a l uj : □ J CD ' <=> I -C i -J I tr ; ■3 -03 --^ 15 .i THE TERATOLOGY OF FISHES PUBLISHED BY JAMES MACLEHOSE AND SONS, GLASGOW, $abUsture to the Bnibcreitfi. MACMILLAN AND CO., LTD., LONDON. New York, Toronto, • London, • Cambridge, Edinburgh, Sydney, • T/te Macmitlan Co. The Macmillan Co. of Canada. Simpkin, Hamilton and Co. Bowes and Bowes. Douglas and Foulis. Angus and Robertson. s^l 0- ■ THE TERATOLOGY OF FISHES BY JAMES F. GEMMILL, M.A., M.D., D.Sc. LECTURER IN EMBRYOLOGY, GLASGOW UNIVERSITY, AND IN ZOOLOGY, GLASGOW PROVINCIAL TRAINING COLLEGE GLASGOW JAMES MACLEHOSE AND SONS PUBLISHERS TO THE UNIVERSITY I 9 I 2 GLASGOW: PRINTED AT THE UNIVERSITY PRESS BV ROBERT MACLEHOSE AND CO. LTD. INSCRIBED TO MY HONOURED TEACHER JOHN CLELAND, M.D., LL.D., D.Sc, F.R.S., EMERITUS PROFESSOR OF ANATOMY, GLASGOW UNIVERSITY, NOT LESS IN AFFECTION AND ESTEEM THAN IN ADMIRATION OF HIS DEEP INSIGHT INTO THE PROBLEMS OF TERATOLOGY AND THE LASTING VALUE OF HIS CONTRIBUTIONS TO ITS STUDY Embrtological Laboratory, Glasgow Univerbity, June, 1912. CONTENTS PAGE Introduction - - - - vii Bibliography - - ... . - xi Chapter I. Double Monstrosity A. Literature - - ........] B. Occurrence, records, and general outline ....... \ C. Classification - ....... 3 D. Causation ........ 4 E. Development and early growth in fishes, and in vertebrates generally .... 6 F. Structure Early Stages of Double Monstrosity in the Sahnonidae and in Esox ■ 9 Normal Advanced Embryos of Salmo fario - - 11 Advanced Stages of Double Monstrosity in Salmo fario, etc. Class I. Union in head region, the twin brains uniting at the optic lobes 12 II. Union in head region, the twin brains uniting at the medulla oblongata - 15 III. Union in pectoral region, adjacent pectoral fins not being present - - 18 IV. Union in pectoral region, adjacent pectoral fins being present, but united and reduced in size --------- 19 V. Union by the body or tail, the united portion ending as normally in a single symmetrical tail ----- - - 20 VI. Union by the body or tail, the united portion ending in a composite tail with various structures still doubled - - - 21 VII. Union by the yolk-sac only, or by the yolk-sac and the ventral-edge membrane close behind it - - 23 VIII. The condition of imperfect doubling exhibited by (a) the (hemi)inesodidymi and (6) (hemi)katadidymi ... - 25 IX. Longitudinal or parallel union - 29 Double Monstrosity in other Fishes - - 30 Chapter II. Triple Monstrosity Description of a specimen - - - 33 Other instances in fishes - ....... - 35 Mode of development in fishes - - .... - 36 Appendix on Triplicity in other vertebrates, and on Classification in Germinal Triplicity - 37 kJO^SI vi CONTENTS Chapter III. Cyclopia paok Classification and structure ...-....-.40 Summary and comparison - - - - 42 Other records in fishes - - ----43 Causation ----------- .44 Chapter IV. Minor Abnormalities I. Hermaphroditism -----------46 II. Abnormalities affecting skull or vertebral column (a) Shortening and curvature of the snout ; the pug-headed condition - - 48 (b) The round-headed condition .--.----50 (c) Defects of the lower jaw --------- 51 (r/) Hump-back, twisted body, and allied conditions - - - - - 51 (e) Local reduplication and deficiency of the notochord - - - 54 III. Abnormalities affecting fins (a) Excess ------ ... 54 (6) Defect ... .... - - 55 (c) The so-called "tailless" condition - - - .... 56 IV. Abnormalities of coloration (a) In fishes generally ----------56 (b) In flat-fishes, associated with certain other defects ----- 56 V. Parasitism --------- --59 VI. Pathological conditions of the ovum or early embryo ------ 60 VII. Abnormalities having reference for the most part to single organs 61 Index to Structures and Subjects in Text and Plates - - - 63 Index to Species, Genera, etc., of Fishes Referred to in Text - 70 Index of Authors - .... 72 Summary of Contents of Plates - 74 Plates I.-XXVI. Figs. 1-114 INTRODUCTION. The primary object of this Memoir is to throw light on the structural aspect of the major abnormalities occurring in fishes, particularly in the trout and salmon. Nearly half of the text, and practically all the illustrations, are devoted to this purpose. The Plate-figures consist of photomicrographs of sections and other specimens, and of reconstruction diagrams which, like the anatomical descriptions in the text, are based on the study of numerous series of sections. I have to express equal indebtedness and thanks to the Trustees of the Carnegie Bequest for the support which has made possible the publication of the work in its present form. The greater part of my trout material was obtained some years ago from a Hatchery at Lochwinnoch, through the good offices of Mr. John Peden who was then in charge. Specimens were also received from the Loch Lomond Hatchery and from various private sources. In addition I had the privilege of examining at leisure the Salmonid material in the collection of the Eoyal College of Surgeons of England. I desire to acknowledge indebtedness to the kindness and skill of my friend, the late Thomas Pieid, M.D., LL.D., for greatly-valued assistance in the preparation of a number of the photographs. My best thanks are also due to Mr. J. G. Connell, F.R.M.S., for help in compiling the Indexes, of which there are three, namely, one of Structures and Subjects, one of Fishes, and one of Authors. The first includes full references to the various Plates and Figures in which individual structures receive illustration. It is through the courtesy of the Council of the Zoological Society of London that illustrations from papers of mine which appeared in their Proceedings, 1903, Vol. II., and 1906, Vol. I., have been reproduced in Plates XVII.-XX. and XXV, and this courtesy is herewith gratefully acknowledged. Whilst, as stated above, the main content of the work is structural, occasion has also been taken to deal briefly with other aspects of major monstrosity in fishes, and to point out relationships with higher vertebrates. In addition, the minor abnormalities form a large and varied group, many sections of which might well receive separate and lengthy discussion. My aim has been to describe these as briefly as is consistent with moderate adequacy of treatment, and to make it easy for future workers to get into touch with the chief present sources of information in each subject. It is hoped that this latter object has been attained through the literature-references and the Bibliography. The study of monstrosities among fishes merits a distinct and important place in the biological field. Although this study has come latest in point of time, its data are now in process of incorporation with those of general vertebrate teratology, the foundations of which they have not failed to deepen and extend. The following circumstances favour this result : (1) In the bony fishes, which provide by far the greatest number of monstrosities, the ova are abundant and the processes of fertilisation and development take place outside the body of the parent. Plentiful material is available at all stages for observation and experiment. Facts of the greatest value have thereby been ascertained, particularly with reference to mode of origin and early development. (2) Although the major types of monstrosity in fishes do not survive the period of nutrition by the yolk sac, still at the end of this stage the cartilaginous skeleton and practically b viii INTRODUCTION all the other structures except the bony framework have been laid down in their final form, and already exhibit their adult relations. Each specimen examined thus provides us in small compass with a compendium for its type, the morphology and histology of which can be studied at the same time and from the same series of sections. (3) The vertebral column is of relatively great length, while the limb masses are small. Accordingly, in the most interesting types of monstrosity, namely those in which there is axial union of two or more embryos, we may expect to find the characteristics of such union exhibited in simplest form. The law that homologous structures become joined together, always holds good, and the full series of double and multiple forms in fishes provides us with examples in which " compositeness," due to primary or secondary fusion, is exhibited by practically every organ and structure in the body. (4) In the great majority of fishes segmentation is partial ; we can speak of a circumscribed blastoderm resting on the yolk mass and giving rise at one part to the rudiment of the embryo. In these points there is correspondence with avian and reptilian development, and with those characteristics of mammalian ontogeny which indicate an ancestral highly yolked condition of the ova. Accordingly we find that in their major abnormalities the fishes show surprisingly close relationships even with the mammals, From the general point of view the most interesting teratological questions are those which are concerned with origin and cmisation. A great deal of the data necessary for forming properly grounded views on those questions is still in process of accumulation. Meantime, I would venture to put forward the following propositions, from my reading of the available evidence, particularly as it has presented itself to me in connection with the teratology of fishes. The underlying ideas have long been part of the currency, if not of the accepted currency, of biological thought. I. Each of the recognised types of monstrosity, major as well as miiwr, can arise in a spontcmeous or autogenetic manner, by abrupt germinal variation. These terms are used with the meaning that (a) the results are not due to the action of environmental factors on the germ-cells directly concerned, or on the embryo itself ; (b) should survival and reproduction be possible, the malformations tend to be transmitted to descendants. The course of ordinary epigenetic development must depend in the main on the nature and characters of the fertilised ovum. Although these are remarkably steadfast, there is evidence that along various lines they are subject to a certain measure of insecurity. Using terms of analogy, we may speak of nodes of instability in the intimate constitution of the germ-cells. The instability is not indeterminate, but tends in each case to open up definite teratological potentialities, and occasionally, from some unknown cause, one of these is followed instead of the normal course of development. The aberration may involve the whole organism as in duplicity or multiplicity, or it may only affect groups of organs or single organs, as in herrnaphroditisrn, pug-head, various fin abnormalities, etc. Monstrosities which arise in this manner may be described as spontaneous or autogenetic. II. Most of the recognised teratological types, and particularly the major ones, are also capable of being produced by environmental factors acting during the course of development. Malformations arising in this manner may be termed acquired, and there is no evidence that they are transmissible. It is obvious that the environmental factors in question are not causal in any real sense. They can neither supply motive power nor guidance in the production of any type. All they can do is to cause the course of development to deviate along certain lines which the organism or structure is capable of following. These lines were already marked out through the potentialities referred to under I. The latter are primary to, and indicate the limits of, the former. At the same time, as regards ordinary frequency of occurrence, it is quite possible that, for some types, the instances of acquired may outnumber those of autogenetic abnormality. III. On the whole the two groups (namely those of autogenetic and of acquired abnormalities) tend to coincide with one another. At the same time the former will be found to have a wider content than the latter, if induced pathological conditions are left out of count, but here the boundary between teratology and pathology becomes difficult to define. In any case, the fact that a partictdar abnormality appears spontaneously is an argument for, and not against, the INTRODUCTION ix probability that the same abnormality can be artificially produced. The converse proposition carries with it an even stronger degree of likelihood. Teratological variation is thus not essentially different in kind from ordinary variation. Owing, however, to infrequency or non-survival or absence of reproductive faculty, the major teratological variations will not, as a rule, have any influence or evolution. That this is not always the case may be surmised from the facts relating to hermaphroditism (p. 46), and can be shown to be practically certain from a consideration of the phenomena which are connected with germinal duplicity and multiplicity, that is the production of more than one embryo from a single ovum. Amongst these phenomena we have (a) on the purely teratological side, the occurrence of double or multiple monstrosities in all classes of animals ; (b) intermediate between teratology and normal ontogeny, the occasional production in the higher animals of complete and separate unioval twins or triplets ; (c) in normal ontogeny, the polyembryony which occurs in the development of the edentate mammal Tatusia (p. 38) and the alternation of generations which has taken a definite place in the life-history of various groups of animals ; (d) Lastly, on the experimental side and in illustration of what was said under II. above, it need hardly be pointed out that we have now numerous well-ascertained instances in which double or multiple forms have been artificially produced through the (virtual or complete) separation of individual cells or of cell masses during early stages in the development of a single ovum. 12 LITERATURE RELATING TO THE TERATOLOGY OF FISHES. The following index has been compiled from Zoological Records, Jahresbcrichle, and many other sources of a more general nature. Work on variation, experimental zoology, and regeneration has been included, only when it seemed to have a direct bearing on the main theme. It is hoped that the lists may not show greater defects than are pardonable in a first bibliography on a wide and scattered subject, and that they may fulfil the primary purpose of being useful to future workers. Throughout the succeeding parts of this Memoir, the papers, etc., to which reference is made, are cited under the authors' names and the list numbers, the latter in italic figures. 10 11 13 U 15 16 Albrecht, P. Ueber eine in zwei Zipfel auslaufende rechtsseitige Vorderflosse bei einem Exemplare von Protopterns annectens. Berlin SitzBer. Ak. Wiss. 1886 (645-6). Aldrovandi, Ulysse. Monstrorum historia. Bologna 164-2 (429). Allis, E. P. Au abnormal musculus obliquus superior in Carcharias. Anat. Anz. Jena 16 (605-7). Audeville, A. d\ Un caa singulier de teratologic sur uu salmonide monstrueux. Bull. Soe. d'Acclimat. Ser. 4 6 1888 (990-993). v. Baer, K. E. Ueber doppelleibige Missgeburten oder organische Verdoppelungen in Wirbeltieren. St. Peter- burg Mem. Ac. Sc. Ser. 6 4 1845 (79-178). Barbieri, C. Sull' origine delle moatruosita embrionali doppie nei teleostei. Milano Atti Soc. ital. sc. nat. 46 1906 (100-106). Barbieri, C. Intorno ad una interessante moatruosita embrionale doppia nei salmonidi. Rivista mensile di Pesca Milano 8 1906 (89-91). Bariurtli, D. Eine Larva von Pelromyzon planeri mit drei Schwanzspitzen. Arcliiv EntwMech. Leipzig 8 1899 (27-31). Barfurth, D. Die Erscheinungen der Regeneration bei Wirbeltierembryonen. O. Hertwig's Handbuch der Entwicklungslehre Jena 1903 3 Teil 3 (1-129). Barrington, Dainea. A letter ...on some particular fish found in Wales. London Phil. Trans. R. Soc. 57 1768 (204). Bataillon, E. La pression osmotiquo et les grands pro- blemes de la biologic Arch. EntwMech. Leipzig 11 1901 (149-183). Bataillon, E. Blastotomie spontanee et larves jumclk-s chez Pelromyzon. Paris C. R. Acad. Sc. ISO (1201-2). BataiUon, E. Recherches experimentales sur revolution de la lamproie. Ibid. 130 (1413-5). Bataillon, E. Pression osmotique de l'oeuf et polyem- bryonie experimentale. Ibid. 130(1480-2). Bateson, W. Pilchards with the number of scales abnor- mally increased. London Proc. Zool. Soc. 1890(586-8). Bateson, W. On specimens of the common pilchard {Clupea pilchardus), showing variation in the number and size of the scales. Ibid. 18S4 (164). 17 Bateson, W. On two cases of colour variation in flat- fishes, illustrating principles of symmetry. Ibid. 1894 (246-9). IS Bateson, W. Note in correction of a paper on colour- variation in flat-fishes. Ibid. 1896 (890-1). 19 Bateson, W. Materials for the study of variation. London Macmillan 1894 (46, 58-9, 466-473, 563). 10a Bean, B. A. A remarkable carp. Forest and stream, New York N.Y. 73 1909 (1022). 20 Bellottl, C. Di una notevole varieta de colorazione della Tinea comune. Milano Atti Soc. ital. sc. nat. 44 1905 (218-220). :?1 Bellotti, C. Di una notevole varieta de colorazione della Tinea comune. Riv. mens. Pesca Milano 8 1906 ( 12-13). 22 Berg, L. S. Sur un exemplaire de la truite (Salmo trutta) d'une coloration etrange. St. Peterburg Ann. Mus. zool. 13 1908. 23 Bert, P. Reproduction de l'extremite caudale enlev^e chez des poisaons osseux. Paris C. R. soc. biol. Ser. 3 6 1863 (100-101). 24 Bird, M. C. H. A question of coloration. Zoologist London Ser. 4 6 (150-151). 25 Blanchard, R. Anomalies des nageoires chez le pro- toptere. Paris Bui. Soc. zool. 19 1894 (54). 26 Bolan, H. A yellow variety of Anguilla vu/garis. Arch. Naturg. Berlin 47 1881 (136-9). (Transl. in Ann. Mag. Nat. Hist. London Ser. 5 9 (65-7).) 27 Borcea, J. Sur un cas de conformation anormale de l'oviducte droit chez une petite roussette (Scyllium eanieula). Paris Bui. soc. zool. 39 1904 (138-148). 2S Boulenger, G. A. Renewed left pectoral limb of a Protopterns annectens. London Proc. Zool. Soc. 1891 (147-8). 29 Boulenger, G. A. Note on the variations of the lateral shields in the three-spined stickleback. Ann. Mag. Nat. Hist. London Ser. 6 11 1893 (228-9). 30 Boulenger, G. A. Exhibition of, and remarks upon, a remarkably malformed plaice {Pteuronectes platessa). London Proc. Zool. Soc. 1908 (161-164). 31 Braus, H. 1st die Bildung des Skcletca von den Muskel- anlagen abhangig 1 Eine experimentelle Unter- suchung an der Brustflosse von Haienembryonen. Morph. Jahrb. Leipzig 36 (240-321). xii LITERATURE RELATING TO TERATOLOGY OF FISHES 32 33 Brindley, H. H. Abramis blicca, Bl. : on a specimen without pelvic fins. London Proc. Zool. Soc. 1891 (108-9). Brindley, H. H. Note on some abnormalities of the limbs and tail of dipnoan fishes. Cambridge Proc. Phil. Soc. lO 1900 part 6 (325-327). 33a Buckland, F. Log-book of a fisherman and naturalist. London 1891 (151). 34 Bugnion, E. Description de quelques alevins de truite monstrueux. Bulletin de la Society Vaudoise des Sciences Naturelles Ser. 2 16 1880 (463-466). 35 Canestrini, R. Nota sui pesci mostruosi. Atti Soc. Ven. Trent. Sc. Nat. 8 fasc. 1 1883. 36 Carlet, M. G. Sur une truite inopse. J. Anat. PhysioL Paris 15 1879 (154-160). 37 Clark, F. Reproduction of a fish's tail. Amer. Nat. New York 8 1874 (363-4). 38 Cleland, J. On birds with supernumerary legs and on abcaudal fission and acephalus, with biological reflec- tions. Memoirs and Memoranda of Anatomy, Edin- burgh and London 1889 (1-13). 39 Cleland, J. Teratology speculative and causal and the classification of anomalies. Ibid. (127-136). 40 Coggi, A. Un'anomalia in un embrione di selacio. Bologna Mem. Ace. Ser. 5 2 1892 (763). 41 Cobbold, T. S. Description of a malformed trout, with preliminary remarks. Edinb. New Phil. Journal 73 1855 (238-242). 41a Cobbold, T. S. Notice of a variety of cod termed the Lordfish. Edinburgh Proc. Roy. Phys. Soc. 1 (51). 42 Coolldge, A. Monstrosities among trout. Amer. Nat. New York 3 1870 (288-290). 43 Coste, M. Origine de la monstruosite double chez les poissons osseux. Paris C. R. Acad. Sc. 40 1855 (868-876, 931). 44 Couch, J. Irregularities of structure in fishes. The Student and Intellectual Observer etc. London 1 1868 (328-336). 45 Couch, J. A history of the fishes of the British islands. London 1865 4 (238-9). 46 Cuenot, L. Presentation d'une sole a deux faces colorees. Paris C. R. soc. biol. 68 1905 (914-16). 47 Cuenot, L. Sur une sole a deux faces colorees. Arcachon Trav. soc. sci. station biol. 8 1905 (1906) (82-89). 48 Cunningham, J. T. An experiment concerning the absence of colour from the lower sides of flat-fishes. Zool. Anz. Leipzig 14 (27-32). 49 Cunningham, J. T. A peculiarly abnormal specimen of the turbot. Plymouth J. Mar. Biol. Ass. 8 1907 (44-46). 50 Cunningham, J. T. On a peculiarly abnormal specimen of turbot. London Proc. Zool. Soc. 1907 (174-1S1). 50a Cunningham, J. T., and MacMunn, C.A. On the colora- tion of the skins of fishes. London Phil. Trans. R. Soc. 184 1893. 51 Cuvier et Valenciennes. Histoire naturelle des poissons. Paris 1828-49 20 (72) ; 21 (335). 52 Dareste, C. Sur l'origine et le mode de formation des monstres doubles. Archives de Zool. exper. Paris 3 1874 (114). 53 Dareste, C. Recherches sur la production artificielle des monstruosites. Paris 1891 (469 et seq.). 04 56 57 5S Day, F. British fishes. London 1880-4 vol. ii. (68, 98- 101, 102, also pi. clxxi. fig. 2). Dean, B. Albinism, partial albinism and polychromism in hag-fishes. Amer. Nat. New York 37 (295-296). Dean, B. Reminiscences of holoblastic cleavage in the egg of the shark (Heterodontus (Cestracion) japonicus). Annotat. Zool. Jap. Tokyo 4 (35-41). Dohrn, A. (A double Torpedo embryo.) Studien zur Urgeschichte des Wirbeltierkorpers. Mitth. Zool. Stat. Neapel IS 1902 (31 Taf. v. fig. 2). Donnadieu, A. L. Sur une heuiiterie de volume observed chez une carpe. Paris C. R. Acad. Sc. 70 1870 (200-20). Variation und Asymmetrie bei Pleuro- Wiss. Meeresuntersuch. Kiel Ser. 2 3 Duncker, A. L. nectes flesus. (333-402). 60 61 Duncker, A. L. Uber Regeneration des Schwanzendes bei Syngnathiden. Arch. EntwMech. Leipzig 20 1906 (30-39) ; ibid. 24 1907 (656-661). Duncker, A. L. Syngnathiden-Studien. I. Variation und Modifikation bei Siphonostoma tt/phlc L. Hamburg Jahrb. wiss. Anst. 25 (1907) Beih. 2; 1908 (1-115). 61a Dyce, R. On the identity of Morrfma punctata and ilorrhua vulgaris. Ann. Mag. Nat. Hist. Loudon I860 (366-389). 62 Eigenmann and Cox. Some cases of saltatory variation. Amer. Nat. New York 36 (33-38). 63 Eigenmann and Kennedy. Variation notes. Biol. Bull. Wood's Holl Mass. 4 (227-229). 63a Eismond, Joseph. Ueber Regulationserscheinungen in der Entwickelung der in Teilstiicke zerlegten Rochen- keimscheiben. Arch. EntwMech. Leipzig SO ii. 1910 (411-436). 63b Ekman, G. Ueber einen Fall von Ruckbildung der letzten Kiemenspalte bei Squalu-s acanthias L. Helsingfors Ofvers. F. Vet. Soc. 62 No. 9 1901 (1-6). 6!f Elmhirst, R. Ambi-coloured flat-fish. Zoologist London 1911 No. 835 (30); Ann. Scott. Nat. Hist. Edinburgh 1911 (77-79). 66 Emeljanov, P. Zwillinge von Oirardinus caudimaculalus. Naturfreund St. Peterburg 2 1907 (239-242). 66 Fasciolo, Alba. Due cas di deformazione uel Labrax lupus. Atti Soc. Ligustica 16 (92-99). 67 Federley, H. MonstrOsa torskar (Monstrose Dorsche). Helsingfors Medd. Soc. Fauna et Fl. Fenn. 34 1908 (68-74). 68 Fiebiger, Josef. Ein Karpfen mit fehlender Schwanz- flosse. Oest. Fischerei-Ztg. Wien 6 1907 (83-85). 69 Filhol. {Rhombmmaximus Cnv., an anomalous specimen). Bull. Soc. Philom. Paris Ser. 8 2 (54). 70 Freund, L. Anomalien des Fischskeletes. Ergebn. allgem. Pathol, u. pathol. Anat. herausgeg. v. Lubarsch u. Ostertag. Jahrg. 11 Abt. 2 (709-729). 71 Fuhrmann, O. Un cas d'hermaphrodisme chez un vengeron (Leuciscus rutilus) du lac de Neuchatel. Neuchatel Bui. Soc. Sci. Nat. 36 (82-85). 78 Qadeau de Kerville, H. Note sur une plie franche et un flet vulgaire atteints d'albinisme. Paris Bull. Soc. zool. France 20 (155 and 156). 73 Gadeau de Kerville, H. Description d'un poisson et d'un oiseau monstrueux (Aiguillat derodyme et Goeland melomele). J. Anat. Physiol. Paris 28 1892 No. 5 (563-6). 74 Garman, S., and Denton, S. F. Abnormal embryos of trout and salmon. Science Observer Boston 6 1886 (1-7). LITERATURE RELATING TO TERATOLOGY OF FISHES xni 75 Garstang, W. Malformation of the mouth in tho common sea-bream. Plymouth J. Mar. Biol. Ass. Ser. 2 6 (345-347). 76' Gemmill, J. F. A contribution to the study of double monstrosities in fishes. London Proc. Zool. Soc. 1903 (4-23). 77 Gemmill, .1. F. The anatomy of symmetrical double monstrosities in the trout. Loudon Proc. Roy. Soc. 68 No. 444 (129-134). 78 Gemmill, J. F. On cyclopia in osseous fishes. London Proc. Zool. Soc. 1906 (413-449). 79 Gemmill, J. P. Notes on supernumerary eyes and loca deficiency and reduplication of the notochord in trout embryos. London Proc. Zool. Soc. 1906 (449-452). 80 Gervais. (Supernumerary pectoral fins on back of Iiaia clavata.) Paris C. R. Acad. Sc. 59 1864 (882). 81 Girdwoyn, Michel. Pathologie des poissons. Paris 1880. 52 Goehli. E. A. Further notes on the Amazonian Lepido- siren. Loudon Proc. Zool. Soc. 1898 (S55). 53 Grimm, Oskar. (Ein Stor mit drei Rogensacken. ) Ve^t. rybopromysl St. Petersburg 21 1906 (21-37). Si Grochmalickl, Jan. Uber die Linsenregeneration bei den Knocheufischen. Zeitschr. wissensch. Zool. Leip- zig 89 1908 (164-172). 86 Grosser, G., aud Przlbram, H. Einige Missbildungcn beim Dornhai (Acanthiai vulgaris). Arch. EutwMech. Leipzig 22 1906 (21-37). 56 Gunther, A. 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Descriptio monstrorum avium, amphibiorum, piscium quae extant in Museo Berolini 1824 8 p. 34 (quoted from 5 p. 88). 05 Hilgendorf, F. Ein krankhaft verandertes Gebiss eines Haifisches (Galeux naleus). Berlin SitzBer. Ges. Natf. Freunde 1891 (64-67). 06 Hofer, B. C'ber Missbildungen beim Hecht. Allg. Fischerei Zeitg. Miinchen Jahrg. 26 No. 1 (14-15). 06a Hofer, B. Haudbuch der Fischkrankheiten. Miinchen 1904. 07 Holt, E. W. L. Note on a young specimen of Zoarces vivipants. Ann. Mag. Nat. Hist. London Ser. 6 6 (256). OS Hopley, C. Observations on a remarkable development in the mudfish. Amer. Nat. New York 26 1891 (+87). 00 Houghton, Uev. W. British fresh-water fishes. London 1879. 100 Howes, G. B. Variation in the kidney of the common thornback (Ilaia clavata): its nature, range, and probable significance. J. Anat. Physiol. London 24 (407-422). 101 Howes, G. B. On some hermaphrodite genitalia of the codfish (tladus morrhua), with remarks upon the morphology and phylogeny of the vertebrate repro- ductive system. London J. Linn. Soc. 23 (539-558). 10J Howes, G. B. On the heads of two lampreys and a hag, showing some remarkable variations of the respiratory organs. London Proc. Zool. Soc. 1893 (730-733). 102a Howes, G. B. On synostoses and curvature of the spine in fishes, with especial reference to the sole. Loudon Proc. Zool. Soc. 1894 (96-100). 102b Hyrtl. Ueber Wirbelsynostesen u. Wirbelsuturen bei Fischen. Wien. Denkschr. XX. 1862 (95-110). Quoted from preceding. 103 Iherlng, H. Ueber Wirbelverdoppelung bei Fischen. Zool. Anz. Leipzig 1 (72-4). 104 Ishikawa, C. On the variations of the proportional lengths of the head, etc., as to the total length in our common eel. Ann. Zool. Jap. Tokyo 2 (125 and 126). Iwanzoff, N. Ein Fall von scheinbarem Hermaphrodi- tismus bei den Barsch (Perca fluviatilis). Moskva Bull. Soc. Nat. 1893 (119-205). 105 106 Jablonowski, J. Ueber einige Vorgange in der Ent- wickeluog von Salmonidenembryonen, etc. Jena Anat. Anz. 14 1S98. 107 Jackel, Hr. (An hermaphrodite carp.) Niirnberg Abhandl. Naturhist. Ges. 3 1867 (245). 10S Jacobi, S. L. (On the breeding of trout by impregna- tion of the ova : a letter to the editor of the Hannoversches Magazin 1765, pt. 62.) Translation in Yarrel 275 (87-96). 100 Jaquet, M. Anomalie de la region posterieure du corps chez un Silurus glanis. Bucharest Bull. Soc. Sc. 8 (786-791). 110 Jaquet, M. Ligne lat^rale supplemental chez un Acipenser ruthenus. Ibid. 8 (791-792). 111 Jaquet, M. Anomalie de la nageoire anale chez des Hebetates dactyloptera. Bull. Inst. Ocean. Monaco No. 79 1906 (1-6). 112 Jaquet, M. Description de l'extremite posterieure du corps anomale chez deux Motcllafusca. Ibid. No. 90 1907 (1-9). 113 Jarvi, T. H. Ein Fall von Hermaphroditismus bei Lota vulgaris. Helsingfors Medd. Soc. Fauna et Fl. Fenu. 36 1909 (226-227). 114 de Jeude, T. W. van Lidth. On deformities of the head in Salmonidae. Notes Roy. Zoolog. Museum Nether- lands Leyden 6 (259-261). 115 Johnson, R. H. The individuality and variation of the pyloric caeca of the Ctntrarchidae. Madison Trans. Wiss. Acad. Sci. 16 Part 2 1907 (713-732). 116 Johnstone, J. A malformed plaice. Trans. lav. Biol. Soc. 18 1903-4 (111-112). 117 Johnstone, J. Ichthyological notes. Ibid. 20 1906 (330-335). US Johnstone, J. Ichthyological notes. (1) An herma- phrodite hake. (2) Gurnard with malformed lower jaw. Ibid. 21 1907 (309-315). 110 Johnstone, J. An abnormal specimen of the brill Ibid. 23 1908-9 (200-202). xiv LITERATURE RELATING TO TERATOLOGY OF FISHES 120 Joseph, H. Ein Doppelei von ScyUium. (Nebst Bemerkiuigeii iiber die Eientwicklung.) Anat. Anz. Jena 29 (367-372). 121 Jussieu. (Two double monster fish.) Histoire de l'Acad. Roy. des Sc. 1754 (30). Quoted from 5 (88). 122 Kershaw, J. A. A colour variety of the common eel (Anguilla axistralis). Victorian Natural. Melbourne 20 (140). 123 Klaussner, F. Mehrfachbildungen bei Wirbeltieren. Miinchen 1890. 184 Knauthe, K. Ueber Melanismus bei Fisehen. Zool. Anz. Jena 16 1892 (25). 125 Knauthe, K. Ichthyologische Mitteil. Zool. Anz. Leipzig 14 (59-61, 259-261) ; 16 (109). 126 Knauthe, K. Zwei Falle von latenter Vercrbung der Mopskbpfigkeit bei Cyprinoiden. Biol. Centralbl. Leipzig (friiher Berlin) 13 1893 (766-767). 157 Knoch. Ueber Missbildungen betreffend die Embryonen der Salmonen und Corerjonus Geschlechts. Moskva Bull. Soc. Nat. 46 No. 2 1873. 12S Kohler, 0. Ein Analogon zu der bei Regeneration wiederholt beobachteten Gabelschwanzbildung der Eidechsen. Wochenschrift Aquarium-kunde, Braun- schweig 1 1904 (215). 129 Kopsch, F. Experiment. Untersuch. iiber den Keim- hautrand der Salmonidcn. Verhand. Anat. Ges. Berlin lO 1896 (113-121). 130 Kopsch, F. Experimented Untersuchungen am Primi- tivstreifeu des Hulmchens und an ScyUium Em- bryonen. Jena Verb.. Anat. Ges. 12 1898 (49-67). 131 Kopsch, F. Die Entwickelung des ausseren Form des Forellenembryonen. Arch. f. microsk. Anat. 51 1898. 132 Kopsch, F. Die Organisation der Hemididymi und Anadidymi der Knochenfiscbe und ihre Bedeutung fiir die Theorien iiber Bildung und Wachsthum der Knocheufischembryos. Internat. Monatschr. Anat. und Phys. Leipzig 16 1899 (221-263). 133 Kopsch, F. Gastrulation und Embryobildung bei den Chordaten. I. Leipzig 1904. 134 v. Krauss. (A malformed head.) Stuttgart Jahreshefte Ver. Natk. 42 (345). 135 Kyle, H. M. Note on the reproductive organs of a hermaphrodite ling. Glasgow Rep. Fish. Board Scot- land 15 Part 3 (396-39S). 136 Lachmann. (On a renewed pectoral limb.) Zool. Garten Frankfurt a. M. 32 (129). 136a Laurence, G. W. On a mouthless fish. Philadelphia Proc. Ac. Nat. Soc. 1876 (125-126). 137 Lavenier. IJne carpe monstre. Angers Bui. Soc. ^tud. sc. 24 (169-170). 13S Leger, L. Observation concernant une anomalie du cervelet d'un Alopias vidpes. Paris Bull. Soc. Philom. Ser. 7 11 (160-163). 139 Leger, L. Mutilation pathologique et regeneration chez le Protoptere. Paris C. R. soc. biol. Se>. 10 4 1897 (543-545). 140 Leonhardt, E. Ueber die Mopskopfbildung bei Abramis vimba. Zool. Anz. Leipzig 31 1906 (53-60). 141 Lereboullet, A. Sur la monstruosite' double chez les poissons. Paris C. R. Acad. Sc. 40 1855 (916, 1028, 1063). 142 Lereboullet, A. Recherches sur le developpement de la truite, etc. Ann. Sci. Nat. (Zool.) Paris Ser. 4 16 1861 (359-36S). 143 Lereboullet, A. Sur les monstruosiWs du brochet. Ibid. Ser. 4 20 1S63 (177-271) ; Ser. 5 1 1864 (113- 199, 257-320). 144 Levison, F. (A double-headed shark.) Nord. Med. Ark. 10 Hit. 2 No. 9 1878. Quoted from 247 2 (57). 144a Lichtenfelt, H. Literatur zur Fischkunde. Eine Vor- arbeit. Bonn 1906. 145 Loeb, J. Uber die Entwicklung von Fischembryoneu ohne Kreislauf. Arch. ges. Physiol. Bonn 54 1893 (525). 146 Lowne, B. Thomson. Catalogue of the teratological series in museum Royal College Surgeons, England. London 1893. 147 Luther, A. Hermafroditiska exemplar af Lota vulgaris. Helsingfors Medd. Soc. Fauna et Fl. Fenn. 35 1909 (227). 147a Malloch, P. D. Life-history and habits of the salmon, sea-trout, trout, and other fresh-water fishes. London 1910 (239-245). 14s Malm. Note sur la reproduction de parties de l'orga- nisme et sur leur multiplication chez certains animaux, et particulierement chez un syngnathe a deux queues. Ann. Sc. Nat. (Zool.) Paris Ser. 4 18 1862 (356- 358). 149 Martens, E. von. Hermaphroditische Fische. Berlin Naturforscher 1879 (116). 150 Masterman, A. T. On hermaphroditism in the cod. Glasgow Rep. Fish. Board Scot. 13 part 3 (297- 301). 151 Matthews, J. D. Oviduct in an adult male skate. J. Anat. Physiol. London 19 (144-149). 152 Mazza, F. Sulla rigenerazione delta pinna caudale in alcuni pesci. Atti Soc. Ligust. 1 1890 (318-321). 153 Mazza, F. Eteromorfie di alcuni pesci marini. Ibid. 4 1893 (427-435). 154 Mazzarelli, G. L'origine di un paio di pinne pettorali sopranonumerarie asimmetriehe in un avanotto di Salmo irideus. Acquicoltura lombarda Milano 7 No. 1-2-3 1905 (13-15). 155 M'Intosh, W. D. Notes from the Gatty marine labora- tory, St. Andrews. XXII. 1. On abnormal coloration in the Pleurotiectidae, and XXX. 2. On an abnormal plaice with a precaudal fin-frill on the left side. Ann. Mag. Nat. Hist. London 1902 (291-296) and 1908 (525-528). 156 Meckel, J. F. De duplicitate monstrosa commentarius. Halle u. Berlin 1815. 157 Mencl, E. Ein Fall von beiderseitiger Augenlinsenaus- bildung wahrend der Abwesenheit von Augenblasen. Arch. EntwMech. Leipzig 16 (328-339). 158 Mencl, E. 1st die Augenlinse eine Thigmomorphose oder nicht? Anat. Anz. Jena 24 No. 5-6 (169-173). 15Sa Miall, L. C. Malham Tarn and its fish. Handbook prepared for meeting of British Association, 1890. York (3-4). 159 Moenkaus, W. J. Material for the study of the varia- tion of Etheostoma caprodes and Etheostoma nigrum in Turkey Lake and Tippecanoe Lake. Indianapolis Ind. Proc. Acad. Sc. 1697 (207-228). 160 Moenkaus, W. J. An aberrant Etheostoma. Ibid. 1901 (115-116). 161 Morgan, T. H. Experimental studies on telcost eggs. Anat. Anz. Leipzig 8 1893 (803-814). 162 Morgan, T. H. The formation of the fish embryo. Journ. of Morph. Philadelphia Pa. IO 1895. 163 Morgan, T. H. Regeneration. New York Macmillan Company 1901. LITERATURE RELATING TO TERATOLOGY OF FISHES xv 164 Morrill, C. V., Jun. Regeneration of certain structures in Funduhis hettrocHtrua. Biol. Bull. Woods Holl Mass. 12 No. 1 1906. 165 Moser, P. Beschreibung einer Duplicitas anterior der Uachforelle und Bespreehung der Theorie von Fr. Kopach fiber Hildung des Waohstumszentrums fur Rnmpf und Schwanz. Auat. Anz. Jena 30 191 <1 (33-52, 81-106). 1GG Hudge, Geo. P. An abnormal dogfish (Scyllium cam- cula). Zool. Anz. Leipzig 30 (278-280). 1G7 Newman, H. H. A significant case of hermaphroditism in fish. Biol. Bull. Woods Holl Mass. 16 1908 (■2H7-214). IGS Ninnl, E. Sopra alcuni Pesei mostruosi raccolti nolle vulli del Veneto esturio. Boll. Soe. Zool.Ital. Ser. 2 4 (117-121). 1G9 Ninnl, E. Sopra due casi d'arresto dclla migrazione oculare (P/euronectes italicus, Solea vulgaris). Milano Atti Soc. ital. sc. nat. 44 1905 (193-197). 170 Ninnl, E. Metacromatismi in pesci raccolti nel mare e nelle lagune di Venezia. Atti del congresso dei Naturalisti italiani Milano 1907 (585-589). 171 Nusbaum, J. Zur Kenntniss der Heteromorphose bei der Regeneration der alteren Forellenembryonen (Salmo irideux). Anat. Anz. Jena 22 (35S-363). 172 Nusbaum, J., und Seymon, S. Beitrage zur Kenntnis der Regenerationsvorgiinge nach kunstlichen Ver- letzungen bei alteren Bachforellenembryonen (Salmo fario). Arch. EntwMech. Leipzig lO 1900 (615-684). 173 Nusbaum, J., und Seymon, S. Zur Teratologic der Knochenfische, zugleich eiu Beitrag zu deren Re- generation. Ibid. 24 1907 (114-123). 174 Nystrom, E. Coitus scorpius L. : on a monstrous speci- men. Bih. Sv. Ak. Haudl. 14 No. 10. 175 Oellacher, J. Ueber einen Doppelembryo von Trulia fario. Innsbruck Berichte des naturwiss. medic. Vereins 3 1 Heft 1873. 17G Oellacher, J. Terata mesodidyma von Salmo salvelinus. Wien Sitzber. Ak. Wiss. 68 1873 Naturw. (299-324). 177 Orlandl, S. Sopra un caso di ermaphroditismo nel Mwjil ckelo. Atti Soc. Ligustica Sc. 13 (3-6). 17S Otto, A. G. Monstrorum sexcentorum descriptio ana- tomica. Vratislaviae 1841 (71-73, 133, 266, Tab. iii. 5, xxiv. 6). 170 Panceri, P. Intornoall'albinismodel Clarias anqwllaris. Naples Rend. Ace. Nap. 12 1873 (110-114). ISO Panum, P. L. Beitrage zur Kenntniss d. physiol. Be- deutung der angebor. Missbildung. Archiv (Vir- chow's) path. Anat. Berlin 72 1878 (69-91, 165-197, 289-324). 181 Paoluccl, L. Sopra una forma mostruosa della Mylio- batis noctula. Atti della Soc. It. di sc. nat. 17 fasc. 1 Milano 1874. 18S 1S3 Pappenhelm, P. Ueber Augenverlust nnd Schadel-Ver- bildung bei einem Fisch. Berlin SitzBer. Ges. natf. Freunde 1905 (7-8). Pappenheim, P. Ein zweiter Fall von Mopskopfigkeit bei einem Lumpenus lampetriformis (Walb. ) aus der Apenrader Fohrde. Ibid. 1907 (349-350). lS3a Parker, T. J. On the intestinal spiral-valve in the genus Raia. London Trans. Zool. Soc. pt. ii. 1880 (49-61). 184 Patterson, A. Malformed codfish. Zoologist London 56 (130); ibid. 1911 (448, hermaphroditism). 186 Peach, C. W. On the so-called tailless trout of Islay. British Assoc. Report 1871 Transactions of the Sections p. 133. 1SG Pellegrin, J. Noto sur une anomalie des rayons ipineux du Proteracanthus. Bull. Mus. Paris 1899 (356- 357). 187 Pellegrin, J. Sur une raie monstrueuse de la famille des cycloccphaliens. Paris Bui. Soc. zool. 26 (106- 108). 1SS Pellegrin, J. Presentation de quelques cas de devia- tions rachidiennes chez les poissons. Ibid. 27 1902 (215-219). 1S9 Pellegrin, J. Sur une race monstrueuse de perches dauphins observee en Seine a Port-Villez. Paris Bui. soc. cent, aquicult. 20 1908 (42-46). 100 Pellegrin, J. Sur une race monstrueuse de perches. Paris Bui. soc. zool. 33 1908 (25-27). 101 Pettis, C. R. Albino brook trout. Science New York Ser. 2 19 (867-868). 10.7 Philllpeaux, J. M. Experiences demontrant que les nageoires des poissons ne se regenerent qu'a la con- dition qu'on laisse au moins sur placo leur partie basilaire. Paris C. R. Acad. Sc. 68 (C69). 103 Poey. (Hermaphroditism in Mesoprion and Chrysurus.) Ann. Lye. New York 9 (309). 104 Pouchet, G. Sur des cyprins monstrueux (C. auratua) venant de Chine. J. Auat. Physiol. Paris 71 1870 (561-569). 195 Przlbram, Hans. Experimental-Zoologie. II. Regenera- tion. Leipzig u. Wien 1910. 19G Punnett, R. C. On the composition and variations of the pelvic plexus in Acanthias vulgaris. London Proc. R. Soc. 68 (140-142) ; 69 (2-26). Formation des monstres doubles Paris C. R. Ac. Sc. 40 1855 (626- 197 Quatrefages, A. d\ chez les poissons. 8, 872, 925, 993). 7.95 Quatrefages, A. d'. MiSmoire sur la monstruosite double chez les poissons. Mem. pnbl. par la Soc. philom. a l'occasion du centenaire de sa foudation 1788-1888 Paris 1888. 100 Rathke. Abb. z. Bild. u. Entw. d. Mensch. a. Thiere 2 (62). Quoted from 5 (89). 200 Rauber, A. Die Theorien der excessiven Monstra. Virch. Arch. 71 1S77 (133-206); 73 1878 (551-594). 201 Rauber, A. Gastrodidymus des Lauhses. Ibid. 76 1879 (553). 202 Rauber, A. Formbildung und Formstorung Entwicklung von Wirbelthieren. Morph. Leipzig 5 (662-702) ; 6 (129-184). 203 Rennie, John. Accessory fins in Raia batis. Anz. Jena 28 1906 (428-431). 204 Riggio, (i. Sopra un caso di notevale ramificazione dei ciechi pilorici di Oentrolophus pompi/us. Natural. Sicil. Palermo 13 1894 (206-211). 205 Risso. Ichthyol. de Nice. Paris 1810 (49). Quoted from 213 (III. 202). 20G Rltcbie, J. An ambicoloured turbot with eyes approxi- mately normal in position. Ann. Scott. Nat. Hist. Edinburgh 1908 (146-150). 207 Ritchie, J. A hump-backed trout from Stranraer. Ann. Scott. Nat. Hist. Edinburgh 1908 (223-227). S08 Rosmini, Olga. Ricerche intorno alia variazione del Petromyzon planeri. Torino Boll. Musei Zool. Anat. 16 No. 390 (29 pp.). in der Jahrb. Anat. xvi LITERATURE RELATING TO TERATOLOGY OF FISHES 209 Roth, W. Doppelte Regeneration eines Bartfadens bei einem Panzerwelse. Blatter fur Aquarien- und Ter- rarienkunde 16 1905 (428). 210 Ryder, J. A. The inheritance of modifications due to disturbances of the early stages of development, especially in the Japanese domesticated races of gold carp. Philadelphia Pa. Proc. Acad. Nat. Sc. 1893 (75-94). 211 Sacchi, Maria. Un caso d'arresto deU'emigrazione oeulare, con pigmentazione del lato cieco in uu Rhombus maximus. Genova Boll. Musei zool. anat. comp. 1898 No. 67 (4 pp.). 212 Sacchi, Maria. Altre casi d'anomalie uel pleuronettidi. Ibid. 1899 No. 82 (3 pp.). 213 St.-Hllaire, Is. Geof. Histoire des anomalies de l'orga- nisation. Paris 1836 (I. 283-7 PI. I. 4-6 ; III. 159, 202 PI. XIV. 5). 214 Sandman, J. A. (Hermaphroditische Exemplare von Clupea harengus.) Fisk. Tidskr. Finl. Helsingfors 16 1906 (126-127). 215 Schmincke, Alexander. Die Regeneration der quer- gestreiften Muskelfasern bei den Wirbeltieren. I. Ichthyopsiden. Eine vergleichend pathologisch ana- tomische Studie. Verb, physik-med. Ges. Wiirzburg N.F. 39 No. 2 (15-130). 210 Schmltt, F. Systematische Darstellung der Doppelem- bryonen der Salmoniden. Arch. EntwMech. 13 1901 (34-134). 217 Schmitt.F. Ueber die Gastrulation der Doppelbildungen der Forelle mit bes. Beriicksichtigung der Kou- kreszenztheorie. Verh. d. Deutsch. Zool. Ges. Leipzig 12 1902 (64-83). 21S Schneider, G. Ueber einen Fall von Hermaphrodi- tismus bei Lota vulgaris. Helsingfors Medd. Soc. Faun, et Fl. Fenn. 29 1904 (103-105). 219 Schneider, G. Ueber einen Fall vou Hermaphrodi- tismus bei Qasterosteus aculeatus. Ibid. 30 1904 (7-8). 220 Schneider, G. Farbenvariationen des Flussbarsches (Perca fiuviatilis). Riga Korr-blt. Naturf. Ver. 61 1908 (41-46). 221 Schondorff, A. Ueber den Farbenwechsel bei Forellen. Ein Beitrag zur Pigmentfrage. Arch. f. Naturg. Berlin 69 (399-425). 222 Schwalbe, Ernst. Die Morphologie der Missbildungen des Menschen und der Tiere. I. Allgemeine Miss- bildungslehre Jena 1906 ; II. Die Doppelbildungen 1907. 223 Scott, G. G. Further notes on the regeneration of the fins of Fundidus heteroclittts. Biol. Bull. Woods Holl Mass. 12 1906 (3S5-400) ; see also 17 (343- 353). 224 Secques, F. Deux monstres gasteropages adultes de salmonides. Paris Bui. Soc. zool. 20 1895 (119- 123). 225 Segre, Rosetta. Ricerche intnrno alia variazione della Tinea vulgaris. Torino Boll. Mus. 17 No. 429. 220 Seligmann, C. G. Supernumerary dorsal fin in a trout. Journ. of Pathology and Bacteriology Edinburgh and London October 1898 (2 pp.). Also, Trans. Path. Soc. Lond. 49 (388-9 ; 390-3). 220a Simpson, J. Y. Article " Hermaphroditism " in Todd's Cyclopaedia of Anatomy and Physiology II. p. 697. London 1836-9. 927 Smallwood, W. M. Notes on the atrophy of the eye of Raia erinacea. Science New York N.Y. (N. Ser.) 28 1908 (930-931). 22S Smith, J. A. Notice on the occurrence of double or vertical hermaphroditism in a common cod-fish (Morrhua vulgaris) recently taken in the Firth of Forth. Proc. Roy. Phys. Soc. Edinb. 1864-5 (300- 302). 22Sa Smith, J. A. (Deformed codfish from the Firth of Forth.) Ibid. (302). 229 Smith, J. A. Notice on true hermaphroditism in the codfish (Morrhua vulgaris) and in the herring {Clupea harengus). J. Anat. Physiol. London 4 1870 (256-258). 230 Smith, W. R. A case of hermaphroditism in a haddock (Gadus aeglifinus). Glasgow Rep. Fish. Board 9 Pt. 3 (352). 231 Smitt, F. A. Description d'un hareng hermaphrodite. Arch. biol. (Paris-Bruxelles) 3 1882 (259-275). 23? Southwell, T. On a hermaphrodite example of the herring {Clupea harengus). Ann. Mag. Nat. Hist. London Ser. 7 9 (195, 196). 233 Spemann, H. Ueber experimentell erzeugte Doppel- missbildungen mit cyklopischem Defekt. Zool. Jahrb. Suppl. 7 Festchr. f. Weismann 1904. SS4 Stach, Jan. (Sandats a tete de carlin.) Okoln. Tow. ryb. Krakow 1906 (9-12). 23',n Steindachner, F. Uber das Vorkommen monstroser Kopfbildungen bei dem Karpfen. Wien Yerh. zool. hot. Ges. 13 1863 (485-487). 