BIOLOGY
LIBRARY

EX LIBRIS.

Bertram C. a.

1LILJB., IB.Sc., S.S.©., JF.».«.

the Auihors Compliments.

THE TERATOLOGY OF FISHES

PUBLISHED BY

JAMES MACLEHOSE AND SONS, GLASGOW,

MACMII.LAN

New York, - -

Toronto, • • •
London, • -

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AND CO., LTD., LONDON.

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Simpkin, Hamilton and Co.

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Douglas and Foulis.

Angus and Robertson.

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
1912

<?"<?(

UiOLOGY
UiJKARY

GLASGOW : PRINTED AT THE UNIVBRSITY PRESS
BY ROBERT MACI.EHOSE 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

EMBRTOLOOICAL LABORATORY,

GLASGOW UNIVERSITY,

June, 1912.

418496

CONTENTS

PAGE

INTRODUCTION . . - vii

BIBLIOGRAPHY - ...... xi

CHAPTER I. DOUBLE MONSTROSITY

A. Literature - . . . .. . 1

B. Occurrence, records, and general outline ......... 1

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 Salmonidae and in Eiox - 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)mesodidymi and

(b) (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

vi CONTENTS

CHAPTER III. CYCLOPIA PAGK

Classification and structure 40

Summary and comparison - 42

Other records ill 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

(6) The round-headed condition 50

(c) Defects of the lower jaw - - 51

(d) Hump-back, twisted body, and allied conditions - 51

(e) Local reduplication and deficiency of the notochord - 64

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

(6) 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 Royal
College of Surgeons of England.

I desire to acknowledge indebtedness to the kindness and skill of my friend, the late Thomas
Reid, 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

Vlll

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 mammalst

From the general point of view the most interesting teratological questions are those which
are concerned with origin and causation. 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. Hack of the recognised types of monstrosity, major as well as minor, can arise in a
spontaneous 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 hermaphroditism, 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 particular abnormality appears spontaneously is an argument for, and not against, the

INTRODUCTION ix

probability that the same abnormality can lie artificially produced. The converse proposition carries
with it an even stronger degree of likelihood.

Teratologieal 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.

LITERATURE RELATING TO THE TERATOLOGY OF FISHES.

THE following index has been compiled from Zoological Records, Jahresberichte, 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.

1 Albrecht, P. Ueber eine in zwei Zipfel auslaufende

reohtsseitige Vorderflosse bei einem Exemplare von
Protopterus annectens. Berlin SitzBer. Ak. Wiss.
1886 (545-6).

2 Aldrovandi, Ulys.se. Monstrorum historia. Bologna

1642 (429).

3 Allis, E. P. An abnormal musculus obliquus superior in

Carcharias. Anat. Auz. Jena 16 (605-7).

4 Audeville, A. d'. Uu cas aingulier de teVatologie sur un

aalmonide monstrueux. Bull. Soc. d'Acclimat. Ser. 4
6 1888 (990-993).

5 v. Baer, K. E. Ueber doppelleibige Missgeburten oder

organische Verdoppelungen in Wirbeltieren. St. Peter-
burg Mem. Ac. Sc. Ser. 6 4 1845 (79-178).

6 Barbieri, C. Sull'origine delle mostruosita embrionali

doppie nei teleostei. Milano Atti Soc. ital. so. nat.
46 1906 (100-106).

7 Barbieri, C. Intorno ad una interessante mostruosita

embrionale doppia nei salmonidi. Riviata mensile di
Pesca Milano 8 1906 (89-91).

8 Barfurth, D. Eine Larva von Petromyzon planeri mit

drei Schwanzspitzen. Archiv EntwMech. Leipzig
9 1899 (27-31).

9 Barfurth, D. Die Eracheinungen der Regeneration bei

Wirbeltierembryonen. O. Hertwig'a Handbueh der
Entwicklungslehre Jena 1903 3 Teil 3 (1-129).

10 Barrington, Dainea. A letter... on some particular fish
found in Wales. London Phil. Trans. R. Soc. 67
1768 (204).

/ / Bataillon, E. La pression osmotiquo et les grands pro-
blemes de la biologie. Arch. EntwMeoh. Leipzig 11
1901 (149-183).

12 Bataillon, E. Blastotomie spontaniSe et larves jumelles

chez Petromyzon. Paris C. R. Acad. So. 13O (1201-2).

13 Bataillon, E. Recherches expe>i men tales sur Involution

de la lamproie. Ibid. 13O (1413-5).

14 Bataillon, E. Pression osmotique de 1'reuf et polyem-

bryonie experimentale. Ibid. 13O (1480-2).

15 Bateson, W. Pilchards with the number of scales abnor-

mally increased. London Proo. Zool. Soc. 1890(586-8).

16 Bateaon, W. On specimens of the common pilchard

(Clupea ptichardus), showing variation in the number
and size of the scales. Ibid. 1894 (164).

17 Bateson, W. On two cases of colour variation in flat-
fishes, illustrating principles of symmetry. Ibid. 1894
(246-9).

IS Bateaon, W. Note in correction of a paper on colour-
variation in flat-fishes. Ibid. 1896 (890-1).

19 Bateaon, W. Materials for the study of variation. London

Macmillan 1894 (46, 58-9, 466-473, 563).

1'ia Bean, B. A. A remarkable carp. Forest and stream,
New York N.Y. 73 1909 (1022).

20 Bellotti, C. Di una notevole varieta de colorazione della

Tinea comune. Milano Atti Soc. ital. so. nat. 44 1905
(218-220).

21 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 do la truite (Salmo trutta)

d'une coloration etrange. St. Peterburg Ann. Mus.
zool. 13 1908.

23 Bert, P. Reproduction de I'extremiM caudale enlev^e

chez des poisaons osseux. Paris C. R. aoc. biol.
Ser. 3 6 1863 (100-101).

2!f Bird, M. C. H. A question of coloration. Zoologiat
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 wlgaris. Arch.

Nature. Berlin 47 1881 (136-9). (Transl. in Ann.
Mag. Nat. Hist. London Ser. 5 9 (65-7).)

27 Borcea, J. Sur un caa de conformation anormale de

1'oviducte droit chez une petite roussette (Scylliutn
canicula). Paris Bui. soc. zool. 29 1904 (138-148).

2S Boulenger, G. A. Renewed left pectoral limb of a
Protopterus annectens. London Proc. Zool. Soc. 1891
(147-8).

20 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 (Pleuronectes platessa).
London Proc. Zool. Soc. 19O8 (161-164).

