 6 weeks. In the next chapter special attention is directed to this comparison to assist the reader in identifying familiar parts of the human brain as they are seen in the simplified amphibian arrangement.

In our comparison of the amphibian brain with the human, two features are given especial emphasis, both of which are correlated with differences in the mode of life of the animals in question, that is, with the contrast between the amphibian simplicity of behavior with stereotyped total patterns of action predominating and the human complexity of movement in unpredictable patterns. The correlated structural differences are, first, in Amblystoma the more generalized histological texture to which reference has just been made, and notably the apparent paucity of provision for well-defined localization of function in the brain; and, second, the preponderant influence of motor patterns rather than sensory patterns in shaping the course of differentiation from fishlike to quadrupedal methods of locomotion and somatic behavior in general.

Sources and Material

The material studied comprises gross dissections and serial sections of about five hundred specimens of Amblystoma from early embryonic to adult stages. About half these brains were prepared by the Golgi method and the remainder by various other histological procedures. Most of these are A. tigrinum, some are A. maculatum (punctatum), and a few are A. jeffersonianum. In early developmental stages some specific differences have been noted in the embryological papers of 1937-41, but no systematic comparative study has been made. The late larval and adult brains under consideration in this book are of A. tigrinum. In former papers there are comments on this material and the methods of preparation ('25, p. 436; '35a, p. 240; '42, p. 193).

In the study of these sections the cytological methods of Nissl and others are less revealing than in more highly differentiated brains because of the unspecialized structure of the nervous elements. Some modifications of the method of Weigert which decolorize the tissue sufficiently to show the myelinated fibers and also the arrangement of cell bodies prove to be most useful for general orientation. Other details can then be filled in by study of reduced silver preparations and especially of Golgi sections. A favorable series of transverse Weigert sections (no. IIC; see p. 3'21) has been chosen as a type or standard of reference, and the median section as reconstructed from this specimen (fig. 2C) has been used as the basis for many diagrams of internal structure. For reference to published figures of this brain and other details concerning it see page 321. Figures 2A and B are similar diagrams of the median section of the specimen from which figures 25-36 were drawn. The topography shown in these median sections is the basis for the descriptive terms used throughout this text.

Except for scattered references to details, the only systematic descriptions of the brain of Amblystoma are in my papers, Bindewald's ('14) on the forebrain, and Larsell's ('20, '32) on the cerebellum. Mention should also be made of Roofe's account ('35) of the endocranial blood vessels and Dempster's paper ('30) on the endolymphatic organ.

Kingsbury's admirable paper on Necturus in 1895 may be taken as a point of departure for all further investigation of the brains of urodeles, including my monograph of 1933 and several preceding and following papers. Some of the more important descriptions of the brains of other urodeles are cited in the appended bibliography, notably the following: Salamandra (Kuhlenbeck, '21; Kreht, '30), Proteus (Kreht, '31; Benedetti, '33), Cryptobranchus (Benzon, '26), Gymnophiona (Kuhlenbeck, '22), Siren (Rothig, '11, '24, '27), and several other urodeles in Rothig's later papers, Hynobius, Spelerpes, Diemyctylus (Triturus), Cryptobranchus, Necturus.

For the Anura the excellent description of the frog by E. Gaupp in 1899 laid a secure foundation for all subsequent work, and the time is now ripe for a systematic restudy of this brain with the better methods now available and the correlation of the histological structure with physiological experiments specifically designed to reveal the action of this structure. Aronson and Noble ('45) have published an excellent contribution in this field.

Development of the Brain

No comprehensive description of the development of the brain of Amblystoma has been published. The difficulties met in staging specimens by criteria defined by Harrison, Coghill, and others I have discussed elsewhere ('48, chap. x). Griggs ('10) described with excellent illustrations the early stages of the neural plate and neural tube. Baker ('27) illustrated dorsal and ventral views of the open neural plate, and Baker and Graves ('32) described six models of the brain of A. jeffersonianum from 3 to 17 mm. in length. Burr ('22) described briefly the early development of the cerebral hemispheres.

Successive stages of the brain of A. punctatum have been illustrated by Coghill and others (some of which I have cited, '37, p. 391, and '38, p. 208), and at the Wistar Institute there are other models of the brains of physiologically tested specimens. Coghill's papers include a wealth of observation on the development of the mechanisms of the action system, and these were summarized in his London lectures, published in 1929. His reports were supplemented by a series of papers which I published from 1937 to 1941, but these fragmentary observations (of the younger stages particularly) were based on inadequate material and are useful only as preliminary orientation for a more systematic investigation. In Coghill's papers there are accurate projections of all mitotic figures and neuroblasts of the central nervous system in nonmotile, early flexure, coil, and early swimming stages and the arrangement of developing nerve fibers of the brain in the last-mentioned stage (Coghill, '30, Paper IX, fig. 4). On the basis of these data he divided the embryonic brain in front of the isthmus into sixteen regions, each of which is a center of active and characteristic differentiation. These regions are readily identified in our reduced silver preparations of these and later stages. Using a modification of this analysis, I have distinguished and numbered twentytwo such regions in the cerebrum and cerebellum ('37, p. 392), and the development of each of these can be followed through to the adult stage. In my papers of 1937-39 some salient features of these changes are recorded; but this account is incomplete, and more thorough study is urgently needed. In the present work some details only of this development are given in various contexts as listed in the Index under "Embryology."