235 Stewart, C. On a hermaphrodite trout (Salmo fario) ; and on a hermaphrodite mackerel (Scomber scomber). London J. Linn. Soc. (Zool.) 24 (69-70, 70-71). 230 Stockard, C. R. The development of Fundidus heteroclitus in solutions of lithium chloride, with appendix on its development in fresh water. J. Exper. Zool. Philadelphia Pa. 3 1906 (99-120). 237 Stockard, C. R. The artificial production of a single median cyclopean eye in the fish embryo by means of sea water solutions of magnesium chloride. Arch. EntwMech. Leipzig 23 1907 (249-258). 23S Stockard, C. R. The question of cyclopia. Science New York N.Y. (N. Ser.) 28 1908 (455-456). 239 Stoddart, T. T. Art of angling. Edinburgh 1836 (75). 240 Btoll, A. A. (Ueber Bastarde von Teleskopen und anderen Varietaten des Goldfisches.) Naturf reund St. Peterburg 2 1907 (391-400). 21,0a Storch, O. Untersuchungen iiber die paarige Afterflosse der Schleierschwanze. Wien Arbeiteu Zool. Inst. Universitat 19 2 (1-24). 241 Storrow, B. A case of spinal curvature in a codling. Northumberland Sea Fisheries Report for 1909 (37- 39). 24.' Sumner, F. B. A study of early fish development, experimental and morphological. Arch. EntwMech. Leipzig 17 1904 (92-149). 243 Suomalalnen, E. W. (Rote Form von Perca fiuviatilis.) Helsingfors Medd. Soc. Fauna et Fl. Fenn. 34 1908 (33). C44 Supino, F. Considerazioni sulla teratologiaspirimentale. Bull. Soc. Veneto-Trent. 6 (43-48). 245 Sutton, J. B. Evolution and disease. London 1890 (121). 240 Suworow, E. K. Ueber die Regeneration der Flossen bei Knochenfischen. Arbeiten der kaiserlichen Gesell- sehaft der Naturforscher Petersburg 33 1904 Lieferung 4 (1-49). 247 Taruffl, O. Storia della teratologia, I. -VIII. Bologna 1894. LITERATURE RELATING TO TERATOLOGY OF FISHES xvii 24S Terni, C. Esoftalmia epizootica in avanotti di Salmo fario L. Riv. mens. Pesca Milano lO 1908 (1-3) (pathological). 249 Tornier, G. Vorlaufiges iiber das Entstehen der Gold- tisehrassen. Berlin SitzBer. Ges. natf. Freunde 1908 (40-45). 250 Tornier, C. Ueber experiment elles Hervorrufen und Naturentstehn von Mopskopfen, Cyclopen und andere vorgebiirtlichen Kopfverbildungen bei Wirbelthieren. Ibid. 1908 (298-315). 961 Traqualr, R. H. On the 60-called tailless trout of May. J. Anat. Physiol. London 6 (411-6). 251a Traquair, R. H. On the Asymmetry of the Pleuro- nectidae. London Trans. Linn. Soc. 25 ('263-296). 96S Traquair, R. H. On specimens of tailless trout from Loch Enoch, Kirkcudbrightshire. Edinburgh Proc. Phys. Soc. 1882 (221-3). 253 Traquair, R. H. Note on an abnormally developed thornback (Rata clacata). Ann. Scott. Nat. Hist. Edinburgh 1892 (29, 30). 254 Traquair, R. H. On malformed trout from Scottish waters, No. 1. Ibid. 1892 (92-103). 255 Traquair, R. H. An unusually coloured example of the thornback (Rata clavata). Ibid. 1893 (25). 256 Traquair, R. H. On a peculiar Charr from Inverness- shire. Ibid. 1898 (78-79). 257 Trois, E. F. Sopra un esemplare di Anguilla con spiccato metacromatismo. Venezia Atti 1st. ven. 67 1907-08 (65-66). 25S Trois, E. F. Nota sopra una forma di metacromatismo osservata in un esemplare di Pleuronectes italicus. Ibid. 67 1907-0S (221-222). 259 Trois, E. F. Sopra 1' anomale colorazione della pelle osservata in un esemplare mutilato di Lophius piscatorius. Ibid. 68 1908-09 (43-45). 260 Trois, E. F. Contributo alia conoscenza di forme di metacromatismo osservate in pesci raccolti nella laguna di Venezia, Ibid. 68 1908-09 (113-115). S61 Vaillant, L. Monstruosite de la limande commune (Pleuronectes limanda). Bull, de la Soc. philomat. de Paris 4 No. 2 (49). 262 Vaillant, L. Sur un individu monstrueux nycteridoide du Raia clavata. Bui. Museum Paris 1908 (112- 113). 963 Valentin, G. G. Ein Beitrag zur Entwickelungs- geschichte der Doppelmissgeburten. Archiv fur physiolog. Heilkunde 10 Jahrgang 1851 (1-40): see also Paris C. R. soc. biol. 4 1852 (92-102). 264 Vayssiere, A. (Double egg in porbeagle shark.) Paris C. R. soc. biol. 1909 (872-3). 265 Vogt, C. Notice eur un hareng hermaphrodite. Arch. Biol. (Paris-Bruxelles) 3 1882 (255-8). 266 Vrolik, W. Tabulae ad illustrandum embryogenesin hominis et mammalium. Amsterdam 1849 Tab. 61 Fig. 6. 267 Warpachowslri. Silurus glanis, L. : on a specimen with supernumerary ventral fin. Anat. Anz. Jena 1888 (379). 267a Watase, S. On the caudal and anal fins of goldfishes. Tokyo Journ. Coll. Sc. Japan 1887 (247-267). 26S Weber, M. Ueber Hermaphroditismus bei Fischen. Amsterdam Ned. Tijdschr. Dierk. Ver. pt. 1 (21-43) ; ibid. Ser. 2 1 (128-134). 269 Williamson, H. C. On two cases of hermaphroditism in the cod (Gadus callarias). Glasgow Rep. Fish. Board 24 part 3 1906 (290-292). 269a Williamson, H. C. (Influence of cold on herring fry.) Ibid. 27 part 3 (119). 270 Williamson, H. C. Notes on abnormalities in Lophius, Oadus, Raia. Ibid. 28 part 3 1911 (53-56). 271 Windle, B. C. A. Notes on certain malformations in fishes. Birmingham Proc. Nat. Hist. Phil. Soc. 6 (318-321). 572 Windle, B. C. A. On double malformations amongst fishes. London Proc. Zool. Soc. 1895 (423-9). 273 Woodward, A. S. Rhinoptera, Kuhl : note on abnormal dentition. Ann. Mag. Nat. Hist. London Ser. 6 1 (281-283). 274 Woodward, A. S. Note on a case of subdivision of the median fin in a dipnoan fish. Ibid. Ser. 6 11 1893 (241-242). 275 Yarrell, W. A history of British fishes. Second ed. London 1841 Vol. II. (87-96, 107, 109). 276 Tung, E. Note sur un cas de monstruosite' de la tete chez une truite. Rev. Suisse zool. Geneve 9 (307- 313). / CHAPTER I. DOUBLE MONSTROSITY. (For Contents see p. vii.) A. LITERATURE. References will be found under the following index figures (pp. xi-xvii) : Salmonidac — d'Audeville 4', Barbieri 6-7; Bugnion 34-', Coolidge 42; Coste 4-3; Dareste 52; Garman and Denton 74; Gemniill 76-77; Girdwoyn 81; Jacobi 108 ; Klaussner 123; Knoch 127; Kopsch 132-133; Lovme 146; Moser 165; Oellacher 175-176; Panum 180; Quatrefages 197-198; Rauber 200-202; Schmitt 216-7; Schwalbe 222, ii. p. 297 et scq.; Secques 224; Sutton 245; Taruffi 247; Windle 271-2; Yarrell 275, ii. p. 107. Other Fishes : — Perca — v. Baer 5; Leuciscus — Bataillon 11; Annrrichas — Buckland 33a; Girardinus — Emeljanov 65 ; Esox— Klaussner 123, Lereboullet 141-143, Rauber 200, Rauber 202, Valentin 263 ; Blcnnius — Rathke 199 ; Scomber — Sutton 245 ; Selachoidei — Aldrovandi 2, v. Baer 5, Gadeau de Kerville 73, Heusner 94, Klaussner 123, p. 12, Levison 144, Lowne 146, Quatrefages 198, Risso 205, St. Hilaire 213, Sutton 245; Torpedo— Dohrn 57; Pctromyzo7i—Ba,ta.i\lon 12. See also pp. 30-32. B. OCCURRENCE, RECORDS, AND GENERAL OUTLINE. From the scattered data available, it would seem that the frequency with which double monstrosity makes its appearance in the development of fishes varies in different species and in broods from different parents within the same species. The following figures apply to the Salmonidac: 1 in 50, and 1 in 280 (Rauber 200 and 202); 1 in 600, none in 600, and 68 in 900 (Schmitt 216); 1 in 200, and 1 in 350 (author's observation); over 100 in 400,000 (Coste 43). Oellacher (176) notes a remarkable brood in which the proportion probably reached as high as fifty per cent., or at any rate twenty to thirty times more than the average. Here, how- ever, the duplicity was of the peculiar and imperfect type described by this author as mesodidymus. On the other hand, v. Baer (5) examined over 3000 eggs of Cyprinus blicca without result. In Perca fluviatilis he obtained two double embryos out of a set of forty eggs, although a very much larger number (over 1000) in other sets provided none. Lereboullet (141-143) examined in all 203,962 eggs of the pike (Esox lucius), obtaining 222 double monsters, an average of 1 in 920. In the same species Rauber (202) found a single example in a set of 325 eggs, and Valentin (263) six out of 917 hatched embryos. In Petromyzon, Bataillon (12) records an extraordinary case where forty out of a hundred eggs developed twin gastrulae. We owe to Lereboullet (143) the illuminating statement that within the same species (Esox lucius), the prevalent types of monstrosity as well as the frequency with which these occur vary somewhat in different groups of eggs. Much work, however, remains to be done on frequency and type in relation to parentage, alike on the male and on the female side, though some interesting figures bearing on the latter are given by Rauber (202 6, 129-184). It is worthy of note that the frequency with which double monstrosity appears in the eggs of fishes, A 2 DOUBLE MONSTROSITY— OCCURRENCE, RECORDS, ETC. 13 not far from corresponding with its frequency in those of the fowl. Dareste's (53) figures showed out of 10,000 eggs, 38 examples of double and 2 of triple monstrosity, an average of 1 in 250. Eecords of double monster fish do not seem to have come down to us from the ancients, and although the Fish has been employed in symbolism and mythology since earliest times, no shape it has received seems to indicate such acquaintance with its teratology as is undoubtedly implied for that of the higher animals in the Janus, Cerberus, Cyclops, and other fables. The reason is not far to seek. With the rarest possible exceptions, monster fish die off so soon as the store of food contained in the eggs on which they develop has been exhausted, and at this period they are still small in size and remote from ordinary observation. The first notice of a double monster fish appears to be that contained in the Monstrorum Historia of Aldrovandi (2), a work brought out some thirty years after Aldrovandi's death by his loyal pupil Ambrosini. The notice bears out that a two-headed fish almost as large as a crocodile was caught in the Nile near the town of Latislana. The figure appended is that of a monster shark-like fish. No shark-like fish, however, seems to be the inhabitant of the Nile (Boulenger, Brit. Mus. Cat. " Fresh- water Fishes of Africa," i. 1909). Examples of double-headed shark or dog-fish embryos are not unknown (see under Selachoidei, above, and on p. 31), and in all proba- bility some such specimen supplied the substratum of the record and story. The only instances of survival or reputed survival of fish exhibiting a major type of monstrosity which I have been able to come across are the following: (1) The story in Aldrovandi given above; (2) Perfectly authentic accounts by Secques (224) of a gasteropagous twin Salmo lacustris and of another similar twin Salmo fario. Both examples were about fifteen months old. In the first, one of the twins was larger than the other and measured about 6 J inches. In the second, the twins were of equal size, each being rather more than 5 inches in length. (3) A notice in Yarrel (275, ii. p. 107) from a Cambrian newspaper of 28th November, 1829, regarding the capture of a " fine Salmon with two heads and two tails. The heads are joined on one neck and the tails meet about the centre. The fish is now to be seen alive in a small pool at Llangattock." (5) Account of a cyclopic ray by Paolucci (181), referred to further on p. 43. F. Buckland (33a) also mentions that he " once read an account of a double-headed catfish (Anarrichas lupus) having been caught in the North Sea." Jacobi (10S) seems to have been the first not only to make systematic observations on the artificial breeding of trout, but also to note and describe the monstrosities which made their appearance in the broods, and to speculate on the causes of their occurrence. A translation of his highly interesting original letter on the subject will be found in Yarrell (275, ii. 87-96). As teratology developed into an exact science in the hands of Meckel (156), St. Hilaire (213), v. Baer (5), Yrolik (266), and others, careful attention was paid to the study of monstrosities in fishes, particularly since their growth and development could be observed in the living condition with the aid of the microscope. It is probably to St. Hilaire's influence that we owe the studies of Valentin (263), Quatrefages (197-8), Coste (4-3), and Lereboullet (14-1-3), which followed each other closely towards the middle of last century. Lereboullet's work is particularly valuable, and still remains as containing standard results. Some of these have been already noted, while others are referred to under Causation (p. 5) and Structure (p. 10). Later, the careful studies in normal embryology, of which His's observations on the develop- ment of the bony and cartilaginous fishes are an example, had their influence in stimulating renewed observations by means of the newer methods on the origin and structure of monstrosities in fishes (Oellacher 176, Bauber 200-2). Very useful work along descriptive, classificatory, and historical lines has also been done by Knoch (127), Panum (180), Bauber (200-2), Klaussner (123), and more lately by "Windle (272) and Schmitt (216). The methods of experimental embryology, particularly in the hands of Kopsch (129-133) and Morgan (161-2), have supplied evidence of the greatest value in regard to the processes of normal and teratological development. On the structural side, our knowledge of detail has gradually been elaborated by the work of Bauber (202), Oellacher (176), Schmitt (216), Moser (165), Barbieri (6-7), and the author (76-77). DOUBLE MONSTROSITY— CLASSIFICATION 0. CLASSIFICATION. In Teratology, the terms (1) anadidym/us, (2) katadidymus, (3) anakatadidymus, and (4) meso- didymus are accepted as indicating respectively (1) doubling at the anterior end, (2) doubling at the posterior end, (3) doubling at both anterior and posterior ends, and (4) doubling in the middle region of the body. As regards fishes, it should be noted that the first (anadidymus) is much the most important ; the third is practically confined to examples of union by the yolk-sac only ; while the second and fourth, so far as seems to be known at present, do not provide examples of com- plete duplicity (p. 25). "We owe the first three terms (anadidymus, katadidymus, and anakatadidymus) to Forster (Missbildungm des Mrnschen, Jena, 1861, pp. 22, 29, 34), but it is to be noted that this author assigned to the first two of them a meaning exactly the converse of that in which they are now employed, having apparently applied the initial preposition to indicate the region of union instead of the region of separation. His anadidymus is thus the katadidymus of later authors. The fourth term, mesodidymus, we owe to Oellacher (176), but for reasons that will appear afterwards (p. 25) this term (as well as katadidymus) may with advantage be replaced by hemididymus, the substitute suggested by Eauber (200 79) and adopted by Kopsch and others. Of the groups above named, only the anadidymi, by reason of their number and variety, require detailed subdivision. The various schemes proposed need not be given here. Most authors dealing with the subject have adopted simpler or more elaborate methods of classification depending on the level at which union takes place. For external and descriptive purposes the scheme given by Windle (272) will be found convenient and inclusive. His classes are as follows: 1. Head union, three eyes present, all of the same size. 2. Do. three eyes present, the median one being larger than the others. 3. Do. four eyes present. 4. Two quite separate heads. 5. Fission extending to the pectoral region. G. Duplicity extending to the posterior border of the yolk-sac, the caudal extremity being quite single. 7. Duplicity extending a short distance behind the posterior border of the yolk-sac, the caudal extremity being quite single. 8. Duplicity extending to the posterior border of the yolk-sac, the caudal extremity not, however, being perfectly single. 9. Union by the caudal extremities alone. 1 0. Union by the ventral aspects at the site of attachment of the yolk-sac. 1 1 . Parasites, one member becoming an appendage to the other. A less convenient, though logically sound system, is proposed by Schmitt (216), who would divide double monsters in fishes according to the manner in which they come together at the region of union. He thus distinguishes : (1) Union by the yolk-sac only. (2) Simple ventral union. (3) Union principally ventral, but partly lateral. (4) Union half lateral and half ventral. (5) Union principal] v lateral, but partly ventral. (6) Simple lateral union. (7) Union so intimate that externally the duplicity is concealed. The classification which will be adopted in this work is given below. It has reference alike to external features and to important points of internal structure. Class I. Union in head region, the twin brains uniting at the optic lobes.1 Class II. Union in head region, the twin brains uniting at the medulla oblongata. 'Regarding the possible need for a Class to come in advance of my Class I., as showing a still less marked degree of anterior duplicity, see the second last paragraph under Reduction and Parasitism (p. 59). f) 3 J> 4 » 5 if 6-7 it 8-9 >j 10 no t represented, 4 DOUBLE MONSTROSITY— CLASSIFICATION, CAUSATION Class III. Union in pectoral region, adjacent pectoral fins not being present. Class IV. Union in pectoral region, adjacent pectoral fins being present, but united and reduced in size. Class V. Union by the body or tail, the united portion ending as normally in a single symmetrical tail. Class VI. Union by the body or tail, the united portion ending in a composite tail with various structures still doubled. Class VII. Union by the yolk-sac only, or by the yolk-sac and the ventral edge membrane close behind it. Class VIII. The condition of imperfect doubling exhibited by (a) the (hemi)mesodidymi and (b) (hemi)katadidymi (p. 25). Class IX. Longitudinal or parallel union. Comparing this scheme with the one given by Windle it will be seen that Class I. corresponds (approximately) with 1-2 of Windle. „ HI- IV v VI „ VII. „ VIII.l „ ix. J With regard to parasitic forms, though it may be convenient for descriptive purposes to group them together, as in Class 11 of Windle's scheme, they should more properly be distributed among the other classes according to the attachment of the parasite. Schmitt's scheme and mine do not so readily fit into one another. From the practical point of view the former is chiefly useful in the analysis of the forms which are grouped under Classes V and VI. of the latter (pp. 20-23). It must be kept in mind that any scheme for the classification of anadidynii can only aim at dividing up into convenient groups a series of forms which is essentially continuous and unbroken, the members differing from one another by characters which depend mainly on the distance which separated the component twins at the time of their first appearance on the margin of the blastoderm. As will be explained under E (p. 6), these components are brought together during the natural elongation of their embryonic axes, and unite in what is designated primary fusion. On the other hand, it is probably as a result of the working of secondary fusion that such discontinuities as can be recognised in the series take their origin — for example, that twins united in the head region can always (so far as my experience goes) be grouped under Class I. or Class II., the final union of their brain cavities being fixed either in the region of the optic lobes or in that of the medulla. In the same way, though in a less striking degree, secondary fusion has helped to mould the structural details in the succeeding groups as well as to accentuate the external characters which have been employed above in their classification. Other references are made to secondary fusion on pp. 11, 15, 30, 43. D. CAUSATION. There is strong reason for believing that the occurrence of double monstrosity is due in the main not to environmental factors, but to conditions which are inherent hi the fertilised germ cell. Lereboullet's {US) observations on the pike went to show that in spite of sameness of environment, different broods of eggs give rise to different prevailing types, as well as to different propor- DOUBLE MONSTROSITY— CAUSATION 5 tionate numbers of monstrosities. So far as one can gather, this seems also in a general way to be the experience of fish hatcheries, though full and careful records from these are not as yet available. Von Baer's (-5), Oellacher's {17 G), and Rauber's {203) observations point to the same conclusion, which may also help to explain the exceedingly wide variations in the other frequency records (p. 1). Lereboullet farther notes that samples of the same brood of eggs tend to exhibit similar types and numbers of monstrosities even under differences of environment. Indeed, the only constant result which could be ascribed to the action of external factors was the production of defects in development. It will be noted that this corresponds on the whole with the data afforded by Dareste's {53) experiments on the eggs of birds. Evidence may also be adduced on the general question from a consideration of the relative frequencies of double and triple monsters (see note on p. 33). The likelihood cannot, of course, be excluded, that external factors sometimes induce the production of double monstrosity in the developing eggs of fishes. As is well known, this has been demo experimentally in the case of many invertebrate and of some vertebrate ova. The result has usually been obtained through virtual or complete separation of individual cells or of cell masses in the earlier segmentation stages. It will obviously be difficult to produce the requisite degree of separation in typical meroblastic ova, and particularly in those of the Sahnonidae, where the earliest divisions affect the nuclei alone, and the first stages in segmentation are syncytial ones.1 The above circumstances may help to explain the common failure of experiments on twin production in the eggs of osseous and cartilaginous2 fishes as compared with the fruitful results of similar experiments in the Amphibia, and to some extent also in the lamprey (Batadlon 12). This last author's {14) apparent success in producing polyembryony in the eggs of Leuciscus, through differences of osmotic pressure, should also be noted (see p. 30), but it does not by any means invalidate the view that the ordinary occurrence of duplicity in fishes is germinal rather than environmental in its origin. The same thing holds good regarding the occasional production of katadidymus under experimental conditions (Lereboullet 14-3, Stockard 236, Kopsch 132). In fishes this condition provides an entirely peculiar type of duplicity (p. 25). The production from a single ovum of twins or of twin embryos more or less united has long been known to occur from time to time in all classes of vertebrate animals. Eecently polyembryony has been shown to be the normal condition for the edentate mammal Tatusia? in which the seven or eight young produced at a birth all develop from a single egg. The view has often been suggested that the blastoderm may be looked upon as a stock, able to give rise vegetatively, so to speak, to more than one embryo. The natural comparisons have been drawn between this faculty and the alternation of generations which occurs normally in some groups of lower animals and in plants. It has even been sought to recognise alternation of generations in the development of all animals. More probably, however, in animals, twinning, double and multiple monstrosity, polyembryony, and alternation of generations, provide instances in which a common " potentiality " has become realised, and beyond that are not necessarily connected by any nexus of a direct or phylogenetic character. 'Kopsch (F.) in Arch. Mikr. Anat. 78, 1911 (618-659). 2 Recently Eismond (G3a) has given an interesting account of experiments on the eggs of Raia clavala and Raia alba (luring the early segmentation stages. He finds that separated portions of a blastoderm often show remarkable activity in reuniting to form a whole, which may then proceed to normal development. On the other hand, the separated portions may remain apart or even undergo further spontaneous division, and a number of embryonic rudiments may arise in the complex thus produced. The number of rudiments in one particular case was four. Two of these were found on one of the fragments ; the other two occurred each on a single fragment, whilst two of the fragments provided none. Later, the clefts between the fragments disappeared completely, and the blastoderm, now single to all appearance, proceeded to extend over the yolk. Two of the rudiments were, to begin with, at an angular distance of about 55° from one another on the margin of the blastoderm. A day later these rudiments were found to have become closely approximated. They were, however, no longer normal in appearance, but seemed to be in process of degeneration, and aceordinglj' it was judged best to fix and preserve the whole specimen at this stage. 3 Fernandez (M.), " Beitrage zur Embryologie der Gurteltiere," Morph. Jahrb., Leipzig, 30 (302-333). DOUBLE MONSTROSITY— DEVELOPMENT, ETC. E. DEVELOPMENT AND EARLY GROWTH. It will be remembered that in the normal development of most fishes the segmentation of the egg is partial, and leads to the formation of a cap-like or disc-like blastoderm, the edge of which becomes thickened at one part and gives rise to the rudiment of the embryo. Gastrulation takes place here, a " primitive streak " being formed, in part at least, by apposition of adjacent portions of the blastodermic margin (Kopsch ; see further on p. 27). According to the concrescence theory (associated particularly with the name of His), the body of the embryo is formed by the coming together of the thickened margins of the blastoderm during the process by which this layer gradually overgrows the yolk. The concrescence theory in its original form is no longer tenable, but the fact still remains that the thickened margins of the blastoderm, close to the developing embryonic axis, are used up during the early growth of the embryo, not in producing the axial organs, but in contributing material for the formation of the muscle plates and the lateral and ventral body walls. This fact is of cardinal importance in connection with what follows. A second point of great importance is that, while the blastoderm rapidly extends and grows over the yolk, the extension sooner or later slackens markedly in the region of the blastopore. The recorded observations indicate that double monster fishes always arise on a single yolk, and from a single blastoderm, at the margin of which two more or less separate centres of gastrulation (Hertwig 93a) and embryo-formation have made their appearance. (We are leaving mesodidymus and katadidymiis out of count till later (p. 25), because in fishes they constitute types of peculiar and imperfect duplicity. What is said on p. 28 regarding longitudinal or parallel union should also be borne in mind.) The twin centres of embryo-formation men- tioned above may be classed in two groups (a) and (b), according to the distance which separates them from one another. («) In the first and most important group the interval is not too great to prevent approximation and union of the two embryonic axes from taking place during the natural course of their growth in length (see Text- figs. 1, 2 on this page). Approximation and union are due to the two factors to which attention was directed above, namely, the utilisation during growth of the blastodermic margins near the primitive streak, and the slowness of expansion on the part of the blastoderm over the yolk in this same region. The twin adjacent axes are inevitably brought together posteriorly through disappearance of the interval between them. The process may be called one of primary fusion, in contrast with a process which often supervenes later, and which consists in the secondary fusion of organs or structures already laid down. Primary fusion takes place earlier or later, i.e. in the head-, body-, or tail-region, according to the interval which separated the embryonic rudiments when they first appeared. In other words, the degree of duplicity varies directly with the original distance between the two centres of embryo- formation. When the union is a purely lateral or an approximately lateral one, the posterior united part finally becomes simply and perfectly bilateral. This takes place through the gradual fusion and disappearance of structures belonging to the left and right halves respectively of the right and left component embryos. Thereafter the right and left halves of the right and left embryos unite naturally to form a normal bilateral body or tail. On the other hand, when the twin bodies come Fig. i. Fig. i.— After Lereboullet (143 1863, PI. III. fig. 14). Egg of the pike observed fifty-three hours after fertilisation, a, blastoderm spreading over the yolk ; ?>, the portion of yolk not yet covered by the blastoderm ; c C*, the two embryonic rudiments. Fig. 2. — After Lereboullet (as above, fig. 15). Anterior part of the same double embryo observed twenty-six hours later (at com- mencement of fourth day), c c't the two heads showing optic vesicles and uniting in the hind-brain region. Further back is seen the commencement of the muscle somites. DOUBLE MONSTROSITY— DEVELOPMENT, ETC. together by ventral rather than by lateral union, the formatinn posteriorly of a perfectly single lnnly or tail becomes impossible, since the necessary readjustments of right and left structures in the twin embryos can no longer take place. (b) In the second group, the twin centres of embryo-formation are so far apart that there is no compelling influence of the kind described above which would lead to the approximation and union of their growing embryonic axes. Accordingly the twin bodies remain separate, except for the adventitious union supplied by the layers forming the wall of the common yolk-sac. Text-fig. 3 illustrates an early example in which the twin embryos are almost at opposite sides of the blastoderm. In the higher animals the corresponding type of doubling may give rise to completely separate unioval or homologous twins. As is well known, in the higher animals well-developed unioval twins are always of the same sex, and they tend to show a remarkable degree of resemblance even in minute characteristics. In fishes, however, one of the twins is not infrequently smaller than its neighbour, or otherwise defective, no doubt through injury or want of room for growth. The defect sometimes takes the form of a pure cyclopia. Accordingly we have here a reason for believing that the natural occurrence of cyclopia need not be occasioned by conditions inherent in the fertilised egg, since other- wise both heads ought to be similarly affected. There do not seem to be any direct observations enabling one to say what amount of angular distance must separate the twin embryonic rudiments on the egg of the trout in order that the twin bodies may remain apart from one another throughout their whole length. Taking it as a matter of chance at what part of the blastodermic rim each of the twin rudiments appears, one might seek evidence on the question by comparing the relative frequency with which union of the twin bodies and union by the yolk-sac only {anakatadidymus) are found to occur. Windle's figures (272) show two anakatadidymi out of a total of forty-six double forms. My own numbers are seven out of seventy-one. Summation of both gives an average of one in thirteen. Dividing the whole circle of the blastoderm (3G0°) by thirteen we get rather less than 30°. In the trout it would seem accordingly (if one assumes the correctness of the principle on which the calculation is founded) that, in order to remain axially separate, the twin embryonic rudiments must have appeared on the rim of the blastoderm not less than 165°, i.e. 180°— 15", from one another. Probably this estimate of distance is too great. A larger number of instances to average from would likely reduce the figure, but Kopsch's (133-3) experiments seem to show that in any case it is over 90°. All double monsters in fishes which are produced in the manner described above will be examples either of anterior duplicity or of union by the yolk-sac only. Further, in all cases the two heads will point in the same direction. Klaussner, however (123, PI. V., Fig. 34), figures an example of union by the yolk-sac only, in which the twin heads are pointing in opposite directions, i.e. the head of the one lies alongside the tail of the other. This seems to be the solitary recorded instance of the kind, and Windle (272), in calling attention to it, notes that it is "difficult to account for by any of the theories now holding the field." Klaussner 's figure is not a perfectly convincing one, but if correct could readily be explained by assuming the presence of two originally separate blastoderms on the parent egg. Such a condition, although it has never been observed in the eggs of fishes, is not unknown in those of other vertebrates. A number of instances are given below. The foundation of our knowledge regarding the mode of growth and union of double monster fish embryos was laid by Lereboullet's work (14-3) on the living eggs of the pike (see p. 10). Very early stages (see pp. 9-10) have also been figured for the same species by Bauber (2U0), and for Fig. 3.— After Rauber (202 6, Taf. VIII. fig. 14). Egg of trout fourteen days after fertilisation, showing two em- bryonic rudiments, almost, but not quite, opposite to one another on the margin of the blastoderm, a, blastoderm spread- ing over the yolk ; b, yolk uncovered by blastoderm ; c c', the twin embryonic rudiments. 8 DOUBLE MONSTROSITY— DEVELOPMENT, ETC. the Salmonidae by Eauber {202), Kopsch (132) and Schmitt (217). Panum (180, p. 309) seems to have been the first to suggest in so many words that His's observations on normal development, which led up to the theory of concrescence, might be employed to explain the characteristic forms of double monster fish. On the experimental side we owe chiefly to Kopsch (130, 133), Morgan (161-2) and Sumner (24-3) the crucial evidence that an embryonic body of full length and with the proper number of segments, but deficient in certain lateral and ventral structures, may develop after section or destruction of the thickened margin of the blastoderm to either side of the embryonic rudiment (see p. 27). The fact that the extension of the blastoderm over the yolk is delayed at, or near, the blastopore has long been known, and recently Kopsch's (133) experimental work has provided us witli exact data as to the progress of this extension in the eggs of the Salmonidae. Origin of double and multiple, forms in vertebrates generally. Whatever be the causation, we may recognise, in vertebrates generally, four somewhat different modes of origin, whether for double (and multiple) monstrosities, or for double (and multiple) unioval separate embryos. These different modes are: (1) The appearance of two (or more) embryonic rudiments on a single blastoderm (germinal area in the case of the mammalian blastocyst). (2) The presence in the egg of two (or more) separate blastoderms (germinal areas in mammalian blastocyst). (3) Fission or dichotomy on the part of a single embryonic rudiment. (4) Forma- tion of certain axial structures in two parallel sets on a single embryonic rudiment. (1) The first mode is universal in fishes, whilst in birds it holds good for the great majority of recorded instances. No doubt the same thing is true for reptiles. As regards mammals, direct observational evidence is awanting, but it is impossible to understand how the majority of double (or multiple) monstrosities can arise in any other way. (2) The second mode is possibly illustrated in fishes by the unique example from Klaussner referred to above. It does not appear to be excessively rare in birds. Thus Dareste (53, PI. I. fig. 4) figures two young blastoderms lying close together which have arisen from the development of two separate cicatriculae. In the succeeding figure the same author shows two blastoderms at a considerable distance from one another, each exhibiting traces of a degenerating embryo. Besides these, the first and fourth examples of triplicity in the chick quoted from Dareste (see p. 37 of this work) are marked by the presence of two separate blastoderms. But perhaps the best example in the chick is a seven days' embryo figured by Panum, which shows two well-developed embryos a considerable distance apart from one another, each surrounded by its own vascular area. Schwalbe reproduces this figure (222, II. p. 37), and on the same page he illustrates also a six-day incubated egg (described by C. W. Wolff) with two quite separate embryos on a single large vascular area. He also states on Panum's authority that Fabricius ab Acquapendente had observed two cicatriculae on the yolk of an unincubated egg. It should be added that Wolff's specimen only doubtfully indicates original doubling of the blastoderm. In reptiles we have the Troindonotus example figured by Wetzel (see p. 37 of this work), in which there are three centres of segmentation on the yolk, and one of the centres is itself double. Grundmann also has described an egg of Laccrta with a double blastoderm (quoted from Schwalbe, 222, II. p. 29). In mammals (see p. 38) the presence of more than one germinal area on the blastocyst is splendidly illustrated by the edentate Talusia in which plurality is normal, while Assheton's observation shows that doubling of the germinal area can also occur in the blastocyst of the sheep, a mammal the development of which proceeds normally on more ordinary lines. (3) The third mode accounts for certain examples of duplicity (chiefly posterior) in birds. There is every reason to believe that it occurs in a similar fashion among mammals as well, but the full discussion of this subject falls outside the scope of our work. In fishes the mode in question is represented only by the peculiar and imperfect doubling characteristic of the hemididymous condition (p. 25). DOUBLE MONSTROSITY— STRUCTURE, EARLY STAGES 9 (4) The fourth mode is illustrated by the examples of longitudinal or parallel union which are referred to on p. 29. A notice regarding the prevalence of the different types of double monstrosity in fishes and in other vertebrates is given at the end of the description of hemididymi (p. 28). F. STRUCTURE. Early Stages of Double Monstrosity in the Sulmonidae and in Esox- p. 9 Normal Advanced Embryos of Salmo fario - - - p. 1 1 Advanced Stages of Double Monstrosity in Salmo fario : p. 21 - p. 23 p. 25 - p. 29 - p. 30 Class I. Class II. - Class III. - Class IV. - Class V. - - p. 12 - p. 15 - p. 18 - p. 19 p. 20 Class VI. Class VII. Class VIII. Class IX. Double Montrosity in other Fishes - EARLY STAGES IN THE SALMONIDAE AND IN ESOX. From the anatomical point of view, the stages of chief interest are those in which a large number of structures have become differentiated. Accordingly, the oldest available examples will be selected for detailed study. The anatomy of a normal trout embryo of corresponding age will also be described. But it may be well first to refer briefly to the earliest instances observed, beginning with the Sahnonidae. Rauber (201), in 1879, described an egg of Salmo salar, which is of normal size and has the blastoderm covering a small area at one pole. On opposite sides of the blastoderm two quite similar centres of embryo formation are appearing. Each shows a normal developing head rudiment. The bodies have not yet begun to form. In the following year the same author (202) described seventeen Salmonid ova showing twin embryonic rudiments, and one showing a triple form. See Text-fig. 3 on p. 7, and Text- fig. 5 on p. 36. The whole provides an excellent series illustrating the different distances from one another at which twin embryos may appear, some pairs being exceedingly closely approxi- mated, while others are at directly opposite sides of the blastoderm. Sections were also made by Rauber of several of these monstrosities. Kopsch (133 p. 233 Taf. XVII. fig. 21) figures and describes an extremely young stage in the trout. The centres of the two embryonic rudiments are separated from one another by an angular distance of something like 90°. Schmitt (217) gives an account, with drawings, of two equally young stages in the same species. In the one case the centres are on exactly opposite sides of the blastoderm, while in the other they are almost so (155-165°). These two eggs would probably have given rise to twins with union by the yolk-sac alone. The next earliest record for the Sahnonidae seems to be that of Barbieri (6). It has reference to an egg of Salmo irideus fixed seven days after fertilisation, showing a double embryo 3 mm. long. This specimen is referred to further on p. 29. Not much older than the above are two other instances of twin embryos recorded by Schmitt (217). They are at the stage when the blastoderm is just completing the overgrowth of the yolk, and they give valuable evidence that the process of overgrowth (as was indicated above) is delayed at, and near, the region where gastrulation began to take place. The two specimens which Rauber (200 71) examined by the method of serial sections have next to be mentioned. They are somewhat older than the foregoing. One is an example of union at the level of the optic lobes, and the other of union in the pectoral region. The figures given go some way B 10 DOUBLE MONSTROSITY— STRUCTURE, EARLY STAGES towards illustrating the behaviour of the more important organs (central nervous system, notoehord, and Wolffian ducts) at the region of transition from the double to the single condition. Of about the same age is the single specimen sectioned and described by Moser {165). The two central canals of the spinal cords unite near the fourth body somite, but the notochords only come together at the level of the vent. Inner or adjacent auditory organs are present, but reduced in size, and enclosed withiii the same cartilaginous capsule. The pronephros is peculiar, having three much reduced pronephric chambers and no proper glomerular tuft. It gives origin to three Wolffian ducts. Moser's paper also contains a discussion regarding the origin of double monstrosity in the light of recent experimental work bearing on the question of concrescence. Still later stages are dealt with by Schmitt (21 6), and by the author (76-77). In the specimens which they describe, practically all the organs and structures found in the adult, except the osseous skeletal parts, have already made their appearance. Schmitt's work covers only union by the tail and extreme posterior part of the body, and will be referred to later in connection with the anatomy of these types. Esox. The eggs of the pike equally with those of the trout and salmon have provided classical material for the study of double monstrosities. The first account of the growth from day to day of a developing double fish was given by Valentin (263) from observations on an example which he watched from the fifth to the thirteenth day. To Lereboullet's work on the same species we owe the fullest investigation which has yet been made regarding the occurrence, and mode of growth, of double monsters in any species. Out of a total of over two hundred thousand eggs examined, he obtained rather more than two hundred double monsters, very many of which he not only observed at an extremely early stage, but also followed in development till their final form became established. A seventy-two hours' stage in the pike was also figured and described by Eauber (202 6 Taf. IX. fig. 23), while two exceedingly early examples are illustrated by Klaussner (123 Taf. I, figs. 1, 2). Lereboullet (H-3 20 254 ct scq.) classified the different sequences observed by him in the growth of anomalous forms into the following six types : 1. Instances in which there are, to begin with, two separate embryonic rudiments. These tend to unite posteriorly, the resulting forms having either separate heads and bodies, or separate heads with united bodies. 2. Instances in which there is one broad embryonic rudiment showing more or less extensive doubling of the anterior axial structures. Later, by fusion and disappearance of inner elements, evidence of duplicity may disappear almost completely. Or one of the sets of structures may become atrophic. 