31 Braus, H. 1st die Bildung des Skeletes von den Muskel-

anlagen abhangig ? Eine experimentelle Unter-
suchung an der Brustflosse von Haienembryonen.
Morph. Jahrb. Leipzig 36 (240-321).

Xll

LITERATURE RELATING TO TERATOLOGY OF FISHES

32 Brindley, H. H. Abramis blicca, Bl. : on a specimen

without pelvic fins. London Proc. Zool. Soc. 1891
(108-9).

33 Brindley, H. H. Note on some abnormalities of the

limbs and tail of dipnoan fishes. Cambridge Proc.
Phil. Soc. 10 1900 part 6 (325-327).

33a Buckland, F. Log-book of a fisherman and naturalist.
London 1891 (151).

Slf Bugnlon, E. Description de quelques alevins de truite
monstrueux. Bulletin de la Soci4t6 Vaudoise des
Sciences Naturelles Ser. 2 16 1880 (463-466).

35 Canestriui, R. Nota sui pesci mostruosi. Atti Soc.

Ven. Trent. Sc. Nat. 9 fasc. 1 1883.

36 Carlet, M. G. Sur une truite mopse. J. Anat. Physiol.

Paris 16 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. Aco. 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 Lordfisb. Edinburgh Proc. Roy. Phys. Soc.
1 (51).

43 Coolidge, 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. 4O 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 Cnenot, 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. 19O7 (174-181).

BOa 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 2O (72) ; 31 (335).

52 Dareste, C. Sur 1'origine et le mode de formation dea

monstres doubles. Archives de Zool. exper. Paris 3
1874 (114).

53 Dareste, C. Recherches sur la production artifioielle

des monstruosites. Paris 1891 (469 et seq.).

54 Day, F. British fishes. London 1880-4 vol. ii. (68, 98-

101, 102, also pi. clxxi. fig. 2).

55 Dean, B. Albinism, partial albinism and polychromism

in hag-fishes. Amer. Nat. New York 37 (295-296).

56 Dean, B. Reminiscences of holoblastic cleavage in the

egg of the shark (Heterodontus (Gestracion) japonicus).
Annotat. Zool. Jap. Tokyo 4 (35-41).

57 Dohrn, A. (A double Torpedo embryo.) Studien zur

Urgeschichte des Wirbeltierkiirpers. Mitth. Zool.
Stat. Neapel 16 1902 (31 Taf. v. fig. 2).

58 Donnadieu, A. L. Sur une hemiterie de volume observed

chez une carpe. Paris C. R. Acad. Sc. 7O 1870
(200-20).

50 Duncker, A. L. Variation und Asymmetrie bei Pleura-
nectes Jlesus. Wiss. Meeresuntersuch. Kiel Ser. 2 3
(333-402).

60 Duncker, A. L. Uber Regeneration des Schwanzendes

bei Syngnathiden. Arch. EntwMech. Leipzig 2O 1906
(30-39) ; ibid. 24 1907 (656-661).

61 Duncker, A. L. Syngnathiden-Studien. I. Variation

und Modifikation bei Siphonostoma typhle L. Hamburg
Jahrb. wiss. Anst. 26 (1907) Beih. 2; 1908 (1-115).

61a Dyce, R. On the identity of Morrhua punctata and
Morrhua vulgaris. Ann. Mag. Nat. Hist. London
1860 (366-389).

63 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-
keimschciben. Arch. EntwMech. Leipzig SO ii. 1910
(411-436).

63b Ekman, 6. Ueber einen Fall von Ruckbildung der letzten
Kiettenspalte bei Squalus acanthias L. Helsingfors
Ofvers. F. Vet. Soc. 62 No. 9 1901 (1-6).

64 Elmhirst, R. Ambi-coloured flat-fish. Zoologist London

1911 No. 835 (30); Ann. Scott. Nat. Hist. Edinburgh
1911 (77-79).

65 Emeljanov, P. Zwillinge von Girardinus caudimaculatus.

Naturfreund St. Peterburg 2 1907 (239-242).

66 Fasciolo, Alba. Due cas di deformazione nel Labrax

lupus. Atti Soc. Ligustica 16 (92-99).

67 Federley, H. Monstrosa torskar (Monstrose Porsche).

Helsingfors Medd. Soc. Fauna et Fl. Fenn. 34 1908
(68-74).

68 Fiebiger, Josef. Ein Karpfen mit fehlender Sehwanz-

flosse. Oest. Fischerei-Ztg. Wien 6 1907 (83-85).

69 FUhol. (Rhombus maximus Cuv. , 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 (Leuciacus rutilus) du lac de Neuchatel.
Neuchatel Bui. Soc. Sci. Nat. 36 (82-85).

72 Gadeau de Kerville, H. Note sur une plie franche et un

flet vulgaire atteints d'albinisme. Paris Bull. Soc.
zool. France 2O (155 and 156).

73 Gadeau de Kerville, H. Description d'un poisson et d'un

oiseau monstrueux (Aiguillat de'rodyme et Goeland
melomele). J. Anat. Physiol. Paris 28 1892 No. 5
(563-6).

74 Gannan, S., and Denton, S. F. Abnormal embryos of

trout and salmon. Science Observer Boston A 1886
(1-7).

LITERATURE RELATING TO TERATOLOGY OF FISHES xiii

la Garstang, W. Malformation of the mouth in the
common sea-bream. Plymouth J. Mar. Biol. Ass.
Ser. 2 5 (345-347).

76' Gemmill, J. F. A contribution to the study of double
monstrosities in fishes. London Proc. Zool. Soc. 19O3
(4-23).

77 Gemmill, J. F. The anatomy of symmetrical double

monstrosities in the trout. London Proc. Roy. Soc.
68 No. 444 (129-134).

78 Gemmill, J. F. On cyclopia in osseous fishes. London

Proc. Zool. Soc. 18O6 (443-449).

70 Gemmill, J. F. Notes on supernumerary eyes and loca
deficiency and reduplication of the notochord in trout
embryos. London Proc. Zool. Soc. 19O6 (449-452).

SO Gervais. (Supernumerary pectoral fins on back of Raia
davata.) Paris C. R. Acad. Sc. 69 1864 (882).

81 Girdwoyn, Michel. Pathologic des poissons. Paris 1880.

82 Goeldi, E. A. Further notes on the Amazonian Lepido-

siren. London Proc. Zool. Soe. 1898 (855).

S3 Grimm, Oskar. (Ein Stor mit drei Rogensacken.) V^st.
rybopromysl St. Petersburg 31 1906 (21-37).