The most detailed description of the development of the urodele brain is the paper by Sumi ('26) on Hynobius. Soderberg ('22) gave a brief description of the development of the forebrain of Triturus (Triton) and a more detailed account of that of the frog, and Rudebeck ('45) has added important observations.

The successive changes in the superficial form of the brain can be interpreted only in the light of the internal processes of growth and differentiation. The need for a comprehensive study of the development of the histological structure of the brain of Amblystoma, including the differentiation of the nervous elements and their fibrous connections, is especially urgent in view of the very large number of experimental studies on developmental mechanics which have been in progress for many years and will probably continue for years to come. Amblystoma has proved to be an especially favorable subject for these studies, and in many of them a satisfactory interpretation of the findings cannot be achieved without more complete knowledge than we now possess of the development of both the nervous tissues and other bodily organs.

The Evolution of Brains

The nervous systems of all vertebrates have a common structural plan, which is seen most clearly in early embryonic stages and in the adults of some primitive species. But when the vertebrate phylum is viewed as a whole, the nervous apparatus shows a wider range of adaptive structural modifications of this common plan than is exhibited by any other system of organs of the body. In order to understand the significance of this remarkable plasticity and the processes by which these diverse patterns of nervous organization have been elaborated during the evolutionary history of the vertebrates, it is necessary to find out what were the outstanding features of the nervous system of the primitive ancestral form from which all higher species have been derived.

Since the immediate ancestors of the vertebrate phylum have been extinct for millions of years and have left no fossil remains, our only recourse in this search is to examine the most generalized living species, compare them one with another and with embryonic stages, and so discover their common characteristics. This has been done, and we are now able to determine with a high degree of probability the primitive pattern of the vertebrate nervous system.

The most generalized living vertebrates (lampreys and hagfish) have brains which most closely resemble that of the hypothetical primordial vertebrate ancestor. The brains of the various groups of fishes show an amazing variety of deviations from the generalized pattern. The paleontological record shows that the first amphibians were derived from one of the less specialized groups of fishes; and there is evidence that the existing salamanders and their allies have preserved until now a type of brain structure which closely resembles that of the most primitive amphibians and of the generahzed fishes ancestral to them.

The internal texture of the brains of the generalized amphibians which are described in this work closely resembles that of the most primitive extant fishes; but the brain as a whole is organized on a higher plane, so that it can more readily be compared with those of reptiles, lower mammals, and man. For this reason the salamanders occupy a strategic position in the phylogenetic series. This examination has brought to light incipient stages of many complicated human structures and some guiding principles of both morphogenesis and physiological action that are instructive.

When the first amphibians emerged from the water, they had all the land to themselves ; there were no living enemies there except one another. During aeons of this internecine warfare they carried protective armor; but in later times, during the Age of Reptiles, these more efficient fighters exterminated the clumsy armored amphibians. The more active frogs and toads survived, and so also did the sluggish salamanders and their allies, but only by retiring to concealment in sheltered places.

In Devonian times, probably about three hundred million years ago, various species of fishes made excursions to the land and acquired structures adapted for temporary sojourn out of water. Some of the primitive crossopterygian fishes went further and, after a fishlike larval period, experienced a metamorphosis into air-breathing tetrapods. They became amphibians. These were fresh-water species, and the immediate cause of this evolutionary change was extensive continental desiccation during the Devonian period. While their streams and pools were drying up, those fishes which had accessory organs of respiration in addition to the gills of typical fishes, were able to survive and, through further transformations, become airbreathing land animals. An excellent summary of the paleontological evidence upon which the history of the evolution of fishes has been reconstructed has been published by Romer ('46).

Two prominent features of this revolutionary change involved the organs of respiration and locomotion, with corresponding changes in the nervous apparatus of control. These systems of organs are typical representatives of the two major subdivisions of all vertebrate bodies and their functions — the visceral and the somatic. The visceral functions and the visceral nervous system will receive scant consideration in this work, for the material at our disposal is not favorable for the study of these tissues. Here we are concerned primarily with the nervous apparatus of overt behavior, that is, of the somatic adjustments.