3. One instance of a triple monstrosity. There are two bodies, one of which carries a pair of closely joined heads. An even younger triple Salmonid specimen has been described by Eauber (see p. 36). 4. Instances of the type afterwards called mesodidymus by Oellacher (176), in which the head and tail are single but the body is double. Each component tends to be defective as regards its inner or adjacent elements (p. 26). 5. Instances of thinness and slenderness of the embryonic rudiment followed later by defective formation of various organs, notably of the brain and organs of sense. 6. Instances in which no proper embryonic rudiment is formed at all. The blastoderm, however, succeeds in covering the yolk mass, and then a small tubercle makes its appearance, which represents the tail of the fish. In his explanation of types 4 and 6, Lereboullet comes very near to enunciating a definite theory of concrescence. Lereboullet's youngest example dated back to fifty-three hours after fertilisation, and showed two embryonic rudiments very close to one another, and imited together at their posterior ends. A medullary groove was present in each. His next earliest observation had reference to an egg fifty-four hours after fertilisation. Here he described the embryonic rudiment as being single but broader than normal, and showing two parallel medullary grooves separated slightly from one DOUBLE MONSTROSITY— STRUCTURE OF NORMAL EMBRYOS 11 another in front. Two heads were thus formed, but in later development one of them remained as a rudiment, forming a knob attached to the other and containing traces of an eye. Nothing is more striking in these and other observations by Lereboullet, than the amount of what may be called secondary fusion which can take place between closely approximated twin sets of structures, provided that they come together at a sufficiently early stage. For example, two head rudiments may, in the end, be converted into a single apparently normal head devoid of inner eyes or olfactory pits. Even two inner series of composite niesoblastic somites may unite, become absorbed, and disappear. Doubling of the notochord, however, seems never to be recalled, though the twin notochords may come to lie close against one another through absorption of the intervening tissue. The period when the power of fusion becomes lost in the case of mesoblastic somites is near the time of appearance of the heart. ANATOMY OF NORMAL TROUT EMBRYOS.1 PI. I. fig. 1 (external appearance); III. figs. 13-1 G, IV. figs. 17-20, VI. figs. 24-26 (transverse sections); V. figs. 21-23 (horizontal sections); diagrams of skeleton, heart, etc., in PI. XVII. figs. 5G, 57, G2, XVIII. fig. 69, XIX. figs. 73, 76, XX. fig. 83. In normal trout embryos of the same age as the monstrosities, the cartilaginous skeleton has long been laid down and the process of ossification is about to commence. Cranial Skeleton. The parachordal cartilages are uniting round the anterior part of the notochord and have already joined with the trabeculae cranii, which, coalescing in front of the pituitary space, run forwards as a median flattened bar to meet the nasal cartilages. The pituitary space gives passage to the choroidal and internal carotid arteries and to the back part of each rectus oculi externus. On either side, the parachordals have grown upwards in the form of laminar plates, which meet in the mid-dorsal line over the upper part of the medulla, but leave a narrow V-shaped fontanelle over the lower part. The auditory capsules are firm bosses of cartilage, moulded on the labyrinth, closed externally, but widely open towards the brain. Dorsally, they are connected with each other by a thin vault of cartilage roofing the cerebellum ; anteriorly, they are continuous with the supraorbital bars to be afterwards described ; ventrally, they join the trabecular and parachordal cartilages; and externally they articulate with the hyomandibulars. The 5th nerves emerge through deep grooves between the trabeculae and the auditory capsules, while the vagus and glosso-pharyngeal nerves pass out together through a foramen in the cartilage connecting the auditory capsules with the parachordals. A single opening in the floor of the capsules on either side gives passage to the internal jugular vein and the facial nerve. The nasal cartilage is connected with three pahs of bars: (1) the trabeculae cranii, (2) the palato-quadrates, and (3) the supraorbitals. These last pass backwards along the dorso-lateral angles of the brain to join the anterior part of the auditory capsules on either side. Over the pineal body and the third ventricle the supraorbitals are connected together by a bridge of cartilage, but no such tegmen exists over the cerebral and the optic lobes, the spaces left uncovered being the anterior and the middle fontanelles. Visceral Arch Skeleton. The hyomandibulars are connected with (1) the outer aspect of the auditory capsules, (2) the posterior ends of the palato-quadrates, and (3) the interhyals. The Meckelian bars are slender, and meet below the mouth in a symphysis. Posteriorly they articulate with the palato-quadrates, but not with the hyomandibular or interhyal cartilages. In the hyoid arches, glossohyals, hypohyals, ceratohyals, and interhyals can be distinguished. The branchial cartilages are five in number, and have the usual forms and relations. Pectoral Girdle, Notochord, etc. The pectoral girdle is represented by a comparatively short bar of cartilage on either side — the coraco-scapular — and is far from being a complete arch ventrally. The limb-cartilage is an unsegmented plate continuous with the coraco-scapular bar. The notochord consists of pith-like tissue surrounded by a very firm capsule, and its anterior end is embedded in 'Full references are given in the Index of St>-uctures, etc. (p. 63) to the various Plates and figures in which iudividua structures receive illustration. 12 DOUBLE MONSTROSITY— STRUCTURE, CLASS I. the fused parachordal cartilages. Its posterior end still shows a rapidly narrowing upturned " heterocercal " portion. In the position of each future vertebra there are four cartilaginous nodules, placed respectively at the dorso-lateral and ventro-lateral corners of the notochord, and prolonged into processes for the neural and haemal arches. Nervous System, Heart, etc. The anatomy of the central nervous system and of the organs of special sense, and of the heart and blood-vessels, is, with certain differences in the relative size of parts, practically the same as in the adult condition. With regard to the aortic roots, it may be stated that the first root, i.e. the first branchial vein, gives off the hyoid and carotid arteries and then passes backwards to join the second root. The resulting trunk bends inwards to the middle line, and, meeting with its fellow from the opposite side, forms the upper part of the aorta. This part is next joined on either side by a trunk formed by union of the third and fourth roots. The carotids pass forward beneath the parachordals and, traversing the pituitary space from below, reach the base of the brain. The hyoid artery arises from the first aortic root at its ventral end, perforates the hypohyal, runs up along the hyoid bar, and, after passing through a foramen in the hyomandibular, is continued mainly into the pseudobranch. The efferent vessel of the pseudobranch passes forwards and inwards, traverses the pituitary space from below, and, after running alongside the optic nerve, ends in the choroidal gland of the eye. Of the two posterior cardinal veins, the left is usually the larger. Kidneys. The head-kidney, or pronephros, contains a single median glomerular tuft of con- siderable size, supplied by a branch directly from the aorta. The Wolffian ducts begin by a funnel-shaped opening from the glomerular cavity on either side. Then, bending forwards, they become convoluted, and are embedded in highly vascular lymphoid tissue. They next arch back- wards, remaining convoluted for a short distance, and end in a small urinary bladder. The mesonephros is just beginning to develop in connection with their middle and posterior parts. The urinary bladder opens by a mesial pore situated just behind the vent. The intestinal canal is completely shut off from the yolk-mass, and there is an open diverticulum for the air-bladder. Body Segments. The total number of body segments is approximately sixty, the vent being at, or near, the 36th segment. The posterior edge of the dorsal fin reaches to the 9th or 10th segment in front of the vent, and the anterior border of the adipose fin to the 7th segment behind that opening. Median Fins. The median fins characteristic of the adult, viz. dorsal, adipose, caudal, and anal, are already well defined. These are still connected, by a low membranous ridge, which is best marked between the caudal fin on the one hand, and the adipose and anal fins on the other. A similar ridge is found between the anal fin and the vent, and also in front of the vent to near the level of the pelvic fins. In tracing the continuity of the mid-dorsal and mid-ventral lines along the transitional region in double monstrosities, it will occasionally be preferable to speak simply of dorsal and ventral edge membranes rather than to specify the particular fins. ANATOMY OF DOUBLE MONSTROSITIES.1 CLASS I. Union in Head Region, the Ttvin Brains uniting at the Optic Lobes. PL I. figs. 4, 5 (external appearance); Pis. VIII.-IX. figs. 35-38 (transverse sections); Pis. X.-XII. figs. 39-46 (horizontal sections); diagrams of skeleton, vessels, etc., in PI. XVII. figs. 58, 59, XVIII. figs. 64, 67, 70, 72, XX. figs. 84, 85. In this Class, the region of transition from the double to the single condition involves the brain, the cranial nerves, and the organs of special sense, as well as the cranial, mandibular, and branchial cartilages. The twin heads are placed symmetrically, side by side, and lie in the same horizontal plane. Union is thus of the simple lateral type. 1 See note at foot of p. 1] . DOUBLE MONSTROSITY— STRUCTURE, CLASS I. 13 Cranial Skeleton. There are two notochords in front, and therefore, potentially at least, two pairs of parachordal cartilages, but the four cartilages are united to form a single basilar plate. In front, the two nasal cartilages are placed widely apart ; each contains a right and left olfactory pit, and is continuous behind with its own trabeculae cranii. The two pairs of trabeculae converge as they pass backwards ; their inner * or adjacent elements unite to form a median flattened bar, which joins the basilar plate mentioned above as being formed by the parachordals. At the same time, the outer elements of each pair of trabeculae have diverged from the inner elements to enclose a pair of pituitary spaces, the latter lying one on each side of the median bar formed by union of the inner trabecular elements. There are only two auditory organs, and their cartilages are continuous with the outer trabecular and parachordal elements in the floor of the skull. Dorsally, the auditory capsules are connected over the cerebellum by a vault of cartilage, which is narrower antero- posteriorly than in the normal condition. Over the medulla the laminae of the parachordals nowhere form a complete vault. Accordingly, the posterior fontanelle is much larger than in a normal case. Dorsally, each nasal capsule is continuous with a pair of supraorbital bars, of which the outer elements pass backwards on either side to join the auditory capsules, while the inner or adjacent elements are connected with each other and with the outer bars over the pineal body. Behind this the adjacent bars disappear, the result being that over the region of the cerebral lobes there are two small anterior fontanelles, while over the optic lobes there is a single large fontanelle. Visceral Arch Skeleton. The inner or adjacent palato-quadrate bars converge, fuse, and end abruptly without being attached to a suspensorium ; the outer bars are continued backwards on either side, and articulate with the hyomandibulars attached to the auditory capsides. Articulating with the united part of the inner palato-quadrate bars is a small twisted piece of cartilage, which passes downwards in the septum between the two mouth-openings, and represents an inner or adjacent pair of Meckelian cartilages. The corresponding outer Meckelian cartilages meet in the septum between the two mouth-openings, but do not form a symphysis. They are continued back- wards to articulate each with a normal suspensorium attached to the outer side of the correspondin°- periotic capsule. The hyoid bars and the branchial cartilages are normal, except that they diverge rather more widely than is usual. Each arch, however, may be looked upon as being composite, namely, as consisting of the outer components of a double set of arches, the inner components having been lost. Notochords, etc. Two notochords are present as far back as from the 14th to the 20th body-segment. These converge at an acute angle and finally unite. The disposition of the neural and haemal arch cartilages is as follows. Where the two notochords are some little distance apart, the inner or adjacent neural arches are displaced so as to form a floor for the transversely- expanded spinal cord, while the inner haemal arches lose their ventral processes and become smaller. As the notochords come closer together and unite, the inner neural and haemal arches disappear, while the outer arches gradually assume a normal form and position. Brain and Cranial Nerves. There are two pairs of cerebral lobes and two thalamencephala, diverging forwards from a single composite optic lobe region. The cerebral lobes and thalamen- cephala, besides diverging, are rotated slightly in such a way that they lie closer together dorsally than ventrally. There are two pairs of olfactory nerves, two pairs of optic nerves, and two pineal diverticula. There are also two infundibula, which converge as they pass downwards and backwards. Each ends in a hypophysis sac after giving off the usual diverticula for the hypoaria. The inner hypoarium on either side, owing to want of space, is smaller than normal, and lies above and in front of the outer hypoarium. The optic lobes show a remarkable transition between the double and the single condition. Their cavity and roof-parts are single, while the basal structures are doubled. There are thus two pairs of 3rd nerves. But only a single pair of trigeminals is found, the components of which represent, respectively, the right and left 5th nerves of the right •In examples of lateral union like the present one, the terms inner or adjacent refer to those structures which gradually become approximated as the twins converge. Here, accordingly, the left trabecula of the right twin head and the right trabecula of the left twin head constitute the inner or adjacent elements in question. 14 DOUBLE MONSTROSITY— STRUCTURE, CLASS I. and left twin heads. The succeeding cranial nerves are also normal, in the sense that there is only a single pair of each. A rudiment of inner or adjacent pairs of trigeminal ganglia may be recognised in the form of a thin elongated band of tissue containing small nerve-cells and lying in the middle line underneath the region of the pons. This band of tissue has no central or peripheral nerve-fibres. The pons and cerebellum are single, but their internal structure shows traces of duplicity, especially in the case of the pons. The medulla oblongata is slightly expanded transversely, but is otherwise normal in form. Spinal Cord. In the anterior part of the spinal cord there is a curious and interesting reappearance of duplicity, coextensive with the duplicity of the notochords, and with the presence, ventral to them, of a median composite muscular mass representing united adjacent lateral muscles. In this region, the spinal cord is greatly expanded in a transverse direction ; its cavity is spindle- shaped, and, in addition to the usual nerve-roots, it gives off, on the ventral aspect in each segment, a pair of small additional motor roots which are distributed to the median muscular mass just mentioned. Organs of Sense. There are two pairs of olfactory organs and nerves. The outer eyes (right eye of right twin head and left of left head) are normal, but lie further back than usual, so that their optic nerves pass backwards as well as outwards from brain to eyeball. The inner or adjacent eyes may, or may not, be fused with one another. In the former case (which is also much the commoner) there is a single lens, which is sometimes smaller than normal, but sometimes larger and showing evidence of duplicity ; the sclerotic and choroid coats are single ; the retinae never fully unite, each showing its own choroidal fissure, optic nerve, and choroidal gland. In all cases the external recti muscles belonging to the inner eyes are absent ; the superior obliques are absent or rudimentary, but the remaining ocular muscles are present in two sets. Heart and Vessels. The heart and the ventral aorta are normal, but the dorsal aorta and its roots, as well as the choroidal and carotid arteries, require description. The coming together of the main collecting-trunks on either side to form the dorsal aorta is carried backwards for a very con- siderable distance, and takes place only at the level of union of the notochords. The pseudobranch on either side receives a branch from the hyoid artery, and its efferent vessel passes to the choroidal gland of the corresponding (i.e. outer) eye. The choroidal glands of the inner or adjacent eyes are supplied by blood which has not passed through the pseudobranch. In the specimen from which PI. VIII. fig. 35 is taken, the arrangement of vessels is quite symmetrical and is indicated in PI. XVIII. fig. 67. A transverse arch vessel connects the upper aortic roots, and gives off a common choroidal artery which soon bifurcates. The two resulting vessels pass through the separate pituitary spaces, and are distributed to the choroidal glands of the adjacent eyes. As is well known through the work mainly of Johannes Muller,1 the pseudobranch and the choroidal gland are so related to one another in osseous fishes that a pseudobranch is never present in species which have no choroidal gland, while in rare instances only is a choroidal gland present where there is no pseudobranch. It is therefore of considerable interest to note that the choroidal glands of the inner or adjacent eyes, in the type of monstrosity under consideration, derive their blood-supply directly from the first aortic root. There is a certain amount of variation in the exact mode of origin of the choroidal and carotid arteries. For instance, cases occurred in which these vessels all arose from the first aortic root on one side only, instead of both roots participating equally as in the specimen figured (PI. XVIII. fig. 67). Kidneys, etc. The mesonephros, ureters, bladdei-, and urinary pore are normal, but the pronephric glomerulus is composite, or may be double. An example of the composite condition is figured in PI. XIX. fig. 84. The glomerulus is larger than normal, and contains two vascular tufts between which is a median compartment that obviously corresponds to the fused adjacent halves of a pair of glomeruli, but has no Wolffian ducts in connection with it. In the specimen from which Fig. 85 is taken the two glomeruli are separate, and one of them has a rudimentary Wolffian duct arising from its inner side, while the other has none at all. The two figures should be com- 1 Verghichende Anatomie dcr Myxinoiden, Berlin 18351845; 3'° Fortaetz. 1841 (41-99). DOUBLE MONSTROSITY— STRUCTURE, CLASSES I. AND II. 15 pared with Figs. 86 and 87 on the same Plate, which show a still greater degree of duplicity in the pronephros. Alimentary Canal. There are two mouth-openings, but the pharynx and the rest of the canal are single, the only evidence of duplicity being the presence of two air-bladder diverticula. Muscles. Of composite muscles, the most important are contained in the median mass mentioned above as underlying tbc twin notochords, and as being innervated by the small additional motor roots of the composite spinal cord. This muscular mass is segmented serially by septa which correspond exactly with the septa of the outer (normal) lateral muscles. In the head-region some small and intricately arranged muscles are found connected with the cartilages which represent the reduced adjacent Meckelian and palato-quadrate bars. These muscles are obviously rudiments of adjacent mandibular and temporal muscles. It has been noted previously, that there are no external recti muscles in connection with the adjacent eyes, and that the superior obliques are either rudimentary or entirely absent. Summary for Class I. Union of simple lateral type (p. 6), the twin brains uniting at the optic lobes ; notochords wider apart in the cervical than in the cranial region, and uniting between the 14th and the 20th segments; body thereafter having a normal bilateral structure; skeletal elements in floor of cranium doubled from pituitary region forwards ; doubling less marked in roof of cranium, and practically absent in visceral arch skeleton ; greater doubling in the floor than in the roof of the brain, and in the first part of the spinal cord than in the medulla ; four separate olfactory pits ; inner eyes more or less united ; inner auditory organs not represented ; heart normal ; vessels slightly modified ; pronephros much modified ; gut single, but with two air-bladder diverticula. The chief uniting and composite structures noted are : the trabecular, parachordal, palato-quadrate, Meckelian, supraorbital, neural, and haemal arch cartilages ; the eyes ; the central nervous system from mid-brain to upper end of spinal cord; the inner muscle plates, and the pronephric glomerulus. Secondary Fusion. The part played by primary fusion in the very early growth of double monster fish was previously described (p. 6). It was stated also that secondary fusion had to do with the union of tissues already laid down and the attempted production of a more or less normal bilateral condition out of a double set of structures. In Class I., which has just been described, we may trace the working of secondary fusion in the following, among other, characteristics : (1) the practically normal condition of the visceral arch skeleton ; (2) the fact that the roof parts of the cranial skeleton, optic lobes, medulla, and first portion of spinal cord are less modified than the corresponding floor parts; (3) the fact that the first portion of the spinal cord shows greater duplicity than the medulla; (4) the single condition of the pharynx, oesophagus, and heart, as contrasted with the doubling of the air-ducts ; (5) the delayed union of the dorsal aortae, and (6) the double or composite character of the pronephric glomerulus. Influence of Sensory Nerves on Growth. Important evidence on this vexed question is afforded by the monstrosities just described. No trace exists of inner or adjacent 5th and 7th cranial nerves. Yet the inner sides of both heads are perfectly well developed wherever they have sufficient space for growth. It is thus evident that, here, the absence of a sensory nerve-supply has neither hindered nor disturbed the natural course of development. CLASS II. Union in Head Region, the Twin Brains uniting at the Medulla Oblongata} PL I. fig. 6 (external appearance); Pis. XIII.-XIV. figs. 47-51 (transverse sections); diagrams of skeleton, vessels, etc., in PL XVII. figs. 60, 61, XVIII. figs. 65, 68, 71, XX. figs. 82, 86. Cranial Skeleton. The structure of the cranial skeleton in this type agrees generally with that in the class last described, except that the place of union of the skeletal elements is carried further back. This gives room for greater development on the part of the inner or adjacent elements in the 'See note at foot of p. 11. 16 DOUBLE MONSTROSITY— STRUCTURE, CLASS II. twin heads, but the union is again of the simple lateral type. The two pairs of trabeculae converge posteriorly, but are separate along their whole length, and unite with corresponding parachordals. The latter are separate in front, but posteriorly the adjacent elements in each pair unite, so that a single composite basilar plate of cartilage containing two notochords underlies the posterior half of the medulla oblongata. The inner or adjacent palato-quadrates converge posteriorly and coalesce. The united part articulates, (1) below, with a small bifid cartilage representing fused adjacent Meckelian bars; and (2) higher up, with a small cartilage representing fused adjacent hyomandibulars. The inner or adjacent supraorbital bars converge posteriorly, unite with one another, and end by becoming continuous with the roof of a small box of cartilage, wedged into the apex of the angle between the twin heads, and representing fused adjacent periotic capsules. This structure will be described later, but it may be mentioned here that its roof is continuous posteriorly with a vault of cartilage which connects the two outer (normal) periotic capsules over the cerebella. In this way double sets of anterior and middle fontanelles are left over the cerebral lobes and mid-brains respectively of the twin heads. The inner or adjacent auditory capsular cartilages are extremely rudimentary, being completely united, compressed from side to side, and wedged into the position above indicated. They contain a single distorted labyrinth, and receive small auditory nerves on either side, which are distributed symmetrically over the labyrinth. This composite auditory capsule is connected anteriorly with the fused adjacent supraorbital bars, and ventrally with the adjacent trabecular and parachordal cartilages on the inner sides of the two pituitary spaces. Behind it a small triangular opening is left, bounded on either side by the converging parachordals. These unite posteriorly, but leave a narrow foramen between them for the exit of a small nerve, which represents a reduced adjacent pair of vagus and glosso-pharyngeal nerves. It will be seen from what precedes, that there are five fontanelles, one over each pair of cerebral lobes, one over each mid-brain, and one over the composite medulla oblongata. Mouth. There are two mouth-openings placed side by side, separated from one another by a thick dorso-ventral septum. This septum contains (1) remains of the adjacent mandibular and hyoid cartilages, (2) much confused muscular tissue, and (3) two arteries which will be afterwards described, one being a continuation of the ventral aorta, and the other a small artery for the supply of the inner or adjacent pseudobranchs. The two mouth-openings lead into separate buccal cavities, but the oesophagus is single, the septum above mentioned ending opposite the second branchial cartilage. Visceral Arch Skeleton. The inner or adjacent hyomandibulars are extremely rudimentary, and are fused together to form a small bifid piece, which articulates (1) anteriorly, with the fused adjacent palato-quadrates, (2) posteriorly, with the fused adjacent periotic capsules, and (3) inferiority, with a rudiment of the fused adjacent hyoid bars. The small artery for the supply of the inner or adjacent pseudobranchs passes up through the notch at the anterior end of this cartilage. The mandibular apparatus may be described as consisting of a composite arcade underlying the two mouth-openings. The outer portions of this arcade are formed by normal (outer) Meckelian bars (i.e. right bar of right twin head and left bar of left head), while a small mesial portion of the arch is formed by rudimentary adjacent Meckelian bars. These latter bars are united at their proximal ends, and there articulate with the fused adjacent palato-quadrates. Distally, each of the inner bars unites in a symphysis with its corresponding outer Meckelian bar. As the inner bars are extremely reduced in size, the two symphyses lie close together in the tissue of the septum separating the two mouth-openings. The hyoid apparatus may also be described as forming a composite arcade, the main part of which consists of the outer arches of the twin heads, while in the middle are interposed the fused remains of the inner arches. These remains consist of (1) a single twisted piece of cartilage articulating, without the intervention of an interhyal, with the fused adjacent hyomandibulars, and representing cerato- hyals ; and (2) two incompletely separated hypohyals, articulating below with two glossohyals which are also incompletely separated. Connected with the outer sides of these glossohyals are the DOUBLE MONSTROSITY— STRUCTURE, CLASS II. 17 hypohyal pieces of the outer arches. No adjacent elements are interposed in the series of branchial cartilages. Here, the only evidence indicating duplicity is to be found in the second copular piece, i.e. that succeeding the glossohyals. This piece is double anteriorly, but it becomes single opposite its articulation with the second branchial cartilages. The succeeding copular pieces are single, but they are a little broader than normal, especially in front. Notochords, etc. The notochords remain separate as far back as the twentieth to the twenty- eighth somite. The arrangement of the neural and haemal arch cartilages in the transition region is the same as that described on p. 13. Brain and Spinal Cord. There are two sets of brain cavities and masses as far back as the level of the fourth ventricle. The fourth ventricle is single posteriorly, but it bifurcates in front into two canals leading into the separate mid-brains. The posterior part of the medulla and the anterior part of the spinal cord are composite, and show the following characters: (1) they are much drawn out transversely, and (2) they give origin to small inner nerve-roots. In the medulla these roots are extremely rudimentary, and their ultimate distribution could not be traced, but in the spinal cord they are better developed and form a regular series of pairs of nerves coming off from the ventral aspect of the cord and distributed to the somites of the median muscular mass which lies ventral to the notochords (compare p. 15). In this monstrosity, as in the one previously described, the anterior part of the spinal cord, though it lies nearer to the place of union of the twin bodies, shows greater structural duplicity than does the medulla oblongata. All the outer cranial nerves belonging to the twin heads are normal, and need no further mention. Of the inner or adjacent nerves, the 1st, 2nd, 3rd, and 4th are normal, while the 5th are reduced in size. The inner or adjacent 7th and 8th nerves with their ganglia are very rudimentary ; while only a remnant of the adjacent glosso-pharyngeal and vagus remains. I was unable to follow out the 6th pair, but the presence of well-developed external recti muscles makes it probable that these nerves are present. Auditory Organs. As has already been mentioned, the inner or adjacent auditory capsules are much reduced in size, their auditory sacs being completely united and forming a single labyrinth, symmetrical in shape, compressed from side to side, and receiving the two adjacent auditory nerves. The arrangement of the sensory epithelium inside the various parts of this labyrinth is also bilaterally symmetrical. A diagram of this labyrinth is given hi PI. XX. fig. 82. Utricle, anterior, and posterior semicircular canals are all represented, but there is no trace of a horizontal semicircular canal. Heart and Vessels. The heart is normal and gives origin to a single ventral aorta, which, for a short distance upwards, has a double cavity, owing to the presence of a median antero-posterior septum, which, however, disappears further forwards. The gill-arteries on either side are normal, but, in addition to them, the ventral aorta gives rise to several small irregular branches which ramify in spongy tissue surrounding the ventral ends of the branchial cartilages, and may be taken to represent a very rudimentary set of inner or adjacent gill-arteries. But the most striking feature of the ventral aorta is that, instead of ending in the first gill-arteries, it is continued forwards and arches dorsally in the tissue of the septum between the two mouth-openings. Passing through the space between the adjacent glossohyals and the succeeding copular piece, it comes to lie behind the small cartilage which represents adjacent ceratohyals. Then, reaching the base of the skull, it bends backwards, and divides into two equal branches which join the upper aortic roots on either side. In the first part of its course, this continuation of the ventral aorta gives off (1) two inner or adjacent carotid arteries, which, after running forwards and outwards, pass through their corresponding pituitary spaces ; and (2) two arteries for the supply of the inner or adjacent pseudobranchs. These arteries run at first forwards and dorsalwards behind the adjacent glossohyals. They next curve forwards and unite in front of the adjacent ceratohyals ; then, separating again, they pass between the adjacent palato-quadrates and the hyomandibulars, and are distributed each to its corresponding pseudobranch. On either side the first aortic root gives off (1) a hyoid artery which sends a branch to the corresponding outer pseudobranch, and (2) a carotid artery. It then joins the second aortic root, and shortly afterwards meets the continuation of the ventral aorta previously mentioned. The c 18 DOUBLE MONSTROSITY— STRUCTURE, CLASSES II. AND III. resulting vessel is next joined by the third and fourth aortic roots, but it remains separate from its fellow on the other side, so long as the notochords are separate, i.e. back to the twentieth somite. The arrangement of vessels which has just been described and is illustrated in PI. XVIII. fig. 68, is somewhat remarkable. Mixing of arterial with venous blood must have taken place in no slight degree, and certain parts in each twin head must have been supplied by blood coming directly from the ventral aorta. There is no trace of a pair of adjacent jugular veins, and it is difficult to make out from sections the course of the venous blood coming from the adjacent sides of the twin heads. But as there is a considerable amount of spongy tissue below the base of the skull and in the septum between the mouth-openings, it is probable that the blood in question found its way into the median and the main jugulars. The presence of this spongy tissue no doubt indicates congestion. Kidneys. The pronephric glomerulus is composite. It is remarkably large, and is divided into three compartments by two vascular tufts each of which has an afferent and an efferent vessel. Normal Wolffian ducts arise from the outer compartments, while the middle compartment gives origin to a sacculated tubule which passes backwards a short distance to end blindly, and represents fused adjacent Wolffian ducts. Alimentary Canal. There are two buccal cavities, but the oesophagus and the rest of the alimentary tract are single except for the presence of two air-bladder diverticula. Summary for Class II. Union of the simple lateral type occurring at the hind-brain ; notochords further apart in the cervical than in the cranial region, and uniting from the 20th to the 30th body-segment; body thereafter having a normal bilateral structure; skeletal elements in floor of cranium doubled from parachordal region forwards ; very little doubling in the visceral arch skeleton ; greater duplicity in the floor than in the roof of the medulla and first portion of the spinal cord ; all four eyes separate ; inner auditory organs present but reduced ; heart normal ; vessels and pronephros greatly modified ; mouth partly double but pharynx and gut single ; two air- bladder diverticula. The chief uniting and composite structures noted are : the parachordal, palato-quadrate, Meckelian, hyoid, supraorbital, auditory, neural, and haemal arch cartilages ; the medulla and first part of the spinal cord ; the ventral aorta and various large vessels ; inner muscle plates ; pronephros. That secondary fusion has played an important part in the ultimate moulding of the transitional region in this type is evidenced by a set of characters almost exactly comparable with those referred to in the description of Class I. (p. 15). What is said on p. 4 and p. 11 should also be referred to in this connection. CLASS III. Union in Pectoral Region, the Inner or Adjacent Pectoral Fins not being represented} PI. II. figs. 7, 8 (external appearance); Pis. XV.-XVI. figs. 52-55 (horizontal sections); diagrams of various structures in PL XVII. fig. 63, XIX. fig. 75, XX. fig. 87. Structures completely doubled. The brains, organs of sense, cranial skeletons, and anterior portions of the vertebral column are completely double and separate, as also are the two visceral arch systems, except for union on the part of the dorsal ends of the inner or adjacent last, and some- times second last, branchial cartilages. The general appearance is illustrated in PI. II. fig. 7. It will be seen that the twin bodies do not meet in perfectly simple lateral union, but tend to come together first by their ventral aspect. This tendency, however, is not strongly marked, and is easily coiTected at the transitional region, behind which the body has a normal bilateral structure. Notochords, etc. The notochords and spinal cords are still widely separate opposite the pectoral region. They remain separate to near the 32nd body-segment, but ultimately fuse, so that the posterior part of the body and the tail contain a single notochord and a single spinal cord. The union of the spinal cords is well in advance of that of the notochords. 1 See footnote on p. 11. DOUBLE MONSTROSITY— STRUCTURE, CLASSES III. AND IV. 19 The behaviour of the neural and haemal arches and of the median muscular mass corresponds to the description on pp. 13, 15. These structures, however, may be studied to greater advantage in the type at present under consideration, as the whole transitional region is open for observation. The ventral ends of the two coraco-scapular bars fail by a wide interval to meet each other below the pericardium. Alimentary Canal. The gullets and stomachs are separate, but union is found to take place (in the specimens sectioned) at the very commencement of the small intestine. The liver forms a single mass transversely drawn out and showing, on either side, the ramifications of separate bile ducts. The two duets unite just prior to opening by a single aperture into the small intestine. Heart and Vessels. Much interest centres round the heart and blood-vessels. A recon- struction drawing of the heart and origins of the vesselu of a typical specimen of this group is given in PL XIX. fig. 75. The whole of this double heart lies inside a large composite peri- cardial cavity, which is prolonged a little forwards on either side round the origin of the ventral aorta. The ventricles are separate, the auricles communicate with one another, and there is a single large sinus venosus opening by a wide ostium into the auricles at their junction. The sinus venosus receives blood («) on either side from the duct of Cuvier formed by union of the outer cardinal and internal jugular veins of the twin embryos (i.e. from the right duct of Cuvier of the right embryo and from the left duct of Cuvier of the left embryo) ; (b) from two separate middle jugular veins ; and (c) from a large trunk formed by union of the inner or adjacent internal jugular veins of the twin heads. This last trunk obviously corresponds to fused adjacent ducts of Cuvier which receive anterior cardinal veins only and have no corresponding posterior cardinals. The hepatic veins also enter the sinus venosus, from below, but are not shown in detail here, their relations having been obscured owing to injury to the liver during removal of the yolk sac. Kidneys. The head-kidney of a specimen belonging to this class is illustrated in PI. XX. fig. 87. It resembles the type described under Class II. (p. 18). CLASS IV. Union in Pectoral Region, the Inner or Adjacent Pectoral Fins being present but united, and reduced in size.1 Parts completely doubled. Doubling extends so far back that the two heads and the gill arch regions are entirely separate. Ventral Convergence. The tendency to ventral convergence of the sagittal planes of the twin bodies now manifests itself somewhat strongly, and leads to slightly earlier union on the part of certain ventral than of dorsal structures. Thus the dorsal fin, while sometimes single along its whole length, is in other instances doubled anteriorly, although the ventral structures are all single at a corresponding level. The pelvic fins always form a single pair. Notochords. The degree of ventral convergence is not so great that it cannot become rectified in the posterior body region, which, accordingly, exhibits a normal bilateral structure. This condition is finally established at, or near, the 36 th body segment. Neural and haemal arch cartilages are disposed as already described for Class I. (p. 13). Spinal Cords. Anteriorly the spinal cords are further apart than the notochords, but at the transitional region they rapidly approach each other and finally succeed in uniting, two or three segments in front of the notochords. The united part is at first composite, and shows the characteristic features already described (p. 17). Inner or Adjacent Pectoral Fins. In typical specimens belonging to this class, fusion of the pectoral limb-cartilages tends to be more complete towards the posterior, than towards the anterior, border of the fins. Thus, in the specimen from which PI. XVIII. fig. 66 is taken, near the anterior border there is only a small bridge of cartilage between the fins ; further back the connecting bridge is broader but shorter; while at the posterior border the limb cartilages are united along 1 See footnote on p. 1 1. 