84 Grochmalicki, Jan. Uber die Linsenregeneration bei
den Knochenfischen. Zeitschr. wissensch. Zool. Leip-
zig 89 1908 (164-172).

So Grosser, ('•., and Przibram, H. Einige Missbildungen
beim Dornhai (Acanlhias vulgaris). Arch. EntwMech.
Leipzig 22 1906 (21-37).

86 Giinther, A. Catalogue of fishes in the British Museum.

London 6 1866 (67).

87 Gunther, A. Black variety of Platyglossus notopsis.

London Proc. Zool. Soc. 1871 (667).

88 Gunther, A. The study of fishes. Edinburgh 1880

(188).

89 Halbertsma, H. J. Normaal en abnormaal hermaphrodi-

tismus by de visschen. Versl. en Mededeel Akad.
Wet. Amsterdam 16 1864 (165-178).

90 Hefford, A. E. Note on a hermaphrodite cod (Oadus

morrhua). Plymouth J. Mar. Biol. Ass. 8 1908
(315-317).

91 Hefford, A. E. Note on a conger with abnormal gonad.

Ibid. 8 1908 (318-319).

92 Heincke, F. Variabilitat und Bastardbildung bei

Cyprinoiden. Leuckart Festschrift 1892 (64-73).

93 Herdman, W. A. Catalogue of the "fisheries collec-

tion " in the Zoological Department, University
College, Liverpool. P. Liverpool Biol. Soc. 11 (1896-
7) (113).

93a Hertwig, O. Allgemeine Biologie. Jena 1909 (580).

94 Heusner. Descriptio monstrorum avium, amphibiorum,

piscium quae extant in Museo Berolini 1824 8 p. 34
(quoted from 5 p. 88).

95 Hilgendorf, F. Ein krankhaft verandertes Gebiss eines

Haifisches (Oaleus galeus). Berlin SitzBer. Ges. Natf.
Freunde 1891 (64-67).

06 Hofer, B. Uber Missbildungen beim Hecht. Allg.
Fischerei-Zeitg. Miinchen Jahrg. 26 No. 1 (14-15).

96a Hofer, B. Handbuch der Fischkrankheiten. Miinchen
1904.

97 Holt, E. W. L. Note on a young specimen of Zoarces

viviparus. Ann. Mag. Nat. Hist. London Ser. 6 6
(256).

98 Hopley, C. Observations on a remarkable development

in the mudfish. Amer. Nat. New York 36 1891
(487).

99 Houghton, Rev. W. British fresh-water fishes. London
1879.

100 Howes, G. B. Variation in the kidney of the common

thornback (llaia davata) : its nature, range, and
probable significance. J. Anat. Physiol. London 24
(407-422).

101 Howes, G. B. On some hermaphrodite genitalia of the

codfish (Gadus morrhua), with remarks upon the
morphology and phylogeny of the vertebrate repro-
ductive system. London J. Linn. Soc. 23 (539-558).

102 Howes, G. B. On the heads of two lampreys and a hag,

showing some remarkable variations of the respiratory
organs. London Proc. Zool. Soo. 1893 (730-733).

102a Howes, G. B. On synostoses and curvature of the spine
in fishes, with especial reference to the sole. London
Proc. Zool. Soc. 1894 (96-100).

102b Hyrtl. Ueber Wirbelsynostesen u. Wirbelsuturen bei
Fischen. Wien. Denkschr. XX. 1862 (95-110).
Quoted from preceding.

103 Ihering, 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).

105 Iwanzoff, N. Ein Fall von scheinbarem Hermaphrodi-

tismus bei den Barsch (Perca fluvialilis). Moskva
Bull. Soc. Nat. 1893 (119-205).

106 Jablonowski, J. Ueber einige Vorgiinge in der Ent-

wickelung von Salmonidenembryonen, etc. Jena
Anat. Anz. 14 1898.

107 Jackel, Hr. (An hermaphrodite carp.) Niirnberg

Abhandl. Naturhist. Ges. 3 1867 (245).

108 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).

/'/'' Jaquet, M. Anomalie de la region posterieure du corps
chez un Silurus glanis. Bucharest Bull. Soc. Sc. 8
(786-791).

110 Jaq.net, M. Ligne late>ale supptementaire chez un
Acipemer ruthenus. Ibid. 8 (791-792).

/// Jaquet, M. Anomalie de la nageoire anale cbez des
Sebastes daclyloptera. Bull. Inst. Ocean. Monaco
No. 79 1906 (1-6).

112 Jaquet, M. Description de 1'extre'mite' posterieure du

corps anomale chez deux Motella fusca. Ibid. No. 90
19O7 (1-9).

113 Jarvi, T. H. Ein Fall von Hermaphroditismus bei

Lota miigaris. Helsingfors Medd. Soc. Fauna et FI.
Fenn. 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 Centrarchidae. Madison Trans.
Wiss. Acad. Sci. 16 Part 2 1907 (713-732).

116 Johnstone, J. A malformed plaice. Trans. Liv. Biol.

Soc. 18 1903-4 (111-112).

117 Johnstone, J. Ichthyological notes. Ibid. 2O 1906

(330-335).

118 Johnstone, J. Ichthyological notes. (1) An herma-

phrodite hake. (2) Gurnard with malformed lower
jaw. Ibid. 21 1907 (309-315).

119 Johnstone, J. An abnormal specimen of the brill.

Ibid. 28 1908-9 (200-202).

xiv LITERATURE RELATING TO TERATOLOGY OF FISHES

130 Joseph, H. Ein Doppelei von Set/Ilium. (Nebst
Bemerkungen iiber die Eientwioklung. ) Aiiat. Anz.
Jena 29 (367-372).

121 Jussieu. (Two double monster fish.) Histoire de

1'Acad. Roy. des Sc. 1764 (30). Quoted from 5 (88).

122 Kershaw, J. A. A colour variety of the common eel

(Anguilla australis). Victorian Natural. Melbourne
2O (140).

123 Klaussner, F. Mehrfachbildungen bei Wirbeltieren.

Miinchen 1890.

/.'/ Knauthe, K. Ueber Melanismus bei Fischen. Zool.
Anz. Jena 16 1892 (25).

125 Kuautlie, K. Ichthyologische Mitteil. Zool. Anz.

Leipzig 14 (59-61, 259-264); 16 (109).

126 Knauthe, K. Zwei Fiille von latenter Vererbung der

Mopskopfigkeit bei Cyprinoiden. Biol. Centralbl.
Leipzig (fruher Berlin) 13 1893 (766-767).

127 Knoch. Ueber Missbildungen betreffend die Embryonen

der Salmonen und Coregonus Geschlechts. Moskva
Bull. Soc. Nat. 46 No. 2 1873.

