“ert cere ae er ke deve \ the (ans worry ery ary lene me bebe is { ite: tre herd a y > aan ehee ee ea y . # ve : M \* pienso? : eat A rep panied Meee a + ie ve age? ; of partons " Pra beet ‘abel gte ere rf aia tee ped bay beth Oe f ogni donne p oye ried oe i Ny. * anny | w uphet Sy rh He a” ' pees? , 7 ‘ , eh i cee ee oy one 1 a a ee Lovak area Fla pthc ee” ‘ gall #7 A li Japon ye Ae de ales aa : ~~ Sia ered Dar Vat ite aS my Mie te hy ] ne re ( te H Tag br he ghey ae Ng. tins 9 penis pha! a aoe ei 4 hep eM pibad> a VRE TN atone Hit es EE Wh Se pebbae eh yalaed or ov ye Sri phar pe epeoas & > vogqee uate wereyel! " ye Hanae " : nt } at bae Hier Me ttt teste iY ) nay Behy rheae ghpestttes St che ¥ a Ay r vm ty : enone oiegse ] i Aes bbe on Y ba fe tt ie pee lone ule Ge ay wert 4 ah gee vtos yet aa meri tae itepariie ge yee byoie nd py teat sat 0 04 iy phgn 1h #9 tt Z plows shia ent rr Lileks ret! an i hahaeen 7 4 aa whe 4 mae jobel . Laseet a jaye nie a eae) ¢ cane So aneree OP me A! eat » pee Ap ieee oe eens Py feted Sausage eier 4 a - > t jayne ve presen rn wa tanh ot fee batornne agit Sey OS ree ve oly eet eS ae eget ages regan ai ‘ Te Abe! ae dyone | 0) eae beobriag at: fasagto tres} ae pa he Buh bey “ee OF “oye ‘ qneey . ba oop oft ae wet 4 doe a ae one ee 4 BT a as é : Sie rye srr barter a ny ms ep Ve iteg varity es $r" rh oe ev 24 WINS . : ve : FOR THE PE OREE FOR EDVCATION FOR SCIEN'GE LIBRARY OF THE AMERICAN MUSEUM OF NATURAL HISTORY SGP 10) 1D) 1 1B fs FROM THE cae STUDIES BIOLOGY FROM THE BIOLOGICAL DEPARTMENT OF ie OWENS, COLEEEr: VOLUME IIT. PUBLISHED BY THE COUNCIL OF THE COLLEGE AND EDITED BY PROFESSOR SYDNEY J. HICKSON, MANCHESTER: J. E. CoRNISH. 1895. PRICE TEN SHILLINGS. ww. PANE ho ba PM eh OUE ENA) YR OUR AA ay ~ PREFACE. THE present volume of the Studies contains a series of papers that were either written or published before [ entered upon my duties as Beyer Professor of Zoology, and, with the exception of the essay by Dr. Hurst on the Structure of Archzopteryx, they were all published during the lifetime of my distinguished predecessor, the late Professor Arthur Milnes Marshall. I cannot claim therefore that any of the work recorded in this volume was done at my advice or under my guidance and _ super- vision, nor can I, on the other hand, be held responsible for the accuracy of the observations or for the opinions expressed in their essays by their several authors. The .delay that has occurred in the publication of the volume, consequent on the sudden death of Dr. Marshall and the lapse of time before appointment of his successor, was unavoidable; but I trust that the appearance of the III" volume of the Studies will be none the less welcome to those who are interested in the work that is being carried on in the scientific laboratories of our College. I would take this opportunity of reminding those Zoologists who read this volume that on the death of Dr. Marshall his relatives very generously gave his scientific books and pamphlets to the Owens College, for the use of the students and teachers of the same for all time. It would add very greatly to the value of this collection if Zoologists would kindly forward to me, for inclusion in ‘the Marshall Library,’ any separate copies of their publications that they can spare. Such donations would be most gratefully received, as certain to prove of very considerable assistance to those of us who are engaged in original work in this laboratory. The third volume of the Studies, although it is the last in which any part was taken by Professor Marshall, does not, by any means, record the end of his work. The stimulus of his teaching and the energy of his character must remain with us in the Owens College for many years to come, and his influence will long be felt in the writings both of his pupils and successors. SYDNEY J. HICKSON. September, 1895. WARE Ae 10 ory TMEV MOTE Bi VOLE IARI AT _« CONTENTS. PAGE Professor MARSHALL.—‘‘ Address to the Biological Section of the British Association, 1890.” Reprinted from the Reports of the British Association ar side ste ‘ins eats aie ie st Mr. O. H. LATTER.—‘‘ Notes on Anodon and Unio.” Plate I. Reprinted from the Proceedings of the Zoological Society, 1891 ... Mr. W. GARSTANG.—‘‘ Report on the Tunicata of Plymouth,” with Plate II. Reprinted from the Journal of the Marine Biological Association, Vol. II., No. 1 ae oe aoe : a Dr. ROBINSON and Mr. R. ASSHETON. ‘‘The formation and fate of the primitive streak, with observations on the Archenteron and germinal layers of Rana temporaria.” Plates III. and IV. Re- printed from the Quarterly Journal of Microscopical Science, 1891. Mr. W. GARSTANG.—‘‘On some Ascidians from the Isle of Wight.” Plates V. and VI. Reprinted from the Journal of the Marine Biological Association. Vol. II., No. 2 AS Mr. F. W. GAMBLE. —‘“‘ Observations on two rare British Nudibranchs.” Plate VII. Reprinted se om the Annals and EEE: of Natural History, 1892 oF : wes ‘ Mr. R. ASSHETON.