20 DOUBLE MONSTROSITY— STRUCTURE, CLASSES IV. AND V. their whole length. As regards the inner or adjacent coraco-scapular bars, they are quite separate, except at their ventral ends, which join together and project downwards into a septum between the two pericardial sacs. The ventral ends of the two outer coraco-scapular bars are very widely distant from one another. Heart and Vessels. The same specimen may serve to illustrate the typical arrangement of the heart and vessels in this group. There are two pericardial cavities separated by a septum of connective tissue, which is thin posteriorly, but in front is thick and contains the fused ventral ends of the adjacent coraco-scapular bars just mentioned. Auricles and ventricles are completely separate, and the sinus venosi communicate only by a narrow neck. Each sinus venosus receives a pair of ducts of Cuvier, the inner or adjacent ducts being smaller than the outer. This difference depends mainly on the fact that the inner or adjacent posterior cardinals are small and short. They can be traced backwards inside the substance of the head-kiduey, but are soon found to unite and to break up into venules in the lymphoid tissue (PI. XIX. fig. 74). Kidneys. The glomerulus of the head-kidney is shown in PL XX. fig. 86. It is greatly elongated in a transverse direction. The tubule from its middle compartment, representing fused adjacent Wolffian ducts, passes forwards so as to lie between the two adjacent cardinal veins which have just been referred to. It ends blindly and is so much sacculated as to suggest a certain degree of pressure in the fluid secreted by the glomerulus. A similar point will be noted later (page 21), where one of the urinary bladders in a double monstrosity has no urinary pore. Alimentary Canals. The gullets, stomachs, and first portions of intestine are separate, union taking place a short distance beyond the duodenum. There are two bile ducts and livers, the latter, however, being confluent on their adjacent sides. The dorsal mesentery of the intestine remains double for a considerable distance after the intestinal canals have united. A mesial triangular pocket or body-cavity thus lies dorsal to the first single portion of intestine. CLASS V. Union by the Body or Tail, the United Portion ending, as normally, in a Single Symmetrical Tail. PI. II. fig. 9 (external appearance, one of the components reduced); XVII. fig. 63 (notochords, etc.); XIX. fig. 77 (Wolffian ducts, etc.). Ventral Convergence. At the beginning of the description of Class IV., ventral convergence of the sagittal planes of the twin bodies was referred to as a factor interfering with simple lateral union at the region of transition. In the class at present under consideration this factor is better marked and operates in a higher degree. Instead of simple lateral union we have a condition which is typically found in Schmitt's 1 group D (union half lateral and half ventral) (p. 3), but which is exhibited also by those examples of his groups C (union chiefly ventral but partly lateral) and E (union chiefly lateral but partly ventral) that approach nearest to the border line between their own groups and group D. Eoughly speaking, the degree of interference with bilateral symmetry is to be measured by the distance backwards at which union takes place, the difficulties in the way of simple adjustment being greatest where union is longest deferred. This will be seen to be only natural if one remembers that, to begin with, the twin embryonic axes appear tangentially on the surface of the yolk sphere and are situated far apart from one another on the margin of the same blastoderm (pp. 6-7). My classification is based simply on the final result, i.e. on whether the ventral convergence ultimately becomes rectified or not. In Class V. rectification does take place and is so complete that, ventrally, only one pair of pelvic fins occurs and the anal fin is single along its whole length, while dorsally the adipose fin is single and the spinal cords complete their union almost as early as the notochords. The dorsal fin, however, is partly or completely double. The level at which the notochord becomes 1 Schmitt's paper {'216) describes carefully and at length the structure of examples of Classes V. and VI., and may with advantage be referred to, for supplementary details. DOUBLE MONSTROSITY— STRUCTURE, CLASSES V. AND VI. 21 normal varies somewhat, but may be put down at from the 38th to the 46th body segment. As regards the caudal veins and arteries, in one particular specimen the former united five segments in front of the vent, and the latter in the second pre-anal segment. Intestinal Canals. As a rule, the twin intestinal canals become united a considerable distance (9-11 body segments) in front of the vent, the united portion being provided with two dorsal mesenteries which come from below the two notochords. More rarely, union of the intestinal canals takes place a few segments (6-4) in front of the vent, and I have not come across a single example of Class V. in which the intestinal canals are separate along their whole length. Kidneys. The two head-kidneys are quite apart from one another, each being normal and giving rise to a pair of Wolffian ducts. As a rule, the inner or adjacent ducts end blindly in the mesonephric region, often being distended as if by the pressure of excreted fluid (PI. XIX. fig. 77). In one example the inner or adjacent ducts open into a much swollen bladder which, like the anterior one in PL XIX. fig. 79, is destitute of an external opening. The outer Wolffian ducts on either side pass backwards as the right and left ducts of the single portion of the body, and open into a urinary bladder which communicates with the exterior by the usual single opening behind the vent. Number. The number of double monsters belonging to this Class is relatively small. My material only provided five examples, in contrast with over twenty which fell to be included in Class VI. CLASS VI. Union by Posterior Part of Body, the united portion ending in a Composite Triangidar or Quad- rangular Tail, in which various structures are still doubled. PI. II. figs. 10-12 (external appearance); VI.-VII. figs. 27-31 (transverse sections); XIX. figs. 7S-81 (Wolffian ducts, etc.). The class is characterised either by (a) very marked ventral convergence of the two sagittal planes, or by (b) pure ventral union of the twins. Both of these conditions make it impossible for inner or adjacent structures to disappear gradually at the region of transition, and to leave just the complement of outer structures needed to make up a single bilateral region, as occurs in the earlier Classes, in the manner which has already been abundantly illustrated. CLASS VI. (a). This division includes all the well-marked and typical members of Schmitt's group C (union chiefly ventral but partly lateral). The posterior part of the body and the tail are characteristically triangular, there being two dorsal edge membranes and a single composite ventral edge membrane (PI. VI. figs. 27-29; VII. fig. 31). The latter is produced by the coming together of the outer halves of twin ventral structures from which the inner halves have gradually disappeared. In the earlier and middle members of this division there is only a single pair of pelvic fins, which is situated on the ventral aspect of the composite body and is itself composite in the sense that its units represent the outer pelvic fins of the twin embryos. The vent opens behind these. In the later members of the division the corresponding inner pelvic fins may be also represented, forming a pair on the upper aspect of the monstrosity. They are reduced in size, and either set close together or actually united with one another. Notochords. The notochords either unite a varying number of body segments (12 to 1) in front of the rapidly thinning terminal " heterocercal " portion, or they unite in the heterocercal portion itself. The difference depends on how closely the ventral convergence of the sagittal plane approaches to direct ventral union, delay being naturally greatest as the latter condition is neared. However, even in extreme cases, the notochords always unite an appreciable distance in front of their actual termination. There thus remains a longer or shorter common pointed portion the tip of which always tends to curve in the mean dorsal direction, that is away from the ventral edge of the triangular tail. The caudal veins unite near the level of the vent and the caudal arteries several segments further back. 22 DOUBLE MONSTROSITY— STRUCTURE, CLASS VI. Spinal Cords. For the first three-fourths or so of the transitional region the spinal cords are always wider apart than the notochords. This condition persists in cases where the notochords unite in the heterocercal part, so that, here, union of the spinal cords is later than union of the notochords. However, when the notochords come together earlier, i.e. in the neighbourhood of the 12 th last body-segment, the spinal cords may approach each other suddenly towards the end of the transitional region, in such a way that they finally unite with one another as early as the notochords. In no instance of Class VI. (a) did the spinal cords or the notochords fail to unite ultimately. Intestine. The intestinal canals may unite anywhere within the last six or seven pre-anal segments, but do so usually quite close to the vent. As in previous classes the united portion has a double dorsal mesentery. The vent itself is always single. Kidneys, etc. The Wolffian ducts, the bladders, and the urinary pores exhibit a number of variations, and will most conveniently be described along with the corresponding organs, under Class VI. (h). CLASS VI. (b). In the ventral union characteristic of this division, the tail shows four angles or keels which are continued more or less regularly right to the end of the caudal fin. Of these keels, two (one opposite to the other) are simply the dorsal edge membranes belonging to the mid-dorsal lines of the component tails. The other two, also opposite to one another, are ventral edge membranes and of composite derivation, half of each belonging to each twin. The manner in which these composite edge membranes are formed may best be understood by supposing that the blastema destined for the ventral portion of each embryo has been split into widely separated halves, and that in the process of primary fusion two new wholes (of composite nature) have resulted from the union of halves belonging to different embryos. A quadrangular tail with two (normal) dorsal and two (composite) ventral edge membranes is shown in PL VII. fig. 30. A comparable result is seen in the tail of the triple monstrosity, sections of which are shown in PI. VII. figs. 32-4. There are two pairs of pelvic fins, i.e. a pair in front of each of the composite ventral edge membranes on opposite sides of the quadrangular body. Obviously, each pair of fins is made up of components which belong to different embryos. Usually one of the pairs is smaller and more closely set than the other, and in such cases the vent, or pair of vents, opens behind the larger pair. Occasionally what should be the smaller pair is deficient altogether. In the remarkable specimen from which PI. XIX. fig. 81 is taken, the two pairs of pelvic fins are exactly alike. This provides an instance of perfectly symmetrical ventral union. Notochords. As regards the notochords, the rule is that they should unite for the whole, or for part only, of their thinner terminal " heterocercal " portions, each of which tends naturally to bend dorsalwards in its proper sagittal plane. "When the portions in question unite at their commence- ment, the two opposing tendencies to dorsal flexure neutralise one another so that the united portion may project straight backwards. When union takes place near the extremity of the " heterocercal " portions, a certain amount of divarication may always be noted before these bend round to meet one another. This divarication is due to the fact that the tendency towards dorsal flexure has had scope for some degree of independent manifestation in each of the notochords. My own material has not furnished me with any examples in which the notochords never united, but instances of the kind are stated to occur by Schmitt (216). Spinal Cord. The spinal cords either unite the shortest possible distance in front of their extreme posterior ends, or they never unite at all, and their union is typically later than that of the notochords. Alimentary Cancds. Three variations may be found: (1) Union occurring within the last three or four pre-anal segments; (2) two separate vents opening side by side in front of one of the composite ventral edge membranes; (3) two separate vents opening on opposite sides of the body, one in front of each of the composite ventral edge membranes. In the first instance the dorsal mesentery is double along the whole length of the united portion ; in the second instance the two anal openings lie in a common field, and the last parts of the two recta are surrounded by a common DOUBLE MONSTROSITY— ANAKATADIDYMUS 23 connective tissue sheath. The third variation is the most interesting, and it is illustrated in PI. XIX. fig. 81. Here the union of the twin bodies has been so directly a ventral one, that the separation of ventral blastema (referred to above in connection with the formation of the composite ventral fins) has affected also the walls of the last portion of the rectum, as well as the anal pits. The two vents are accordingly to be looked upon as being composite structures. As will be seen from the figure just quoted, the Wolffian ducts, bladders, and urinary pores have undergone a corresponding rearrangement. Wolffian Ducts, etc. Very great variation is found in the arrangement of the ureters, bladders, and urinary pores in Classes VI. (a) and VI. (b). All my specimens have two bladders, which sometimes communicate with one another and sometimes are quite separate. In practically all cases, the right ureter of one twin and the left ureter of the other open into one of the bladders, while the two remain- ing ureters go to the second bladder. Thus, each bladder receives a right and a left ureter derived from different embryos, and, except in cases of symmetrical ventral union, the ureters which go to the one bladder may be recognised as the inner or adjacent pair, while those which go to the other may be recognised as the outer pair. In such cases, the first bladder lies anterior and ventral to the second, with which also it frequently communicates, especially when destitute itself of an external opening. Attention may be drawn to Figs. 78-81 on PI. XIX., which are reconstruction diagrams illustrating the principal variations referred to above. Fig. 78 is drawn from a specimen in which the bladder in connection with the inner or adjacent pair of Wolffian ducts has no urinary pore, but opens into the bladder connected with the outer pair of Wolffian ducts. Fig. 79 illustrates a case in which the first bladder has no opening and is enormously expanded, as are also the lower ends of its ureters. Fig. 80 is from a specimen in which the bladders are separate and have urinary pores which open in the mid-ventral line, one behind the other. It will be seen that, in this case, the two bladders lie in the same plane and have corresponding right and left sides. But the right side of BL' is in connection with a left Wolffian duct, while the left side is in connection with a right Wolffian duct. Such a transposition is exceedingly rare in double monstrosities. Two further variations are noted by Schmitt {216), namely an instance in which there was a single large bladder bifurcating into two horns, one for each embryo, and an instance in which all four Wolffian ducts opened by separate pores after each had widened out into a small bladder-like dilatation. Fig. 81 of PI. XIX. is taken from the case of symmetrical ventral union already selected for special reference, inasmuch as the alimentary canals are united posteriorly, but open by two vents on opposite sides of the body. Two urinary pores are also present, one behind each of the vents. In this case, accordingly, the vents and the urinary pores lie in a plane at right angles to the sagittal planes of the twin bodies. This arrangement has many parallels in teratology, e.g. in cases of ischiopagous double monstrosity in the higher animals. It preserves the natural correspondence between rights and lefts in the ducts and bladders, which, as has just been seen, is inverted in the case of which Fig. 80 provides an illustration. Caudal Vessels. Traced posteriorly, the caudal veins usually unite behind the vent or vents, and then divide into two vessels which pass backwards in the new plane of symmetry mentioned above, and are equal or unequal according as the type of union is purely ventral or not. The caudal arteries are similarly disposed, their place of union being, however, a short distance behind that of the veins. CLASS VII. Anakatadidymi (Doubling at Anterior and Posterior Ends). A typical example is illustrated in PI. I. fig. 2. Fig. 3 of the same plate shows a second example, which presents the very interesting anomaly that one of the twin heads shows the condition of semi-cyclopia (p. 44). The anakatadidymi have to be subdivided into the two following groups : ( 1 ) with union by the walls of the yolk-sac only ; (2) with union by the walls of the yolk-sac and, in addition, for a short stretch behind that region, by contiguous ventral structures. Examples of the second group 24 DOUBLE MONSTROSITY— ANAKATADIDYMUS are very rare. I have come across none, and must borrow the following details from a description by Schmitt (216, pp. 52-3). The pre-anal edge-membranes are not united, and accord- ingly a triangular gap is left immediately behind the yolk-sac. Union of the ventral edges of the bodies occurs at the level of the vent, and is continued backwards throughout the first portion of the anal fin. Apart from this, the two tails are quite separate, one of them overlapping the other. There are two vents, close together, both looking towards the same side. Four urinary pores are present, each of the Wolffian ducts having remained independent and developed a small bladder-like dilatation near its posterior end. All four urinary pores open near one another just behind one of the vents. In other respects this small second group exactly resembles the first or main one, in which there is union by the yolk-sac only, and, accordingly, apart from the details just given, the following general description applies to both. The twins tend to rest on their sides (PL I. figs. 2-3), the right side of one and the left side of the other being for the time uppermost. Sometimes the twin bodies are so exactly opposite to one another on the yolk that their sagittal planes coincide, and it then becomes a matter of indifference which sides are uppermost. In such cases, on external examination, an equal area of yolk will be visible in either position, and if the circulation be watched, each embryo will occasionally be found to possess right and left anterior yolk-veins of approximately equal size. The yolk store disappears on both sides at an equal rate, and leaves the twins united exactly opposite to one another by their ventral walls. Sometimes, however, the embryos do not lie on exactly opposite meridians, so that their sagittal planes do not quite coincide. Accordingly, a rather larger area of free yolk will be visible between the twins on one side as compared with the other. They still come to rest on their sides, but now those sides tend to be underneath between which the area of free yolk is the larger. Towards the end of its absorption the yolk is visible on this aspect rather longer than on the opposite one. In healthy twins the movements of breathing go on at equal rates, and it would appear that these movements are usually synchronous, but this rule is by no means without exceptions. The frequency of cardiac pulsation also tends to correspond in the twin hearts, but, as a matter of fact, the individual beats are independent, that is neither necessarily synchronous nor alternating with one another. This same point was observed by de Quatrefages in the double-headed shark which he described and figured (198 PI. VIII. figs. 1, 2). Coste's view (4-3) that in double monster fish the hearts aid each other in maintaining an active circulation by having an alternate rhythm does not apply to healthy specimens, though possibly it holds good in cases where one embryo is degenerating and about to become parasitic on the other. Over the yolk-sac, free anastomosis occurs on the part of the finer vessels belonging to the two embryos. Usually also, the anterior yolk-veins are connected by one or more larger anastomotic branches, but on the whole, each embryo keeps the major part of its own blood to itself. The arrangement of the yolk-veins presents many variations. Each embryo tends, however, to have a right and left yolk-vein, but these are seldom of equal size, and one or other of them may be absent. A furrow on the anterior aspect of the yolk-sac commonly marks the boundary between the two circulations. It was this furrow which Quatrefages and others interpreted as indicating the fusion of two separate yolks. The furrow is not visible at all in very young double monsters, and is only well marked during the middle stages of yolk absorption. After absorption of the yolk is completed, the body cavity shows a large mesial common chamber, which is prolonged backwards and forwards within each embryo, thus forming four horns. The intestinal canals are quite separate, and probably also, as a rule, the livers. Union of the ventral body-walls exists from just behind the breast fins to a short distance in front of the pelvic fins. The right and left abdominal recti in each embryo become separated from one another as they approach the region of union, and then each arches backwards, keeping to its own side of the junction line. Reaching the hinder DOUBLE MONSTROSITY— HEMIDIDYMUS 25 end of the united region, the individual right and left recti next rejoin one another within their proper embryos, in which they are continued backwards to their posterior termination (Schmitt 216). The following points relating to anakatadidymi receive mention elsewhere : Frequency of occurrence, p. 7 ; relation to homologous twins in higher animals, p. 7 ; survival after exhaustion of yolk, p. 2 ; orientation of the developing embryonic axes, p. 7. CLASS VIII. Hemididymi (Mesodidymi and Katadidymi). Under these terms are grouped certain forms which are due to similar developmental aberra- tions, and present a characteristically imperfect degree of duplicity. In mesodidymus there is apparent doubling of the middle region of the body, the anterior and posterior ends being single, while katadidymus is marked by apparent posterior duplicity. As far back as 1863, Lereboullet (1S/.3), dealing with living material at very early stages, described various examples taken from the pike. Careful descriptions were afterwards given by Oellacher (176) of more advanced stages in Salrno salvelinus, examined both superficially and by the method of serial sections. This author proposed the term mesodidymus as being supplementary to ana- and katadidymus already introduced by Forster. Eauber (202 5) next added a description of very early specimens in Salrno fario. He had previously suggested that Oellacher's specimens should be called hemididymi to indicate the imperfect character of the doubling which they exhibited (200 71). Almost from the first, the problem of the origin of hemididymus was closely connected with the theory of concrescence. Lereboullet enunciated this theory, practically though not formally, in the course of his observations on the pike. Oellacher did not consider it needful to adopt the theory in question, which had meantime been definitely formulated by His. Eauber was a supporter of concrescence, which seemed to him to be alike confirmed by, and explanatory of, the hemididymous condition. Later, the experimental work of various observers, notably Kopsch (132-133), Morgan (162), and Sumner (242), settled the major question of concrescence in the negative for fishes (see below, p. 27), and at the same time threw much light on the origin of the abnormalities at present under consideration. These have also received illustration from the study of similar defects in other animals, particularly in the Amphibia. Structure. We may begin with a summary for mesodidymus as described by Oellacher (176) in Salmo salvelinus. There is more or less complete doubling throughout a longer or shorter portion of the middle region of the body, while the anterior or posterior ends both externally and internally are perfectly single. The complete doubling involves the mesial organs alone, namely the nervous system, the notochord, the gut, and also in certain cases the liver. On the other hand, all the lateral paired organs, namely muscle plates, Wolffian ducts, auditory organs, and peritoneal cavities, are present only in the total number characteristic of a single embryo. They lie on the outer side of the twin embryonic axes, and are deficient (except for occasional rudiments of the muscle plates) on the inner or adjacent sides. The duplicity of the central nervous system and of the notochord always extends further back and further forward than that of the gut. The length of the hiatus or space between the two components varies greatly. It may only affect the middle of the body (and that not very deeply), or it may affect the whole body from the mid-brain region right back to deep in the tail. In early stages while the yolk-sac is large, the breadth of the space between the components varies proportionately to its length, but in later stages when the yolk is absorbed the twin axes come to he close to one another, and the space in question is reduced to a very narrow slit, the bodies at the same time becoming greatly contorted. In the floor of the hiatus, the yolk appears, not with a free surface, but covered by ectoderm, beneath which a thin sheet of mesenchyme containing blood-vessels seems also to occur. At the posterior end of the hiatus there is nearly always to be found a peculiar tubercle, made up of large epidermal cells, and situated just in front of the place of union of the two spinal cords. 26 DOUBLE MONSTROS ITY— HEMID1DYMUS Oellacher also observed and described several specimens showing posterior duplicity (kata- didymi). In these specimens the division, instead of being confined to the middle of the body as in mesodidymus, is carried back through the tail region. The posterior part of the body and the tail are accordingly doubled, although the doubling still remains of the same imperfect kind which has been described above as characteristic of mesodidymus. The most complete example of kata- didymus recorded by Oellacher is one in which the duplicity begins as far forward as the region of the optic lobes. Variations of the typical condition are noted, for example the two components may be unequally developed, one of them being short, defective in parts as regards the notoehord and other structures, or even completely interrupted. Or again, small inner muscle plates may be present at various points. In the pike, Lereboullet (lJf-3 20 p. 218 et seq.) described thirteen instances of mesodidymus, and two of katadidymus. As regards mesodidymus, his first example and the example which he illustrates in PI. III. figs. 26, 28, are perhaps the most important. The former was observed on the sixth day after fertilisation, and lived till about the fifteenth day, but never hatched out. The fore part of the head was single, but mesial separation began as far forward as the optic lobes, and continued back to the tail, which became normal. The two components were somewhat unequally developed, but each had a notoehord as well as a spinal cord and an outer set of muscle plates. Eemarkable features in this specimen were the presence in each component of a heart and of a small inner otocyst besides the large outer one. The hearts commenced to beat regularly. Inner otocysts did not occur in Oellacher's specimens of the trout. He and Eauber suggest that Lereboullet's observation is likely to have been a faulty one. It seems to me, however, that the for- mation of small inner otocysts is just as likely to occur as that of occasional small inner muscle plates. The example illustrated by Lereboullet in PI. III. figs. 26, 28, is interesting since it shows the presence of a thin mem- brane lying between the divergent components and covering the yolk. The latter, however, still appears through a small round gap in the membrane at the place where overgrowth is just about to be completed. We may compare this witli the covering membrane described by Oellacher, and we must assume that here the advancing blastodermic margin has con- sisted of epiderm only. Eauber (202 5 pp. 694-699) described sLx early examples in Salmo fario — all of them evidently katadidymi. One is at a stage sixteen days after fertilisation. Normally at this stage the various regions in brain and body are beginning to be definite, and the outgrowth for the tail is raised up from the surface. The specimen in question shows dehiscence of the two halves of the embryo from the origin of the optic vesicles backwards (see Text-fig. 4 on this page). Almost symmetrical markings occur on the two sides of the gap and indicate corresponding differentiating cerebral structures. Posteriorly, on either side, a small tubercle is seen which the author thinks must represent the separated halves of a tail bud. The hiatus between the two body-halves is bridged over by a thin layer of ectoderm — the menibrana intermedia — resting on the primary entoderm, or yolk with nuclei but without separate cells. The definitive entodermic layer is, of course, awanting. Of the other five cases described by Eauber, one also exhibits dehiscence affecting the whole length of the body, but in the rest, the dehiscence affects the middle and posterior parts only. One of these is of particular interest, since it shows a gap interrupting the continuity of the right component. Kopsch (132) succeeded in producing two hemididymi in eggs of the trout by applying electrical Fig. 4.— After Rauber (202 5. PI. XLI. fig. 19). An extreme example of the hemididymous condition (dehiscence, Rauber), in an egg of the trout sixteen days after fertilisation, h, the bending round anteriorly of the germinal thick- ening gr; oc, region of the primitive optic outgrowths ; m, membrana intermedia ; a, blastodermic surface; bt surface of yolk uncovered by blastoderm ; gm, margin of blastoderm. DOUBLE MONSTROSITY— HEMIDIDYMUS 27 stimulation immediately behind the embryonic rudiment. In one example, the body-halves commenced to separate in front of the first muscle plates, while in the other the separation began at the anterior border of the auditory vesicles. An outer series of sixteen to eighteen muscle plates was present in each component in both cases. At certain places small inner muscle plates were found, while various portions of the notochords were absent or defective. The presence of inner muscle plates in the one component corresponded, on the whole, with their absence in the same region of the other component. There was similar mutual correspondence between normal and defective formation of the notochord in the two components. Moser (165), who lays stress on these alternations, would seek to throw light on the structure of hemididyini in general by supposing that three kinds of fission on the part of the growing embryonic axis may take place, viz. (1) ordinary equal, producing similar components; (2) ordinary unequal, producing components one of which is larger than the other; and (S) zig-zag, producing components with certain alternating structures. References to minor abnormalities observed by the author, and traceable to mesodidymus or katadidymus, will be found on pp. 52, 54 of the present work. Causation. Mesodidymus and katadidymus in fishes may be produced experimentally through the same kinds of agencies as cause defective conditions in general. Thus Knoch (127) found duplicity of the posterior kind to occur much more frequently in eggs hatched out in water which was stirred up by an active current than in still water. Jablonowski (106) was able to produce splitting of the embryonic rudiment by the action of common salt solutions. A similar result occasionally appeared in Stockard's (236 p. 105) experiments with lithium chloride, and Kopsch (132) induced the formation of two hemkhdymi by electrical stimulation. A satisfactory account of the mode of origin of hemididyrnus can, however, hardly be given at present. Probably the condition may result from different factors. Before discussing these, it will be necessary to refer again briefly to what is known regarding the earliest stages in the formation of a normal embryo. Kopsch's experiments seemed to show that the embryonic blastema for the head and body is spread out, to begin with, over a small segment of the blastodermic margin, the head blastema occupying a middle position and being flanked on either side by the body blastema, which is thus divided into two separate masses. Next, the two masses of body blastema come together behind that of the head, and, having united, grow backwards. The region formed by their actual union is called by Kopsch the primarily formed portion of the body, and we see that it has resulted from a very limited sort of concrescence. It produces the rest of the body by backward growth, and, as was stated on p. 6, during this growth material is drawn from the adjacent thickened margin of the blastoderm for the formation of the lateral and ventral body-walls, and in particidar for the muscle plates. On the other hand, Morgan (161) and Sumner (24.2) hold that the blastema for the whole embryo is disposed from the first in a spatially single centre, and that this centre produces the definite body and tail by backward growth, accompanied by utilisation of material from the thickened adjacent margin of the blastoderm in the manner already noted. These authors, accordingly, deny even the limited degree of concrescence which Kopsch would admit. If we adopt the view of Morgan and Sumner, we may look on hemididymus as capable of being produced either (A) by factors which tend to draw apart into two diverging horns the backwardly growing mass of blastema which should give rise to a normal body or tail ; or (B) by factors which hinder the direct backward growth of the blastema in question, causing it to split, so to speak, into two streams which flow round the obstacle ; or (C), by any of the causes, innate or external, which tend towards excessive growth and fission on the part of a growing system. If we adopt Kopsch's view as to the mode of formation of the embryo we shall have to say in addition that the causes detailed under (A) and (B) may operate at the very start by preventing concrescence on the part of the masses of blastema from which the p)%imarily formed portion of the body arises. 28 DOUBLE MONSTROSITY— HEMIDIDYMUS (A) Seeing that material is drawn in considerable amount from the adjacent margin of the blastoderm on either side to aid in the formation of the lateral parts of the body, any cause which fixes this material or diminishes its mobility will induce a corresponding outward pull or drag on to the two sides of the embryonic rudiment. This may well be strong enough to make the growing end of the rudiment divide into two horns, each of which would then naturally proceed to grow backwards and would attempt to become complete and bilateral in itself, according to a potentiality which seems to be inherent in all embryonic apical systems. Completeness on the inner or adjacent sides would not be attained, however, there being no material at hand to aid in the formation of the muscle plates on these sides. Under normal conditions it is the gradual extension of the blastoderm enclosing the yolk which continually brings the required marginal material into serviceable position, and accordingly any condition which prevents the spreading of the blastoderm or increases the area to be covered may cause the pull or drag referred to above. Hemididymus may therefore be expected to appear (1) in ova the vitality of which has been lowered from any cause, e.g. hatching under unfavourable conditions, as in agitated water, in water deficient in oxygen, or having foreign substances in solution, and (2) in ova which have become partially swollen through differences of osmotic pressure or other cause. It is a common occurrence in ova with lowered vitality that definite organs are the first to suffer, whilst the undifferentiated superficial blastoderm, and later even the mesenchyme, may continue to spread. Here we have a possible explanation of the fact brought out by Lereboullet, Oellacher, and Eauber, that a thin epidermal layer bridges the space between the diverging halves of the body, and also of the additional fact noted by Eauber that the underlying yolk in this region contains nuclei without cell boundaries, e.g. that the primary entoderm may be present although the definitive entoderm is never formed. The thickened ectodermal tubercle noted by Oellacher at the posterior angle of the cleft between the two body-halves in mesodidymus is difficult of explanation, but may perhaps be due to the crowding together of surface tissues which will naturally take place in the angle of re-union, and which will be augmented afterwards through the supervention of secondary fusion. That the last named process occurs in an appreciable degree is evidenced by the fact that the doubling of the notochords extends very considerably further back than doubling of the spinal cords, or of the alimentary canals. (B) The factors grouped under (B) have to do with postero-mesial obstruction or defect, and this may be caused by mechanical injury to the yolk or to the tip of the growing tail bud. The hemididymi produced by Kopsch {132) through electrical stimulation were no doubt of this kind. Each of the divisions thus produced will attempt to complete itself as described under (A), but here again the absence of a supply of lateral material on the inner or adjacent sides prevents the realisation of the perfect bilateral condition. Alike in (A) and (B) the two axes will now naturally follow in their growth the thickened margin of the blastoderm. If the extension of the latter has been greatly interfered with, the axes may never again be brought together, and thus the katadidymous condition may result. Or if vitality has been only slightly lowered, the axes may become approximated posteriorly, through delayed completion of the natural process whereby the yolk is covered. The tail buds now becoming united may give rise to a single posterior portion, and thus the mesodidymous type described by Lereboullet and Oellacher will result. (C) Mention of the causes under this heading is made on chiefly theoretical grounds, since observations on their nature and action are awanting. But there is every reason for believing that the growing embryonic axis of a fish possesses that potentiality of fission which manifests itself from time to time in the growing axes of other vertebrates, as in apical growth-systems generally. Eauber's " posterior radiation " theory attempts to express and formulate the potentiality in question, which was admirably enunciated by Cleland (39), as a principle even wider in its nature and application. Prevalent Types of Monstrosity in Fishes and in other Vertebrates. The chief difference between fishes on the one hand and birds and mammals on the other, as regards the types of monstrosity DOUBLE MONSTROSITY— PARALLEL UNION 29 which they exhibit, is that in fishes posterior duplicity is rare, and when it does occur, is always of the imperfect hemididymous kind. By contrast, the other two groups both provide excellent examples of posterior doubling. In this connection, it should be added that no example of posterior triplicity is known among fishes, while undoubted instances of this condition have been recorded among mammals (see p. 39). We have seen that the imperfect doubling characteristic of katadidymus in fishes arises in all probability from the circumstance that inasmuch as the posterior end of the embryo remains continuous with the edge of the blastoderm, the inner sides of a posteriorly dividing embryonic rudiment abut on the yolk, and, accordingly, are unable to obtain the peripheral cell material normally employed in building up the lateral and ventral body walls. For want of this, the two diverging limbs of the divided axis are unable to complete themselves by becoming bilaterally symmetrical in the full degree. In birds, on the other hand, the groove of the sickle early becomes obliterated, the blastoderm spreading rapidly behind it. The posterior end of the primitive streak (and afterwards of the growing embryonic axis) is thus removed from the margin of the blastoderm, and lies entirely within the area of the latter. Should the posterior end of the primitive streak (or of the embryonic axis) for any reason undergo fission or dichotomy, the new growth-systems thus produced will find their inner or adjacent, as well as their outer, sides abutting equally on the various blastodermic layers. Nothing will then hinder the systems in question from completing themselves so as to become bilaterally symmetrical, in virtue of that power and tendency towards the realisation of the whole, which, as we saw reason above to believe, may be manifested at very early stages by all embryonic growth-systems should opportunity and necessity arise. On the whole, the reptiles will tend to resemble the fishes as regards the types of monstrosity which they exhibit, since in them the primitive streak and the posterior end of the embryo maintain a modified continuity, for a considerable time at least, with the margin of the blastoderm. It is true that, as far as I am aware, no examples of the hemididymous condition have been recorded among reptiles, but neither (with one very doubtful exception x) are there any instances of true posterior duplicity. On the other hand, anterior duplicity is far from being rare, considering the relatively small numbers of reptilian ova and embryos that come under observation. CLASS IX. Longitudinal or Parallel Union. My own material has not provided me with any examples of this class, and indeed the sole instance on record in the Salmonidae appears to be Barbieri's (6), which has reference to an egg of Salmo irideus fixed seven days after fertilisation and showing a double embryo 3 mm. in length. There is a marked tendency for the organs of the ventral part of this embryo to show duplicity in a greater degree than the dorsal ones. For example, the brain cavity in cross section looks like a three-rayed star. One of the rays is directed dorsally, while the other two point downwards and to the sides, and represent the ventrally-doubled central cavity. The spinal cord is triangular in section, and has a notochord below each basal corner, while ventrally and mesially a series of united inner mesoblastic somites is to be found. There are two pairs of optic vesicles, the inner ones being more or less fused together. One of the twin sides is rather smaller and less well developed than the other, the mesoblastic somites in particular being deficient in size. Two vesicles of Kupffer can be made out, the one belonging to the smaller twin being situated a little in front of the other. In the figures there is no sign of duplicity on the part of the gut. It will be remembered (see p. 10) that one of the types into which Lereboullet classified 1 A young blastoderm of Lacerta ocellala showing on a single embryonal shield two primitive streaks converging gently in front (Tur, quoted from Schwalbe 222, II. p. 30). 