BIOLOGICAL LECTURES DELIVERED AT THE MARINE BIOLOGICAL LABORATORY OF WOOD'S HOLL IN THE SUMMER SESSION OF 1890 t^tt%^^^ N^^*° BOSTON, U.S.A. PUBLISHED BY GINN & COMPANY I 89 I Copyright, 1890, Bt GINN & COMPANY. All Rights Reserved. Typography by J. S. Gushing & Co., Boston, U.S.A. Presswork by Ginn & Co., Boston, U.S.A. PREFACE. The addresses and lectures contained in this volume, with two exceptions, were delivered at the Marine Bio- logical Laboratory during the summer session of 1890. They are a continuation of the Evening Lectures begun in the previous session. The educational value which such lectures may be pre- sumed to have, and the consideration that through them the aims, the needs, and the possibilities of biological work might, in some measure, be made better known to the public, especially to those whose liberal benefactions have enabled the Laboratory to carry forward its work, suggested the propriety of publication. This step, how- ever, was not decided upon until late in the session of this year, after most of the lectures here presented had been delivered. The preparation of the Mss. for this purpose has been an extra tax upon the time of the contributors ; and, as this was done at my solicitation, I desire to acknowledge here my great obligation to them for this part of their invaluable co-operation in the work of the Laboratory. There are one or two points in the raisojt d'etre of this course of lectures, which do not lie wholly on the surface, but which deserve to be made clear. It was iii IV PREFACE. hoped, through such a course, to bring speciahsts into mutually helpful and stimulating relations with one another, and at the same time to make their work and thought intelligible and useful to beginners. It was not intended to take the place of systematic lectures, such as are given in the regular courses of instruction ; it stands rather for the higher and the more general needs of the science. Its leading pur- pose, if I may be permitted to define it more with reference to the possibilities of its future development than to its present attainment, was to meet the rapidly growing need of co-operative union among specialists. Specialization has now reached a point where such union appears to be an essential means of progress. Speciali- zation is not science, but merely the method of science. For the sake of greater concentration of effort, we divide the labor ; but this division of the labor leads to interdependence among the laborers, and makes social co-ordination more and more essential. This is the law of progress throughout the social as well as the orgfanic world. An oro^anism travels towards its most perfect state in proportion as its component cell-indi- viduals reach the limit of specialization, and form a whole of mutually dependent parts. Scientific organi- zation obeys the same law. As methods of investiga- tion improve, specialization advances, and at the same time the mutual dependence of specialists increases. Isolation in work becomes more and more unendura- ble. Comparison of results, interchange of views and ideas, and a thousand other advantages of social contact, become of paramount importance to the highest devel- opment. PREFACE. V In such considerations may be found the leading motive for this course of lectures. While directed in the main to the higher needs of investigators, they deal, as a rule, with subjects of present, and quite gen- eral interest to beginners ; and considerable pains has been taken to put them in a form that would be readily understood by such readers. In general, it may be said that the authors undertake to set forth what has been accomplished in their special fields of research, to give the conclusions of the best work and thought, to point out general bearings, and to state the problems that await solution. Obviously, such a course of lectures admits of un- limited development. Those here offered may serve to emphasize our need, and perhaps may contribute some- thing towards the eventual realization of that more perfect organization which we look for in the establish- ment of a permanent station with an endowment equal to our great opportunities for marine biological research. C. O. WHITMAN. CONTENTS. — •<>• — LECTURE ^^^^ I. Specialization and Organization, Companion Principles of All Progress. — The Most Important Need of American Biology. C. O. Whitman i II. The Naturalist's Occupation: i. General Survey. 2. A Special Problem. C. O. Whitman .... 27 III. So>ne Problems of Annelid Morphology. E. B. Wilson, 53 IV. The Gastrcza Theory and its Successors. J. P. McMuRRiCH 79 V. Weismann aud Maupas on the Origin of Death. Edward G. Gardiner 107 V\. Evolution and Heredity. Henry Fairfield Osborn, 130 VII. The Relationships of the Sea-Spiders. T.H.Morgan, 142 VIII. On Caryokinesis. S. Watase 168 IX. The Ear of Man: its Past, Present, and Euture. Howard Ayers ^^^ X. The Study of Ocean Temperatures and Cur refits. William Libbey, Jr 231 vii FIRST LECTURE. SPECIALIZATION AND ORGANIZATION, COMPANION PRINCIPLES OF ALL PROGRESS. THE MOST IMPORTANT NEED OF AMERICAN BIOLOGY. By C. O. whitman. A HEALTHY faith in the progress of biology pre- supposes a correct understanding of the tendency to speciaUze. It is important to know not only that special- ization is a necessity, but a necessity that need not be feared. It may sound a little paradoxical to assert, that this tendency means union as well as separation ; but it is only a truth illustrated in the most familiar facts of science and of every-day life. Let us look at some of the broader aspects of this tendency, in order to learn whither it is carrying us and what its implications are. Naturalists are long accustomed to the idea that the living body represents a commonwealth of cells. The metaphor is based, not upon superficial or fanciful re- semblances, but upon analogies that lie at the very foundation of organic and social existence. On the same grounds that the sociologist affirms that a society I 2 MARINE BIOLOGICAL LABORATORY. is an organism, the biologist declares that an organism is a society. A society is an organized whole, the unity of which consists in, and is measured by, the mutual dependence of its members. The living body is an organization of individual cells with the same bond of unity. The principle of organization in both cases is the division of labor or function. The primitive social aggregate • — the undifferentiated germ of society — is composed of practically like units, with like simple needs. Every ore 4,8 a factotum, fulfilling all needs in and for himself. I , ill self-dependence and no mutual dependence. 1 he coherence of the whole is so slight that it can break up into as many parts as there are individuals, without the sacrifice of a single tie or condition essential to existence. In course of time, division of labor comes into play, and with it social organization has its beginning. The different members, instead of doing all sorts of work, and aiming only to supply their own individual wants, begin to limit themselves to such work as their tastes, capacities, surroundings, etc., commend to them. This concentration of effort, which Coleridge, in his theory of life, has defined as "the tendency to individuation," both strengthens and improves the productive power, thus enabling a few to do the work of many. Each class of specializers produce in excess of their own needs, and through the exchange of these surplus prod- ucts the needs of all are supplied. The social integration that accompanies such division of labor may best be seen under conditions conceived as simple as possible. Let it be assumed that we have SPECIALIZATION AND ORGANIZATION. 5 an aggregate of a hundred individuals, equal in compe- tency and capacity for work, and all living under like conditions. Let us assume that the necessities of exist- ence for each member require ten kinds of labor in equal quantities. Now as long as each individual fulfils all ten needs, there will be no division of labor, but rather a divison of energy and correspondingly inferior products. The aggregate will represent a mere chance collection of independent individuals, not a whole of mutually depend- ent parts. But introduce the division of labor, and see how social integration follows. To take a simple form of division, we will suppose the aggregate divided into ten equal groups, one for each kind of work. We still have the same workers, the same energy expended, the same work accomplished, and the same needs fulfilled ; all we have done is simply to divide the labor instead of the time, and distribute it in such a way that . each person gives his entire time to one work instead of dividing it among ten. The change, in itself considered, looks extremely simple and insignificant ; but, when measured by the consequences entailed, its importance becomes at once apparent. Each work is now accomplished by ten men instead of a hundred, with the result that each individual fulfils only one-tenth of his own needs, and depends upon his fellows for the rest. Instead of jacks-at-all-trades, we now have specialists working under a social compact, which makes each indi- vidual the indispensable servant of every other. The co-ordination of individuals is such as to maintain a complete consensus of functions; which is the funda- mental trait of a perfectly organized community, and 4 MARINE BIOLOGICAL LABORATORY. its chief distinction from a purely gregarious aggre- gate. If the members of such a community, in adaptation to the conditions it imposes, should become so far differ- entiated as to lose the power of providing for more than one or two of the ten necessities of existence, the social unity would become as inviolable as the physiological unity of the higher organisms. Break it, and every member would soon perish, just as certainly as every cell would die if the body were irreparably injured. The social organism might endure the loss of a Ijmited number of its members, as the animal organism survives the death of individual cells and even the loss of certain organs. These losses may be repaired, in the one case by substitution, in the other by regeneration. If reparation fails, all the remaining parts suffer in proportion to their dependence on the parts lost. In either case, the more complete the division of labor, broadly speaking, the more perfect becomes the unity of parts, the more complete the coincidence of the indi- vidual with the general welfare. The parallel is more complete than our assumed case is suited to illustrate. We have considered only the two extremes of a series, and have not allowed for any connection through intermediate stages of development. But division of labor is not an artificial affair arbitrarily fashioned to our convenience ; it is not a thing of human device, for it antedates both the written and the un- written history of our race. In the organic world, its development has been as slow and as long as the rise of the beings now inhabiting the earth ; in the human SPECIALIZATION AND ORGANIZATION. 5 race, its period of growth coincides with that of civiliza- tion itself. We cannot know the circumstances of its first intro- duction. We assume that opportunities for the first steps in the division of labor presented themselves for- tuitously, and that, the opportunities being given, the inherent advantages of the principle in the struggle for existence would be quite enough to secure it the aid of natural selection. The principle carries with it two grand advantages — two primary conditions of progress. First, the concen- tration of energy ; and secondly, the economical com- bination of energies. The one holds the possibilities of intensifying and improving ; the others, the possibili- ties of utilizins: and aus^mentino^. These conditions and their contained possibilities, given with the division of labor, are the possibility, not only of all social, but also of all organic evolution. We may now go still further and assert that the evo- lution of the cell, the relatively simple structural unit of the organic world, would have been an impossibility without the division of labor. Imperfect as our knowl- edge of the cell still is, it is now certain that it has an organization based upon a division of function. There is already an overwhelming amount of concur- rent evidence to show that the nucleus is the real seat of the hereditary tendencies ; and the deeper we pene-. trate into the complexities of its structure, and the more we study its internal transformations and movements, the more evident it becomes that the nucleus has had its evolution, which carries the subdivision of labor still farther back. 6 MARINE BIOLOGICAL LABORATORY. Our knowledge, so far as it goes, points to the con- clusion that division of labor is not only co-extensive with life, but also coeval with it. Indeed, we should be on the side of all the probabilities, in assuming that the simplest possible form of living matter presupposes this principle. We are not, of course, to confound the prin- ciple with life itself, nor with the cause of life ; it is only a condition or means to an end. The universal corre- late of division of labor is union of the laborers. It always means specialization, and always implies organ- ization. Thus the paradox resolves itself. As Herbert Spencer long ago pointed out in his Social Statics, ''progress is toward complete separateness and complete union," and *'the highest individuation is joined with the greatest mutual dependence." As you see, the principle is one which may re-inaugu- rate itself, as often as a new order of units is evolved with needs that can be most economically and efficiently served by a co-operative union. We do not know how many times this may have happened before the cell order of beings arose ; but the general course of devel- opment following this stage, we are now very confident about. Some of these cells, finding independent nomadic life congenial, have persistently declined every temptation to part with individual freedom. They have kept their freedom, but with it the low estate of unaided individual effort. Precious freedom that, which excludes all those larger possibilities of life which we see unfolded in the organic world. Others preferred company to isolation, and herded SPECIALIZATION AND ORGANIZATION. 7 together in roving colonies. Some of these were dominated by a gregarious instinct only, and have clung tenaciously to self-freedom, refusing to make any sacrifice of personal independence for the sake of a physiological union. A few such aggregates, whose freedom has been the forfeit of all advancement, still survive, as exemplified in some members of the Volvox family. In Gonium, for example, the colony consists of a few (4-16) flagellate cells, adhering together in plate-like form, each self-moving, self-feeding, and self- propagating. Among these colonial aggregates, there were some, however, which found out how to take one or two simple steps in labor partnership, and thus advanced to a rudi- mentary kind of composite individuality. An interesting example is seen in the famous Volvox of Leeuwenhoek, in which the evolutionists of last century found a con- firmation of their idea, that the germs of plants and ani- mals are preformations in miniature, incased one within the other. The division of labor is here of such an elementary order, that, as BiitschU has suggested, we may look upon a Volvox colony as a near ally of those simple forms from which all the higher plants as well as the Metazoa arose. The Volvox colonies, composed of numerous individ- uals, often more than a thousand, are attached to the inner surface of a colonial envelope, at equal distances. In each colony we find two kinds of cells ; one with two fiagella for locomotion, the other without such appendages, fulfilling the work of reproduction. This sinsfle division of labor makes one class of individuals the propagators of the species, the other the preservers 8 MARINE BIOLOGICAL LABORATORY. of the colony. Neither class can dispense with the services of the other ; and this dependence of part upon part gives the colony a certain physiological unity. But the integration of the colony is of such a simple order, that w^ might conceive it splitting up into as many independent colonies as it contains times the least number of cells of both sorts necessary to maintain the physiological connexus. Although in practice, we could not carry the division so far, still we know that artifi- cial, if not spontaneous, division would be possible with- out destroying the physiological unity necessary to the continued existence of the severed parts. One feature of labor-division in Volvox deserves men- tion here, chiefly as foreshadowing more complex condi- tions seen in hisfher forms. It is the alternation of agamic with gamic generation. The agamic reproduc- tive cells are all alike, and correspond to parthenogenetic ova ; while the gamic generation is represented by two distinct kinds of cells answering to ova and spermatozoa, and conjugation is necessary to development. This alternation of parthenogenesis with hermaphro- ditic gamogenesis — is not, we may be sure, an acquisi- tion of the colony ; it is rather to be regarded as a combination of features that originated separately and successively among the unicellular ancestors of the col- ony. Parthenogenesis must have been the primitive mode of reproduction ; gamogenesis undoubtedly origi- nated secondarily in adaptation to infusorial conditions 'of life. This sequence of generations is common enough among the unicellar Protozoa ; and the colonial forms exhibit it as an inheritance of their component cells. SPECIALIZATION AND ORGANIZATION. 9 The parthenogenetic colony presents itself, then, as an ao-^reo-ate of individuals with differentiated, but undi- vided, reproductive work ; the hermaphroditic colony explains itself as an aggregate of individuals with differ- entiated and sexually-divided reproductive work. The association of both sexes in the same colony is an acci- dent of aggregation ; for obviously we might have, in fact do have, dioecious as well as monoecious colonies. The protozoan colony of the Volvox type, represents the old infusorial system of labor-division and, super- added thereto, the colonial stage of what we may call the intercellular system — which runs through all the higher organisms. With the intercellular system is given a higher order of units, capable of combining and recombining to form successively higher orders, each carrying all previous systems of labor distribution with its own superimposed thereon. The ascending series ranges through all diversities of form and all complexities of structure between the simplest cell-colony and man. Cells com- bine into tissues, tissues into organs, organs into organ- isms, organisms into organic as well as social aggregates, and these in turn into higher units. While the higher units are entering into new combinations, their compo- nents of the next order below, of the next below that, and so on to the lowest, may be undergoing simul- taneously special modifications, each struggling to keep up its own internal and external adjustments, but always in subordination to the welfare of the entire organism. W^hen we contemplate the finished mechanism, the prod- uct of all these consentaneously and yet unconsciously directed energies, these millions of individual minim lO MARINE BIOLOGICAL LABORATORY. workers, uniting in such intimate fellowship as to consti- tute an indissoluble whole — a real conscious intelligent unity — with powers so far transcending those of its units that we can form no conception of the special combinations from which they result, — when we con- template this miracle of co-adjustments among myriads of units, among these systems of units, and these sys- tems of systems, we are not disposed to ridicule the judgment that once refused to believe that natural forces could produce such wonders, and took refuge from the difficulties that beset every mechanical theory in the doctrine of preformation. If our microscopical aids have enabled us to know that organisms are not simple unfoldings of pre-existing structures, and have revealed the fact that every devel- oping germ actually re-enacts the wonders of a new creation, still it is no less an unscrutable mystery than before. Indeed one must credit the preformationists with having perceived and emphasized the real difficulty in the way of any rational theory of generation. We endeavor to meet it, by assuming, not pre-existing rudi- ments, but pre-existing hereditary units ; not predelinea- tions, but potentialities, of structure. Predeterminations of some kind or other are a logical necessity, and so there is some analogy between our position and that of Bonnet, Haller, and Cuvier, and other evolutionists of the old school, although we are compelled to regard the process of development as one of epigenesis, as conceived by Aristotle, Harvey, John Hunter, and Casper Fried- rich Wolff. The difference between the two schools re- duces itself to the difference between potentialities and actualities ; and although the difference as understood SPECIALIZATION AND ORGANIZATION. II by the contending parties is utterly irreconcilable, still we can understand how, by modifying our potentialities in one direction and our actualities in another, the dif- ference might be brought near a vanishing point. Whether we look at the successive stages connecting the relatively homogeneous germ with the completed organism, or at the paleontological succession of forms, we see that progress in the organic world is always from the less to the more heterogeneous. As division of labor advances, complexity of structure increases, and the ties of mutual dependence multiply and strengthen. In a word, the most characteristic trait of evolution is, that increasing division of labor conditions increasing union of the laborers. Division and union, differentiation and integration, specialization and organization, march hand in hand. The same truth comes perhaps more clearly into view, when, taking the protozoan colony for our starting-point, we run up the scale of animal organizations. Passing on from the Volvox colony, we soon come to an instruc- tive stage represented in the common fresh-water Hydra. Although we now know that the organization of this animal is far from being as simple as was supposed by its discoverer, Trembley, and by other naturalists of his time, who regarded it as a connecting link between plants and animals, still it affords a striking illustration of the fact, that physiological nnity is a thi^ig of degrees^ inco7nplete according as the division of labor is low. A single division of labor, in advance of what we saw in Volvox, makes Hydra an unmistakable Metazoon, plac- ing it fairly on the main line of animal evolution. It is the separation of the digestive from the other functions 12 MARINE BIOLOGICAL LABORATORY. which characterizes the Hydra stage. In correlation with this important step, we have one grand and several minor structural features introduced. The digestive cells arrange themselves together in the form of a tubular sack open at one end, thus taking the first step towards a rudimentary alimentary canal. Around this sack, the remaining cells station themselves, forming another sack inclosing the first. The sacks are in close contact, and the walls of the outer one are continuous with those of the inner one at its open end, so that the inner sack may be regarded as an infolding, such as we might rudely ' represent by pushing in the end of a glove-finger. If the material were elastic, so that we could draw out the double wall around the open end into a number of arm- like extensions, we should have a fair model of the Hydra body with its tentacles. The cells constituting the inner sack, called the ento- derm, are in the most favorable situation for attending to the food-supply of the entire cell community ; and natural selection has constrained them to specialize in this direction until they have become inoperative in other ways, and even incapable of doing anything else. Trembley succeeded in turning these creatures inside out ; and as they lived on after such treatment, he in- ferred that the functional differentiation of the two lay- ers was so slight that ectoderm and entoderm could exchange places and works. The mistake has only recently been corrected by a Japanese naturalist. Dr. Ischikawa of Tokyo has shown conclusively that Hydra cannot live long turned inside out, and that, if left to itself after the operation, it soon turns itself back into its normal condition. This act of recovery escaped the SPECIALIZATION AND ORGANIZATION. I3 observation of Trembley, and of others who repeated his experiments ; and hence the unity or individuaUty of the Hydra community of cells has generally been estimated too low. That the two layers carry functions fundamentally dis- tinct and non-interchangeable, and that the co-operative combination of the two sets of functions is necessary to existence, — is, in fact, the very essence of the Hydra personality, — is shown by still another fact brought out by Mr. Ischikawa. It is generally taught that Hydra may be divided ad libitum, and that each fragment will have the power to regenerate the whole individual. But it turns out that there is a very definite limit to such possibilities, which cannot be overstepped without anni- hilating individuality and extinguishing even the germ of it. An isolated piece of either layer is incapable of regenerating the individual. The mutual dependence of these two layers is such that they must go together or perish. Carry artificial division in any direction that does not sunder these complementary parts, and repeat the operation as often as you like, the smallest frag- ments in which this vital connection is preserved will represent, potentially at least, the personality of Hydra. This personality comprises a certain number of func- tional powers ; and hence the minimum number of cells combining these powers in vital relations represents all the essential elements of individuality. The fertilized ovum unites all these powers as potentialties, and it is therefore the individuality in germ. If the ectoderm cells, like the entoderm, were all alike, the essentials of a Hydra might be said to exist in a single pair of cells, one from each layer ; but of course we could not hope to isolate such a pair of cells in vital union. 14 MARINE BIOLOGICAL LABORATORY. The remarkable thing about such an individuality is, that a hundred of them may be added together and the sum total will be but one ; and yet you may divide this one into a hundred ones. Mr. Ischikawa succeeded in forcing two individuals into complete and permanent coalescence ; and for aught we can see, the experiment might be repeated indefinitely. One Hydra was turned inside out, and then pushed into the mouth of another until the diges- tive sacks of both were brought together one within the other. A bristle was then thrust transversely through both bodies to prevent separation. In the course of a few days, the two bodies were completely merged in one, and the resulting individual was a perfect personal unity, bearing two sets of tentacles as the only mark of its double origin. Having seen in what the essential unity of Hydra consists, we can readily understand why such an indi- viduality may not be weakened by division or strength- ened by doubling. A society of a hundred individuals with ten labors, distributed as we supposed, would rep- resent a unity with ten essential points of union. Now we could double the number of members without in- creasing the points of union ; and we could divide the whole community into ten communities, each with as complete a functional unity as that of all combined. In the case of Hydra, we could divide more freely, because the points of union are fewer. Now what I wish to emphasize here is this : TJie mo7'e the points of tmion midtiply in a social or an oi'ga7tic body, the more complex ajtd extended becomes the integration of its parts, and the less susceptible it is to such divisions and fusions SPECIALIZATION AND ORGANIZATION. 1 5 as we have described. Bear in mind as we go on, that every point of union is a point of division, or specializa- tion in labor. Our supposed social aggregate, as I have said, admits of division into ten independent communities. If, how- ever, we multiply the points of union by ten, i.e., if we suppose each labor sub-divided into ten specialties, each member of the aggregate will fulfil only one-hundredth of his own needs, and will depend upon his ninety-nine associates for the rest. The mutual dependence is not only ten times as great, it is also ten times as extensive, for each individual is now a necessity to ninety-nine instead of nine others, and the entire aggregate becomes an indivisible whole. The same processes are followed by like results among the cell-constituents of an organism, only here we rarely find such simple, and never such complete, uniformity in numerical relations. We find no organ- ism in which the division of function exactly coincides with the number of its component units. Both the division of labor and its distribution here tend to adjust themselves, first of all, in harmony with the primary necessities of existence ; and secondly, in correspond- ence with that complex of relations, conditions, and needs, both internal and external, which hold all the possibilities of improving existence, and rising above it to conscious life and intelligence. In the organic association of cells, nutrition and reproduction take precedence in determining the direc- tion of development. The needs which centre in them are, as a rule, best served, not by giving the whole of a given kind of work to a single cell, but by l6 MARINE BIOLOGICAL LABORATORY. dividing it more or less equally among many cells, scattered or grouped according to the nature of the work. Nature is provident as well as bounteous, and so she determines the number of workers not only with reference to ordinary needs, but also with a view to emergencies. The cell cannot work on indefinitely. Exhaustion follows exertion ; rest and recuperation are as necessary to the cell as to an individual ; hence the need of relays. The cell has its own term of existence, which is usually much shorter than that of the organ- ism ; hence the need of substitutes. So each class of specialized cells may greatly exceed in number the actual needs of the moment. One of the best illustrations of this fact is seen in the reproductive cells, which are often so enormously in excess of use, that they are scattered in the water or the wind, with not one chance in a thousand of ever fulfilling the purpose of their existence. All such profuseness, however, has its mean- ing, even though it only neutralize accident, and so insure a few the realization of their proper destiny. But these hosts of cells suffice for only one of the many varieties of reproductive work. They are called reproductive cells, not to indicate monopoly of the entire work, but pre-eminence merely in one important branch of it. Propagation of the species is their task ; but this becomes a monopoly only among the higher forms of life. The same work may be accomplished by budding and fission, processes which prevail very largely among plants and many of the lower animals, usually supplemented, however, by the more general process of reproduction by means of specialized cells. But the generation of the species, which follows such SPECIALIZATION AND ORGANIZATION. \J different courses, each of which goes on into almost endless sub-division, is not the whole of reproduction. Reproduction of the species of course includes all other kinds of reproduction ; nevertheless it is as distinct from them as the individual is from its component cells. The individual may be the product of a single cell, but, once formed, the heterogeneous components must sev- erally have their own methods of reproduction, other- wise the organism could not keep up its reserves, nor supply the places of exhausted, disabled, or worn out laborers. These specialized modes of reproduction, as varied and as distinct as the histolos^ical elements of the organism, although derived from the process which continues the species, yet differ from it in this impor- tant respect, that their products are isogeneous rather than heterogeneous. This distinction is already well marked in Hydra, where we find the entoderm cells so specialized that they can reproduce only cells of their own kind. How different it is with the ova, which reproduce all kinds of cells represented in the Hydra community. Certain kinds of work exclude the power of reproduc- tion, and such cases call for special provisions of still another class. The loss of such power by any class of cells is generally made good by a closely allied class, or by the younger cells of the same class. Such reserves may play a relatively passive part, until the time arrives for them to take the place of their prede- cessors ; and they may be capable of assuming any one of several different roles. Again Hydra furnishes us with a simple illustration. The superficial ectoderm cells of Hydra, consisting of nettle-cells, nerve-muscle l8 MARINE BIOLOGICAL LABORATORY. cells, etc., are replaced by deeper and younger cells, called ''intermedial." The reproductive cells have the same origin. The work of reproduction then is not confined to any one or two classes of cells ; it is divided and sub- divided in endless detail, carried all through the organ- ism, and distributed independently of most other labors. Its elaboration in this or that direction may be cor- related with a system of morphological differentiations, so extensive and involved that a whole course of lectures would be required to elucidate the subject. Take the genital system of the vertebrates, or that of forms no higher than the annelids, and you will find no end of problems yet to be settled. What complicated cycles of generation have been followed by many parasitic forms, especially among the worms, and what wholesale modifications of structure in answer thereto. How devious have been the paths of generation in insects, and how wonderful the metamorphoses attending them. How diverse the ways of multiplication among the Tunicates and Coelenterates, and what puzzling suc- cessions and combinations of forms have here tried the sagacity of naturalists. What peculiar corporation aggregates are represented in Siphonophore colonies, of which we have a most beautiful example in the Portuguese-man-of-war. How long it has taken to decide between the ''poly-organ theory" of Eschscholtz, Huxley, M tiller, and Met- schnikoff, and the "poly-person theory" of Vogt, Leuckart, Kolliker, Gegenbaur, and Haeckel. The complex of reproductive processes in one such colony would still bear a life-time of research, and not be SPECIALIZATION AND ORGANIZATION. IQ exhausted then. Who is able to trace out the reproduc- tive alternations and metamorphoses of such simple yet strange forms as the Dicyemids and Orthonectids, supposed by some, though probably erroneously, to stand as an intermediate group between the Protozoa and the Metazoa ? And when we descend to the Protozoa themselves, we find the reproductive cycles specialized in ways as varied as the forms of life. Then behind and under- lying the whole of this reproductive work, from Proto- phyte to Phanerogam, from Protozoa to man, are those intensely interesting phenomena of caryokinesis, which are at once the most varied and the most uniform ex- pression of reproductive energy that modern research has yet revealed. I need not dwell longer on this subject. We have not found the beginning of reproductive work, nor can we see an end either to its divisions or to the structural unities and divergencies correlated therewith. We might now turn to nutrition and sketch the gen- eral features of its specializations and coordinations ; but we should only find the same principles illustrated in new directions, and might get weary without getting wiser. I will therefore at once try to bring the leading thought to a focus, and then very briefly point out its application. Division of labor is the principle underlying all or- ganic as well as all social progress. The development of the principle brings with it mutual dependence of the working units ; and hence, every step in advance leads necessarily to that closer integration of the units 20 MARINE BIOLOGICAL LABORATORY. which merges their individualities into an individuality of a higher order. The tendency towards unity, as specialization advances is nowhere more strikingly illus- trated than in the progress from the lower to the higher segmented animals. In the lower annulose types, the individual represents a chain of segments or somites, which we may regard as so many individuals which have arisen as buds, one after the other, from before backwards, but have remained connected in the order of origin. These somites retain their individuality to such an extent that they are not killed by artificial separation, and indeed often undergo spontaneous fission. As we glance along the line of forms terminating in the Myriopods, the Crustaceans, the Insects, and the Arachnids, we find the individualities of the somites more and more subordinated to that of the chain they compose. There is a progressive consolidation, which, in its extreme phases, more or less completely obliterates the traces of articulation. The illustration might be extended to the vertebrates, but that would be needless. Let me add only, as one of the broadest conclusions to be drawn from such facts as we have been considering, that tJie grade of specialization attained in any group of 07ganisnis determines its rank in the scale of life and in- telligence. In each order of units, specialization seems to have its limit in the highest possible integration of its com- ponent elements. When this limit is reached, progress is arrested. The only way then open for advance lies in combining these units into units of a higher order. In this combination is given the possibility of a SPECIALIZATION AND ORGANIZATION. 21 new system of specializations and integrations, with correspondingly high grades of life. The final stage of each order of units represents a complete individuality, which cannot be divided without destruction. So we advance from certain " physiological units " to the indivisible cell, from the cell to the indivisible organism composed of cells, from the highest units here to those of the social order. It is unnecessary to enlarge further upon these facts. It remains only to point out their bearing in relation to the biological sciences. The days when naturalists could presume to take all nature for a subject of study and meditation are a long way behind us. The cosmogonists of olden times en- gaged single-handed with all the mysteries of the universe. We honor them for their heroic efforts, ineffective as they were from misdirection. At the expense of centuries of baffled efforts, the lesson began at last to be learned, that division of the problem facilitates progress. That a knowledge of the whole presupposes a knowledge of the parts, was a simple enough fact ; but it took a long time to turn it into practice. Division of labor in the sciences, as elsewhere, has been a thing of slow growth, self-originating, self-per- petuating, and self-regulating. It has taken possession of the biological sciences, and presides over their onward march, just as it determines and directs social and industrial progress. It is simply an economical principle, the growth of which began with, advances with, and will always have its limitations in, an actual need — the need of concentrated attention. This need 22 MARINE BIOLOGICAL LABORATORY. in turn has its limits in our power to improve the methods of investigation. The tendency, then, is regu- lated by the necessities and advantages of the investi- gator; and although we may not be able to fix definite limits to its growth, we are not the less certain that it has such limits, and that there is no danger either of a wholesale reaction or of our ever specializing to pieces. As in the organic and social worlds, so in the scientific, there are centripetal forces that keep pace with the centrifugal ones ; and the danger of any science flying into disconnected atoms is about as dreamy and remote as the dissolution of the earth itself. The movement in the direction of separation is gen- eral and, as it now seems to us, rapid. Cuvier thought that division of labor characterized the natural science of his day ; but the movement was then in its earliest infancy. If you wish to know how extensive it has now become, you should look at the ponderous volumes of the *' Zoological Record" or the ''Naples Jahresbericht." When you reflect that it requires such massive volumes to record the bare titles and a brief abstract of the work of a single year, you realize how impossible it is for any one naturalist to cover the whole ground, or even to read the hundredth part of what his collaborators have to report. Naturalists then are no longer cosmogonists, but specialists. This being the fact, what is to be done in view of it.-* Where lies the remedy for every dan- ger of narrowness that may lurk in the tendency to specialize ? How is the range of vision to be kept free and broad while focussing attention on some one point of the field .-* If one specialty absorbs our whole SPECIALIZATION AND ORGANIZATION. 23 time and energy, how are we to keep its general bear- ings and relations in full view ? Will organization of any kind within our reach effect this ? If our special- ties are parts of a whole, then this whole must be representable by one or more modes of combination. That kind of organic association which permits each unit to work for itself while making it the servant of all the rest, must be a possibility. It must be evident to every one who is capable of understanding the situation, that luiion is just as essen- tial a part of the law of progress as division. If spe- cialization is a necessity, so is organization. But there is this difference between the tendencies, — that the one precedes the other and comes into recognition first. Specialization has already forced its way to the front, and is nearly everywhere recognized as a necessity ; organization follows, but lags lamentably behind the needs of the times. The general principle of cooperation has long been at work. The naturalists of all countries are brought into cooperative relations through journals and other scientific publications. Every year multiplies these points of union, and draws the scattered workers into closer mutual dependence. Cut off these indispensable media of communication, and that unity of action on which progress now depends would at once come to an end. Of course the unity of action in so extended a body cannot be complete. Duplication of work will now and then occur, but the waste in this direction is fast becoming reduced to a minimum. The tendency to specialization is rapidly developing among our journals. This is seen especially in such 24 MARINE BIOLOGICAL LABORATORY. journals as the Zoologischer Anseiger, the Biologisches Centra lb latt, \.h.Q Anat07nischer Anzeiger^ the Journal of the Royal Mic. Soc, the Zeitschrift f. wis. Mikros- kopie, &c. Manifestly all such specialization is led by the cooperative spirit. But it is not to the general tendency so much as to our own special need that I would now direct attention. We have now reached a point where our advance, both individually and collectively, depends, far more than ever before, upon the privileges, the opportunities, and the many peculiar advantages inherent in the prin- ciple of cooperative work. Among the ways of bringing together our scattered forces into something like organic union, the most important, and the most urgent at this moment, is that of a national marine biological station. Such an establishment, with a strong endowment, is unquestion- ably the great desideratum of American biology. There is no other means that would bring together so large a number of the leading naturalists of the country, and at the same time place them in such intimate helpful relations to one another. The larger the number of specialists working together, the more completely is the organized whole represented, and the greater and the more numerous the mutual advantages. Just consider what such an organization implies. It means, first of all, a permanent staff of investigators, with laboratories equipped for special research, and with facilities for extending observation to different points of our varied coast. It means boats, and all needful appli- ances for collecting, dredging, etc. It means a corps of trained collectors at the service of the investigators. SPECIALIZATION AND ORGANIZATION. 2$ It means a comprehensive working library ; ample funds for serial and monographical publications ; funds for travelling research ; and resources for cooperative work with similar stations in other parts of the world. It means, further, all those important aids and accessories of investigation, such as conservators of material, assistants in microtomical and other mechan- ical work, skilled draughtsmen, photographers, lithog- raphers, and so on to the end of all the needs of such an organization. Create such conditions of work, and how biology would flourish. Specialization would characterize the individual members ; but organization would dominate the aggregate. In place of the weakness of isolation, we should have all the power of union. There would be economy of time, money, service, physical and intellect- ual strength. The productive power of each individual would be intensified and augmented beyond calculation by that of every co-laborer; for while the units minis- tered to the whole, the power and influence of the whole would redound to the benefit of each. The difference between isolation and a union of this kind, is like that which separates the solitary cell from a highly de- veloped organism, or the nomadic aggregate from civ- ilized society. We need viva voce contact for incentive, for stimulation, for inspiration, and especially for main- taining that " moving equilibrittm " of our specialized forces which constitutes progressive scientific life ; and organization in the direction I have roughly indicated will accomplish all these ends. In conclusion, let me say that the establishment of such a station as I have sketched will probably never 26 MARINE BIOLOGICAL LABORATORY. be effected through the unaided effort of any one per- son. But individual effort, though weak alone, has in the growing aggregate a cumulative power that often surprises expectation. Let every one feel, therefore, that his or her personal interest in the matter may be just what is required to make the combined effort of all effective in converting possibilities into actual realiza- tions, and in giving to specialization its consummation in organization. SECOND LECTURE. »o>^o<. THE NATURALIST'S OCCUPATION.i By C. O. whitman. I. General Survey. I SCARCELY need remind you that the domain of Bi- ology is a broad one, and that it has long since become impossible for one person to master the different prov- inces of knowledge embraced in it. The most that I can hope to do, is to take you into one small section of the great realm of life, and try to give you an inside view of some of the problems now occupying attention. As this many-sided occupation may be approached with almost equal advantage along any one of many intersecting paths, a hasty general survey may be the best means of getting the points of the compass. Let us take systematic biology as our starting point, and from this as a centre find our way into the other provinces of biology, with a view to understanding their general features and relative positions. What is the chief end to be reached in the classifica- tion of plants and animals } The general drift of bio- ^ 1 Delivered at the opening of the Evening Lectures, July 9, 1889. 