—‘‘ On the development of the optic nerve of Verte- brates, and the choroidal fissure of embryonic life.” Plates VIII. and IX. ae Srom the Quarterly Journal cae Microscopical Science, 1892 . My. F. W. GAMBLE.—‘‘ Contributions to a knowledge of British Marine Turbellaria.” Plates X., XI., and XII. “Roar inted ie om the Quarterly Journal of Microscopical Science, 1893 : MirsstEii= ©: CELDT —“QOn an abnormal specimen of Antedon rosacea.” Plate XIII. noe inted from the Annals and ee of Natural History, 1893 . Suc “e : Dr. C. H. Hurst.—‘“‘ The structure and habits of Archzeopteryx.” Plates XIV., XV., XVI. Reprinted from Natural Science, 1895 ... 49 58 82 . 129 . 155 . 163 . 181 . 263 . 267 "0 ‘) att ADDRESS TO THE BIOLOGICAL SECTION OF THE BRITISH ASSOCIATION (1890), By Professor A. Minnes Marswaut, M.A., M.D., D.Sc., F.R.S., President of the Section. As my theme for this morning’s address I have selected the Development of Animals. I have made this choice from no desire to extol one particular branch of biological study at the expense of others, nor through failure to appreciate or at least admire the work done and the results achieved in recent years by those who are attacking the great problems of life from other sides and with other weapons. My choice is determined by the necessity that is laid upon me, through the wide range of sciences whose encouragement and advance- ment are the peculiar privilege of this Section, to keep within reasonable limits the direction and scope of my remarks ; and is confirmed by the thought that, in addressing those specially interested in and conversant with biological study, your President acts wisely in selecting as the subject-matter of his discourse some branch with which his own studies and inclinations have brought him into close relation. Embryology, referred to by the greatest of naturalists as ‘one of the most important subjects in the whole round of Natural History,’ is still in its youth, but has of late years thriven so mightily that fear has been expressed lest it should absorb unduly the attention of zoologists or even check the progress of science by diverting interest from other and equally important branches. Nor is the reason of this phenomenal success hard to find. The 2 PROFESSOR A. M. MARSHALL. actual study of the processes of development ; the gradual building up of the embryo, and then of the young animal, within the egg ; the fashioning of its varions parts and organs; the devices for supplying it with food, and for ensuring that the respiratory and other interchanges are duly performed at all stages: all these are matters of absorbing interest. Add to these the extraordinary changes which may take place after leaving the egg, the conversion, for instance, of the aquatic gill-breathing tadpole—a true fish as regards all essential points of its anatomy—into a four-legged frog, devoid of tail, and breathing by lungs ; or the history of the metamorphosis by which the sea-urchin is gradually built up within the body of its pelagic larva, or the butterfly derived from its grub. Add to these again the far wider interest aroused by com- paring the life histories of allied animals, or by tracing the mode of development of a complicated organ, e.g. the eye or the brain, in the in the various animal groups, from its simplest commencement, through gradually increasing grades of efficiency, up to its most perfect form as seen in the highest animals. Consider this, and it becomes easy to understand the fascination which embryology exercise over those who study it. But all this is of trifling moment compared with the great generali- sation which tells us that the development of animals has a far higher meaning; that the several embryological stages and the order of their occurrence are no mere accidents, but are forced on an animal in accordance with a law, the determination of which ranks as one of the greatest achievements of biological science. The doctrine of descent, or of Evolution, teaches us that as individual animals arise, not spontaneously, but by direct descent from pre-existing animals, so also is it with species, with families, and with larger groups of animals, and so also has it been for all time; that as the animals of succeeding generations are related together, so also are those of successive geologic periods; that all animals, living or that have lived, are united together by blood relationship of varying near- ness or remoteness ; and that every animal now in existence has a pedigree stretching back, not merely for ten or a hundred generations, but through all geologic time since the dawn of life on this globe. The study of Development, in its turn, has revealed