30 DOUBLE MONSTROSITY— IN VARIOUS FISHES his series of anomalous pike embryos is distinguished by the presence of a single broad embryonic rudiment showing more or less complete duplicity of the axial structures especially in front. No doubt this type approaches the anadidymous condition, but following Lereboullet's description one must infer that it will also include examples of parallel union. Dohrn's (57) Torpedo embryo, of which a short account is given on pp. 31-32, seems an almost perfect example of the class. Unfortunately details are not available regarding the condition of the anterior and posterior ends of this embryo. Mode of Origin. There is little evidence to go upon, but if, as seems certain, the class is really separable from the anadidymi, and therefore justifiable on logical grounds, we must assume that there has been, to begin with, a single centre of gastrulation, and that in connection with this centre there have appeared two sets of axial structures (notochord and nerve cord) lying parallel with one another. Ultimately, posterior union of these axial structures, or entire reduction of one set of them, cannot fail to occur during the progress of growth, since no reserve of cell material is available for the formation of inner or adjacent parts (see p. 6 and p. 27). In the end therefore the condition will become an anadidymous one, but from the disposition of the twin components, the tendency to loss or concealment of duplicity through secondary fusion will have the chance of operating very fully, and Lereboullet's observations show that this actually takes place. DOUBLE MONSTROSITY IN OTHER FISHES. Blennius. Kathke (199). An anakatadidymous specimen: two separate embryos, one larger than the other, the smaller joined to its neighbour and to the yolk-sac by a short connecting band (quoted from v. Baer 5 89). Cyprinus Blicca. v. Baer (5). Two anadidymi: (1) two heads united in neck region and diverging at an angle of 60°; (2) union behind the middle of the body, divergence at 110°. Both specimens were observed 2\ days after fertilisation. Scomber. Sutton {'24-5). Mention of double monstrosity in the mackerel. Anarrichas. Buckland (33a). A more than doubtful instance, referred to on p. 2. Lcnciscus. Bataillon (11) describes fragmentation of ova temporarily exposed to differences of osmotic pressure. This was followed by an attempt at independent development on the part of the fragments. The results, which were experimentally obtained, are of somewhat doubtful nature. Girardinus Caudimaculatus. An example of anakatadidymus is recorded by Emeljanov (65) in this species. The twins were united together ventrally behind the second pair of fins by means of cutaneous structures. They were not so large as normal newly hatched embryos, one of them in particular being smaller and less active than its neighbour, to the movements of which it proved a hindrance. They lived for about a week after hatching. Petromyzon. Bataillon (1:3) describes a particular group of about a hundred artificially fer- tilised eggs of the lamprey. In 40 per cent, of these the first segmentation furrow — a vertical one — went deeper into the substance of the egg than the later ones, so that the whole mass was to some extent divided into two halves by a vertical constriction. These halves gave rise each to a blastula and gastrula, the blastoporic openings being on the under side. In later development most of the eggs died off, but four managed to survive, twin larvae hatching out from each. Experimentally Bataillon (11) succeeded in obtaining imperfect imitations of the above, by exposing lamprey eggs at suitable stages to temporary changes of osmotic pressure. Selachii. There are a fair number of records relating to double monstrosity in the cartilaginous fishes. All are examples either of anterior duplicity or of union by the ventral abdominal walls (yolk-sac union). The proportion of the latter to the former is much greater than in the trout, namely, 1 to 2-25 in contrast with 1 to 13 (see p. 7). This is what might be looked for, consider- ing the relative sizes of the blastoderms and yolks in the eggs of cartilaginous and osseous fishes. The tendency to primary fusion will operate over a smaller proportion of the whole blastodermic DOUBLE MONSTROSITY— IN CARTILAGINOUS FISHES 31 margin. Taking the figures as they stand, they would mean that in order to remain separate the two embryonic rudiments would have to be located, to begin with, at an angular distance of not less than 129'50 from one another on the margin of the blastoderm. The corresponding distance in the trout, as estimated in the same manner, amounts to not less than 165° (p. 7). Classification of Double Monster Cartilaginous Fishes. Probably if a full series were available for comparison, the same types of double monstrosity would be found to occur in the cartilaginous as in the osseous fishes. The mode of origin will be essentially the same in the one group as in the other, and the degree of duplicity will again depend on the distance which originally separated the two embryonic rudiments on the margin of the blastoderm. On these points, however, we must speak by inference, since direct observations are a wanting. Altogether I have only been able to collate thirteen recorded instances in the cartilaginous fishes. The chances are that this number should be reduced to twelve or possibly to eleven, owing to duplications, no longer traceable, in the records. As a rule few descriptive details are given, even regarding external characters, and in no case is the internal anatomy adequately outlined. None of the specimens is described as katadidymous, although that figured by Gadeau de Kerville (see below) somewhat resembles the later stages of this condition in the trout. No instance of triplicity seems yet to have been observed except under experimental conditions (see note on p. 5). The records which 1 have been able to obtain are given below. In regard to some of the specimens, the species and even the genus cannot now be ascertained, but all such specimens may be put down as being sharks or dogfish. Union in the Read Region. An example is figured by St. Hilaire (313, PL XIV. 5) in the atlas attached to his great work. The two inner eyes are close together, but do not seem to be actually united. As far as one can judge, inner spiracle-openings are absent. Probably the twin brains are united with one another towards the anterior part of the medulla. Union in the same region or a little further back is perhaps exemplified by the instance recorded by Risso (205), in which two mouth-openings were present, one above the other. St. Hilaire, however (III. 202), throws some doubt on the validity of this record. Union in Pectoral Region. Gadeau de Kerville (73, PI. XVIII. figs. 1,2). A young Acanthias vulgaris with the umbilical vesicle still well marked externally. Inner pectoral fins are not present. The posterior part of the body is irregular and distorted in a manner resembling, superficially at least, the condition frequently seen in advanced katadidymous trout embryos. Union just behind the Pectoral Region. Quatrefages (198 pp. 11-12) gives a careful description of a young Acanthias showing anterior duplicity. The union occurs just behind the pectoral region. Inner pectoral fins are present, the one belonging to the right twin being markedly displaced down- wards, while the other is correspondingly displaced in an upward direction. Union near the middle of the Body. Aldrovandi (;?). The shark referred to on p. 2. Heusner (94 8 p. 34). " Dr. Barclay of Edinburgh had a new-born specimen, double to the umbilicus, while there is a similar specimen in the Hunterian Museum " (quoted from v. Baer 5 p. 88). Lowne (14-6 y>. 11 No. 23). A foetal dog-fish exhibiting anterior dichotomy. The dichotomy extends to the umbilical region, and the ventral convergence of the sagittal planes is so well marked that the component embryos may almost be described as facing one another. Union by the yolk-sac only, or, after absorption of the yolk, by the abdominal walls. Heusner (94- 8 p. 34). " Professor Brookes in London had a specimen double anteriorly and posteriorly, but united in the middle of the body " (quoted from v. Baer 5 p. 88). Levison. A sternopagous double monster (quoted from Panum 180 p. 72). Lowne (14-6 p. 11 No. 22) catalogues two small sharks united by the abdominal walls. The remains of a single yolk-sac is seen between the pectoral fins. The specimen was obtained from a female shark taken by the donor (mate of a vessel) in the Indian Ocean. When brought on deck 32 DOUBLE MONSTROSITY— IN CARTILAGINOUS FISHES and cut up, about thirty living young escaped from its abdomen. The specimen presented lived for about two days in a bucket of water. Klaussner (123 p. 12) makes mention of an instance in which there were two embryos on opposite sides of the egg of a cartilaginous fish. The record was given verbally to him by Dr. Riickert. Longitudinal or Parallel Union. Dohrn (57) describes and figures an interesting malformation in an embryo of Torpedo marmorata, one mm. in length. The specimen, which had been cut into horizontal sections, shows two notochords lying parallel with one another, and a short distance apart. The mesoblast on the outer side of each notochord is normal in arrangement and shows division into seven muscle somites. Between the notochords is a narrow unpaired strip of mesoderm with eight separate somites, forming a series which is out of line with the normal lateral ones. Details are given regarding the size, etc., of the somites, but the condition of the nervous system and of the alimentary canal is not described. Parasitism. No definite instances seem to be on record, but the suggestion is not an unreason- able one that the accessory fins noted by Gervais (80) and Rennie (203) in the skate are remains of rudimentary twins (see p. 54). CHAPTER II. TRIPLE MONSTROSITY. Description of a specimen, - ... - p. 33 Other instances in fishes, - - - - p. 35 Mode of development in fishes, - - ■ p. 36 Appendix on Triplicity in other vertebrates, and on Classification in Germinal Triplicity, - - - - - • - p. 37 DESCRIPTION OF A SPECIMEN. TmrLE monster fishes are extremely rare. In the trout, I have only come across one example, as contrasted with over seventy double forms. In birds, on the other hand, Dareste's figures show one example of triplicity to nineteen of duplicity, the latter condition in turn occurring on an average in one out of every two hundred and fifty eggs incubated.1 My specimen came from a brood of young trout which had been hatched for about five weeks, and were still subsisting on the egg-yolk. The general appearance is shown in PL XXII. fig. 94. Figs. 95-99 in Pis. XXII.-XXIII. and Figs. 32-34 in PL VII. also have reference to this specimen, and illustrate the alimentary canals, the Wolffian ducts, and various transverse sections. It will be seen that only one of the component embryos is properly developed. In what follows, this component will be designated the principal embryo. The other two, which may be called " A " and " B," are much smaller, and are defective in various important respects. Posteriorly they become united first with one another and then with the principal embryo in the manner to be afterwards described. Principal Embryo. This embryo is slightly smaller than ordinary examples of the same age, but is otherwise normal except in the posterior fourth of its length, where it becomes distorted through the addition of tail elements belonging to the defective embryos. It retains along its whole length its own spinal cord, notochord, neural and haemal arch cartilages, muscle masses, aorta, caudal vein, and alimentary canal. 1 It will be seen that in birds the proportion of triple to double monsters is thirteen times greater than the proportion of double monsters to normal forms. This fact seems to indicate that blastoderms with two centres of embiyo formation show a greater tendency to exhibit a third centre than can be accounted for by the theory of probability applied to the results of simple random distribution. We may suppose that in ova generally there is a certain liability to the incidence of duplicity or multiplicity. The result is to produce a certain proportion of cases in which there are at least two centres of embryo formation. Within this latter group of cases it may naturally be expected that the incidence will show intensification if the liability be fundamentally duo to a predisposition depending on inherent germinal factors. Daresto's figures indicate that such an intensification is actually present, the final result being that the ratio between double and triple monstrosities is so far lessened as to be approximately equal to the square root of the ratio between normal and double forms. In the trout we do not possess so complete a series of observations dealing with early stages, as is that of Dareste for birds. However, Rauber (202 6 139-184), examining a large number of ova ten to sixteen days after fertilisation, obtained one triple rudiment as compared with eighteen which showed duplicity. If we take I in 300 (see p. 1) as something like the average ratio between double and normal forms, it will be seen again that this ratio is approximately equal to the square of that indicated by Rauber's observations as existing between double and triple monstrosity. All this is in consonanco with the view that the formation of double or multiple embryos has behind it something of the nature of germinal predisposition. E 34 TRIPLE MONSTROSITY— STRUCTURE The distortion affects chiefly the ventral aspect of the tail and the tail fin. The ventral edge membrane (including the anal fin) may be described as having been split widely apart into two halves. These unite with a moiety from the corresponding structures of the defective embryos, to form two composite ventral edge membranes. (Compare with ventral union in double monstrosity, pp. 22-23.) A transverse section near the commencement of the tail (PI. VII. fig. 32) shows five angles, viz. three referable to the dorsal edge membranes of the three embryos, and two to the composite structures which have just been named. Nearer the tail (PL VII. figs. 33-34) the number becomes reduced to four through fusion of the dorsal membranes of the two defective embryos. The alimentary canal and the Wolffian structures are referred to, after the description of " B." Defective Embryo "A." The cerebral lobes, the pineal body, and the 3rd ventricle region are entirely absent. The optic lobes are small and distorted, while the cerebellum, the 4th ventricle and the medulla are present but reduced in size. The spinal cord consists of irregular nervous tissue without a central canal, and is not separated by a membranous capside from the surrounding structures. It joins the spinal cord of the other defective embryo a short distance in front of the vent. The united part becomes rapidly smaller, and disappears without passing back into the tail. There is no trace of olfactory organs or of eyes, but two small otocysts are found lying against one another below the medulla. The left otocyst has a main chamber without distinction into saccule and utricle. It gives rise to a single large semicircular canal with an ampulla and crista acoustica, besides two other small pockets destitute of sensory cells. The right otocyst shows traces of division into saccule and utricle. The saccule has a sensory patch, while the utricle has one well-developed semicircular canal besides a small blind evagination. A single small auditory nerve is seen passing down from the medulla between the adjacent surfaces of the pair of otocysts. The notochord is absent except between the otocysts and for a short stretch below the medulla. There are three or four irregular gill arches separated by slits and carrying gill tufts. Pectoral fins are present, but reduced in size. There is a small single pelvic fin on the right side. The embryo is acardiac, and its tissues are supplied with blood from the yolk-sac vessels of the principal embryo. Defective Embryo " E." The structures found in " B " are practically the same as in " A." The spinal cord, however, is even more irregular, and the left notochord is entirely absent. A single pelvic fin is seen on the left side of " B," and is in all probability a composite structure formed in conjunction with the principal embryo, in the manner already described for the ventral edge membranes. The dorsal edge membranes of " A " and " B " are separate in front, but unite posteriorly, where they form one of the angles of the quadrangular tail fin. Alimentary Canal and Urinary Organs. The monster has two vents, which lie close together just behind the pelvic fins of the principal embryo and of embryo " A." One of these leads into the intestinal canal of the principal embryo, which is entirely separate from that of the others. The second vent is the orifice of a portion of intestine common to " A " and " B." This common portion bifurcates anteriorly near the middle of the small intestine, giving rise to separate stomachs, gullets, air-sacs and livers, for the two embryos " A " and " B." There is a single urinary pore behind the vent. This pore leads into a bladder which is common to all the embryos. The right Wolffian duct of the principal embryo has a separate opening into the bladder ; the left Wolffian duct of the principal embryo and the right duct of embryo " A " unite before opening into the bladder, as also do the left duct of " A " and the right duct of " B." The left duct of " B " ends blindly. The alimentary and the urinary systems are illustrated in PI. XXII. figs. 95 and 96. TRIPLE MONSTROSITY— RECORDS IN FISHES 35 OTHER INSTANCES OF TRIPLICITY IN FISHES. 1. Esox hums. An egg, observed four and a half days after fertilisation, showed two embryos uniting posteriorly behind the pectoral region, and diverging widely in front. The left component was normal, but the right one had two heads, the outer of which was somewhat defective. In later development the latter showed only a single eye, viz. its outer one. The specimen lived ten or eleven days and developed two hearts, one belonging to the left embryo and the other to the (doubled) right component. The two hearts were independent as regards pulsation (Lereboullet, US 1863 p. 217, PI. III. figs. 24-25). 2. Sal mo far io. This is a blastoderm showing three embryonic rudiments. Rather more than half of the yolk still remains uncovered. Two of the rudiments are extremely close to one another, and, indeed, are in process of uniting posteriorly. The third is at an angular distance of approxi- mately 60° to the left along the edge of the blastoderm. This is the youngest triple Salmonid of which we have any record. Fuller development would no doubt have produced a form closely resembling the preceding (Eauber, 202 6 p. 145, PI. VIII. fig. 15). Text-fig. 5 on p. 56 of this work illustrates the specimen just described. 3. Salmo salvdinus. Also a very early example. Only 18 to 20 mesoblastic somites are present, and the yolk is not yet completely covered. The embryonic axis, which is single posteriorly, bifurcates in front, and the right branch again divides (Klaussner, 123 p. 31, Taf. VI. fig 38). 4. Salmo salvdinus. Similar to the preceding, but much older. The main bifurcation extends to near the pectoral region. The second bifurcation affects the left component, and only reaches to the optic lobe region (Klaussner, 123 p. 32, Taf. VI. fig. 40). 5. Salmo fario. A specimen in which the absorption of the yolk is well advanced. It belongs to the same type as the preceding ones. The main bifurcation reaches behind the pectoral region, a pair of inner pectoral fins being present in its angle. The anterior bifurcation affects the right component from the pectoral region forwards. Of the twin heads produced by this latter bifurcation, the right one is reduced and on the way to becoming a parasite on its neighbour (Klaussner, 123 p. 33, Taf. VII. figs. 42-43). 0. Salmo fario. Also at a stage well advanced as regards absorption of the yolk. An apparently simultaneous tripartite division takes place just behind the pectoral region. Inner pectoral fins are present in both angles thus produced (Klaussner, 123 p. 34, Taf. VII. figs. 44-45). The preceding cases all belong to P>. 1 of the appended Classification Table (p. 38). 7. Salmo fario. A moderately early stage. Two embryos are seen coming together at their extreme posterior ends. The right one bifurcates just behind the region of the medulla. The posterior ends of the two main components diverge in such a manner that one cannot consider them as exhibiting axial union, though probably, had they survived, their tails would have remained united by paraxial structures. Accordingly, the specimen belongs to B. 2 of the Classification Table on p. 38. It is recorded by Klaussner (1:23 p. 39, PI. VI. fig. 39). 8. Salmo fario. The specimen described in this paper. The axes of the two defective embryos are united. These last, however, do not undergo true axial union with the principal embryo, being united with the latter only by paraxial structures. The specimen is accordingly to be referred to B. 2 of the table. 9. Salmo fario. The verbal description of this specimen (Klaussner, 123 p. 34, PI. VIII. figs. 4G-47) bears out that there are three completely separated embryos, all being smaller than normal, and one very markedly so. However, both of the illustrations seem to indicate union on the part of the tails of two of the embryos. If the description is right, this specimen will belong to Division I. of the Classification Table, and allowing for differences in the general conditions of development, will be in line with those instances among mammals in which three separate embryos are found within the same chorionic sac. On the other hand, if the figures are correct, the specimen will have an important bearing on the question as to how far apart on the blastodermic margin embryonic 36 TRIPLE MONSTROSITY— DEVELOPMENT rudiments must appear in order that the growing embryonic axes may remain separate along their whole length (see p. 7 and p. 30). 10. Petromyzon plancri. The example of caudal triplicity recorded by Barfurth (S) in a larval Petromyzon should be referred to here. The larva has three tails, each of which contains a spinal cord, notochord, and caudal artery, as well as muscle plates and skin. Barfurth, however, considers that probably two of the tails originated by a process of supra-regeneration after injury. If this be so, the specimen is not properly referable to B. 1 of the table, which includes examples of true posterior axial triplicity occurring in the mammals alone. 11. Salmo fario. Klaussner describes a remarkable instance of anterior quadruplicity. The posterior end is single. The main bifurcation extends to behind the pectoral region, and each of the components is divided as far back as the optic lobe region. The four heads are all defective, being unprovided with eyes (Klaussner, 133 p. 32, Taf. VI. fig. 41). 12. Schmitt ( 316 p. 37) mentions the occurrence of several triple forms and of one quadruple form in the set of 900 eggs (Salmo fario) referred to on p. 1 as having provided 68 double monsters. Descriptive details are, however, awanting. MODE OF DEVELOPMENT. Regarding the mode of growth, and the causation of triple monsters, little can be added to what was said under these headings in the part dealing with double monstrosity (pp. 6-8). All the available evidence goes to show that double and triple forms arise in essentially the same manner, that is, by the appearance of different centres of embryo- formation at a greater or less distance from one another on the margin of the blastoderm. The example (Instance 1, above) recorded and illustrated by Lereboullet, and the other early examples noted by Ilauber (see Text-figure on this page) and Klaussner leave no room for doubt on this matter. We are speaking here, it will be understood, of anterior duplicity and of union by the yolk-sac only (anadidymus and anakatadidymus), no triple form showing mesodidymus or katadidymus having been put on record. The heads invariably point in one direction, as occurs also in the double monsters, for a precisely similar reason (p. 7). f,g. .5.-Af.er Rauber (m 6, Taf. vi it. U wU1 be remembered from the part dealing with double fig. 14). Egg of trout sixteen days after monstrosity (p. 7) that two embryonic rudiments which are to fertilisation, showing three embryonic nidi- .. > » ,, , , ments which are slightly unequal in size remain entirely separate from one another need to arise at a very S^y^(^CL^SSS*SS2 considerable angular distance (approx. 165") from one another on apart along the margin of the blastoderm, the edge of the blastoderm. If the description attached to The specimen is described under (2) (p. 35). t- . #» * i ■ 1 • f -l t -ii Instance 9 of triple monstrosity in fishes be literally followed, our estimate of the distance in question will have to be reduced at least to 120°. On the other hand, if the illustrations in connection with Instance 9 are correct, they supply evidence that the previous estimate was not far from being true. Triple monster fish showing simultaneous tripartite division are possible forms. Instance 6 of the preceding series may have been an example. The mode of formation could be explained by assuming that the two outer centres of embryo formation appeared at an equal distance from the middle one on the edge of the blastoderm. The chances are that this will happen very rarely indeed, while, even when it does happen, there is always the likelihood that the egg as a whole, or one or other of the components, will not survive long enough to give the monstrosity a chance of being observed. There do not appear to be any records, ancient or modern, indicating survival, or reputed survival, of a triple monster fish beyond the embryonic period. Even as regards that period we still know of examples only from Esox and the Salmonidae. TRIPLE MONSTROSITY— IN OTHER VERTEBRATES 37 TRIPLICITY IN OTHER VERTEBRATES. Amphibia. Although re-duplication of parts is fairly common in the Amphibia (Gemmill ]) examples of triple monstrosity appear to he exceedingly rare. The nearest approach to the condition that the author can find occurs in the Pdobutcs larva described by Bruch,2 in which the axial parts of the tail divide posteriorly into three. This condition, however, as in Barfurth's (8) Pelromyzon larva, was probably the result of injury. Reptilia. A very early stage in the ring snake is figured by Wetzel,3 in which there are three centres of segmentation on the yolk, and one of the centres is itself double. In the older records, a three-headed snake is stated to have been seen at Lake Ontario (Mitchill, quoted from Bruch, as above, p. 172). Then there is also the account reported in Aldrovandi (# 427) of a snake with three heads that was killed in the Pyrenees. The occasional formation, by way of supra-regeneration after injury, of a triple tail rudiment (in Lizards), may also be mentioned here, as under Amphibia. Birds. Examples of triplicity in developing eggs have been recorded by : Dareste (58 p. 456, PI. XIV. fig. 5). Two blastoderms on the same yolk, one with a single embryo, the other with two separate embryos, of which one is smaller than its neighbour and has no heart. Dareste (as above, but Fig. 4). Three separate embryos on a single blastoderm. Dareste (as above, but only mentioned in note on p. 45 G). One area pellucida, with three embryos united by the heads. Dareste (as above). Two blastoderms united together, one of them with a single embryo, and the other with an embryo having two heads. Eauber. A common vascular area surrounding two areae pellucidae, one of which is small and bears a single embryo, while the other has two embryos lying close together and uniting by their adjacent body-walls. (Morph. Jahrb. Leipzig 5 167-190 Pis. XII. XIII.) Moriggia. A single blastoderm carrying three separate embryos. (Quoted from 247 3 p. 464.) Koch. An area pellucida carrying two embryos, one of which shows anterior duplicity. (Quoted from 247 3 p. 465.) A case recorded by Tur should also be referred to here. I have only access, however, to the very brief description of it given by Windle.4 " There were four separate centres of development. On three of these there was a primitive groove, and one of these grooves was double." Mitrophanow also figures a chick blastoderm with several (at least three) primitive streaks. M.'s figure is reproduced by Schwalbe (222 ii. p. 26). Mammals. A few examples of unioval triplicity, but none of triple monstrosity, seem to have been recorded since the publication of Taruffi's work on Teratology (247). This author very properly emphasises the occurrence of unioval or homologous triplets in the human subject and in other mammals, and refers such cases to an origin similar to that of identical twins, namely, the development of more than one embryo from a single ovum (24-7 3 p. 450-460). More recent instances of this kind are given by (a) Walla, (b) Breitschneider, and (c) Watson.5 Of triple monstrosity, Taruffi has collated eight examples in the human subject, four in the dog, and four in the sheep (247 3 p. 465 et scq.). The accounts of these records need not be reproduced here, and, indeed, the older records are often defective in respect of detail. Some of the cases will be referred to in the notes appended to the following scheme suggested for the classification of examples of Germinal Triplicity. 'Supernumerary limb in a Frog, J. Anat. Physiol. London, 40 (387-395). 2Ueber Dreifaehbildungen, Jenaische Zeitschr. Natw. 7 (142-175). 3Drei abnorm. gebildete Eier von TropiJonolus natrix, Anat. Anz. Jena, 18 (425-440). 4 Report on Teratological Literature, /. Anat. Physiol. London, 40 p. 296. °(a) and (6). Quoted from Windle's Reports in J. Anat. Physiol. London, 37 p. 302, and 38 p. 365. (c) British Medical Journal, Jan. 9, 1904 p. 75. 38 TRIPLE MONSTROSITY— CLASSIFICATION CLASSIFICATION IN GERMINAL TRIPLICITY. Three embryos from one ovum. II All the bodies separate. (Trisomi dieriti of Taruffi, 247 III. p. 458) — The two united bodies showing axial union. A Only two of the The bodies not all separate. (Trisomi sineriti of Taruffi, III. p. 464) B bodies united. -The two united bodies not showing axial union. 1 — All three bodies showing axial union. — Only two of the bodies showing axial union. All three bodies united. -Axial union not present. The divisions A. 1, B. 1, and B. 2 may be further subdivided according as the union of the bodies is anterior or posterior (anadidymus and katadidymus). A single condition of the neural axis and the axial skeleton, at least for some part of their length, should be taken as their criterion of axial union. Notes to the Classification Table. Under I. will be included the cases of identical triplets already referred to. These are probably much less rare (l2/+7 III. p. 460, 19, 20) in the human subject than are cases of triple monstrosity. Instances of quadruple, and even of quintuple, pregnancies with single chorion and placenta, and presumably of unioval origin, have also been recorded (&£7 III. p. 459). The remarkable observation by Assheton (J. Anat. Physiol. London, 32 362) of two germinal areas on a single blastocyst of the sheep, threw very striking light on the probable mode of origin of identical plural embryos. Still more remarkable is the recent work of Fernandez1 on the embryology of the Edentate Tatusia hybrida Desm. Confirming and greatly extending previous observations by v. Kblliker, Milne Edwards and v. Jhering, this author finds that normally in Tatusia all the eight or nine foetuses in a pregnancy arise from a single ovum, are unisexual, and are enclosed within the same chorionic sac. In the lower vertebrates, the mode of development does not, in the end, allow so complete a separation of the embryonic bodies as is possible in the mammals. Taruffi, however, points out with justice that, in the chick, such examples as the second quoted from Dareste, and that described by Moriggia, are similar in their essentials to his mammalian trisomi. So also is the triple salmon of Klaussner, according to the description stating that the three embryonic bodies were quite separate except for their attachment to the common yolk-sac. A. 1. Examples are the fourth of the chick embryos noted above as described by Dareste and that described by Koch. A. 2. Had it survived, the first of the chick specimens from Dareste would have come to 1 Beitrage zur Embryologie der Gurtelthiere, I. Morph. Jahrb. Leipzig, 39 Heft 2. TRIPLE MONSTROSITY— CLASSIFICATION 39 be of this type through inclusion of the smaller acardiac embryo in the body-wall of its more vigorous neighbour. B. 1. This is the commonest kind of triple monster in the fish. See cases 2, 4, 5, 6, p. 35. The type usually exhibits bifurcation of one of the branches of an already divided axis, but it seems possible that simultaneous tripartite division may have been present in case 6 of the fish series, and perhaps also in some of the three-headed monsters recorded by older authors, to which reference is made in Taruffi (247 III. 466 d seq.\ Oss. 4, 5, 6, 7, 9, 12, 14). However, very careful dissection will be required before tripartite division is definitely shown to have occurred. In the case of the fish embryo, the examination would need to be done by serial sections. The triplicity in the cases just mentioned is anterior, but posterior axial triplicity seems also to occur in mammals (247 III. p. 471 ; Oss. 15, 16, 17). B. 2. Specimens 7 and 8 of the fish series belong to this subdivision, which also includes what may be called the classical instance in the human subject, viz. that described by Keina.1 B. 3. Examples are recorded by von Froriep 2 in the sheep, and by Fiedler 3 and Bettoli and Fattori4 in the human subject. ' Atti dell. Accad. Oioen. 8 p. 203. 'Gurlt'a Lehrbuch der Pathologischen Anatomie, II. p. 201, Berlin 1832. 'Quoted from 247 III. p. 465. ' Quoted from Bruch, Joiaische Zeilschr. 2?atw. 7 p. 158. CHAPTER III. CYCLOPIA. PI. XXIV. fig. 100 (external appearance, see also Pis. I. fig. 3 and II. fig. 11); XXIV. figs. 101- 104 (various structures); XXV. figs. 107-108 (sections of head), and figs. 109-110 (semicyclopia). Classification and structure, - - - - p. 40 Summary and comparison, ... - - - p. 42 Other records in fishes, ... - p. 43 Causation, - ... - - p. 44 Cyclopia rarely manifests itself in developing trout eggs. Under ordinary fish-hatchery con- ditions it may be put down as being ten or fifteen times less frequent than double monstrosity. Altogether I have obtained only six examples. It is noteworthy that two of these were provided by double monsters in which one or other of the twin heads showed the defect in question (PI. I. fig. 11, cf. I. fig. 3). The total number of specimens is hardly large enough for systematic purposes, but I find that here, as in the first groups of double monsters, the condition of the nervous system affords a basis for classification into at least two main divisions. The first (A) is characterised by fusion, more or less complete, of the cerebral lobes (two specimens) ; while the second (B), in addition to fusion of the cerebral lobes exhibits fusion on the part of certain structures belonging to the third ventricle and the mid-brain regions (three specimens). The sixth specimen was not cut into serial sections, and accordingly I cannot say for certain to which division it belongs. In view of the probable causation of cyclopia (p. 44) it will be understood that while the condition is always associated with cerebral defects, these defects need not be of a perfectly uniform character, nor need the fusion or reduction of the two eyes always correspond in amount with the fusion or reduction present in the two sides of the brain. (A) Cyclopia with Fusion of the Cerebral Lobes (two specimens). The external appearance is illustrated by PI. XXIV. fig. 100. The front of the head is wedge- shaped, its size being reduced in the transverse and increased in the vertical line. The large median eye is overarched by a mesial frontal process carrying a pair of small closely approximated olfactory pits (Fig. 101). Upper and lower jaw arches are present. The posterior part of the head and the body are normal. Cranial Skeleton. The skeleton is greatly modified in front of the pituitary region. The trabeculae cranii pass downwards so as to lie below the median eye. They are widely separated from the base of the brain and they take no part in the formation of an olfactory capsular cartilage. Anteriorly, they articulate with short palato-quadrate bars. In the normal Trout embryo at a corresponding stage, the trabeculae, though united, still show evidence of their double origin. But in all my cyclopic specimens the trabeculae form an absolutely single piece right back to the pituitary space. A rudimentary olfactory capsule is derived from the united anterior ends of the supra-orbital bars. This united portion lies in the frontal process and is perforated by the two small olfactory CYCLOPIA— STRUCTURE 41 nerves. Posteriorly, the supra-orbital bars separate and pass along the dorso-lateral aspects of the brain to join the auditory cartilages, as in the normal condition. Near their place of separation, each gives origin to an obliquus oculi superior muscle. The mandibular, hyoid, and palato-quadrate bars are appreciably shortened in accordance with the small transverse measurement of the mouth. Brain. The cerebral lobes are slightly smaller than normal, and are in great part united along their inner faces. The median longitudinal fissure penetrates for only a third of their depth in front at the place of origin of the olfactory nerves, while posteriorly, close to the third ventricle, the fissure in question appears simply as a shallow groove. The third ventricle region and the optic lobes are well developed, pineal diverticulum, optic recess, hypophysis, and hypoaria being present as in the normal condition. A transverse section at the origin of the olfactory nerves shows an area of brain which is -66 of normal size as judged from a specimen of the same age. The proportion to normal in a similar section at the back part of the cerebral lobes is -95, while the middle region of the optic lobes shows a sectional brain-area which is slightly in excess of normal. There is no dropsy of the central cavity of the brain or of the meninges. The cranial nerves are all present and are normal, with the exception of the first two pairs, the olfactory nerves being small and closely approximated, while the optic tracts unite at the commissure to form a single optic nerve. Eye. The globe is large and has its transverse diameter increased, as also has the lens. The lens-cavity is not completely occupied by fibres, a space being left anteriorly which is filled by small round cells. Ketina, choroid, cornea, vitreous humour, and sclerotic are well developed. The single choroidal fissure leads back to a large optic nerve formed, as above stated, by the union of the two optic tracts. There are two choroidal glands, one on either side of the optic pore. They are supplied, as usual, by choroidal arteries coming from the pseudobranchs. The following eye-muscles are present : two superior obliqui, arising from the supra-orbital bars ; two superior recti, arising along with two inferior recti from the fibrous capsule of the brain in front of the hypophysis; two external recti, which are normal in origin and are inserted into the right and left sides respectively of the eyeball. The inferior recti are united close to their insertion into the eyeball. Inferior obliqui and internal recti are absent. (B) Cyclopia with Fusion of the Cerebral Lobes and of Structures in the Mid-brain. Three of my specimens exhibit this condition, two of them possessing a single median eye, while the third, although showing the other essential features of cyclopia, has a pair of small closely- approximated eyes. 1. The specimens which have a single eye resemble type (A) in general appearance, except as regards their mouth-parts. In place of the upper jaw there is a membranous flap on either side projecting downwards and forwards from below the eye. In place of the lower jaw arcade there is a narrow mesial process projecting forwards to end just between the flaps. Microscopic examination of the flaps shows that they contain externally a number of young teeth and internally a commencing membranous ossification. They are probably to be compared with un-united maxillary processes, while in appearance they resemble the horn-like structures found by Paolucci {181) in his cyclopic ray. The mesial process above mentioned contains a much elongated symphysis of the lower jaw, the Meckel's bars of which diverge little from one another and articulate with suspensoria which are similarly approximated. Skeleton. The trabeculae cranii are represented by a single exceedingly short bar projecting downwards and forwards towards the wall of the pharynx. Quite separate from this are the palato- quadrates, the anterior ends of which, uniting below the eye, form a mesial plate replacing the defective trabeculae. The supra-orbitals are different in the two specimens : in one, they unite anteriorly in the frontal process, giving rise to a small olfactory capsule; in the other, they are short and extend no further forward than the middle of the fore-brain. In this latter case, the olfactory F 42 CYCLOPIA— STRUCTURE region is destitute of cartilage and there is no tegmen over the third ventricle. In both specimens the supraorbitals are displaced downwards so as to be ventro-lateral to the brain. The auditory cartilages are displaced similarly but in a slighter degree. Brain. The cerebral lobes are markedly reduced in size and are fused together, the longitudinal fissure being almost entirely absent. The central cavity is slightly enlarged and extends downwards on the outer sides of the lobes further than in the normal condition. The pineal diverticulum is a small unstalked pouch. The optic lobes are of considerable size and are normal as regards their dorsal parts, but internally the mesial furrow of the central canal is only slightly marked, and there is absence alike of the optic recess, of the hypophysis, and of the hypoaria. Optic tracts and nerves are absent. As in type (A), the olfactory nerves are small and closely approximated. A transverse section at the origin of the olfactory nerves shows an area of brain which is only ■32 of normal size as judged from a specimen of the same age. The proportion to normal in a similar section at the back part of the cerebral lobes is '7, while the middle region of the optic lobes is very distinctly (I "3 times) in excess of normal. Compare with the corresponding figures for the (A) type of cyclops. Eye. The single small deeply embedded eyeball has no choroidal fissure, vitreous humour, or optic nerve. The lens and the retina are, however, fairly well developed and there are two choroidal glands. The position of the optic pore is marked inside the eyeball by an interruption of the retina exhibiting a few nerve-fibres, which fail to pierce the hexagonal pigment layer or the sclerotic (PI. XXIV. fig. 104). Two external and two superior recti muscles are present. The other eye-muscles arc awanting, with the exception of a pair of small superior obliques occurring in the specimen mentioned as having its supra-orbital bars extending forward into the frontal process. Mouth. The mouth-opening is represented by a minute canal, beginning at the bottom of the groove between the maxillary flaps and extending backwards above the symphysis of the lower jaw. In one case this canal ends blindly, in another it joins the pharynx. 2. The specimen which had two small eyes closely approximated but un-united shows the following characters : cerebral lobes well developed, deeply cleft anteriorly, but united posteriorly ; pineal diverticulum small ; third ventricle almost obliterated ; incomplete fusion of structures in the floor of the optic lobes ; rudimentary hypophysis and hypoaria ; optic tracts and nerves absent ; eyes small, embedded, almost touching one another, without choroidal fissure, vitreous humour, or optic nerve, but with well-developed lens, retina, and retinal pigment-layer ; superior obliqui as well as superior and external recti present for each eye ; inferior recti and obliqui wanting ; no mouth, the upper and lower jaws being sealed together ; trabeculae cranii extremely short, forming a single bar projecting downwards and forwards into wall of pharynx ; olfactory capsules absent ; supra-orbital bars ending separately in front, the tip of each giving origin to an obliquus ocidi superior ; olfactory pits approximated and supplied by small olfactory nerves. Summary. Olfactory Organs. — Olfactory nerves and pits, reduced in size, are present in all my specimens. The olfactory pits lie close together on the inferior aspect of the mesial frontal process. Brain. — Fusion of the posterior parts of the cerebral lobes is found in all my specimens. By itself, as in type A, this condition is compatible with the presence of a well-developed cyclopic eye possessing vitreous humour and an optic nerve, as well as with the presence of pineal diverticulum, hypophysis, and hypoaria, and of optic tracts and optic recess. Fusion of the basal structures in the mid-brain, as in type B, is associated with greater defects, viz. reduction in the size of the eye-ball ; absence of choroidal fissure, optic nerve and optic tracts ; absence or rudimentary condition of hypophysis and hypoaria. The absence of dropsy of the central cavity of the brain is remarkable. Eye. — As seen in type A, the eye may be remarkably well-developed, possessing lens, retina, vitreous humour, retinal pigment, and optic nerve. A double set of normal eye-muscles, excepting only the internal recti, may be present. Paired superior and external recti are constant, while the superior obliqui and the inferior recti are variable. The remarkable set of conditions which CYCLOPIA— COMPARISON AND RECORDS 43 accompanies fusion of mid-brain structures has been mentioned above in connection with the brain. Skeleton. — The trabeculae cranii appear as a single bar of cartilage underlying the median eye. Either they formed a single structure from the first, and this seems to me most probable, or their fusion was remarkably early and complete. Olfactory capsular cartilages may be present or absent; when present, they are developed in connection with the anterior ends of the supra- orbital bars. The palato-quadrate, the mandibular, and the hyoid bars tend to be shortened, in correspondence with the general transverse narrowing of the mouth-parts. Comparison with Cyclopia in Mammals. 