27 24 MARINE BIOLOGICAL LABORATORY. journals as the Zoologischer Anseiger, the Biologisches Centralblatt, the Anatomischer Anzeiger, the Jotirnal of the Royal Mic. Soc, the Zeitschrift f. wis. Mikros- kopie, &c. Manifestly all such specialization is led by the cooperative spirit. But it is not to the general tendency so much as to our own special need that I would now direct attention. We have now reached a point where our advance, both individually and collectively, depends, far more than ever before, upon the privileges, the opportunities, and the many peculiar advantages inherent in the prin- ciple of cooperative work. Among the ways of bringing together our scattered forces into something like organic union, the most important, and the most urgent at this moment, is that of a national marine biological station. Such an establishment, with a strong endowment, is unquestion- ably the great desideratum of American biology. There is no other means that would bring together so large a number of the leading naturalists of the country, and at the same time place them in such intimate helpful relations to one another. The larger the number of specialists working together, the more completely is the organized whole represented, and the greater and the more numerous the mutual advantages. Just consider what such an organization implies. It means, first of all, a permanent staff of investigators, with laboratories equipped for special research, and with facilities for extending observation to different points of our varied coast. It means boats, and all needful appli- ances for collecting, dredging, etc. It means a corps of trained collectors at the service of the investigators. SPECIALIZATION AND ORGANIZATION. 2$ It means a comprehensive working library ; ample funds for serial and monographical publications ; funds for travelling research ; and resources for cooperative work with similar stations in other parts of the world. It means, further, all those important aids and accessories of investigation, such as conservators of material, assistants in microtomical and other mechan- ical work, skilled draughtsmen, photographers, lithog- raphers, and so on to the end of all the needs of such an organization. Create such conditions of work, and how biology would flourish. Specialization would characterize the individual members ; but organization would dominate the aggregate. In place of the weakness of isolation, we should have all the power of union. There would be economy of time, money, service, physical and intellect- ual strength. The productive power of each individual would be intensified and augmented beyond calculation by that of every co-laborer; for while the units minis- tered to the whole, the power and influence of the whole would redound to the benefit of each. The difference between isolation and a union of this kind, is like that which separates the solitary cell from a highly de- veloped organism, or the nomadic aggregate from civ- ilized society. We need viva voce contact for incentive, for stimulation, for inspiration, and especially for main- taining that " moving equilibrium " of our specialized forces which constitutes progressive scientific life ; and organization in the direction I have roughly indicated will accomplish all these ends. In conclusion, let me say that the establishment of such a station as I have sketched will probably never 26 MARINE BIOLOGICAL LABORATORY. be effected through the unaided effort of any one per- son. But individual effort, though weak alone, has in the growing aggregate a cumulative power that often surprises expectation. Let every one feel, therefore, that his or her personal interest in the matter may be just what is required to make the combined effort of all effective in converting possibilities into actual realiza- tions, and in giving to specialization its consummation in organization. SECOND LECTURE. *o>«4o<. THE NATURALIST'S OCCUPATION.i By C. O. whitman. I. General Survey. I SCARCELY need remind you that the domain of Bi- ology is a broad one, and that it has long since become impossible for one person to master the different prov- inces of knowledge embraced in it. The most that I can hope to do, is to take you into one small section of the great realm of life, and try to give you an inside view of some of the problems now occupying attention. As this many-sided occupation may be approached with almost equal advantage along any one of many intersecting paths, a hasty general survey may be the best means of getting the points of the compass. Let us take systematic biology as our starting point, and from this as a centre find our way into the other provinces of biology, with a view to understanding their general features and relative positions. What is the chief end to be reached in the classifica- tion of plants and animals } The general drift of bio- ^ 1 Delivered at the opening of the Evening Lectures, July 9, 1889. 27 28 MARINE BIOLOGICAL LABORATORY. logical research is in the direction of a genealogical system of classification — a system based upon, and expressing, the kinship which underlies the whole or- ganic world. From this standpoint, the myriad forms of organisms that have arisen since the dawn of life, genealogically arranged and viewed as a whole, would present the branching figure of a tree. The trunk and branches of this great tree, representing ancestral forms, have been buried in the sand and mud of geo- logic ages, and preserved only as an imperfect fossil frame-work, so that we see only the terminal buds of its topmost twigs in the plants and animals of to-day. To trace out and reconstruct such a tree is a work of magnitude, scarcely dreamed of in the philosophy of cabinet naturalists. The best classification within the range of present possibilities can only have a tentative value. It can have not a single hour's security against the invasion of newly discovered facts — an invasion that is advancing along a thousand lines with plenary authority to spare nothing fictitious. The goal of sys- tematic botany and zoology is not then the terminus of any one line of research, but rather a focal point of all the biological sciences. Having noted the principal aim of classification, we have now to glance at its position, scope, and functions. The low standards followed by many systematic writers have brought reproach upon this department of knowl- edge ; but the reproach is certainly misplaced, and we must accord to systematic biology the high position to which its true aims and functions entitle it. Its first business, obviously, is to ascertain what forms of life now exist, and to describe, name, and catalogue them THE NATURALIST S OCCUPATION. 29 for the purposes of easy and certain identification. Although much of this work can have only a provisional value, it is, nevertheless, quite indispensable ; for there is not a single department of biology that does not continually profit by its acquisitions, nor indeed is there one that can make any great progress without its aid. The process of coining names and labelling new species must continue for a long time to come ; but, it does not of course follow, because systematic names are indispensable, that we can profitably spend our time in committing them to memory. That is the delusion of inexperience and the conceit of charlatan- ism. Time was when the knowledge of a thousand names secured one the title of botanist or zoolosfist, and when the capacity for ten times that number was esteemed the measure of a great naturalist ; but if we may believe a celebrated German botanist, Schleiden, such qualifications fell below par more than half a century ago. In the beginner, and in the general observer, we frequently meet with a superstitious regard for names that blinds them to the real character and aims of natu- ral history. With them, an ideal naturalist is supposed to have an encyclopedic knowledge of names, and to be ready for any worm, beetle, or butterfly, that may be laid before him. If he has the courage to say he does not know the name of the form presented, the inquirer is amazed at the confession of ignorance ; if a vernacu- lar name is offered, the information is received with evident disappointment ; but if some unintelligible, poly- syllabic, cacophonous Greek or Latin compound is glibly 30 MARINE BIOLOGICAL LABORATORY. enunciated, the awe-stricken recipient retires, feeling profoundly edified, and credits his informer with having fulfilled the function of a great naturalist. This mis- chievous delusion is too often encouraged by those who are able and willing to impose upon it, or who have not the courage to follow the injunction so often given by Professor Agassiz at Penikese, — " Learn to say you do not know." Pitiable as is this fear of appearing ignorant, and des- picable as is the impostor's pretence of knowledge, there is no ground in either for prejudice against sys- tematic names. Whoever reflects on what the binary nomenclature, introduced by Linne, has done for zoology and botany, will scarcely need to be told that no misuses, excesses, or abuses to which systematic work is liable, can detract from its importance. Indeed, it may be said that time will increase rather than diminish the value of such work. Thirty years ago our systematic names stood for differences and resemblances, the deeper significance of which had only been caught by Darwin and Wallace. The idea of the genetic unity of the organic world set the whole field of systematic work in a blaze of light, imparting to it an interest and a dignity of the highest order. The second important function of systematic biology is to arrange its forms in a genealogical system. But for the fulfilment of this function, systematic biology requires the aid of all the sister branches of knowledge, and in return renders the important service of recording their verdicts along with its own. The record, represent- ing as nearly as possible the consensus of all the mor- THE naturalist's OCCUPATION. 3 1 phological and physiological sciences, shows how far the reconstruction of the tree of life has been carried, and thus furnishes a chart which is invaluable as a guide in the selection of subjects for investigation. When the classifier, or taxonomist as he is sometimes called, has taken account of morphological features, modes of reproduction, habits, instincts, and distribu- tion, he has exhausted the resources of his special province. The conclusions reached and the questions raised are then to be submitted to other departments for revision and further investigation. Let us suppose that the preliminary work of naming and describing has been completed, and that the taxono- mist undertakes with purely descriptive data to map out the genealogical tree. With superficial characters alone, it is evident that he could not advance very far, although, according to the supposition, he would have the immense advantage of knowing precisely what the task is. What such an advantage means, becomes clear when we re- member that, with all the light of all the sciences, we waited until the last half of the nineteenth century for the formulation of the problem. With this key to the situation, a quarter of a century has outdone the blind plodding of all previous centuries, and the old landmarks have been left with a speed that threatens to make Rip Van Winkles of us all. Armed with such an advan- tage, the investigator would certainly be able to find in external characters important clews to genetic relation- ships. But if limited to those methods and means which naturally belong to surface observation, he would remain in absolute ignorance of a great part of the animate world, and would be utterly powerless to discover in 32 MARINE BIOLOGICAL LABORATORY. what the bond of unity actually lies. He would have no conception of what Huxley has called ''the physical basis of life," and the structural unit of all organisms would lie wholly beyond the range of his perception. That isthmus of small life between the animal and vege- table kingdoms, his unaided vision would never discover. In searching for intermediate forms, he would inevitably be led astray by those deceptive appearances under which adaptive development and degeneration have con- cealed so many ancestral relationships. Sessile animals, like the sponges, the hydroid polyps, the sea-anemones, the polyzoa or moss polyps, the ascidians, and many others from the higher as well as the lower classes, would be separated from animals having the power of locomotion, and be regarded either as plants, or as forms representing both plants and animals. Where immobil- ity is combined with the branching form, as in the hydroid polyps, the disguise would be complete. Even Linne, the great lawgiver in systematic biology, de- scribed such forms in the tenth edition of his " Systema Naturae" as ''plants with animal flowers"; and in the twelfth edition, which concluded his systematic work, he held to the opinion that the stock of the hydroid col- ony is a true plant, while its " flowers " are true animals. This idea was embodied in the word zoophytes, plant- animals, a word that has done varied service in system- atic zoology from the middle of the i6th century. The utter insufficiency of external characters as a guide to genetic affinity, is well seen when we come to such forms as the so-called compound ascidians, which are found encrusting the rocks along the shore. At first sight one would not even detect any signs of life here, THE NATURALIST S OCCUPATION. 33 and appearances would suggest relationship to the li- chens sooner than to the vertebrates. If we cut open the fleshy encrustation, and examine under the micro- scope the contents of some of the little sacks found in it, we discover some minute tadpole-like beings, repre- sentinsf the larvae of the ascidian. The structure of these remarkable creatures repeats the fundamental features of the vertebrates so perfectly that we are com- pelled to place them in the same great family. They have a chordal axis with a nerve-tube on one side and the alimentary tube on the opposite, with gill-slits per- forating the throat, features common to all vertebrates in early life. Although the adult ascidian bears not the remotest resemblance to a vertebrate, the combination of these characters in its larva proves that it belongs to the vertebrate stock. The larva reaches the adult con- dition by a process of degeneration. It fixes itself to a stone by its head, then loses its tail, its only organ of locomotion, and sinks into a purely vegetative existence. So completely are its original features obhterated, that its vertebrate nature would never have been suspected, had not embryology brought to light its developmental history. The striking agreement with the development of the curious worm-like fish, Amphioxus, as made known by Kowalevsky, a Russian embryologist, led Professor Haeckel of Jena to regard the ascidian as the ancestor of the vertebrate stock. Startling as such a proposition was, it was favorably received at first, and was approved, though with reserve, by no less a logical and critical thinker than Huxley. Most authorities now concur with Lankester and Dohrn in regarding both Amphioxus and the ascidian as our degenerate vertebrate cousins. 34 MARINE BIOLOGICAL LABORATORY. Thus you see how far from the surface the truth may- lie, and how, in the systematic position of a single form, we tnay find a problem which only yields to solution after exhausting the resources of nearly every depart- ment of animal biology. In order to correct and extend the results of surface observation, the investigator appeals first to internal structure, and is thus led into the province of anatomy. Here fundamental features of relationship are brought more clearly into view ; and, following the general law that animals or plants of like structure have de- scended from common ancestors, it becomes possible to outline, in a rough way, a genealogical system. It is here that the investigator would begin to grasp the meaning of those deeper resemblances, called homolo- gies, and learn to distinguish between these and decep- tive analogies. But nature has concealed many of her more important homologies under disguises that a study of adult structure could not penetrate. Comparative anatomy, in the hands of such men as George Cuvier, Friedrich Meckel, Johannes Miiller, Richard Owen, Thomas Huxley, and Carl Gegenbaur, has accomplished wonders in this direction, but it owes many of its greatest discoveries to the aid of embryology and paleontology. Its greatest achievement was the .reduction of the animal world to four great types, and the same high elevation was reached independently by comparative embryology. But the type system of George Cuvier and Carl Ernst von Baer did not finish the reconstruction of the genea- logical tree ; for it failed to grasp the full meaning of like development and like structure. Comparative anatomy THE NATURALIST S OCCUPATION. 35 found ascending grades of organization in the vertebrates of the present ; paleontology discovered a corresponding gradation in the vertebrates of the past ; and embryology revealed the same serial gradation in developmental stages. The discovery of this most remarkable parallel- ism between the three series, the anatomical, the pale- ontological, and the embryological, is one of the most brilliant in the whole history of biology, and one which with pride and admiration we place to the credit and honor of Louis Agassiz. It is remarkable that these three of the leading biologists of the century, after laying the foundation of the theory of transformation, remained to the end its most determined opponents. It was left for Charles Darwin to show that the coincidence pointed out by Agassiz between the geological succes- sion, the embryonic development, and the zoological gradation, held also in the geographical distribution of animals in the past and the present, and to find the interpretation of the fact now universally accepted. The recognition of so fundamental a truth as that of community of descent, at once raised every department of biology to a new plane, gave new aims to each, and profoundly altered their relations to each other. Descent was seen to be ''the hidden bond of connection " so lona: sought for under the '' natural system " of classification. Embryonic development came to be regarded as the epitomized history of ancestral development. As Dar- win puts it, the embryo is "a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class." '' In two or more groups of animals, however much they may differ from each other in structure and habits 36 MARINE BIOLOGICAL LABORATORY. in their adult condition, if they pass through closely similar embryonic stages, we may feel assured that they are all descended from one parent-form." Thus embry- ology came to have a higher value in classification than anatomy, and to take the place assigned to it by v. Baer more than half a century ago, as ''the true torch-bearer in the investigation of organic bodies." Embryology and paleontology have become comple- mentary sciences, associated in the common aim of determining the genesis and the history of life. The peculiar charm of embryology is, that it brings us into direct contact with living forms, places us face to face with the phenomena of life, and reveals in the history of the individual the principal events in the history of the race. It holds the key to many a problem that has exhausted the resources of all the sister branches of biology, and promises to contribute more than all the rest towards the solution of the great mystery of life. In order to illustrate the relative position of embry- ology, and at the same time the nature of the naturalist's work, let us now look at one of the problems before him. II, A Special Problem. Naturalists are familiar with the efforts of compara- tive anatomy to determine the number of segments in the vertebrate head. At the beginning of this century, Germany's great poet, Goethe, and one of her most gifted naturalists, Oken, came independently to the idea that the skull is only an enlarged and otherwise modified portion of the backbone ; that is, that it is composed of THE NATURALIST S OCCUPATION. 3/ a number of segments, each of which is the structural equivalent of a vertebra. The idea was suggested by the sutural lines in the mammalian skull, which appeared to mark the boundaries of successive segments. Thus arose the so-called "vertebral theory" of the skull, which was widely accepted and which was made the corner stone of Richard Owen's great work on the com- parative anatomy of vertebrates. If the skull could be regarded as three, four, or more modified vertebrae, it followed that the brain might be considered as a portion of the spinal column, and that the cranial nerves were the equivalents of the spinal nerves. It was impossi- ble to settle these questions by compartive anatomy, and the assistance of embryology was invoked. The dis- covery by Jacobson, that the bony skull is preceded in development by a so-called ^^ primordial cranium^' con- sisting of a cartilaginous case, which, although a direct continuation of the cartilaginous basis of the backbone, yet differs from it in not being divided into segments, and the fact that the adult skull is really a double brain case, the inner portion representing the primordial cra- nium and its derivatives, or by bone that has replaced it, while the outer portion consists of the so-called dermal bones that have been added externally and secondarily, have been used with great force by Huxley, against the vertebral theory of the skull. If the skull ever con- sisted of segments comparable with the vertebrae, the proof of this should appear in the primordial cranium, as it is found to-day in the lower fishes, or in the course of its development. Since the time of Goethe and Oken, we have learned the important lesson, that the place to look for primitive 38 MARINE BIOLOGICAL LABORATORY. vertebrate characters is in the lower rather than the higher forms, and in the embryos rather than in the adults. The original expounders of the vertebral theory pursued just the opposite course, and were thus deceived- by superficial analogies. This theory, wide of the mark as it was in its origi- nal form, contains a germ of truth, which embryology has brought to light. Its fundamental idea, that the head, in whole or in part, is composed of segments morphologically equivalent to those of the body, may now be said to be an established fact. The problem as it now stands is this, — How far is the segmenta- tion of the body carried forward into the head ? in other words, how many segments are represented in the head ? Let us look a little further into the nature of the problem, and the methods and means of approaching its solution. The subject is a difficult one to make intelligible to those who are not familiar with the main features of development ; but it is so full of instruction, that it can- not fail to yield some points of interest even to a most superficial examination. Possibly the idea of segmen- tation of the head, or, to use the technical expression, the metamerism of the head, may appear to some of you quite devoid of general interest or importance. The principal charm of the subject of course lies in its envi- ronment, if I may use such a term, to express its general relations or bearings. In attempting to decipher the metamerism of the vertebrate head, we are really en- gaged in tracing the history of the origin of the great vertebrate stock or phylum. It is simply a question of the genesis or the pJiylogeiiy of the vertebrate type. THE naturalist's OCCUPATION. 39 at the head of which stands man himself. We do not know when nor how this metamerism of the vertebrate arose ; but, both embryology and paleontology afford ample evidence that it existed long before it took the form of vertebrae. Among the earlier and extinct forms of fishes are found some without bony vertebrae, but still divided into segments ; and in the development of the fishes and other vertebrates of to-day, we find that the vertebrae are preceded and predetermined by a primor- dial division of the trunk into a series of uniform seg- ments. This division appears very early in the embryo, long before there is any cartilage or bone, and before there is any trace of limbs, or indeed of any distinctive vertebrate organ, except the cordal axis referred to in speaking of the ascidian larva. Now this primordial segmentation carries us back to a stage in the evolution or phylogeny of vertebrates, so full of meaning that its contemplation would seem to be enough to arouse the interest of the most worldly-minded. This is a stage through which every vertebrate passes on its way from the ^^^ to the adult, a stage in which the fish, the amphibian, the reptile, the bird, the beast, and man find a common level, and in which every title to superior rank lies in unexpressed potentialities. But more than this ; for it is here that the vertebrate is an invertebrate, and stands beside its prototype, the seg- mented worm. On the same metropolitan plain, the lobster, the crab, the insect, in short all the members of the great arthropod group, meet and acknowledge their community of descent. Thus, the great branches of the genealogical tree represented in the higher types first defined by Cuvier converge and unite in a common 40 MARINE BIOLOGICAL LABORATORY. stem, which bears the deep and enduring mark of metamerism. So much for the general significance of metamerism. Let us now return to the vertebrate head. If the me- tamerate type of structure precedes and forms the foundation of the vertebrate type, then the question how many primordial segments are represented in the head takes precedence of the question how many ver- tebrae compose the skull. The inquiry takes us back to that interesting stage in which the embryo becomes divided into a chain of segments. But here we find that the transverse lines marking the boundaries of the segments do not extend into the region of the head, or at most only into its hinder portion. But we are not yet satisfied that the head is a thing siii generis, built upon a plan fundamentally different from that of the body. Baffled in the attempt to find direct evidence sufficient to demonstrate the unity of plan which we suspect underlies both the head and the trunk, we next resort to indirect or circumstantial evidence, and begin to question whether the records of ancestral development have been perfectly preserved in the em- bryonic development. It is here that the towering difficulties of the problem come into view, in scaling which investigation rises to its sublimest heights. Before the division into segments, there is nothing in the embryo to show even approximately where the head ends and the body begins ; the part which is destined to become the head forms with the rest a continuous whole, as shown in the external form and in the con- tinuity of like structural elements. The cordal axis before alluded to is the precursor of the backbone, and THE naturalist's OCCUPATION. 4I this structure extends through the greater part of the head region of the embryo, from which we may safely infer that, at least, so much of the primordial cranium as possesses this structure must be regarded as a direct continuation of the vertebral axis, even though no distinct outlines of segments appear in it. In the ab- sence of such outlines, the inquiry turns upon indica- tions which may betray their former existence. For evidence of this sort, the investigator continues his search, first of all, in the posterior region of the head, since this is demonstrably the least modified. Now it is just here that embryology has been able to demon- strate, in some of the lower fishes, the existence of at least one genuine vertebral segment. In a remarkable shark from Japanese waters, which Mr. Garman of Harvard has recently baptized with the name, Chlaniy- doselachns aiigidnens, as I learn from Dr. Ayers, who has lately studied its cranium, there are unmistakable evidences of from three to five cranial vertebrae. Indi- cations of a considerable number of primordial seg- ments, or protovertebrae as they are called, have been discovered in the hind head of the embryo of the amphibian, the reptile, the bird, and even the mammal. In the lowest representative of the fishes of to-day, the much-talked-of Amphioxus, the segments and the chordal axis extend from end to end ; and the head merges so completely in the trunk, that the most search- ing examination has scarcely yet been able to fix any boundary line. Although Amphioxus takes an isolated position, and may have sacrificed some elements of its head in exchange for the material enjoyments of a semi-vegetative existence, still it must be admitted as 42 MARINE BIOLOGICAL LABORATORY. an eligible witness to the metamerism of the verte- brate head. We are indebted mainly to recent studies on the development of the nervous system for the views now held on this subject. In the trunk we find each seg- ment provided with a pair of so-called spinal nerves, both of which spring from the spinal cord by two short roots, known as the anterior and the posterior root. The posterior root bears, just before its union with the anterior root, a spinal ganglion, and is thus stamped as something different from its fellow. This anatomical distinction is the basis of a physiological distinction, the discovery of which, by Sir Charles Bell, in the early part of this century, has been regarded as the most important acquisition of physiology since the time of Harvey. Bell determined by experiment that the posterior roots are appointed for sensory, the an- terior roots for motor, work. Thus both structure and function suggest that the spinal nerve is not one nerve, but two nerves united ; and this point is settled beyond dispute, first, by the independent and unlike develop- ment of the two roots, and second, by their complete and permanent separation in such fishes as Amphioxus and Petromyzon. Each segment of the trunk may therefore be said to have two pairs of nerves, a sensory pair with ganglia, and a motor pair without ganglia. Now we come to a question of absorbing interest not only to the embryologist but also to the anatomist and physiologist. Are the metameric arrangement, the divi- sion of labor or function, and the mode of development, essentially the same for the cranial as the spinal nerves? The several inquiries into which the question resolves THE NATURALIST S OCCUPATION. 43 itself have not yet been fully answered ; but the inves- tigation of the last ten years has heaped up affirmative evidences until the final answer has been in the main anticipated. Although a Dutch embryologist, Van Wijhe, has shown that Bell's law must be modified for the cra- nial nerves, yet we know from the researches that started with His, Balfour, and Marshall, that these nerves fol- low the same general law of development as the spinal nerves. We find posterior nerves with ganglia and ante- rior nerves without ganglia ; and the latter are purely motor as in the trunk, while the former are sensory. Some of these posterior cranial nerves, however, are mixed nerves ; that is, they have in addition to the regu- lar sensory fibres motor fibres, and in this respect they appear to depart from the spinal nerve type. But this difficulty, which still remains to be cleared up, loses its force as an objection, when placed beside an overwhelm- ing amount of evidence in favor of the homology of the two sets of nerves. The posterior nerves of the head and trunk have the same origin ; and the early develop- ment runs so exactly parallel in both cases, that their fundamental equivalence can no longer be seriously ques- tioned. The cranial ganglia, according to the researches of Beard, receive, secondarily, some elements that are not added to the spinal ganglia ; but homologies are settled by original conditions, not by adventitious differ- ences, and hence no objection can be raised on this score to the identification of the nerves. That the cranial nerves agree with the spinal in having a metam- eric arrangement is made evident by their relations to undoubted segmental structures of the head, such as the gill-arches and the head-cavities. 44 MARINE BIOLOGICAL LABORATORY. I must now invite you to the very borders of the beaten ground of investigation ; and I hope you will have the fortitude to follow me even to the brink of a precipice or two, should it be necessary, in order to get a view of the steep ascent which now challenges further advance in this direction. I have endeavored to give you the salient points in the historical development of the subject, and it remains for me to define the position now occupied, and so far as possible, by way of anticipa- tion, the path which investigation is destined to take in the immediate future. In the anterior region of the head, into which the cordal axis or "primitive backbone," as Lankester has called it, does not extend, there are two sensory nerves, the olfactory and the optic, which investigation has thus far failed to reduce to the type of the spinal nerves. No corresponding motor nerves exist ; and no decisive evi- dence of metamerism has yet been discovered in their development, or adult condition. Foremost authorities in anatomy and embryology, like Gegenbaur, Balfour, and Kolliker, have declared that here a dividing line must be drawn, separating the head into two distinct regions, one of which bears with the trunk the common stamp of metamerism, while the other is built upon a plan of its own. It is here that Balfour, looking back into the remote ancestral history of the vertebrates for clews, recognized what appeared to him a primitive boun- dary line, corresponding to what now divides the head and trunk in many invertebrate forms. According to this view, the fore-brain would represent the whole of the ancestral brain, while the mid-brain and hind-brain would represent a number of segments belonging origi- THE NATURALIST S OCCUPATION. 45 nally to the trunk, but now pressed into the service of the head. This conversion of trunk into head, in answer to the greater and greater demands made upon the brain, as the vertebrate line rose in the scale of development, is just that kind of economy which nature everywhere practises, and which we find exhibited in most instruc- tive grades of elaboration in the nervous system of in- vertebrates. That this has been the history of the mid and hind portions of the vertebrate brain, is a truth resting upon so many convergent lines of evidence that there is no longer room for scepticism. The fore-brain, in which the problem culminates, is still enveloped in a dense cloud of uncertainty, pierced by so few and feeble rays of light that we are compelled to accept the lead of conjecture, or to abandon the hope of further advance. We are limited to three hypotheses : We may assume with Balfour, that the fore-brain is the unsegmented brain inherited from an invertebrate ances- tor ; or with Kolliker, that it is a new formation, repre- senting an outgrowth from the unsegmented anterior end of the primitive nervous axis; or with Kleinenberg, that it represents a number of fused trunk segments, in which the ancestral brain — the " head-glanglion " of annelid worms — has either been absorbed beyond the hope of identification, or totally suppressed. Balfour's view marks the level of investigation ten years ago. Since that time the progress of discovery has been steadily in the direction of Kleinenberg's view. But we have reached a point where direct, demonstrative evidence appears to vanish, and it is only by the circui- tous route of circumstantial evidence that we can push onward. The solution we are looking for does not lie 46 MARINE BIOLOGICAL LABORATORY. in the skull, the primordial cranium, the cranial nerves, the head-cavities, or the gill-clefts, nor, in short, in any one organ or system of organs that could be named in the head. As Professor Dohrn has insisted, both by word and example, nothing less than a complete analysis of the whole head and trunk can furnish a safe founda- tion for speculation on this subject. But the task does not end with the vertebrates. The present vertebrate head represents the cumulative development of unnum- bered aeons, and its ancestral history is only very imper- fectly recorded in its embryonic development. Our analysis must therefore be extended to the worms, the arthropods, the molluscs, and, as it now appears, even to the coelenterates. The history of metamerism must be traced upwards, and the lessons of the simpler types must be our stepping-stones to a knowledge of the higher. There is little prospect of ever knowing precisely how many segments the ancestor of the vertebrates possessed. The number varies in the different branches of a common stock ; and we know that this variation is the result of loss in many cases, and suspect that it may be due to addition in others. But we know that this variability in number has very definite limitations in the laws that control the formation of segments. The possibilities in this respect are by no means the same for all regions of the segmented axis. Although the head segments have undergone the greatest modi- fications in form, fixity in munber is he7'e the rule, while variation, if we except degenerate forms, is con- fined to the posterior trunk segments. In the embryo the anterior segments are invariably first in formation, THE NATURALIST S OCCUPATION. 4/ and generally so in definition, the addition of new- segments taking place from behind. TJiere is not the least ground for supposing that a single segment has ever been, or can ever be, added to the anteiHor e7id. If, in the course of development, segments disappear, the loss is borne by the posterior end, as we see when the tadpole lays aside its fish-like tail in rising to the estate of frogdom. The direction of loss is the reverse of that of acquisition, the one travelling away from, the other towards, the head. Thus the point of maximum variability in number is always most remote from the head. Both the lav/s of development and the condi- tions of continued existence tend to strengthen the dis- tinction. The head segments developing first, have the advantage in the struggle for existence, and their su- preme importance is the guarantee of their perma- nence. Although there is not the least probability, and scarcely a possibility, of adding or interpolating entirely new segments in the head region, and although the chances of loss appear to come to a vanishing point a long way behind this region, still the shadow of un- certainty is not dispelled, and we have to acknowledge that we do not yet know how far the transforming influence of functional changes and substitution of organs has here been felt. In looking around us for a possible foothold, we in- quire, first of all, if there are not some structures con- nected with the fore-brain on which the seal of metam- erism has left an indelible impression. One pair of these so-called cerebral nerves, the olfactory, have fast been losing their high claims to a position of isolation, 48 MARINE BIOLOGICAL LABORATORY. until at last, stripped of one disguise after another, they have been almost, if not quite, reduced to the level of the sensory nerves of the trunk and hind head, through the researches of Marshall, His, Beard, and others. The identification of this pair of nerves with the rest of the segmental sensory nerves, on the basis of de- velopment and structural features, is a triumph of investigation so near at hand that it is scarcely pre- mature to proclaim it. The chain of discoveries bear- ing on this subject has still many links to be supplied, and here is one of the opportunities of the hour. The optic nerves still hold undisputed possession of the very pinnacle of isolation ; and even to question their claim to such a position may appear to betray a woful superabundance of speculative audacity that would be less unbecoming to a romancing visionary than to the sober investigator. But without hesitation or misgivings, and without any special claim to scien- tific prevision, I venture to predict that these nerves and their sense-organs will yet fall into line with the other sense-nerves and sense-organs. I cannot here enter very far into the question of the origin of the vertebrate eyes, but the subject is one of such great interest, that, at the risk of overtaxing your forbearance, I venture to ask your indulgence for a few general remarks. The evidence in favor of the derivation of the organs of the special senses from a common basis, has been growing during the past few years with such astonishing rapidity, that the hypothesis of independent origin has no longer a respectable claim to attention. If the eyes have been derived from some simpler form of sense-organs possessed by the ancestors THE NATURALIST S OCCUPATION. 49 of the vertebrates, we can only expect to find out what those primitive organs were by searching among inverte- brate animals of the ancestral type. By common con- sent we turn to the annelids, or segmented worms. Here we find sense-organs of a low order, segmentally arranged, and supplied by nerves bearing ganglia, which correspond in position and general relations to the spinal gangha of vertebrates. That the segmental nerves and ganglia of the annelids are the morphological equivalents of the spinal nerves and ganglia of vertebrates, is a proposition that now admits of little doubt. If the ar- gument holds for the nerves and ganglia, the basis is given for the comparison of sense-organs. But are there any sense-organs in the vertebrates that can be said to agree in structure and function v/ith the segmental tactile organs of annelids ? Leydig and Eisig have given an affirmative answer to this question, and their views have already met with general acceptation. The sense- organs of the lateral line of fishes and amphibia, rudi- ments of which have been found by Froriep and Beard in the higher classes of vertebrates, have essentially the same structure, the same or a closely allied function, and, so far as known, fundamentally the same mode of development. Allowing then that these organs are the homologues of the segmental sense-organs of annelids, there arises the very important question, is it possible for such organs to develop into those of the special senses, taste, smell, sight, and hearing .? In the vertebrates we meet with no serious difficulty until we come to the eye. The sensory impressions received by a visual organ differ so radically from those received by a tactile organ, that it seems 50 MARINE BIOLOGICAL LABORATORY. almost incredible that cells devoted to one of these func- tions could ever serve the other. Nevertheless, this marvellous transformation and change of function have actually taken place, and the fact still admits of ocular demonstration in a very large group of annelids. Some- times all the tactile cells are converted into visual cells ; at other times only a part of the cells assume the new function, while the rest continue to serve the old. The result is that we have at one end of the series pure visual organs, at the other end pure tactile organs, and between the two extremes every grade of mixture repre- sented in veritable compound sense-organs. The picture is a revelation that gives swift wings to suggestion. If such is the path of evolution in one case, the best ground is given for suspecting that the same economy has been practised elsewhere. The discovery of these facts in the leeches, led naturally to the anticipation of a similar origin for the eyes in other annelids and in those groups that have had a common origin with the annelids, before all the arthropods and vertebrates. The existence of segmental sense-organs, as I have said, is well known in other annelids than the leeches, and the origin of eyes from them is fairly well indicated in many cases. It is a most promising subject of investigation, which, like a thousand others, still waits for the encouragement which the wealth of this country will not long, I trust, refuse to supply. The close relationship between the annelids and ar- thropods rendered it probable that in the latter the eyes were also derived from segmental sense-organs, and the probability was strengthened by the arrangement of the eyes in successive pairs, as in the larvae of many insects. THE NATURALIST S OCCUPATION. 5 I My anticipations seem to have been in the right direc- tion, so far as I can judge from the observations of Dr. Patten, which have been carried on durins: the last three years through the generous support of Mr. AlUs, of Milwaukee. So far then, as we now understand the genesis of sense-organs, both in the vertebrates and in the inver- tebrates, the evidence all points to the derivation of the paired eyes of vertebrates from segmental sense-organs. The development of the vertebrate eyes has never been studied from this stand-point ; but the subject is a most inviting one, and offers a broad field for observation and reflection. The existence of an unpaired median eye in verte- brates, which has been claimed by a number of recent investigators, is rendered doubtful by Professor Ley- dig's careful researches. If the pineal organ turn out to have been a visual organ, it will present a difficulty not easy to dispose of on the hypothesis of derivation from segmental sense-organs. All such sense-organs are paired, and a single median eye could arise from them only through the fusion of at least one pair of eyes. We have examples of such fusion in the inver- tebrates ; but it might be extremely difficult to find any evidence favoring a double origin of the pineal organ. Investigation must lead with a searching analysis of structure and development in every group of verte- brates, while keeping up the search for a homologous structure in the invertebrates. We have now followed the subject of the metamerism of the vertebrate head- far enough to get a clear idea of its essential features and general bearings. We started 52 MARINE BIOLOGICAL LABORATORY. with a special problem and found it to be the centre of inquiries, leading in all directions into the unknown. So it is with all special subjects in biology. The farther we pursue them the broader and more interesting they become. Nothing could be farther from the truth than the idea that such questions are isolated, and devoid of interest to all except the specialist. 