to us that each animal bears the mark of its ancestry, and is compelled to discover its parentage in its own development ; that the phases through which an “PRESIDENTIAL ADDRESS, 3 animal passes in its progess from the egg to the adult are no accidental freaks, no mere matters of developmental convenience, but represent more or less closely, in more or less modified manner, the successive ancestral stages through which the present condition has been acquired. Evolution tells us that each animal has had a pedigree in the past. Embryology reveals to us this ancestry, because every animal in its own development repeats this history, climbs up its own genealogical tree. Such is the Recapitulation Theory, hinted at by Agassiz, and suggested more directly in the writings of von Baer, but first clearly enunciated by Fritz Miller, and since elaborated by many, notably by Balfour and by Ernst Haeckel. It is concerning this theory, which forms the basis of the Science of Embryology, and which alone justifies the extraordinary attention this science has received, that I venture to address you this morning. A few illustrations from different groups of animals will best explain the practical bearings of the theory, and the aid which it affords to the zoologist of to-day ; while these will also serve to illustrate certain of the difficulties which have arisen in. the attempt to interpret individual development by the light of past history—difficulties which I propose to consider at greater length. A very simple example of recapitulation is afforded by the eyes of the sole, plaice, turbot, and their allies. These ‘flat fish’ have their bodies greatly compressed laterally ; and the two surfaces, really the right and left sides of the animal, unlike, one being white, or nearly so, and the other coloured. The flat fish has two eyes, but these, in place of being situated, as in other fish, one on each side of the head, are both on the coloured side. The advantage to the fish is clear, for the natural position of rest of a flat fish is lying on the sea bottom, with the white surface downwards and the coloured one upwards. In such a position an eye situated on the white surface could be of no use to the fish, and might even become a source of danger, owing to its liability to injury from stones or other hard bodies on the sea bottom. No one would maintain that flat fish were specially created as such. The totality of their organisation shows clearly enough that they are true fish, akin to others in which the eyes are symmetrically placed one on each side of the head, in the position they normally hold among vetebrates. We must therefore suppose that flat fish are descended from other fish in which the eyes are normally situated. 4 PROFESSOR A. M. MARSHALL, The Recapitulation Theory supplies a ready test. On employing it, 2.€., on Studying the development of the flat fish, we obtain a conclusive answer. ‘The young sole on leaving the egg is shaped just as any ordinary fish, and has the two eyes placed symmetrically on the two sides of the head. It is only after the young fish has reached some size, and has begun to approach the adult in shape, and to adopt its habit of resting on one side on the sea bottom, that the eye of the side on which it rests becomes shifted forwards, then rotated on to the top of the head, and finally twisted completely over to the opposite side. The brain of a bird differs from that of other vetebrates in the position of the optic lobes, these being situated at the sides instead of on the dorsal surface. Development shows that this lateral position is a secondarily acquire! one, for throughout all the earlier stages the optic lobes are, as in other vetebrates, on the dorsal surface, and only shift down to the sides shortly before the time of hatching. Crabs differ markedly from their allies, the lobsters, in the small size and rudimentary condition of their abdomen or “tail.” Develop- ment, however, affords abundant evidence of the descent of crabs from macrurous ancestors, for a young crab at what is termed the Megalopa stage has the abdomen as large as a lobster or prawn at: the same stage. Molluscs afford excellent illustrations of recapitulation. The typical gastropod has a large spirally-coiled shell; the limpet, however, has a large conical shell, which in the adult gives no sign of spiral twisting, although the structure of the animal shows clearly its affinity to forms with spiral shells. Development solves the riddle at once, telling us that in its early stages the limpet embryo has a spiral shell, which is lost on the formation, subsequently, of the conical shell of the adult. Recapitulation is not confined to the higher groups of animals, and the Protozoa themselves yield most instructive examples. A very striking case is that of Orbitolites, one