1. While olfactory nerves are not found in the typical mammalian cyclops, they are present in all my trout specimens, being traceable from the fore-brain to the small olfactory pits on the under surface of the frontal process. If, as seems certain, this process represents the " proboscis " of a cyclopic mammal, the " proboscis " in question can have no relation with parts of the brain behind the cerebral lobes, and, in particular, none with the hypophysis. 2. Dropsy of the central cavity of the brain or of the space between brain and skull is not characteristic of cyclopia in fishes. This may lie contrasted with the usually dropsical condition of the hemispheres and the meninges in cyclopic mammals. 3. The relatively good development of all parts of the brain, particularly in type A among fishes, is remarkable. Indeed, there seems to be no absolute bar to prevent a specimen of this kind from surviving and obtaining food for itself as in the remarkable case recorded by Paolucci. Other Records in Fishes. The literature of cyclopia in fishes is by no means extensive. Small as it is, however, it provides two most interesting facts. One has reference to the growth and survival of a cyclopic fish, and the other to an agent in the production of cyclopia, which seems almost specific in its action. Lereboullet (14-3) figures a pike embryo with a cyclopic eye which is furnished with a single lens, and has probably arisen by the fusion at an early stage of two separate eye rudiments. Girdwoyn (SI) also figures a typical cyclopic salmonid embryo well advanced in development, and resembling the specimen I have illustrated in PL XXIV. fig. 100. The remarkable instance of survival referred to above was recorded by Paolucci (J SI). The specimen, caught in the Adriatic not far from shore, and placed in the Natural History Collection of the Technical Institute at Ancona, was identified by P. Paolucci as probably belonging to the species Myliobatis noctula Dum., though it presented certain characters intermediate between this species and Myliobatis aquila. The dimensions, as judged from the author's illustrations, are: length of head and body, not including the tail, (H inches; breadth between tips of pectoral fins 17 inches. There is a single very large median eye, set in the place of what should be the fleshy anterior border of the head. This border is divided into two parts sticking out like horns, " 3 cm. long, and as thick as a finger," one on either side of the eye. The nostrils and the fleshy upper lip are both awanting. The above record, regarding the authenticity of which there seems not the slightest reason to express doubt, is probably the only one which indicates that any vertebrate animal has ever been found in actual adult life embodying the fabled Cyclops type. A very interesting set of experiments has been carried out by Stockard (237) on the production of cyclopia in the eggs of Fundulus heteroclitus by the addition of magnesium chloride to the sea-water in which the eggs were being hatched. With certain strengths of solutions (\ m. sea-water solutions of MgCL), one-eyed embryos occurred with surprising i-egularity in 50% of the eggs. The remaining embryos in these solutions were apparently normal so far as the eyes were concerned. The structure of the compound eye corresponded almost exactly with that described above under cyclopia (A). Stockard does not give particulars regarding the brain, but from his figures, particularly Figs. 4-5 p. 253, it may be inferred that some degree of fusion existed in the region of the cerebral lobes and the third ventricle. As regards the coming together of the two eyes, Stockard considered it to be through an antero-medio-ventral fusion of the elements of the two optic vesicles at an early developmental 44 CYCLOPIA— CAUSATION, ETC. stage, this fusion being more or less complete in the different embryos. He thought it probable that the resulting large optic cup induced the formation of a single lens, the lens being formed of ectoderm different in position from that of the normal lens-forming cells. He adds that probably there is no localisation of lens-forming substance in the ectoderm of the fish embryo, and that it is the underlying optic cup which induces actual lens formation on the part of the ectoderm. This is more or less in harmony with the result of recent experimental work on the development of the amphibian eye. On the other hand, it would render difficult of explanation those cases of defective heads in embryo fish which possess one or more lenses without any trace of corresponding optic cups. Semicyclopia and allied conditions. Alike in single embryos and in double monsters, one may come across examples in which the heads show lateral compression. In extreme instances, the whole anterior part of the head may be atrophied, the mouth being deficient, the brain pro- foundly malformed, and the eyes absent or represented only by the lens. (See PL XXV. fig. 106, XXVII. fig. 112.) When the defective condition is moderate in degree, the eyes and the olfactory pits are brought together, the mouth narrowed, and the trabeculae cranii ventrally displaced, so that they lie below the approximated eyes and fail to join the olfactory cartilage in front. This condition, which may be called semi-eyclopia, is illustrated in PL I. fig. 3 and XXVI. figs. 109, 110. Similar specimens are figured by Girdwoyn (SI p. 112) and by De Quatrefages (198 PL VII. fig. 2). See also p. 51. Causation. 1. It seems probable that the causes which produce cyclopia in fishes are sometimes purely mechanical, and that under favourable circumstances they may exercise a minimal degree of interference with the development of other structures, in particular with that of the brain. Under ordinary circumstances, and apart from such experiments as those of Stockard, pressure, or at any rate want of space for development, may be put down as the commonest causal factor. The egg-membrane of the trout is tough and strong, while the cavities of the optic bulb and stalk, and even that of the central nervous system, are developed secondarily in solid masses of cells. It may be supposed that undue lateral pressure, from whatever cause (e.g. partial coagulation of the yolk, swelling due to differences of osmotic pressure, want of space from twinning of the head), may bring the optic buds together and cause them to unite during their actual outgrowth. If only moderate in degree this pressure might next allow a central cavity to form in the single bulb and stalk. Such a central cavity would permit the development of the secondary optic vesicle with its choroidal fissure. The choroidal fissure would enable mesenchymal cells to pass into the interior of the eyeball and form a vitreous body, and would enable also nerve-fibres growing from the retina to escape from the eyeball, pass along the optic stalk, and form an optic nerve and tracts such as are actually found in type A. The effect of moderate pressure on the brain may perhaps be recognised in the fusion of the posterior parts of the cerebral lobes characteristic of this same type. A greater amount of lateral pressure might lead to such further degrees of fusion affecting the third ventricle and the mid-brain as are illustrated in type B. In the eye it may greatly hinder the formation of a central cavity in the primary optic vesicle and stalk. This might prevent the formation of a choroidal fissure by the usual method of ventral cupping. In the absence of a choroidal fissure, mesenchyme could not enter behind the lens to form a vitreous humour, and nerve-fibres formed in the retina would have no exit from eyeball to stalk, and the stalk itself would degenerate. The condition in type B might then be realised, i.e. an eye, reduced in size, without choroidal fissure, vitreous humour, or optic nerve. 2. The apparently specific action of magnesium chloride in producing cyclopia is at present impossible of explanation if, as Stockard seems to have proved, it does not depend on differences of osmotic pressure. Stockard (:237), however, points out that probably other substances are capable of exercising other specific effects on developing eggs, and calls attention to the characteristic Lithium larvae of Herbst, to Morgan's frog larvae and to his own fish embryos. 3. Although positive evidence is a wan ting, I think it extremely likely that cyclopia may sometimes have a spontaneous or autogenetic origin. In this respect it would resemble various CYCLOPIA— CAUSATION, ETC. 45 other abnormalities which are also capable of being produced by the action of external factors, and do not depend directly on atavism. The remarkably perfect and symmetrical nature of the anatomical adjustments that is sometimes found both in the eye itself and in the surrounding parts, seems to point to this conclusion. According to the view already expressed (pp. ix-x), the fact that cyclopia may be produced by environmental factors, is rather an argument for, and not against, the probability that it may also arise by abrupt spontaneous variation. CHAPTER IV. MINOR ABNORMALITIES.1 I. Hermaphroditism, ... p. 46 II. Abnormalities affecting skull or vertebral column. (a) Shortening and curvature of the snout; the pug-headed condition, - - p. 48 (b) The round-headed condition, - p. 50 (c) Defects of the lower jaw, - - p. 51 (d) Hump-back, twisted body, and allied conditions, - - - p. 51 (e) Local reduplication and deficiency of the notochord, - - p. 54 III. Abnormalities affecting fins. (a) Excess, - - • p. 54 (b) Defect, ----- - p. 55 (c) The so called " tailless " condition, - - - - - - p. 56 IV. Abnormalities of coloration. (a) In fishes generally, - - ------- p. 56 (b) In flat-fishes, associated with certain other defects, - - - - p. 56 V. Parasitism, ------------ p. 59 VI. Pathological conditions of the ovum or early embryo, - - - ■ - p. 60 VII. Abnormalities having reference for the most part to single organs, - - - p. 61 HERMAPHRODITISM. Under this heading we have to deal almost entirely with true hermaphroditism, since the Teleosts (which provide the vast majority of instances) do not, on the whole, possess accessory genital structures which are peculiar to either sex. Among other fishes, the only instance of spurious hermaphroditism I have come across is provided by the adult male skate described by Matthews {151) as possessing an oviduct on the left side. The specimen measured over two feet in length and had practically perfect glands and ducts of the male type on either side. In addition there was a Mullerian duct of full adult size on the left side. This duct was furnished with a large oviducal gland and contained numerous living spermatozoa in its anterior (Fallopian tube) portion. It may he noted in this connection that the male Raia elavata is occasionally found with teeth as much blunted as those of the female (Day 5J, II. p. 344). True hermaphroditism, normal. As is well known, the condition occurs normally in a number of fishes. Following Howes (101) with slight modification, we may distinguish three types of 1 The following paragraph from Stoddart ("39 p. 75) is worth quoting at this stage. It shows the frequency with which abnormalities of many kinds occurred in trout from Scottish waters before artificial hatching was practised, and at a time when the amount of sewage pollution must have been vastly less than it is now. " We have caught them hump-backed and covered with sores, blind of one eye, wanting a gill or a fin and crooked up in all shapes. On the Tay we once took an individual with a short round upper head like that of the bull-dog and the lower lip projecting beyond it. We understand that a variety of this kind is to be found in Loch Dow in Inverness-shire. On the Water of Leith we saw a friend capture three successively out of one stream all of which wanted the tail ; this defect was most probably occasioned in winter, the water from which they were taken happening to be extremely shallow and the frost before being somewhat severe." MINOR ABNORMALITIES— HERMAPHRODITISM 47 the condition: (1) successive unicyclic {monocyclic Masterman 150); (2) successive multicyclic {poll/cyclic Masterman); and (3) synchronous. To the first group belong those fish which, like Myxine, ripen only sperm when young and afterwards only eggs; to the second, those which are protandrous in each season ; and to the third, those in which the ripening of both kinds of sexual products takes place at much the same time in each period of breeding. True hermaphroditism,, abnormal. A very large number of instances have been recorded, chiefly from the Teleosts. References to the more important of these will be found under the following index numbers. Fuhrmann 71; Halbertsma 89; Hefford 90; Howes 101; Iwauzoff 105; Jiickel 107; Jarvi 113; Johnstone 118; Kyle 135; Lowne 146, pp. 164-5; Luther 14.7; Martens, 149; Masterman 150 ; Newman 167; Orlandi 177; Patterson 184; Poey 193; Sandman 214; Schneider 218-9; Simpson 226a ; Smith 228-30; Smitt 881 ; Southwell 232 ; Stewart 235; Vogt 265; Weber 268; Williamson 269-270. The limits of this work make short treatment of the present subject necessary, but certain of the papers named above may readily be consulted for supplementary details. In particular, Weber gives a valuable resume" of what was known on the subject up till 1884. Howes adds some cases, and also initiates a very interesting discussion on the general questions involved. Stewart contributes the solitary well-described instance in the Salmonidae, and an interesting example in the mackerel. Masterman adds two cases, and continues the discussion begun by Howes. More recently five other instances have been supplied by Williamson (269-270). Abnormal hermaphro- ditism has now been recorded in examples of the Percidac, Squamipinnes, Sparidae, Scombridac, Mugilidae, Gasterosteidae, Labridae, Gadidae, Plcuronectidac , Cyprinidae, Esocidae, Salmonidae, Clupeidae. The condition seems also to have been recorded in Acipcnser, and among the Sclachii (see Howes 101, p. 543). Classification. Decisive evidence is not easy to obtain, but I believe that, on the whole, abnormal hermaphroditism in Teleosts belongs to the third or synchronous type. The cases recorded by Stewart (235) in the trout and mackerel were marked by the presence at the same time of fully developed ova and sperm. No doubt various other recorded examples have shown apparent protandry, and this circumstance, together with the fact that normally the females are larger than the males, has led some authors to consider abnormal teleostean hermaphroditism as belonging to the successive polycyclic type. It must be noted, however, in the case of many dioecious animals, that during each season motile sperm is present abundantly in the testis of the male, months before the contents of the ovary are fully ripe (e.g. sea-urchin, limpet, and other mollusca). The argument regarding size is hardly relevant to cases in which the sexes are united. Structure. It will be remembered that in the majority of osseous fishes the ovaries are paired, and that the ova when ripe dehisce into their central cavities, which unite posteriorly in a single efferent duct. In the males also, the efferent channels, though frecpiently different in form, are built essentially on the same plan. The Salmonidae, Muraenidae, Cobitis, and some others, are exceptional in that the ova when ripe dehisce into the body cavity, from which they escape by means of oviducal pores. In such fish, however, the males exhibit the typical arrangement as regards their efferent ducts. In hermaphroditic conditions, very great differences in the relations of the two kinds of tissue occur. For example : (1) One or more masses of spermatic tissue may be contained in, or attached to. some part of one or both of the ovaries. In such cases the testicular ducts open into the cavity of the main gland and are thereby placed in communication with the exterior. Or, mutatis mutandis, the converse conditions may occur. The vast majority of the instances may be grouped in this division. (2) The gonad on one side may be entirely male and that on the other entirely female. The gonads unite posteriorly just as two ovaries or two testes do, and their ripe products escape by one genital aperture (cf. Williamson 269). (3) There may be an ovary as well as a testis on one or both sides of the body. In the almost 48 MINOR ABNORMALITIES— HERMAPHRODITISM symmetrical instance recorded by Stewart {235) from the mackerel, the two testes lay dorsally to the ovaries. Judging from the figures, all four glands opened posteriorly into a single efferent duct. (4) In the Salmonidae hermaphroditism appears to be exceedingly rare. Its occurrence is mentioned by Simpson (Todd's Cyclopaedia of Anatomy and Physiology, London 1836-9, II. p. 697), but otherwise the only example I have come across is that described by Stewart (235) from preserved material in the trout. This case merits separate mention, on account of the differ- ences occurring normally between the sexes in the Salmonidae, and also because, on other grounds, it presents features of very great interest. The right genital gland is ovarian in structure. The left, which is the larger, consists chiefly of ovary, but has a middle portion with the structure of a testis. There are two genital ducts, one on either side, leading from the posterior ends of the gonads to the uro-genital chamber. The specimen was therefore regarded as being a fundamentally male fish possessing also partly the female character. The previous history bore out that on two occasions ripe eggs had been extruded from its belly by artificial pressure, and these eggs, although kept completely isolated, developed normal young. At the time of examination several ova were lodged in the right duct close to its termination in the uro-genital chamber. A short distance in front of this point the duct in question was ruptured or defective. There were between two or three dozen loose ova in the body cavity, but these had apparently escaped from a rupture of the right ovary or from the opening in the wall of the right duct alluded to above. Nothing in the description indicates the presence of oviducal pores leading from the body cavity into the uro-genital chamber, and presumably in this case the ova dehisced into the interior of the gland and not into the body cavity. The frequency with which hermaphroditism appears in fishes, normally or abnormally, and the fact that the Ascidia are hermaphrodite, have been taken as evidence that the chordate ancestor was monoecious (Haeckel, Howes, Masterman and others). It does not appear that the conclusion is necessary on either ground, any more than that the alternation of generations found in many widely different animals is due to common ancestry, or that the hermaphroditism of the Cirripedes indicates a primitive monoecious condition in the crustacean stock. I believe that here we have an instance (see p. 5) in which a certain "potentiality" (i.e. hermaphroditism on the one hand or unisexuality on the other) has become fixed as normal in certain forms (e.g. hermaphroditism in Ascidia, Myxine, Serranus, etc. ; unisexuality in Amphioxus and most fishes), while in these forms the converse potentiality still manifests itself in a sporadic and " teratological " manner, through some inherent variation tendency the persistence and strength of which are independent of atavism. This view, while not directly supporting Pilseneer's l position that the hermaphroditic is always derived from the unisexual condition, points to a property inherent in animal life whereby either state may conceivably receive the opportunity of gradually replacing the converse one through the ordinary working of natural selection. II. SHORTENING AND CURVATURE OF THE SNOUT. (Pug-head ; Bulldog-head, etc.) References : Salmonidae. — Carlet 36; Couch £5 ; Day 54. II. p. 102; Lidth de Jeude 114; Lowne 146 p. 55; Malloch 147a; Saint Hilaire 213 I. pp. 283-287; Stoddart 239; Tornier 250; v. Krauss 134- Specimens have been sent to the author by Henry Lamond, Esq. (Salmo fario), and Peter M'Nair, Esq. (Salmo salar). As regards early stages, Girdwoyn 81 fig. 112, illustrates a trout embryo with shortened upper jaw and defective also as regards the eyes. A somewhat similar specimen is figured by de Quatrefages (19S, PL VII. Fig. 2). 1 "Hermaphroditism in Mollusea," Q.J. Microsc. Society, London, 37 (19-46). MINOR ABNORMALITIES— PUG-HEAD 49 Other Fishes: Examples in Cyprinus are furnished by Knauthe {125-6), Lowne (146 p. 55), Otto (17S thirteen instances), St. Hilairo (SIS PI. I. figs. 4-6), Tornier (250); in Abramis vimba by Leon- hardt (140) ; in Esox by Otto {178), and Vrolik (266); in Anrjuilla by Lowne (11+6 p. 55), and Tornier (250); in Lumpenus by Pappenheim (183); in Pe?-ca by Pellegrin (189); in Mugil chelo, M. capito, Gobius ophiocephalus, Labrax lupus, Gadus minutus, Pleuronectes italicus, Merluccius vulgaris, by Ninni (168); in Cottus by Nystrom (174), aQd in Mugil capito by Canestrini (35). Leonhardt (14-0) refers to records in Cyprinus by Neydeck, Steindachner, and Hofer; in Lumpenus lampetrae- for mis, G■; (hemididymus) : 54-55 (various abnormalities). Caudal vein, SO (CI. v.) : .7, .'.,' (CI. vi.) : VI. 26 (s. trans, norm.) : VII. 31 (s. trans. CI. VI.), 32 (triple monster). 64 INDEX TO STRUCTURES AND SUBJECTS Causation, 4 (of double monstrosity): 33 (triple monstrosity) : 44 (of cyclopia) : 4S (of pug-head) : 52-03 (of twisted body) : 55 (of tin abnormalities). Cavity, cerebral lobes, 41-43 (cyclopia): III. 13 (s. trans. norm.) : VIII. 35 (CI. I.) : X. 39-40, XII. 43, 44 (a. horiz. CI. I.): XIII. 47 (s. trans. CI. II.): CFB in XVIII. 69 (normal), 70 (CI. I.), 71 (CI. n.) : XXI. 91-93 (diagrams) : XXV. 102 (cyclopia). Cavity, optic lobes, III. 14-16 (s. trans, norm.): V. 21 (s. horiz. norm.) : VIII. 36, IX. 37 (s. trans. CI. I.) : X. 39 40 (s. horiz. CI. I.) : XV. 52-53 (s. horiz. CI. in.) : COL in XVIII. 69 (normal), 70 (CI. I.), 71 (CI. n.), 72 (CI. I.): XXI. 91-93 (diagrams): CO in XXIV. 106 (parasitism): XXV. 109-110 (semi-cyclopia). Ceratohyal, 11 (normal): 10 (CI. n.): XVI. 54 (s. horiz. CI. in.). Cerebellum, Gl (anomaly of) : IV. 17 (s. trans, norm.) : V. 21 (s. horiz. norm.): XV. 52 (s. horiz. 01. in.): XXVI. Ill (semi-cyclopia). Cerebral lobes, 13 (CI. I.): 41-42 (cyclopia): neural epithelial organ connected with, Gl : III. 13 (s. trans, norm.) : V. 21-22 (s. horiz. norm.) : VIII. 35 (s. trans. CI. i.) : X. 39 (s. horiz. CI. I.) : XIII. 48 (s. trans. CI. II.) : XXV. 107- 108 (cyclopia) : XXVI. 112 (s. trans, atrophic head). Chick. See under birds. Chordate, 43 (ancestry). Choroidal arteries, 12 (normal): 14 (CI. I.) : 41 (cyclopia): AChor in XVIII. 67 (CI. I.), 68 (CI. n.): XXVI. Ill (cyclopia). Choroidal fissure of eye, 14 (CI. I.) : 41-4' (cyclopia): XL 41-42 (s. horiz. CI. I.): XXIV. 104 (cyclopia), 105 (supernumerary eye) : XXV. 110 (semi-cyclopia). Choroidal gland, 12 (normal) : 14 (CI. I.) : 41, 42 (cyclopia) : XXV. 108 (semi-cyclopia). Circulation, 24 (in anakatadidymus) : GO (embryos without). See also heart, etc. Cirripedes, 4S (hermaphroditism). Classification, 3-4, 31-32 (in double monstrosity) : 3S (in triple monstrosity): 40 (in cyclopia): 47 (in herma- phroditism). Coalescence of vertebrae. See under synostosis. Cold, iufluence of, in producing twisted body, 5.'. Coloration, 56. Concrescence, theory, G, 10, 25-2S. Copula visceral arch II. (glossohyal), 11 (normal): 1G (CI. II.): III. 13 (s. trans, norm.): XVI. 54 (CI. III.): OH in XVIII. 64 (CI. i.), 65 (CI. II.). Copula visceral arch III., VIII. 36 (s. trans. CI. I.): XVI. 54 (s. horiz. CI. in.): Cop in XVIII. 64 (CI. I.), 65 (CI. II.). Cranial skeleton, 11 (normal) : 13 (CI. I.) : 15 (CI. II.) : 40,4- (cyclopia) : 44 (semi-cyclopia) : 4S (pug-head) : 49 (round- head). See also XVII., XXI. Cranio-facial defects, 40-44 (cyclopia) : 4S-51 (pug-head, etc.) : 59-GO (parasitism) : 61 (atrophy) : 62 (absence of mouth, etc.). Crustacea, 5S (as food of flatfish). Curvature, 25, 53 (of body and tail in hemididymus) ; 51-53 (do. in hump-back, etc.): 43-50 (of snout in pug-head and round-head). Cuvier, duct of, IS (CI. in.) : 20 (CI. IV.) : XIX. 73 (normal), 75 (CI. in.), 74 (CI. iv.). "Cyclopean" position of eye in ambicoloured flatfish, 57. Cyclopia, 7 (in double monsters) : 40-4$ (classification, struc- ture, comparison, etc.). Cyclopic ray, 2, 43. Cyclops, 1, 43. Defects, 10, GO (in formation of embryonic rudiment) : 25-29 (of inner structures in hemididymus) : 33-34 (in triple mon- ster) : 40-42 (in cyclopia) : 43-51 (cranio-facial) : 52 (of lateral muscles) : 54 (of notochord) : 55, 56, 61 (of fins and fin rays, etc.), 57 (in ambicoloration) : 62 (in gonads): 51, 62 (of mouth, jaws, etc.) : 59, 60 (in parasitism, etc.). Dehiscence, of body-halves (Rauber), 26 : of ova into body- cavity, 47. Development, G, 27 (fishes) : 6, 8, 10 (double monstrosity in fishes and other vertebrates) : 36 (triple monstrosity) : 42-43 (of cyclopic embryos). Diagrams of cranial skeleton illustrating the planes of the sections figured in VIII. -IX. (CI. I.): XIII. -XIV. (CI. n.). Dichotomy or fission, S, 27, 28. Dioecious condition in fishes, etc., 46-43. Distance between centres of embryo-formation on blastoderm, 6, 7, 30, 35, 36. Dog, 37 (triplicity) : SO (pug-head). Dorsal edge membrane, inch dorsal fins, 1.' (normal): 20 (CI. v.) : 21, 22 (CI. vi.) : 34 (triple monster) : 52 (hemi- didymus) : 54, 55 (various abnormalities) : 57 (on head of flatfish) : VI. 24, 25 (s. trans, norm.) : VI. 27-29, VII. 30, 31 (s. trans. CI. vi.) : VII. 32-34 (triple monster). Double eggs, 61. Double monstrosity in fishes, passi?n in Chap. I. pp. 1-32 (see Contents, vii-viii) : 8, 2S (in vertebrates generally). Drops}' of central nervous system and cyclopia, 40. Duct. See under air, bile, Cuvier, Wolffian. Duplicity. See under double monstrosity. Also 25 (imperfect in hemididymus) : 54 (of notochord). Early stages, 6, 9 (of double monstrosity in Salmonidae and Esox) : 35 (of triple monstrosity) : 43 (of cyclopia) : 60 (pathological conditions in). Edentate. See under Tatusia. Edge, apparent dorsal, of head in flat-fishes, 57. See also under membrane. Efferent ducts of gonads in hermaphroditic fish, 47 : do. in Salmonidae, 47. Embryo and embryonic rudiment. See under blastoderm, margin, and rudiment. Entoderm, primary, in mesodidymus, 26, 2S. Environment, and causation of abnormalities, xi, 5, 43, 44, 50, 51, 53, 55, 5S. See also under experimental. Ethmoidal bones in pug-head, 49. Excess, 6-9, 27, 28, 35 (as regards embryonic rudiments, blastoderms, or through fission) : 54 (in abnormalities of fins) : 57 (associated with ambicoloration). See also supernumerary. Experimental work on the production of various abnormalities, xi, 2, 5, 25, 26, 27, 28, SO, 43, 44, 53. See also environ- ment. External factors. See under environment. Eyes, 5, 11: 14 (CI. I.): 26 (hemididymus) : 5C(quadruplicity): 40-45 (cyclopia) : 49 (pug-head) : 50 (round-head) : 56 (in ambicoloration) : 59 (supernumerary) : 60, 61 (absence) : GO, 61 (represented by lens) : III. 13-15 (s. trans, norm.) : V. 21-23 (s. horiz. norm.): VIII. 36, IX. 37 (s. trans. INDEX TO STRUCTURES AND SUBJECTS Cl. 1.) : X. 39-40, XI. 41-42, XII. 43-40 (s. lioriz. CI. I.) : XXIV. KKJ (cyclopia), 105, 100 (supernumerary eyes): XXV. 1(17-1 10 (cyclopia and semi-cyclopia). Eyes, muscles. See under obliquus and rectus. Fins. See under anal, caudal, membrane dorsal and ventral edge, pectoral, pelvic. 54-56, 61 (abnormalities of) : 55, 56, 61 (fin rays). Fishes. See special Index, 68. Fission, S (of embryonic rudiment) : ..'7, 88 (of embryonic axis in hemididymus) : 54 (of single axial structure). Fontanelles, 11 (normal) : tS (Cl. i.) : 16 (Cl. a.): f\,/2,/3 in XVII. 57 (normal), 59 (Cl. i.), 61 (CL u.) : XXI. 89, 92 (diagrams). Fowl. See under birds. Fragmentation of ova, SO. Frequency, 1 (of double monstrosity in fishes and birds) : 33 (of triple monstrosity in fishes and birds) : ,"■, ?8(ot differ- ent types of double monstrosity) : 41 (of cyclopia). Frog larvae, 4 1- Fusion, 4, 6 (primary) : 4, 11, 16, SO (secondary) : 40-44 (of cerebral lobes, mid-brain, eyes, etc., in cyclopia) : 13-89 (of homologous structures at transitional region in double monstrosity, passim). Ganglion, nerve V., 14 (Cl. I.): V. 22 (s. horiz. norm.): XXVI. Ill (semi-cyclopia). Ganglion, nerve VIII., /," (Cl. II.) : IV. 17 (s. trans, norm.) : XIV. 51 (s. trans. Cl. II.). Ganglion, vagus, 17 (Cl. II.): IV. 18 (s. trans, norm.): IX. 38 (s. trans. Cl. I.). Gastrulatiou, 1, 30 (double in Petromyzon) : 6 (centres in double monstrosity). Germ cell, predisposition in, 5, 33. See also under autogenetic. Germinal area, mammalian, ,'.> (pug-head): 50 (round-head) : IV. 17 (s. trans, norm.): V. 23 (s. horiz. norm.) : XI. 41, 42, XII. 45, 46 (s. horiz. Cl. I.): XIII. 48, XIV. 49, 51 (s. trans. Cl. II.): i in XVII. 56 (normal), 58 (Cl. I.), 60 (Cl. n.): XXI. S8, 89 (diagrams): XXVI. Ill (cyclopia). Hypoaria, IS (Cl. I.): 41, 41 (cyclopia): IX. 37 (s. trans. Cl. i.): XI. 41, 42, XII. 45 (s. horiz. Cl. I.): XIV. 50 (s. trans. Cl. u.) : Hyn in XVIII. 72 (Cl. I.). Ifypohyal. See under basihyal. Hypophysis, IS (Cl. (.) : 41, 4 J (cyclopia) : 69 (double) : V. 23 (s. horiz. norm.) : XI. 41, 42, XII. 45, 40 (s. horiz. Cl. i.) : Hyp in XVIII. 72 (Cl. I.). Induced, abnormalities. See under acquired. "Individualities," mixing of, in double monster, 60. Infundibulum, 13 (CI. I.) : III. 10 (s. trans, norm.) : V. 23 (s. horiz. norm.): VIII. 36 (s. trans. Cl. I.): XII. 45, 46 (s. horiz. Cl. i.) : XIV. 49 (s. trans. Cl. II.) : In/ in XVIII. 72 (Cl. I.) : XXV. 109, 110 (semi-cyclopia). Injury, mechanical, 58, 68. See also under experimental. Intestinal canal, 18 (normal): 16 (Cl. 1.): IS (Cl. II.): 19 (Cl. in.) : SO (Cl. iv.) : ,.'/ (Cl. v.) : 83 (Cl. VI.) : 85 (hemi- didymus) : 34 (triple monster) : 68 (hernia) : IV. 20, VI. 24 (s. trans, norm.) : VI. 27, XXVI. 114 (s. trans. Cls. v., vi.): XVI. 54 (s. horiz. Cl. in.): XXVI. 113 (absent notochord) : XXII. 95, XXIII. 97-99 (triple monster). Invertebrates, 5 (ova). Islay, tailless trout from, 56. Iter or Sylvian aqueduct, HI. 16 (s. trans, norm.) : IX. 37 (s. trans. Cl. I.) : XIV. 50 (s. trans. Cl. n.) : XV. 53 (s. horiz. Cl. in.): CIt in XVIII. 69-71, XXV. 110 (semi- cyclopia). Jaws. See under visceral arches. Also 41, 4~ (cyclopia) : 40 (pug-head) : 60 (round-head) : 51 (lower absent) : 50 (accessory) : 68 (gill tufts instead of teeth) : 68 (mouth absent): VIII. 35 (s. trans. Cl. (.): XVI. 54 (s. lioriz. Cl. in.): XXV. 107, 108 (cyclopia). Jugular vein, 18 (CI. n.): 10 (Cl. Hi.): IV. 18 (s. trans. norm.): IX. 3S (s. trans. Cl. I.): XIV. 51 (s. trans. Cl. ii.): VI in XIX. 73 (normal), 74 (Cl. m.), 75 (Cl. iv.). Katadidymus, .>, 5, 6, 3, .'.7. See also under hemididymus. Kidney, 68 (variations in). See also head-kidney, Wolffian body. Kirkcudbright, tailless trout from, 56. Kupll'cr, two vesicles of, ."'. 66 INDEX TO STRUCTURES AND SUBJECTS Labyrinth, V. 22 (s. horiz. norm.): XII. 43-44 (s. horiz. CI. I.): XIV. 51 (a. trans. CI. II.): XXVI. Ill (semi- cyclopia). See also under auditory and periotic. Lacerta, S (double blastoderm) : 29 (double primitive streak) : 37 (trifid tail). Laminae, parachordal, 11 (normal) : 13 (CI. I.). Lateral line, 57 (curvature of, in ambicoloured flat fish) : 62 (interrupted) : 62 (supernumerary). Lateral muscles, 25, 52 (hemididymus). See also under muscle plates. Lateral union, S, 6, 1 .'. Lens, lb (CI. I.): 41-44 (cyclopia and semi-cyclopia): lib (ectoderm for) : 60, 61 (without other eye structures) : III. 14 (s. trans norm.) : V. 22 (s. horiz. norm.) : VIII. 36, IX. 37 (s. trans. CI. i.) : X. 40, XII. 44 (s. horiz. CL I.): L in XXIV. 106 (parasitism): XXV. 107, 110 (cyclopia and semi-cyclopia) : XXVI. 112 (s. trans, atrophic head). Light, influence of exposure to, 5S. Limpet, 47. Lip, lower. See under jaw. Also XIII. 47-48 (s. trans. CI. n.). Literature, general index of, xi-xvii. See also under the various chapters and sections on particular malforma- tions. Lithium chloride, 44- Liver, 19 (CI. HI.): 20 (CI. IV.) : 34 (triple monster): 62 (in reversed flatfish) : XII. 46 (s. horiz. CI. I.) : XXLI. 95, XXIII. 97-98 (triple monster) : XXVI. 1 14 (s. trans. CI. v.). Lizard. See under Lacerta. Longitudinal or parallel union, 4, 29, 30. Magnesium chloride, fS, 44- Maltham Tarn, trout from, 61. Mammal, 5, S (double and multiple forms) : 35, 36, 37 (triplicity) : 39, 4J (cyclopia) : 51 (facial defect). Man, 37, 38, 39 (triplicity). Margin, 6 (of blastoderm in relation to embryo formation and double monstrosity) : 26 (do. in mesodidymus) : 36 (do. in triple monstrosity) : 60 (in pathological conditions). Meckel's cartilage. See under visceral arch I. Medulla oblongata, 3, 4, 17 (union at) : IV. 18 (s. trans, norm.): V. 21-22 (s. horiz. norm.): IX. 38 (s. trans. CI. i.) : X. 39-40, XII. 43-44 (s. horiz. CI. I.) : XIV. 51 (s. trans. CI. n.) : XXIII. 97-98 (triple monster) : XXVI. Ill (semi-cyclopia). Membrane. See under dorsal edge, ventral edge. 25, 26 (intermediate, in hemididymus) : 23 (preanal edge, in anakatadidymus). Meroblastic ova, xi, 5. Mesentery, 20, 21, 22 (double in Cls. iv., v., vi.) : VI. 28-29 (s. trans. CI. via.). Mesial muscular mass. See under muscle segments. Mesodidymu3, 1, 3, 6, 10, 25, 55, 60. See also under hemididy- mus. Mesonephros. See under Wolffian body. Metamorphosis, and coloration in pleuronectids, 58, 59. Migration of eye in flat-fishes, 58. Minor abnormalities, 46-62. Mode of origin, of double and multiple monstrosity, xi, 6-S : of hemididymus, 27 : of triple monstrosity, 36. See also under cyclopia, etc., etc. Mollusca, 47-48 (hermaphroditism, etc.). Monocyclic hermaphroditism, 47. Monoecious condition, whether ancestral, 4S. Mouth, 15 (CI. I.): 16 (CI. II.) : 41-42 (cyclopia): 44 (semi- cyclopia) : 49 (pug-head) : 50 (round-head) : 51 (defective) : 61, 62 (absence of mouth, etc.) : III. 15 (s. trans norm.): XIV. 49, 51 (s. trans. CI. n.) : XXII. 95 (triple monster). Mucous groove or canal, III. 13-15 (s. trans, norm.) : V. 21-22 (s. horiz. norm.): VIII. 36 (s. trans. CI. I.): XIII. 47 (s. trans. CI. II.): XXV. 108 (cyclopia). Miillerian duct. See oviduct. Multiple forms, xi, 5, 8. See also triple monstrosity. Muscle. See under lateral, obliquus oc, rectus oc. , muscular raphe, muscle plates. Muscle plates or segments, 11 (at transition region): 12 (normal number): 15 (CI. I.): 17 (CI. II.): 19 (CI. in.): 25-27 (hemididymus) : 29 (parallel union) : 52 (absence) : VI. 26 (s. trans, norm.): V. 21-23 (s. horiz. norm.) : VI. 27, VII. 31, XXVI. 114 (s. trans. Cls. v., vi.) : VII. 32-34 (triple monster) : IX. 38 (s. trans. CI. I. ) : X. 39-40, XI. 41, XII. 43-44 (s. horiz. CI. I.): XV. 52-53, XVI. 54-55 (a. horiz. CI. in.) : XXIII. 97-98 (triple monster). Muscular raphe, 54, XXVI. 113 (s. trans, absence of noto- chord). Mythology and teratology, 1. Nasal cartilage. See under olfactory. Nerve II., 13-14 (CI. I.): 41, 42, 44 (cyclopia): 59 (super- numerary eye): III. 15 (a. trans, norm.): VIII. 35 (s. trans. CI. I.) : XI. 41-42, XII. 46 (s. horiz. CI. I.) : XXV. 108 (cyclopia) : XXV. 110 (semi-cyclopia). Nerve III., 13 (CI. I.) : III. 16 (a. trana. norm.). Nerves of pelvic plexus, 62 (variationa). Nerves, sensory, and growth, 15. Neural arch cartilage, 12 (normal) : 13 (CI. I.) : 17 (CI. II.) : 19 (CI. in.): 52 (twisted body, etc.): 54 (absent noto- chord) : IV. 19 (s. trans, norm. ) : X. 39-40 (a. horiz. CI. i.) : XV. 52, XVI. 55 (a. horiz. CI. iil) : Na in XVII. 62 (normal), 63 (duplicity) : XXVI. 113 (a. trana. absence of notochord). Nile, two-headed shark (?) from, 2. Notch, 52 (on dorsal edge of body) : 57 (in dorsal fin of flat- fishes) : 62 (on ventral edge of do.). Notochord, 11 (normal) : 13 (CI. I.) : 17 (CI. II.): 18 (CI. III.): 19 (CI. iv.): 20 (CI. v.): 21-22 (CI. VI.): 25 (hemididy- mus) : 29 (parallel union) : 34-36 (triple mon8ter) : 54 (absent) : 54 (double) : IV. 17-19, VI. 24-26 (s. trans, norm.): V. 22 (a. horiz. norm.): VI. 27-29 (s. trans. CI. via.): VII. 30 (a. trana. CI. vi b.), 31 (a. trans. CI. via.): XXVI. 114 (s. trans. CI. IV.): VII. 32-34 (triple monster): IX. 38 (s. trans. CI. I.): X. 40, XI. 41-42, XII. 44-46 (e. horiz. CI. I.) : XIV. 51 (s. trana. CI. II.) : XV. 53, XVI. 54-55 (a. horiz. CI. in.) : XXVI. 113 (abaence of) : XXIII. 97, 98-99 (triple monster) : XXI. 89-90 (diagrams) : n in XVII. 56 (normal), 58 (CI. I. ), 60 (CI. II.): nch in XVII. 62 (normal), 63 (duplicity): IV. 20 (a. trans., reduplication). Obliquus oc, inferior, 14 (CI. I.): 41-4- (cyclopia, etc.) : V. 23 (s. horiz. norm.) : VIII. 35 (s. trana. CI. i.) : XII. 46 (a. horiz. CI. I.) : XIII. 47-48 (s. trans. CI. n.). Obliquus oc, superior, 14 (CI. I.): 41-42 (cyclopia, etc.): 62 (abnormality of): III. 13 (s. trans, norm.): V. 22 (a. horiz. norm.): VIII. 35 (a. trana. CI. I.): X. 39-40 (a. horiz. CI. i.). INDEX TO STRUCTURES AND SUBJECTS 07 Oesophagus, 16 (CI. II.) : 19 (CI. III.) : 34 (triple monster) : 63 (hernia into) : IV. 19 (s. trans, norm.) : XII. 45 (s. horiz. CI. I.) : XXIII. 97 (triple monster). Olfactory capsular cartilage, 11 (normal): IS (CI. I.): 34 (triple monster): 40, 41, 4-.' (cyclopia): V. 22-'23 (s. horiz. norm.): XI. 41-42, XII. 45-46 (s. horiz. CI. I.) : XIII. 47-48 (s. trans. CI. n. ) : XXI. 88-89 (diagrams) : C in XXIV. 101 (cyclopia). Olfactory nerve, 41-44 (cyclopia): XIII. 47 (s. trans. CI. II.). Olfactory pit or organ, 49 (pug-head) : XII. 45 (s. horiz. CI. I.): XIII. 47-48 (s. trans. CI. n.) : XXIV. 100-101 (cyclopia). Optic chiasma, V. 23 (s. horiz. norm.). Optic lobes, 3 (union) : IS (CI. X.) : 41-42 (cyclopia) : 59 (super- numerary eye): III. 14-16 (s. trans, norm.): V. 21 (s. horiz. norm.): VIII. 35, 36, IX. 37 (s. trans. CI. I.) : X. 39-40, XII. 43-44 (s. horiz. CI. i.): XIII. 48, XIV. 49-50 (s. trans. CI. n.): XV. 52 53 (s. horiz. CI. in.): XXV. 109-110 (semi-cyclopia) : 01 in XXIV. 103, 106 (cyclopia). Optic nerve. See under nerve II. Optic vesicles, 6 (in young double monsters): $6 (hcmididy- mus): 29 (parallel union) : 43-44 (cyclopia) : 59, 60 (super- numerary eye). Osmotic pressure, 5, 27, 30, 44, SO- Otocyst, 26 (mesodidymus) : 34, XXIII. 97 (triple monster). See also under auditory. Ovaries, in hermaphroditism, 47. See also gonads. Overgrowth of blastoderm over yolk, 6, 9, 27-29. Oviducal, 40 (gland) : 41 (slits). Oviduct, 46 (in male skate): ^T(in hermaphroditic conditions): 62 (abnormal). Paired fins, possible origin of, 54. See also pectoral, pelvic. Palato-pterygoid, or palato-quadrate cartilage, 11 (normal) : 13 (CI. I.) : 16 (CI. II.) : 40-41 (cyclopia) : 49 (pug-head) : 59 (supernumerary eye): III. 13-16 (s. trans, norm.): VIII. -IX. 35-37 (s. trans. CI. I.): XIII. 47-48, XIV. 49-50 (s. trans. CI. II.): XXVI. Ill (cyclopia): XXV. 108 (cyclopia) : XXI. 88-89 (diagrams) : /and/' in XVII. 56 (normal), 58 (CI. I.), 60 (CI. II.): PI in XXIV. 105 (cyclopia). Pancreatic tissue, IV. 20 (s. trans, norm.): XVI. 54 (s. horiz. CI. III.). Parachordal cartilage, 11 (normal) : 13 (CI. I.) : 16 (CI. II.) : 53 (doubled notochord) : IV. 18 (s. trans, norm.) : V. 22 (s. horiz. norm.): IX. 38 (s. trans. CI. I.): XI. 41-42, XII. 44-46 (s. horiz. CI. I.) : XIV. 51 (s. trans. CI. II.) : XXI. 88-89 (diagrams): C in XVII. 56 (normal), 58 (CI. I.), 60 (CI. ii.). Parasitic forms, 3, 4 (classification) : 54 (as accessory fins) : 59, 60 (certain examples of). Parasphenoid in pug-head and round-head, 49, 50. Pathological conditions of ovum or early embryo, 60. Pectoral fin, 4, IS (CI. in.) : 4, 19 (CI. iv.) : 64 (supernumer- ary): 55 (notched, fused with skin): 56 (in "tailless trout"): 61 (defective): IV. 19 (s. trans, norm.): XVI. 54 (s. horiz. CI. in.): XXIII. 97-98 (triple monster). Pectoral girdle, 11 (normal): /;/ (CI. in.): 19 (CI. tv.) : IV. 18 (s. trans, norm.): IX. 38 (s. trans. CI. i.). See also XVIII. 06. Pelvic fin, 19 (CI. IV.): 20 (CI. v.): 21-22 (CI. vi.): Si (triple monster) : 52 (displaced) : 54 (supernumerary) : 55, CI (absence or defect): 56 (in "tailless trout"): VI. 25 (s. trans, norm.): VI. 28 (s. trans. CI. vi.): XXIII. 97-98 (triple monster). Periotic cartilage, /; (normal): 13 (CI. I.): 16 (CI. n.): 34 (triple monster): IV. 17 (s. trans, norm.): V. 21-23 (a. horiz. norm.): IX. 38 (a. trans. CI. I.): X. 39, 40, XI. 41, 42, XII. 43, 44 (a. horiz. CI. I.) : XIV. 49, 50, 51 (s. trans. CI. n.) : XV. 52, 53 (s. horiz. CI. in.) : XXV. 110, XXVI. Ill (semi-cyclopia) : XXI. 88-92 (diagrams) : k in XVII. 56 (norm.) : 58 (CI. I.), 60 (CI. n.). Peritoneal lining, pigmentation of, 57. Pharynx, 15 (CI. I.): /',' (CI. II.): IV. 17 (s. trans, norm.): IX. 38 (s. trans. CI. I.). Pigment layer of retina, 4- (cyclopia) : 59 (supernumerary eye): XXIV. 104 (cyclopia): XXV. 109 (semi-cyclopia). Pigmentation, abnormalities of, 56-5S. Pineal body, or sac or diverticulum, 13 (CI. I.): 41, 42 (cyclopia): III. 13 (s. trans, norm.): VIII. 35 (s. trans. CI. i.): XIII. 48 (s. trans. CI. n.): XV. 53 (s. horiz. CI. in.) : Pin in XXIV. 102 (cyclopia) : XXV. 107, 108 (cyclopia). Pituitary space, 11 (normal) : /.; (CI. I.) : 41 (cyclopia) : IX. 37 (s. trans. CI. I.) : XXI. 88, 90 (diagrams) : m in XVII. 56 (normal), 58 (CI. I.), 60 (CI. n.). Planes of certain sections. See under diagrams. Pneumatic ducts. See under air-ducts. Polychaeta, 68 (as food of flatfish). Polycylic hermaphroditism, 47. Polyembryony. See under multiple forms. Pons, 14 (CI. i.) : V. 22 (s. horiz. norm.). Potentiality in teratology, xi, 5, 2S, 4S. Predisposition. See under germ. Preopercular in ambicoloured flatfish, 57. Pressure, 44- See also under osmotic and experimental. Primarily formed portion of body, 27. Primary fusion. See under fusion. Primitive streak, 6. Proboscis, in cyclopia, 40, 43- Pronephros. See under head-kidney. Protandry in hermaphroditism, 4"i- Pseudobranch and vessels, 12 (normal) : 14 (CI. 1) : 17 (CI. II. ) : 41 (cyclopia). Pug-head, f8. Quadruplicity, 36, 38. Radiation, posterior (Rauber), 2S. Records, of double monstrosity in osseous fishes 1, in carti- laginous fishes 30-31, and in other fishes SO ; of survival of monstrosities 2, 43. See also sections dealing with parti- cular abnormalities. Rectum, VI. 25, 26 (s. trans, norm. ) : VI. 28, 29 (s. trans. CI. via.). See also under vent. Rectus abdominis, .:', (in anakatadidymus). Rectus oculi, externus, 11 (normal) : 14 (CI. 1.) : 17 (CI. II.) : 41, 42 (cyclopia) : V. 22, 23 (s. horiz. norm.) : XI. 41, 42, XII. 45, 46 (s. horiz. CI. I.) : XIII. 48 (s. trans. CI. II.) : XXI. 88, 89 (diagrams) : XXVI. Ill (semi-cyclopia). Rectus oculi, inferior, 14 (CI. I.) : 41, 42 (cyclopia) : III. 15, 16 (s. trans, norm.) : VIII. IX. 35, 36, IX. 37 (s. trans. CI. 1.) : XIII. 47, XIV. 49, 50 (s. trans. CI. 11.): XXV. 108 (cyclopia) : XXV. 110 (semi-cyclopia). 68 INDEX TO STRUCTURES AND SUBJECTS Rectus oculi, interims, 14 (CI. 1.) : 4U 4$ (cyclopia) : III. 13- 16 (s. trans, norm.) : V. 23 (s. horiz. norm.) : VIII. 35-36 (s. trans. CI. 1.): XI. 41, 42, XII. 44 (s. horiz. CI. 1.): XIII. 48, XIV. 49 (s. trans. CI. 11.). Rectus oculi, superior, 14 (CI. I.): 41-4 ■• (cyclopia) : III. 14- 16 (s. trans, norm.): VIII. 35-37 (s. trans. CI. I.): X. 39-40, XI. 41-42, XII. 45-46 (s. horiz. CI. 1.) : XIII. 47-48, XIV. 49-50 (s. horiz. CI. n.) : XXV. 107-108 (cyclopia) : XXV. 109-110 (seini-cyclopia). Reduplication, local, of notochord, 53 : IV. 20. Reptiles, 8, 89, 37 (double and multiple forms). See also Lacerta, snake, Tropidonotus. Retina, 14 (CI. I. ): 41-4~ (cyclopia): 59 (supernumerary eye): III. 14-15 (s. trans, norm.): V. 21-23 (s. horiz. norm.): VIII. 36, IX. 37 (s. trans. CI. 1.) : X. 39, 40, XI. 41, 42, XII. 43 46 (s. horiz. CI. I.) : XXIV. 104 (cyclopia) : XXV. 107-108 (cyclopia) : XXV. 109-110 (semi-cyclopia). Reversed flatfish, 57, 58, 6?. Reversion, in descendants of pug-headed fish, 50. See also atavism. Rhythm, of respiration and circulation in anakatadidymus, 84. Round-head, 49-50. Rudiment, embryonic, 6, 10 (in double monstrosity) : 87 (in mesodidymus) : 29 (in parallel union) : 10, GO (pathologi- cal conditions). Saccule, 34 (triple monster) : IV. 17 (s. trans, norm.) : V. 23 (s. horiz. norm.). Scales aud tubercles on under side in ambicoloured flatfish, 57 : increased number, '..'