4 I THIRD LECTURE. SOME PROBLEMS OF ANNELID MOR- PHOLOGY. By EDMUND B. WILSON. I SHALL endeavor in this address to consider, in an elementary way, some of the broader morphological questions that are suggested by a study of the develop- ment of annelids. It is a subject that has a very special and technical side ; yet it is also, as I shall try to show, a subject that at every step suggests wider and deeper problems, some of which extend so far beyond the limits of the particular group of annelids as to stand among the most interesting general questions of comparative zool- ogy. They are, moreover, questions which I believe may be made intelligible and suggestive to those who are not specialists — who, I had almost said, are not even zoolo- gists. To this end, however, it is of primary importance to indicate the point of view from which the subject is considered and the pathway by which it is approached. And hence I may perhaps be pardoned for a few intro- ductory remarks on the purposes and methods of morpho- logical inquiry, and the considerations that lend interest to the group of annelids from a general point of view. 53 54 '. MARINE BIOLOGICAL LABORATORY. Of the numberless genealogical inquiries raised by the theory of organic evolution, none has a higher interest or has attracted more general attention than the deriva- tion of vertebrates, involving, as it does, the origin of all the highest manifestations of vital structure and action. So long as the evolution tneory remained an unproved and comparatively vague hypothesis, as it was left by Lamarck and St. Hilaire sixty years ago, the origin of vertebrates, like other genealogical inquiries, could have no more than a speculative interest, and could produce little direct effect upon morphological investigation — which indeed had quite enough to take care of at home, without following speculative zoology in her erratic excur- sions. When, however, the field of action had been cleared by such pioneers as von Baer, Johannes Miiller, Remak, and Kowalevsky, when speculative zoology had been redeemed and vitalized by Darwin, and the theory of organic descent established on a firm footing, mor- phological research entered upon a new phase. A broad foundation of known facts had been laid ; a splendid working hypothesis had been found. The central ques- tion in every morphological investigation became two- fold ; it was no longer simply wJiat is f it was also Jiow came it to be? And this second question, be it observed, is not properly a speculative matter at all, but an his- torical one ; it relates not to an ideal or hypothetical mode of origin, but to a real process that has actually taken place in the past and is to be determined like any other historical event. " Speculative zoology " thus, by slow degrees, became the guide and leader of research, and every morphological inquiry became, in the last analysis, a genealogical one. SOME PROBLEMS OF ANNELID MORPHOLOGY. 55 Now, under the evolutionary interpretation of nature, every higher and more complex form has arisen from a lower and simpler one, presumably now extinct, but possibly more or less similar to forms still existing. In any case the key to the genealogy of higher forms must be sought in the organization of lower but related forms. In morphology, as in every field of research, the inter- pretation of complex phenomena must be sought through the study of simpler phenomena. And so it comes about that the indispensable basis for every inquiry respecting the derivation of vertebrates is an accurate knowledge of those invertebrate forms, if any there be, that possess any features in common with the vertebrate type. I scarcely need to add that no zooloirist would look for the actual o progenitor of vertebrates among existing invertebrates. We seek only for forms more or less nearly similar to the ancestral proto-vertebrate. The character of the ancestral type must be largely a matter of inference, not of direct observation. Now, what are the most fundamental and interesting features of the vertebrate body.^ Let us leave aside such characters as the presence of a corda dorsalis, the relation of the central nervous system to the alimentary canal, and the double-tubular composition of the body — all of which are peculiar to the vertebrates or their im- mediate allies. Let us consider only those broader characteristics on which the distinctive vertebrate feat- ures are, as it were, moulded. I think most morphologists will agree that the most striking feature of the vertebrate body as regards struc- ture is its metamerism (or segmented structure) ; and its most remarkable feature as regards development is 56 MARINE BIOLOGICAL LABORATORY. the fact that growth takes place mainly at one end (the posterior) of the embryo, differentiation becoming more marked as we proceed forwards along the antero-poste- rior axis of the body. For the sake of brevity, this may be called apical or tuiipolar growth. To these funda- mental morphological peculiarities we may add a third — that in all vertebrates the body appears to be built up, in a greater or less degree, by the union along the median line of parts that are laid down in the embryo as separate bilateral foundations. This I shall term coiiC7'escence} Vertebrate morphology therefore presents three fundamental problems for solution, viz., the origin of (i) metamerism, (2) apical growth, and (3) concres- cence ; and these three lie at the root of all others. Let us now turn to the invertebrate types. Do any of these possess the three characteristics in question ? There are two such types, namely, the artJiropods (in- sects, arachnids, Crustacea, etc.) and the annelids (earth- worms, leeches, and a great number of marine worms). The arthropods may, however, be left aside, since the annelids are in every respect simpler and less specialized, and there is strong reason to believe that the leading features of arthropods are inherited from annelid-like ancestors. The annelids have a typical metamerism and they are the lowest of segmented animals ; apical growth appears among them in its clearest and simplest form ; the phenomena of concrescence are nowhere so ^ The term is here used in a wider sense than is ordinarily employed. The occurrence of concrescence in the more restricted sense among the vertebrates is not generally admitted to be a fact, and very eminent authority can be cited both for and against it. However, this very division of opinion in regard to so deep-lying a question only serves to render its investigation more interesting and important. SOME PROBLEMS OF ANNELID MORPHOLOGY. $/ striking, nowhere less open to dispute. It is therefore clear that from a comparative point of view a peculiar interest attaches to these animals. It is nearly certain that they are closely related to the ancestors of the arthropods ; many zoologists regard them as closely approaching the progenitors of the vertebrates. In any case, no one who wishes to gain any insight into the morphology of the higher segmented types can afford to pass by the annelids, even though their remarkable similarities to the higher types in organization and development be regarded merely as analogies and not as evidence of direct genetic connection. The importance of the annelids is heightened by another remarkable fact. All annelids, in the course of development, pass through a larval stage (Fig. 2) known as the trochospJiere or trocJiophore, often disguised but always present in some form. This larval type, under many different modifications, occurs in many other groups of invertebrates, though nowhere so clear and typical as among the annelids. Its significance is one of the most vexed questions of comparative mor- phology, and opinion is at present nearly equally divided between two opposing schools. According to one view the trochophore is the embryological (or ontogenetic) representative of an ancestral (phylogenetic) type, the "Trochozoon," from which all forms passing through a trochophore stage have actually been derived by evolu- tion— just as a bird, for instance, is supposed to have arisen from a fish-like ancestor because it passes through a fish-like stage of development within the Q%^. If this view be well founded, and if (observe the double condition) the similarities between annelids and verte- 58 MARINE BIOLOGICAL LABORATORY. 3Y0.\>ft* } WtOo^ or \\u.NvV, "Vtcj^y ,iow brates indicate real affinity, then the trochophore larva must be taken to represent, in a certain sense, the an- cestor of all the higher forms of life. According to a , second school, however, the trochophore has no such re- markable significance, but is a ** secondary" or ''adaptive" larval form — i.e., one second- arily interpolated into the de- velopment and representing no ancestral group ; as is the case, for example, with the larvae of insects. Whichever view be taken, a precise knowl- edge of the trochophore is essential to the investigation of the general problems indi- cated; and I shall show further on that the study of this re- markable larva raises a num- ber of very singular questions regarding the nature and origin of the higher forms of life. I trust that this introduction will suffice to make clear the general considerations which, in my opinion, render the annelids, and particularly their trochophore larvae, worthy of especial attention ; and which give value to the investigation of every detail of their morphology. We turn now to a more special account of the annelids. The body of an annelid (Fig. i, which may be taken OAXYVVJL.VX a\'ai4 Fig. I. SOME PROBLEMS OF ANNELID MORPHOLOGY. 59 as a diagram of the common earthworm) is divisible into two widely different regions, though the grounds for making the division do not very clearly appear until the embryological development is taken into account. The first, known as the head or prostoniiiim, lies anterior to the mouth. It is unsegmented, contains no organs of reproduction, excretion, or circulation (minute blood- vessels excepted), and is devoted to the higher functions of sensation and coordination. It contains the brain (cerebral ganglia), and is often, though not always, pro- vided with eyes, antennae, or other highly organized sensory apparatus. The second portion, known as the body or trunk, lies posterior to the mouth. It is much larger than the head, and forms an elongated cylinder divided into a large number of segments {inetameres or somites). The trunk is segmented internally as well as externally, nearly all of the internal organs being divided into segments, or repeated in the successive somites throughout its whole extent — as for example, the ribs or the spinal nerves are repeated in a vertebrate. As regards function, the trunk is in the main given over to nutrition, circulation, excretion, motion, and repro- duction ; its actions are, however, regulated by a series of ganglia, a pair to each somite, that form a "ventral nerve-cord " along the middle line of the body. It is also, as a rule, provided with sense-organs ; these, how- ever, are in most cases less highly organized than those of the head. It has been proved that, in many cases at any rate, the brain exercises a directive action over the other ganglia ; so that from a physiological point of view the body may be regarded as subordinate to the head. We shall find a somewhat similar morphological subordi- 6o MARINE BIOLOGICAL LABORATORY. nation of body to head in studying the development of an annelid. Upon this simple plan all annelids are constructed, though they are almost as varied in the details of their orofanization as the vertebrates themselves. The head may be provided with the most elaborate lobes, tentacles, cirrhi, branchiae, eyes, etc. ; or it may lose all of these special organs and become reduced to an insignificant rudiment, as in the earthworm. The trunk is no less diversified ; sometimes a simple jointed cylinder, some- times provided with lateral appendages of the most diverse character in different forms and performing many functions — locomotion of many varieties, respira- tion, sensation, etc. Let us now turn to the development ; I select Poly- gordms, a form generally regarded as one of the sim- Vw)\w I'Yu.Yy.V. Fig. 2. plest and most primitive of the annelids. Polygordins has a perfectly typical trochophore larva (Fig. 2), shaped SOME PROBLEMS OF ANNELID MORPHOLOGY. 6l almost like a bi-convex lens, with a circle of powerful cilia (prototroch) running around the edge, a mouth at one point near the edge, a retort-shaped alimentary canal (dotted in the figure) and a brain (cerebral ganglion) at the upper pole. The trochophores of other annelids have the same general structure as this, but are often more rounded in form, sometimes being nearly or quite spherical, with the prototroch at the equator. The anus is always at the opposite pole from the brain. The subsequent history of this larva reveals the sin- gular fact that the parts thus far described become, almost entirely, converted into the Z^^^^^aT or prostomium of the adult. The trunk is a later formation, growing down from the lower pole of the larva — like a bud, as it were — and becoming divided into segments. (Fig. 2, B and C). At the posterior (lower) end of the body there is a kind of growing point, like the terminal bud of a plant, at which rapid cell-formation takes place, so that the tip is carried further and further down and the body steadily elongates in one direction. The segments are formed successively, those in front being the oldest while new segments are continually in process of forma- tion, one after another, at the growing point. This is a typical case of apical or unipolar growth. Examining the structure of the growing point more narrowly we find that the internal tissues (mesoblast) are arranged in two widely separated lateral bands, which, as the trunk grows older, widen out and grow together along the median line, ultimately giving rise to muscles, blood-vessels, ex- cretory organs, reproductive organs, etc. This process is a form of concrescence ; but this mode of growth will be seen more clearly in the development of the leech, 62 MARINE BIOLOGICAL LABORATORY. described further on. The newly formed trunk ulti- mately becomes so large that the original substance of the trochophore forms the head only,^ which is in Poly- gordins scarcely larger than one of the trunk-segments. The cilia of the prototroch disappear, the animal sinks to the bottom, burrows into the sand, and assumes the adult condition. Let us examine the significance of these facts. It appears that the head is the oldest part of the body, and that in a certain sense the trunk is its offspring, — as a branch is an offshoot from a tree. In other words, the Qgg develops into what we may regard as a free- swimming head, and this after a while buds off the body — an afterthought, as it were. Observe now the genealogical question that is at once raised. If this mode of development be in any manner a repetition or representation of the ancestral development, then the ancestors of the annelids, and of all the higher metam- eric types, are represented to-day in the head. And the head must, therefore, be historically, as well as em- bryologically, the oldest part of the body, and the trunk is a later acquisition.^ We may go farther than this. The trunk, I have ^ This statement demands some qualification, since the extreme lower pole of the larva, bearing the anus, is carried down with the growth of the trunk, and remains as the so-called telson of the adult. It is, how- ever, unnecessary to complicate the discussion by bringing in this fact. 2 The " head " of arthropods and vertebrates probably represents the prostomium plus a certain number of trunk somites, closely united and devoted to special functions. Indeed, these somites have so far usurped the functions of the original prostomium that it is doubtful whether this can any longer be distinguished in the arthropod or vertebrate embryo. This, however, does not affect the essence of the historical question under consideration. SOME PROBLEMS OF ANNELID MORPHOLOGY. 63 said, appears to be budded off from the head. How ? By the successive formation of a series of somites, each of which contains its own segment of the ahmentary canal and of the circulatory apparatus, a pair of excre- tory organs, a pair of ganglia and nerves, and in some cases gills, locomotor organs, sense-organs — eyes, it may be — tactile organs, and the like. Each somite has a complete, or nearly complete vital apparatus of its own ; and in some annelids (though these cases are rare) the somites may become separate, lead indepen- dent lives, and develop finally into complete individuals like the original worm. These facts irresistibly suggest the question : is not the trunk to be regarded as a linear colony of sexual in- dividuals, successively budded off from the asexual head .'^ — 2^^^cisely as sexual medusae are budded off from the asexual scyphistoma, or as proglottides are seg- mented off from the scolex of a cestode worm. If this question be answered in the affirmative, then all metam- eric animals, vertebrates and man included, must be colonial organisms, comparable, in point of individuality, with a hydroid or polyp colony. A number of eminent zoologists do not hesitate to accept this conclusion. One of the latest and best students of annelid develop- ment (Kleinenberg) regards the trochophore as being simply a modified asexual medusa ( ! ). He finds in it the characteristic medusan nerve-ring, the modified velum (prototroch), the umbrellar and sub-umbrellar regions. He does not state definitely his conception of metamerism, which we are left to infer ; but other morphologists have not hesitated to interpret his views in accordance with the colonial theory. 64 MARINE BIOLOGICAL LABORATORY. This theory of metamerism is an old and familiar one ; it has been adopted by many eminent morpholo- gists, both of the older and the newer schools. That it is a plausible and fascinating hypothesis must be ad- mitted. Yet there is strong reason to doubt whether it can be sustained, either on general or on special grounds. I have not space for a discussion of the ob- jections to the theory, but I will mention a few of the principal difficulties. We do not, as a matter of fact, find in the lowest and most primitive annelids, as we should find if the theory were true, that the somites show clearly marked individuality, or any tendency to become separate individuals. On the contrary, the metamerism of these forms {Polygordius, etc.), is less pronounced than in higher forms. It is in the highly organized Polychaeta that the repetition of similar parts is most marked, and in the highly modified Oligochaeta that the independence of the somites is greatest. A second, and much more fundamental difficulty, is that the trochophore, according to Kleinenberg, has at first no middle germ-layer (mesoblast). How then is it pos- sible, on any theory of budding, to account for the ori- gin of the trunk-mesoblast } Again, as a recent writer (Meyer) has pointed out, if the somites are budded off successively from the head, the anterior somites should be the youngest, which is the reverse of the truth ; and finally, the somites are not strictly homodynamous, since the alimentary canal of the anterior and posterior somites (stomodacum and proctodDsum, respectively), differs entirely from that of the middle section. These various objections, with others that might be given, are in my opinion fatal to the entire theory. SOME PROBLEMS OF ANNELID MORPHOLOGY. 65 It is not my intention to review the various theories that have been put forward in place of the colonial theory. Some of them are exceedingly ingenious ; none of them are adequate explanations of metamerism. But I wish to show that the study of this question is very closely bound up with that of certain others which need to be carefully studied in the embryology of annelids, and which offer a very inviting field for investigation. The segmentation of the trunk first arises in the inter- nal parts of the embryo — i.e., in the mesoblast — and upon this internal segmentation the external segmenta- tion is, as it were, moulded. Now, the mesoblast, as every embryologist knows, arises in all annelids in two separate lateral masses (mesoblastic bands) that extend lengthwise along nearly opposite sides of the trunk and sooner or later join each other both in front and behind, so as to form, as it were, a longitudinal ring, lying be- tween the two primary germ-layers. In all cases the bands grow mainly at their posterior ends, where, in many cases, each terminates in a large pole-cell (or *'telo- blast ") from which the entire band is derived (see Fig. 6). As development proceeds the two bands widen out, forcing their way between the ectoblast and entoblasf, and ultimately grow together along the median line, first below and afterwards above the alimentary canal, which is thus entirely surrounded (Fig. 3, A., B., and C). Metamerism, I repeat, first appears in these mesoblastic bands, and is only secondarily extended to the other parts. And it is interesting to observe, further, that the segmentation of the bands is perfectly distinct long before they unite in the median line. Each somite, therefore, is formed by the union in the middle line of 66 MARINE BIOLOGICAL LABORATORY. two halves, which are at first completely separate, i.e.y by concrescence. And each band grows at its posterior )atii- i&. V ^ 1 / \t : / 1 ^ 1 h A \ /^ \— A 1° / \ Fig. 3. tip, the somites being progressively developed from this point forwards, i.e., by apical growth. I think it must be clear, therefore, that the investiga- tion, from an embryological point of view, of metamer- ism, of apical growth, and of concrescence is insepara- ble from the study of the whole series of phenomena relating to the mesoblast formation ; and that a close study of the origin of the mesoblast in annelids is of great importance to comparative embryology. If we turn now to the literature of the subject, we find the utmost confusion, the most extraordinary differences of opinion among the various authors, respecting almost every detail of the subject. We find the mesoblast de- scribed by one author as arising wholly from the ecto- blast, by another as arising from the entoblast, by others as arising from both layers, or again as arising SOME PROBLEMS OF ANNELID MORPHOLOGY. 6/ from neither directly, but being differentiated in the course of cleavage. It is described as arising from a single pair of pole-cells, from several pairs of pole-cells, from no pole-cells at all — and so on through a long list that might be given did space suffice. It is, in short, simply impossible, at present, to reconcile the various modes of mesoblast-formation in annelids as described by various good authorities, and there is scarcely a more confused subject in comparative embryology, or one which more pressingly demands revision. It is no wonder that Kleinenberg, who has been largely influ- enced by the study of annelids, attempts to cut the Gordian knot by denying the very existence of the mesoblast as a "germ-layer" — ''es gibt gar kein Meso- derm." Nevertheless very few forms have as yet been adequately studied. Indeed, scarcely a single case has been exhaustively worked out ; and while this is the case, we need not despair of reducing the various modes of mesoblast-formation to a common type. Until this has been accomplished, however, it will, I believe, be premature to speculate on the origin and meaning of metamerism. We may come now to closer quarters with the prob- lem of concrescence and the meaning of the trochophore, questions that have thus far only been alluded to in passing. I am obliged to treat this part of the subject in a more technical manner, though I fear it will be at some sacrifice of intelligibility to those not especially interested in the subject. If we examine the embryo of a leech {Clepsine), in the middle period of development (Fig. 4, A), we find that the future alimentary canal is represented by three macromeres (yolk-cells), distended 68 MARINE BIOLOGICAL LABORATORY. with food-yolk, on the top of which the remaining parts are spread out in a fiat disc, the margins of which are notched at two opposite points. These notches mark, respectively, the anterior and posterior ends of the future body ; and it is therefore possible to distinguish the right and left sides of the embryo, even at this very early period. The margins of the disc are thickened on each side to form a structure known as the gom- band. The germ-bands join anteriorly above the notch to form the head ; posteriorly each ends in a group of five large pole- cells which form the growing point of the band. As development pro- ceeds, the disc extends over the yolk- cells and finally encloses them com- pletely ; its edges grow together in a seam (the beginning of which is shown in Fig. 4, B.), which extends alons^ the median ventral line of the embryo. The body is formed by the fusion of the two germ- bands, which are at first completely separate except at their foremost ends. But more than this, a close examination of the germ-bands shows that each consists of several distinct elements. Each is covered by the outer ectoblast ; it contains a cord of nervous matter, from which the correspond- ing half of the ventral nerve-cord is derived ; it has a cord of cells which appears to be concerned in the development of the excretory organs (nephridia) ; and each contains internally a mesoblastic band like that of Fig. 4. SOME PROBLEMS OF ANNELID MORPHOLOGY. 69 PoIygQvdiiis, which gives rise to muscles, blood-vessels, etc., and which, by segmentation, first blocks out the metamerism of the trunk. Thus, with exception of the alimentary canal, every system of the body — circula- tory, excretory, muscular, nervous, reproductive — is laid down in two completely separate halves. And the union of the two germ-bands, which form the two halves of the trunk, is a typical and unquestionable case of concrescence. This extraordinary phenomenon is ex- hibited in its greatest perfection in the leeches and some of the fresh-water annelids (" naids "). It occurs in a striking form in the development of the earth- worm, though modified by the very different structure of the gastrula. In Polygordiiis the two halves of the body are never as completely separated as in the leech, yet the primary separation and subsequent growing together of the mesoblastic bands is clearly enough a simplified form of the same general phenomenon ; and the same is true of many other marine annelids. Among the arthropods complete concrescence, — i.e. the com- plete separation of the two halves of the body on the ventral side — has been observed in a single case only ; but a partial concrescence, comparable with that of Polyg07'dius, probably occurs throughout the entire group. Whether complete concrescence occurs among the vertebrates or not is still a disputed question. It is asserted, on very high authority, to take place in some of the lowest vertebrates (sharks and bony fishes) in nearly as typical a form as among the leeches, but this is disputed by many observers. It is, however, unquestionable that a partial concrescence — that for instance in the mesoblast and the central nervous sys- 70 MARINE BIOLOGICAL LABORATORY. tern — takes place throughout the group, and the phe- nomena are in some cases nearly as striking as in the annelid types. What is the interpretation of concrescence ? Is it a secondary adaptive mode of growth, necessitated by some mechanical condition of development ? Or is it a primary, ancestral process, which means that all un- paired organs (such as the heart or the spinal cord) now formed by concrescence were originally double ? Both sides of this alternative have their adherents. The first view, which is represented by very eminent authority, regards concrescence as a process of resto- ration (to use Professor Whitman's apt expression) by which the two halves or the embryo, which have been mechanically separated in the course of development, are brought together again. I have not time to go fully into the nature of the causes that are supposed to have produced this separation. Broadly speaking, however, the main cause is supposed to have been the excessive accumulation of food (yolk) in the lower and middle part of the egg, for the use of the developing embryo. This mass of food, lying as it does in the median line, is sup- posed to have temporarily bisected the embryo, as it were ; so that a subsequent concrescence became a mechanical necessity in the construction of the body. According to the second view concrescence has a far deeper meaning, though the probability is not lost sight of that accumulation of yolk may have modified its char- acter or heightened its effect. The origin of concrescent growth is to be sought, from this point of view, in the origin of bilaterality itself — an inquiry which brings us to a deep-lying problem concerning the mode of deriva- SOME PROBLEMS OF ANNELID MORPHOLOGY. 7 1 tion of bilateral animals from the radiate (coelenterate) forms, which by common consent are considered to have been their progenitors. At present, apparently, the data do not exist for a trustworthy decision between these two conflicting views ; though, as a matter of fact, most practical em- bryologists adopt one or the other as a working hypoth- esis. Speaking for myself alone, and judging from the development of annelids, the view that concrescence is a wholly secondary process seems inadequate and opposed to many important facts. There are forms — the earthworm, for instance — in which there is little food-yolk, and yet a nearly typical concrescence takes place. Furthermore, a nearly complete series may be traced, from such typical cases of complete concrescence as Clcpsine or R/iyjicheimis, to the opposite extreme of PolygGrdiiis, in which there are no "germ-bands" and no concrescence save that of the mesoblastic bands. It is precisely these bands, however, that form the most important element of the germ-bands in C/epsi7ze, etc., inasmuch as the development of the other parts is, as I have said, moulded upon them. There is no logical jus- tification for making any fundamental distinction be- tween complete concrescence (i.e. of the germ-bands), and partial concrescence {i.e. of the mesoblastic bands). The latter process is, however, one of the most charac- teristic features in the development of all annelids, whether possessing food-yolk or not ; and this, in my opinion, is fatal to the theory in question. Let us turn, therefore, to the second view. If it be true that the origin of concrescence goes back to the origin of bilaterality, then our inquiry must be extended 72 MARINE BIOLOGICAL LABORATORY. to include the genealogy of the Bilateralia — 2>. of all forms above the Coelenterata. It is generally agreed that the bilateral type of structure arose by the modifi- cation of a radial type, but as regards the mode of transi- tion two totally different views are maintained. Under the older view, still maintained by some morphologists, the long (or principal) axis of the radiate body — e.g. of a Hydra or a sea-anemone — corresponds to the long or antero-posterior axis of the bilateral body; and hence the oral face corresponds to the anterior end, and the aboral face to the posterior. The mouth must therefore corre- spond in the two cases, and the anus of Bilateralia is a new formation. Under the more recent view — which is held by the greater number of morphologists — the long axis of the bilateral body corresponds to one of the transverse axes of the radiate body, and the oral face of the latter is represented by the ventral aspect of the former.^ The justification of this view — which to me appears the only possible one — lies in the facts of em- bryological development. The gastrula stage of devel- opment is all but universally regarded as being, in a broad sense, the embryonic representative of the radial, two-layered, ancestral type. The blastopore (or gas- trula mouth) represents the ancestral mouth (or protos- tome). Hence the mode of transition from the radial gastrula to the bilateral adult should give us decisive evidence in regard to the ancestral transition. Now, it has been shown in the clearest manner that, in the great majority of cases, at any rate, the blastopore occupies 1 This applies, of course, to the bilateral invertebrates only. The verte- brates are left out of consideration, as having, in all probability, arisen from forms to which the statement would equally apply. SOME PROBLEMS OF ANNELID MORPHOLOGY. 73 e^AW .NNOVVW. the ventral surface of the embryo (Figs. 5, 6), and that the elongation of the body takes place approximately in the plane of the blastopore — i.e. that the long axis of the adult coincides with one of the transverse axes of the gastrula. This appears with especial clearness in the development of annelids (Fig. 5), where the bilaterality is thrown back, in a measure, upon the gastrula itself, and the blastopore is more or less elongated, its anterior part persisting as the mouth ; or in that most primitive arthropod, Peripatus, where the elon- gated blastopore closes in the middle, the two openings thus left persisting as mouth and anus respectively (Fig. 6). In these facts lies, as I believe, the key to the problem of concrescence. We see from such cases as the earthworm and Peiipatus, that the separation of the two sides of the body (germ-bands), may be caused, not by a mass of food- yolk, but by the blastopore itself, and concrescence is a sequence of the closure of the blastopore, modified more or less ex- tensively by accumulation of yolk. In forms like the leeches or vertebrates concrescence is modified and exaggerated by the fact that the region of the blastopore is occupied by the enormous mass of yolk. But this should not blind us to the fact that the primary cause of concrescence lies in the position of the blastopore, not of the food-yolk. From a genealogical point of view I believe this must be Figs. 5 and 6. 74 MARINE BIOLOGICAL LABORATORY. taken to mean that bilateral animals have arisen from radial forms by elongation in one of the transverse axes of the latter, the oral face becoming the ventral aspect, and the aboral face the dorsal. The mouth, meanwhile, shifted its position so as to lie near the anterior extremity of the new long axis, and the lateral portions, growing together more or less completely along the region formerly occupied by the mouth, gave rise to the process of concrescence in the ontogeny. What, then, was the origin of the anus of bilateral forms ? Here again the answer of embryology appears to be nearly or quite conclusive. The blastopore gives rise sometimes to the mouth, sometimes to the anus, sometimes to both. The only possible interpretation of these facts would seem to be that the blastopore originally gave rise to both mouth and anus, the case of Peripatiis being an interesting and apparently iso- lated remnant of the ancestral mode of development, or perhaps a reversion to it. Under any other view, as has often been pointed out, we should be reduced to the absurdity of regarding the mouth of one animal as homologous with the anus of another, perhaps closely related, form ; or we should be involved in other difficul- ties, equally great. The original mode of closure of the blastopore has been secondarily modified in the great majority of cases, but the mesoblastic bands and the neural cords still follow the original mode of develop- ment, being laid down separately on either side of the region of the blastopore, and growing together along its line of closure. Let us now turn, in the last place, to the significance of the trochophore. A very little consideration will SOME PROBLEMS OF ANNELID MORPHOLOGY. 75 show that if the foregoing discussion of concrescence has any weight the trochophore cannot possibly be an ancestral larval form, but is one that has undergone very great secondary modification. For concrescence takes place throughout the trunk-region ; and if the Hne of concrescence represent the original line of closure of the protostome the trunk cannot be of later origin than the head, since, by the hypothesis, the ancestral radiate body gave rise, by transverse elongation, to both head and trunk. From which it follows that the suppression of the trunk-region, which is the essential feature of the trochophore, must be a secondary matter. In other words, the anterior part for some reason develops more rapidly than the posterior part, which lags behind and only makes its appearance after the anterior part has acquired highly developed organs of locomotion, sensa- tion, and coordination. Strong confirmatory evidence of this view appears to me to be afforded by the follow- ing facts : If the suppression of the trunk region be a secondary character, we should expect to find in the larva some rudiment of the trunk, present, but in an undeveloped state ; and, in point of fact, I believe such a rudiment is always present. If we examine the poste- rior portion of the trochophore we find on each side of the body, near the end of the alimentary canal, a small group of cells — or, it may be, a single cell ('* primary mesoblast "), lying in the cavity of the body. It is this cell or group of cells that in later stages gives rise to the mesoblastic band — i.e. to the basis of one-half of the trunk. The trunk is not present, but its germ is ; and hence it is not strictly correct to say that the tro- chophore represents the head alone. It is a highly de- ^6 MARINE BIOLOGICAL LABORATORY. veloped, individualized head, which carries within itself a minute, rudimentary trunk — just as in a seed two huge modified leaves, the cotyledons, carry between them the minute germ of the stem, root, and foliage- leaves. If the cells in question can be shown to be always present the trochophore is not a diploblastic or- ganism, but a triploblastic one, and Kleinenberg's com- parison of the trochophore to a medusa falls to the ground. Kleinenberg asserts that the trochophore con- sists at first of the ectoblast and entoblast alone, the mesoblast being a later formation. This conclusion is based upon the study of Lopadorhynchus, in which the mesoblast is apparently not present at the start but is afterwards split off from the ventral ectoblast. This result has always seemed a very puzzling one, which could not be harmonized with what is known of the mesoblast formation in the earthworm, the leeches, and many other forms. I have recently been able, however, to examine the development of an annelid {Nereis) which I believe solves the puzzle and shows how Lopa- dorhyncJms is connected with the other forms. The early trochophore seems to be diploblastic, as in Lopa- dorhyiicJins — i.e. to consist of ectoblast and entoblast only, without any trace of mesoblast. In later stages the mesoblast arises from the anterior ventral portion of the outer layer (''ventral plate"). A study of the early stages of development however — which Kleinenberg did not succeed in following — shows that the cells of the ventral plate are differentiated from the remaining outer layer cells almost from the beginning of development, and even without the use of reagents can be easily distin- guished from the remaining outer layer cells in the fully established ''diploblastic " trochophore. These cells have SOME PROBLEMS OF ANNELID MORPHOLOGY. 7/ tlie same 07itogcnetic origin^ in the cleavage-process, as the viesoblast in other annelids. In other words, the meso- blast is differentiated during the cleavage in essentially the same way as in other forms, but is not removed from the surface until a very late period. It forms, in fact, a part of the outer layer of the larva, which has accord- ingly a deceptive appearance of being two-layered. In reality the third layer is already present ; and it con- stitutes here, as elsewhere, the germ of the trunk. It appears, therefore, that in all cases the trochophore contains a rudiment of the trunk, the presence of which means in my opinion, that the larva once possessed a fully developed trunk, which is now temporarily reduced in favor of the head. A somewhat analogous case is that of the Nauplius larva of the Crustacea. This larva has but three pairs of functional appendages, which be- come highly organized and of great functional impor- tance while the remaining appendages are represented by mere rudiments, or, it may be, by mere groups of cells near the posterior extremity of the larva. Now, it is nearly certain, in the opinion of the best authori- ties, that the Nauplius is a secondary form ; that the posterior appendages formerly developed in uniform succession to the three anterior pairs ; and that their temporary suppression in the Nauplius has been sec- ondarily brought about. This case, though not entirely parallel to that of the trochophore, will serve to illus- trate the general character of the change which I believe the latter form has undergone. Let us finally pass in brief review the principal points we have considered. Of the three main problems sug- gested by the development of annelids, only one can at present be brought under a satisfactory working yS MARINE BIOLOGICAL LABORATORY. hypothesis, and this solution is one that many embry- ologists would be unwilling to accept. I think the great majority of morphologists will agree that no satisfactory explanation of metamerism has yet been given ; and the problem of apical growth is still farther from a solution. Concrescence stands on a very different footing, since clear and definite causes for it can be assigned ; yet even here a complete solution of the problem will only be possible when comparative embryology has advanced far beyond its present standpoint. As regards the trochophore, opinion is still divided ; and I am giving only a personal view in stating that the accumulating evidence seems to favor, in the main, the view that it is a secondary larval form, which gives no clue to the ancestory of the segmented animals. To those whose interest in science lies in the consideration of its posi- tive results only, the outcome of this discussion will doubtless seem rather unsatisfactory ; and it must be admitted that in some respects the fundamental prob- lems of annelid and vertebrate morphology seem to be as far from a solution as in the time of von Baer. To the investigator, however, it is the unsolved problems that call forth the deepest interest. It is the very vagueness and uncertainty of the subject that impress upon us how much remains to be done in the embry- ology of annelids, and arouse the interest with which we look forward to the results of future investigation in this field of study. That the problems of metamerism and apical growth will ultimately be solved, there can be little doubt ; but the present need is for new facts, not for new theories. When the facts are forthcoming, the theories will take care of themselves. FOURTH LECTURE. -<>0j:^0-0- THE GASTRyEA THEORY AND ITS SUCCESSORS. By J. PLAYFAIR McMURRICH. When morphological science had emancipated itself from the influence of the Cuvierian doctrine of types, a result mainly due to the publication of Darwin's "Origin of Species," morphologists turned their attention to the problem of tracing out the phylogeny of the various animal groups and forms. During the last thirty years much has been accomplished along this line, but one of the greatest of the difficulties which presented themselves in the way of a completion of the phylogenetic scheme, was the lack of facts upon which to base a satisfactory explanation of the manner of origin of the Metazoa from the lower unicellular organisms. From time to time, however, theories have appeared which attempted an explanation, but, with a single exception, they have been weighed and found wanting. These theories may be classed in two groups, (i) those which take for their starting point a multinucleate proto- zoan, such as Opalina (von Ihering), and (2) those in which a colonial flagellate is regarded as the ancestral 79 80 MARINE BIOLOGICAL LABORATORY. form (Haeckel, Lankester, Balfour, Blitschli, and Met- schnikoff). Von Ihering suggests^ the possibility of the trans- formation of a multinucleate protozoan into a metazoan by the segregation of the protoplasm around the various nuclei, whereby the organism becomes multicellular, the original Infusorian mouth becoming the mouth of the multicellular animal, and the contractile vacuole its excretory system. This theory, however, has not been receiv^ed with any degree of favor, inasmuch as it lacks confirmation from the developmental phenomena of the Metazoa, the cases in which the segmentation results in a syncytium (Crustacea, Insecta) being evidently a secondary modification due to the accumulation of food- yolk. There can be little doubt but that the segmenta- tion of the ovum and the resulting formation of a morula or blastula are most readily comparable to the development of a colonial protozoan, and the theories based upon this idea are more worthy of consideration than that advanced by von Ihering. The first of these theories in point of time, and the one which has had the greatest influence upon embryo- logical investigation, is Haeckel's well-known Gastraea theory. This made its appearance in 1872, and was the outcome of the researches embodied in the classic " Mono- graph of the Calcareous Sponges." The simplicity of structure of the lowest calcareous sponges, and their apparent similarity to the gastrula of the higher forms, a similarity all the greater to the mind of Haeckel on account of his erroneous conception of the structure of 1 H. von Ihering. Vergl. Anatomic des Nervensystems und Phylogenie der Mollusken. Leipzig. 1877. THE GASTR.EA THEORY AND ITS SUCCESSORS. 8 1 the sponges, were the leading causes in the evolution of the theory. Haeckel entirely overlooked the presence of the flattened ectodermal layer (first discovered by F. E. Schulze in 1875, and since demonstrated by other investigators in many different groups of sponges), and homologized with the ectoderm of the Coelenterata and of the Gastrula the mesogloea, or rather the mesogloea phis the unobserved ectoderm of the sponges. Tiius a sponge was to him a diploblastic organism, the OlyiitJnts being '^nur eine festsitzende Gastrula." A complete exposition of the theory and of the facts upon which it was based is to be found in the Jenaische Zeitschrift.^ The starting point in the line of evolution, according to the theory, was a simple mass of protoplasm destitute of a nucleus, the Monentla, a representative of which is found VixProtaniceba, and the disappearance of the nucleus of the ovum previous to its division to form the polar globules, was considered to be the reproduction of this stage in the individual development. It is probable, however, that the nucleus is represented in Protamoeba by scattered particles of chromatin disseminated through the cytoplasm and not yet aggregated into a definite mass, in which case the Moners, as Haeckel understood them, do not exist. Granting the disappearance of the nucleus in the ovum, the explanation that it is a return to an ancestral condition is most unsatisfactory. The ^ E, Haeckel. Die Gastrsea-Theorie, die phylogenetische Classification des Thierreichs und die Homolugie der Keimblatter. Jenaische Zeitschr. Bd. viii. 1874. E. Haeckel. Die Gastrula und die Eifurchung derThiere. Jen. Zeitschr. Bd. ix. 1875. E. Haeckel. Nachtrage zur Gastrsea-Theorie. Jen. Zeitschr. Bd. xi. 1877. 