of the most complex of the porcellanous Foraminifera, in which each individual during its own growth and development passes through the series of stages by which the cyclical or discoidal type of shell was derived from the simpler spiral form. \ In. Orbitolites tenuissima, as Dr. Carpenter has shown,! ‘the whole 1 W. B. Carpenter, ‘On an Abyssal Type of the Genus Orbitolites,’ Phz/. Trans, 1883, part ii. p. 553. PRESIDENTIAL ADDRESS, 5 transition is actually presented during the successive stages of its growth. For it begins life as a Cornuspira, ... . its shell forming a continuous spiral tube, with slight interruptions at the points at which its successive extensions commence ; while its sarcodic body consists of a continuous coil with slight constrictions at intervals. The second stage consists in the opening out of its spire, and the division of its cavity at regular intervals by transverse septa, traversed by separate pores, exactly as in Peneroplis. The third stage is marked by the subdivision of the ‘‘ peneropline ” chambers into chamberlets, as in the early forms of Orbiculina. And the fourth consists in the exchange of the spiral for the cyclical plan of growth, which is characteristic of Orbitolites ; a circular disc of progressively increasing diameter being formed by the addition of successive annular zones around the entire periphery.’ The shells both of Foraminifera and of Mollusca afford peculiarly instructive examples for the study of recapitulation. As growth of the shell is effected by the addition of new shelly matter to the part already existing, the older parts of the shell are retained, often unaltered, in the adult; and in favourable cases, as in Orbitolites tenurssima, all the stages of development can be determined by simple inspection of the adult shell. It is important to remember that the Recapitulation Theory, if valid, must app!y not merely in a general way to the development of the animal body, but must hold good with regard to the formation of each organ or system, and with regard to the later equally with the earlier phases of development. Of individual organs the brain of birds has been already cited. The formation of the vertebrate liver asa diverticulum from the alimentary canal, which is at first simple, but by the folding of its walls becomes greatly complicated, is another good example; as is also the develop- ment of the vomer in Amphibians as a series of toothed plates, equiva- lent morphologically to the placoid scales of fishes, which are at first separate, but later on fuse together and lose the greater number of their teeth. Concerning recapitulation in the latter phases of development and in the adult animal, the mode of renewal of the nails or of the epidermis generally is a good example, each cell commencing its existence in an indifferent form in the deeper layers of the epidermis, 6 PROFESSOR A. M. MARSHALL. and gradually acquiring the adult peculiarities as it approaches the surface, through removal of the cells lying above it. The above examples, selected almost haphazard, will suffice to illustrate the Theory of Recapitulation. The proof of the theory depends chiefly on its universal applicability to all animals, whether high or low in the zoological scale, and to all their parts and organs. It derives also strong support from the ready explanation which it gives of many otherwise unintelligible points. Of these latter a familiar and most instructive instance is afforded by rudimentary organs, 2.¢., structures which, like the outer digits of the horse’s leg, or the intrinsic muscles of the ear of a man, are present in the adult in an incompletely developed form, and in a condition in which they can be of no use to their possessors ; or else structures which are present in the embryo, but disappear completely before the adult condition is attained, for example, the teeth of whalebone whales, or the branchial clefts of all higher vertebrates. Natural selection explains the preservation of useful variations, but will not account for the formation and perpetuation of useless organs ; and rudiments such as those mentioned above would be unintelligible but for Recapitulation, which solves the problem at once, showing that these organs, though now useless, must have been of functional value to the ancestors of their present possessors, and that their appearance in the ontogeny of existing forms is due to repetition of ancestral characters. Such rudimentary organs are, as Darwin pointed out, of larger relative or even absolute size in the embryo than in the adult, because the embryo represents the stage in the pedigree in which they were functionally active. Rudimentary organs are extremely common, especially among the higher groups of animals, and their presence and significance are now well understood. Man himself