128 Kohler, 0. Ein Analogon zu der bei Regeneration

wiederholt beobachteten Gabelschwanzbildung der
Eidechsen. Wochenschrift Aquarium-kunde, Braun-
schweig 1 1904 (215).

129 Kopach, F. Experiment. Untersnch. ttber den Keim-

hautrand der Salmoniden. Verhand. Anat. Gea.
Berlin 1O 1896 (113-121).

130 Kopsch, F. Experimentelle Untersuchungen am Primi-

tivstreifen des Huhnchens und an Scyllium Em-
bryonen. Jena Verb. Anat. Ges. 12 1898 (49-67).

131 Kopsch, F. Die Entwickelung des ausseren Form des

Forellenembryonen. Arch. f. microsk. Anat. 61
1898.

132 Kopsch, F. Die Organisation der Hetnididymi und

Anadidymi der Knochenfische und ihre Bedcutung
fur die Theorien iiber Bildung und Wachsthum der
Knochenfischembryos. 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 16 Part 3 (396-398).

1S6 Lachmann. (On a renewed pectoral limb.) Zool.
Garten Frankfurt a. M. 32 (129).

JS6a Laurence, G. W. On a mouthless fish. Philadelphia
Proc. Ac. Nat. Soc. 1876 (125-126).

137 Lavenier. Une carpe monstre. Angers Bui. Soc. dtud.

so. 24 (169-170).

138 Leger, L. Observation concernant une anomalie du

cervelet d'un Alopias wipes. Paris Bull. Soc. Philom.
Ser. 711 (160-163).

139 Leger, L. Mutilation pathologique et regeneration chez

le Protoptere. Paris C. R. soc. biol. Ser. 10 4 1897
(543-545).

140 Leonnardt, E. Ueber die Mopskopfbildung bei Abramit

vimba. Zool. Anz. Leipzig 31 1906 (53-60).

HI Lereboullet, A. Sur la monstruosite double chez les
poissons. Paris C. R. Acad. Sc. 4O 1855 (916, 1028,
1063).

142 Lereboullet, A. Recherches sur le deVeloppement de
la truite, etc. Ann. Sci. Nat. (Zool.) Paris Ser. 4 16
1861 (359-368).

143 Lereboullet, A. Sur les monstruosites du brochet.

Ibid. Ser. 4 2O 1863 (177-271) ; Ser. 5 1 1864 (113-
199, 257-320).

144 Levison, F. (A double-headed shark.) Nord. Med.

Ark. 1O Hft. 2 No. 9 1878. Quoted from 247 2
(57).

144a Llchtenfelt, H. Literatur zur Fischkunde. Eine Vor-
arbeit. Bonn 1906.

14o Loeb, J. Uber die Entwicklung von Fischembryonen
ohne Kreislauf. Arch. ges. Physiol. Bonn 64 1893
(525).

146 Lowne, B. Thomson. Catalogue of the teratological
series in museum Royal College Surgeons, England.
London 1893.

lift Luther, A. Hermafroditiska exemplar af Lota mdgaris.
Helsingfors Medd. Soc. Fauna et Fl. Fenn. 36 1909
(227).

IJfla, Malloch, P. D. Life-history and habits of the salmon,
sea-trout, trout, and other fresh-water fishes. London
1910 (239-245).

148 Halm. Note sur la reproduction de parties de 1'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).

140 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 della 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 asiminetriche 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 Pleuronectidae, and XXX. 2. On an abnormal
plaice with a precaudal fin-frill on the left side.
Ann. Mag. Nat. Hist. London 19O2 (291-2%) 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 niyrum
in Turkey Lake and Tippecanoe Lake. Indianapolis
Ind. Proc. Acad. Sc. 1897 (207-228).

160 Moenkans, W. J. An aberrant Etheostoma. Ibid.

19O1 (115-116).

161 Morgan, T. H. Experimental studies on teleost eggs.

Anat. Anz. Leipzig 8 1893 (803-814).

162 Morgan, T. H. The formation of the fish embryo.

Journ. of Morph. Philadelphia Pa. 1O 1895.

163 Morgan, T. H. Regeneration. New York Macmillan

Company 1901.

LITERATURE RELATING TO TERATOLOGY OF FISHES xv

1G4 Morrlll, C. V. , Jun. Regeneration of certain structures
in Fundulus hcteroclitits. Biol. Bull. Woods Holl
Mass. 12 No. 1 1906.

165 Hoser, F. Beachreibung einer Duplicitas anterior der
Bachforelle und Besprechung der Theorie von Fr.
Kopsch iiber Bildung des Wachstumszentrums fiir
Rumpf und Schwanz. Anat. Anz. Jena 3O 1907
(33-52, 81-106).

1GG Mudge, Geo. P. An abnormal dogfish (Scyllium cani-
cvla). Zool. Anz. Leipzig 3O (278-280).

1G7 Newman, H. H. A significant case of hermaphroditism
in fish. Biol. Bull. Woods Holl Mass. 16 1908
(207-214).

1GS Ninul, E. Sopra alcuni Pesci mostruosi raccolti nelle
valli del Veneto esturio. Boll. Soc. Zool.It.al. Ser. 2
4 (117-121).

169 Ninnl, E. Sopra due casi d'arresto della migrazione

oculare (Pleuronectes italicus, Solea vulgaris). Milano
Atti Soc. ital. sc. nat. 44 1905 (193-197).

170 Ninnl, E. Metacromatismi in pesci raccolti nel mare e

nelle lagnne di Venezia. Atti del congresso dei
Naturalisti italiani Milano 10O7 (585-589).

171 Nusbaum, J. Zur Kenntniss der Heteromorphose bei

der Regeneration der alteren Forellenembryonen
(Salmo mdem). Anat. Anz. Jena 22 (358-363).

173 Nusbaum, J., und Scymon, S. Beitrage zur Kenntnis
der Regenerationsvorgiinge nach kunstlicheu Ver-
letzungen bei alteren Bachforellenembryonen (Salmo
fario). Arch. EntwMech. Leipzig 1O 1900 (645-684).

173 Nusbaum, J., nnd Seymon, S. Zur Teratologie der

Knochenfische, zugleich ein Beitrag zu deren Re-
generation. Ibid. 24 1907 (114-123).

174 Nystrbm, E. Cotlm scorpius L. : on a monstrous speci-

men. Bih. Sv. Ak. Handl. 14 No. 10.