. Sea urchin, 47 (apparent protandry in). Secondary fusion. See under fusion. Segmentation, syncytial in Salmonidae, 5 ; more than one centre of, S, 37. Segments, normal number, etc, /?. See also under muscle plates. Semicircular canal, 17 (CI. 11.): 84 (triple monster): IV. 17 (s. trans, norm.): V. 21 (s. horiz. norm.): X. 39-40 (s. horiz. CI. 1.): XIV. 49-51 (s. trans. CI. 11.): XV. 52-53 (s. horiz. CI. 111.): ASc, PSc in XX. 82 (CI. n.): XXV. 110 (semi-cyclopia). Semi-cyclopia, 24 (in anakatadidymus) : 4'i (general account). See also 51. Sheep, 8, 37, 38, 39 (two germinal areas). Shortening, 4S (of snout) : of body or tail, see under hump- back : 56, 57 (of attachment of dorsal fin). Sinus venosus, 19 (CI. m.): SO (CI. rv.): IX. 38 (s. trans. CI. 1.): SFinXIX. 73 (normal), 75 (CI. IV.). Skeleton, cranial. See under cranial, neural and haemal arch, notochord, etc. Snake, 37 (three headed?). Snout, 40-43 (in cyclopia): 49-51 (in pug-head, etc.): ft? (bifid). Specific action of salts in the production of cyclopia, etc., 48. Spinal cord, U (CI. I.) : 17 (CI. 11.): «(Cl.m.): 19 (CI. iv.): 20 (CI. v.) : 21 (CI. VI.) : 25 (hcmididymus) : 29 (parallel union): 34 (triple monster): IV. 19-20, VI. 24-26 (s. trans, norm.): V. 21 (s. horiz. norm.): VI. 27-29, VII. 30-31, XXVI. 114 (s. trans. Cls. iv.-vi.): VII. 32-34 (triple monster): X. 39-40, XI. 41, XII. 43-46 (s. horiz. CI. 1.): XV. 53, XVI. 55 (s. horiz. CI. in.): Spc in XVIII. 69 (normal), 70 (CI. I.), 71 (CI. 11.): XXIII. 97-99 (triple monster) : XXVI. 113 (s. trans, absence of notochord). Spines. See under tubercles. Spiuous processes of vertebrae in twisted body, 52 : absence of, in a vertebra, 52. Spurious hermaphroditism, 4G. Stimulation, electrical, producing hemididymus, 20. See also under experimental. Stomach, 19 (CI. in.) : 34 (triple monster) : 62 (hernia) : XII. 46 (s. horiz, CI. 1.) : XXII. 95, XXIII. 98 (triple monster). See also under intestinal canal. Structure of monstrosities. See under the various types described. Supernumerary, barbels, 61 : eyes, 59 : fins, 54 : lateral line, 62 : ovary, 6! : scales, 62. Supraorbital cartilage or bar, 11 (normal): 13 (CI. 1.): 16 (CI. 11.): 41-42 (cyclopia): III. 13-16 (s. trans, norm.): V. 21 (s. horiz. norm.): VIII. 35-36, IX. 37 (s. trans. CI. 1.): X. 39-40, XII. 43-44 (s. horiz. CI. 1.): XIII. 47-48, XIV. 49-50 (s. trans. CI. 11.): s and s in XVII. 57 (normal), 59 (CI. 1.), 61 (CI. 11.) : XXI. 90-91 (diagrams) : XXV. 109-110 (semi-cyclopia). Supra-regeneration, 36, 37, 55, 01. Survival of monstrous fishes, 2, 43. Swim-bladder. See under air-sac. Sylvian aqueduct. See under Iter. Symbolism and teratology, 1. Synchronous hermaphroditism, 47. Synostosis, of vertebrae, 51. Tables of classification in double monstrosity, 3, 4 : in triple monstrosity, 3S. See also under classification. Tail bud in mesodidymus, 26. Tail, 86 (in Tetromyzon, trifid) : 37 (in Pelobates larva, do.) : 37 (in lizards, do.): 3, 4, 20-23 (union by): SI, 5$ (abnormalities). See caudal. Tatusia, xi, 5, S, 3S (polyembryouy in). Teeth, 46 (blunted in male ray) : 62 (abnormal). Tegminal cartilage, 11 (normal): VIII. 35 (s. trans. CI. 1.) : X. 39 (s. horiz. CI. 1.) : A' in XVII. 57 (normal) : XXI. 90-92 (diagrams) : XXV. 107-10S (cyclopia) : XXVI. 112 (a. trans, atrophic head). Telescopic eyes of goldfish, 62. Testes in hermaphroditism, 4~- Trabeculae cranii, 11 (normal) : 13 (CI. 1.) : 16 (CI. II.) : 40-43 (cyclopia) : 44 (semi-cyclopia) : 59 (supernumerary eye) : III. 13-16 (s. trans, norm.): VIII. -IX. 35-37 (s. trans. CI. 1.): XI. 41-42 (s. horiz. CI. I.): XIII. 47-48, XIV. 49-50 (s. trans. CI. n.) : 0 to d in XVII. 56 (normal), 58 (CI. 1.), 60 (CI. 11.) : XXI. 88, 91 (diagrams) : XXV. 108, XXVI. Ill (semi-cyclopia). Transition, region of, from the double to single condition. See under the various classes of anadidymus. Transmission, of abnormalities, see under hereditary. Tripartite division of embryonic axis, 36, 39. Triple monstrosity, S (general) : 33 (description of specimen) : 35 (other instances in fishes) : 36 (mode of origin in fishes and other vertebrates) : 3S (classification) : VII. 32-34 (s. trans, tail). See also XXII., XXIII. 94-99. Trisomi, 38. Tropidonotus, S, 37 (three centres of segmentation). Truncus arteriosus, 14 (CI. I.) : 17 (CI. 11.) : IV. 17 (s. trans, norm.): XIV. 51 (s. trans. CI. II.): XVI. 54 (s. hoiiz. CI. in.) : TA in XVIII. 68 (CI. 11.). Tubercle, posterior, in mesodidymus, 85, .'>'. INDEX TO STRUCTURES AND SUBJECTS G9 Tubercles, spines, and rough scales present on underside of ambicoloured flatfish, 58. Twinning, xi, 5, 7, SO, 37. Twisting of body or tail. See under humpback. Types of double monstrosity, 1, 3, ~>, 8, S8 : of anomalous development in eggs of pike, 10. Unioval, duplicity, xi, 7, S : triplicity, 33-5.°. Urinary bladder or opening, /.' (normal) : 21 (CI. v.) : 22-23 (CI. vi.): 23 (anakatadidymua) : 34, 48 (hennaphroditio trout) : Bl and BV in XIX. 77-81 : XXII. 94, 96 (triple monster). Utricle, 17 (CI. n.): 3/, (triple monster): IV. 17 (s. trans. norm.): V. 21 (s. horiz. norm.): XIV. 50 (s. trans. CI. II.): Utr in XX. 82 (CI. II.). Valve, spiral, variation, G2. Vein. See under cardinal, caudal, jugular, yolk-sac. Vent, 20-23 (Cls. v.-vr.): SS (anakatadidymua) : 34 (triple monster): XIX. 81 (CI. vi.b): XXII. 94-95 (triple monster). Ventral edge membrane (inch anal tin), 1'J (normal): 20 (C\. v.) .'/-.'.'(CI. vi.): .'3 (auakatadidymus): 34 (triple monster) 54 (doubling in goldfish) : SS (absence or malformation) VI. 2G (a. trana. norm.): VI. 29, VII. 30, 31 (s. trans. CI. vi.) : VII. 32-34 (triple monster). Ventral union, 3, 7, 22, 23. Ventricle III., 41-42 (cyclopia) : III. 14-15 (a. trans, norm.) : V. 21-22 (s. horiz. norm.) : X. 39-40, XII. 43-44 (a. horiz. CI. i.): C3rd Tin XVIII. 69-71 (normal, CI. i., CI. n.): XXI. 91-93 (diagrama): XXIV. 106 (parasitism) : XXV. 107-108 (cyclopia). Ventricle IV. See also medulla. IV. 17-18 (s. trans, norm.) : V. 21 (s. horiz. norm.): X. 39, XII. 43-44 (s. horiz. CI. I.) : XV. 52-53 (s. horiz. CI. in.) : C4th V in XVIII. 69-71: XXI. 90, 93 (diagrams): XXVI. Ill (semi- cyclopia). Ventricle of heart, 10-20 (Cls. m.-iv.): IV. 18 (s. trans. norm.): XVI. 54 (a. horiz. CI. in.): V in XIX. 73 (normal), 74 (CI. in.), 75 (CI. n.). Vertebral column, in twisted body, etc., 51-53. See also notochord, etc. Vertebrate, 5 (ova) : S, 2S, 31, 88 (double and multiple mon- strosity). Visceral arch I. (incl. Meckel's cartilage), 11 (normal) : IS (CI. i.): 15 (CI. II.): 41-42 (cyclopia): III. 13-16 (a. trans, norm.) : VIII.-IX. 35-37 (a. trans. CI. I.) : XIII. 47-48, XIV. 49-50 (a. trana. CI. n.): XVI. 54 (s. horiz. CI. in.): g and in XIX. 76-81, XX. 83-87: XXII. 96, XXIII. 98 (triple monster). Yolk, IV. 19, 20, VI. 24 (s. trans, norm.): VI. 27 (s. trans. CI. VI.): IX. 38 (a. trana. CI. I.) : XVI. 54 (a. trans. CI. III.): XXIII. 97-99 (triple monster): XXVI. 114 (a. trans. CI. v.). Yolk-sac, 2 (exhaustion of yolk) : 3-4 (union by yolk-sac) : 6-7 (overgrowth of blastoderm) : S (more than one blasto- derm) : 23-24 (in anakatadidymus) : 25 (in hemididymus) : 50 (yolk and osmosis) : 67 (doublo-yolked egga). Zig-zag fission, 27 (supposed in katadidymus). INDEX OF FISHES. Abramis blicca, 55. Abramis I'imba, 49. Acanthias, 31, 51, 54, 55, 61, 62. See also under Selachii and Selachoidei. Acipenser, 47, 62. Alopias, 61. Amciurus natalis, 55, 61. Amia, 52. Amphioxus, 48 (unisexuality). Anarrichas, 1, 2, 30. Angler-fish, 61. Amjuilla, 49, 51, 61. Bleak, 50. See also Abramis and Cypri- nus blicca. Blennius, 1, 30. Bony fishes. See under Osseous fishes. Brill, 58. See Rhombus laevis. Carcharias, 62. Carp. See under Cyprinus. Cartilaginous fishes (Selachii), 1, 2, 5, 30, 31, 47. Catfish. See under Anarrichas. Cenlrolophus, 62. Clupeidae, 47. See Herring, Pilchard. Cobitis, 47, 52. Cod, 51, 52, 53. See Oadus morrhva. Conger, 62. Cottus, 49. Crenilabrus, 52. Cyprinidae, 47, 61. Cyprinus, 49, 50, 51, 55, 61, 62. See also Goldfish. Cyprinus aphya ? 50. Cyprinus blicca, 1, 30. Dipnoan. See under Fishes. Dogfish, 2, 31, 51, 62. See Scyllium, Acanthias. Eel. See under Anguilla. Esocidae, 47. Esox lucius, 1, 9, 10, 35, 36, 49, 52, 55. See Pike. Fishes (general), 1, 8, 33 (double and triple monstrosity) : 46-48 (herma- phroditism) : 56 (coloration). See also Cartilaginous fishes, Flatfish, Lung-fishes, Osseous fishes, Shark- like fishes. Flatfish, 46, 56, 62. See also Pleuronec- tidae. Flounder. See Pleuronectes Jlesus. Fundulus heteroclilus, 43. Oadidae, 47. Oadus aeglefinus. See Haddock. Oadus merlangus, 49. Oadus minutus, 49. Oadtis morrhva, 49. See Cod. Gasterosteidae, 47. Oasterosteus, 61. Oirardinus, 1, 30. Oobius ophiocephalus, 49. Goldfish, 50, 52, 54, 55, 61. Gurnard, 51. See Trigla. Oymnotus, 52. Haddock, 50, 62. Hag. See Myxine. Halibut. See Hippoglossus. Herring, 52, 53. Hippoglossus, 57. Labrax lupus, 49, 51. Labridae, 47. Lamprey, 1, 5, 30, 62. See Pelromyzon. Lepidosiren, 54. Leucaspius delineatus, 50. Leuciscus, 1, 5, 30, 52, 61. Lophius. See Angler-fish. Lord fish, 51, 53. Lumpenus, 49. Lung -fishes, 54, 55. Mackerel, 1, 47, 48. See Scomber. Merluccius vrdgaris, 49. Mormyrus, 52. Motella, 55. Mugilidae, 47. Mugil capito, 49, 51. Mugil chelo, 49, 51. Mullus, 52. Muraenidae, 47. Myliobatis aquila, 43. Myliobatis noctula, 2, 43, 55. See Ray. Myxine, 47, 48, 62. Osseous fishes, 2, 5, 30, 46, 47. See under Fishes. Perca, 1, 49, 52, 61. Percidae, 47. Petromyzon, 1, 30, 36, 37. See also under Lamprey. Phoxinus, 49, 52, 61. Pike, 1, 4, 10, 26, 29, 54, 60. See Esox. Pilchard, 62. Plagusia, 57. Plaice, 55, 57, 58, 62. See Pleuronectes platessa. Pleuronectes cynoglossus, 56. Pleuronectes flesus, 56, 57. Pleuronectes italicus, 49, 57. Pleuronectes limanda, 56, 57. Pleuronectes microcephalus, 56. Pleuronectes oblongus, 56. Pleuronectes platessa, 54, 56, 57. See also under Plaice. Pleuronectidae, 47, 55, 58. See also Flatfish. Polypterus, 52. Proteracanthus, 61. Protopterus, 54. Raia alba, 5. Raia balis, 54. Raia clavata, 5, 46, 54, 55, 62. See under Skate. Ray (Myliobatis), 2. Rhinoptera, 62. Rhombus laevis, 56, 57, 58. See Brill. Rhombus maximm, 56, 57. See Turbot. "Rinkin " goldfish, 54. Salmon, 2, 10, 38, 49, 60. Salmonidae, 1, 5, 8, 9, 29, 36, 47, 48, 50, 51, 52, 56, 60. INDEX OF FISHES 71 Salmo fario, 2, 9, 25, 26, 35, 36, 48, 54. See Trout. Salmo kideut, 9, 29, 49, 54. Salmo lacustru, 2. Salmo salar, 9, 48. See Salmon. Salmo salvelinvA, 25, 35. SaJmo trutta, 52. Sclerognathux, 62. Scomber, 1, 30, 52. See Mackerel. Stombridac, 47. Scyllium, 61. See Dogfish. Sea-bream, 62. Sea-trout. See Salmo trutta. Sebastes, 55. Selachii. See under Cartilaginous fishes. Selachoidei, 1, 2, 31. Serranun, 48. Shark (Porbeagle), Lamna cornubica, 61. Shark-like fishes. See under Selachoidei. Siluriis glanis, 54, 61. Skate, 32, 46. See under Rata clavala. Solea vulgaris, 51, 52, 56, 57. Sparidae, 47. Sparua ctntrodontus. See Sea-bream. Squalius cavedanus, 61. Squatus, 61. Squatus acanthias, 61. See under Acan- Ihias. Sqiiamipinnes, 47. Sturgeon, 62. See Acipenser. Telcoslei. See under Osseous fishes. Tinea, 52. Torpedo, 1, 29, 31, 61. Trujla gnrnardus, 52. Trout, 2, 7, 9, 10, 11, 30, 33, 40, 46, 47, 48, 53, 54, 55, 59, 61. See Salmo fario. Turbot, 57. See Rhombus maximus. Zoarccs, 52. INDEX OF AUTHORS. Ahlfeld, 59. Albreeht, 54. Aldrovaudi, 1, 2, 31, 37. Allis, 62. Ambrosini, 2. Assheton, 8, 38. d'Audeville, 1. v. Baer, 1,2,5, 30, 31. Barbieri, 1, 2, 9, 29. Barclay, 31. Barfurth, 36, 37. Barrington, 51. Bataillon, 1, 5, 30. Bateson, 54, 56, 57, 58, 61, 2. Bellotti, 39, 56. Berg, 56. Bird, 56. Bolan, 56. Borcea, 62. Boulcnger, 2, 54, 55. Brandt, 57. Breitschneider, 37. Brindley, 55. Brookes, 31. Bruch, 37, 39. Bruyant, 62. Buckland, 1, 2, 30. Bugnion, 1. Canestrini, 49. Carlet, 48, 49. Cleland, 28. Cobbold, 51, 52. Coggi, 61. Coolidge, 1. Cori, 61. Coste, 1, 2, 24. Couch, 48, 51, 55. Cox, 55, 61. Cuenot, 57. Cunningham, 56, 57, 58, 62. Dareste, 1, 5, 8, 33, 37, 38. Day, 46, 48, 51, 53, 54, 55, 61, 62. Dean, 56. Denton, 1. Dohrn, 1, 29, 31. Donnadieu, 51, 62. Duncker, 54, 55. Dyce, 51, 52. Edwards, Milne, 38. Eigenmann, 55, 61. Eismond, 5. Ekman, 61. Klmhirst, 56, 57. Emeljanov, 1. Erdl, 51. Fabricius ab Acquapendente, 8. Fattori, 39. Fernandez, 5, 38. Fiebiger, 55. Fiedler, 39. Filliol, 57. Forster, 3, 25. Freund, 49, 51, 61, 62. v. Froriep, 39. Fuhrmann, 47. Gadeau de Kerville, 1, 31. Carman, 1. Garstang, 62. Gemmill, 1, 2, 10, 37, 54. Gervais, 32, 54. Giraldus, 61. Girdwoyn, 1, 43, 44, 48, 50, 60. Goeldi, 54. Grimm, 62. Grosser, 54, 55, 62. Grundmann, 8. Giinther, 51, 56. Gurlt, 39, 59. Haeckel, 48. Halbertsma, 47. Hefford, 47, 62. Herbst, 44. Hertwig, 6. Heusner, 1, 31. His, 2, 6, 8, 25. Hofer, 49, 52, 55, 61, 62. Holt, 52, 53. Houghton, 51. Howes, 46, 47, 48, 51, 52, 53, 62. Hubrecht, 52. Hyrtl, 51, 52. Iwanzoff, 47- Jablonowski, 27. Jackel, 47. Jacobi, 1, 2. Jaquet, 55, 62. Jarvi, 47. de Jeude, 48, 49. v. Jhering, 38. Johnstone, 47, 51, 55, 57, 62. Joseph, 61. Kershaw, 56. Kishinouye, 61. Klaussner, 1, 2, 7, 8, 10, 31, 35, 36, 38. Knauthe, 49, 50, 61. Knoch, 1, 2, 27. Koch, 37, 38. v. Kfalliker, 38. Kopsch, 1, 2, 3, 5, 6, 7, 8, 9, 25, 26, 27, 28. v. Krauss, 48. Kyle, 47. Laurence, 62. Leger, 61. Leonhardt, 49, 50. Lereboullet, 1, 2, 4, 5, 7, 10, 11, 25, 26, 28, 29, 30, 35, 36, 43, 54, 59, 60. Levison, 1, 31. Loeb, 60. Lonneberg, 49. Lowne, 1, 31, 47, 48, 49, 51, 52, 55, 61, 62: Luther, 47. M'Intosh, 54, 55, 57, 58. MacMunn, 56. Malloch, 48, 51. INDEX OF AUTHORS 73 Martens, 47, 61. Masterman, 47, 48, 58. Matthews, 46, 62. Mazzarelli, 54. Meckel, 2. Mencl, 61. Meyer, 53. Miall, 61. Mitchill, 37. Mitrophanow, 37. Morgan, 2, 8, 25, 27, 44. Moriggia, 37, 38. Moser, 1, 2, 10, 27. Mudge, 62. Newman, 47. Neydeck, 49. Ninni, 49, 51, 56, 57. Nusbaum, 55. Nystrtim, 49. Oellacher, 1, 2, 3, 5, 10, 25, 26, 28. Orlandi, 47. Otto, 49, 55. Panceri, 56. Panum, 1, 2, 8, 31. Paolucci, 2, 41, 43, 55. Pappenheim, 49. Parker, 62. Patterson, 47. Pavesi, 61. Peach, 55. Pellegrin, 49,52,61. Pettis, 56. Pilseneer, 48. Poey, 47. Pouchet, 61. Przibram, 54, 55. Punnet, 62. de Quatrefages, 1, 2, 24, 31, 44, 48, 50. Rathke, 1, 30. Rauber, 1, 2, 3, 5, 7, 8, 9, 10, 25, 26, 28, 33, 35, 36, 37, 39, 60. Heina, 39. Rennie, 32, 54, 55. Riggio, 62. Risso, 1, 31. Ritchie, 51, 52, 57, 62. Roth, 61. Ryder, 53, 61. Sacchi, 57. St. Hilaire, 1, 2, 31, 48, 49, 62. Sandman, 47. Schiemenz, 62. Schmitt, 1, 2, 3, 4, 8, 9, 10, 14, 21, 22, 23, 24, 36. Schneider, 47, 56. Schondorff, 56. Schwalbe, 1, 8, 29, 37. Secques, 1, 2. Seligmann, 54. Simpson, 47, 48. Smith, 47, 51, 52. Smitt, 47. Southwell, 47. Stannius, 51. Steenstrup, 56. Steindachner, 49, 62. Stewart, 47, 48. Stockard, 5, 27, 43, 44, 60. Stoddart, 46, 48, 51, 55, 61. Stoll, 61. Storch, 54, 61. Storrow, 51, 52. Sumner, 8, 25, 27. Suomalainen, 56. Sutton, 1, 30, 59. Taruffi, 1, 37, 38, 39, 59. Todd, 48. Tornier, 48, 49, 50, 52, 53, 55, 56, 61. Traquair, 55, 56, 57. Trois, 56. Tur, 37. Vaillant, 55. Valentin, 1,2, 10. Vayssiere, 61. Vogt, 47. Vrolik, 2, 49. Wagner, 56. Walla, 37. Warpachowski, 54. Watase, 61. Watson, 37. Weber, 47. Wetzel, 8, 37. Williamson, 47, 52, 53, 55, 61. Windle, 1, 2, 3, 4, 7, 37. Wolff, 8. Woodward, 62. Yarrell, 1, 2, 51. SUMMARY OF CONTENTS OF PLATES. Pis. XVII.-XXI. figs. 56-93, PI. XXII. figs. 94-96, PI. XXIV. figs. 100-106, are from drawings made by the author, and are based on the study of numerous series of sections. All the other plates and figures are from photomicrographs of specimens or sections. Pis. I. -II. figs. 1-12. Surface views of normal trout embryo, and of specimens belonging to various Classes of double monstrosity. Pis. III.-IV. figs. 13-20 ; also PI. VI. figs. 24-26. Transverse sections of a normal trout embryo at the same stage in development as the monstrosities described. Compare with the sections, etc., of the latter, shown in the succeeding Plates. PI. V. figs. 21-23. Horizontal sections of normal trout embryo, as above. PI. VI. figs. 27-29 ; also PI. VII. figs. 30-31. Transverse sections of double monsters with union in the posterior part of the body (Class VI. (a) and (b) ). PI. VII. figs. 32-34. Transverse sections through the triple monster described on pp. 33-34 ; see also Pis. XXII.-XXIII. figs. 94-99. Pis. VIII. -IX. figs. 35-38. Transverse section through the head of a double monster with union of the twin brains in the region of the optic lobes (Class I.). Pis. X.-XI. figs. 39-42. Horizontal section through the head of a double monster with union of the twin brains in the region of the optic lobes (Class I.). PI. XII. figs. 43-46. Horizontal sections, as above, but in a specimen showing a different disposition of various parts. Pis. XIII.-XIV. figs. 47-51. Transverse sections through the head of a double monster with union of the twin brains in the region of the medulla (Class II.). Pis. XV.-XVI. figs. 52-55. Horizontal sections of a double monster with union in the pectoral region, inner pectoral fins not being present (Class III.). PI. XVII. figs. 56-63. Diagrams to illustrate : the cartilaginous cranial, hyo- mandibular, and vertebral, skeletal elements in normal trout embryos, and in double monsters belonging to Classes I. and II. (union of the twin brains in the region of the optic lobes or of the medulla). PI. XVIII. figs. 64-72. Figs. 64 and 65 illustrate the visceral arch cartilages, and Figs. 67 and 68 the vascular system in double .monsters belonging to Classes I. and II. Fig. 66 shows the inner girdle and fin cartilages in an example from Class IV. (union in pectoral region, inner pectoral fins being present but united). Figs. 69-72 are diagrams to illustrate the cavities of the brain in a normal embryo and in double monsters belonging to Classes I. and II. PI. XIX. figs. 73-81. Figs. 73-75 illustrate the heart in a normal embryo and in double monsters belonging to Class III. (union in pectoral region, adjacent pectoral fins not being repre- sented) and Class iV. (union in pectoral region, adjacent pectoral fins being present but united). Figs. 76-81 illustrate the posterior ends of the Wolffian ducts, the bladders and urinary pores in a normal embryo and in double monsters belonging to Classes V. and Vl. (union by the body or tail). PI. XX. figs. 82-87. Fig. 82 shows the inner composite otocyst in a double monster belonging to Class II. (union of the twin brains at the medulla). Figs. 83-87 illustrate the glomerulus and the head-kidney and the first portions of the Wolffian ducts in normal embryos and in specimens belonging to Classes I. II. III. and IV. of double monstrosity. PI. XXI. figs. 88-93. Diagrams with transverse lines to indicate the planes and levels of certain typical sections of which photo- micrographs are shown in the earlier plates. The struc- tures particularly illustrated are the brain cavities and the cranial and hyomandibular skeleton. PI. XXII. figs. 94-96. These figures illustrate the external appearance and the alimentary and urinary structures of the triple monster described on pp. 33, 34. PL XXIII. figs. 97-99. Transverse sections of the body of the triple monster ; see also PI. VII. figs. 32-34. PI. XXIV. figs. 100-106. The figures illustrate cyclopia (pp. 40-45) and super- numerary eyes (pp. 59, 60). PI. XXV. figs. 107-110. Transverse sections through the ocular region in cyclopia and sera icy clopia (p. 44). PI. XXVI. figs. 111-114. Fig. Ill follows Fig. 110 in illustrating semicyclopia ; Fig. 112 shows the atrophic head referred to on p. 61 ; Fig. 113 is a seotion showing absence of notochord ; and Fig. 114 a section behind the pectoral fins in a double monster belonging to Class IV. XX. tig. 83. nilar normal sp<- ption on p| 1 uilh Figs. . 59 ; XVII : ■ OF PI XXII i n the study o trout ambry o, and of speciu i double monstrosity. tit embryo at the same nent as the monstrosities described. us, etc., of the latter, shown in utal sections of normal trout embryo, as above. i: also PI. Vll. figs. 30-31. Transverse sections of double monsters with union in the posterior part of th ss VI. (a) and (?<)). PL VII. figs. 32-34. Tiansverse sections through the triple monster described on pp. 33-34 ; see also Pis. XXII. -XXIII. figs. 94-99. ■ Ira wings made aid figures pie from Class IV. i' fins being present llustrate the double sented) o oral lins being present I illustrate the posterior ends < bladders and urinary pores in a normal embryo and in double monsters belonging to Classes V. and VI. (union by the body or tail). i 87. Fig. 82 shows the inner-composite otocyst in^a do III. a id of the medulla B II.). P!s. XV. -XVI. figs. 52-o5. Horizontal sections of a double monster with union in the pectoral region, inner pectoral fins not being present (Class II . lernal appearance and the id urinary structures of the triple monster iii. 33, 34. PI: XXIII. figs. 97-99. Transverse sections of the body of the triple monster ; see also PL VII. figs. 32-34. PL XXIV. figs. 100-106. ■es illustrate c Fig. i. Side view ol a n .il trout embryo of same age .i3 the monsters described. Sections ol similar normal specimens are shown in PI. III. riys 13-16, IV. figs. 17-20, VI figs. 24-26 (trans- and in PI. V. figs. 21-23 1 Diagrams of skeleton, etc. in PI. XVII. Rrs. 56, 57, 62, Will fig. 69, XIX. figs. 73, 76. XX. fig. 83. Description on pp. 11 12. Contrast with Figs. 2-12. I I 1 1'h,. 4.— View in. in above ol a double :>" l" Class I., i.e. with union of the twin brains in the region of the optic i 1 .Transverse sections of a similar specimen are shown in P VIII. -IX., figs. 3$-s&. horizontal sections in Pis. X.-XI1 figs. 39-46, ami diagrams ol ci i eleton, etc., in PI. XVII. figs.58,59; XVIII. figs. 64, 67, 70, 72. Descripl i I on pp. 1 2- is. ( -Side view of a double trout embryo, with union by the yolk-sac only. See pp. 23-24. ( x 3.) Fig. 5. — View from ventral aspect of the double monster illus- trated in Fig. 4. Two mouth-openings are seen, separated by a septum containing the remains of the adjacent Mei I For references, see under Fig. 4. ( X3.) Fig. 3 S ■ ew o( a specimen simitar t" the last, but v> of the I See pp. 7, 44. Figs. 109-111, in Pis. XXV.-XXVI., arc from sections of this specimen. ( X3.) FlG. 6. View from below of a double monster belonging t<> Class II., i.e. with union of the twin brains in the region of the medulla oblongata. The two adjacent eyi ; also the two mouth-op* mings, separated by 1 thick septum containing the adjacent hyomandibular and Meckelian cartilages. S shown in PI. XIII. iigs. 47, 4:-;, and XIV. figs. 49-51. Diagrams of cranial skeleton, etc., in I'l. XVII. tigs. 60, 61. XVI II. figs. 65,68. 71. Descripl ' 1 Kiss II. on pp. 15-18. ' PI.. Fig. 7.— View from ventral aspect of double monster belonging to Class III., i.e. with union in pectoral region, the adjacent pectoral fins not being represented. Horizontal sections in PI. XV. figs. 52, 53, XVI. figs, sr, ^;. Diagrams of vertebral column, heart, etc., in PI. XVII. fig. 63, XIX. fig. 75, XX. fig. 84. Description under Class III. on pp. 18-19. The body wall over the yolk is ruptured along the middle line. ( ■ 3. ) Fig. 10. — Double monster with union in posterior part of body. Tli.- union is obliquely by the ventral aspect of each twill, and the tail has three angles. See pp. 21-22. Compare PI. VI. fiss. 27 20. and I'!. VII. fig. 31. (x3.) Fig. 8. — View from above of double monster, with unioi pectoral region, the adjacent pectoral fins not being represented. For references see under Fig. 7. ( :■: 3, ) I'll,. 11. — Double monster with union by posterior part of body. The tail is irregular, but resembles that of the preceding specimen. One of the twin heads is smaller than the other, and shows the condition of cyclopia. See pp. 7, 40 and PI. XXIV. figs. 100-104, also PI. XXV figs. 107, 108. 1 j 1 ^ — ^tf^F^V FlG. 9. — View from the side of a double monster, with union behind the pectoral region. One of the twins is much reduced in size. The posterior part of the body is single and symmetrical. For description see under Class V. on pp. 20-21. ( x 3.) Fig. 12.— Double monster with symmetrical ventral union of the posterior parts of the twin bodies. Two pairs of pelvic fins are present, each pair being made up of components derived from different twins. The tail has four angles as in the section shown in PI. VII. fig. 30. Description under Class VI. {/>) on pp. 22-23. (X3-) PL. II. ■rnuS :h I [ ■ optic lobes '. bar elc. , in and the .) i and PI. XXIV. figs. 100 104, XXV. Fig. behind the pi size. "1 PL. 11 Mucous canal Pineal body Supraorbital bar ( a ity cerebral lobes Cerebral lobes Obliquus oculi superior Retina and lens Rectus oculi internus Trabeculae cranii Palato-plerygoid bar Copula visceral arch II Visceral arch I Cavity optic lobes . Optic lobes Supraorbital bar . _ ' Rectus oc. superior--. Ventricle III . Optic recess Retina and lens •• Rectus oculi internus ..** Trabeculae cranii •-■ Nerve II -••* Palato-pterygoid - - " Copula visceral arch II .-■■ Visceral arch I • - ' Cavity optic lobes Optic lobes Mucous canal Supraorbital bar Ventricle III (infundibulum) Retina Nerve II Rectus oculi internus Trabeculae cranii Rectus oc. inferior Palato-pterygoid bar Mouth cavity Visceral arch II Visceral arch I Cavity optic lobes -.... Optic lobes Sylvian aqueduct .... *.\\. "* ?/7 Supraorbital bar. /\ \ \'\ Nerve III..... /V '^V/^^- Infundibulum — ■- J -ps, .' :\\- :~$£~ Recti oculi intern, super, and infer. - •-- I -V?. ^v^, I i abeculae cranii V ■■'..y.:iX^^'::^> Palato-pterygoid bar ■■ V ■ 7/z~7-^T Visceral arch III- \ ' " * f rOO^'V^r ,, ,, ii Vv*^..®^, [•••■•■ ■ Copula visceral arch I\' 1. l-\ G. HI nao auosuM ixid J i '■.' i" ■ i] r' fifuqcO'"' aadol Dijqo ^)i i 29do! DlJqO . . ij5»f [jsjidv nU>- loraqua .so iulDsS - Ill sloronsV 289D3T SllqO J-; anol bne EniisJ! .1."^.. zurmlni ilioo 3ifJ ■ irnnio 9cIu-j3d£iT oilqO |j;nKj 2UO01/1/ Tjsd LsJidio£iqi;2 (inuli : - ' 1 ab'nlnaV II 9 :uni3ini iluoo su)39>I iin/no 3«lu lohaitai .DO buidsJI i«d biogyiolq-oJElfil -"^-x ^^ "j dluoM II rial]; IfiTS I don a-xioi • iilril ■ iRd bioj ,W^-JPjJks/ I! .. ■ 1 ■VI rior' Ill Fig. 13. — Tran >versi through head of normal trout embryo (PI. I. fig. 1), showing eyes, cerebral [1 ibe . ■ni'l pinea] sac. ( I pare \\ ith I'!. VIII. fig. 35 (union .11 optic lobes) anil PI. XIII. fig. 47 (union at medulla). For othi r references »ei nix lei PI. I. fig. 1. ( x 15.) I'D,. 14. — Transverse section through head of normal embryo (as in last figure), aero-, anterior part of optic lobes ami entrances of optic Compare with PI. VIII. fig. 36 (union at optic lobes), and with PI. XIV. fig. 50 (union at medulla). For other references see under PI. I fig 1. 1 .5 I Fig. 15 Transverse section as above, through middle of optic lobes, and through optic commissure. For other references see under PI. 1. fig. 1. 1 i'. 16. — Transverse section as above, through back part ol lobes and infundibulum. Compare with PI. IX. fig. 37 (union al and with PI. XIV, tig. 50 (union at medulla). For other n PI. I. fig. 1. I 15.) PL. III. FlG. 17. — Transverse section through head of normal trout embryo, showing cerebellum, auditory organs, medulla, pharynx, gill chambers, etc. Compare with PI. XIV. fig. 51 (union at medulla). For other references see under PI. I. fig. r. ( x 15.) FlG. 18. — Transverse section through head of normal trout embryo, -1). wing the posterior part of the medulla, the gill chambers, and the anterior part of the heart. Compare with PI. IX. fig. 38 (union at optic lobes). For other references st'e under pi. 1. fig. 1. (X15.) FlG. 19. — Transverse section of nor- mal trout embryo in pectoral region. The section passes through the head- kidney, oesophagus, and pectoral fin. Other references under PI. I. fig. 1. (xi50 Fig. 20. — Transverse section of nor- mal trout embryo, across middle of body, showing swim-bladder, intestine, and pancreatic tissue. The specimen, which is otherwise normal, shows local reduplication of the notochord. See p. 5^. Other references under PI. I. fig. 1. (xi5.) PL. IV sbhlU III VI "/I sbhlnav ." biodooJoU zugEv noils i V ,VI zsdoiB lei-*D=iV sbiiln bioo (EniqS agjslinno rfo-i£ frnu^'/I biorfooJoW sgelinr.D d^ifi IjsmscH nnfftlo'// ioub njsrftloW Y.nibiKo lorajacfl clqoaaO nn I:. jIIoY innsKo'I 3 anitojnl VI .O .-I .1. Cerebellum Cavity ventricle IV Periolic cartilage Semi-circular canal Utricle Ganglion nerve VIII Saccule Hyomandibulai Notochord and Rectus oculi externus (origin) Visceral arch 1 1 1 ., IV ,, V Pharynx Truucus arteriosus Cavity ventricle IV Medulla oblongata Parachordal cartilage Notochord Ganglion vagus Jugular vein Aorta Aortic arch III Visceral arch III Visceral arches IV, V Visceral arch VI Auricle Cartilage pectoral girdle Ventricle Spinal cord Neural arch cartilage Notochord Haemal arch cartilage Aorta Wolffian body Wolffian duct Posterior cardinal vein Oesophagus Pectoral fin Yolk Spinal cord ... (Ti~\ Notochord (double) •■•■ ' (~)C~\ Wolffian duct J /?" Posterior cardinal vein ■■ - Swim-bladder Pancreas • Intestine • Yolk J. F. G. IV Mucous canal Supraorbital cartilage Ventricle 1 1 1 Retina Cavity optic lobes Supraorbital cartilage Optic lobes Cerebellum Ventricle I V Semicircular canal Utricle Periotic cartilage Medulla oblongata Canal of spinal cord Spinal cord Muscle segments Mucous canal Olfactory capsular cartilage Obliquus oculi superior Cerebral lobes Retina Lens Ventricle III Rectus oculi exte'rnus Ganglion nerve V Pons Periotic cartilage Parts of labyrinth Medulla Parachordal cartilages Notochord Muscle segments Olfactory capsular cartilage Rectus oculi internus • Obliquus oculi inferior Retina Optic chiasma Infundibulum . Rectus oculi externus Hypophysis Hyomandibular Periotic cartilage Saccule Gill cover Visceral arch cartilages Head-kidney Glomerulus of head-kidney Posterior cardinal vein Muscle segments Body cavity Swim-bladder J. K. G. 9gjs(i)1K-j lKlirhoj;iqu£ III shhuia'/ . lEiiiqa 1o Iehe') bioo Icriiqg tonaqua ilu : flOO» • V avian noilgnaO.. Etiol ... 93&Iii-M» oi)oii3lI . dlni-rvdfil to aiiEl riftnbsl/ . iSgBlillBJ (fib KlfbElEl . biorisoJoH airiamgaa aloeuM - 1 B3 leluaqso '(ioJobIIO ■-. TOiTjtni ilu Emanitb DiJqO 21)111") ■ iqoqvH .. -usludibaso* sgBli; 9tU33 asgEliiiEo d :»B3H . ni9V Ifinib 'i. IqzujM ■ Vlivi; wbbfild-miv/g O .M .[ Fig. 21. — Horizontal section of not ni.il troul embryo (PI. I. fig. i). I he se( tion passes through the uppei parts of the eyes, and the auditory organs, as w< II as through the various cavities of the brain, and the i entral anal ol the spinal cord. I ■ with PI. X figs. j9, 40 (union at optic lobes I, M l- fig. 4 ) (union .it medulla), and with PI. XV. fig. 53 (union in pectoral region). For other refen n< es see under PI. 1. lis;. 1. ( ■ 13. 1 01* ■ ■ Fig. 22.- Horizontal section as above, but passing below the cavities of the optic lobes and the medulla, and cutting through the middle of the eye and the auditory organs. Com- parewith PI XII. figs. 44.45 (union at medulla). Other references under PI. I. fig. 1. (X13.) Fig. 23. — Horizontal section as above, but passing through the lower parts of the eyes, brain, and auditory organs, .is well as through the head- kidney, the duct of the swim-bladder, and the dorsal portions of the gill .itches, and of the body cavity. Com- pare with Pis. XL. XII. figs. 42. 45, 46 (union at optic lobes, and at medulla). Other references under PI, I. fig. 1. (X13.) PL. V. Fig. 24. —Transverse section through normal trout embryo, a con- siderable distance in front of the vent. Compare with Fig. 27 (union by pos- terior part of body). Other references under PI. I. fig. 1. ( v 14.) Fig. 27. — Transverse section corresponding with Fig. 24, but through a double monster of the type illustrated in PI. II. fig. 10. Figs. 28, 29, and 31 are from the same series. At this level the two intestinal canals are widelv separated. For relations of Wolffian ducts, etc., see pp. 22. 23 and PI. XIX. figs 76-79. (x 14.) Fig. 25. — Transverse section as m Fig. 24, but a little further back. Compare with Fig. 28 (union by pos- terior part of body). Other references under PI. I. fig. 1. (x 14.) I i'.. 28. — Transverse section of the same double monster a- m Fig. 27, but a little further back in the series. The intestine is now single, but has two mesenteries. The Wolffian bodies and ducts are coming together bv then inner, 01 adjacent, sides. Compare with Fig. 25 (normal). Other Fig. 27. ( x 14.) Fig. 26. — Transverse section, as in the two preceding figures, but passing just m front of the vent. The Wolffian ducts have opened into the bladder. which is seen lying dorsal to the rectum. Compare with Fig. 29 (union by posterior part of body). Other references under PI. I. fig. 1. i 14.) FlG. 29. — Transverse section through same double mon- sti 1 as in Figs. 27 and 23, but further back in the series. Compare with Fig. 26 (normal). The adjacent Wolffian ducts have opened into a first bladder, while the outer ducts have not yet reached the second one, which lies posterior to the other. These relations are explained on p. 23. Other references under Fig. 27. (xi4.) PL. VI. tnoil ] < biodaoioX bio'i sasidmsin IcaioU Irigh) eJaub nfirfiloW nfi >oloV. . . . il iod nntftloV/ r/b nEtftloV/ lift o'vh'i biod bioo ;'.bbn!M-- muJ ,o Dorsal membrane Spinal cord ' Notochord" Aorta • ■ Wolffian ducts Muscle segments Aorta Notochord Dorsal membrane Spinal cord Wolffian duct Intestine Yolk Dorsal membrane ' Spinal cord - ■ ') /-\ Notochord • • ■ Aorta Wolffian body Wolffian ducts- Rectum -(•"■V'V'f "(J Pelvic fin Notochord Spinal cord Dorsal membrane Aorta Wolffian ducts (right and left) Mesentery (double) Rectum Pelvic fin Muscle segment? /.. Spinal cord ' £*) Notochord Aorta Caudal vein • Bladder Rectum Ventral membrane -.... -■-Notochord Spinal cord Dorsal membrane Aorta Wolffian ducts Bladder Mesentery Rectum Ventral membrane 1. F. G. VI Ventral membrane (composite) Dorsal membrane Spinal cord Notochord — Spinal cord Notochord Dorsal membrane Ventral membrane (composite)- — '^O0 Dorsal membrane i.ents Spinal cord Notochord (composite) Aorta Caudal vein Ventral membrane Spinal cord >rds, and d - ng in part to Muscle segments of principal . Spinal cord Notochord .... Aorta }y^ \ Caudal vein Muscular tissue of defective embryos Ventral membrane (composite) \^4J Dorsal membrane of defective embryo B A Dorsal membrane Muscle segments .Spinal cord ) aC%f Notochord / SV Ventral membrane (composite) Ventral membrane (composite) Dorsal membranes of \ embryos A and BJ Naming of parts same as in last figure. The dorsal mem- branes of A and B are now united. J. F. G. VII Dorsal 9(11; i jSJIfflq! 1119V I (9)iao mom lsilnsV B b-iogbojotf mbrane bio?" Isniq? biorfsolo'/I griEirlmein Ir.f-rofl obi gnEidrrom I/nJnsV ov.idrri9 Inqoinq lo.airwng?* stoull bloo UniqS • 1I0X jsnoA :>urD .. ■1o 9U£2(! IfiiuOPuM (gliaoqrfioD) 9nj5idoi9m IsunV/ a oyidms svijosteb to 9nsidm9m IestoQ L. A Notochord Kcctum Pelvic fin ni n, ?m£2 snsq to gnimaH -msrti [fisiob 9fiT won 9i£ 3 bns A to ssncid sriEidmgm l£2ioCJ _ a)n9mg98 gfoaul/ . Ijiod [eniqS V \ biodoooZ (9)iaoqmo9) sneidrrom laijno'/ --"(iliEoqmoo) srundnwm talosV I to »n£idARm Ifiaioa IIV .3 .•? .1 Fig. 30. — Ti msvei ■> section through tail : embryo belonging to the type illustrated in PI. II. fig". 12 (ventral union by posterior part 1 if body). There .in.- two spinal cords, notochords, and dorsal edge membranes Two ventral edge membranes ire also present, but these are coiup derived from the widely separated halves of ventral membranes belonging to each of the twins. This is an example of almost pure ventral union. See pp. 22-23. Compare with Fig. 31. (xi.f.) FlG. 1 I 1 section tin tail of an embryo belonging to the general type illustrated in PI. VI. figs. 27-29. rhi two notocl fusi d together, and the ventral edge membrane is composite and single. Compare with Fig. 30. See also under Fig. 27. I 1 \.. 1 Fig. 32 ; section through com- mencemi tail of the triple monster trout described on pp. 33-34 See also PI. XXII. figs. 94-96 and PI. XXIII. figs 97-99. The section is taken up chiefly with structures belonging to the principal embryo, but it shows also, near the middle line on the under ■ i-l" 1 t . the two dorsal tins of the defective em- bryos. A little further up on either side is a composite ventral edge membrane (anal tin I. belonging in part to the principal embryo and inpart to one or othe! of the defective en ibrvo-.. (xi8.) Fig. 33. — Trans ■ ■ n thn >ugh 1 pecimen as in last figure, but considerably further back. All the fin membranes are stijl present, but the two dorsals of the defective em- bryos have almost united. The section, as well as the succeeding one, may be compared with Figs. 30 and 31. Other references under Fig. 32. • Fig. 34. — Transverse section as above. The dorsal fins of the defective embryos have now united. (xi8. ) PL. VII. Fig. 35. — Transverse section through anterior part of head of a double monster belonging to Class I., the type illustrated in PI. I. figs. 4 and 5. The posterior walls of the adjacent eyes are just grazed by the section, and several of the ocular nerves and muscles are seen. The cerebral lobes and their cavity, the pineal sacs, and a forwardly projecting part of the com- posite optic lobes come into the section. Noticeable in the middle line, ventrally, is a large cartilage consisting <>f the fused adjacent hyomandi- bulars, with which articulates a small bar representing the adjacent Meckelian cartilages. Compare with PI. III. fig. 13 (normal) and with PI. XIII. fig. 48 (union at medulla). Pis. X.-XII. figs 39-46 are also from sections of a double monster similar to this one, i.e. with union at optic lobes. For plane of section see PI. XXI. figs. 88-90. Other references are given under PI. I. fig. 4. (X20.) *.;W*- f ! 11. j6. Transverse section from same series as Fig. 35, but passing across the middle of the outer eyes. There are two infundibula, but the composite optic lobe region has a single cavity. The trabeculae cranii are doubled. A few of the muscles belonging to the adjacent eyes are cut across near their origins. In the floor of the mouth, the two first visceral arch bars and the copula of the third are seen. Compare with PI. 111. figs. 14 and 1 j (normal). For other references see under PI. I. fig. 4, and for plane of section see PI. XXI. figs. 88-90, line 36. (>;20.) PL. VIII. •jg/.linBo Lb ?wiol jiiqo aJiaocnB vbo - »•<■• - (asvt) rami 3-^Y--Re;;na //K S;)\ l ^ A 7 • ■ ("" -AY- Lens ' v ' ^~ '~-/-■' , sr^. J ^-^^^A-" \^i~?^y\- ' " Rectus oc- inl- z&23^Z~^~s^£i&-----~,-/- .... Rectus inf. and sup. of one of the I • '- ~~~'—~^»~^ inner eves \^-k/J v '"CiJ>" ■-■ Trabeculae cranii Meckel's cartilage (visceral bar I) Hyoid cartilage (visceral bar II) Viscera! bar III "• ■ — Copula bar III scles belonging I 20.) J. F. G. PI. VIII. VIII with union ■•- Optic lobes ■■ Cavity of optic lobes Supraorbital bar "" Commencement of iur \- H>: ''" 7) \ Rectus sup. and inf. j\ / '",.[ _^' j Lens and ret 0 ']•©'' " "V — '~>?< '' ' fibecular unit -^ X" W-?/ Pituitary space of right twin head ^■sr zy — Tgy .... A^-j ' '" " Palato-pterygoid bar 1 \£ ISfy -* . '^J' ' Inner trabecular units, fused //$ I ) 1 ' snaJ linu iK!uD9dBi) i3juO b£t>ri niwp . q? yii,!iLTii'l' lEd biogpaiq-OJsl'Bl beaut .iiiii III 93/ilh'U.j rin;; fjwSbgjy (e'fejbsl/.j I (bio? I., i.e. the type illustrated in PI. I. figs. 4-5 (surface views), and 111 Pis. VIII. -IX. figs. 35-38 (transverse sections). Compare with PI. V. fig. 21 1 section of a normal embryo at the corresponding level). The section passes through the upper parts of all four eyes of the two auditory organs. The two forebrains are cut somewhat obliquely, while the large composite optic lobes and their central cavity are seen in horizontal section, as also are the fourth ventricle anil the medulla oblongata. Other refer- ences are given under PI. I. fig. 4. I 1 Fin. 40. — Horizontal section of the same double monster as in last figure, but at a slightly deeper level. The section passes through" the middle of the outer eyes, but only through the upper segments of the adjacent eyes. The composite cavity of the optic lobes will be seen to bifurcate in front, and lead into the cavities of the two forebrains. For other references see under PI. I. fig. 4. ( x 16.) PL. X. dvod£ to esionsqua rlasH aadol Icidai^o )o aadol Ml rah-" o^idms Jdj "ViHI asdol jiiqi) lo (liveS"' esdol oitqQ^" Ib(ieo -rrdimiaimsg VI sbhjn-j / (sjiaoqmoa) slbfbsM esgRlinKO dai£ Ii; u (sJieoqmcn) bnoo t(;V (isniii) ainarrtgaa biotto (9ii?oqrnOD) bio'j I R The inner eyes Recti superiores of above Tegminal cartilage Cavity of cerebral lobes Supraorbital cartilage Optic lobes Ventricle III Obliquus superior Rectus oculi superior Outer eye of right embryo Cavity of optic lobes Optic lobes Periotic cartilage Semicircular canal Ventricle IV Medulla (composite) Muscle segments Neural arch cartilages Spinal cord (composite) bes, and bel' ding •out '' • The inner eyes - Recti superiores of above / J uS>-"-Sy " Supraorbital cartilage 'ffi*—~?\\ "" Cavity of cerebral lobes ■— Obliquus superior Ventricle III 01 7'"^>^.'i "" Optic l°Des Outer eye of right twin //^Vj!// -Lens Retina Rectus superior Cavity of optic lobes '■■ Optic lobes Periotic cartilage Semicircular canals Medulla Muscle segments Neural arch cartilages ;u. _•: j Muscle segments (inner) Notochord • Spinal cord (composite) ■ tion of the J. F. G. P1 X The two inner or adjacent ■ Choroidal fissures ■ Obliq. inf. of an inner II of an inner eye ■Olfactory cartilage and beginning of trabeculae ■ Rectus int. inner eye • Rectus sup. inner eye - Obliq. inf. outer eye '- Rectus int. outer eye ■ Optic nerve and commissure •The two sets of hypoaria and hypophyses Rectus ext. of right outer eye Hyomandibular Periotic cartilage The paired notochords and parachordals The two aortae Inner muscular segments Head-kidney with glomerulus - The two notochords . Lateral muscle segments •■The composite spinal cord n. For otl .. The two inner or adjacent Choroidal fissures inf. of an inner eye Xerve 1 1 of an inner - ("Vv V,.. '"" I. Olfactory cartilage and beginning of trabeculae f} \ ._ Rectus int. inner eye Rectus sup. inner eye (7 ■■- Obliq. inf. outer eye _/ /^Tf^^V'V"* Rectus int. outer eye <'v £•"" Jfc_ lEa^m' i Optic nerve a°d commissure — ^5^"T — """^c— Choroidal fissure ^^S«^5-'- - The two sets of hypoaria and hypophyses • • Rectus ext. of right outer eye •Periotic cartilage \. .-Hyomandibular \\ _.. Upper ends of gill arches - The paired notochords and parachordals il-Vtl ..The two aortae An aortic root (third) J. F. G. XI. XI i.isofilbr. 10 i9iiai ov/1 sriT asiijaaft IfibioioriD 379 isnni njs lo .lni ,p 9'{9 isani an to II 503/- 9s(u39dK'ii lo gninnigsd bar, 3gE[iriE9 '(TO»3,;irO' ■-... 379 i9fim .Jai ;»:/l?9>{ 979 ismu .qu 379 tsjuli Jni >: 3iij=sirnrnoD bru j 'q'(fl bnB EnBoqvrf'lo elss ov/J 9riT | 19100 Jllph'tO .1X9 2UJD3/I 1 I alfibiori9Bi£q bns abiorbojon bgiiBq 9riT 9EITOE 07/) sriT 3Jn3fn§93 iKluoaum i9nnl .-r/hmrnolg dun zbiorfooion ov/i 9riT a]n3tng9e 3b2uin IbtjieJ bTOD Isniqa sjiaoqrnoo 9fi T .ijosite) e973 )fl93B(bE TO 13(1111 07/1 9fiT 29iu2aft IsbioiorO 973 i3nni hbIo ."1. • isnni riKlo II 9/t . / sslmsdBil lo gninnigsd bnr. sgEliliED (ioidbIIO 3'(3 ISnili ,)ni 2UI99M 3V3 igririi .qua aulosJI Jni 2H109.3'— / 9iu2airnrnoo bns svran jbqO •-'■ 9'IUe 29r(riqoq{ri bnE f.iiEoq/ri lo 2I9» ov/j griT 973 13JU0 Jtigi'i lo .1X9 32e[:j : lEludibnEtno^H.. E9ri91E lliglo ebfi9 -raqq'J-. alfibiodoBiEq bnr, ebioriDOJon b9iitq 9riT . J 9riT.. (biidl} Jooi oijioE nA iperioref of. ibes cle III ■ Optic lobes O lobes a/cord IX .O .1 I A - Fig. 41. — Horizontal section through same double monster as in last two figun . The section is at .1 still deeper level, and passes below the auditory organs, through the basal parts of the brain and the lower segments of t It* - eyes All four optic nerves and fissure- are seen. Owing to the curvature of the specimen, the body is cut through somewhat obliquely, and shows the two notochords and head-kidneys, as well as the composite spinal cord and the median muscular mass. There is greater apparent duplicity in the forepart of the body than in the posterior cerebral region. For other references see under PI. I. Fig. r. (xi2.) Fig. 42.