affords numerous and excellent examples, not merely in his bodily structure, but by his speech, dress, and customs. For the silent letter 6 in the word doubt, or the w of answer, or the buttons on his elastic-side boots are as true examples of rudiments, unintelligible but for their past history, as are the ear muscles he possesses but cannot use, or the gill-clefts, which are functional in fishes and tadpoles, and are present, though useless, in the embryos of all higher vertebrates, which in their early stages the hare and the tortoise alike possess, and which are shared with them by cats and by kings. PRESIDENTIAL ADDRESS. if Another consideration of the greatest importance arises from the study of the fossil remains of the animals that formerly inhabited the earth. It was the elder Agassiz who first direvted attention to the remarkable agreement between the embryonic growth of animals and their paleontological history. He pointed out the resemblance between certain stages in the growth of young fish and their fossil representatives, and attempted to establish, with regard to fish, a correspondence between their palzeontological sequence and the Successive stages of embryonic development. He then extended his observations to other groups, and stated his conclusions in these words: ** It may therefore be considered as a general fact, very likely to be more fully illustrated as investigations cover a wider ground, that the phases of development of all living animals correspond to the order of succession of their extinct representatives in past geological times.’ This point of view is of the utmost importance. If the development of an animal is really a repetition of its ancestral history, then it is clear that the agreement or parallelism which Agassiz insists on between the embryological and paleeontological records must hold good. Owing to the attitude which Agassiz subsequently adopted with regard to the theory of Natural Selection, there is some fear of his services in this respect failing to receive full recognition, and it must not be forgotten that the sentence I have quoted was written prior to the clear enunciation of the Recapitulation Theory by Fritz Miiller. The imperfection of the geological record has been often referred to and lamented. It is very true that our museums afford us but fragmentary pictures of life in past ages; that the earliest volumes of the history are lost, and that of others but a few torn pages remain to us; but the later records are in far more satisfactory condition. The actual number of specimens accumulated from the more recent forma- tions is prodigious; facilities for consulting them are far greater than they were; the international brotherhood of science is now fully established, and the fault will be ours if the material and opportunities now forthcoming are not rightly and fully utilised. By judicious selection of groups in which long series of specimens can be obtained, and in which the hard skeletal parts, which alone can be suitably preserved as fossils, afford reliable indications of zoological affinity, it is possible to test directly this correspondence 1 L. Agassiz, Essay on Classification, 1859, p. 115. 8 PROFESSOR A. M. MARSHALL. between paleontological and embryological histories, while in some instances a single lucky specimen will afford us, on a particular point, all the evidence we require. Great progress has already been made in this direction, and the results obtained are of the most encouraging description. By Alexander Agassiz a detailed comparison was made between the fossil series and the developmental stages of recent forms in the case of the Echinoids, a group peculiarly well adapted for such an investigation. The two records agree remarkably in many respects, more especially in the independent evidence they give as to the origin of the asymmetrical forms from more regular ancestors. The gradually increasing complication in some of the historic series is found to be repeated very closely in the development of their existing represen- tatives ; and with regard to the whole group, Agassiz concludes that, ‘comparing the embryonic development with the paleontological one, we finda remarkable similarity in both, and in a general way there seems to be a parallelism in the appearance of the fossil genera and the successive stages of the development of the Echini.’ Neumayr has followed similar lines, and by him, as by other authorities on the group, there seems to be general agreement as to the parallelism between the embryological and paleontological records, not merely for Echini, but for other groups of Echinodermata as well. The Tetrabranchiate Cephalopoda are an excellent group in which to study the problem, for though no opportunity has yet occurred for studying the embryology of the only surviving member of the group the pearly nautilus, yet owing to the fact that growth of the shell is effected by addition of shelly matter to the part already present, and to the additions being made in such manner that the older part of the shell persists unaltered, it is possible, from examination of a single shell—and in the case of fossils the shells are the only part of which we have exact kuowledge—to determine all the phases of its growth ; just as in the shell of Orbitolites all the stages of development are manifest on nspection of an adult specimen. In such a shell as Nautilus or Ammonites the central chamber is the oldest or first formed one, to which the remaining chambers are added 1 A, Agassiz, Paleontological and Embryological Development. ‘An Address before the American Association for the Advancement of Science.’ 1880. ° PRESIDENTIAL ADDRESS. 9 in succession. If, therefore, tae development of the shell is a repetition of ancestral history, the central chamber should represent the palzeontologically oldest form, and the remaining chambers in succession forms of more and more recent origin. Ammonite shells present, more especially in their sutures, and in the markings and sculpturing of their surface, characters that are easily recognised, and readily preserved in fossils; and the group, con- sequently, is a very suitable one for investigation from this standpoint. Wiirtenberg’s admirable and well-known researches* have shown that in the Ammonites such a correspondence between the historic and embryonic development does really exist; that, for example, in Aspidoceras the shape and markings of the shells in young specimens differ greatly from those of adults, and that the characters of the young shells are those of paleeontologically older forms. Another striking illustration of the correspondence between the paleontological and developmental records is afforded by the antlers of deer, in which the gradually increasing complication of the antler in successive years agrees singularly closely with the progressive increase in size and complexity shown by the fossil series from the Miocene age to recent times. Of cases where a single specimen has sufficed to prove the paleontological significance of a developmental character, Archeeopteryx affords a typical example. In recent birds the metacarpals are firmly fused with one another, and with the distal series of carpals ; but in development the metacarpals are at first, and for some time, distinct, In Archzopteryx this distinctness is retained in the adult, showing that what is now an embryonic character in recent birds, was formerly an adult one. Other examples might easily be quoted, but these will suffice to show that the relation between Paleontology and Embryology, first enunciated by Agassiz, and required by the Recapitulation Theory, does in reality exist. There is much yet to be done in this direction. A commencement, a most promising commencement, has been made, but as yet only a few groups have been seriously studied from this standpoint. It is a great misfortune that paleontology is not more generally 1 L. Wiirtenberger, ‘Studien iiber die Stammesgeschichte der Ammoniten. Ein geologischer Beweis fiir die Darwin’sche Theorie.’ Leipzig, 1880. 10 PROFESSOR A. M. MARSHALL. and more seriously studied by men versed in embryology, and that those who have so greatly advanced our knowledge of the early development of animals should so seldom have tested their conclusions as to the affinities of the groups they are concerned with by direct reference to the ancestors themselves, as known to us through their fossil remains. I cannot but feel that, for instance, the determination of the affinities of fossil Mammalia, of which such an extraordinary number and variety of forms are now known to us, would be greatly facilitated by a thorough and exact knowledge of the development, and especially the later development, of the skeleton in their existing descendants, and I regard it as a reproach that such exact descriptions of the later stages of development should not exist even in the case of our commonest domestic animals. The pedigree of the horse has attracted great attention, and has been worked at most assiduously, and we are now, largely owing to the labours of American paleontologists, able to refer to a series of fossil forms commencing in the lowest Eocene beds, and extending upwards to the most recent deposits, which show a complete gradation from a more generalised mammalian type to the highly specialised condition characteristic of the horse and its allies, and which may reasonably be regarded as indicating the actual line of descent, of the horse. In this particular case, more frequently cited than any other, the evidence is entirely paleontological. The actual development of the horse has yet to be studied, and