175 Oellacher, J. Ueber einen Doppelembryo von Trutta
fario. Innsbruck Berichte des naturwiss. medic.
Vereins 3 1 Heft 1873.

17G Oellacher, J. Terata mesodidyma von Salmo salvelirma.
Wien Sitzber. Ak. Wiss. 68 1873 Naturw. (299-324).

177 Orlandl, S. Sopra un case di ermaphroditismo nel

Mwjil chelo. Atti Soc. Ligustica Sc. 13 (3-6).

178 Otto, A. G. Monstrorum sexcentorum descriptio ana-

tomica. Vratislaviae 1841 (71-73, 133, 266, Tab. iii.
5, xxiv. 6).

179 Panceri, P.

Naples Rend. Ace. Nap. 12 1873 (110-114).

180 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.

182 Pappeuheim, P. Ueber Augenverlust und Sehadel-Ver-

bildung bei einem Fisch. Berlin SitzBer. Ges.
natf. Freunde 19O8 (7-8).

183 Pappenheim, P. Ein zweiter Fall von Mopskopfigkeit

bei einem Lumpenus lampetriformis (Walb. ) aus der
Apenrader Fohrde. Ibid. 19O7 (349-350).

ISSa Parker, T. J. On the intestinal spiral- valve in the genus
Baia. London Trans. Zool. Soc. pt. ii. 1880 (49-61).

184 Patterson, A. Malformed codfish. Zoologist London

66 (130); ibid. 1911 (448, hermaphroditism).

185 Peach, C. W. On the so-called tailless trout of May.
British Assoc. Report 1871 Transactions of the
Sections p. 133.

18G Pellegrin, J. Note sur une anomalie des rayons <(pineux
du Prater acanthi*. Bull. Mus. Paris 1809 (356-
357).

187 Pellegrin, J. Sur une raie monstrueuae de la famille

des cyclocephaliens. Paris Bui. Soc. zool. 20 (106-
108).

188 Pellegrin, J. Presentation de quelques cas de devia-

tions rachidieunes chez les poissons. Ibid. 27 1902
(215-219).

189 Pellegrin, J. Sur uue race monstruense de perches

dauphins observde en Seine k Port-Villez. Paris
Bui. soc. cent, aquicult. 2O 1908 (42-46).

190 Pellegrin, J. Sur une race monstruense de perches.

Paris Bui. soc. zool. 33 1908 (25-27).

191 Pettls, C. R. Albino brook trout. Science New York

Ser. 2 19 (867-868).

198 Phillipeaux, J. M. Experiences dijmontrant que lea
nageoires des poissons ne se regent-rent qu'i la con-
dition qu'on laisse au moins sur place leur partie
basilaire. Paris C. R. Acad. Sc. 68 (669).

193 Poey. (Hermaphroditism in Mesoprion and Chrysurus.)

Ann. Lye. New York 9 (309).

194 Pouchet, G. Sur des cyprins monstrueux (G. auratus)

venant de Chine. J. Anat. Physiol. Paris 71 1870
(561-569).

195 Przibram, 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).

197 Quatrefages, A. d'. Formation 'des monstres doubles

chez les poisaons. Paris C. R. Ac. Sc. 4O 1855 (626-
8, 872, 925, 993).

198 Quatrefages, A. d'. Memoire sur la monstruosit^ double

chez les poissons. Mem. publ. par la Soc. philom. a
1'occasion du centenaire de sa fondation 1788-1888
Paris 1888.

199 Ratlike. Abh. z. Bild. u. Entw. d. Mensch. u. Thiere 2

(62). Quoted from 5 (89).

200 Rauber, A. Die Theorien der excesaiven Monstra.

Virch. Arch. 71 1877 (133-206); 73 1878 (551-594).

201 Rauber, A. Gastrodidymus des Lachses. Ibid. 76

1879 (553).

%0% Rauber, A. Formbildung und Formstorung in der
Entwicklung von Wirbelthieren. Morph. Jahrb.
Leipzig 6 (662-702) ; 6 (129-184).

SOS Rennle, John. Accessory fins in Ilaia batis. Anat.
Anz. Jena 28 1906 (428-431).

&04 Rlggio, G. Sopra un caso di notevale ramificazione dei
ciechi pilorici di Centrolophus pompilus. Natural.
Sicil. Palermo 13 1894 (206-211).

SOS Risso. Ichthyol. de Nice. Paris 1810 (49). Quoted
from SIS (III. 202).

206 Ritchie, J. An ambicoloured turbot with eyes approxi-

mately normal in position. Ann. Scott. Nat. Hist.
Edinburgh 19O8 (146-150).

207 Ritchie, J. A hump-backed trout from Stranraer.

Ann. Scott. Nat. Hist. Edinburgh 19O8 (223-227).

SOS Rosmlnl, Olga. Ricerche intorno alia variazione del
Pe.trom.yzon planeri. Torino Boll. Musei Zool. Anat.
16 No. 390 (29 pp.).

xvi LITERATURE RELATING TO TERATOLOGY OF FISHES

509 Roth, W. Doppelte Regeneration einea Bartfadens bei

einem Panzerwelse. Blatter fiir Aquarien- und Ter-
rarienkunde 16 1905 (428).

510 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. 1803
(75-94).

511 Sacclii, Maria. Un caso d'arresto dell' emigrazione

oculare, con pigmentazione del lato cieco in un
Rhombus maximus. Geneva Boll. Musei zool. anat.
comp. 1898 No. 67 (4 pp. ).

512 Sacclii, Maria. Altre casi d' anomalie nel pleuronettidi.

Ibid. 1880 No. 82 (3 pp.).

SIS St.-Hllalre, Is. Geof. Histoire des anomalies de 1'orga-
nisation. Paris 1836 (I. 283-7 PI. I. 4-6 ; III. 159,
202 PI. XIV. 5).

514 Sandman, J. A. (Hermaphroditische Exemplare vou

Ghtpea harengus.) Fisk. Tidskr. Finl. Helsingfors
15 1906 (126-127).

515 Schmincke, Alexander. Die Regeneration der quer-

gestreiften Muskelfasern bei den Wirbeltieren. I.
Ichthyopsiden. Eine vergleicheud pathologisch ana-
tomische Studie. Verb, physik-med. Ges. Wiirzburg
N.F. 30 No. 2 (15-130).

S1G Schmltt, F. Systematische Darstellung der Doppelem-
bryonen der Salmoniden. Arch. EntwMech. 13 1901
(34-134).

517 Schmitt, F. Ueber die Gastrulation der Doppelbildungen
der Forelle mit bes. Berucksichtigung der Kon-
kreszenztheorie. Verb. d. Deutsch. Zool. Ges. Leipzig
12 1902 (64-83).