— The anterior part of last specimen shown in higher magnification. ( x 21.) PL. XI. Figs. 43-46. — Horizontal sections of double monster (union at optic lobes), belonging to the same type as the specimen illustrated in PI. I. figs. 4, 5, Pis. VIII., IX. figs. 35-38, and Pis. X. , XI. figs. 39-42. The moulding of the adjacent eyes and brain parts presents, however, some important differences when compared with the two last-named sets of figures. The eyes are turned towards one another, so that the common lens lies buried below the surface in the centre of the globe. The optic lobes show greater evidence of duplicity, as also do the parts round the iter and the fourth ventricle. Owing to curvature of the specimen, the body is cut through obliquely, as in Pis. X., XI. figs. 39-41, and shows a double condition of the notochord and of certain other structures. The gullet and rest of the alimentary canal are single, but there are two swim-bladders and air-ducts. For general references see under PI. I. fig. 4. Fig. 43. — Horizontal section of the embryo described above, passing through upper parts of eyes, brain and auditory organs. The fused adjacent sides of the optic lobes form a rounded mass, projecting into the central cavity. Dorsally. the sagittal planes of the twin fore-brains are much inclined towards one another. ( ■ 14.) FlG. 44. — Horizontal section of the embryo described above, passing through the middle of the outer eyes and of the auditorv organs. The mesial optic lobe mass, referred to under Fig. 43, is now larger. The mesial muscular mass between the two notochords is also to be noted. ( x 14.) gffft? r \; Fig. 45. — Horizontal section of the embryo described above, at a deeper level than in the last two figures. The adjacent olfactory pits and the two adjacent eyes with single lens in the middle of the globe are seen, as also are the two sets of in- fundibular outgrowths and the origins of the two swim- bladders. (X14.) Fig. 46. — Horizontal section of the embryo described above, at a deeper level than in Fig. 45. The globe made up of the fused adjacent eves is cut in median section, while the outer eyes show their choroidal fissure and places of entrance of the optic nerves. (X14. ) PL. XII. bsjin'J XldSIS'jIo [1 ^T^Vc-S .Ini .'JO 2U)3*->3 111 sgsliJ-uso »i«i 71 sbhUBV- ■• -Tl.jS Hnii i J/ /; tllobsl/ ! ' ilsbiodaj nlooion owl i ir.luoauin 1 ■ • Jin lo esii9S isIuO"" bioo Ifiniqe sliaoqmc'J • • 111 sloi IejuIio) sado! oilqO •;. iBifi-jtri bsJin'J "••bio ;moO I j*»<^\ C A ygnliliBO vioIdkIIO •-.... f/\ \ \ ^? •cl"' ! -. /»~'->.'"0 .1X9 .so au)395l ..I "<5nA 1 eludibnulni o-«) sri 1 V '"" ••• ' Vyi Q f\ o -\ / ,5C-' ^v ^ .-■£/ K, inlmlibni.ii: ■ : '. .\~^^ /\ '^-vi" .^Ct. ^S lorisoion ov/j urlT y*y .R .^\ ry^.jTN Q/ c ■rbiR llij \ /Zw/lP *Oo ■hod lo gain J y! V^ "\^-^ yO i iflluoaurn \\Ac->\f. — '_''... \\. . . •• ■ \ ^-3^ sbiotboJon ow) silT \\ \ C«V hioa Ij.niqe sJieoqmoo sriT ■ ■jnul luo n£. .. .qua .oo aulosJl .. icludibnfiir Ion ow) srfT ignriqoasO bioo kniqa slieoqtt- 11/ • i :i .y er evidence other struct;. genera ■.•low the The '1 and of cei i acts. For United inner supraorbitals Outer supraorbital-- . Cavity of cerebral lobes ■-- An outer eye - ■- Ventricle 1 1 1 — Outer supraorbital Optic lobes - — " Periotic cartilage _.. Ventricle IV Parts of labyrinth -Medulla- United haemal arch cartilages. Mesial muscular mass--'" Composite spinal cord •-"" United inner supraorbitals ^\^^\%JC\ Outer supraorbital •]- x" - "__^T Cavity of cerebral lobes v^K^V / / ^ — )] \ "Rectus oc. int. 'Ventricle III ' Optic lobes ' Periotic cartilage i^ f77 " Ventricle IV /1K*\ J Parts of labyrinth j) |K3l] Medulla ' Parachordals 'The two notochords 'Mesial muscular mass 'Outer series of muscle segments "" Composite spinal cord Inner olfactory organs Olfactory cartilage United inner eyes and lens ••- An outer eye . . . Rectus oc. sup. ., Rectus oc. ext. .. The two infundibula and .." their outgrowths Hyomandibular ... Upper parts of gill arches . The two notochords and their parachordals Oesophagus ginning of the two air ducts Head-kidney . . The two notochords — The composite spinal cord . Fig. 45.— It at a deeper level olfactory pits middle of th< fundibular outgrow bladders. (X14.) ,011 of the < ■ ' 1 Inner olfactory organs - Olfactory cartilage .- Obliquus oc. inf. . Rectus oc. sup. - Optic nerve ■ Rectus oc. ext. The two infundibula and their outgrowths Hyomandibular The two notochords and their parachordals .-•■-■ Upper parts of gill arches Beginning of body cavity Beginning of stomach Part of liver Lateral muscles ■■-• Mesial muscular mass The two notochords • The composite spinal cord J. F. G. PL XII. XII Mucous groove Inner eye of right twin head Supraorbital bar Rectus oc. ext. Cavity of cerebral lobes Olfactory cartilage Olfactory nerve and pit Rectus oc. inlernus Obliq. inf. of oute' Palato-pterygoid Rectus oc. inf. Trabeculae cranii Inward dip of surface epithelium Commencement of an inner or adjacent lower lip Anterior ends of united inner Meckel's bars Commencement of optic lobes Supraorbital bars Pineal body ■ Rectus oc. ext. Rectus oc. sup. Cerebral lobes Olfactory cartilage Rectus oc. int. ' "J Q-- .\/\ Olfactory organ ^""-'-ix. ") Obliq. oc. inf. of outer eye *-» ~* Trabeculae cranii "" Palato-pterygoid cartil I An inner lower lip : ^ Anterior end of united inner hyomandibulars i Upper end of united inner Meckel's cartilages Inward dip of surface epithelium J. F. G. XIII I FJwfl • i«JnT . - "^^^ / — ^— — i \~ — - — "^ ^-^ I : II1X .O 1 I Fig. 47. — Transverse section through anterior part of head of double monster belonging to Class II., i.e. the type illustrated in PI. I. fig. 6 (union at medulla). Compare with the series begun in PL VIII. fig. 35 (union at optic lobes). The two adjacent eyes are cut through in their posterior halves, near the places of entrance of the optic nerves. The anterior ends of the cerebral lobes belonging to both heads are seen in oblique section, as are also various skeletal elements and ocular muscles. For plane of section see PL XXI. figs. 91-93, line 47. Other references are given under PL I. fig. 6. ( x 18.) Fir,. 48. — Transverse section from same series as last figure, but a short distance further back. Compare with PL III. fig. 13 (normal). The section passes through the posterior portions of the adjacent eyes, and through the extreme anterior part of the composite otocyst which represents the fused adjacent auditory organs of the twin heads. The outer eyes lie some distance behind the plane of the section, which is indicated by line 48 in PL XXI. tigs. 91-93. Other references are given under PL I. fig. 6. ( x 18.) PL. XIII. Fig. 49. — Transverse section from the same series as Figs. 47, 48, but through the outer eyes, the optic lobes, and the hyomanrlibular suspensorium on the inner or adjacent sides of the twin heads. Below, in the middle line, is the septum between the two mouth-openings, and in it the inner or adjacent Meckel's cartilages. The moulding of the adjacent united periotic capsules and of the otocysts is extremely interesting. See also the two following figures. Other references are given under PI. I. fig. 6, and the plane of section is indicated by line 49 in PI. XXI. figs. 91-93. ( x iS.) Fig. 50. —Transverse section from the same series as the last three figures, but through the posterior part of the optic lobes, the commencement of the iter, the inner hypoaria, the entrance of the optic nerve on one side, and the back part of the mesial oral septum containing the united adjacent hyoids. Other references under PI. I. fig. 6. (xi8.) Fig. 51. — Transverse section from same series as the last four figures, but through the outer auditory organs and the posterior part of the brain, which is obviously made up of two converging medullae. The notochord is double, and the inner parachordals have a small mesial mass of muscle wedged in between them. Compare this section with PI. IV. fig. 18 (normal), and with PI. IX. fig. 38 (union at optic lobes). Other references under PI. I. fig. 6, and plane of section in PI. XXI. figs. 91-93, line 5t. (xi8.) PL. XIV mOO .q»2 ,oo aujosS? i ibfll iBsn ii.-.'i jsanl ir.Ijsl ' i ■■'' / \ (9lizoqrnoo. ilohs1? 2-ifiIudibnsinoyd !ri30/.|b£ t> ebioyd >asOftib£ bojin'J agninoqo riJuom ov/j iA\ 9E • isnni 1o sIe-oro Tfili/o-n esdot oiJqO - j^lf^' "\-*"~' biogrislq-O! rr^Z&^-c^Zs^ / ~~~) "" — *^"T~~~ — — v? -O^ '1 rmwlad mujqoa yrfj rii asg/sliiiBo s'fojIosM Jn30£(b£ owl sr!T iljuoin ov/J 9ri) : : : I eogElilito lBbiod»ciBcI .... isIudibxu>iaoxH 2)00 •' V d-j OEjagBlincobioyH -•" viz . Semicircular canals of inner compressed double otocyst Optic lobes Periotic cartilage Supraorbital bar Infundibulum "■■-Rectus oc. sup. nner recti near their origins Rect. int. outer eye Rect. inf. outer eye Palato-pterygoid Trabeculae cranii Periotic cartilage (composite) United adjacent hyomandibulars United adjacent hyoids The two adjacent Meckel's cartilages in the septum between the two mouth openings Semicircular canals of inner compressed double octocyst Optic lobes ......The two utricles, here separate Iter _ -.-Supraorbital bar .... Rectus oc. sup. .V.. Cavity of hypoarium Floor of periotic capsule Trabeculae cranii Rect. inf. outer eye Palato-pterygoid United adjacent hyoids An outer Meckel's cartilage The two adjacent Meckel's cartilages in the septum between the two mouth openings. Mesial muscular mass Medulla (almost double) Parts of inner composite labyrinth • ; United inner jugular veins Periotic capsule ..."..- Parts of an outer labyrinth ... Ganglion, auditory nerve ... Parachordal cartilages Notochord Hyomandibular Upper end, visceral arch III (ist branchial) Aortic arch and roots Visceral arch IV Visceral arch V Copula arch V ""'-- Hyoid cartilage (an outer arch) Truncus arteriosus Irregular septum between the two mouth openings J. F. G. XIV Optic lobes 1 i optic lobes Hum Periotic capsule Semicircular canal —•Mesial muscular mass —Ventricle IV Dilated central canal of upper part of spinal cord Lateral muscles Neural arch cartilages section pas Pineal divertic. Optic lobes Cavity of optic lobes Iter Ventricle IV Mesial muscular mas* Medulla Periotic cartilage Semicircular canal Lateral muscles Notochords Mesial muscular ; cord (composite) I F. G. PL asdol jilqO aadol silqolo >i s I rrmlbdsisD sluaqEO oijoha^ [£fj£0 iR[trDiioroi33 e^fan 1J31U02rjJ VI 3bnln-;tV - - naqqu lo Ifinno urilnaa bslsliG- bioo Isniqalo jtr.q aabcii ■ 23gElrm:> rbiE l/;iu-.'/i trials of inner ed double ct .age im I rabeculae The t" the two mouth openings mals of inn — _^; '" Optic lobes helwoutri! .-Iter .^ .-Supraorbital bar :el's cartilage the septum between .aiti9vifa aodol DijqO asdol Dilqo lo - libjfviadeV ner jugular \ > iptoX III Aortic arch and roots Visceral arch IV pula arch V cartilage (an outer Tn. two mouth open :i VX J. F. G. . ' . 1 I xiv FlG. 52. — Horizontal section of double trout monstrosity belonging to the type illustrated in PI. II. figs. 7, 8 (union in pectoral region). Adjacent pectoral fins are not present. 1 he section passes through the optic lobe masses, the cerebellum, and the upper parts of the periotic cartilage? in either head, while, further Lack, the spinal cord is seen to have its central cavity much dilated and bifurcating into the two fourth ventricles. Compare with PI. V. fig. 21 (normal), and with PI. X. fig. 39 and PI. XII. fig. 43 (union at optic lobes). For other references see under PI. II. fig. 7. ( X20. ) I i'.. 53. Horizontal section belonging to the same series as Fig. 52, but at a slightly deeper level. The section passes through the cavities of the pineal bodies, of the optic lobes, the iter, and fourth ventricle. \- in Pis. X.-XII. figs. 39-46, owing to the curvature of Q, the trunk i-, cut through obliquely. The section here shows the composite spina! cord, the two outer series of mi j in ins, and a mesial similar series between the two notochords. I >r other references see under PI. II. fig. 7. ( x2o.) PL. XV. Fig. 54. — Horizontal section belonging to the same series as the last, but at a distinctly deeper level. Some of the gill-arch structures are seen in either head in front, and behind these certain chambers of the double heart, which lies inside a single large pericardial cavity. The alimentary canals are double down to the level of the stomach. Two bile ducts enter the first part of the composite intestine. For references see under PI. II. fig. 7. ( x 20. ) Fig. 55. — Horizontal section further back in the same series as the last. The section passes through the sharply curved portion of the body behind the place of union. The spinal cord is cut in two places ; its proximal section is drawn out transversely and is obviously composite, while the distal section is almost normal. Both regions have a pair of notochords. The mesial muscular mass between the notochords is well illustrated here, and is seen to be divided into a series of segments corresponding with the two outer series of muscular segments. The disposition of the adjacent neural arch cartilages is also indicated. References under PI. II. fig. 7. (X28.) PL. XVI. i'>qqu lo zsuttfl tit i rij-jyJlo 2im»g riliw //b( iswol \S Ii ■ ■■!;>■■ ■•• l«.vrlnq vJl Ill ,( ■•••■ i ' : / ■ ■ - rrioJBIsO • ..^ S "^ III ,11 I i/iiinT-v jUoY.... aJoubslid bll£ »u- ,.1 ... iiioLtnoli srlj hue. ; p Xfaod :i. ftfc //. __ eeustn tKlirjsoni fbuin ,bto? ab-r> i J.U.I •• iM ■'"" BSbeinn telsljsj abiorioolon ov/i srIT ■-'" "... bio > i. i q£ ivy .O .1 .1 . Soft tissues of upper jaw ■ Soft tissues of lower jaw with germs of teeth •'Glossohyal • Hvpohyal "'Copula' visceral arch III - Meckel's cartilage (visceral arch I) "Ceratohyal Meckel's caitilage Ceratohyal "Gills, I, II. Ill Truncus arteriosus Ventricle of heart Auricle of heart Inferior cardinal veins , Gill cover . Cartilage of pectoral fin Yolk Pancreatic tissue and bile ducts JE ) The two intestines just beyond the stomachs Single portion of intestine formed by junction ot the two former Wolffian body The two notochords Mesial muscular mass Lateral muscles ^4. — Horizontal section i - as the last. double Composite spinal cord, much drawn out transversely ■■' The two notochords ■•• Inner neural arch cartilages • Haemal arch cartilages - Aort.u :: m bodies Mesial muscular mass Lateral muscles The two notochords Spinal cord in th ly curved portion behind its proximal nd is obvio;.- hile the th regions ha i ichords is v. ;mei i J. F. G. PL. Xtfvh x4. IIVX 3TAJS mS>£ IfitnsBri bn« Isiion Id Inacaa^neitB IsDi'qvT— .jd .oil glduob 9dl moil noi)ianEi)lo noigsi srb je a9gEli)i£D rbi£ gnilididy.9 igiznoiri slduob e ni .noijibnoo glgnia arfJ oJ .aaibod niv/j grf) lo noinu I .q yril no bsDfilq A ni e33«) sluaqao Dilonaq lo TOoft nO .(sluaqBO ailoiisq ln$u;{ gniaajsq aslDaum 11337 Ismsl/s dliy/ 33Bqa yiEliulil )i dguoiril Ifiiusn JnsoiibK to isnni ,VA) sgEliliBs doir. Ieius'/I .(sgaliliBD dOTB .biorisoloVl .3gBlinB3 iBlnaqBD vtoIdbIIo lo nsq loofl loidTOBiqUi InsDEibB to tw. 'uid-ioBiqua .{nnd .bioj iBniqg alfiaqr 1 oijoiisq lo sliruO .nimnJ -3133 brie sbhin-jv Midi MiJ -jvoclj; zUgiljrtEo iBnimgsT qasi mulhxl A .A .fcV. .foVi •xij 'jvoac 23g«un/ iBludibnKfnoyrl bri£ Ifiiocn 1 -d? •■'il 19)1e ajl93v, zip. juodt .oyidnre juoi: kirno/rslo noJgbJa bgnirnE/.g agiliaoilanom oil) E£ 931; oin^.to bnr. .-niri^jnd svodfi moi\ )s bsjloot sd ol btjaoqqu? 31c ^aiujjmJa aril" -gniaasq noijose r, yd asgBWJiflo looi 3d} lo Icvorrm isflc .esgEliJiBD oijonaq bnr, Uzcn 9fl) dguoid) yllfijnosiiorf 99« 233n9i9l3i isrijo ioI btw ,M q 998 noijqiiaagb 10^ .1 .^rt .1 .11 igbnu jlfirmon £ lo llu>Ie grf) lo z9g£liJi£D looi 9riT— .\i .ail '0) egongialg^ .9/od£ moil 3e bgilool .oyidmg Juoi) .d{ .gi^ i9bnu 2B enoilEDeulli bn£ noijqhoegb liiludibnemoyrl bnj; IfiinBiD lo nifiigjsifj -- .85 .oil iifi(J) gqy) 9rl) lo .oyidmg i9)enorn gldoob £ lo nojglade niw) 9d)lo aoinu dliw .•>.« ,f ,^ ^yrt .1 .I'l ni bgjr.Daulli (.1 e9iuJoui)2 9d) ,dz .gi'-l ni aA .eadoJ nilqo 9rii )£ enicid b;vom9T i9il); »vod£ mod i£ bsJooi ad o) b9i-oqqoa 9i£ yllfiJnosiiod grigezsti id roai;Iili£D lorn 9d) lo '<)bi[qub 9flT .c95?t,Itnr,-j ,.iJoii9q bnfi Icenn 9fi) riguoiril •P? -^'^ oalc 98c; .ylno asiujornJa iori9)ri£ ajogUfi .1 .1*1 i9bnw a9Dfi939l9i ]3[]jo bin- .,'1 .q no noilqiioagQ ■*■ gn 9rf) lo aggiilij-tE > looi 9d) jjniwoda rri£7gi>i< 'I— .1 ■? ..ni .9-iujjft anib939Tq ad) ni b9)£i)ao!ii i9)*nom 9lduob -oyri btiE. IfiinEio gdl ani)ai)aulli mEiafiia— .od .Oil o) gnignotod i9)anorn glduob s ni nolalgjJa lEludibnErn Ellnbgm gd) )£ anieid turn 9d) lo noinu djhw .v'« ..II aar.13 9ri) lo Uvoai9i I9)l£ 9vod£ moil ai wsiv 9dT .£}E§noIdo 998 noilqiioagb ioM .3? ,dj .a3i"? ni as a9§£iin£3 looi .0 yn .1 .11 a93n9i9l9i i9d)o id boe ,di .q -ijir.o looi 9d) 9/odE rnoil gniwode m£ig£iCI— .id ..'H'-i ni b91£i)aulli i9)anotn glduob 9dl lo llujla 9dj lo aggfil .od .31"? i9bnu 99a agongialgi Ifiigngg io"? .od .gi"? aggEfiliED Aytn Icmgcrl bnr, Icwd/iIo rnEigEiCI — sd .oil .gnidalr.d i9)1e 2^997/ iia .oyidrng luoi) (crmon £ ni 1:1 .q 99c! IfrKrt ^). PLATE XVII FIG. 56. — Diagram of cranial and hyomandibular skeleton of a normal trout embryo, about six weeks after hatching, and of same age as the monstrosities examined. The structures are supposed to be looked at from above after removal of the roof cartilages by a section passing horizontally through the nasal and periotic cartilages. For description see p. 11, and for other references see under PI. I. fig. 1. Fig. 57. — The roof cartilages of the skull of a normal trout embryo, looked at from above. References to description and illustrations as under Fig. 56. Fig. 58. — Diagram of cranial and hyomandibular skeleton of a double monster embryo, of the type (Class I.) illustrated in PI. I. figs. 4, 5, i.e. with union of the twin brains at the optic lobes. As in Fig. 56, the structures are supposed to be looked at from above after removal of the roof cartilages by a section passing horizontally through the nasal and periotic cartilages. The duplicity affects anterior structures only. See also Fig. 59. Description on p. 13, and other references under PI. I fig. 4- Fig. 59. — Diagram showing the roof cartilages of the double monster illustrated in the preceding figure. Fig. 60. — Diagram illustrating the cranial and hyo- mandibular skeleton in a double monster belonging to Class II., i.e. with union of the twin brains at the medulla oblongata. The view is from above after removal of the roof cartilages as in Figs. 56, 58. For description see p. 16, and for other references PI. 1. fig. 6. FlG. 61. — Diagram showing from above the roof carti- lages of the skull of the double monster illustrated in Fig. 60. For general references see under Fig. 60. FlG. 62. — Diagram of neural and haemal arch cartilages in a normal trout embryo, six weeks after hatching. See p. 12. 1: bh. FlG. 63. — Typical arrangement of neural and haemal arch cartilages at the region of transition from the double to the single condition, in a double monster exhibiting union of the twin bodies. See p. 13. Lettering in Pi. XVII. Figs. 56-63. a. Olfactory capsular cartilage. Trabeculae cranii. Hypohyal cartilage. (The glossohyal is omitted ; see PI. XVIII. fig. 64.) Parachordal cartilages. Is placed on the trabeculae cranii just in front of the pituitary space in Kig. 56, or the two pituitary spaces in Figs. 58, 60. Is placed on the parachordals just behind the pituitary space in Fig. 56, or the two pituitary spaces in Figs. 58, 60. Palato-pterygoid bar (/', inner or adjacent palato- pterygoid bar). Anterior, middle, and posterior fontanelles. Meckel's cartilage of loyyer jaw (g' inner or adjacent Meckel's cartilage of lower jaw). Hyoid bar. Haemal arch cartilage [/m' inner or adjacent haemal arch cartilage). Hyomandibular (;', inner or adjacent hyomandibular). On floor of periotic capsule (£', on floor of inner or ad- jacent periotic capsule). Pituitary space with external recti muscles passing through it. Neural arch cartilage (Na\ inner or adjacent neural arch cartilage). Notochord. Roof part of olfactory capsular cartilage. Supraorbital bars (/. inner or adjacent supraorbital bars). Spinal cord. Cartilage of roof of periotic capsule. Laminar part of parachordals. Tegminal cartilages above the third ventricle and cere- bellum respectively. A.A.A g- h. ha. Na. Xch. spc. t. w. xy. Sp.c. M Na 62. I. 1 . G. PL. XVII. Meek VII 64- Scap- 66 AChor AChor.' A a. , A Chor Psbr A a. 67 CF.B . C 3 'J V CF.B. C F B 70 COL J. F. G. PL. XVIII. PLATE XVIII Fir.. 64.— Ventral ends of the Meckelian, hyoid, and succeeding visceral arch cartilages of a double monster belonging to Class I. For general references see under PI. I. fig. 4. Lettering as in the succeeding figure. Fig. 65.— Ventral ends of the Meckelian, hyoid, and succeeding visceral arch cartilages of a double monster belonging to Class II. For general references see under PL II. fig. 6. Meek. Outer Meckelian bars. Meek'. Inner or adjacent Meckelian bars united and greatly reduced. C,H. Glossohyal. BH. Hypohyal. LH. Ceratohyal. BH'. Inner or adjacent hypolvyals paitly united: LH'. ,, ,, ceratobyals united. Ill, IV, V, VI, VII. The branchial cartilages. Cop. Copular pieces, the anterior one being bifid. Fig. 66. — Transverse section of skeleton of composite pectoral fin from a double monster belonging to Class IV., in which union of the twin bodies took place just behind the pectoral fins. Description on p. 19. Scap.. Cor. Scapular and coracoid parts respectively of the coraco- scapular bars. The ventral ends of the coracoids are united. ' /.. The limb-cartilages united for the greater part of their length. R. The rays (not cartilaginous). FIG. 67. — Diagram illustrating the dorsal aorta and its roots in the double monster belonging to Class I., from which PI. XVIII. fig. 64 was taken. For references see under PL I. fig. 4. Act. Aorta, the two limbs uniting further back. II, I, III, IV. Aortic roots, i.e. branchial veins. Car. Carotids. Aa. Artery to pseudobranch, a branch of the hyoid artery. Psir. Pseudobranch. A Char. Choroidal artery. A.Chor'. The arteries for the choroidal glands of the inner or adjacent pair of eyes. They arise by a single stem from the middle of a vessel connecting the aortic roots on either side, and go to the choroidal glands without passing through a pseudobranch. FlG. 68.— Diagram illustrating the ventral aorta, the dorsal aorta, and the aortic roots in the double monster belonging to Class II., from which )'!. XVII. fig. 60 was taken. For references see under PI. II. fig. 6. Letter- ing as in Fig. 67 with additional : I A. Ventral aorta and its branches 1, 2, 3. 4. ' ar>. Inner or adjacent carotids, giving off Aa'. arteries to the inner or adjacent pseudobranchs Psbr1. It will be seen that there are two sets of carotid and afferent pseudobranch arteries, the inner sets being derived directly from the ventral aorta. The ventral aorta arches dorsally in the septum between the two mouths of the monstrosity, reaches the base of the skull, and then divides into two limbs which are continued backwards to join the aortic-collecting roots on either side. Fir,. 69.— Outline of central cavity of brain of normal trout embryo. Lettering as in Fig. 71. For references see under PI. I. fig. 1. Fig. 70.— Outline of central cavity of brain of the double monster (Class I.) illustrated in PI. XVII. fig. 58. Letter- ing as in Fig. 71. For references see under PI. I. fig. 4. PlG. 7'— Outline of central cavity of brain in the double monster (Class I.) illustrated in PI. XVII. fig. 60. For references see under PI. II. fig. 6. C.F.B. Cavity of the hemispher< Cyd V. ,, 3rd ventricle. COL. ,, optic lobes. '-,l- .. Her a tertio ad quartum ventriculum. C^thV. ,, 4th ventricle. Spc. ,. spinal cord. FIG. 72.— Outline of transverse section of central cavity (in anterior part of optic lobe region of the brain illustrated in Fig. 70 (r/.T-.). There are two infundibula and two sets of hypoarial cavities, while the main optic lobe cavity is single. ' OL. Cavity of optic lobes. Inf. Infundibulum. Hyp. Hypophys Hya. 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H ..:!,..., il 1o 3llO .aiV'. 9 l6iin»V- bn£ ,bioY<"I ,nfiil3}Io3M srfj to abns iKiJnsV — .^.d .Oil isJanom slduob b to aagfiliJiBD daiB Ifiisoaiv gnib33D3ua isbnu ssa asonsistsi iBisnsg lol .1 aafilD o) gnignobd .siugrl gnib333Due sri) ni aB gniiausJ .(. .git .1 .11 bnfi ,bioi(d .nfiilsjljal^ sdl to abns lEiJnaV — .jd .Oil islanom slduob b to asgEliliED dais IfiisDaiv gnibssDDua isbnu 93a aa^naistsi Uisnsg io'-I .II aasD ol gnignobd .d.git .II .11 i neiteAoM ialuO .4«Vi. nu iiBd nfiils>l'j3l/: In^Dsibfi io isnnl .'i.AI. .baoubai .l£"(rioaao[0 .\\i> .If.vrioqxH Ma .[£(lloI£13D .WA './iloqvrl I(i3-j£ibr> io ismil .'V\a .ivjiinu dl/.^rioJBTj-j ., ,, .VWA Itn&o laiifenljTd 9dT .IIV ,IV ,V ,71 ,111 il 3iio loralns 'jril .aaosiq xefuqoD A.i ) sliaoqnioo to nolsbjla to noilosa saigvariEiT — .do . '/I BEfilD ol gnignolsdislsnorn sldnob b moit nit Ifiiologq bnifl9d )au^ 3 >Blq -Jool asibod niv/1 sril to noinu tbidw ni .91 .q no noilqioaaG ea& kiomq sdl ■OOfiiOD sdl 10 1 i.q biojr.100 biimfilnqK-jS ,-W.l ,.V>V. abiooBio.. jril 'to d nsluqEoa , .btilinu 9tb ibdJ to nr,q 13IE91, il ariT .dlgnal iitKiUr^i.Ulir.j Ion) svbi 'j((T ali bnfi eIioe Icaiob sdl gnilfiilaulli mfiigfiia — s.d .Oil moit ,.I aafil'J ol gnignobd islanom glduob aAi ni alooi 99a 39Dn3i9t9i ioI .n9>(61 zb'ii j>d .git .IIIVX .14 doidw 4 .git .1 .11 igbriu .jlotd isriliut gnilinu edmil out sd) ,£!io/. . il .2013/ UiriodBid .t.'\ ,51001 ainoA. .'/I .III .1 ,11 ebiJoifiD .11O donfiid b .doiiBidobusaq oJ -(isnA .rijHBidobirasI Its (BbioiodO iBlg Ifibioiorio 3rd iot eaiislTB ariT .10 lisq ln33B|bt 210015ilTOB3f! 1 , Juoriliy abrifilg Ifibioiodo sri) 01 05 bus ,3bi? r.dli3 no .dollBldobueaq 1 f\ :^r.q . \' I .V>L .aK 75 XIX HTAJ4 fans loub afifAloW sdi gniJfiiJeuIIi msTgfiiU — .d\ .Oil ojjaJ j: Im9 jutnJ Ifimioas Ini isbbfild viiriiuj I I'l isbnu 095 ssonsiglgi -I ni es gni toln-j 9rlj fnJtfiiJauILi -niriL. -- -.>H Jrwisftib orl) o) [ffftlo// 9ilj 3riJ Jsd ii rc-u .djq no badi-pgb agq<i Wolffian ducts. 1 1 'D, Wolffian ducts. II /''■'. Wolffian ducts belonging to one of the tw.i. of the figures. WD,!'. Wolffian ducts belonging to the other twin 11 1 I of the figures. Int. Intestine. /. . Re mm. /'. Vent. P. Urinary pore. 75 / fy.c. so WO a. 77 WDa I. F. G. PL. XIX. A So. P5c. WD HK WO 84. 87. 86. J. F. G. PL. XX. PLATE XX Fig. 82. — Lateral view of the composite auditors sac present in the angle between the twin heads of a double monster belonging to Class II.. i.e. with union of the twin brains at the medulla oblongata. Description on p. 17, and other references under PI. I. fig. 6. I tr. ft a.sc. An anteriorly placed semicircular canal. p. !C. A posteriorly , . >"]<;. S3. — Diagram of head-kidney of 'a normal trout embryo (p. \z). (',L. Glomerular tuft of vessels. I \fferent in V. Efferent yi ssel. apsule of glomerulus. I i>. Commencer :ian duct JS-7 HK. Coils of Wolffian duct in head-kidney. F)G. 84. — Composite head-kidney in a double mon- strosity, of the type described on p. 14. Lettering as in ■ . Two glomerular tufts are present in a cavity ded into three chambers. 0. Mesial chamber. FIG. 85. — Head-kidney in a double monstrosity of same general type as last, but here the glomeruli have not united er, though the adjacent halves of each give origin to no Wolffian ducts or only to a short blind one. Lettering as in Fig. 82. Figs. 86 and 87. —Head-kidney glomeruli of other two double monstrosities showing a degree of-duplicity greater than that illustrated in Fig. 85. A blind sacculated tube conies off fiord the middle compartment of the composite glomerulus and represents united adjacent Wolffian ducts. XX 3TAJ4 -nom alduob e ni ^e'abijJ-bfiad t>; , .(Jj ..ii-l ni be gniiaJJsJ I >] : srti to .Yliaoita <; ni )n^3K| 9'if> altul isluia .aSsld nli oini babivib .0 arriEe to v)i<(.ruanoui sfduob r, /: bgjinu Ion 9/Brl li -id) 913d jud ,Jasl 2£ aqvi (fi'iansg 01 ni- 1 asvlfid JnsoE^bB arfl rfguoriJ ,i9ri gnhattsJ .ano fjnflcf ttoda e 01 <(Ino 10 sl^ub nerttloV/ on ni se ov/J od)o to ilimrnolji o dqubto aj-iygb B gniwoda asiJraoiJenom si sdut baJfiluDDEa bnild A .j8 .^i"i ni bslinieuHi tfirfl m.di 9)iaoqrno3 ad) to JnarnliEqmoo albbim 3d) rnoittto esmoD .aJ mb nfifnlo'// )ri90E(bB bajinrj alngasiqgi briE auluisrnofjj yrtgnolad istanom sm sd) IE enifiid .lOJtbuE ajiaoqrnoo sd) to //9r Hei3)eJ — s.'i . >[ I alduob £ to ebi;od cutis sd) nsa >■ ne 3d) ni jnaaaiq niv/i ad) to ,^I .q no noj^q '.ft .1 .IS tsj 11913'! 13((l' feoq Juoii -bead to ,, .(ii .q) ovicltna flirt ihlu-rernoIO .19283/ InSlsflA .' \ ismorg to ••[' .WII loub nsfflloWto ir.^iniotiammoO .CI' 11 .vsnbijl-bfisrf ni s ,'AVi 87. PL. XX. Fig. ■ i. Fig. 90 Figs. 88, 89. — Reconstruction diagrams of the cranial cartilaginous skeleton in a double monster belonging to Cla I , . with union in the region of the optic lobes. Fig. 88 is a view from above of the skeletal parts after removal of the rool cartilages b} a section passing horizontally through the olfi and the auditory capsular cartilages. Fig. 88 also includes the mandibular and hyoid arches, which for diagrammatic purposes an n ted as wider than normal. The transverse lines numbered 35, 36,37, 38, indicate respectively the planes and levels of the various trans' tions shown in Pis. VIII., IX., figs. 35, 36, 37, 38. Fig. 89 represents the roof parts of the skull with transverse lines having the same meaning a n Fig. 88 (see < planatton under PI. XVII. fig. 59). The general references to (.'lass I. will be found under PI. I. rig. 4. ( xapprox. 15.) FlG. 90. — Diagrammatic outline of the central cavities of the brain in the same double monster as that from which Figs. 88 and 89 were taken. The transverse lini nave the ^.mie meaning as in these figures. The large composite cavity of the mid-brain (optic lobes) is seen to lead, in front, into the 1 nines of the two fore!. rains, and, behind, into that of the single medulla I see under PI. XVI 1 1, tig 70). 47 48 40 - - IG. 92 Fig. .1 Fit 93- FIGS. 91-93. —Diagrams ol the cartilaginous cranial skeleton and ol the brain cavities in .1 double monster belonging to Class II., i.e. with union in tin- region of the medulla. The numbered transverse lines are for the same purpose, and have the sane in. ig as in PI. XXI. tigs. 88-90. The genei il references to I b- II. are given und. ill: ■ ... See also under PI. XVII. tig. 61, XVIII. fig. 71. ( ■ approx. 1; I olfactory capsular cartilage, trabeculae 1 racl ordal cat tilages. m trabei ulae in from 1 J the tv 0 pituitai j pai 11 parachordals behind the pituitary spaces and the tendon of rect. oc. ext. I.I I n KIM. IN All. I III FlGl Rl S. 1 r kk' il il ■ out. r uid inner palat. 1 ptei hyomandibulai period, '.hi in the two pituitary outer and innei Meckel's l»ars. ,. hyoid ,11 I. F. G. PL. XXI. Jh, Ijjv v*>lnu.iUl l*lr]<> (Lit SiC 1 ■,-'.: (rinaj-J Mlrjo lA-r-1 MctL t\ iK-trj' A ..... '. ■ •- FlG. 95. — Diagram of alimentary canals of the triple monster illustrated above. The whole canal of the principal embryo is quite separate from the canals of the other two, and has its own air-duct, air-sac, liver and bile-duct. The two defective embryos have the anus and last part of the intestine in common, but their gullets, stomachs, first portions of intestine, and liver out- growths are separate. Other references are given under Fig. 94 ■ n J,*'t t - - i ■ Fig. 96.— Diagram of the urinary apparatus in the triple monster. The head-kidneys and, for the greater part of their length, the Wolffian ducts are separate, but there is only a single bladder and urinary pore. The left duct of the principal embryo, and the'right duct of embryo A, unite before opening into this bladder, as also do the left duct of A, and the right duct of />'. The left duct of B ends blindly. Other references are given under Fig. 94. J. F. G. PL. XXII. Lateral muscle mass of principal embryo Pelvic fin ,, ,, Notochord ,, ,» ' Spinal cord „ ,, ' Intestine „ M - Head-kidney of embryo •/ Brain tissue ,, -^ "^ Pectoral fin (left) ,, ~'\\.- Oesophagus ,, " Air-duct ,, The common yolk mass - Right pectoral fin of embryo B ,i|-^\ , Oesophagus ,, - . Head-kidney „ Liver tissue ., "(y-r Spinal cord ,, i Medulla ,. OtOCySts „ Pectoral fin (left) Lateral muscle mas ; of prim pal < mbryo Spinal cord ...Notochord Wolffian ducts Intestine ,, ,, Common yolk mass Left pelvic fin of principal embryo Hepatic tissue of embryo B Hepatic tissue of embryo A Stomach of embryo B Air- ,, Wolffian ducts ,, -■ Spinal cord ,, Medulla Stomach of embryo .-/ Air-sac , , Medulla Left pectoral fin of embryo A Lateral muscle mass of principal embryo Spinal cord „ „ ) J. \ ""~' — // Aorta ,, ...-_ ....(....., W i I Wolffian body ,, ,, .- \' " /}T>) Intestine ,, ,, \. j \5-/" Common yolk mass -i" > ~'~ Intestine of embryo A (j Wy^"^) ''""v- ...... (i yC\;, Commencement of the portion of intestine / t/'"|V 7 \-r , ^ // common to A and B I ( ' " "I \v - Bodies of 7 and B uml \ / 0 j. r. o. XXIll O^ttSias leqianhq to agfim sbaum IbtcjeJ it it ntt ohhl « ,, bioriooJoW ,, ,, bioo Inniqg " ■, „ anilealnl K o/idrns to <(9nbi>I-bi;9H suai] nisia ,, (itol) n3 IeioJosI EUgfiriqoegO ti Joub-iiA 2f£m illoy (joramoD 9dT a Ofidrngto nft Isiolagq irigifl ,, augisrfqcMaO „ x3nb'>'-bB3H .. straail 19/iJ blew lisniqa BIIub9l/ • . 5I2YDOIO . il->l) m1 Ij.ioJosH oyidmn [fiqionhq to seun sbsom Ui9)jsJ ,, bioo [jsniqS biorfooioK--. 9ni)a9)nl aajsrrt jftov nornmo'J [fiqioniiq to nil Divbq 1I9J a ox'idrns to 9uaait oilr.qaH K ovidrne to suaaii Air.q-iH to h ksmolS ., h IO0 k O. [I ,, K ovidms to nft Ijiiomq itoJ to 2d/.m sfozum li -^ Q} /■■ ,, bio:> I biorboJoK bJtoA / — .. ., /hod ncrftloW ., -iniissjnl - ?-i.m jKov nom /'T'x f /^? i It V. oipdnreto sniJaslnl y#.| • * •• 1 a bflK K o) nomrno-j — - ~~~ 3 St-H i ■ idoH IIIXX PL. XXII. .0 . i .! 4* 't 1 ection through the ! i illustrated in the three pre- Lcrossthe middle of i the ] pal embryo, ■ . ion, and lin, Ml three ; x '" ■ ■ eparate, egion of '' ' a well- developed head-kid;: princip es some 'i .tarn ■ 0 eferences i" given under Fig, 94, 1 ■ 10.) Fig, 98.— 1 ransverse section belonging to the same sei ies ,1- the last figure, but .1 little further back. The body of embryo . / is taken up chiefly with the nerve tissue ol the medulla and spinal cord, while B don 01 ih" tiny fourth venti icle and medulla. Below the bodies of A and /■' are seen the Wolffian ducts, air-sai , stomachs, and the hepatic tissue In l< ing to these embryos. A pelvic fin of the principal embryo comes into the section on its left side. Other references are given undei I ig. 94. ( ■ 14-) FlG. 99. — Trans vi 1 -■■ ■-.. . don from same sei ies as the la t two figures, but a con- siderable distance further back. The of the two defective embryo now uniting, and they are seen to be made up ol very irregular tissues, the spinal cords being recognisable. I hi intestinal can ah ol thi are joining together, the pai t seen the middle being the commencement of tti<- ci minion terminal portion. References 94. 1 ■ 17.) PL. XXIII. 100. 102 104 103. Ro- 106. J. F. G. PL. XXIV PLATE XXIV FlG. ioo. — Cyclopic trout embryo of type A (p. 40), seen from left *ide and showing the large single eye overarched by the mesial frontal process. FlG. ioi. — Transverse section of the frontal process referred to above, showing the small approximated olfactory pits (o), and the reduced olfactory capsular cartilage (C). Fir.. 102. -Transverse section through the posterior parts of the cerebral lobes in the same specimen, showing the pineal diverticulum (Pin.), and the cerebral lobes (F.B.), the latter being intimately united along their inner surfaces. The central cavity of the brain (C.F.B.) is not markedly dropsical, and the cerebral lobes are almost normal in their transverse measurements (p. 41). Fin. 103.— Section through mid-brain of a cyclopic trout embryo belonging to type B (p. 41), showing almost complete obliteration of the mid-ventral groove which should lead downwards into the infundibulum. Infundi- bulum and hypophysis are absent. Fig. 104. — Section through back part of retina of cyclopic eye (type B) described on p. 42, showing failure of development on the part of optic nerve and choroidal fissure. Pg. . . . hexagonal pigment layer ; Ret. . . . retina ; n ... a few nerve fibres not perforating the pigment layer. Fig. 105. — Transverse section of head of trout embryo with supernumerary eye (see p. 59). L. R. R.O. N. C. Tr. PL. S.o. !:■. I -ens of supernumerary eye. Retina ,, ,, Right noi trial eye. Right optic nerve receiving fibres from retina of super. numerary eye. Central cavity of brain with two deep grooves in its floor, each of which leads downwards into an infundibulum and a hypophysis. Trabeculae cranii displaced towards left. t'alatoquadrate bar on left side ; the right bar is absent. Supra-orbital liar. Anterior corner of auditory capsular cartilage. Fig. 106. — Horizontal section of head of trout embryo with supernumerary eye (see p. 60). J L. Lens lielongiog to the left (aborted) twin head. CL'. Cerebral lobes belonging to ditto. 3rd V. 3rd ventricle belonging to ditto. C. Cranial cartilages belonging to ditto. O. Functional left eye. CL. Functional cerebral lobes. CO, Cavity of optic lobes. OL. Optic lobes. Au. Auditory cartilage. •V \ \ VI XX aTAjq oyidmg looi) lo bBarilo noilsaa aeiavanEiT — .501 .Ol"5 .(qj q aaa) ays yiBiamumaqug riliw .ays yunsrnurmqua lo anaJ ,, BnilaK tannon idgrfi -nsqua lo £nil9i moil 29idft gnivi9D9i avian oilqo Irlgifl .979 V*l£19inijn ,iooft ali ni eavooig q99b ov/1 dliw nitnd lo ylivEo IfiDnsO rnuliidibmilni ns olni abiEwnwob ab£9l doidw lo ria£9 .aiavriqoqyd £ bnB .ll9l 8blJT»OT b99£^ib iiri£19 9£(U99d£lT .Ingedfi ai ifid idgh 9ril ; 9biz jlaf no isd aiEibfiup-olBlfiS .tfid l£lidio-£iqu3 .9g£liJT£j ifiluaqeo yiolibii£ ^° I9m09 lonalnA .A A .O.i M 3 A*. oyidma JuoiJ lo bsarf lo noilDsa IfilnoshoH — .doi .Oil .(od .q 992) aya yiBiarnumaque riliw .bfiarf a'f«y (baiiodEf Jl5f »dl-ol gnignahx) 2n»J .A .ollib o) jjnignolad asdol iBidaiaO .'AO .ollib ol gnignotsd gbnlnsv bi£ 7A Vt\, .ollib oj gnignofacV aagEliliBo (eiiunO . ) DV9 llsl fMK'iDnul .0 .asdol Ieid9i99 fjinoilanuT .A^> .89.161 aijqo lo yiivED ■cfol oilqO ,A0 .ajjnliliK'j viuiibn/. .uK ,(oji .q) A aqyl lo oyidma luoi) aiqobyD — .001 .oil aya algnia agifil adl gmworii bne abia llal moil naaa .88930iq IfiJnoit Ifiiaam griJ yd barfoifiiavo 889DOiq klnoi! aril lo noilDag aaiavsnfiiT^-.ioi .Oil balBmixoiqqfi Hem? 9ri) gniworfa ,avodE ol banalai lEluaqe.) yiolofillo baoubai 9ril brjB ,(0) eliq yioloElIo f e .p) 98BiinB3 lohalaoq aril riguoiril noilD98 aaia/anBiT— .£01 .oil gni-worfa .namioaqa amfie aril ni 8adoI Uidaiaa aril lo 8lifiq eadol Isidaiaa aril bnr.^ittVY) rnuluDilia/ib leaniq aril liadl ynoU balinu ylalEmilni gniad iallr.1 aril ,(!>\r\) (.a.."^."^) nicid aril lo yiivso Uilnaa ariT .eaafiliue lanni 9iB aadol Uidaiao aril bne rissiaqoib ylbajliBm Ion ei .(iji .q) alnamaiuasam aaiavanBil liari] ni iBrmon laomls Diqobyo e lo niBid-bim rfguoirfl noilDa?.— .£01 .oil laomlB gniworia ,(>)> .q) 3 aqyJ ol gn'Snol9d oyidma Juoil rioiriv/ gvooig lEilnav-bim aril lo noiiBialildo alalqmoD -ibnulnl .muludibnulni sell olni ebiBv/irwob bsal bluoria adfi aifi eieyriqoqyri bn£ mulud \o Bnil9i 1o li£q J^ixl riguoiril noilD98— +01 .Ol"? aiulifi"! gniworfa fit jq no bsdir^ab (!I aqyl) 9yg aiqobyo Ifibioiorfo bnr. avian Dilqo lo lueq aril rto'lnamqolavab lo . . . .\s*<\ ; i9■— — * ( Recti i ^- Qf\ Tissue of upper jaw with \ 1... anterior ends of fuy " yJ Palato-quadrates and . Trabeculae brand' Optic lobes I '.v\ it) ol optic lobes Infundihulum. . . . Supraorbital bar Recti oc. sup Pigment layer (continuous)...... Retinae Lenses,... Optic lobes Cavity of optic lolies Her (beginning) Periotic cartilage Supraorbital bar Semicircular canal Infundibuluni . ■ Recti oc. sup. Retinae Optic nerve Choroidal fissures Lenses Recti inferior I. K. G. XXV , OYidrn. injjlusiltevib ir.inin .... inulusimvib Iehbo euoouM III sbrdosV ^^doI [sidsia .qua .DO JJj&H Hi: min JiJqO .lni .to i!3itf to abns»V baa asWihaup-oMlsl, SfilO i » : lo finiln lo JSStt.1'.- 1332 .3fllS 'yj.hlnrj IfinirnggT to ^)ivj33 IffllasO bsmitef Ho .. ... , bmfl -'■"'• .(JJJd .30 IJJJM ■I I Mo irle asdol jilqO d-xlol silqo 1o (JrvjsD (gninnig jd) ^«\ jgcliJxtJ H xsd InJidiOiriquri lnnB3 uiluoibimsr! rnjjluJibiinl.il .qOE .30 ijDvH 9£niJ •>! 0/13JI jilqO asmaatl UbiotoiK > \\ f/ ~~n^^o TOiralni iioaJl asdol Jrjql 1 ssdoJ utqu la <(ji/»'> inuludibniilnl if,d Islidrocr ..qua .3o ti '■'>! -•ij;uij3M ...aaen^J vy.x „> Figs. 107 and 108.— Sections through 1 1 a cyclopii trout embryo, the former through the middle of the median eyi and its lens, and the latter through the entranci ol the single optic nerve, This nerve is -een to divide at onci inl branches for the two partly separate retinae. The fused condition ol the cerebral lobes will be noted, particularly in f ig 01 details of structure see pp. 4042. Compare also with the two succeeding figures, and with PI. XXVI. fig. tit. ( Figs. 109 and 1 to. ["ran 1 1 ection ol [he head of a trout embryo showing the condition ol semi-cyclopia I 1 eyes, but they are close together, and the optii nervi is single, although there are two choroidal fissures, rhe internal recti are absent. A* in typical 1 yi lopia, the 1 ne between the bas 1 the brain and the trabeculae cranii, the tnterioi ends of which are separated by a wid< gap from th< rud mtary olfactory capsular 1 rhe latter are nol shown in this section. Compare with the two preced and with PI. XXVI. fig. in. See also p. 44. I -i.> PL. XXV. Fig. hi— Section through head of a semi-cyclopic trout embryo, from same series as PI. XXV. figs. 109. 110, but further back in the series. The section pas 1 almost horizontally along the trabeculae cranii, which here form a single mesial rod. See pp. 40, 44. Anteriorly, the trabeculae are seen to articulate with the palato-pterygoids; behind them the two external recti of the eye are passing through the pituitary space in company with the two choroidal arteries. The head is somewhat compressed in the s.igut.il plane. ( x 24. ) Fig. 113. — Transverse section of bod) of trout embryo just behind the pectoral tins, showing iocal absence of the noto- chord. Theadjacent neural and haemal arch cartilages fuse together to form a series of half rings below the spinal cord. The lateral muscles meet in a mesial raphe above the dorsal aorta and below the spinal cord. See p. 54. ( ■ 20 ) FlG. 112. — Transverse section through atrophic head of an embryo trout, in which the lenses alone of al> the eye struc- tures have survived. Even the mouth, the Meckel's cartilages, the trabeculae, and the palato-pterygoids are absent. See pp. 60, 61. ( x 16.) FlG. 114 — Transverse section of double monster belonging to the type described under Class V. on pp. 20-21. The section passes across the middle of the bodv. and shows the twin sets of structures coming together t" unite in lateral union. The intestinal canals are here separate, as also are the livers, air-sacs, Wolffian bodies, etc. (x 16.) PL. XXVI. bio j I' akj8 • . ../ ^V 9311 u-jW .. / fyfu^ woladsgBfiftsolozgnh'ilEH / . '.'\^~f7 ,, bnoo limiqe sril BJioA- '(bod nfirftlo*// MriMOl.. .-V-/ g^i) ■ in-./ ihni i ■ elkrl \_'-y\S L. . 0 H abiog7TJli|-o]i.[i.'l IsklSBOm I/.1911.I iimnibti srfT gahinu ?\>ze£m zb' ?bioil5oJori i/.ifil.i'// owl 3dT ilj;(iE3 tnnuaolni ow) 9(1T zisvil OV/J DrfT mo sriT .. agslinso li.imnii- ■ 1 -■• esdol Ij.t* ... 3ozei) wfuosuM BOsJ l/X/ ... .-I .1 Cerebellum ... Ventricle IV ti Periotic capsule Parts of labyrinth -.•"/'"/'/' C 'I [ Medulla . r ' V\ "^ Ganglion nerve V..„.J — -VlJ Recti, oc. ext. and choroidal j arteries Hyomandibular ... A Trabeculae cr. Palato-pterygoids. ough the pil i -^ \ ... Spinal cord Cf~JT\k ' Neural archcartilage ^rfj. S\ Half rings of cartilage below the spinal cord . Muscular raphe •Aorta • Wolffian body !/ hA "\--/ Intestine I he lati ral aorta and > Tegminal cartilage Cerebral lobes Muscular tissue Lens The adjacent lateral muscular masses uniting The two spinal cords The two notochords The two Wolffian bodies The two intestinal canals The two livers The common yolk mass h the lenses alone of all tl tures have survive the Mecke and the palato-] ... ring to J. F. G. PL. XXVI. XXVI