82 MARINE BIOLOGICAL LABORATORY. ovum is a cytode, the next stage in the process of evolu- tion according to Haeckel's scheme, and that it returns for a few minutes to a lower grade of organization simply to indicate its ancestry is certainly an idea at variance with all morphological principles. Thanks, however, to our more perfect technique, we now know that the original nucleus of the ovum does not disappear but is the direct ancestor of the nuclei of all the cells compos- ing the organism resulting by development, and there is therefore no Monerula stage in the developing ovum. The next staoe of evolution was the formation of a nucleus, by which the Monerula was converted into a cell or cytode, the ancestral form being a Cytula equiva- lent to the existing Amceba. Following this came the Moj'ula, represented ontogenetically by the morula stage which occurs in the development of certain forms, and during which the embryo consists of an undifferentiated solid mass of cells. That this stage can be considered primitive, and the early appearance of a segmentation cavity which is found in so many forms a secondary condition seems however very doubtful. The evidence at our disposal points the other way. No living repre- sentative of the morula stage is known, and to fill this gap Haeckel proposes an hypothetical ancestor, the SynaniosbiiLin. The fourth stage is the blastiila, a hollow sphere pro- duced by the usually contiguous cells of the morula secreting a fluid, which, passing to the interior, forces the cells to the periphery. The hypothetical ancestor corresponding to this ontogenetic stage is termed the Plaiicea, The fifth stage is the gastnda, formed from the THE GASTR^A THEORY AND ITS SUCCESSORS. 83 blastula by the invagination of certain of its cells, and represented ancestrally by the hypothetical Gastrcea. This forms the last stage common to all the Metazoa ; from it various paths branch off : '* sie f iihren von der monaxonien Gastrula einerseits zu den monaxonien Spon- gien und den stauraxonien Acalephen, anderseits zu den dipleuren oder bilateralen Bilaterien ; und zwar zunachst zu den Wiirmern, aus denen sich die vier typischen Stamme der Mollusken, Echinodermen, Arthropoden, und Vertebraten erst spater hervorgebildet haben." Such is in outline the Gastraea theory, the first attempt to plan out from embryological data the phylogenetic origin of the Metazoa. One of the greatest difficulties in the way of its acceptance is the occurrence of delami- nation ; that is, the conversion of the monoblastic blastula into a diploblastic organism, not by invagination, but by the separation off, by karyokinetic division, of the inner ends of the blastula cells. Some of Haeckel's followers have endeavored to overcome this difficulty by rejecting as improbable Fol's observations on the delamination in Gejyojiia, but renewed study of the development of the Trachymedusae by Metschnikoff and Brooks have in- contestably demonstrated the occurrence of the process. Haeckel, however, met the obstacle more fairly, and, rely- ing on the fact that in comparatively closely related forms both modes of endoderm formation may be found, held the view that delamination is a secondary condition derived from invagination, failing however to explain how it has been derived, and thus leaving the difficulty as great as before. Shortly after the publication of the Gastraea theory Ray Lankester brought out his PlanuUi theory, the key- 84 MARINE BIOLOGICAL LABORATORY. note of which is the formation of the endoderm primarily by delamination, invagination being thus a secondarily acquired phenomenon. Thus Lankcster's views stand in direct contrast to those of Haeckel. Like Haeckel, Lankester attempts^ to give the dif- ferent historical phases recapitulated in ontogenetic development. He starts with the ovum, the Monoplast, corresponding to Haeckel's second stage, the Cytula. This is succeeded by ih.Q Polyp last, equivalent to Haeck- el's third and fourth stages, for Lankester recognizes two different forms of Polyplast, — one in which it is solid, the Morula, and another in which it is hollow, the blast- ula, derived from the morula in the manner indicated by Haeckel. Up to this point the only difference between the two authors is the omission by Lankester of the Monerula stage. To the Polyplast succeeds the diplo- blastic Planula, for which Salensky's more convenient term DiblasUda may be employed. This is a two-layered vesicle without a mouth, the inner layer of cells (endo- derm) having been formed by delamination. The cavity of the blastula {blastoccel, Huxley) has now become the digestive cavity, or archenteron, the cells lining it having gradually acquired a digestive function while they still formed the inner ends of the blastula cells, the acquisi- tion of the function leading to their differentiation from the non-digestive or ectodermic portion. Delamination is, however, a relatively unusual occur- rence, invagination replacing it in a large number of cases. How can its occurrence be explained on the basis of the Planula theory 'i The changes which led to the ^ E. Ray Lankester. Notes on Embryology and Classification of the Animal Kingdom, etc. Quart. Joiirn. Micr. Sci. vol. xvii. 1877. THE GASTR^A THEORY AND ITS SUCCESSORS. 85 formation of the diblastula must be regarded as having been primarily adaptive, but later became dependent upon heredity. The physiological molecules composing the ovum tended to become differentiated earlier and earlier in the ontogenetic history, those destined to form the entoderm being early set apart for that purpose. This is Lankester's doctrine of precocious segregation, equiv- alent to the law laid down by Haeckel as "heterochrony in the palingenetic phenomena of ontogeny." Lankester supposes that in invaginate types the segregation of the endoderm extends to the first division of the ovum, one spherule containing the ectodermal molecule and the other the entodermal (Fig. i). By the continued division of these spherules a num- ber of cells are produced, the entodermal ones ar- ranging themselves with- in the ectodermal, a gas- trula being thus formed (Figs. 2, 3) without the intervention of a blastula. This structure, which oc- curs so frequently in the typical formation of an invaginate gastrula, Lankester considers to be an alto- gether different structure from that which precedes the diblastula, and to have been secondarily acquired by the mechanical accumulation of fluid between the cells of the forming gastrula, whereby the endoderm cells are forced out from their position within the ectoderm, and a cavity is thus formed between the two layers (Fig. 4). This cavity is, however, not equivalent to that of the Figs. 1-4. S6 MARINE BIOLOGICAL LABORATORY. delaminating blastula, inasmuch as it does not become the archenteron on the formation of the endoderm, but is obUterated by the invagination. It is necessary then to distinguish between the delaminating blastula and the invaginating " pseudo-blastula," and between the archenteric blastocoel of the former and the *'pseudo- blastocoel " of the latter. The ingenuity of this theory is its strong point, but simplicity can hardly be considered one of its charac- teristics. It has not met with the general acceptance which greeted its predecessor and rival, nor has it had the same influence on embryological investigation, — a result owing to the fact that no evidence in support of such an origin of the gastrula can be found. A third theory is due to Balfour, and may be termed the AmpJiiblastula theory.^ It is founded upon the peculiar blastula of the calcareous sponge Sycandra, the cells of which at one pole are columnar and ciliated, while those of the other pole are larger and granular. Balfour thinks it possible to consider this larva as a colony of Protozoa one-half of the individuals of which have been specialized for locomotor and respiratory pur- poses, while the others are essentially nutritive. In the later stages, however, the ciliated cells become invagi- nated within the granular ones, a fact which seems at variance with the theory if the granular cells are to be homologized with the endoderm of other forms, but which Balfour explains in the following manner. On the set- tling down and fixation of the sponge embryo the ciliated cells, being partly locomotor in function, become, to a 1 F. M. Balfour. A Treatise on Comparative Embryology, vol. i. Lon- don. 1880. THE GASTRiEA THEORY AND ITS SUCCESSORS. 8/ great extent, useless, and are therefore invaginated, the nutritive granular cells being thus able to expose their full surface for the acquisition of food particles. The ciliated cells are enabled to carry on their respira- tory function by the formation of an osculum and pores. According to this idea the nutritive function ought to reside in the ectodermal cells of the Sponge, and in the fiat cells lininsf the walls of the canals which Balfour took to be derived by invagination from the ectoderm, and the collared cells of the ciliated cham- bers should be purely respiratory, and Metschnikoff's re- searches appeared to confirm this view to a large extent. The recent extensive observations of von Lendenfeld ^ are, however, in direct opposition to it, demonstrating that it is the collared cells which are ingestive, thus confirming the earlier statements of Carter, In addition to this, the fact that the development of Sycandra cannot be regarded as primitive, and therefore as throwing light on the ancestry of the Metazoa, indicates that the AinpJii- blastiila theory is not founded on a secure basis. The speculations of Biitschli, which led to what that author has denominated the Plakiila theory,^ had their starting point in a study of an existing flagellate, Goniiim, which consists of a single-layered plate of cells. The transverse division of all the individuals of such a colony would result in the formation of a two- layered plate which Biitschli terms the Plakiila. At first in all probability there would be no difference in 1 R. von Lendenfeld. Experimentelle Untersuchungen iiber die Physi- ologic der Spongien. Zeit. fur wiss. Zool. Bd. xlviii. 1889. 2 O. Biitschli. Bemerkungen zur Gastrsea Theorie. Morph, Jahrb. Bd. ix. 1884. 8S MARINE BIOLOGICAL LABORATORY. the cells of the two layers, but later one layer might specialize for nutritive purposes, and the other for locomotor (Fig. 5). From such a condition, by the gradual bending of the plate so that the nutritive layer becomes concave (Fig. 7), a gastrula could readily be pro- duced, and in this con- nection Biitschli points out that the concavity would be useful to the colony, serving as a trap for food -particles and also allowing a larger number of cells to come into contact with a larger food body. The blastula which so frequently precedes in- vagination could be pro- duced from this by the accumulation of fluid between the two layers ; and furthermore, delamination may have been brought about by the plate, while still one-layered, becoming concavo-convex (Figs. 6, 8 and 9), and finally a hollow sphere (Fig. 10), the transverse division of the constituent cells then taking place. Unfortunately for this theory it does not find general support in embryological phe'nomena, nor any more than the Planula theory does it explain the formation of the blastula in a manner in accordance with the actual facts. There is a series of phenomena which none of these theories attempt to explain. Many of the authors who have recently contributed to our knowledge of Coelen- FiGS. 5-10. THE GASTR^A THEORY AND ITS SUCCESSORS. 89 terate development have felt strongly the insufficiency of these theories to explain the phenomena occurring in the early stages of the ontogeny of forms belonging to that group, and have been led either to throw doubt upon their applicability to these cases (Brooks, von Lendenfeld, Goette), or have exhibited considerable intellectual elasticity in endeavoring to bring about a harmony. Let us glance at the methods of formation of the diploblastic embryo in the lower Metazoa, before passing on to a consideration of the next theory. In Sponges there seems little room for doubt that the solid embryo or planula, as we may call it, is of much more frequent occurrence than the invaginate gastrula. In Ascetta a blastula results from segmentation, and by the migration of cells situated at one pole of this struc- ture, a solid central mass of cells is produced, and a similar process (perhaps assisted by delamination) prob- ably occurs in Halisarca, Reniera, Esperia, and other forms ; at all events, there is nothing in the formation of the central cells of the embryos of these forms which indicates the occurrence of invagination. This process is exceptional in the Sponges, and so far as is known at present occurs only in some of the simpler calcaregus sponges and in Oscarella, and in these cases presents some peculiar characteristics which throw doubt upon the homology of the Gastrula of these forms with that of such a form as Sagitta. In the Cnidaria the absence of an invaginate gastrula is quite as striking as in the Sponges. It is unknown in the Hydrozoa, in which migration of cells of the blastula resulting in the formation of a solid planula may be regarded as the rule, though in the Trachyme- 90 MARINE BIOLOGICAL LABORATORY. dusae delamination may produce a hollow diploblastic embryo without the intervention of a solid condition. The formation of a solid morula during segmentation which occurs in such forms as Hydractinia, Clava, etc., may readily be regarded as a precocious immigration, finding its counterpart among the Sponges in Chalinula. (Kellar). In the Scyphomedusae some forms such as Pelagia^ Cyajiea, and Chiysaora have long been considered to have an invaginate gastrula. Regarding Pclagia there is no doubt that this is really the case, and Metschnikoff has described the occurrence of invasfination in Natisi- tJioe. As regards the other forms, however, the observa- tions are not sufficiently complete to render the occur- rence of this process certain. Sections are absolutely necessary for a correct determination of the processes which occur, and Goette's observations on Aiirelia and Cotylorhiza show that a structure exactly resembling an invaginate gastrula may be produced from a solid planula formed by migration. My own observations on Cyanea arctica demonstrate the occurrence in its development of a solid planula formed apparently by immigration, and it seems probable from Claus' figures that such is also the case in Chrysaoi-a. Among the Anthozoa it is very doubtful if invagina- tion ever occurs. In the majority of forms investigated delamination is the process by which the inner cells are formed, but invagination has been stated by Kowa- lewsky to occur in Ceriaitt/ius mevtbranaceus and Acthzia, sp. (.'*) In Metridium marginatum I have found what renders doubtful the statements of that author, who studied optical sections only. A hollow blastula results THE GASTR.'EA THEORY AND ITS SUCCESSORS. QI from segmentation ; delamination then occurs, produc- ino- a diblastula. At the same time, however, or even previous to the delamination, a certain amount of dis- integration of the inner ends of the cells takes place, the central cavity becoming more or less filled with granules of food-yolk. These are later on either ab- sorbed, or pass out through the mouth -opening, which is formed later, and we get a structure resembling closely in general appearance a typical gastrula. The fact that Kowalewsky found some food -yolk in the sup- posed gastrula cavity of Cerianthus seems to indicate that we have in that form not an invagination, but a delamination such as occurs in Metridium. To sum up briefly, in the lowest Metazoa there is pro- duced a solid embryo, either by immigration of certain of the blastula cells or by delamination, a hollow diblastula being formed in a few cases by the latter process, and still more rarely invagination occurs. Ac- cordingly the difficulties in the way -of the Gastrsea theory are very great, and the other theories fail to explain the immigration phenomena. It is hardly logi- cal to take phenomena occurring in comparatively few cases to be the most typical, and consequently we must assume that the formation of a soUd planula and a sub- sequent hollowing out of the central mass is typical in the Porifera and Cnidaria. The Parenchymella or Phagocytella theory proposed by Metschnikoff ^ seems much more in accordance with the facts than any of the theories hitherto discussed. He starts with a spherical hollow colony of flagellate Infusoria similar to Volvox, and supposes that in it " cer- 1 E. Metschnikoff. Embryologische Studien au Medusen. Wien. 1886. 92 MARINE BIOLOGICAL LABORATORY. tain of the superficial cells became amoeboid and mi- grated into the interior of the colony just as we find them doing to-day in Pivtospongia, and that, in addition, certain other cells divided transversely, one of the cells so formed passing into the interior, while the more peripheral one retained its position at the surface. . . . While transverse division became predominant in some forms, longitudinal division and consequently the immi- gration of superficial cells prevailed in others. In this manner from mixed delamination, primary delamination branched off on the one hand, and multipolar immigra- tion on the other." Is it necessary though to assume that this " mixed delamination," i.e. a mixture of delamination and immi- gration, was characteristic of the ancestral flagellate colony.? May we not claim that immigration is the more primitive method, and that delamination has been secondarily acquired after the group Metazoa had been well established "^ There is evidence in the colonial Protozoa in favor of such a view ; as, for instance, in Protospongia in which cells, originally seated superfi- cially in the jelly in which the individuals of the colony are imbedded, pass to the centre, losing their flagella and collars, and becoming, according to Saville Kent, reproductive. Volvox again, when mature, is a hollow sphere with reproductive cells lying freely in the central cavity ; these cells were originally at the surface, but, losing their flagella, they migrated to the centre. The same process too is found in the sponges in Ascetta, and Metschnikoff has demonstrated its prevalence over delamination in the metagenetic Hydromedusae. If this idea be accepted, however, how can delamina- THE GASTR^A THEORY AND ITS SUCCESSORS. 93 tion be explained ? Metschnikoff imagines that both delamination and immigration have arisen simulta- neously as a natural sequence of the modes of non- sexual multiplication found in the Flagellata, where transverse division and longitudinal division both con- tribute, sometimes one sometimes the other occurring, or, as in certain Chlamydo-monadinae, both processes occurring together, colonies being thus built up. If in the lower Metazoa longitudinal division predominates, we get immigration, but if transverse division also oc- curs delamination results. This seems, however, hardly to explain what happens ; it does not explain why longi- tudinal division, i.e. the division in a plane perpendicular to the outer surface of the blastula, prevails exclusively during the conversion of a large-celled blastula into one with columnar narrow cells, the transverse division then suddenly appearing. It seems quite possible to explain the origin of delamination in another way. We know that in many ova food material may be aggregated at one pole, and if such a polar storing-up of food should have occurred in the various cells of the blastula, or of an ancestral form corresponding to it, it is easy to under- stand how it would be to the advantage of the organism for the inner portion of its cells to divide off, to delami- nate, instead of migrating in toto. A certain amount of migration might accompany this process, as in Coty- lorJiiza and Aurelia, or the new delamination might entirely replace the earlier process as in Renilla and Metriditini, and if this specialization were carried a little further, typical delamination such as occurs in Geryojiia and Lii'iope would result, the embryo in these cases being not a solid planula but a hollow diblastula. 94 MARINE BIOLOGICAL LABORATORY. Invagination can readily be deduced from immigra- tion. Metschnikoff points out that immigration may- take place irregularly over the entire surface of the blastula (multipolar immigration), or may be confined to the posterior extremity, especially in free-swimming blastulas (unipolar immigration). It is interesting to note, in this connection, that according to recent ob- servers there is a tendency towards unipolar immigra- tion in the case of the sexual cells of Volvox. From the polar immigration Metschnikoff deduces invagina- tion. In the Hydroid Laodice a few cells at the poste- rior extremity of the blastula are clearer than the rest and gradually migrate into the interior. If these cells should migrate en masse, instead of individually, we should have such an invagination as occurs in Nansithoey the alteration of the manner of migration being an abbre- viation of the original process. The simplicity of this explanation recommends it, and it serves to clear up the sporadic appearance of invagi- nate gastrulae in the Sponges and Scyphomedusas. But the occurrence of these gastrulse suggests the question, when once the invaginate gastrula has been established, does invagination continue to be the mode for endoderm formation, other processes, such as epibole, being de- rived from it } A negative answer to this question involves the assertion that the invaginate gastrula may have been developed several times independently. Is this impossible t We have invagination occurring in the Sponges and in the Discomedusae. The gastrula of Sycandra cannot be regarded as having been the caenogenetic ancestor of the gastrula of Oscarella, and neither of these that of THE GASTR^A THEORY AND ITS SUCCESSORS. 95 Pelagia. Too many forms with the characteristic coelenterate soUd planula intervene phylogenetically, to say nothing about the individual differences of the gastrulae. If the Sponges are included among the Coelenterata it is necessary to homologize the osculum of such a form as OlyntJms or Sycajidra with the mouth of a Cnidarian. This being the case the gastrulae of Sycajidra and Oscarella cannot logically be considered homologous with the gastrula of Pelagia, since, whereas in the latter the blastopore becomes the mouth of the adult, in the Sponges it closes, the embryo settling down upon it, and a new mouth (the osculum) breaking through at the opposite pole of the embryo. The relation of the gastrulae of Pelagia and NaiisitJioe to the invaginate gastrula of the higher Metazoa is also highly improbable. Whatever scheme of Coelen- terate ancestry for the higher Metazoa we accept, whether the Actinozoan, the Ctenophoran, or the Me- dusan, it is improbable that such a form as one of the higher Discomedusae ever came into the series. Is it probable too that the invaginate gastrula of EiipODiatus is without genetic relation to that of Ar- bacia, of Sagitta, and of AvipJiioxiisf Let us try the phylogenetic method of solving this question also. What form of gastrulation is most prevalent among the lower forms, and how far can it be considered an- cestral .-* Some years ago, while studying Molluscan embryology, I was struck with the similarity which exists between the segmentation and gastrulation of certain forms of that group, and what has been described as occurring in the marine Turbellarians. • The conclusion suggests 96 MARINE BIOLOGICAL LABORATORY. itself that the epiboUc gastrulation which is found in these two somewhat widely separated groups can hardly have arisen independently, and that its occurrence is due to its having been the mode of gastrulation in the ancestors of both groups. It is also to be found in members of other groups, such as the Annelida, having been first described by Kowalewsky in Euaxes, and it occurs more or less typically in many Polychoetous forms. I endeavored to express this idea in the follow- ing statement : " The modes of segmentation of the Platyhelminths, Annelida, Mollusca, and Molluscoidea, can be referred to a common type, indicating that the ovum (so to speak) in all these groups has been derived from an ovum possessing a considerable amount of nutritive yolk aggregated more or less completely at one pole." A necessary corollary of such a proposition I also stated as follows : *' The regular and equal segmen- tation which occurs in certain forms in several of these groups cannot be considered the original mode, but has been secondarily brought about by the loss of a food-yolk originally present." It would not be suitable in a lecture of this kind to review the various accounts of segmentation in these groups, but I would simply point out that if this idea is correct it follows that epibolic gastrulation is more primitive than embolic, the latter having been derived from the former by the loss of food-yolk. Can epibole have arisen from immigration .'* It can be regarded as a process of migration, the migrating cells being special- ized very rarely in the development, having stored up within them a large amount of yolk. The protoplasm separates more or less from the large yolk spherules, THE GASTR^A THEORY AND ITS SUCCESSORS. 9/ forming the micromeres, and the large spherules project into and fill what would otherwise be the blastula cavity. In consequence of this there can be no im- migration, but . the same result is achieved by the micromeres growing round and enclosing the large micromeres. An indication of the possibility of such a process can be seen in some of the Hydromedusae as Laodice, where the migrating cells are much larger and of a different structure than their fellows, and Met- schnikoff has also described a more pertinent case in Polyxeiiia, in which the segmentation is as a rule equal, resulting in the formation of a morula by precocious immigration, but occasionally the segmentation becomes decidedly unequal, so much so that the process of endoderm formation resembles closely epibole. I have noticed in Cyanea, in which the segmentation is appar- ently equal, an exceptional case in \Vliich one pole of the rotating blastula consisted of a few large cells, while the other was formed by a number of smaller ones. The invaginate gastrula I do not then consider to be a phylogenetic form — the Gastraea never existed. The ancestor which must take its place is the Parenchy- mella. This ancestor is perhaps reproduced in the ontogeny of the Metazoa in the structure which results from the closure of the blastopore. This is a phe- nomenon which frequently occurs, which is difficult to explain under the Gastrasa theory. It is on the other hand what might be expected from the Parenchymella theory, as I have endeavored to extend it. Where epibole occurs, owing to the nature of the process, the breach made in the walls of the blastula by the immi- gration is not closed at once as in typical immigration. 98 MARINE BIOLOGICAL LABORATORY. A large section of the blastula wall migrates at once, and it takes some time for the opening so made to be obliterated. This obliteration is the '* closure of the blastopore." The stage succeeding this is the Paren- chymella stage, though owing to caenogenetic modifica- tions and acceleration in the development of organs, the comparison is not perfect, but the embryo consists of an ectodermal layer with a central mass of cells in which differentiation has commenced. The blastopore or prostoma of the Turbellarians and Gasteropods has no exact counterpart in the Cnidaria ; it is the result of the method of enclosure of the central cells. It is the later formed mouth, or mouth and anus, which corre- sponds to the Coelenterate mouth, and just as the mouth forms in the Coelenterates at the pole where the immi- gration occurred, so in higher forms the mouth, or mouth and anus, appears at the pole of the embryo formerly occupied by the larger yolk bearing cells. In embolic gastrulas, where the invaginated inner layer of cells forms a hollow sack, the archenteron, it may be to the advantage of the embryo, little or no food-yolk being present, for the blastopore to persist, complete closure never taking place, a portion of the blastopore being converted into the mouth or anus as the case may be. Even in the Echinodermata, however, in which as as a rule the blastopore persists as the anus, a complete closure of it occurs in the Crinoids. This view of the relation of the blastopore to the mouth and anus does not necessarily conflict with the theory first advanced by Biitschli, and later supported by Adam Sedgwick and E. B. Wilson, to the effect that the mouth and anus of the higher Metazoa correspond to THE GASTRiEA THEORY AND ITS SUCCESSORS. 99 the two extremities of the mouth of the Polyps, but a discussion of this theory would occupy an undue amount of time. I have hitherto omitted all reference in this discussion to the Arthropoda and the Vertebrata. In the former group a considerable amount of study of the early stages of development is still necessary before the processes of formation of the germ layers can be homologized with accuracy with those of other Metazoa. In one group, however, that of the Arachnida, the re- searches of Metschnikoff on Chelifer, and Morgan on the Pycnogonids, demonstrate the formation of an inner layer of cells by delamination, and it seems not improb- able that typical centrolecithal segmentation may be derived from such a process by the extensive accumula- tion of food-yolk in the inner portions of the blastula cells. As reo^ards the Vertebrates there is an almost uni- versal concensus of opinion among embryologists that the peculiarities of their endoderm formation are to be explained on the hypothesis of an ancestral invaginate gastrula. It seems quite possible that invagination secondarily derived from epibole may have become so impressed upon the ontogeny of the Vertebrate ances- tors as to leave its imprint on the development of the later groups. There is still another point to be considered in con- nection with the Parenchymella theory, viz., the causes which operated to bring about the transformation of the solid planula into the hollow gastrula. Metschnikoff's views on this are as foUows : — It is clear that it would be to the advantage of locomotor colonies of more or 100 MARINE BIOLOGICAL LABORATORY. less similar cells, that those individuals which were laden with food matter should not remain at the periphery, but should pass to the centre, not only to equalize the weight of the various parts of the colony, and to allow the more active cells free action at the periphery, but also to permit these food-laden cells to carry on their assimilative functions without disturbance from external conditions. *' For a long time the individuals of any colony prob- ably differed only quantitatively ; the locomotor cells attracted food particles by the movement of their fla- gella and also absorbed some of the smaller particles, just as the ectoderm cells in some Coelenterates of to-day occasionally ingest food-matter. The inner amoeboid cells, however, were on the other hand capable of swal- lowing larger food particles. Probably when so engaged the amoeboid cells approached the periphery, and gained possession of food particles lying on the surface of the colony by means of the numerous pores between the cells of the superficial layer. . . . Gradually the differentiation in this direction progressed ; the loco- motor cells lost more and more their food-ingesting function, which concentrated itself in the amoeboid phagocytes; the occasional fine pores between the loco- motor cells enlarged and became openings similar to those so numerous on the surface of a sponge. . . . With the increased activity of the Metazoa, now pro- vided with two primitive organs, there must also have been an increased necessity for food, and larger plant and animal organisms must have served as prey. To make this possible one or more larger openings arose, which led to the formation of a mouth." THE GASTR^A THEORY AND ITS SUCCESSORS. 101 According to this scheme the central cells are diges- tive and ingestive ab initio. It seems strange, however, that cells specializing themselves for a purely digestive function should withdraw themselves from practically all contact with the surrounding medium, the source of the food supply, and there seem to be mechanical difficulties in the way of the central cells obtaining particles of food, larger than those which could be ingested by the locomotor cells, and through minute pores many times smaller than the ectodermal cells. Furthermore the explanation of the formation of the mouth is not at all in harmony with its ontogenetic development. The cells which migrate are well nourished and there- fore in a suitable condition for reproduction. This is actually their function in Volvox, and in Protospongia, which Metschnikoff cites, the only observations we have indicate that the cells which leave the periphery of the colony become reproductive. May this not be the origi- nal function of the migrating cells, the formation of the parenchymella being regarded as the migration to the centre of eutrophic cells capable of being reproductive } It does not necessarily follow that all the cells which migrate must become reproductive ; some of them may become specialized along other lines ; but the idea is sim- ply that primarily it was cells in a condition suitable for reproduction that migrated, and when only a few cells underwent this change of position they all became repro- ductive, but when a large number migrated some might become differentiated to subserve other purposes. It seems to me that what we find in the development of such a sponge as Halisarca indicates the manner in which the solid embryo becomes converted into a hollow 102 MARINE BIOLOGICAL LABORATORY. organism with a mouth, endoderm, and mesogloea, and also giv^es a clue to the causes which have brought about the change. The central cells of the hollow planula become transformed into the mesogloea and endoderm, the number of cells contributing to the latter layer being relatively few. The mesogloeal cells have various func- tions, some producing spicules or horny fibres in some sponges, but a large number remain indifferent or un- specialized to a greater or less extent, some becoming reproductive. We find too that the endoderm becomes differentiated from the central mass long before the cavity it encloses has any communication with the exterior, and cannot very efficiently function as a nutritive layer since it is separated from the outside world not only by the thin ectoderm, but also by the thick mesogloea. It is neces- sary that respiration should be carried on throughout the entire mass of the sponge. While it is small this is readily effected through the ectoderm, but as it grows, the mass increases so much more rapidly than the sur- face, that this simple method no longer suffices. Cavi- ties appear here and there in the mesogloeal mass, and later communicate with one another, the cells lining them becoming ciliated to produce a more rapid circulation of the water. A large central space into which the various cavities finally open makes its appearance, and last of all this space breaks through to the exterior, forming the Osculiim. The water which at first reached the cavities by filtration through the tissues, later on has ingress by pores, and the complicated canal system of the sponge is established. In its first inception, then, the canal system and the THE GASTRiEA THEORY AND ITS SUCCESSORS. IO3 ciliated chambers with their collared cells are respiratory in function. The recent observations of von Lendenfeld, however, demonstrate that the collared cells are inges- tive, and that ingestion takes place in them solely. Even so, however, they are also the cause of the currents of water which pass in through the pores, bringing them food particles, and are therefore also respiratory in func- tion. It seems quite possible that their ingestive func- tion has been secondarily acquired, they being in the best situation for obtaining food. As we pass higher in the scale of animal life we find a o-reater and g-reater differentiation of the function of the cells corresponding to the central mass, and we find the endoderm cells assuming more and more a purely digestive function. Except in a few cases the reproduc- tive cells arise from the mesoderm or endoderm (in the Scyphozoa), both of which structures may be considered as derivatives of the central mass of the solid planula. Instances having an important bearing upon the idea here proposed are offered by the Orthonectids and Dicyemids. As is well known, in these forms there is a single layer of ectodermal cells enclosing a mass of reproductive cells, or, in the case of the Dicyemids, a single large " endodermal " cell which is the source of the reproductive elements. In development this cell is separated off very early, and is enclosed by the ecto- dermal cells by a process of epibole. The stage preced- ing the complete enclosure of the central cell is generally spoken of as the gastrula stage, but under the present view a Dicyemid or an Orthonectid would be regarded as an adult parenchymella, whose central cells retain their original function, being entirely reproductive. 104 MARINE BIOLOGICAL LABORATORY. Other cases might be given having a more or less direct bearing on this question, but the time at my dis- posal prevents a fuller treatment of the subject. I would like, however, to refer to one case, namely, Grob- ben's account of the development of the Phyllopod Moina. We have in this form three distinct invagina- tions : (i) an invagination of certain cells to form the endodermal midgut ; (2) of certain cells to form the general mesoderm ; and (3) of four cells to form the re- productive organs. Only one of these, however, the first, can be considered a true invagination ; the others are more correctly immigrations. The entire process can, I think, be referred to the formation of a parenchy- mella in which there has been a precocious segregation of certain important organs. The germ plasma has been early segregated into a certain spherule of the develop- ing ovum and accordingly immigrates independently of the general mesoderm cells, whose perfect segregation, like that of the endoderm, is postponed to a slightly later period, these two last-named structures likewise immigrating independently. There are some cases, however, which seem to throw serious obstacles in the way of the view as to the origin of the endoderm and the reproductive elements which I have advanced. In the Hydrozoa and Ctenophora, the reproductive cells have been shown by Weismann, and the Hertwigs especially, to be derived from the ectoderm. The latter authors endeavored, on this account, to asso- ciate the Hydrozoa and Ctenophores together as dis- tinct from the Scyphozoa, in which the reproductive cells have an endodermal origin. Other structural pecu- liarities, such as the presence of an ectodermal stomato- THE GASTRyEA THEORY AND ITS SUCCESSORS. IO5 dseum, seem to indicate, however, that the Hydrozoa and Ctenophores have Uttle in common and must be regarded as two widely divergent stocks. If this be so, the occurrence of an ectodermal origin for the reproduc- tive cells in these two groups cannot be regarded as a primary arrangement, but for some unexplained reason has been secondarily acquired. We have seen that in certain lower Crustaceans the " Keimplasma " may be- come very early specialized, and immigrate quite inde- pendently of the mesoderm and endoderm, the other constituents corresponding to the central cells of the Coelenterate planula. We may perhaps conceive of a somewhat similar segregation of the germ-plasm occur- ring in the Hydrozoa and the Ctenophores, the cells containing it, however, remaining in the ectoderm, and not migrating with the other cells, which are to form the mesogloea cells and the endoderm. In conclusion I must express my admiration of the masterly manner in which Metschnikoff has treated the exposition of his theory. Probably no one has such an acquaintance with the embryological phenomena of the Coelenterates, or has contributed so extensively to our knowledge of these phenomena, as Professor Metschni- koff, and few possess that insight into the bearings of facts which he has exhibited in all his studies. The Parenchymella theory places our ideas of the relation- ships of the Protozoa and Metazoa upon an entirely new basis and enables us to overcome many great and per- plexing obstacles. It is founded on facts and explains them satisfactorily. The ideas advanced in this lecture which are not em- bodied in Metschnikoff' s statement of his theory do not I06 MARINE BIOLOGICAL LABORATORY. modify it in its essential points in the least. They are merely suggestions which have developed during the consideration of the application of the theory, and must be regarded simply in that light. For a complete exposi- tion of them much more detailed description, unsuitable for an occasion of this kind, would be necessary, and I present them here merely in the hope that they may incite to more thorough and perfect acquaintance with the problems connected with the early development of invertebrated animals, and the bearings of the onto- genetic phenomena on the question of the origin of the Metazoa and of their organs. I cannot do better than quote in conclusion from Metschnikoff, '' Es vmsste gerade im Bereiche der niederen Metazoen eiii fester Bo den fiir das Verstdndnis der primitiven Organe gewon- neii werden!' FIFTH LECTURE. -ooXKo"- WEISMANN AND MAUPAS ON THE ORIGIN OF DEATH. By EDWARD G. GARDINER. Heredity and Variation are among the most interest- ing subjects which attract the attention of naturaUsts, and any theory which attempts to explain these phenom- ena is worthy of consideration. Why is it that all organisms tend to repeat them- selves in their descendants, and why do the offspring always differ somewhat from their parents ? That this invariably occurs has long been a matter of common knowledge, and yet a satisfactory explanation of why it should occur is still to be sought. Darwin founded his theory of evolution on the facts of Heredity and Variation, but the explanation by which he sought to account for these phenomena was offered more as a "provisional hypothesis" than as a com- plete theory. Many valuable modifications of, and additions to, his theory have been proposed from time to time by dif- ferent authors, but the main idea of gemmules from all the different cells of the body aggregating in the 107 I08 MARINE BIOLOGICAL LABORATORY. generative glands, and being thus transmitted to the young, seemed too cumbersome and complex for general acceptance. Quite recently Professor Weismann of Freiburg has advanced a theory of Heredity which seems by far the best hitherto offered. This theory is the culmination of a train of thought which he has put forward in essays from time to time during the last few years. These show the gradual growth and development of the theory in the mind of the author, and though some of the facts from which he argues may be open to dispute, yet the ideas which he suggests are so interesting that they are entitled to consideration, even though subordinate to the main plan of his theory. In one of his earlier essays he points out that the manner of reproduction among the Protozoa is such that death does not normally occur in this group, for the animal reproduces by merely dividing itself into halves. Thus an adult animal ceases to exist as such, by be- coming: two animals instead of one. It does not die during this process, for there is no corpse, but the whole animal as such has completely disappeared, and in its place we find two individuals so similar that it is impossible to regard them as parent and offspring. Indeed, they cannot be parent and offspring, for they are of the same generation, — it is more natural to call them twins. They are both young animals, for they increase in size, and when adult each of them ceases to exist by dividing itself into two new young ones, and so on indefinitely. Hence it would appear that the life-history of such an animal may be di\ided into two periods, — youth and ON THE ORIGIN OF DEATH. lOQ adult life. There is no old age; there is no death. Clearly, then, since these forms do not die, they may be said to be potentially immortal. The Uving matter of which they are composed passes over without break into a younger generation, and in it life is continued. These facts have long been known, and earlier investi- gators have pointed out the potential immortality which this mode of reproduction implies ; but Weismann was the first to develop this knowledge into a scientific theory which may throw light on other facts. When Weismann calls these animals immortal he draws a proper distinction between the terms immortal and eternal. Eternity reaches back into the past as well as out into the future. With eternity he has nothing to do. Neither does he use immortality in the sense in which it is used in theology, — as applied to something which can never die, but must exist throu2:h all future time. His claim is not that the life of a Protozoon is such that it must under any circumstances exist forever, but that it will exist as long as the proper physical conditions exist ; in other words, that death is not inherent in life. He compares the life cycle of a Protozoon to the circulation of water which evaporates, gathers in clouds, and falls to the earth only to evaporate again. There is no inherent cause in the physical and chemical properties of water which will bring this cycle to an end. As long as the present physical conditions exist the cycle must continue. So it is, he claims, with the life cycle of a Protozoon ; i.e. division, growth by assimi- lation, division again — and so on without end; there being: no inherent cause in the constitution of the no MARINE BIOLOGICAL LABORATORY. protoplasm which will cause it to fall short of its cycle, and to physiologically decline. He does not mean that such unicellular forms cannot be starved to death, crushed out of existence, devoured, or killed by disease. These are rather accidental than natural deaths. He claims only that since life has existed in these forms, it has passed unbroken from one generation to another down to to-day. The material of which the individual is made may change, but in all cases it is animated by the same life. Now no one doubts that the Metazoa have at some time in the remote past been evolved from such poten- tially immortal Protozoa. But the life of all of the Metazoa may be divided into tJirce