218 Schneider, G. Ueber einen Fall von Hermaphrodi-
tismus bei Lota vulgaris. Helsingfors Medd. Soc.
Faun, et Fl. Fenn. 29 1904 (103-105).

319 Schneider, G. Ueber einen Fall von Hermaphrodi-

tismus bei Oasterosteus aculeatus. Ibid. 3O 1904

(7-8).
S20 Schneider, G. Farbenvariationen des Flussbarsches

(Perca fluviatilis). Riga Korr-blt. Naturf. Ver. 61

1908 (41-46).

SSI Schondorff, A. Ueber den Farbenwechsel bei Forellen.
Ein Beitrag zur Pigmentfrage. Arch. f. Naturg.
Berlin 60 (399-425).

S22 Schwalbe, Ernst. Die Morphologic der Missbildungen
des Menschen und der Tiere. I. Allgemejne Miss-
bildungalehre Jena 1906 ; II. Die Doppelbildungen
1907.

223 Scott, G. G. Further notes on the regeneration of the

fins of Fimdulus heleroclitus. Biol. Bull. Woods
Holl Mass. 12 1906 (385-400); see also 17 (343-
353).

224 Secques, F. Deux monstres gastdropages adultes de

salmonides. Paris Bui. Soc. zool. 2O 1895 (119-
123).

826 Segre, Rosetta. Ricerche intorno alia variazione della
Tinea vulgaris. Torino Boll. Mus. 17 No. 429.

286 Seligmann, C. G. Supernumerary dorsal fin in a trout.
Journ. of Pathology and Bacteriology Edinburgh
and London October 1BOB (2 pp.). Also, Trans.
Path. Soc. Lond. 49 (388-9 ; 390-3).

2S6a Simpson, J. Y. Article " Hermaphroditism " in Todd's
Cyclopaedia of Anatomy and Physiology II. p. 697.
London 1836-9.

tS7 Smallwood, W. M. Notes on the atrophy of the eye of
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38 1908 (930-931).

22S Smith, J. A. Notice on the occurrence of double or
vertical hermaphroditism in a common cod-fish
(MorrJi.ua vulgaris) recently taken in the Firth of
Forth. Proc. Roy. Phys. Soc. Edinb. 1864-6 (300-
302).

S2Sa Smith, J. A. (Deformed codfish from the Firth of
Forth.) Ibid. (302).

839 Smith, J. A. Notice on true hermaphroditism in the
codfish (Morrhua wlyaris) 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

(Qadus 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).

232 Southwell, T. On a hermaphrodite example of the

herring (Clupea, harengus). Ann. Mag. Nat. Hist.
London Ser. 7 0 (195, 196).

"33 Spemann, H. Ueber experimentell erzeugte Doppel-
missbildungen mit cyklopischem Defekt. Zool. Jahrb.
Suppl. 7 Festchr. f. Weismann 1904.

234 Stach, Jan. (Sandats a tSte de carlin.) Okoln. Tow.

ryb. Krakow 1OO6 (9-12).

23ja Steindachner, F. Uber das Vorkommen monstroser
Kopfbildungen bei dem Karpfen. Wien Verh. zool.
bot. 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.) 84 (69-70, 70-71).

236 Stockard, C. R. The development of Fundulus

heteroditus in solutions of lithium chloride, with
appendix on its development in fresh water. J.
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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 33 1907 (249-258).

238 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).

S40 Stoll, A. A. (Ueber Bastarde von Teleskopen und
anderen Varietaten des Goldfisches.) Naturf reund
St. Peterburg a 1907 (391-400).

240a Storch, O. Untersuchungen uber die paarige Afterflosse
der Schleierschwanze. Wien Arbeiten Zool. Inst.
Universitat 1O 2 (1-24).

841 Storrow, B. A case of spinal curvature in a codling.
Northumberland Sea Fisheries Report for 1909 (37-
39).

S48 Sumner, F. B. A study of early fish development,
experimental and morphological. Arch. EntwMech.
Leipzig 17 1904 (92-149).

243 Suomalainen, E. W. (Rote Form von Perca fluviatilis.)

Helsingfors Medd. Soo. Fauna et Fl. Fenn. 34 1908
(33).

244 Supino, F. Considerazioni sulla teratologia spirimentale.

Bull. Soc. Veneto-Trent. 6 (43-48).

545 Button, J. B. Evolution and disease. London 1890

(121).

546 Suworow, E. K. Ueber die Regeneration der Flossen

bei Knochenfisclien. Arbeiten der kaiserlichen Gesell-
schaf t der Naturforscher Petersburg 33 1904 Lieferung
4 (1-49).

S47 Tarnffl, C.
1894.

Storia della teratologia, I. -VIII. Bologna

LITERATURE RELATING TO TERATOLOGY OF FISHES xvii

S48 Terni, C. Esoftalmia epizootica in avanotti di Salmo
fa.no L. Riv. mens. Pesca Milano 1O 1908 (1-3)

(pathological).

£49 Tornier, G. Vorlaufiges iiber das Entstehen der Gold-
fischrassen. Berlin SitzBer. Ges. natf. Freunde
1908 (40-45).

550 Tornler, C. Ueber experimentelles Hervorrufen und

Naturentstehn von Mopskiipfen, Cyclopen und andere
vorgebiirtliehen Kopfverbildungen bei Wirbelthieren.
Ibid. 1908 (298-315).

551 Traqualr, R. H. On the so-called tailless trout of May.

J. Anat. Physiol. London 6 (411-6).

S51a Traqualr, R. H. On the Asymmetry of the Pleuro-
nectidae. London Trans. Linn. Soc. 26 (263-296).

252 Traquair, R. H. On specimens of tailless trout from
Loch Enoch, Kirkcudbrightshire. Edinburgh Proc.
Phys. Soc. 1882 (221-3).

K5S Traquair, R. H. Note on an abnormally developed
thornback (Raid, clacata). Ann. Scott. Nat. Hist.
Edinburgh 1892 (29, 30).

554 Traquair, R. H. On malformed trout from Scottish

waters, No. 1. Ibid. 1892 (92-103).

555 Traqualr, R. H. An unusually coloured example of the

thornback (Raia davata). Ibid. 1893 (25).

556 Traqualr, R. H. On a peculiar Charr from Inverness-

shire. Ibid. 1898 (78-79).

557 Trois, E. F. Sopra un esemplare di Anguilla con

spiccato metacromatismo. Venezia Atti 1st. ven.
67 1907-08 (65-66).