it is greatly to be desired that it should be under- taken speedily. Klever’s' recent work on the development of the teeth in the horse may be referred to as showing that important and unexpected evidence is to be obtained in this way. A brilliant exception to the statement just made as to the want of exact knowledge of the later development of the more highly organised animals is afforded by the splendid labours of Professor Kitchen Parker, whose recent death has deprived zoology of one of her most earnest and single-minded students, and zoologists, young and old alike, of a true and sincere friend. Professor Parker’s extraordinarily minute and painstaking investigations into the development of the vertebrate skull rank among the most remarkable of modern zootomical 1 Klever, ‘Zur Kenntniss der Morphogenese des Equidengebisses,’ Morphologisches Jahrbuch xv. 1889, p. 308. PRESIDENTIAL ADDRESS. 11 achievements, and afford a rich mine of carefully recorded facts, the full value and bearing of which we are hardly yet able to appreciate. If further evidence as to the value and importance of the Recapitulation Theory were needed, it would suffice to refer to the influence which it has had on the classification of the animal Kingdom. Ascidians and Cirripedes may be quoted as important groups, the true affinities of which were first revealed by embryology ; and in the case of parasitic animals the structural modifications of the adult are often so great that but for the evidence yielded by development their zoological position could not be determined. It is now indeed generally recognised that in doubtful cases embryology affords the safest of all clues, and that the zoological position of such forms can hardly be regarded as definitely established unless their development, as well as their adult anatomy is ascertained. It is owing to this Recapitulation Theory that Embryology as exercised so marked an influence on zoological speculation. Thus the formation in most, if not in all, animals of the nervous system and of the sense organs from the epidermal layer of the skin, acquired a new significance when it was recognised that this mode of development was to be regarded as a repetition of the primitive mode of formation of such organs ; while the vertebral theory of the skull affords a good example of a view, once stoutly maintained, which received its death- blow through the failure of embryology to supply the evidence requisite in its behalf. The necessary limits of time and space forbid that I should attempt to refer to even the more important of the numerous recent discoveries in embryology, but mention may be very properly made here of Sedgwick’s determination of the mode of development of the body cavity in Peripatus, a discovery which has thrown most welcome light on what was previously a great morphological puzzle. We must now turn to another side of the question. Although it is undoubtedly true that development is to be regarded as a recapitulation of ancestral phases, and that the embryonic history of an animal presents to us a record of the race history, yet it is also an undoubted fact, recognised by all writers on embryology, that the record so obtained is neither a complete nor a straightforward one. It is indeed a history, but a history of which entire chapters are lost, while in those that remain many pages are misplaced while others are so blurred as to be illegible ; words, sentences, or entire paragraphs 12 PROFESSOR A. M. MARSHALL. are omitted, and worse still, alterations or spurious additions have been freely introduced by later hands, and at times so cunningly as to defy detection. Very slight consideration will show that development cannot in all cases be strictly a recapitulation of ancestral stages. It is well known that closely allied animals may differ markedly in their mode of develop- ment. The common frog is at first a tadpole, breathing by gills, a stage which is entirely omitted by the West Indian Hylodes. A cray fish, a lobster, and a prawn are allied animals, yet they leave the egg in totally different forms. Some developmental stages, as the pupa condition of insects, or the stage in the development of a dogfish, in which the cesophagus is imperforate, cannot possibly be ancestral stages. Or again, a chick embryo of, say the fourth day, is clearly not an animal capable of independent existence, and therefore cannot correctly represent any ancestral condition, an objection which applies to the developmental history of many, perhaps of most animals. Haeckel long ago urged the necessity of distinguishing in actual development between those characters which are really historical and inherited and those which are acquired or spurious additions to the record. The former he termed palingenetic or ancestral characters, the latter cenogenetic