periods, — youth, adult life, and old age, during which latter there is clearly a physiological decline in vigor, which is termi- nated normally by death. Old age and death then would appear to be something which have been acquired with the development of the Metazoa from the Protozoa. Exactly for what purpose, and how, death has been instituted are questions which Weismann endeavors to answer. But first let us compare the life-history of a Metazoon with that of a Protozoon, and see whether there is any- thing in the Metazoa which is comparable to Protozoa immortality. All Metazoa start their individual lives from an ovum, which is a single cell, and may well be compared to a Protozoon. After fertilization this cell or ovum divides into two, then into four, then into eight cells, and so on, thus giving rise to a very large number of cells which, as development progresses, ON THE ORIGIN OF DEATH. Ill differentiate to form the tissues and organs of the em- bryo. Now of these cells in the embryo, Weismann distinguishes two different kinds ; viz. the germ-cells which lie in the generative glands of the animal, and the somatic cells, which form all other organs and the body itself. During early youth the germ-cells remain dormant. When, however, adult life is reached, they develop, and under proper conditions, such as fertiliza- tion, etc., each one is capable of producing a new organism, with germ-cells and body, while the body itself grows old and dies. The germ-cells of the second generation do not die, but produce a third, and the third a fourth generation, and so on. The body of each of these generations must grow old and die, but the germ-cells themselves, if allowed the proper physical surroundings, do not. They go on germinating, and so produce generation after generation. Hence it would appear that the germ-cells of the higher organisms are comparable to the entire body of unicellular forms, and like them are endowed with potential immortality ; and that this immortality has never been broken by death since life has existed on this earth. In other words, a Metazoon equals a colony of Protozoa ////j" a perishable body. This theory he calls the '* continuity of the germ- plasm." He compares the germ-plasm of the Metazoon to the rhizome of a fern which runs along underground, the perishable body to the green frond which grows up, withers, and dies, without affecting the life of the rhizome from which it springs. Death, then, is something secondary ; an ad^tation 112 MARINE BIOLOGICAL LABORATORY. which has been acquired through natural selection dur- ing the evolution of the Metazoa from the Protozoa. Whether this conclusion is just is to a certain extent a side-issue to the theory of the continuity of germ- plasm, and further on the matter will be more fully discussed. Among the Protozoa it is very evident that the young must resemble the forms from which they spring, for they are a part of the same stuff. Further- more, if during life an individual becomes changed by its environment, this change must become an hereditary trait. Now we may assume that the Protozoa may be affected by the physical conditions under which they live just as much as are the Metazoa. It is a well- known fact that the bodies of the higher forms may be changed by food and exercise. The muscles of a blacksmith or a sailor are stronger than those of a dude. A Paramaecium which lives in a strong-flowing stream would be apt to acquire stronger cilia than one living in a stagnant pool. The young which spring from a Paramaecium with strong cilia must inherit this charac- ter. Indeed, the cilia of the young are the identical cilia which by constant use have been lengthened and strengthened. In every case of reproduction by fission, the whole body of the parent continues its existence in the younger generation. If the parent is strong and vigorous, the young are so likewise ; if the parent is decrepit and feeble, the young must likewise be de- crepit and feeble. Hence all modifications which any unicellular organism may acquire must be transmitted to its ySung. ON THE ORIGIN OF DEATH. II 3 Among the Metazoa, however, the case is somewhat different. In all the higher forms the germ-cells which contain the potential immortality of the organism have no function but that of reproduction, and are so well protected from the environment that they must be diffi- cult to affect. The old idea that an animal which strengthens its legs by severe labor will have offspring with stronger legs than one which has not so exercised is hard to comprehend. If this be so, then we must imagine that the strain put upon the muscle cells of the legs has changed the molecular structure of the germ-cells. This seems a monstrous supposition. Of course when Weismann says that no acquired character can be in- herited, he does not mean that half-starved animals may not give rise to stunted young. This is, however, a case of direct action on the young itself, for before birth the young is fed from the tissues of the mother, or the mother supplies food in the form of the egg-yolk. Certain diseases are known to be hereditary. These, however, may perhaps be ascribed to the direct passage of the Bacterium from the parent to the germ-cell or to the embryo. Other diseases which are due to structural malformation may, of course, also be inherited. The isolated cases where scars or other such acquired char- acters are said to have been inherited, are never so well authenticated that their accuracy is beyond doubt. Certain it is that where wholesale experiments on animals have been carried on, no inheritance of mutila- tions has been observed. Generations of horses and dogs have had their tails docked without affecting their young. The Chinese women deform their feet, yet the 114 MARINE BIOLOGICAL LABORATORY. young are not affected. Mutilation, among savages, of the nose, face, and ears has been carried on for genera- tions, yet no traveller has reported that either the mutilations or the scars are inherited. All these facts should be borne in mind when it is claimed, as it still is by many, that acquired characters may be and are inherited. Weismann admits that the long-continued effect of climate and food may to some extent act on the germ-cell ; but the idea that species can have originated through the inheritance of the peculiarities acquired by their ancestors, is contrary to his theory. Now the young of the Metazoa inherit the main characters of their parents, for the same reason that the young Protozoon resembles the form from which it springs, — they are made of the same stuff. Germ- cells and body-cells arise from the same germ-cell. During the development of the individual the immortal portion of the germ-cell is set aside to form the next generation. The characters which the body of the individual may acquire during life must be very funda- mental to affect these germs. A young Metazoon is a part of its parent, because the stuff of which it is made is a part of the germ-cell from which its parent was made. It may also resemble a grandparent or a great-grandparent, for its parent sprang from a part of the same germ cell which built its grandparent. Its great -grandparent was in turn a part of its great-great- grandparent. Thus every individual is made up of the same material as its ancestors, in degrees varying with the different generations. In ten generations an indi- vidual may have 1024 ancestors, of all of whom the in- dividual itself is a part. If the theory of the continuity ON THE ORIGIN OF DEATH. II 5 of the germ-plasm be accepted, there is no difficulty in understanding why heredity should occur. Ancestors, parent, and offspring are all a part of the same stuff. Now in regard to variation, and why it should occur. Every individual is the result of the union of the male and female germ-cells, and Weismann holds that the minor characters are due to the manner in which these cells are mixed. He assumes that these cells are of the most complex molecular structure, so that when they unite it is impossible that exactly the same arrange- ment of the molecules should be repeated. Put a hand- ful of currants into two loaves of bread, and after baking see whether there is exactly the same number and arrano-ement in the fifth slice of each. The chances O against this are enormous. All variations which occur in nature, — size and shape of the features, organs, stature, color of skin or hair, — depend on the molecular combinations formed in the union of the two germ-cells. If a single germ-cell should develop without so uniting with another, the young would be the exact duplicate of its parent, for it would be composed solely of the germ-material of which its parent was composed. In essence the male and female germ-cells are identi- cal. Each is potentially capable of producing an em- bryo by itself, if the proper physical conditions could be secured. Weismann says that if it were possible to bring about the conjugation of two ova, fertilization would be accomplished, and development pursue its nor- mal course. Indeed, this, or something very similar, has been done. Boveri succeeded in removing the nucleus of an echinoid ovum, and then induced this Il6 MARINE BIOLOGICAL LABORATORY. ovum to develop by introducing spermatozoa. A new nucleus was then formed by these, and the ovum de- veloped into a free-swimming larva. If before sexes were fully established, two germ-cells of different origin were to unite, the embryo would have a dual origin, and hence would differ from both parents. If the variation thus introduced were favorable, sur- vival of the fittest would preserve it. The germ-cells of this form would, -of course, inherit the tendency to unite with another germ-cell. In this way natural selec- tion would establish sex, to ensure the combining of two germ-plasms in each individual. Without such combina- tion no variation in the Metazoa could occur. Hence all sexual characters — even to the higher mental attri- butes — are due in their origin to the necessity of establishing variation. Variation, then, is a consequence of the union of two sexual germ-plasms, and not some- thing inherent in protoplasm. At first thought it seems strange that variations are not more numerous and marked than they are ; that for thousands of years the germ-plasm of every species of animals has undergone so Httle change that each species still retains all its specific characters. It would seem that if animals are not suited to the environment, they are not modified, but exterminated. Yet when advan- tageous variation has occurred, and the race has been preserved by natural selection as a different species, this must have been brought about very slowly. It is thus evident that the plasm is very conservative and difficult to change ; that the conservative forces far out- weigh all tendencies to vary. Now to return to the question whether death is ON THE ORIGIN OF DEATH. 11/ inherent in germ-plasm. Weismann attempts to show- that to the species death is an advantage. All animals are liable to accidents, and if animals were to live on for all time, many would be maimed and useless. By far his strongest arguments, however, are based on other facts. Favorable variations in nature are seized on by natural selection and preserved, whether they be an advance towards greater specialization or retrogressive. Long since, Darwin pointed out that small wings were an advantage to insects living on islands where strong winds prevail, for those which had large wings would be more apt to be blown out to sea, and so lost to the race. Hence a survival of the fittest has maintained short-winged varieties. Small eyes buried in fur are an advantage to burrowing animals, such as the mole ; hence this variety has been preserved. So, also, the complete disappearance of the limbs of snakes which squirm through small holes and clefts. Limbs would be a disadvantage and a hindrance. When, however, degeneration of disused organs con- fers no benefit on the individual, the explanation is different. Thus the eyes of insects, crabs, fishes, and amphibia, which live in caves, have undergone degenera- tion ; yet this can hardly be of advantage to them, for they could live quite as well in the dark with well- developed, as with rudimentary, eyes. The efficient eyes of allied forms living in the light have been main- tained in their perfection by the survival of those only having good eyes. Is it not evident that when such animals come to live in darkness, natural selection will cease to act, and deficient eyesight result .'* If those with imperfect eyes intermingled with those with Il8 MARINE BIOLOGICAL LABORATORY. perfect eyes, the eyes of the whole race would begin to degenerate. Take another case, such as is offered by birds of prey, which are possessed of exceedingly sharp sight. If some of these birds happened to come into the world with defective eyes, they would starve to death, or at least be less likely to succeed than their brethren which were more fortunate in this respect. Hence sharp sight is maintained by the continued operation of natural selection, which tends to exterminate those with defec- tive sight. But let these birds come to live under con- ditions in which those with defective sight got along as well as those with keen sight, — say in a dark cave, — and it would seem more than probable that the eye- sight would degenerate. Those which varied in the direction of defective eyes would survive as well as the others. Hence they would breed with the keen-sighted ones, and thus gradually the general average would be lowered. Another example must suffice. Short-sighted- ness is not at all uncommon among civilized races, and is generally believed to be hereditary. Short-sighted men succeed in life as well as those with normal or very keen sight. Neither natural selection nor any other selection compels keen sight. But a short-sighted hawk or antelope, or even a short-sighted Indian, would be placed at so great a disadvantage that short-sighted- ness would soon be eliminated from the race. In these last-mentioned forms natural selection com- pels the eyes to come up to a certain standard, while among civilized m^en and cave animals there is no such necessity. Natural selection maintains only what is an advantage, and when it neglects an organ, that organ tends to degenerate. ON THE ORIGIN OF DEATH. IIQ Weismann urges that through the neglect of natural selection immortality has been lost to the Metazoan body. Among the Protozoa, since the whole body is the germ- cell, immortality is inevitable. Among the Metazoa, the body is merely a protector of the immortal germ-cells, and immortality is not essential to the body ; hence natural selection has failed to maintain it. He sug- gests that the Metazoa have been evolved from small colonies of Protozoa which formed clusters. In such groups those on the outer side must obtain food more readily than those in the centre ; hence the colony would become gradually differentiated into feeding cells on the outer side and reproductive cells on the inner side, the feeding cells supplying the reproductive cells with nutriment just as the digestive cells of Hydractinia supply the rest of the colony. The cells that are thus supplied with food would have no use for mouth, cilia, etc. ; hence they would lose them, but might retain their reproductive powers. If these central cells retained their immortality, there would be no necessity for the feeding cells doing so also ; and if natural selection does not compel the retention of a physiological character, it degenerates just as a useless organ degenerates. Certain of the lower forms, such as Volvox, suggest this manner of evolution of the Metazoa from the Protozoa. Volvox is a hollow sphere of cells, each of which is provided with a couple of long flagella, by means of which the colony swims. Some of these cells pass to the centre of the sphere, and there undergo certain changes in form, becoming, in fact, the repro- ductive cells of the colony. When they are ripe the I20 MARINE BIOLOGICAL LABORATORY. rest of the colony withers up and dies. Hence we find in Volvox the first approach to a differentiation into germ and somatic cells. Since Weismann made this startling assertion, that death is not an attribute of all living organisms, much opposing evidence has been brought forward. Most prominent and recent among his opponents is E. Mau- pas, of Algiers, who, after extensive study of some of the Infusoria, asserts that degeneration and death occur as normally among the Protozoa as among the Metazoa. Before entering on his experiments, Maupas first deter- mined very carefully the habits of the different species which he chose for study. He found out the tempera- ture to which they were best adapted, and the kind of food on which they throve best. Then he took a single individual, and isolated it on a glass slide, on which it could be studied. This slide was kept over a dish of water in a warm, damp chamber in which the air was so thoroughly saturated with moisture that evapora- tion was reduced to a minimum. Durins: its confine- ment the animal was fed on its favorite food, and in every particular what seemed to be the most suitable conditions were maintained. He found that at the end of seven days there were no less than 935 Infusoria in his culture. One of these 935 he isolated and kept as he had the first. In four days this single one had in- creased to 230. One of these was isolated in the same way, and this process of isolating and confining one individual of each brood was continued throu2:h a larsre number of generations. He shows the rapidity of in- crease to be something almost incredible. Calculations show that in six and a half days a single Stylonichia . ON THE ORIGIN OF DEATH. 121 might produce by fission a mass of protoplasm which should weigh one kilogram, and that in thirty days the number of kilograms would be represented by i with forty-four zeros, or a mass of protoplasm a million times larger than the volume of the sun. Cultures were made of no less than twenty different species of Infusoria, and were maintained during periods of time varying in different cases from two weeks to between four and five months. He found that after from fifty to one hundred generations had been pro- duced by fission, there was clear evidence of a physio- logical decline, which seemed to indicate the approaching extinction of the culture. He withdrew some of the Infusoria from the culture and allowed them to mix with others of a different origin. With these they conjugated, and their full vigor seemed restored. If, on the other hand, they conjugated among themselves, observation showed that decline was so far advanced that the culture was doomed. Soon, the animals produced by fission were smaller — often being less than half the normal size. At the same time what might be called pathological changes began to appear. The cilia were absent on parts of the body, and the infusoria seemed weaker and less able to digest food. In some species the micro-nucleus under- went changes, finally falling to pieces, a phenomenon which not unfrequently occurs in the cells of the Metazoa when the tissue is undergoing degeneration. Also the macro-nucleus was found to undergo marked pathological changes, finally breaking down and disappearing. When this degeneration, which Maupas calls senile degeneration, has reached its maximum, nutrition be- 122 MARINE BIOLOGICAL LABORATORY. comes impossible, and death follows. Thus it would appear that the life of these animals is cyclic. During the period of reproduction, which is the adult life of the animal, a sort of physiological decUne takes place, and this decline can be repaired only by conjugation. Now if during these experiments the animals have not been injured or poisoned, it would seem that Maupas had proved that death may occur normally among uni- cellular forms. Weismann, however, is not ready to admit this. He claims that conjugation is a necessary condition of the animal's life, just as fertilization is a necessary con- dition for the survival of an ovum, and if conjugation is denied, the death in consequence is accidental and not natural ; further, that the fact that conjugation^ is necessary does not imply that the protoplasm is not potentiaPy immortal. He seems, however, to over- look the fact that a certain physiological decline has taken place, and that if there is any physiological decline the cycle of life is incomplete ; therefore the seeds of death must exist inherent in the life of the animal. It is, then, for the present, impossible to speak of the Infusoria as potentially immortal, and to claim that that portion of the Metazoa which under- goes physiological dechne has no equivalent in these forms. Nevertheless, it is too soon to declare that the idea that death is an adaptation is altogether erroneous. In many well-known unicellular forms, neither a physio- logical decline nor a process of rejuvenation has been observed. The Bacteria, the Cyanophyceae, and Yeast increase by budding, spore building, and fission, and ON THE ORIGIN OF DEATH. 1 23 unless their life-history is much less well known than bacteriologists and botanists think, these forms are potentially immortal. The Infusoria are the highest and most differentiated of unicellular forms. They have organs of locomotion, mouth, pharynx, some sort of excretory apparatus, myophanes (muscle-like struc- tures), trichocysts, etc. ; while in the lowest organisms none of these organs are to be found. Further, the Infusoria have a macro-nucleus which Maupas shows is vegetative in its function, and a micro-nucleus which is generative. If the macro-nucleus is lost, nutrition fails, and if the micro-nucleus is lost, conjugation is im- possible. In the lower Protozoa no such differentiation has been observed. There is merely one nucleus, which is surrounded by a mass of protoplasm. In a recent article Biitschli maintains that in the Bacteria the whole body is the nucleus, and that the surrounding mass of protoplasm, such as characterizes the Rhizopods, is absent. Between the Bacteria and Infusoria there is a wide gap in the zoological scale. Now is it not possible that as the Infusoria were evolved from lower and simpler forms, the process of conjugation was first acquired ? That when, in the cycle of meta- bolic changes the protoplasm fell short of the point from which it started and to which it should return, this deficiency was made up by foreign substance ob- tained from an individual of different origin, and there- fore of different material ? Those of the primitive forms which retained their original immortality have left lineal descendants which we know to-day as Bac- teria. Those which in a measure lost that power have either become extinct or else acquired a habit of re- 124 MARINE BIOLOGICAL LABORATORY. juvenescence by conjugation. In other words, those to which it was an advantage to retain their immortality have retained it, and those which varied in such a manner that immortahty could be advantageously re- placed by rejuvenescence have, by the action of natural selection, undergone this modification. If this is so, Weismann's error is not in claiming that death was an adaptation, but in asserting that all unicellular forms are immortal. Still another and earlier opponent of Weismann has urged serious objections to this theory of the origin of death. Professor Charles S. Minot was the first to maintain — and many have taken up his suggestion — that Weismann is fundamentally wrong in comparing the life-history of a Metazoon, which is a complex colony of cells, with that of a Protozoon, which is a single cell. Minot urges that an individual Metazoon is comparable to a colony of Protozoa, not to a single cell. If this be so, then the death of a Metazoon (a colony of cells) has its only homologue in the degeneration and death of a culture of Protozoa. The Metazoon colony is the product of a single germ-cell, as is also the whole culture of Infusoria. This comparison seems safe between the Metazoa and those forms of Protozoa which conjugate, and in which senile deo:eneration occurs. But how is it when we bring those which do not conjugate under consideration.'' If this view be correct, then a single Metazoon is equiva- lent to all of a species of Bacterium which may arise through generations of fission. As far as our knowl- ed"-e sfoes, these Bacteria are immortal and their num- bers almost infinite. We know, on the other hand, that ON THE ORIGIN OF DEATH. 12$ nothing but the germ-cell of the Metazoa possesses this immortality and vast power of reproduction. But to return to Weismann's views. If death is not something which is inherent in living matter, but which is acquired, how is it that the length of life differs so markedly in different species } Weismann answers that the age which an animal may attain has been deter- mined by natural selection, and also that the power of reproduction and length of life are correlated. In order to understand this view it is necessary to compare the length of life and reproductive powers of different animals. Birds, as a rule, live to a surprisingly great age. Even the smallest singing-birds live for ten years, while some live for twelve or even eighteen years. A par- tridge lives from twenty to twenty-five years. A pair of eider-ducks were observed nesting in the same place for twenty years, and it is believed that these birds often reach the age of a hundred. The same cuckoo was recognized by its peculiar note in the same forest for thirty-two consecutive years. Birds of prey become much older, for they outlive more than one generation of men. A white-headed vulture was kept in a zoological garden in Germany for one hundred and eighteen years ; and many examples of eagles and falcons reaching an age of over a hundred years have been recorded. Hum- bolt mentions a parrot from the Orinoco, of which the Indians told that none could understand it, for it spoke the language of an extinct race. Now let us compare the length of life and reproduc- tive powers of the partridge and an eagle, and see if there is any reason why one should live longer than the other. The partridge Uves a little more than twenty 126 MARINE BIOLOGICAL LABORATORY. years, and each year lays about twenty eggs. Hence a pair of partridges may produce about four hundred eoro:s in their Hfetime. This is at the rate of two thousand in a hundred years. Yet, since the number of partridges in the forest does not increase, three hundred and ninety- eight of these eggs, or young, must be destroyed in twenty years, while but two survive to take the place of their parents. The eggs and young are destroyed by beasts and birds of prey. If these enemies increased very much in number, the partridge would become extinct unless it laid more eggs. It would appear, then, that the partridge lays just eggs enough to ensure the continuance of its race, and this being accomplished, death removes it. Many species have doubtless become extinct through the in- sufficiency of their reproductive powers. The number of offspring which, under ordinary conditions, would have ensured perpetuation, has proved insufficient when their enemies increased or the environment became unfavorable. The supply must be equal to the de- mand. Now for the eagle. The eagle is one of the most powerful of birds, and builds its nest on such inac- cessible cliffs that eggs and young are comparatively safe from marauding animals. Many, however, are de- stroyed by late frosts and snows. To be on the safe side, let us fix the duration of life of the eagle as sixty years, and of this ten years is spent in immaturity. Hence there are fifty years of its life during which it reproduces. If the eagle lays but two eggs a year, a pair of eagles would produce one hundred during their life- time. In a hundred years two hundred eggs against ON THE ORIGIN OF DEATH. 12/ the partridge's two thousand ; therefore the partridge produces ten times as many young as the eagle, and it is safe to say that the partridge has ten times as many enemies. If the life of either were shortened, the race would die out unless the power of reproduc- tion were increased or the struggle for existence became less severe. Many sea-birds, such as the petrel, auk, and gannet, lay but one or at the most two eggs a year. Any one who visits a locality where these birds breed must be struck with the enormous number of eggs or young which are destroyed. The eggs are often laid on the bare rock on projecting ledges of a cliff, so that the slightest movement will precipitate them to the beach below. Every disturbance among the breeding birds is marked by a small avalanche of eggs or young, so that the beach below is strewn with broken eggs and mangled remains. If these birds were not long-lived they would soon become extinct. Now all of these birds live much longer than mammals of a much larger size. The lion lives thirty-five years, the sheep fifteen, the fox fourteen, the squirrel or mouse about six. Most of these animals are much more fertile than the birds, and the young are much less exposed to dangers. The bird's egg is exposed from the time it is laid, while the young mammal is protected during its development. Only the very largest of the mammals, such as the whale, the elephant, and possibly the rhinoceros, live as long as these birds. The elephant may live for a hundred or perhaps a hundred and fifty years, and reaches maturity when about thirty. A pair produce but a single calf about every ten years ; hence, during 128 MARINE BIOLOGICAL LABORATORY. their lifetime, a pair of elephants contribute but ten or a dozen young to the race. Wallace shows that we are living now in a zoologically impoverished world. Almost all of the largest and strangest forms have recently become extinct : in Europe the great Irish elk, the sabre-toothed tiger, cave-lion, rhinoceros, hippopotamus, and elephant ; in North America equally large felines, horses, and tapirs larger than any now living, a llama as large as a camel, mastodons and elephants, besides a large number of huge megatherians ; in South America an even greater number of megatherians, huge armadillos, a mastodon, large horses and tapirs, large porcupines, two kinds of antelopes, numerous bears and felines, beside the sabre- toothed tiger. Remains of all these are found in the recent de- posits, and these animals lived till shortly before the northern continents were encased with the ice of the glacial epoch. It is possible that a change of climate, due to the growing cold from the encroaching ice-belt, affected the flora. This would, of course, affect the food supply, and so tend to lessen the reproductive powers, and shorten the lives of the individuals. Les- sened reproductive power and shortened lives of the individuals would surely result in the extinction of the race, and in this way the destruction of these forms may be accounted for. The starvation of the individual is therefore synonymous with the starvation of the race. The main features of Weismann's theory may be summed up as follows : First, The continuity of the germ-plasm. Second, Variation is due to the different molecular combinations formed in the mixture of the ON THE ORIGIN OF DEATH. 1 2g male and female germ-plasm. Third, Death is an adaptation, and the duration of an animal's life has been determined by natural selection, and is correlated with its power of reproduction. Whether this theory will endure and receive even fuller acceptance in the future than it does to-day, time alone can show. SIXTH LECTURE. -o-d^^oo- EVOLUTION AND HEREDITY. By henry FAIRFIELD OSBORN. I WANT especially to direct your attention to the rela- tions between our present knowledge of the evolution process and the problem of Heredity. The mere truth of the origin and succession of life by evolution may now be demonstrated in every branch of Biology, the argu- ments from palaeontology, embryology, and morphology being equally convincing, but the theory of the evolu- tion process is inseparably connected with some theory of inheritance. If new individuals were simply repetitions, like coins struck from a species mint, there would, of course, be no evolution possible, and we should perforce return to the Miltonic conception of creation, at the same time greatly reducing the number of difficulties in the heredity problem. While in zoology the repeti- tion phenomena are perhaps the most conspicuous, in palaeontology, or, in other words, in the succession of life in time, the variation phenomena are more striking, and we come to realize that the how, why, and wdien of the variations give zest to the study of the fossil series and furnish the crucial test for any heredity hypothesis. 130 EVOLUTION AND HEREDITY. I3I Yet men were slow to see this relation. Lamarck did not study heredity as a special problem in itself, but he boldly postulated the only theory which fitted his views of evolution. Darwin really gave it a compara- tively small share of his thought, and only after he had modified his views of the omnipotency of natural selec- tion,^ did he begin to feel the absolute necessity for a working hypothesis of inheritance. But now the hered- ity problem is no longer the subsidiary one, in fact, just at the present time, it is the chief one, for the whole accepted theory of the process of evolution has been overthrown by a brilliant student of heredity ; and there are two parties, each attempting to throw the onus pro- bandi upon the shoulders of the other. It is clear enough that when we have reached an heredity theory which will explain the phenomena of inheritance, the method of evolution will itself be a problem of the past. No such explanation can be reached, however, so long as students of heredity take only a partial view of the facts of evolution. The present temper of Weismann and his English followers is apparently somewhat exclu- sive ; the same is equally true of some of our friends on this side of the Atlantic. What then is necessary in a complete theory of hered- ity.'* It must account for the repetition phenomena; these were the first to attract attention, for we are always more struck by the features in which the off- spring resembles the parents than by those in which it differs. Under this head are included " reversions." 1 This change of view becomes most evident in his Animals and Plants under Domestication, in the closing part of which Pangenesis is proposed. 132 MARINE BIOLOGICAL LABORATORY. Second, it must account for the non-repetitive phenom- ena, or the appearance of new characters, to the impor- tance of which Darwin first directed our attention. It is clear that anthropology, zoology, botany, and palaeon- tology give an ample field for the above. Thirdly, there are the physical transmission phenomena, the peculiar field of the embryologist. We may approach heredity through either of these channels, but the test must be by the data afforded by them all. Thus we find Lamarck, Darwin, Galton, Spencer, Brooks and others coming to the problem chiefly from the study of living adults in past and present time, while Weismann has come to it from the side of embryogen- esis. There are great difficulties in the embryological problem ; we have to do with particles of protoplasm of minute size, largely composed of water, yet suspending invisible molecules which we must suppose are the actual vehicles of inheritance, for, under the ordinary conditions of nutrition, they will repeat chapter after chapter in the history of the race, and finally take the form of the adult — they are indeed microcosms. We have to consider the part played by the male and female element. And the relation of these specks of protoplasm to the life history of the individual. Democritus, who offered the first hypothesis, supposed that the sperm was secreted from all parts of the body of both sexes at the time of impregnation animated by a bodily force, like parts producing like. When Dar- win proposed his provisional hypothesis of pangenesis he embodied in it a somewhat analogous conception, but he was aided by the v/ell advanced stage of embryo- logical and physiological science. The older views of EVOLUTION AND HEREDITY. 1 33 "evolution of the germ," or the expansion of a minute pre-formecl individual, as opposed to Harvey's ''epigene- sis,"or growth by addition, had given way to a modifica- tion of both ideas: ''That development, which, in its superficial aspect, is epigenesis, appears in essence to be evolution or the expansion of a potential organism ac- cording to fixed laws." ^ Darwin's views were thus stated : ^ " It is universally admitted that the cells of the body increase by self division thus forming the various tissues. Besides this, I assume that the cell units throw off minute granules which are dispersed through the entire system ; these may be called ge^nmules. They are collected from all parts of the system to form the sexual elements, and their development in the next generation forms the new being. They are thrown off by every unit, not only during the adult stage but during each stage of development of every organism. Hence it is not the reproductive organs or buds which generate new organ- isms but the units of which each individual is composed. Gemmules are capable of transmission in a dormant state to future generations and may then be developed." Notice two main features of this hypothesis : that there is no difference of character in the elements con- veyed by the two sexes ; second, that the vehicles of hereditary characteristics are broken down, scattered through the system, and ;r-collected from the soma or body of each individual to form the germ of the new one. There are, therefore, postulated countless numbers of gemmules corresponding to the constantly changing cells of the body ; these gemmules being also imaginary elements. Their migration to and from the body cells is in contrast with the fixity of the repetition of parental ^ Huxley, Art. Evolution, Enc. Brit., p. 746. 2 Variation of Animals and Plants under Domestication, vol. ii., p. 369. 134 MARINE BIOLOGICAL LABORATORY. and ancestral traits, and does not accord with the facts of the early appearance and isolation of apparently com- plete germ cells. Moreover Galton's well known experi- ment of the transfusion of the blood of eighteen silver- gray rabbits with that of other varieties showed that the gemmules, if existent, do not circulate in the blood, for this transfusion of blood was not accompanied by the slightest interchange of characteristics. Passing by Haeckel's suggestion of Perigejtesis, which invests the molecules of organized matter with some of the lower faculties of mind,i let us look at Brooks's in- genious attempt to modify Darwin's hypothesis. His postulates were the actual existence of the gemmules, but not quite as conceived of by Darwin, for there is a fundamental difference between the male and female elements of reproduction. The ovum is conservative, reproducing cells like itself and a storehouse of heredi- tary tendencies, giving rise directly to new ova, while the spermatozoan is the progressive element, the repository of the gemmules. Nor are the gemmules thrown off at random by all the cells of the male body, but especially by those which are disturbed in their conditions by a change in environment. Variability, therefore, is most rapid when external conditions are disturbed, and it springs from the union of the ovum and spermatozoan. This system is open to most of the objections which can be raised against the original of Darwin, but is moreover opposed by the rapidly accumulating evidence for the essential similarity of the ovum and spermatozoan. 1 See Brooks's Heredity, p. 37. " Unconscious memory is the most important characteristic of organized matter. Heredity is the memory of the plastidules; variability, their power of perception." EVOLUTION AND HEREDITY. I35 Observe that all these hypotheses sprang from the evolution standpoint, and were constructed to account not only for the repetition phenomena but for what was by each author considered a prime factor in evo- lution, namely, the transmission of acquired charac- ters, or the inheritance by the offspring of some of the influences which environment and habit have exerted upon the body cells of the parent. All reason- able hypotheses, as these certainly were, have their value in stimulating research ; and the main service ren- dered to science by the pangenesis doctrines has been a negative one, namely, they have shown that it is extremely difficult to construct even an a priori zvorking- hypotJiesis of heredity which will explain the trans- mission of acquired character's. Weismann is the most brilliant of the post-Darwinian writers, and no one at the present time has so great a following or is exerting such a wide influence. He ap- proached the heredity problem purely from the embryo- logical side : '* How is it that a single cell of the body can contain within itself all the hereditary tendencies of the whole organism .? " There can be but two alterna- tives, either the substance of the germ cells is derived from the body of the new individual, or directly from the parent germ cells. His theory of the " continuity of the germ plasm " supports the latter alternative according to which the germ cells of the parent must give rise to " somatic " cells forming the body of the offspring, and to ''germ" cells. Each generation has an identical starting-point in ceUs of the latter kind which are in a sense immortal, the individuals being mere offshoots from the continuous germ plasm stem. Some of the details 136 MARINE BIOLOGICAL LABORATORY. of Weismann's views are, that the nucleus of the germ cell is the sole bearer of the hereditary tendencies ; that the expulsion of each of the two polar bodies of the ovum has a distinct meaning, the first expelling the ovogenetic nucleoplasm, the second reducing the ances- tral germ plasm by one half, and thus rendering the nucleus ready for the accession of the germ plasm of the spermatozoan which has previously been correspondingly reduced. While these subsidiary hypotheses, if correct, would strengthen Weismann's position, I do not see that his theory is dependent upon them. Now look at his main propositions, first, as against Brooks, there is no fundamental distinction between male and female germ cells ; second, as against Darwin and Lamarck, the early separation and isolation of the germ plasm from the somatoplasm, renders it highly im- probable that acquired characters can be inherited, for the changes in the body can exert no definite influences upon the germ cells. It is at once evident that this theory explains admirably the repetition-phenomena, and is strong upon the embryological side. Before criticising it from the non-repetitive or evolution standpoint let us examine the views of Galton. It is certainly very remarkable that there are so many points of agreement between Weismann and Galton, when we consider their diverse fields of research. Galton was the pioneer, and the fact that Weismann indepen- dently reached similar conclusions from entirely different data, affords a strong presumption in favor of their com- mon grounds of opinion — that the physical vehicle of heredity is continuous and feebly or not at all influenced by changes in the individual which conveys it. Some EVOLUTION AND HEREDITY. 13/ of the special biological problems which Galton^ set before himself to solve by anthropology, were : (i) The share contributed to the offspring by each of its several ancestors. The nearness of kinship of different degrees of relation. (2) The causes of stability of type observed in large populations through many generations. (3) Nature v. Nurture, or the share contributed by inheri- tance and environment respectively to personal charac- teristics. I. Stature is an heritage which blends ^ and which therefore enables us to determine with precision the contribution of each ancestor. We find that the mid- parent contributes \ the total heritage ( = father \, mother \ with transmutation for sex). The mid-grand- parent ( = grandfather 3V' grandmother -3^ contributes ■^g, etc. There is in every one an unused or " latent " heritage as shown by the fact of his transmitting ances- tral peculiarities he himself does not exhibit, but the store of latent heritage is not necessarily from all or even from many of his ancestry. As the personal heri- tage from each parent is but one quarter, the total being one half, it follows that both the personal and latent heritages must be reduced by one half. 2. In heredi- tary inquiry we must study fraternities and compare all the members of one generation with those of another. The main line of hereditary connection unites the set of elements out of which both the parent and the child are evolved. The main line is a chain of which the personali- ties ai'e pendants. Stable forms represent the groupings 1 See his " English Men of Science," " Inquiries into Human Faculty," and " Natural Inheritance." 2 Eye color is an example of characters which do not blend. 138 MARINE BIOLOGICAL LABORATORY. of characteristics which have the firmest hold upon the race ; as illustrated by a polygonal block with unequal sides : the chances are that the block will fall upon its longest side, yet it may fall upon its shortest side and represent a new type, which, nevertheless, possesses some small stability of its own. Against such stability, however, works the law of regression, viz. that upon the whole the stature of children will be more mediocre than that of their parents ; that the more exceptional an indi- vidual is, the greater are the chances that his offspring will be unlike him, i.e. nearer to the race type. Near- ness of kinship must therefore be computed in degrees of regression. 3. In the life history of the individual the influences of nature, or inherited constitution, greatly preponderate over those of nurture, or environment and education. It has been generally held that the study of man affords the most striking examples of the law of inheritance of acquired characters. Yet Galton says, in doubtful terms, that such inheritance should be looked for not in the first but in the second and third genera- tions ; that acquired faculties, if at all, are transmitted with dilution ; that the actual evidence for such trans- mission is not very conclusive. It appears to me that if Weismann's system had been especially built up a posterioriiox Galton's anthro- pological laws it could not have been better adjusted, but since he makes no allusion to Galton's work we know this was not the case. The balance between variation and stability, the element of certainty and of chance in the mingling of heritages, the reduction of parental and ancestral characteristics are all explainable by Weismann's views of physical transmission. Upon EVOLUTION AND HEREDITY. 