£58 Trois, E. F. Nota sopra una forma di metacromatismo
osservata in un egemplare di Pleuronectes italicus.
Ibid. 67 1907-08 (221-222).

259 TroiB, E. F. Sopra 1'anomale colorazione della pelle
osservata in un esemplare mutilate di Lophius
piscatorius. Ibid. 68 1908-09 (43-45).

£60 Trois, E. F. Contribute alia conoscenza di forme di
metacromatismo osservate in pesci raccolti nella
laguna di Venezia. Ibid. 68 1908-09 (113-115).

t61 Vaillant, L. Monstruosit^ de la limande commune
( Pleuronectes limanda). Bull, de la Soc. philomat. de
Paris 4 No, 2 (49).

S6S Vaillant, L. Sur un individn monstrueux nycteridoi'de
du Haiti davata. Bui. Museum Paris 19O8 (112-
113).

263 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. 19O9 (872-3).

265 Vogt, C. Notice sur 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 Warpachowski. Silurus glanis, L. : on a specimen with

supernumerary ventral no. Anat. Auz, Jena 1888
(379).

••:'.,'," Watase, S. On the caudal and anal fins of goldfishes.
Tokyo Journ. Coll. Sc. Japan 1887 (247-267).

268 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 (Qadus callarias). Glasgow Rep. Fish.
Board 24 part 3 1906 (290-292).

S(!9a Williamson, H. C. (Influence of cold on herring fry.)
Ibid. 27 part 3 (119).

270 Williamson, H. C. Notes on abnormalities in Lophius,

Gadus, Jiaia. 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).

272 Windle, B. C. A. On double malformations amongst

fishes. London Proc. Zool. Soc. 1890 (423-9).

573 Woodward, A. S. Rhinoptera, Kuhl : note on abnormal

dentition. Ann. Mag. Nat. Hist. London Ser. 6 1
(281-283).

574 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).

S7G Yung, E. Note sur un cas de monetruosit4 de la t€te
chez une truite. Rev. Suisse zool. Geneve 9 (307-
313).

CHAPTER I. DOUBLE MONSTROSITY.

(For Contents see p. mi.)

A. LITERATURE.

REFERENCES will be found under the following index figures (pp. xi-xvii) :

Salmonidae. — d'Audeville 4> Barbieri 6-7; Bugnion 34', Coolidge 4%', Coste 43', Dareste S3;
Garman and Denton 74; Gemmill 76-77; Girdwoyn 81; Jaeobi 108; Klaussner 123; Knoch
137; Kopsch 132-133 ; Lovmel46; Moser 165; Oellacher 176-176; ~Pa,nnml80; Quatrefages
197-198; Rauber 200-203; Schmitt 216-7; Schwalbe 222, ii. p. 297 et seq.; Secques 224;
Sutton 245; Taruffi 247; Windle 271-2; Yarrell 275, ii. p. 107.

Other Fishes: —

Perca — v. Baer 5; Leuciscus — Bataillon 11; Anarrichas — Buckland 33a; Girardiniis —
Emeljanov 65 ; Esox— Klaussner 123, Lereboullet 141-143, Rauber 200, Rauber 202, Valentin
263 ; Blennius — 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; Petromyzon— Bataillon 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
Salmonidae: 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 mesodidymiis.

On the other hand, v. Baer (5) examined over 3000 eggs of Cyprinus Uicca 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 (141-14$) 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.

is 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 he that contained in the Monstrorum
Historia of Aldrovandi (3), 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. Miis. 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 (334) 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£ inches. In the second,
the twins were of equal size, each being rather more than 5 inches in length. (3) A notice in
Yarrel (875, 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 (3 3 a) also
mentions that he " once read an account of a double-headed catfish (Anarrichas hipus) having been
caught in the North Sea."

Jacobi (108) 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 (375, ii. 87-96). As
teratology developed into an exact science in the hands of Meckel (156), St. Hilaire (213),
v. Baer (5), Vrolik (366), 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 (43), and Lereboullet (141-3), 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, Eauber 300-3). Very useful work along descriptive, classificatory, and historical lines
has also been done by Knoch (127), Panum (180), Eauber (300-3), Klaussner (123), and more
lately by Windle (272) and Schmitt (316). The methods of experimental embryology, particularly
in the hands of Kopsch (139-133) and Morgan (161-3), 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 Eauber (303), Oellacher
(176), Schmitt (316), Moser (165), Barbieri (6-7), and the author (76-77).

DOUBLE MONSTROSITY— CLASSIFICATION

C. CLASSIFICATION.

In Teratology, the terms (1) anadidymus, (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
(Missbildmigen des Menschen, 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
witli 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 (#7#) 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.

6. 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.

10. Union by the ventral aspects at the site of attachment of the yolk-sac.

11. 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 principally
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, gee the second last paragraph under Reduction and Parasitism (p. 59).

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

(6) (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.
TT 3

I) J.-L. ,, ,, ,, ,,

)» ill. „ „ „ 4 ,,

TV 5

»l » »t yt •* )J

yy * • yt yy » "~ ' »)

VT 8 Q

yj yy » ;>*-'*' j»

„ VII. „ „ ,,10

TV' \ „ „ not represented.

» IA.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 anadidymi 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 in the fertilised germ cell.
Lereboullet's (IJfS) 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 (176), and Rauber's (%0%) observations point to the same
conclusion, which may also help to explain the exceedingly wide variations in the other frequency
records (p. 1). Lereboullet further 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
done experimentally in the ease 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 Salmonidae, 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 (Bataillon 1%}. 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 143, 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. Recently polyembryony
has been shown to be the normal condition for the edentate mammal Tatusia,3 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).

5 Recently Eismond (GSa) has given an interesting account of experiments on the eggs of Raid clavata and Raia alba
during 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 accordingly it was
judged best to fix and preserve the whole specimen at this stage.

3 Fernandez (M.), " Beitrage zur Embryologie der Gurteltiere," Mcn-ph. Jahrl., Leipzig, 39 (302-333).

DOUBLE MONSTROSITY— DEVELOPMENT, ETC.

E. DEVELOPMENT AND EAELY GKOWTH.

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 katadidymus 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, (a) 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 ; b, the portion of yolk not yet covered by
the blastoderm ; 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', 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 formation, posteriorly of a perfectly single body
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 (#70)
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 (360°) 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 (132-3) 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 (1%3, 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 (143) 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 Eauber (200), and for

FIG. 3.— After Rauber (302 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 Kauber (202), Kopsch (132) and Schmitt (217). Panuin (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 (ISO, 133), Morgan
(161-2) and Surnner (24*2) 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 with exact data as to the progress of this extension in the eggs of the
Salmonidae.