The most important change in these somatic adjustments during the critical evolutionary period under consideration is the transition from swimming to walking. The fossil record of the transformation of fins into legs is incomplete, but it is adequate to show the salient features of the transformation of crossopterygian fins into amphibian legs (Romer, '46). In the individual development of every salamander and every frog the internal changes in the organization of the nervous system during the transition from swimming to walking can be clearly seen. And these changes are very significant in our present inquiry because they illustrate some general principles of morphogenesis of the brain more clearly than do any other available data.

In fishes, swimming is a mass movement requiring the co-ordinated action of most of their muscles in unison, notably the musculature of the trunk and tail. The paired fins are rudders, not organs of propulsion. The young salamander larva has no paired limbs but swims vigorously. This is a typical total pattern of action as defined by Coghill. The adult salamander after metamorphosis may swim in the water like the larva; and he can also walk on land with radically different equipment. Some fishes can crawl out on land, but the modified fins are clumsy and ineflBcient makeshifts compared with the amphibian's mobile legs.

Quadrupedal locomotion is a very complicated activity compared with the simple mass movement of swimming. The action of the four appendages and of every segment of each of them must be harmoniously co-ordinated, with accurate timing of the contraction of many small muscles. These local activities are "partial patterns" of behavior, in Coghill's sense. From the physiological standpoint there is great advance, in that the primitive total pattern is supplemented, and in higher animals largely replaced, by a complicated system of co-ordinated partial patterns. This is emphasized here because it provides the key to an understanding of many of the differences between the nervous systems of fishes, salamanders, and mammals. Motility, and particularly locomotion, have played a major role in vertebrate evolution, as dramatically told by Gregory ('43). This outline has been filled in by Howell's ('45) interesting comparative survey of the mechanisms of locomotion, and I have elsewhere discussed ('48) Coghill's contributions to this theme.

In the history of vertebrate evolution there were four critical periods: (1) the emergence of the vertebrate pattern of the nervous system from invertebrate ancestry; (2) the transition from aquatic to terrestrial life; (3) the differentiation within the cerebral hemispheres of primitive cerebral cortex; (4) the culmination of cortical development in mankind, with elaboration of the apparatus requisite for language and other symbolic (semantic) instrumentation of the mental life.

The extinct ancestors of the vertebrates in early Silurian times were probably soft and squashy creatures, not preserved as fossils. Some of their aberrant descendants may be recognized among the Enteropneusta, Tunicata, and Amphioxi ; but the first craniate vertebrates preserved as fossils were highly specialized, heavily armored ostracoderms, now all extinct.
The salient features of the second critical period have been mentioned, and here the surviving amphibians recapitulate in ontogeny many instructive features of the ancestral history.
Amphibians have no cerebral cortex, that is, superficial laminated gray matter, in the cerebral hemispheres. This first takes definitive form in the reptiles, though prodromal stages of this differentiation can be seen in fishes and amphibians, a theme to which we shall return in chapter vii.
The fourth critical period, like the first, does not lie within the scope of this work, though study of the second and third periods brings to light some principles of morphogenesis which may help us to understand the more recondite problems involved in human cortical functions.

It is probable that none of the existing Amphibia are primitive in the sense of survival of the original transitional forms and that the urodeles are not only aberrant but in some cases retrograde (Noble, '31; Evans, '44); yet the organization of their nervous systems is generalized along very primitive lines, and these brains seem to me to be more instructive as types ancestral to mammals than any others that might be chosen. They lack the highly divergent specializations seen in most of the fishes; and, in both external form and internal architecture, comparison with the mammalian pattern can be made with more ease and security. So far as structural differentiation has advanced, it is in directions that point clearly toward the mammalian arrangement.

Amphibian eggs and larvae are readily accessible to observation and experiment; they are easily reared; they tolerate experimental operations unusually well; and, in addition, the amphibian neuromuscular system begins to respond to stimulation at a very early age, so that successive stages in maturation of the mechanism are documented by changes in visible overt movement. The adult structure is instructive; and, when the embryological development of this structure is compared with that of higher brains and with the sequence of maturation of patterns of behavior, basic principles of nervous organization are revealed that can be secured in no other way. In the absence of differentiated cerebral cortex, the intrinsic structure of the stem is revealed. Experimental decortication of mammals yields valuable information, but study of such mutilations cannot tell us all that we need to know about the normal operations of the brain stem and the reciprocal relationships between the stem and the cortex.

In brief, the brains of urodele amphibians have advanced to a grade of organization typical for all gnathostome vertebrates, Amblystoma being intermediate between the lowest and the highest species of Amphibia. This brain may be used as a pattern or template, that is, as a standard of reference in the study of all other vertebrate brains, both lower and higher in the scale.

Book - The brain of the tiger salamander 1

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