1 39 the question of inheritance of acquired characters, they are almost but not quite in accord ; Galton is in doubt, Weismann thinks it improbable if not impossible. From the standpoint of evolution, or non-repetition, however, there is one point in which Galton's results are de- cidedly adverse to Weismann as we shall see. There is thus strong evidence for the ''continuity of the germ plasm" theory. This does not seem to be necessarily antagonistic to the Lamarckian idea for we can conceive that the germ plasm is continuous and still influenced in definite ways by the body which contains it. Yet Weismann holds that this is not the case: that no special or local life changes in the body can in any way reach or influence the germ cells in such a man- ner as to be inherited. This view throws the whole burden of evolution upon the natural selection or sur- vival of those individuals which possess, by blending or otherwise, that germ plasm which represents the bodily constitution and structure best fitted to environ- ment. In this manner the principle of inheritance of acquired characters, from being one of the dogmas of science has been first questioned, then thrown in doubt, and finally rejected by a large body of zoologists. We really owe it to Weismann that this principle, which forms the mainstay of Lamarckism, was adopted by Darwin as a most important factor in evolution, but has never been critically examined from the heredity side, should now be thoroughly investigated. The existence of this principle in inheritance is the focal point of a discussion in which the authority of Lamarck, Darwin, Spencer, Eimer, and the greater number of American biologists may 140 MARINE BIOLOGICAL LABORATORY. be quoted in the affirmative, while Wallace, Weismann, Lankester, and the majority of the younger English biologists take the negative. Darwin's later views in this matter are most clearly stated in his correspondence with Moritz Wagner : " In my opinion the greatest error which I have committed has been not allowing: sufficient weight to the direct action of the environment, i.e. food, climate, etc., independently of natural selec- tion." I quote this because Lankester has tried to show in a letter to Nature that Darwin never adopted the views of Lamarck. The general drift of my own views is that upon the side of evolution, or non-repetition in inheritance, the Lamarckians have much the best of it ; while upon the side of repetition and of embryology, their opponents are strongest. It has been said, '' Heaven deliver us from our friends," and I must confess that, upon care- fully analyzing the arguments of some of the neo-La- marckians, I find almost as much against as for the principle they are advocating. The palaeontological evidence appears to be the least vulnerable. For ex- ample, the evolution of the horse's foot seems to afford conclusive proof of the inherited effects of use and dis- use ; yet when we consider the enormous period of time which the reduction of the second and fourth di^rits has required since the lower Miocene period, when they became absolutely useless, the force of the argument is somewhat invalidated. Again, in the teeth, the evi- dence for kinetogenesis is not without exceptions. We require more accurate observation and more logical reasoning, especially directed to the facts of transforma- tion in their bearing upon inheritance. EVOLUTION AND HEREDITY. I4I What is your opinion upon the nature of variations ? All my own observation in the field of palaeontology goes to show that they are not fortuitous, but along cer- tain definite lines, as early claimed by Gray and Nageli. Discard the principle of the inherited influences of habit and environment, and you are apparently left without any explanation of this fact. The fortuitous mingling of germ plasms must result in random varia- tions. Granting that they may be of sufficient value to be selected, we still have to eliminate the swamping effect of interbreeding, and the fatal force of the law of regression to race type which, according to Galton, acts even in the offspring of a pair, both of which possess the advantageous variation. In addressing this association of the Marine Biologi- cal Laboratory, I chiefly advocate, not so much my own views, as a liberal and generous spirit of discussion, for there is little prospect of a solution of these irreconcilable opinions in the temper which characterizes both sides at present. I claim, if the Lamarckians can demonstrate by palaeontological or other evidence, that acquired characters are inherited, it rests with the embryologists to furnish a theory of physical transmission. On the other hand, embryologists may show conclusively that such inheritance is impossible. In the meantime let us keep in view, without prejudice, all classes of facts which bear upon this most important biological problem. SEVENTH LECTURE. ^xy^^c THE RELATIONSHIPS OF THE SEA- SPIDERS. By T. H. morgan. Amongst the Hydroids and sea-weeds on the piles of the wharf of the Fish Commission, hving below the tides, and dinging tenaciously by means of the long jointed legs to the surrounding stems, are to be found abundantly small whitish spider-like creatures. And if the sea-weeds, etc., dredged in the ''Hole" be examined, a similar but larger animal may be found.^ The group to which these belong is known technically as the Pycnogonida, or, as Professor Dohrn of Naples has proposed to call them, Pantopoda. Fortunately we have a good enough English equivalent for the terms — Sea- Spiders. It may not be out of place to say here a few words about the structure of the adults of these sea-spiders, as the group is a small, isolated one, and only short accounts are given in the ordinary text-books. The accompanying figure (i) may aid us in understanding some of the principal structures of the group. 1 Phoxichilium maxillare. 142 THE RELATIONSHIPS OF THE SEA-SPIDERS. 143 At the anterior end of the body we see the large for- wardly-projecting proboscis, with the triangular mouth opening at the anterior end. Above and on each side of the proboscis lies the first pair of appendages — the mandibles so-called — each with a pair of forceps at its end — the chelae. Behind the mandibles and some- what on the dorsal side of the animal lies the second pair of appendages — the palps. The next pair of 144 MARINE BIOLOGICAL LABORATORY. appendages following the palps is situated more ven- trally than either of the other two, and hangs down from the ventral side of the animal. These are the ovigerous legs. These anterior three pairs of appen- dages are followed by four pairs of walking-legs. The first pair of walking-legs is attached to what we may call the first body segment. To the same segment are attached the proboscis in the anterior middle line, the pair of mandibles, the palps, and the ovigerous legs. In the middle of the dorsal surface of the same segment lie the four small simple eyes. The second pair of walking-legs (the fifth pair of appendages) is attached to the second body segment, the third pair of walking-legs to the third body seg- ment, and the fourth pair to the fourth body segment. The body of the animal ends posteriorly in a small process pointing upwards and backwards, which carries the anal opening at its distal end, and is known as the abdomen or rudimentary abdomen. Turning now to the internal structure, we find a brain — dorsal to the oesophagus — followed by a ven- tral chain of five pairs of ganglia for the middle region of the body and a pair of small ganglia, continuous with those of the trunk, within the abdomen. The first pair of appendages is innervated from the brain ; the palps and ovigerous legs from the first pair of ventral ganglia. This first pair of ganglia is in reality the partially fused first and second pairs. Each of the four pairs of walking-legs has a pair of ganglia in its segment, which supplies it with nerves ; and there is, as I have said, a pair of ganglia in the abdomen. The diges- tive tract runs as a simple straight tube through the THE RELATIONSHIPS OF THE SEA-SPIDERS. 1 45 body, beginning at the anterior end of the proboscis and ending at the tip of the abdomen. As it passes through the body it sends out long diverticulae into the appendages — the mandibles and the legs — and an anteriorly directed pair into the base of the proboscis. These diverticulae are the most characteristic and in- teresting structures in the group. The reproductive organs likewise extend from the body into the walking-legs, and have their openings on the second (proximal) joints of these appendages. There is a simple tubular heart lying dorsal to the intestine, which receives the blood through lateral ostia to pump it over the body. These are the more important structures of the Pycnogonids, or at any rate those which we will need in our comparisons. The gallantry of the males must not pass unnoticed. Each during the breeding season carries on its ovigerous legs the developing eggs, which have been received, as soon as laid, from the females. Curiously enough this fact led naturalists into a most amusing blunder, inas- much as it was tacitly assumed that those individuals which took charge of the young and eggs must be the females, and were described as such. The problem, and possibly its solution, which I wish to present to-night, is the relationship of the Pycno- gonids to other groups ; and the point of attack is to be largely from the side of the embryology of the group. There have been endless speculations as to the position — zoologically — of these animals, but until within re- cent years little was known of their anatomy, and prac- tically nothing of their embryology. 146 MARINE BIOLOGICAL LABORATORY. Speaking in general we may say that prior to 1880 the group was placed either amongst the Arachnids (spiders, scorpions, etc.), or else amongst the Crustacea. Recent work has shown Crustacea to be out of the question, I think, but at the same time, the alternative was not believed to be the Arachnids. The group has been independently monographed by Dr. Hoek in a report of the Challenger Expedition, and by Professor Dohrn in a volume of the Naples Fauna and Flora. Each of these authors concludes that we are to believe the group to have arisen, independently of other Arthro- pods (Crustacea, Spiders, Insects), from the Annelids. While their work has been very complete as regards the adult anatomy, and inasmuch as any speculations about the inter-relationships of animals must take into account largely the adult structure, yet there remained unworked another equally valuable source of knowledge; viz. the study of the changes which the animal under- goes in its development from ^gg to adult. Information from this source must take equal rank at least with that from adult anatomy and histology, and may throw light upon hidden relationships which could not other- wise be obtained ; and I wish to give hurriedly a few facts about the embryology of these animals, choosing those which seem to me to bear directly upon the re- lationship of the Pycnogonids to another group to which a study of the development has led me to believe the sea spiders to be allied. Without a series of figures to illustrate these stages of development, it will be impossible to do more than merely mention, in the briefest possible way, those points which are to be used in our comparison. Moreover, THE RELATIONSHIPS OF THE SEA-SPIDERS. I47 there are two types of development within the group, and only to one of these have I paid, so far, special attention. The first type is that represented by the genus Tany- stylum, and is to be regarded as the more primitive of the two, which I shall describe. The egg of Tanystylum undergoes a regular equal segmentation into two, four, eight, etc., resulting in a solid mass of cells, each cell pyramidal in shape, with its apex at the centre of the egg, and the base at the surface. Every cell contains a single nucleus lying nearer to its base ; that is, to the outer part of its cell. Each of these cells then divides — the nucleus dividing into two at the same time — into an inner and outer half, so that there results a solid two-layered embryo. This change has been effected by a process of multipolar delamination. The inner cells, or at least some of them, go to form the endoderm of the adult ; the outer form the ectoderm. I have not followed in detail the changes which take place after this, so that we must pass to a stage of development when the young embryo has broken through the egg membranes and has begun to lead an independent life. At this stage it is totally different from the adult, although, as we shall see later, it con- tains many of the essential characteristics of those parts of the adult which it represents. Figure 2 gives an out- line of a larva at such a stage. We see it has three pairs of appendages, of which the first is chelate, and the second and third pairs are simple uniramous struc- tures, which are used in crawling around amongst the 148 MARINE BIOLOGICAL LABORATORY. PANTOPOD-LAR VA hydroids. The first pair are three-jointed, — counting the movable claw of the chelae, — the second and third appendages on each side are two-jointed. These last two appendages represent the palps and ovigerous legs of the adult. The body ends anteriorly in a rounded pro- boscis. The digestive tract, beginning at the mouth at the distal end of the proboscis, passes through the proboscis as a tube triangular in cross- section and opens out into the wide stomach within the ^3 body proper. There is no posterior opening to the digestive tract, but from the stomach runs forward into the mandibles a pair of diverticulae. The larva has a brain lying dorsal to the oesophagus, which is connected by two commis- sures with a pair of ganglia ventral to the digestive tract. There are in all two pairs of these ventral gan- glia, the pairs more or less fused together ; and behind these, and completely detached from them, is a thicken- ins: of ectoderm which indicates the commencement at this place of a third pair. The larva has no heart nor reproductive organs at this stage. It is needless to follow here the changes which this Pantopod-larva (Dohrn) undergoes as it is metamorphosed into the adult. The body elongates, new appendages appear seriatim, the proctodaeum invaginates and puts the digestive tract posteriorly into communication with the outside world, and the second or third pairs of THE RELATIONSHIPS OF THE SEA-SPIDERS. I49 larval appendages change into those of the adult — the palps and ovigerous legs. We may now take up another type of development found in the group, and represented by the genus Pallene, which, I may say in passing, is to be regarded as a much modified and abbreviated development of a form like the last — Tanystylum. The egg has one hundred and twenty-five times the volume of the preceding species. Corresponding with the increase in the size of the egg is a decrease in the number, and correlated with the addition of food-yolk to the egg is the abbreviated development ; so that the young leaves the parent — not in an immature state as in Tanystylum — having almost all of the structures of the adult. The segmentation of the egg is holoblastic as in the last case, but the first cleavao:e furrow divides the e^s: into a small cell — the micromere — and a larger one. The protoplasm surrounding the nucleus is free from yolk in all cases ; but from this purer protoplasm, pro- cesses ramify all through the yolk. It is needless to follow, for our purposes, the egg through the series of segmentation phases which follow, and it will be suffi- cient for us to know that the result of these changfes is a solid mass of cells much as in the case of Tanystylum. There is this difference, however, that in Pallene the cells covering one pole of the egg — those which have come from the micromere — are smaller than those over the rest (two-thirds) of the egg. It is in the region of these smaller cells that the first indications of the embryo appear. After the segmentation is completed follows, as in the 150 MARINE BIOLOGICAL LABORATORY. former case, multipolar delamination ; but owing to the large amount of yolk, the delamination begins first in the region of the micromeres, — each cell of which delaminates into an inner and an outer cell, — and this process progresses slowly over the surface of the Qgg, each cell, both micromere and macromere, delaminating just before it joins the edge of the ever-widening blasto- derm. The outer cells of the blastoderm form a some- what flattened columnar layer, under which lie the nuclei and protoplasm of the inner cells. To the inner cells would belong properly all of the yolk of the pyramidal cells; but as a matter of fact, all traces of cell-outlines are completely lost in the yolk at this time, so that the blastoderm, with its two layers, lies at the surface of a yolk-mass. Soon after this there can be seen at the surface of the embryo a round opaque spot, which I shall call the primitive cumulus. The ectoderm is thickened in this region, and in the centre of the thickening lies the triangular invagination of the stomo- daeum, and around the latter appear the first traces of the mesoblast. There next appear at the surface of the embryo other opaque areas, which are the beginnings of the brain, the ventral ganglia, and four pairs of appen- dages ; — viz. the first, fourth, fifth, and sixth. The embryo still retains its spherical shape. The next stage is shown in ventral view by Fig. 3, By reference to the figure the more important structures can be seen. The proboscis projects forward ; on each side of this are the mandibles. On each side of the ventral median line lie five pairs of appendages, and to the sides of the last three are three pairs of walking- THE RELATIONSHIPS OF THE SEA-SPIDERS. 151 BMBBYO PALLENE >-/- d-" 0 V t 0 1 0 ° , 0 1 , 'J (0 0 J 0 0^ Fig. 3 legs. The second and third pairs of appendages are absent from the embryo at this stage. If we had cross- sections through the body of such an embryo, we should find, lying at the base of each of the walking-legs, a small cavity with a definite boundary of mesoderm cells ; this cavity is the body-cavity. Such sec- tions would further show us that the digestive tract had begun already to push out into the appendages, and cacJi pouch contains yolk, as does also the whole of the mid-gut. It is not necessary to follow the later stages of development of such an embryo, as we have already seen those peculiarities which are to be used in the comparison of the Pycnogonids with other groups. Now in the briefest manner possible I wish to point out the importance of these embryological facts, and their bearing upon the relationship of the Pycnogonids to the group of the Arachnids. First there is the multipolar delamination, which is such a striking feature in the development of the Pycnogonids ; so that when we come across an exactly similar series of phenomena in the false-scorpions (Metschnikoff) and in the spiders (Balfour), we have found a strong point for comparison in the develop- ment of the two groups. We find in the spiders the first external appearance of the embryo to be a round 152 MARINE BIOLOGICAL LABORATORY. opaque area on the surface — the primitive cumulus ; beneath this the first traces of the mesoderm are found, and further, this is the point where the stomodaeum soon invaginates. We have seen an exactly similar series of changes in the embryos of the Pycnogonids. We also find body cavities lying at the bases of the legs of the embryo spider, in an exactly similar position to those which we have seen in the sea-spiders. The digestive pouches found in both the adult and in the embryos of the Pycnogonids, which contain yolk in the young, furnish us with another point of comparison with the Arachnids. In the latter, we have these for a time in the developing young ; and, moreover, in the false-scorpion they contain yolk from the mesenteron. Lastly, I must call attention to the structure, inner- vation, and origin of the first pair of appendages. These are chelate, as in many Arachnids, and the free movable joint of the claw moves outwards and down- wards as in the Arachnids generally (and not upwards and outwards as do the chelae of the Crustacea). Again, these appendages are innervated in the sea-spiders from the brain, and finally in the embryo the appendages are seen to arise at the sides of the stomodaeum, and subsequently move forward and dorsal to it. In both of these last characteristics we can compare the two groups directly together. So much for the embryology : I leave these com- parisons to speak for themselves. And now let us turn to the adult structures of the two groups under comparison. I do not believe any general definition of the group of Arthropods can be made, which, if based on funda- THE RELATIONSHIPS OF THE SEA-SPIDERS. 1 53 mental characteristics, will not of necessity include within it the Pycnogonids. We have the segmented body with its segmented appendages, typically a pair to each segment. There is a supra-oesophageal ganglion — the brain — connected by commissures with a double ventral chain of ganglia. Digestive tract, circulatory apparatus, and to some extent^ the generative organs, fall into line with the other Arthropod characteristics. Comparing the adult structures of the sea-spiders with the Arachnids, we find many common characteris- tics ; and I must point out that many of these similarities are in structures which we must believe the primitive ancestors of the Arachnids to have had more fully de- veloped than the living forms to-day. First, we may consider the digestive tract. In the adult sea-spiders we find long diverticulae from the mid- gut into the appendages ; and we find in the early stages of development of both (false) scorpions and spiders similar pouches. ^ The heart, too, with its lateral ostia resembles that of spiders, though the resemblance is not more striking than to Arthropods in general, and I have spoken above of the resemblance between the chelae and their innervation in the sea-spiders and Arachnids. Further, the external openings of the re- productive organs in Limulus lie on the base of the first 1 The opening of the ducts of the reproductive organs are not as difficult to account for as I had at first supposed. We may imagine them to have secondarily shifted from the body to the legs, and we see in Limulus (" An Arachnid ") the external reproductive orifices on the base of the first pair of abdominal appendages. 2 The structure and arrangement of the so-called livers of Limulus and Scorpions is interesting, inasmuch as they may represent ancestral ab- dominal pouches. 154 MARINE BIOLOGICAL LABORATORY. abdominal appendage, so that the difficulties arising from the existence of similar openings in the Pycnogonid, on most of the appendages, is partially at least overcome. I have also examined the structure of the adult eyes of the Pycnogonid, which seem to furnish, perhaps, an explanation to the early inversion of the eyes of the Scorpion and other Arachnids. Let us turn finally to an examination of the ap- pendages of the Pycnogonids, where we meet with a stumbling-block in the way of our acceptance of the relationship of the sea-spider and Arachnid. If we arrange in two series the appendages of the two groups, beginning with the first pair in each, we get the following table : — Pycnogonid. spi 'der. Scorpion, etc. Mandibles 1st pair Chelicerae. Palps . 2d - Chelae. Ovigerous legs . . 3d " 1st walking-legs. 1st walking-legs 4th " 2d " " 2d 5th - 3d 3d " . 6th " 4th 4th " *♦ 7th '* • • • • In the Pycnogonids there are seen to be seven pairs of appendages, while the spider, etc., have only six pairs in all. There are four of these pairs used by each as walking-legs, and in the Pycnogonids three pairs lie in front of these, and two such pairs in the spiders. We may get out of this difficulty of numbers in either of two ways. In the first place, we may assume that the four pairs of walking-legs are homologous in the two groups, and at first sight this seems very plausible. THE RELATIONSHIPS OF THE SEA-SPIDERS. 1 55 Then the mandibles of the one group seem undoubtedly homologous with the chelicerse of the other ; so also do the palps of the one and the chelae of the other; there- fore the trouble lies with the ovigerous legs. Schimkewitsch tried to solve the problem by sug- gesting that this third pair came in as greatly developed exopodite, or endopodite — I forget which — of either the second or third pairs of appendages, and was there- fore not equivalent to the other appendages. This was ingenious, but wrong. The development of the ap- pendages shows that the third pair is formed in line with, and independently of, the other pairs ; and, more- over, it has a separate pair of ganglia from its earliest appearance. But there yet remains another possible explanation for this third pair of appendages of the sea-spiders, viz. that the spiders have lost a pair of appendages (and ganglia.?) between the chelae and the first pair of walking-legs. Here the objection arises that, although the develop- ment of spiders has been quite fully studied, no evi- dence of such a loss of appendages is forthcoming, and therefore, for the present at any rate, we have to reject this solution.^ The explanation which seems most plausible is, that we cannot properly bring into line all the pairs of walk- ing-legs and compare them, pair for pair, but that the first pair of walking-legs of the spiders corresponds (as in the table given above) with the ovigerous legs of the sea-spiders, and, therefore, the second pair of legs of the 1 Perhaps the development of spiders will bear re-examination in this respect, and more especially should the first pair of ventral-ganglia be examined to see if it is a single or a double pair. 156 MARINE BIOLOGICAL LABORATORY. spiders with the first of the sea-spiders, the third with the second, and the fourth with the third, leaving over the fourth pair of legs of the Pycnogonids as a pair not found in the spiders. If this be true the position to which we are forced is obvious, — that the last segment of the body of the sea-spiders, and its appendages, corre- spond to the first segment of the abdomen of the spiders. Such a transfer of segments is not uncommon amongst Arthropods, and there is nothing unusual in such a process. It here carries with it, however, the assump- tion that the fusion must have taken place at a time when the abdomen still bore appendages serially homolo- gous with those of the thorax. We may imagine, if we like, that this took place at a time when the third pair of appendages appeared and began to carry the eggs, so that the body, by utilizing the first legs as egg- carriers, retained a pair of the abdominal legs for pur- poses of locomotion. I must confess, however, that although this last view seems far the more probable, in the present state of our knowledge, yet the idea is a very attractive one, — that the four pairs of walking-legs are homologous in the two groups, and that therefore the spiders have lost a pair of appendages between the chelae and the first pair of legs. I have, in the above comparison, left to one side the Crustacea, nor is there need to say much in regard to their possible relationship to the Pycnogonids. The characteristics which the two groups have in common are only those of Arthropods in general. Other than this there seem to be few homologies, with the possible exception of a six-legged larval form of which I shall THE RELATIONSHIPS OF THE SEA-SPIDERS. 15/ speak later. And these points of difference in the development may be mentioned. There is no primitive cumulus or its homologue in the Crustacea, and the first invagination of the surface ectoderm of the embryo goes to form the mid-gut, and its point of origin corresponds, approximately (perhaps entirely), with the permanent anus. Further, there is nothing corresponding to the gut-pouches so characteristic of the Pycnogonids and, to some extent, of the Arachnids. If we turn to the insects, which form by themselves so well-defined a group, we see little or nothing new for comparison with the Pycnogonids.^ Returning again to the Arachnids, we have found that the Pycnogonids have, in common with them, not only the general characteristics of the Arthropods but many peculiarities of development and many structures com- mon in the adults. If we accept this we have gone perhaps as far as the facts at present at our command will allow. For it is impossible to say at just what point the Pycnogonids have branched off from the phylum of the Arachnids ; but, inasmuch as several of the adult characteristics of the sea-spiders are found in the em- bryos of higher Arachnids, we may fairly believe that the group arose quite far down amongst the primitive Arachnids, to have retained many of their primitive structures, and to have lost others which the ancestral form possessed becoming simplified, specialized, and degenerate. 1 If Heider's and Wheeler's accounts of the early formation of inner yolk nuclei from the outer (ectoderm) nuclei be true, it looks exceedingly like multipolar delamination for the insects. 158 MARINE BIOLOGICAL LABORATORY. There is another problem, which must arise in any discussion of the affinities of the Pycnogonicls, and which may be treated as a corollary to what has gone before. I refer to the presence within the group of a free-living larval form — the Pantopod-larva which we must believe typically to belong to all the species of the order.^ Was the ancestral form from which the group has arisen similar to this Pantopod-larva, hence its pres- ence as a stage of development to-day ; or is it a purely secondary larval form, an interpolated stage, therefore, unimportant from a phylogenetic point of view ? Before attacking the problem directly, let us under- stand clearly the possible conditions under which such a larval form, or any larval form, may occur within a group of animals. There are at least, two, and perhaps three, such conditions generally recognized. We have first what is known as a primary larval form, by which we mean the young form represents approximately the structure possessed by the ancestor from which the group has arisen. In Fig. 4 this is shown diagrammati- cally by A. The X marks the place at which a species x expanded into a group, and after the group has been evolved each young form in that group first reaches X, and then each in turn develops the peculiar features of its particular family, genus, species. Again we may find all the animals of a group possess- ing a common larval form, but which need not neces- sarily ever have been an ancestor of the group, but may have been interpolated quite early in the history of the ^ Such exceptions, as Pallene for instance, show an abbreviation of this development, and at one time they or their ancestors must have had a free-larval form. THE RELATIONSHIPS OF THE SEA-SPIDERS. 159 group before it (the group) diverged into those forms existing to-day. In Fig. 5 such a condition is shown by B. At X in this figure we may suppose, by some means or other, a stage of development (not an ances- tral one), to have appeared, and the young to have become adapted to an independent existence, or to have been fitted by a process of natural selection to lead a free life. Now since the adult form, which developed this secondary larva, subsequently was the r ■ct -ay x\jz ayz xyz xyz G starting-point for a group of animals, we would expect each species to retain this secondary larval form. Thus in diagram B, Fig. 4, we may suppose a larval form a to appear, and afterwards as the group arose giving x, x, each to have retained the larva a. There is possibly a third condition by which a larval form may appear within a group. Briefly put, it is something like this, an adult animal which subsequently gives rise to a group, may have had at its starting-point l60 MARINE BIOLOGICAL LABORATORY. a free larval form of its own (whether primary or sec- ondary is immaterial), and as the group arose, through divergence of the adult animals, the larval form was itself affected by the new characters of the adult, and so changed into what I shall speak of as a transfomned larva, or, to put it differently, we may suppose the newly acquired characters of the adult to be thrown back upon the larval form already present. This is shown by diagram C, Fig. 4, when a is changed gradu- ally to ay, az, as the adult X changes into Fand Z. There may be many modifications of these three con- ditions I have sketched, but for our present purposes these will suffice. To restate the problem before us : we wish to find out, if possible, in which of these ways the larval form in the Pycnogonids has been evolved. Professor Dohrn believes, and his theory is the natural outcome of the position he has taken as to the origin of the group of the sea-spiders, that the larva of the Pantopoda is a transformed larva, and therefore the explanation of its appearance within the group to be represented by Fig. 4, C. Dr. Hoek, on the other hand, believes the embryo to be a primary larval form, and therefore its presence to be accounted for in some such way as repre- sented by Fig. 4, A. Professor Dohrn further believes that the Trochophore of Annelids represents the first larval form which became modified into the Pantopod- larva ; while Dr. Hoek believed that the Pantopod-larva of the Sea-spiders, the Trochophore of the Annelids, and the Nauplius of the Crustacea, have each given rise to their respective groups, and to each represent a primary larval form ; and moreover, he thought these THE RELATIONSHIPS OF THE SEA-SPIDERS. l6l larval forms themselves to have been inter-related at the time when they diverged into Sea-spiders, Annelids, and Crustacea. My own view of the question is that we have here a condition represented by Fig. 4, B, or, in other words, that the Pantopod-larva represents purely a secondary larval form. I wish now to bring forward the reasons which have led me to such a belief, and at the same time the objec- tions to the views of Dohrn and Hoek. Let us stop for a moment and examine the structures of the three larval forms under discussion. NAVPLIJJS The Nauplius of the Crustacea is shown in Fig. 5. The body is oval in outline, and unsegmented ; it has three pairs of appendages arranged along the sides of the body. The mouth lies under a large upper-lip, a little anterior to the centre of the body, and leads into the oesophagus which in turn opens into the stomach or mid-gut ; then follows the proctodseum with its exter- l62 MARINE BIOLOGICAL LABORATORY. TROCHOPHORE nal opening at the posterior end of the body. There is a brain (supra-oesophageal ganglion) dorsal to the oesoph- agus, connected by commissures with the first pair of ganglia of the ventral chain. An eye lies in the mid- dorsal line over the brain. The first pair of appendages is simple and uniramous and receives its nerves from the brain. The second and third pairs of appendages are each biramous, and the first and second pairs are innervated from ventral ganglia. These I believe are the essential features of the Nauplius. It swims freely in the water, and lives a per- fectly independent life in those forms in which it is best repre- sented. The Trochophore of the Annelids is likewise a free- swimming pelagic animal, and is diagrammatically represen- ted by Fig. 6. The larva has a large oval anterior end, and a somewhat funnel-shaped posterior part. In the centre of the head-region lie two eye specks, and under this the beginnings of the brain. The middle of the animal is encircled by a band of ciliated cells which form the locomotor ap- paratus. Just beneath this band, or where the band is double within it, is the large mouth opening. The oesophagus is short and leads into the large mesen- teron, and from this runs posteriorly the proctodaeum to end in the anus. Beneath the mouth, and lying between it and the anus, is a bilateral plate of cells which is to form the nervous system. In the earliest stages the Trochophore represents a single segment — ■ Fig. 6 THE RELATIONSHIPS OF THE SEA-SPIDERS. 163 the head segment. ^ Soon, however, other segments appear in the anal region, so that we see at one time three segments, two besides the head, and it is at such a stage we may compare the Trochophore with the other forms. The characteristics of the Pantopod-larva have been already described on page 148, and reference to Fig. 2 may serve to recall its more important features. We have seen that Hoek holds the Pantopod-larva to be a primary form, and that Dohrn believes it to be neither primary nor secondary, but what I have called a transformed larva. It is exceedingly difficult to combat these theories, but this much must be against Hoek's position, viz. that the same objections which are nowadays being brought against the Nauplius theory of the Crustacea, must tell at every step against his Pantopod-larval theory for the Pycnogonids ; and I believe morphology is outgrowing the Nauplius theory. To-day it seems to be on its last legs — or rather more legs have been forced upon it than it could conveniently carry. Some of its earliest and most brilliant advocates, including Hatschek, Dohrn, and Claus, have at last thrown it over, so that it seems superfluous to repeat the same arguments against the Pantopod-larva that have been brought against the Nauplius theory. Turning now to Dohrn's theory, we cannot but grant that it at least is a theoretical possibility, even if the Pycnogonids be related to Arachnids. It only remains to examine the facts which we have and see if they 1 This is not quite exact, for the posterior part of this segment contains the rudiment of the future body. 164 MARINE BIOLOGICAL LABORATORY. furnish sufficiently good grounds for rejecting such a theory as exceedingly improbable. First, I must insist upon the fact that those char- acteristics which the Pantopod-larva has in common with the Trochophore are only those features which any two Arthropods have as common characteristics of the first three segments ; or, if we go further and include the Annelids, we may say which any two Articulates possess as common characteristics. They are these: a brain and ventral chord, united by commissures around the oesophagus; serial appendages correspond- ing in number to the number of segments ; a digestive tract with stomodaeum, mesenteron ; and — and here lies the difficulty : not a trace of proctodaeum does the Pantopod-larva possess, while both Nauplius and Trochophore have the digestive tract open posteriorly. In the young Pantopod-larva we have, so far as it goes, a fully formed and presumably functional digestive apparatus, and no reasonable account can, I believe, be given to explain how this posterior opening could have become lost in the transition of Trochophore into Pantopod-larva as Dohrn has supposed. If it be urged that in the young form the anus may have become func- tionless, and hence not developed, it will not mend matters, since we have every reason to believe that in the adult Pycnogonid the proctodaeum is to a large extent useless.^ I think these facts must prejudice us strongly against the position taken by Dohrn ; but let us go further and examine into the other features of the Pantopod-larva. 1 It has been suggested that it may serve for purposes of respiration, but no evidence for this is forthcoming. THE RELATIONSHIPS OF THE SEA-SPIDERS. 165 The chelae are identical with those of the adult, and pass over during the metamorphosis without change into the first pair of adult appendages. The ventral ganglia arise in the young larva, as do the later ones, by a pecu- liar process of invagination, and differ markedly from the origin of similar ganglia in the Trochophore ; and lastly, the ectoderm and its sense organs are the same as those of the adult. Now while, as I have said, it is just pos- sible all of the things (with the exception of the Procto- daeum !) may, as Dohrn believes, have been thrown back upon the larva, such a process seems improbable in itself, and, I think, is an entirely unnecessary supposition. For the necessity of believing that the young forms, in such groups as Annelids and Crustacea, more nearly resemble each other than do the adults seems to me an entirely unwarranted supposition. On the contrary, I think a priori we should expect to find exactly the reverse of this, — that is, that the adults are nearer together ancestrally than are the larvae. If we stop to recall the fact that most of these ani- mals lay great numbers of eggs and that almost all of these are destroyed during the larval stages, and out of several thousands only a few reach the adult condition, then I think we must see that the battle for existence amongst the larvae as compared with the adult is as a thousand to one ; and hence, when there is this vigorous process of natural selection going on, we must expect the embryos to become changed, or adapted to new con- ditions, with great ease and rapidity. And now if we remember that during the time in which the groups of Annelids and Crustacea have been evolved that the larval forms themselves have been 1 66 MARINE BIOLOGICAL LABORATORY. acted upon in an increased degree, there seems every reason to believe that the young may have been much more acted upon and suffered far greater changes. On the other hand, when we see in such a group as the Vertebrates that in the higher forms the young have been removed to a large extent from the action of surrounding conditions, — as, for instance, by being enclosed within a shell as in the Sauropsida, or retained within" the uterus in mammals, — then can we under- stand why the young resemble each other more closely than do the adults, for the obvious reason that the adults have had to adapt themselves to more numerous external conditions while the embryo has remained fixed. Indeed this may be pushed a step further, it seems to me, and explain why such young retain the characteristics of lower forms while the adults have lost such struc- tures. This may be due to the young having been removed to a greater extent than the adults from a process of active selection. Hence in such a group, when we say that the Ontogeny tends to repeat the Phylogeny, we mean that the embryos have retained more of the ancestral features than have the adults. But in such groups as the ones we are discussing, — Annelids, Crustacea, etc., — we ought to expect, if what I have said be true, the reverse of what we find in such a group as the higher vertebrates ; viz. that the young forms diverge far apart, and the adults come nearer together. This will tell strongly against the position taken by Hoek (also against the Nauplius theory), and render unnecessary or even improbable that we need bring together such forms as the Trochophore of the Annelids THE RELATIONSHIPS OF THE SEA-SPIDERS. 16/ and the Pantopod-larva of the Pycnogonids. And although on this supposition we might suppose so great a transitional change as Dohrn believes to have taken place, yet it also carries with it the assumption that a new larval form may have been acquired with ease within such a group independently of any previous (ancestral) stages. Hence we are left in an unpreju- diced position to choose between these two possibilities! And for my own part I am led to the conclusion, from the foregoing facts of development, that the Pantopod- larva is neither a primary form, as Hoek supposes, nor represents a much modified Trochophore, as held by Dohrn. And it seems to me far simpler and much more in accordance with the facts to believe that we have here a clear case of a secondary larval-form. EIGHTH LECTURE. -00>S):^00 ON CARYOKINESIS. By S. WATASE. It has been said that one of the greatest discoveries of modern times is the generahzation that all animals, however complex their structure, arise by the division and subdivision of a single, nucleated cell. The Cell doctrine in its original form, which had its origin in the comparative study of adult tissues, appears in a new light when viewed from the standpoint of the embryo- logical history of these tissues, arising as they do, as the direct products of egg-cleavage, each segment being the exact copy of the original egg-cell, in so far as its general, superficial features are concerned. Here arise two important problems : — (i) How does one egg, which is a single nucleated cell, and which gives rise to one animal, differ from another which gives rise to an entirely different organism .'* (2) What is the essential method of cleavage by which an apparently homogeneous ovum becomes converted into a complicated organism } The first problem is well-nigh beyond the range of our present means of research. Take an egg of a star- 168 ON CARYOKINESIS. 