or acquired. The distinction is undoubtedly a true one, but an exceedingly difficult one to draw in practice. The causes which prevent development from being a strict recapitulation of ancestral characters, the mode in which these came about, and the influence which they respectively exert, are matters which are greatly exercising embryologists, and the attempt to determine which has as yet met with only partial success. The most potent and the most widely spread of these disturbing causes arise from the necessity of supplying the embryo with nutriment. This acts in two ways. If the amount of nutritive matter within the egg is small, then the young animal must hatch early, and in a condition in which it is able to obtain food for itself. In such cases there is of necessity a long period of larval life, during which natural selection may act so as to introduce modifications of the ancestral history, spurious additions to the text. [f, on the other hand, the egg contain within itself a considerable quantity of nutrient matter, then the period of hatching can be post- poned until this nutrient matter has been used up. The consequence PRESIDENTIAL ADDRESS, 13 is that the embryo hatches at a much later stage of its development, and if the amount of food material is sufficient may even leave the egg in the form of the parent. In such cases the earlier developmental phases are often greatly condensed and abbreviated; and as the embryo does not lead a free existence, and has no need to exert itself to obtain food, it commonly happens that these stages are passed through in a very modified form, the embryo being as in a four-day chick, in a condition in which it is clearly incapable of independent existence, The nutrition of the embryo prior to hatching is most usually effected by granules of nutrient matter, known as food yolk, and embedded in the protoplasm of ihe egg itself; and it is on the relative abundance of these granules that the size of the egg chiefly depends. Large size of eggs implies diminution of number of the eggs, and hence of the offspring ; and it can be well understood that while some species derive advantage in the struggle for existence by producing the maximum number of young, to others it is of greater importance that the young on hatching should be of considerable size and strength, and able to begin the world on their own account. In other words, some animals may gain by producing a large nnmber of small eggs, others by producing a smaller number of eggs of larger size—z.e., provided with more food yolk, The immediate effect of a large amount of food yolk is to mechanically retard the processes of development ; the ultimate result is to greatly shorten the time occupied by development. This apparent paradox is readily explained. A small egg, such as that of Amphioxus, starts its development rapidly, and in about eighteen hours gives rise to a free swimming larva, capable of independent existence, with digestive cavity and nervous system already formed; while a large egg like that of the hen, hampered by the great mass of food yolk by which it is distended, has, in the same time, made but very slight progress. From this time, however, other considerations begin to tell, Am- phioxus has been able to make this rapid start owing to its relative freedom from food yolk. This freedom now becomes a retarding influence, for the larva, containing within itself but a very scanty supply of nutriment, must devote much of its energies to hunting for, and to digesting its food, and hence its further development will proceed more slowly. 14 PROFESSOR A. M. MARSHALL. The chick embryo, on the other hand, has an abundant supply of food in the egg itself; it has no occasion to spend time searching for food, but can devote its whole energies to the further stages of its development. Hence, except in the earliest stages, the chick develops more rapidly than Amphioxus, and attains its adult form im a much shorter time. The tendency of abundant food yolk to lead to shortening or abbreviation of the ancestral history, and even to the entire omission of important stages, is well known. The embryo of forms well provided with yolk takes short cuts in its development, jumps from branch to branch of its genealogical tree, instead of climbing steadily upwards. Thus the little West Indian frog, Hylodes, produces eggs which contain a larger amount of food yolk than those of the common English frog. The young Hylodes is consequently enabled to pass through the tadpole stage before hatching, to attain the form of a frog before leaving the egg; and the tadpole stage is only imperfectly recapitulated, the formation of gills, for instance, being entirely omitted. The influence of food yolk on the development of animals is closely analogous to that of capital in human undertakings.