1 69 fish and that of a jelly-fish ; raise them under exactly the same conditions. Both will undergo division and subdivision, but the process will end in the production of two entirely different organisms. The difference in result cannot, therefore, be attributed to difference of conditions under which they develop, but to something inherent in the ova themselves. In other words, the egg-cell of a jelly-fish must have had from the begin- ning the potentiality of becoming a jelly-fish and nothing else ; and similarly, the starfish ovum must have been a potential starfish from the beginning. To imagine, therefore, that all protoplasm is identical, because no difference is recognizable by any means at our disposal, must be an error. Deep within the two particles of protoplasm which give rise to two different organisms, there must be a corresponding difference which lies at the bottom of all differences. In short, the eggs of two different animals must be supposed to differ in their elementary constitution, as much as their adult organ- isms differ in anatomical structure. " From general scientific principles," says Professor Sachs, "we must assume that for each visible external difference of organ, there is a corresponding difference in its material sub- stance, exactly as we regard the form of a crystal as an expression of the material properties of the crystallizing substance." And again, says the distinguished German botanist, '' Even the different shapes of the two sexual cells — of an antherozoid or a pollen grain compared with the oosphere — indicate plainly, that both are con- stituted differently as to material, since the external form as well as the internal structure of any body is the necessary expression of its material constitution. Dif- 170 MARINE BIOLOGICAL LABORATORY. ference of form, always indicates difference of material substance." This doctrine of " Form and Matter," or of " Mechanism and Function," as expressed in the language of physiology, is the basis of our biological inquiries. As is clearly expressed in the words of Pro- fessor Burdon Sanderson, we must assume that ''every appreciable dijference of s timet lire eorrespoiids to a differ- ence of function ; and conversely, each endowment of a living organ must be explained, if explained at all, as springing from its structure"; or in short, ''living material acts by virtiie of its structure, provided we allow the term structure to be used in a sense which carries it beyond the limits of anatomical investigation, i.e. be- yond the knowledge which can be attained either by the scalpel or the microscope." Given protoplasm of defi- nite structure, and we have its definite function or property. Or conversely, we observe a certain property in a given mass of protoplasm and we regard it as springing from a definite structure. Wlien structure varies, the function must vary also ; and when we ob- serve certain peculiar properties we must ascribe them to peculiarities in structure. One rational answer to our first inquiry is possible, viz. the protoplasmic structure of the Q,gg which gives rise to one organism, must differ from that of the ^g^ which gives rise to another different organism, the dif- ferences between the two being relatively as great as those which the two adult organisms display in their anatomical relationships. If the similarities of two organisms must be attributed to the corresponding similarities of the protoplasm of the fertilized ova from which they respectively arise, ON CARYOKINESIS. I/I the source of similarities in the latter must be sousfht for in the community of their hereditary antecedents. Hence, one way to place the doctrine of phyletic kin- ship of two or more organisms upon a scientific basis, would be to demonstrate the molecular or structural affinities of their tissues, or what amounts to the same thing, to demonstrate the molecular or structural affinity of their germ-cells. The embryological phenomena of a developing organism may be expressed in the terms of protoplasmic metamorphosis. Two organisms at the same stage of development would represent the same stage of protoplasmic structure. The budding of a new cell or the formation of a new organ would correspond to the birth of a new phase in the course of the meta- morphosis of the original protoplasm of the egg. To turn to our second problem. What is the cleavage of the ovum ? What is accomplished by it ? Is it ''the mere sundering of material which has no more reference to the future organization of the embryo than the snow- flakes bear to the size and shape of a future avalanche" ? Or is it a ''histogenetic sundering" in which every step in the process has a definite relation to the building up of the future embryo ? These questions have been raised from time to time and have been variously answered. Upon this historical aspect of the question it is not my purpose to enter at present. But that each step of cleavage has some definite significance in relation to the organization of the adult or of the larva, at least in certain forms which have been most carefully studied, there can be no question. Thus in a certain animal, it has been observed that the nuclear substance of the ovum is divided, during the first cleavage, in such a 1/2 MARINE BIOLOGICAL LABORATORY. manner that one of the new nuclei by its division gives rise to the right, and the other to the left side of the adult organism. In another case, it has been main- tained, the first division of the nucleus distributes the nuclear substance into future ectoderm and entoderm. And again, the formation of a certain organ, or of a system of tissues, has been traced in a most definite manner to a particular cell or group of cells in an early stage of cleavage. The more carefully the phenomena are studied, the more astonishing is the regularity and the precision with which the cleavage process is con- ducted and the differentiation of tissues is accompHshed. The occurrence of variations or irregularities in the mode of cleavage in a certain animal — irregularities as judged by the arrangement of the superficial cyto- plasmic furrows — does not invalidate the importance of the conclusion which can be derived from the study of forms where absolute regularity prevails. For the es- sential feature of the cleavage process is the division and distribution of the nuclear substance of the ovum, and in so far as the nuclear substance is distributed in such a manner as to produce a symmetry of growth in the developing organism, it is immaterial whether its total quantity be divided exactly in two equal halves and distributed into right and left at the first cleavage, or whether it be divided into dissimilar portions and the equilibrium of growth be gradually secured during the subsequent stages of cleavage. The distribution of the nuclear substance may have been just as accurate and precise. in one case as in the other. A comparative study of cleavage of different ova affords another example illustrating this point. For ON CARYOKINESIS. 1/3 instance, as my friend Dr. C. Ishikawa tells me, the summer and winter eggs of a certain species of DapJinia undergo different '' types " of cleavage, one being holo- blastic and the other being meroblastic, the difference being produced probably by the conditions of the cytoplasm and its deutoplasmic contents. The same may be said in regard to the cleavages in different species of Peripatits^ as the studies of Sedgwick and others have shown. The same is true in the case of Renilla as was shown by Wilson. In short, if we classify animals by the "types" of cleavage or dif- ferences of cleavage, rather than with reference to the potential qualities of the nuclear substance, we fall into an error of placing nearly related species of organisms in different categories, nay, we even fall into the ab- surdity of separating the individuals of one and the same species into different groups. That the argument based on the arrangement of superficial furrows alone is not entitled to any weight, is further shown by their total absence in several forms of ova, which nevertheless develop into perfect organisms. It has been shown that in a certain plant, the cytoplasm becomes divided without a corresponding division of its nucleus. Such facts seem to point to the conclusion that the division of the cytoplasm and that of the nucleus are two independent phenomena, and that one process can occur without the other, and that when they do occur in close succession, as in ordinary cell-division, it is to be looked upon as a case of coincidence. At any rate, the following conclusion seems to be a valid one, viz. that the division of the nucleus and that of the cytoplasm are due to different causes. 174 MARINE BIOLOGICAL LABORATORY. It is now quite generally conceded that the nucleus of the fertilized ovum contains all the hereditary charac- teristics of the parent organisms. It is this substance in the ovum which stamps the particular characteristics upon an organism of a given species. The study of fertilization has clearly demonstrated the metamorphosis of the sperm-nucleus into a constituent part of the cleavage-nucleus, and thence it is distributed to all nuclei formed in the subsequent cleavages. Morpholog- ically, all the hereditary characteristics which the infant organisms inherit from the parents, must be traced back to a certain number of chromosomes which come from the sperm and egg-nuclei of the fertilized ovum. By cleavage, the potential characteristics become gradually analyzed into their special attributes — the attributes which we assio:n to different tissues of the larval or the adult organism. If, therefore, I may use one word to characterize the whole process of cleavage of the ovum, the term Analysis will perhaps best express our interpre- tation of the phenomenon. It is true, that we know very little as to the essential respects in which the nuclear substance in the entodermic cleavage sphere differs from the similar substance in the ectodermic sphere. In the present state of our knowledge on this subject, we can only infer a structural difference of the protoplasm from the careful study of the fate of the respective segments. If, for instance, one cell gives rise to a sense-organ, the fundamental molecular struc- ture of that cell must be different from another which contains all the germs of an excretory organ, just as we are forced to conclude that the ova of different organ- isms are of necessity different, even if they appear ON CARYOKINESIS. 1/5 identical by the means of observation at our disposal. Thus, instead of inferring function from structure, we infer structure from function and conclude that where- ever we detect a difference in function the protoplasmic structure must be different also. When, therefore, we speak of the analysis of nuclear substances we do not speak from actual knowledge of the substances thus analyzed, but from purely scientific reasoning. It is probable that during cleavage, the original nuclear substance may undergo a series of molecular changes, and split up into a number of protoplasmic substances each of a different molecular structure, and that as a final result of this chain of metamorphoses different kinds of tissue cells come into existence. In short, dif- ferent morphological stages of the developing ovum may be considered as so many different molecular conditions of the protoplasm. And perhaps, the molecular consti- tution of a dividing ovum in its earlier stage may differ more from that of the later larval stasre than two or- O ganisms belonging to different species would differ from each other in their adult condition. Professor Weis- mann's phrase — ''ontogenetic stages of idioplasm" — aptly expresses our meaning on this subject. For the metamorphoses of structures and of embryonic tissues must of necessity correspond to the change in the con- stituent protoplasm. Without change in the nuclear substance, development is impossible ; the Qgg must remain an &gg forever. If all the determining elements of future tissues are contained in the nucleus of the ovum, and if cleavage is the process by which these elements are analyzed into more tangible tissues, the question naturally arises 176 MARINE BIOLOGICAL LABORATORY. as to the method of analysis employed in such a process. Such a method we find in Caryokiiiesis. I will, therefore, describe the process which may be termed the mechanics of nuclear division, as based on my observation on Cepbalopods and Echinoderms. It is now agreed by many foremost investigators of the subject that the essential feature of caryokinesis lies in the division of the chromatic substance of the nucleus among the daughter' cells, and the complicated system of spindle-rays is the mechanism to effect such a division. The development of a spindle clearly shows this, and the following is an attempt towards a further confirmation of the current view on the subject, as held especially by E. van Beneden and T. Boveri. In one important respect my view is entirely different from that of these authors, but this difference lies more in the in- terpretation of phenomena than in the facts themselves. First of all, I will endeavor to describe the anatomy of a well-developed caryokinetic figure in the Cephalo- pod ^ZZ^ upon which my observations have been chiefly carried on. The question of nomenclature presents some difficulty. I will use here a set of terms of a simple descriptive character, descriptive of either of function, of origin, or of topographic relationship of dif- ferent parts. Since scientific nomenclature embodies marks of the progress of our knowledge on the subject, I will use, wherever convenient, such terms as have been introduced quite recently, and represent, in a measure, the latest phase of our information on the subject. The accompanying illustration (Fig. i) shows a cary- okinetic figure in the blastoderm of the squid. ON CARYOKINESIS. 177 Figure 2 shows the same in a more advanced condition as seen in the developing ovum of a starfish. A B C D E F M M^.. Fig. I. — Loligo.i Centre of Archoplasm. Extra-nuclear archoplasmic filaments. Cytoplasm. Interzonal archoplasmic filaments, Intra-nuclear archoplasmic filaments- Remnant of nuclear cavity filled with nuclear fluid. Limit of the original nuclear cavity, sharply separated from the surrounding cytoplasm. n, n' Two daughter chromosome bands. The figure consists of two essential anatomical feat- ures, (i) the central, elliptical body, and (2) the two star- 1 Letters on other figures have the same significance. 178 MARINE BIOLOGICAL LABORATORY. like, radiating structures. The fornner corresponds to the outUne of the original nucleus, as will be shown later, and the latter constitute the asters of Fol, spJieres at- tractive of van Beneden, or, to use a more recent nomen- clature, the arcJioplasviic spJieres of Boveri. The central area of archoplasm (/i), is situated in the substance of J^-z--^.- -''r^-^rr^j Fig. 2. — Asterias. the cytoplasm {C). From the granular archoplasmic substance as a centre, there radiate out in all directions a large number of fibre-like rays, the archoplasmic fila- me7tts {B and E). A portion of these ray-fibres pene- trate into the elliptical part of the figure, and constitute ON CARYOKINESIS. 1/9 the intra-miclear archoplasmic filaments {E) ; while those lying outside of the elliptical body are the extra-nuclear arcJwplasmic filaments {B). The elliptical portion of the figure consists of three parts, two terminal and one intermediate. The terminal portion, which presents different optical properties from the intermediate part, consists of a hemispherical mass of a slightly stainable, semi-liquid substance, which I believe to be the nuclear-sap of the original nucleus. Into this part the archoplasmic rays extend, as has already been mentioned. The two terminal masses of stainable substance are separated from the intermediate non-stainable bundle of filaments by parallel chromatic '* plates " (;/), (;/), — the chromosomes (Waldeyer) of the original nucleus. The non-stainable intermediate filaments above referred to are the interzonal archoplas- mic filaments {D), — ''interzonal filaments" of Mark, "filaments reunissants " of van Beneden, " gubernacu- lum"of Maupas, " Verbindungschlauch" of Strasburger, ''connective filaments," " Verbindungsfaden," etc., of authors. One plate of chromosomes goes to one daughter nucleus, and the other to another. The cytoplasm accumulates around each, and there follows a separation into two cells, each with its distinct nucleus. If one examine a nucleus at a tolerably early stage of caryokinesis, one will see a phenomenon such as is shown in Fig. 3. The nucleus with a network of chromo- somes is intercepted between two archoplasmic spheres. More than this, however. That portion of the archo- plasmic rays which falls on the surface of the nucleus presses that part inward and so flattens that side of the i8o MARINE BIOLOGICAL LABORATORY. nucleus. This polar flattening of the nucleus goes on until the nucleus presents the appearance shown in Fig. 4. Space only forbids the illustration of the further changes, but it may be easily imagined that when this flattening of the nucleus is continued, the whole solid contents of the nucleus are reduced to a single flat sheet M-^-,V-^ Fig. 3. — Loligo. iV Nucleus. X Nucleolus (?) as it were, as shown in Fig. 5, forming the equatorial chromatic "plate." The spindle then, as its history clearly indicates, consists of two cones with their bases turned toward each other, and with their apices in the archoplasmic centres, as was first pointed out by van Beneden. ON CARYOKINESIS. I8l This stage of caryokinesis with its single chromatic "plate" leads to another with two daughter ''plates," — a phase which has been called by Flemming, meta- kinesis. The question naturally arises, How is this separation of a single "plate" into two "plates" effected? With the separation of the two daughter "plates" of chromo- somes, there comes into existence a series of parallel M-r-r Fig. 4. — Loligo. interzonal filaments which lie between the two separat- ing "plates." The separation of the daughter "plates" of chromosomes, and the formation of the interzonal fila- ments, are so intimately connected with one another that we naturally look for a causal connection which underlies the parallel series of phenomena. Any theory which explains the one must also explain the other. 1 82 MARINE BIOLOGICAL LABORATORY. Boveri meets this difficulty by denying the filament- ous nature of the interzonal substance, holding that what appears as filamentous is the optical expression of the longitudinal folds produced by the contraction of the two antagonistic groups of archoplasmic fibrils, whose distal extremities are fastened to the chromosomes. Strasburger ascribes to the substance the function of a wedge which grows in size by the absorption of the cytoplasmic fluid, and pushes apart the parallel "plates" of chromosomes. Platner explains the filamentous ap- pearance of the interzonal substance as the optical ex- pression of a protoplasmic stream. The existence of such a stream in a living dividing cell has, however, been denied by Strasburger. As to the filamentous nature of the interzonal sub- stance, there can be no question, as several observers have abundantly shown. My own studies on Cephalo- pods and Echinoderms have convinced me of the truth of this conclusion. Further, no optical difference could be observed between the archoplasmic fibrils at the poles of the spindle and the filamentous bodies in the intermediate zone, which fact has already been pointed out by several investigators. Observing, then, that the interzonal portion of the caryokinetic figure consists of the bundle of filamentous substance, that this filamentous substance is essentially the same as the archoplasmic filaments of the spindle, that the length of these filaments is exa'ctly the same as the space between the parallel bands of chromosomes in all stages, that the archoplasmic filaments have been growing in length from the poles toward the equator of the nucleus, and, further, that the interzonal filaments ON CARYOKINESIS. IS3 came into existence exactly at the moment when the single equatorial "plate" was dividing into two par- allel daughter "plates," the following view becomes pro- bable, viz. that after the archoplasmic filaments from the two centres have reduced the chromatic contents of the A--- Fig. 5. nucleus into a flat "plate" by gradual lengthening, they continue to grow in the same manner, and push through between each other, just as two brushes would do if their ends were pushed together. Their free ends will dovetail with each other. The distal extremity of each archoplasmic filament being fastened to the chromosome, 1 84 MARINE BIOLOGICAL LABORATORY. the latter will be carried by the former at its tip, and will be pushed forward as long as the filament continues to grow. Two opposing systems of the archoplasmic fila- ments behaving in a similar way, and lengthening in a contrary direction, would reduce the spherical nucleus first to a bi-concave disc, then to a Hat ** plate," and finally, into two parallel ''plates," each ''plate" travel- ling in an opposite direction. The interzonal filaments, then, according to this view, are the actual continuations of the archoplasmic filaments; but, instead of consisting of a single system, as at either end of the spindle, they are composed of two systems each dovetailing with the other, and growing in contrary directions. Intei'zonal filmnents are, therefore, the prolongations of the iiit^-a- miclear filaments. I am further inclined to believe that a certain optical peculiarity of the interzonal region, as, for instance, its aversion to take stains, is due to the existence in it of a proportionally large number of non- stainable archoplasmic filaments. Having briefly sketched the general outline of the process by which the characteristic shape of a caryo- kinetic figure originates, it would be appropriate to devote a few words to the obscure point as to the origin and movement of the archoplasm itself. But as a matter of fact, we know as yet very little in regard to the origin of the archoplasm, sometimes wdth a definite body in its centre — the cciitrosome. A great authority like van Beneden looks upon it as a permanent organ of the cell, equal in value to the nucleus itself ; but the whole ques- tion of its origin and its apparent homologues, which pass by different names in different cells, is too compli- cated and obscure to be discussed in this place.. ON CARYOKINESIS. 185 The later history of the archoplasm is, however, better known. When we examine a cell at the close of caryokinetic division, we see a small nucleus with the archoplasmic sphere at one side of it, appearing some- what like a satellite of a planet. This small nucleus is one of the daughter nuclei of the previous generation, "N-^--, -^S Fig. 6. — Loligo. and is destined to become the mother nucleus of the next. Just as new nuclei arise by the division of the old one, so the new archoplasmic spheres also arise by the division of the previous one. In the Cephalopod blastoderm, the division of the mother archoplasmic sphere into two daughter spheres could be observed with sufficient clearness. In Ascaris, its division has been 1 86 MARINE BIOLOGICAL LABORATORY. most carefully studied by several investigators. At first the two daughter spheres he in close opposition, later they separate more and more widely. As each sphere has a system of radiating filaments, there is formed a little spindle (Fig. 6, S) where they come into contact. This spindle lies outside of the nucleus, and has nothing to do with the larger one which has been described already. The daughter archoplasmic spheres migrate further apart, and finally settle themselves on the oppo- site sides of the nucleus. Their effect on the latter is soon seen. That surface of the nucleus upon which the archoplasm rests soon shows signs of flattening, as was shown in Fig. 3. This polar flattening of the nucleus has been interpreted as due to the pressure exerted by the growing archoplasmic filaments. The growth of the filaments continues, and the effects it produces upon the nucleus, in the arrangement and distribution of the chromosomes have already been de- scribed. Compare in this connection the series of stages shown in Figs. 3, 4, 5, and i. The above is a sketch of the mechanics involved in the distribution of the nuclear substance in the dividing cell. The history of the formation of the spindle has been briefly given, and the mode of its activity has been suggested. The spindle, however, plays only a part in the production of the caryokinetic phenomena. The whole behavior of the chromosomes preparatory to di- vision, such as the transformation from a resting con- dition to a coil stage, followed by the longitudinal splitting of each filament, — phenomena which take place independently of the influence of the spindle, — has received no consideration, and, so far as I can see, ON CARYOKINESIS. 18/ has no causal connection with the behavior of the archo- plasm, although both tend to accomplish the same end, viz. the formation of two nuclei out of one. It is con- ceivable that one mother coil may sometimes split into two different kinds of substances, and the archoplasmic filaments play simply the part of a distributing agent in carrying these into opposite halves of the dividing cell. In view of the general theoretical conclusion regarding the intimate correlation between form and matter, and mechanism and function, such a view does not appear improbable; for, as has already been stated, the differ- ences of two cells lie in their structure, and the struc- ture being the expression of the chemical substance of the protoplasm which compose them, wherever we find the difference of structure we find difference of sub- stance or substances, and wherever we find difference in the substance, we find difference in property or function. It is probable, as has been mentioned already, that the nuclear substance, by its constant metamor- phoses, gives rise to a series of substances to isolate and distribute which is the function of the spindle, and thus give rise to a number of differently constituted cells. In the study of caryokinesis, then, Cytology and Morphology may properly be said to meet ; and the rela- tion which it bears to the broad field of Embryology may not be unlike that which the latter bears to Anatomy. If, in conclusion, I recapitulate what has been said in a few words, the cleavage of the ovum may be charac- terized as Analysis of tissues, caryokinesis as the Method, and the archoplasmic spindle as the Instrument. NINTH LECTURE.^ -»<'X»4«><^ THE EAR OF MAN: ITS PAST, PRESENT AND FUTURE. By HOWARD AYERS. The study of the human ear, its structure and its functions, is not alone a modern endeavor on the part of man to *'know" himself; for history tells us, that the Greek philosophers and anatomists sought, both by dissection and physiological experimentation, to ascer- tain the anatomy of the organ, and how it is that we hear by the ear. The extent of their knowledge we do not know, for most of their anatomical treatises are lost to us. Empedocles, 473 years B.C., referred the perception of sound to the cochlea ; and Aristotle (384 b.c.) was acquainted not only with the internal ear of the higher vertebrates, but with that of fishes as well. The Egyptian school of anatomists does not seem to have progressed beyond the knowledge borrowed from the Greeks. 1 The memoir, of which this lecture is an abstract, will be issued in the jfournal of Morphology ^ Vol. IV., No. 3. 188 THE EAR OF MAN. 1 89 At the present day we owe our knowledge of audition and its apparatus mainly to the French, German and Swedish anatomists of the past half-century. For not- withstanding: the fact that the earlier Italian school had paid much attention to the auditory organ, they did not make any noteworthy contributions to our knowledge. With all honor to the large number of workers in this field who have added facts from this side and that, and without whose labors it would have been far more difficult for the more recent writers to have accomplished their comparative studies and to have formed their generalizations, there are a few names among them which stand out in greater prominence than the rest, and to whose investigations we owe the facts which, when properly combined, furnish us with a solution of the problems of the origin of the ear and of its exist- ing condition. They further allow us to predict with reasonable certainty the course which it will pursue in its further development. In other words, their in- vestigations of the ear enable us to understand its past, its present and its future. First among these names are those of C. Hasse of Germany and Gustav Retzius of Sweden, John Beard of England and E. P. Allis of our own country. The two first mentioned have, by extended investiga- tions into the structure of the adult condition of the internal ear of a large number of species, representing every important group of vertebrates, built up a com- parative anatomy of the internal ear upon an anatomical basis alone. They have also given accurate descriptions and figures of the forms thus studied, which render IQO MARINE BIOLOGICAL LABORATORY. possible the comparison of the embryonic conditions with the adult, and the establishing of important phylo- genetic conclusions based on these relations. John Beard first clearly saw the relation existing between the sense-organs on the surface of the body and the organ enclosed within the head by the for- mation of the auditory vesicle (the auditory labyrinth of the adult), but he failed to apprehend the nature of the involution and the relation of the sense organ to its walls. E. P. Allis, by giving us the first thorough investi- gation of the relations of the sense-organs of the lateral line system to the surface of the body, in the account of which he describes the method of their enclosure within canals, the laws of their increase, and of the fusion and division of canals, the process of their sink- ing below the surface of the body, and of their enclosure within the cartilage (or the bone) of the skeleton, — made known the developmental plan according to which not only the superficial canal sense-organs grow and reach their adult relations, but also, as it is now dis- covered, the plan according to which the ear sense-orga7i divides and forms its canals and other parts. This latter fact is demonstrated by the published results of numerous embryological investigations on animals from the several classes of vertebrates. In order that we may fully recognize the influence these investigations have had on the solution of the problems concerning the phylogenetic history of the human internal ear, it will be necessary first of all to understand the anatomy of the ear in some type form. For this reason I ask your forbearance while I briefly de- THE EAR OF* MAN. IQI scribe the internal ear of one of our American Torpedoes or Electric Rays {Torpedo occidentalis), and bring into comparison with it, part by part, the homologous struc- tures of the ears of Myxine, or the Hag-fish, and Petro- myzon, or the Lamprey Eel, as two representatives of the Cyclostome type ; the Alligator as a representative of the reptilian type, the Thrush {Miimis) to illustrate the avian condition, and the ear of Man as typical for the mammalian group. • You will find the vertebrate internal ear described, in the text-books and special memoirs dealing with this organ, as a paired sense-organ, occupying a position on either side of the head behind the eye. It resembles in the oreneral features of its relations to the head and nervous system the other organs of special sense. One prominent feature is its constant position between the roots of the fifth and tenth cranial nerves, which in the higher vertebrates especially, may be said to be pushed cephalad and caudad respectively by the growth of the ear. The first trace of the ear seen in the embryo is, as shown in Fig. 7, a simple saucer-shaped depression of the ectodermic epithelium on the dorso-lateral region of the head, not far removed from the gills. This thick- ened saucer sinks into the head, and divides into two parts, a superior and an inferior, or an utriculus and a sacculus as they are called. From the former the semi- circular canals are differentiated, while the latter portion gives off the aqueductus vestibuli, or the endolymphatic duct, and the cochlear canal. These parts by further growth are converted into the auditory labyrinth of the adult. 192 MARINE BIOLOGICAL LABORATORY. Leaving out of consideration all other developmental processes connected with the completion of the auditory organ, such as its enclosure in mesodermic tissue which solidifies to form the ear-capsule (the bony labyrinth of the adult ear of higher forms), I wish to call your attention to two points of great importance for the cor- rect understanding of the morphology of the ear. Torbedo .,, loccidenTali Fig. I. — The left internal ear of Tor- pedo occidentalis dissected out of its cartilaginous capsule, and viewed from the outside. The somewhat dia- grammatic figure represents the ear about twice its natural size, as found in a fish five feet in length. A"' Anterior ampulla. A*' Anterior canal. ac Auditory nerve. c.e Endolymphatic canal. CO Utriculo-saccular cone. H^ External ampulla. //< External canal. mu Macula utriculi duct. / Lagena and papilla lagenae. P Surface pore of endolymph canal; jits sac. P* Posterior ampulla. Pt Posterior canal. SC Sacculus and its macula sacculi. U' and U" Utriculus and utriculo-saccular chamber. \lu* O.C I St. The ear vesicle does not divide, as previous investigators have supposed, into superior and inferior portions, but into an anterior and a posterior chamber. 2d. The semicircular canals are not given off from the utriculus alone, but from the saccnlus as well. With these two misconceptions corrected, the way is clear which leads to the really simple-as-the-truth account of the morphology of the vertebrate ear, and we shall never again encounter an auditory labyrinth. THE EAR OF MAN. I93 An inspection of the Torpedo ear (Fig. i) shows it to be composed of a number of more or less curved tubes, three of which approach a semicircular form, and have been on this account, since their discovery by Fallopius in 1 561, called the semicircular canals. These three canals project from the central sac of the ear in three directions, and they lie approximately in the three planes of space. These planes do not coincide with the sagittal, horizontal and transverse planes of the body respec- tively, nor do the planes of any of the canals of the right side form continuations of the planes of the canals of the left side, or vice versa. The importance of these spacial relations will appear when we come to consider the physiology of the organ. One of the canals projects forwards and outwards, and is known as the anterior vertical canal (the terms supe- rior and sagittal canal have also been applied to it), another projects backwards and outwards, and is called the posterior vertical canal (the terms inferior and fron- tal have been given to it), while the third canal projects outwards and downwards nearly in the horizontal plane, and is called the external or the horizontal canal. Although it is usually stated in the text-books, that these canals occupy the three planes of space, and although, if not stated directly, it is none the less implied, that the canals also coincide with the three planes of the vertebrate body, — the sagittal, frontal, and horizon- tal respectively, — yet, we find that the very considerable and universal deviation from this hypothetical condition has been recognized by several investigators and the important bearing of the facts on auditory physiology at least in part recognized. I shall return to this further on. 194 MARINE BIOLOGICAL LABORATORY. Each of these canals opens into the main sac of the ear by its two ends, though this usual relation is not adhered to by all vertebrates, since some of the cartilag- inous fishes show the posterior canal separated almost completely from the main sac, communicating with it only by a small, much reduced canal, the two ends of the semicircular canal having been brought together and its two terminal openings fused into one. In Torpedo the proximal (superior) ends of the canals (Fig. I, near co), are brought together and empty into a common but very short tube, which serves to maintain the communication with the main sac. Before uniting, however, the external and posterior canals unite by communicating with a common ama,^ while the anterior canal swells out into an ama of its own. There is no constancy in the relation of the ends of the canals to each other or to the main sac, and, as we shall see later on, the matter has no great morphological or physiologi- cal importance. The enlargements at the proximal ends of the canals are found among most of the lower forms and occur in the divslopment oi the higher groups, usually disappearing before maturity is reached. The distal (inferior) ends of the canals are swollen into globular bodies, the ampullae, which are usually flattened on one side by the entrance of the nerve branch, which breaks up into a brush of fine fibrils as it enters the ampullar wall. The nerve fibrils are distributed to the sensory cells of the crista acustica or transverse ridge, which ^ I shall call the superior terminal enlargements of the semicircular canals amae, from the Greek a?na, — ?i water-vessel, — since, unhke the ampullae or the inferior terminal enlargements, they contain no nerve end organs and are mere fluid-holders. Their significance is unknown to us. THE EAR OF MAN. 195 projects from the bottom of the ampulla into its cavity and bears on its free surface the hair-bearing sensory cells constituting the percipient elements of the nerve end-organs of the ear. Each ampulla is connected with the main sac by a shorter or longer tube (Figs, i-6), the length of the tubes varying with the canal, and in greater degree with the animal species under consideration. In the Torpedo the ampullar tubes of the anterior and the external canals join before opening into the utriculus (Fig. I, near U'), while the posterior canal has a sep- arate and very large tube which opens into the saccular division of the main sac (Fig. i, near U"). So far as the main sac of the Torpedo ear is con- cerned, it appears when viewed from the side (Fig. i) to be composed of three large communicating chambers, — 6'7 U,'^ and SC. When, however, it is seen from the inside, the chamber U'^ appears as a median vesicle from which are given off cephalad the utricular recess U,' and caudad the saccular recess SC, and the lagena. This central portion, then, is the connecting chamber of the primitive ear-sac, and is composed in part of the original sacculus and in part of the original utriculus. This chamber, as you see, is produced into a conical body, CO, which is continued outward to the surface of the body by the long and slender canal c.e. Shortly before the slender tube or endolymphatic duct reaches the surface it presents an enlargement in the form of a broad sac which lies just below the skin of the dorsal region of the head. The duct is continued beyond the sac where it pierces the skin, and thereby places the cavity of the ear in communication with the surrounding 196 MARINE BIOLOGICAL LABORATORY. medium. This duct communicates with some of the sense-organ canals lying in the skin, and is innervated by one of their nerves. The sense-organs, six in number, with perhaps a trace of a seventh, are placed, three in the ampullae of the semicircular canals, as already described, these being desis^nated the cristae, while the other three are known as the maculae and papillae acusticae ; the one in the recessus utriculi, 6V being the utricular macula vui, while the one in the saccular recess SC is called the macula acustica sacculi. It has budded off the lagenar spot, or papilla acustica lagenae, /. These organs are derived from the division of a single parent organ, as we shall see later, and hence their dif- ferences of form are secondary. The trunk of the auditory nerve (Fig. i, ac) is composed of two branches, each of which gives off three branchlets to the anterior and posterior divisions of the ear respectively. We may thus speak of an utricular and a saccular branch, each supplying the structures belonging to {i.e. de- veloped from) its part of the ear. In Torpedo the anterior branch or the ramus utriculi gives off one branchlet to the crista of the anterior ampulla, another branchlet to the crista of the external ampulla, and a third to the macula utriculi. This latter branchlet is in reality composed of several branchlets ; for the macula utriculi is a compound sense-organ, as the sequel will show. The posterior or saccular branch gives off one branchlet to the crista of the posterior ampulla, a second branchlet to the macula sacculi, and a third to the papilla lagenae, while there is possibly a fourth THE EAR OF MAN. 197 o-iven off from the ampullar branch to an abortive ampullar organ for which Retzius proposed the name of macula neglecta. In the figure of the Torpedo ear this organ would lie in the ampullar tube of the posterior canal about where the horizontal canal crosses it, and its nerve would be in the figure entirely hidden from view. Both Hasse and Retzius, after many years spent in the comparative study of the internal ear, have decided that between the ears of the Cyclostomata and the ver- tebrates of the gnathostome type there was no discern- FlG. 2. — The right internal ear of the Hag- fish {Myxine glutinosa)^ seen from the inside or cerebral face. Figure after G. Retzius. The figure represents the ear somewhat enlarged, and does not show the shape or exact positions of the contained sense-organs. mu,. a Anterior ampulla. ap Posterior ampulla. c Anterior and posterior canals. ^": I Ampullar ends of the same. d Ductus endolymphaticus. ifi7i Macula utriculi et sacculi. n Nerve branchlets. u Utriculo-sacculus. J Sacculus endolymphaticus. ible basis of homology. The gulf separating the two types prevented the recognition of such genetic rela- tionships as might exist. Since my studies lead to a very different conclusion, a short account of the salient features of the Cyclostome ear, introduced here, may serve as a preliminary to the consideration of the mor- phological laws governing the development of the ver- tebrate ear, alike in its ontogeny and its phylogeny. The internal ear of Myxine (Fig. 2) is the simplest known among vertebrates. It is divided by a vertical line passing through the letter c, the sac of the endo- 198 MARINE BIOLOGICAL LABORATORY. lymphatic duct (s), and the duct itself into two quite symmetrical portions. The anterior corresponds to the anterior part of the Torpedo ear, and there is a similar correspondence between the posterior halves of these two ears. The main points of difference are, ist, The sense-organs of the Myxine ear are only three in num- ber, or at most four. 2d, The canals, only two in number, and they do not communicate by their amal ends with the main sac of the ear, or the utriculo- sacculus, for here the chambers are not separated though distinctly marked off. 3d, The endolymphatic duct does not communicate in the adult with the sur- face of the body. The auditory nerve, however, has the same number of divisions, — two, — and they supply the utriculus with its anterior ampulla, and sacculus with its posterior am- pulla, respectively, the same as in the Torpedo ear. Fig. 3. — The right internal ear of a Lamper-eel (^Peti'omyzon Jluviatilis), viewed from its inner or cerebral face. The figure after Retzius. ac Auditory nerve. n Utriculus. c Anterior canal. up Sacculus. cp Posterior canal. s Ductus et sacculus endo- n Ramus utricularis. lymphaticus. oc' " tip Ramus saccularis. In Petromyzoji the ear is much more compact than in any other known form ; the canals, though two in number and well developed, do not rise above the sur- face of the utriculo-sacculus ; and although they fuse together at their proximal ends as in Myxine, they also open into the main sac by a short, small tube. The whole structure is divisible by a vertical passing through the apex and the middle of its base into anterior and THE EAR OF MAN. 199 posterior divisions, and the parts thus separated are very much the same as in Myxine, showing but sUght modifi- cations of the floor of the sac in the direction of a Fig. 4. — The right internal ear of Alligator mississippiensis, seen from the outer face. Figure after G. Retzius. The figure outlines do not bring out the relations of the parts distinctly, and only the more important parts are lettered. ac Auditory nerve. b Pars basilaris cochleae auct. c Anterior canal. ch External canal. cp Posterior canal. / Lagena. n ' Cristae acusticae of the ampullae. s Sacculus. Fig. 5. — The right internal ear of T Urdus inusica, seen from the inner or neural face. F^igure after G. Retzius. The letters are placed only on those parts of the organ plainly visible in the figure. ac Acoustic nerve. c Anterior canal. ch External canal. cp Posterior canal. d Ductus endolymphaticus. / Lagena. ms Macula sacculi. n Lagenar nerve. «' Cristae acusticae of the ampullae. differentiation of the sense-organs contained, and in the formation of chambers for the reception of the new organs thus budded off. Though sUght, this change 200 MARINE BIOLOGICAL LABORATORY. is very important, for it introduces us to a series of changes which have led to the production of the gnathostome type of ear. The reptile, bird, and mammal ears furnish us with a progressive series in which the parts already present in the Torpedo are carried to successively higher degrees of perfection. The most noticeable change from the Torpedo condition is the much elongated lagenar re- gion, or, as it is called in the mammalia, the cochlea, which by means of the high differentiation of its con- tained sense-organ has risen to first rank among the auditory sense-organs. The utricular and saccular regions are relatively much reduced. The semicircular canals have under- gone a similar reduction, and among the birds and mam- mals appear more nearly semicircular or more evenly curved than in either the reptiles or the fishes. All of these ears retain traces of the primitive division into anterior and posterior chambers, and the relation of the nerves described for the Cyclostome and the Tor- pedo obtains throughout the group, except, of course, where a given sense-organ has divided, when, as is fre- quently the case among the higher types, two or more nerve branchlets are present where only one existed in the lower type. In Fig. 6, which is a figure of the developing human ear seen from the outside, the anterior and external ampullce are intimately related to each other and to the utriculus U, while the posterior is given off from the sacculus. A line passed through the space between U and 5 of the figure, and continued so as to pass out just above the ampulla of the posterior canal, divides the ear THE EAR OF MAN. 201 into its anterior and posterior halves, which, owing to the distortion produced by the enormous growth of the Fig. 6. — The left internal ear of a human embryo, 22 mm. in length, seen from without and below. Figure after W. His., Jr. The figure is from a model constructed from serial sec- tions, and represents the ear much magnified. a Anterior canal. am Ampulla. am' Amae. The middle reference line is superfluous. Cochlea. Ductus endolymphaticus. External canal. Sacculus in the restricted sense; really only the recessus sacculi. Utriculus. c d h s u cochlea, now appear as superior and inferior portions of the canal system of the ear. The development of the special sense-organs of the lateral line in A^nm or the ganoid Dog-fish, as made known by Allis, gives us the key to the solution of the problem of the homologies of the parts of the internal ear. In this fish the inclosure of the canals and the formation of the pores and tubes, while it is undoubtedly the primitive process, is essentially a simple and regular process, and when it is fully carried out t/ie canals arise in separate sections^ each of which contains a single sense- organ. In the young Dog-fish, in which the canals have not begun their development, the sense-organs lie below the surface and may be traced as more or less continuous whitish lines. "These lines mark general and exten- sive surface depressions." From the bottom of these 202 MARINE BIOLOGICAL LABORATORY. general surface depressions the sense-organs sink down forming, as they do so, small pits, at the bottom of which the organ lies. The lips of the pit grow upward and inward, and meeting above arch over the pit forming the beginning of a canal. This arch grows away from the pit in two directions until it meets another canal, with which it fuses, or until its energy is spent, when it comes to a standstill and remains a longer or shorter canal open at both ends, possessing somewhere near the middle of its course the canal sense-organ which gave the first impulse to the development of the canal. The sense-organ lies in a pit which represents what we know in the ear canals as the ampulla. The lateral line sense-organs are then supplied with, or lie in, ampullae. The canals being formed in short sections, one to each sense-organ, and fusing as they do to make longer canals, we should expect the compound canal at the point of fusion of its two components would retain a pore connection with the surface from which the canals were formed. Such is the case. Again, in the division of a canal organ and its canal, we should expect to find the canal retaining its surface communication by means of a single pore, if the division did not progress to completion, and we find this occurs regularly in the development of canals. The reverse of this process also occurs, in which the pore divides first, while the parent canal may remain undivided. We shall see later on that the ear canals fuse and retain their communication with the surface by means of a single pore, and that in one instance a canal organ divides, bringing about an incomplete division of the original canal, so that both canal organs communicate THE EAR OF MAN. 203 with the mother surface by means of a single pore. This is true in the case of the anterior and external ear canals of Torpedo (Fig. i, U'). We shall also find that the division of a canal organ may be barren of result so far as the production of a separate canal is concerned, and that in this case the offspring may retain its position in the parent canal. As an instance of this condition in the ear we have the macula neglecta (Fig. 7, ma), which has arisen by the division of the parent organ, the primitive sense-organ of the posterior ampulla. Fig. 7. — The right internal ear of the European Adder {Tropidonotus natrix), seen from the inside. Figure after Dr. Kuhn. This ear shows very distinctly the division into anterior and posterior chambers, especially in the arrangement of its sense-organs. a Anterior ampulla. ca Anterior canal. ch External canal. cp Posterior canal. d Ductus endolympLaticus. / Lagena. via Macula acustica neglecta of Retzius. ins Macula acustica sacculi. n Crista acustica ampullarum. p Ampulla posterior. pb Pars basilaris cochlear auct. u Utriculus. The early history of the Elasmobranch auditory cap- sule does not differ materially from that of other forms except in the rate of growth. In this respect it is a much more favorable object for 204 MARINE BIOLOGICAL LABORATORY. the study of the development of the internal ear than any vertebrate with which I am acquainted. Fig. 8 shows the auditory involution after it has assumed the saucer- shape already alluded to. This condition is the result of a concomitant thickening and sinking of the ecto- derm, which over the area of the saucer represents one of the canal sense-organs of the lateral line system, and the whole process from the beginning on is but a Fig. 8. — The head of an embryo Shark {Acanihias vulgaris), ixovci nature, magni- fied 20 times. The figure shows the saucer- shaped depression containing the insinking sense-organ which is to be converted into the auditory sense-organs of the Shark. a.v Auditory saucer (vesicle). ep Epiphysis. / Fore-brain. g Gill region. ^, I Upper and lower hind-brain region. m Mid-brain. ^ repetition of the formation of the canal organ and its canal, as seen in Amia. As the saucer-shaped thick- ening sinks below the surface, the opening on the surface grows smaller, the bottom of the saucer in- creases in size, and the resulting structure is an auditory vesicle distinctly flask-shaped (Fig. 9, a. v). The neck of the flask grows longer, and finally appears bent backwards and inwards, owing to the increase in size of the head in this region, which causes a transla- THE EAR OF MAN. 205 tion of the auditory capsule forwards and outwards (Fig. 10, d). Fig. 9. — The head of the embryo smooth Dog-fish (^Galeus cajiis), seen from the left side. Figure drawn from nature, magnified about 20 times. The flask-shaped auditory vesi- cle is shown prominently placed above the gill region. Letters as in the preceding figure. a.v Auditory flask. e Eye. y Fore-brain. £ Gill region. h Hind-brain. m Mid-brain. n Nose. Fig. 10. — The head of an older Shark of the same species, viewed from the left side. The figure, which was drawn from the living fish, shows the internal ear well advanced in its development. The rudiments of the semicircular canals and lagena are seen pushing out fcom the auditory sack (respectively the utriculo-sac- culus, which has sunk far below the surface, with which it is, however, still connected by its sickle-shaped ductus endolymphaticus). a.v Auditory vesicle. cp Posterior canal. c Anterior canal. I Lagena. d Ductus endolympha- m Mid-brain. ticus. h Hind-brain. e Eye. n Nose. f Fore-brain. u Umbilical cord. ch External canal. During this lengthening of the neck of the flask, which becomes the surface canal of the adult ear, the body of the flask is much changed in shape. First of all, it becomes compressed laterally, and is 206 MARINE BIOLOGICAL LABORATORY. then drawn out in an antero-posterior direction. During this process the anterior end comes to point outward, while the posterior end is directed inwards toward the median line. There may now be seen several changes in the shape of the vesicle which, while not conspicu- ous, are very important, since they usher in a succession of transformations which ultimately produce the three aw* Fig. 12. — The internal ear of a some- what older Pig embryo, ^;^ mm. long. Figure after Krause. a Anterior canal. atn Ampulla. d Ductus endolymphaticus. I Lagena. P Posterior canal. s Sacculus. u Utriculus. Fig. II. — The ear of a very young Rabbit embryo, 1 1 mm. long. Figure after Krause. a Anterior canal. atn Ampulla. d Ductus endolymphaticus. / Lagena. 5 Sacculus. u Utriculus. semicircular canals and the rudiment of the cochlea. These changes are visible on the outer and upper faces of the vesicle as slight ridge-like elevations of the sur- face, and on the posterior ventral end of the vesicle as a knob-like prominence (Fig. lo, c, c/i, and a.v). Of the former there are two, of the latter a single one. The THE EAR OF MAN. 207 former structures seen from inside the vesicle are merely depressed grooves in the wall of the vesicle, and the latter a sunken but broadly open pit. These grooves grow deeper, their edges grow together, first, along the middle part of their course, thereby producing canals. The lips of the pit fuse by reaching across the middle of the opening, and thus produce the beginning of the posterior canal, which, by its continued growth upward, soon reaches and fuses with the anterior canal. The terminal pores of these two canals, at what proves from later events to be their amal ends, fuse and open into the vesicle by a single pore. Fig. 13. — The left internal ear from a human embryo, 13 mm. long, about the fifth week of development. a Anterior canal. am Ampulla. d Ductus endolymphaticus. h External canal. I Lagena. P Posterior canal. s Sacculus. u Utriculus. The introductory stages in the development of the human ear are but a repetition, as regards essentials, of the process as I have described it in the shark. When fully cut off from the exterior, the auditory vesicle forms a compressed sac having an irregular 208 MARINE BIOLOGICAL LABORATORY. quadrangular shape. This condition is found in embryos of tliree and a half to four weeks. The utricular and saccular divisions are readily distinguished, and even now the semicircular canals have begun to differentiate by the formation of two shallow depressions in the walls of the utriculus. Both the pocket common to the vertical canals and the pocket for the external canal are present. Below the latter is seen the larger and deeper evagination for the cochlea. When the embryo has reached the length of 8 mm., the cochlea has so far developed as to stand out distinctly from the sacculus, and is at the same time bent (Fig. it, /). At both ends of the common pocket of the verticals, and at one end (anterior) of the external depressions, are slight enlargements — the future ampullae. From now on, the parts rapidly acquire individuality. The anterior canal is completed first. Then the posterior becomes cut off, and soon after the external is perfected. The whole vesicle undergoes a marked change in form during this period, for, owmg to the growth of the ductus endolymphaticus from the apex of the primitive vesicle, and the great elongation of the cochlear tube from the bottom of the saccular region, the canal com- plex now appears drawn out in a dorso-ventral direction. The cochlear canal has now, about the fifth week, one half a spiral turn, and a long groove has appeared on the inside of its wall (fold on the outside), from which the nerve end-organ — Corti's organ — arises. In the embryo of 30 mm. (Fig. 6) length the canals are well formed, and the ampullae are quite prominent. The two verticals which from tJieir viode of development have up to this time occupied the same planCy now begin THE EAR OF MAN. 2O9 to diverge, and the planes of the canals now meet in the middle of the common tube which unites them. Their angle at this time is 150°. This method of origin is most interesting and impor- tant in its bearings on the spacial relations of the adult ear canals. The cochlea has now made nearly a whole spiral turn. About the second month the internal ear has assumed the foetal conditions, and its development from this point on consists in the perfection of the parts already marked out. The canals and ampullae have acquired their adult characters, and the cristae are distinctly formed, although the sensory cells have not acquired their mature structure. Each canal has at this stage a noticeable swelling at the opposite end from the ampullae. In these you will recognize the amae which I described in the fish ear as an adult characteristic. In the human ear the ama of the ex- ternal canal is the only one usually persisting beyond the foetal condition. The cochlea has continued its development, and is readily seen in the greater number of spiral turns it has acquired, and the row of sense-organs, formed by the budding of the original cochlear sense-organ or papilla lagenae, has now several hundred discreet sense-organs which are so closely related in structure and function as to pass as one organ, the organ of Corti. One very important matter belonging with the ana- tomical facts I intentionally neglected, for, until we had gained a knowledge of the course of development of the ear as a whole, and of the details of canal formation in particular, we were not likely to fully appreciate its bearings on the problem of the inter-relation of the 210 MARINE BIOLOGICAL LABORATORY. sense-organs of the auditory complex and of their canals. It is the morphological value of that sense-organ which, ever since its discovery by Retzius, has been such a source of discussion and investigation ; viz. the macula acustica neglecta, supposed by some to belong to the utriculus J by others, to the sacculus in the older sense. A B i^J Clo OQ CD €> i> O Fig. 14. A Auditory vesicle. B Utriculo-sacculus. C Utriculo-sacculus -I- two ampullary canals. D Utriculo-sacculus -t- two ampullary canals and lagenar canal. Stage A represents the undivided superficial sense-organ of the verte- brate ancestor, as it is invaginated from the surface and enclosed within the THE EAR OF MAN. 211 auditory vesicle to function as the macula acustica vesiculi ; the only auditory sense-organ of this stage. It is but little removed from the canal organs as they exist in, e.g., Amia, and differs from it mainly in size. This condition is not represented in the adult of living vertebrates. Stage B represents the first division of the macula vesiculi, into its two offspring the maculae acusticse utriculi et sacculi. This stage is likewise not represented among living forms. Stage C is so characteristic of the Cyclosto7naia, so far as we know them, that we w'U call it the Cyclostome stage. Here the cristas acusticae anterior et posterior have made their appearance. Stage D shows the condition of the organs in the Gnathostomata, hence its name the Gnathostome stage. The cristae acusticae anteriores, horizon- tales, posteriores and abortivae are all developed by the division of the two parent ampullary organs of the Cyclostome ancestral stage, while the maculae utriculi et sacculi undergo division, giving rise to the parents of the utricular and saccular complexes of sense-organs. These latter reach their highest differentiation in some rodentia and porcine species. Retzius finally came to the conclusion that this nerve- end organ had arisen from the posterior, ampullar sense- organ, and that among the higher forms, especially the mammalia, it was no longer produced, or, as he expressed it, the macula neglecta in these forms had disappeared in the prista acustica posterior. After an examination of all the evidence bearing on this question, both from the embryological and the anatomical sides, I have solved the problem of the morphological value of the parts of the internal ear and their inter-relationships, by the discovery of the very simple law which governs their origin and suc- cession. Let me give you the whole problem in a nutshell, even at the risk of some slight repetition. The primitive auditory sense-organ is invaginated from the surface of the body, and may be said at this time to be in the vesicular stage (Fig. 14, A). So far as we know, this condition is not retained by any adult 212 MARINE BIOLOGICAL LABORATORY. living vertebrate. And it is quite possible that the auditory saucer may contain the rudiments of the two primary sense-organs of the Cyclostome ear. The parent sense-organ soon divides transversely into two nearly equal parts, which are the anterior and pos- terior sense-organs respectively. The auditory vesicle is at the same time partly separated into two cham- bers (incomplete canals) to accommodate them. These chambers are the utriculus and sacculus. c.r r.p. viyzon. Fig. 15. — The Cyclostome type ear shown here is a very simple structure as compared with the complicated organ found among the Gnatho- stomata, but compared with its an- cestral condition, i.e. the auditory vesicle, it is seen to have gone through many changes, and stands to-day midway between its earliest condition and the highest differen- tiation known. This diagram is con- structed on the basis of the anatomy of the only two known forms of this group, Myxhie and Petro- The canals and their organs retain a very primitive condition of structure. caa Anterior canal. cap Posterior canal. cr. a Crista acustica anterior. cr. p Crista acustica posterior. Endolymphatic duct. The utriculo-sacculus. and 2 Branchlets of the utricular branch of the auditory nerve, and 2 The saccular branches of the same. d n.s w I J I ode mac. 71 mac. s I and 2 Terminal portion of the endo- lymphatic duct, which in neither of these Cyclostomes opens on the surface of the body in adult life. Macula acustica utriculi. Macula acustica sacculi. The portions (i) of the sense- organs that remain in the parent cavity and the portion (2) which migrate into the re- cessus utriculi and lagena respectively. Each of the two sense-organs of the second genera- tion after a while divides into two unequal parts in such THE EAR OF MAN. 213 a manner that the smaller sense-organ appears as a bud from the parent. There are thus formed within the two chambers of the ear four canal sense-organs be- longing to the third generation. The two external organs are soon enclosed within the ampulla of two complete and relatively large canals, which are now formed about them. An anterior vertical and a pos- terior vertical formed in the manner already fully de- scribed. Up to this time the organs have retained the primitive relations to each other — a serial arrange- ment along a line running in an antero-posterior direc- tion. Now, there begins a distortion of the structure as a whole, which continues ever after, and reaches its greatest development in the mammals — viz. a pro- cess of sinking and drawing out ventrally of the posterior chamber of the ear, so that hereafter we might speak of a superior and an inferior portions or chambers. The sense-organs of the third generation play the leading role in these changes, and by means of four sets of divisions, viz. by the bipartition of each sense-organ present in the Cyclostome stage nearly simultaneously there is produced a fourth generation of canal sense-organs. To this fourth generation belong all the sense-organs of the internal ear of the higher vertebrates which in the diagram are numbered eight to fifteen consecutively. During this process of sense-organ differentiation, the canals have been variously modified, and to show the relationship of the third generation to the fourth let us examine the diagram shown in Fig. 15, in the construc- tion of which I have ignored the ventral distortion spoken of. 214 MARINE BIOLOGICAL LABORATORY. The sense-organ numbered six in the Cyclostome stage (the crista acustica anterior, or anterior canal o.d.e. 9ia. t.a.a cr.a.post. Fig. 1 6. — The Gnathostome type ear shown here illustrates the method and the extent of the modifications which the Cyclo- stome ear has suffered m its descent to this group. The an- terior and posterior canal organs have each budded off an organ which, in the case of the anterior canal, produces a well-formed canal, the external, present in all the members of this group, w^hile the posterior bud never produces a well-formed canal. The utricular recess, which con- tains a number of sense-organs derived from the division of the macula utriculi, is represented as forming a tubular prolongation of the utricular cavity, in order to more clearly show the parallelism of this group of organs with the lagenar group. In no known form does the production of the canal proceed to the extent shown in the diagram. The sense-organs of the lagenar group are repre- sented as separated from each other by an interval, whereas the actual relation in nature is much more intimate. # N.vni. ca Anterior canal. cp Posterior canal. ch External canal. c. ab Abortive canal. u Utriculus. rec. utric Recessus utriculi. rec. sac Recessus sacculi. d Ductus endolymphaticus. s Sacculus. o. de Orifice of the same. organ, of Myxine and Petromyzon) has divided to pro- duce sense-organs ten and twelve of the Gnathostome stage, i.e. the cristae acusticas anterior et horizontalis, of the human ear. THE EAR OF MAN. 21 5 The sense-organ numbered four of the Cyclostome stage (the macula utriculi of Myxine and Petromyzofi) divides into the sense-organ eight and sense-organ com- plex fourteen of the Gnathostome stage, the macula utriculi of the human ear, which is, as I stated when describing the Torpedo ear, a compound organ com- posed of the parent organ and its numerous progeny, all of which remain close together. The sense-organ numbered seven of the Cyclostome stage is the sense-organ of the posterior canal of Myxine and PeU'oinyzon^ and has divided to produce sense- organs, eleven and thirteen, of the Gnathostome stage, or the crista acustica posterior, or the canal sense-organ of the posterior canal of the human ear, and the so-called macula acustica 7ieglecta of Retzius, which is the abortive second horizontal canal organ of the internal ear of man. Sense-organ five of the Cyclostome stage has divided to produce the sense-organ nine, and the sense-organ complex fifteen of the Gnathostome stage, or the macula sacculi and the lagenar organ respectively, the organ of Corti of the human ear. With the exception of the generalization that the vertebrate internal ear is derived from branchial sense- organs — i.e. organs of the lateral line system — a gen- eralization which we owe to Beard, but which he did not attempt to establish by any detailed anatomical or em- bryological investigations, the phylogenesis of the ver- tebrate ear has been studied only on the basis of the adult structure. Such structures as the semicircular canals and the divisions of the body of the auditory ves- icle have been used alone as the basis for proofs of the genetic connection of the types of internal ear — even 2l6 MARINE BIOLOGICAL LABORATORY. the nerve supply has been excluded from its just share in forming the conclusions. These canal structures had no special phylogenetic significance for investigators in this field ; for although it was known that the auditory vesicle was invaginated from the surface of the body, the connection of the sensory part of the invagination with the superficial canal organs was not understood. Now that we know that there is a genetic connection, many of the intricate problems receive their solutions, and the genetic rela- tionships appear clear and certain for all the types. The demonstration of these facts deals another blow, and a fatal one, at the degeneration hypothesis which has been so persistently applied to the elucidation of Cyclo- stome anatomy and development, with such pernicious morphological results. (This will apply equally to Amphioxus, though only indirectly in this particular instance.) This solution relieves us from the necessity of explaining away the vertebrate ear on every occasion, when we seek to establish a relationship between the vertebrate and the invertebrate types; for we are led to see the value of very simple, superficial, sense-organs in an undifferentiated group of animals, for the building up of structures of great complexity, and of intricate relations to other parts of the animal body in the members of higher forms, and also to recognize the manner in which the structure and functions of organs may be profoundly changed in the course of time. Now, while it is not known that Amphioxus is pro- vided with an organ of hearing, the next higher forms do possess an internal ear of such structure that not only does it help us to understand the more complex THE EAR OF MAN. 21/ Gnathostome type, but proves conclusively that, so far as the ear is concerned, the Cyclostomes have not suf- fered degradation of structure. Against the view that the completely closed or perfect canal is the primitive condition of this system, so far as existing vertebrates are concer7ied, one may raise the very plausible objection that since all the Holocephala and some Teleosts have the canal incompletely developed, as a more or less open groove on the surface of the body, and since the Cyclostomes do not retain the superficial canals except in very imperfect form during adult life, and further, since the closed canal is produced in ontogeny, and may have appeared in phylogenetic his- tory by the action of the same process,^ it is only reasonable to suppose that the open groove is the primitive condition, and constitutes a phylogenetic stage passed through in the development of the higher (?) types of closed canal. When, however, we consider that the internal ear is a very ancient structure, and that in all known cases the sense-organs developed in it become enclosed in com- plete canals, by a process exactly similar to that known for the formation of the superficial canal organs and their canals in the admittedly primitive ganoid type, we are compelled to admit that such a process could hardly have arisen independently within the closed capsule after its separation from the surface and its removal away from the external influences which originally caused their development, and that consequently there ^ e.g. the grooves present on the surface of the head and body of Chi- mcera and Tetrodons, could readily be converted into tubes by the fusion of their edges. 2l8 MARINE BIOLOGICAL LABORATORY. imist be a genetic relationship between the canal organs of the internal ear and tJie superficial canal organs ; and since there is not the sUghtest doubt that the super- ficial canal organs are the original or parent organs, we are brouo^ht to the unavoidable conclusion that the ver- tebrate intertial ear is a transformed canal organ. It follows from what I have said that the system of canal sense-organs must be a very ancient one, since it must have antedated the origin of the internal ear of the Cyclostomes. Although ontogenetic evidence seems to lead to the conclusion that the auditory organ arose by the in- vagination of a single superficial sense-07gan, it is by no means certain that this is true ; for there are certain facts of comparative anatomy and certain phylogenetic considerations which point to the conclusion that the auditory organ has arisen by the bringing together of two originally distinct sense-organs which were together sunk below the surface. The main facts at present in favor of this view are these : — 1. In all eared vertebrates the auditory organ is sup- plied from two distinct brain centres, one lying in close connection with the facial nuclei, the other intimately related with the glossopharyngeal nucleus. 2. In all eared vertebrates the so-called auditory nerve is composed of two distinct roots — an anterior and a posterior — which supply the anterior and pos- terior chambers respectively. 3. In all these forms the anterior root is external to the brain, united with the facial nerve. 4. In some fishes the nerve to the posterior ampulla is derived from the glossopharyngeal nerve, between THE EAR OF MAN. 2I9 which nerve and the posterior root of the auditory- there exists, however, a more proximal connection. 5. The auditory vesicle is always developed between the facial and glossopharyngeal nerve roots. 6. The ductus endolymphaticus is supplied on its anterior face by a branch of the utricular nerve, while its distal end, in some fishes, opens into a canal con- taining sense-organs innervated by the glossopharyngeal nerve. 7. The so-called eighth cranial or the auditory nerve must have arisen from branches of two distinct cranial nerves, and is not homodynamous with such cranial nerves as the fifth or tenth, as we now understand them. This is true {a) because the auditory sense- organs thus supplied were primarily only a portion of the canal sense-organs innervated by the original nerves of the preauditory condition of these sense-organs ; {b) because the auditory nerve is clearly not a complete nerve, and is not even equivalent to a dorsal root of a cranial nerve, for its two divisions are probably merely branches of the dorsal roots of the seventh and ninth nerves, since they draw off only a portion of the sensory fibres from these two nerves. The primitive division of the auditory chamber and its nerve supply into two so sharply marked portions is thus phylogenetically accounted for, and at the same time the early ontogenetic changes in the auditory vesicle receive their explanation. The two sense-organs, the maculae acusticae of the utriculus and sacculus, are thus derived from two organs terminating two separate canal systems which had, as they may still be seen in Amia, become confluent on 220 MARINE BIOLOGICAL LABORATORY. the surface of the body midway between the roots of the facial and glossopharyngeal nerves as they issue from the brain. At the point of junction the two half- pores united into a single pore, which in some fish forms {e.g. Torpedo) persists as the outer opening of the endolymphatic duct, and the only persisting indica- tion of the separate origin of these organs and their canals is their nerve supply. It is a necessary conse- quence of the great functional differentiation which the ear organs have suffered that their nerves should also become much increased in size, and instead of appearing now as nei've branches they have become really larger than the parent nerves from which they arose. As a prelude to the little I have to say on the physiology of the internal ear, I wish to emphasize the following considerations : We have very slender foundation indeed for correctly judging of the functional relations of the integral parts of the internal ear, and what we have is largely speculative, based on our knowledge of the structure of the parts. What we need at the present time is physiological experimentation. First of all, an extended series of varied, careful, and unbiassed experiments on the sense- organs of the lateral line system of the lower verte- brates, to determine their functions, and then more experiments on the internal ear of the least differen- tiated representatives of our type, to determine what functional modifications have arisen during the trans- formation processes. Finally, the combination of the knowledge thus gained, with the results of experiments upon the human subject. THE EAR OF MAN. 221 If proof were needed for the statement that most of the speculation and experimentation on the auditory function, especially as regards its different phases, has led mostly to negative results, it is easily forthcoming ; for there is not a single investigator who, during the last half-century has written on this subject, but regrets the paucity of facts and the depressing insufficiency of the prevailing theories. The amount of experimental knowledge of the func- tions of the lateral line organs is very limited indeed. Of the many theories which have been proposed Merkel's is by far the most satisfactory. According to this in- vestigator, the function of these organs is in all proba- bility that of receiving and transforming the mecJianical stimuli occurring in the surrounding medium, and it cannot be in any way connected with the perception of chemical changes in the water. Mayser and Emery consider the organs of the lateral line system as forming an accessory auditory organ, and Mayser has proved that these organs find their central brain connections in the immediate neighbor- hood of the auditory nuclei. The functions of the ear are usually separated into two classes by physiologists. The first and most prominent of which is audition, with its several subdivisions ; the second, though less prominent, not for that reason, however, less important, viz. equilibra- tion. The function of audition certainly belongs to the ear ; not so, however the equilibrious function ; for it can be conclusively shown that equilibration is not necessarily affected by the removal of the ear, and that injury to other organs is not unfrequently 222 MARINE BIOLOGICAL LABORATORY. followed by loss of the ability to equilibrate the body. An extended presentation of the arguments for and against the equilibration theory of the semicircular canals would be too long and unprofitable to justify introducing it here, especially so now that there is no longer the slightest evidence in favor of the theory, since the results of experiments by the physiologist Steiner have been published. It is commonly stated that the auditory nerve, besides possessing the func- tion of transmitting auditory impressions, also transmits stimuli to the equilibrious centre, and that the ampullae of the semicircular canals contain the sense-organs which subserve this function. The ampullar sense-organs may, however, be com- pletely severed from their respective nerves without producing any disturbance in the equipoise of the body. The section of the ampullar nerves, even though the greatest care be taken to prevent damage to the connecting nerves, must produce an intense stimulation of the central end cells, and it is wholly unexplained why the ampullar nerves may be thus cut without pro- ducing any visible effect if the equilibration theory were true. Experiments carried out in Hermann's laboratory in 1877, by Fraulein Tomaszewicz, showed that in bony fishes the semicircular canals and their ampuUary sense- organs could be entirely removed without in the least influencing the equilibration of the body. Another set of experiments, even more decisive, were performed by Professor Steiner in 1888, in the physiological department of the Naples zoological station, on the THE EAR OF MAN. 223 common Dog-fish of the Bay of Naples. Steiner's ex- periments proved that so long as the fibres of the acoustic nerve were neither pulled so as to disturb their central relations, nor displaced in a manner in- jurious to their peripheral terminations, disturbances in the equilibrium of the body did not make their appearance. While observing these conditions it was possible for him to cut out one, several, or all six of the semicircular canals witJioiU destroying or in the least disturbing the fisJi s power to equilibrate its body^ though the operation evidently caused the fish very painful sensations, i.e. very intense irritation of cen- tral end cells. Now under the old hypothesis of the equilibration functions of the semicircular canals, these removals without equilibrative disturbances would be impossible. Experiments of a similar nature performed on warm- blooded animals were formerly used as evidence in proof of the theory. In all of these experiments disturbances of the power of equilibration never failed to make their appearance. These disturbances, however, were due to the conditions under which the experiments on warm- blooded animals are necessarily performed, and are not directly related to the injury to the nerve from simple cuttino:. In Pisfeons whose semicircular canals have been more or less injured ^ or destroyed, the loss of coordinating power is certainly to be connected with the injury done to the richly nervous structures ; but it does not follow that this loss of coordination is due solely to the disturbance of the function of the canals, 1 As in the experiments by Elourens, Cyon, and others. 224 MARINE BIOLOGICAL LABORATORY. for it can be clearly shown that during the wounded state the bird is still able to equilibrate itself under certain conditions, and those conditions involve calling into use the tactile sense. The sensation of giddiness produced by injury to or destruction of the canals is due not to the injury of the ampullar sense-organs, but either to a disturbance of the centre of equilibration in the brain by mechanical injury to the cells of this centre, or to the cessation of the perceptions arising in part from the functional activity of the auditory mech- anism. It has been observed that a pigeon operated on by canal section or destruction can, and usually does, steady itself and properly direct and execute coordinated movements, the moment its sense of sight is aided by the tactile sense sufficiently to enable it to form a cor- rect judgment of its position in space and its relation to surrounding objects. Now, barring nausea or other nervous disturbance accompanying section among the warm-blooded animals, it is plainly true that the animal's failure to coordinate its movements lies in the fact that it forms false judg- ments of its spacial relations on an insufficient basis sense-perception, the constant stream of auditory im- pressions having been cut off, even in the milder experiments, by the disturbance of the pressure equi- librium of the endolymphatic fluid. On the basis of the evidence which had been pro- duced by the advocates of this doctrine, Milne-Edwards disposed of the kinetic and statical theories of the equilibration functions of the semicircular canals in jthe following words : " Mais les hypotheses propos^es THE EAR OF MAN. 22$ a ce sujet ne reposent pas sur des bases suffisantes pour que nous y arretions ici." Audition is the resultant of all the wave motions, transformed and transmitted to the auditory centre in the brain by the fibres of the auditory nerve. There are several kinds or conditions of auditory stimuli, depending, first, on the intensity or force of sound {i.e. the amplitude of vibration) ; second, on the pitch of sound {i.e. the number of vibrations in a given unit of time); and third, the quality or timbre {i.e. the result of the form of the sound wave). Upon the com- bination of these conditions, in varying proportions as they occur, depend the character of the auditory im- pressions. The variability in the powers of auditory perception among animals is much greater than is commonly sup- posed. As instances of increased sensitiveness above that of the human ear one may mention mice, cats, and other nocturnal animals. Galton found that the house cat among domestic animals possessed a wonder- ful capacity to perceive shrill tones, and it is a common experience that the cat's ear is super-sensitive to audi- tory stimuli. All vertebrates above the fishes are provided, like man, with a tympanic membrane and a chain of bones, or its functional equivalent, which serve to transmit vibrations to the internal ear ; but while we are accus- tomed to look upon this arrangement of the parts as presenting us with a greater perfection of the auditory apparatus than is present in the lower forms, it is not by any means proven that such is the case, for this 226 MARINE BIOLOGICAL LABORATORY. accessory apparatus is a ponderous mass, which only relatively powerful vibrations can set in motion. All vibrations of the air which do not succeed in moving this conducting apparatus are lost to us. It is for the purpose of making up this great loss that the external ear has been developed. Traces of accessory apparatus to place the internal ear in more favorable communi- cation with the exterior, begin to make their appearance Fig. 17. — A projection of the internal ear on a flat surface, to illustrate the relations of the sense-organs and canals of the Torpedo ear, when the auditory chamber is brought to the surface and spread out flat. c Canal. n" Macula neglecta. po Canal pore. tnl Papilla lagense. n' Crista acustica. tns Macula sacculi. inu Macula utriculi. c. de Endolymphatic duct. among the cartilaginous fishes, whose ears still retain their direct connection wTth the sea-water by means of an endolymphatic duct. This tendency is due to THE EAR OF MAN. 22/ the removal of the ear from the surface of the body, and is greatly increased as soon as the forms emerge into the poorer conducting medium of the aerial ocean. When the structure of the internal ear of the higher vertebrate has been analyzed with reference to deter- minins: its functions, we find that it is easilv reduced to the type of the canal organ on the surface of the body, and we should expect an essential harmony be- tween the functions of these two groups of organs, although in the case of the ear the functions would necessarily be in some degree modified to correspond with its modifications of structure. These latter, we know, have not been extensive, except in the case of the cochlea, and, strange as it may seem, we are not able to assign to the cochlea any function at all in keeping with the great complication of structure which it has undergone during its evolution from the lagena. The Helmholtz piano-string theory is entirely in- adequate to account for perception of musical tones by vertebrate animals, in that it can be ai^plied only to the higher mammalian forms, and leaves us to seek another explanation for the equal, if not greater, powers of musical perception possessed by some birds. What- ever explanation future investigation may disclose to us, it is safe to say that it will prove equally applicable to all vertebrate forms capable of musical perception, while being in perfect harmony with the then' per- fected knowledge of sound (tone) perception by means of lateral line organs. 228 MARINE BIOLOGICAL LABORATORY. In closing, allow me to recapitulate the more impor- tant features of oto-phylogeny, past, present, and to come. In the PAST the ear of man was a canal organ of the lateral line system of sense-organs. A system which has disappeared from the surface of the adult human body; but which still occurs in a reduced condition during embryonic development. All its organs regu- larly produced protective canals, and the auditory organ came to differ from the neighboring organs merely in size and the greater depth of its ampullar pit. At first its functions were identical with those organs of the same system, having the same nerve supply ; but as it was more and more removed from the surface of the body, it acquired greater protection against injuries and concomitantly greater sensitiveness. It increased in size and began the process of division, which has resulted in the organ as it exists at the present day. These changes required untold millions of years for their perfection ; for, since palaeontological science tells us that this same Amia or ganoid Dog-fish which has retained its surface organs in such a primitive condition, existed in its present form, at the very least, twelve mil- lions of years ago, you will readily conclude that the human mind can form no adequate conception of the period of time which elapsed since the ancestors of the Dog-fish were like the Cyclostome fishes in the structure of their ears. For we know that at the present time Amia has an internal ear of greater complication than the Torpedo, THE EAR OF MAN. 229 which I briefly described to you at the beginning of the hour. The auditory organ has played a long and very important role in the phylogenetic history of man. The auditory organ of man has no genetic connection with any of the invertebrate auditory struc- tures. At the PRESENT time the human ear is a canal organ complexus, constructed according to a simple and sym- metrical plan, whose ontogenetic history is a recapitula- tion of many of the stages of its phylogeny. During its development it is badly distorted, and the simplicity and symmetry of the plan of its construction is thus in a measure concealed. In its adult condition it is not the most highly differentiated ear known to us, for some other mammals are far more fortunate in this respect. It is an organ about whose functions we know little, since most of what is written in the text-books is pure speculation, much of which is proven to be without foundation, if, indeed, it is not directly controverted by recent experiments. It subserves the auditory function, and has no more to do with the equilibration of the body than many other organs, especially those of the higher senses. Its canals are not organs of equilibration. The FUTURE of the human ear can only be foreseen in terms of its past and present. It will probably de- velop functionally much more than structurally. Its semicircular canals will gradually undergo reduction, while its utricular and saccular complexes of sense- organs will increase in size and perfection of structural detail, in response to the increasing demands for tonal 230 MARINE BIOLOGICAL LABORATORY. differentiation; it will, nevertheless, remain in the human descendants of the distant future, whatever their form may be, however keen or diversified their auditory function may become, merely A transformed CANAL organ. TENTH LECTURE. -ooJV 5''^j 75» i<^Oj 15O' 200, 300, 400, and 500 fathoms. After these lines had been run out, some of them were repeated under different meteorological conditions in order to note any changes which had taken place. The above observations were accompanied by an hourly set of meteorological records, which were compared with the regular series of the Signal Service offices of New York and Boston. The work was limited to about two months, in which time 136 such stations were studied, 1600 water temperature observations were taken, 300 observations were made upon the specific gravity of the water, both at the surface, the bottom, and at inter- mediate depths; and in addition over 10,000 general meteorological observations were recorded. These ob- servations have all been reduced and plotted, and a report, embodying the results and comparisons, is now in process of publication. The report shows, in brief, that an important influence is exerted by the winds, forcing the warm surface water of the Gulf Stream to a considerable distance northward towards the coast, when blowing from the south, and that this warm body of water swings back again, when the wind blows from the opposite direction. This warm water, which affords the proper temperature conditions for the development of the forms of marine life upon 248 MARINE BIOLOGICAL LABORATORY. which the fish feed, is crossed and followed by the schools of fish. In this way the cold coast current is bridged over, so to speak, and the fish brought within reach of the fishing fleet. As the warm water approaches the coast, it seems to be broken up into bands, which lose their velocity by coming into contact with a current flowing in the opposite direction to that in which they originally moved, and their course is again modified to such an extent that they are found flowing in the oppo- site direction of the Gulf Stream. We have further gained concise ideas of the relations of the cold coast current and the warm waters of the Gulf Stream ; and know something of the changes which take place in these relations through the mechanical influence of the winds. The outline of the problem has thus been obtained. The results were interesting enough to in- duce Professor Mendenhall, the superintendent of the Coast Survey, to cooperate with the commission in the study of these temperature problems; and this summer a still more extended investigation is in hand, in which the Coast Survey steamer Blake and the schooner Gramptis are engaged. The work has progressed to such a point that I may say that we shall have over 375 stations to study, giving us over 4000 serial tempera- tures, 1000 specific gravity observations, and over 14,000 general meteorological records. Last year the only means of comparison for our meteorological observations were the records made in New York and Boston; but this year we shall have in addition for this purpose a station upon the Nantucket New South Shoal light-ship, through the courtesy of the Light-house Board. Here a series of observations OCEAN TEMPERATURES AND CURRENTS. 249 have been made upon the atmospheric conditions, atten- tion paid to wind velocity, tidal movements, and also to the direction and velocity of the currents. This light- ship occupies the most exposed position upon our coast, being twenty-one miles southeast of the island of Nan- tucket, and affords a fine opportunity for the study of these phenomena. Many interesting results are expected from this inves- tigation, which, while they will be of scientific value in other directions, will be welcome sources of information to aid in the solution of the problems of the Fish Com- mission. The physical investigations in Long Island Sound can be described in a few words. For several years a decrease has been noticed in the oyster crop from that body of water. At first it was be- lieved that the natural enemies of the oyster, such as the star fish, the drills, etc., were the main cause of the evil. After investigation it was seen that where the oyster beds were properly cared for, their enemies did not exist in sufficient numbers to cause the trouble, though the oysters continued to disappear from these beds, masses of empty shells being brought up in the dredges. While the Fish Commission steamer Fish Hawk was investigating this matter last year, they occasionally brought up in their dredge great quantities of decomposing matter, which was so offensive that it had to be instantly washed back into the water. It should be remembered that for several years past the river mouths and harbors along the Sound have been dredged, and the matter obtained dumped into the deeper portions of the Sound ; and further, that the 250 MARINE BIOLOGICAL LALORATORY. action of the tides in that body of water do not carry any substance placed in it out to sea, but keep it moving back and forth until it is decomposed. This fact sug- gested the idea that possibly the oxygen in the water could not take care of any such excessive charge of poisonous nitrates and nitrites as would be developed by these decomposing substances. The waters of the Sound are therefore being investi- gated this season with reference to their chemical con- dition at various depths. The currents are at the same time being carefully studied by a member of the Coast Survey, and it is hoped in this way to obtain a complete view of the physical conditions under which the oyster has been evidently forced to live.