Origin of double aiid 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,
PL 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 0. 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 Tropidonotus 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 Lacerta 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 Tatusia 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 Salmonidae and in Esox- - p. 9

Normal Advanced Embryos of Salmo fario p. 11

Advanced Stages of Double Monstrosity in Salmo fario :

Class I. - - - - - p. 12 Class VI. - - . - - p. 21

Class II. - - p. 15 Class VII. - - p. 23

Class III. - - p. 18 Class VIII. - p. 25

Class IV. - - p. 19 Class IX. - - p. 29

Class V. - - p. 20

Double Montrosity in other Fishes - - p. 30

EAELY 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 Salmonidae.

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 Salmonidae 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
(21 7). 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, notochord,
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 within 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 (14-3 20 254 et seq.) 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 united 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 mesoblastic 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. 13-16, 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.
56, 57, 62, 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 pairs 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 legmen 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

1 Full references are given in the Index of Structures, etc. (p. 63) to the various Plates and figures in which individua
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 Twin Brains uniting at the Optic Lobes.

PI. 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, rnandibular, 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. 11 .

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 1 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 capsules. 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 corresponding
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
PL 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, bladder, 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 Vergleichende Anatomie der Myxinoiden, Berlin 1835-1845; 3te Fortaetz. 1841 (41-99).

DOUBLE MONSTROSITY— STRUCTURE, CLASSES I. AND II. 15

paved 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 the 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.1

PI. I. fig. 6 (external appearance); Pis. XIII.-XIV. figs. 47-51 (transverse sections);
diagrams of skeleton, vessels, etc., in PI. 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) inferiorly,
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.

Aiiditory 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 in 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 fen- 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.1

PI. II. figs. 7, 8 (external appearance); Pis. XV.-XVI. figs. 52-55 (horizontal sections);
diagrams of various structures in PI. 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 corrected 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 ducts 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 vessels of a typical specimen of this group
is given in PI. 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 (a) 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. 8 7.
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 36th 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

'See footnote on p. 11.

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-kidney, 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 PI. 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. 6 3 (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's1 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 fia, 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 Triangular 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.
78-81 (Wolffian ducts, etc.).

The class is characterised either by (a) very marked ventral convergence of the two sagittal
planes, or by (J) 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
chieny 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
12th 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, hut 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. (6).

CLASS VI. (6).

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 PL 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 PL 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 (%16).

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 Canals. 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. (&). 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 (316), 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 PL 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 PL 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— ANAKATADIDYM US

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 (PI. 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 PL VIII. figs. 1, 2).

Coste's view (43) 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 21 6).
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 (143), 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 Salmo 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 (20% 5) next added a description of
very early specimens in Salmo 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. Rauber
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 (2^2), 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 lie 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 MONSTROSITY— 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 notochord
and other structures, or even completely interrupted. Or again, small inner muscle plates may be
present at various points.

In the pike, Lereboullet (.7 -£520 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 PL 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 notochord as well as a spinal cord and an outer set of muscle plates.
Remarkable 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. 2 6,
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 with
the covering membrane described by Oellacher, and we must
assume that here the advancing blastodermic margin has con-
FIG 4. -After Rauber (aoa B, PI. XLi. fig. sisted of epiderm only.

19). An extreme example of the hemididymous A • .-•'_«•

condition (dthiscence, Rauber), in an egg of the Eauber (S0% 5 pp. 694-699) described six early examples

trout sixteen days after fertilisation. A, the • n , /. n * .LI -jj.tii.j-j • r\

bending round anteriorly of the germinal thick- m Salmo fano-a.ll of them evidently katadidymi. One is
ening gr-. oc, region of the primitive optic at a stage sixteen days after fertilisation. Normally at this

outgrowths; m, membrana intermedia; a, *

blastodermic surface;*, surface of yolk uncovered stage the various regions in brain and body are beginning to

by blastoderm ; gm, margin of blastoderm. IT/.., T ,. , i /• ,1 j -i • i

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 membrana 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

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 hemididymi 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 (3) 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 hemididymi by electrical stimulation.

A satisfactory account of the mode of origin of hemididymus 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 particular
for the muscle plates.

On the other hand, Morgan (161) and Sumner (84%) 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 primarily 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 (138) 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 *) 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 (#), 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 ocdlata showing on a single embryonal shield two primitive streaks converging gently
in front (Tur, quoted from Sohwalbe $22, 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.

Blenniits. Eathke (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 589).

Gyprinus 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 (245). Mention of double monstrosity in the mackerel.

Anarrichas. Buckland (3 3 a). A more than doubtful instance, referred to on p. 2.

Leudscus. 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 (12) 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 129'5° 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 wan ting.

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 I 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 Head Region. An example is figured by St. Hilaire (213, 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 Eisso (®05\ 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 (2). 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 (146 p. 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® 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. Eiickert.

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 Eennie (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.

TRIPLE 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 PI. XXII.
fig. 94. Figs. 95-99 in Pis. XXIL-XX1II. and Figs. 32-34 in PI. 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 embryo 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 due to a predisposition depending on inherent germinal factors. Dareste'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 1 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 consonance with
the view that the formation of double or multiple embryos has behind it something of the nature of germinal predisposition.

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 (PI. 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 capsule 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 " B." 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 TEIPLICITY IN FISHES.

1. Esox liwius. 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, 143 1863 p. 217, PI. III. figs. 24-25).

2. Salmo fario. This is a blastoderm showing three embryonic rudiments. Bather 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 salvelinus. Also a very early example. Only 1 8 to 2 0 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, 1*23 p. 31, Taf. VI. fig 38).

4. Salmo salvelinus. 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, 183 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).

6. 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 B. 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 (123 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, PL VIII. figs.
46-47) 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).

1 0. Petromyzon planeri. The example of caudal triplicity recorded by Barf urth (<?) 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, 183 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;

Eegarding 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 (Iiistatice 1, above)
recorded and illustrated by Lereboullet, and the other early
examples noted by Kauber (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,-After Rauber (*» 6, Taf. vin. Ifc wil1 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

ri wmain entirely separate from one another need to arise at a very
TWO of these (c S) are : already uniting considerable angular distance (approx. 165°) from one another on

posteriorly, while the third c" is some distance '

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). r * * *• i -^ • p \. ii-i n^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 J)
examples of triple monstrosity appear to be exceedingly rare. The nearest approach to the condition
that the author can find occurs in the Pdobates 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) Petromyzon
larva, was probably the result of injury.

Eeptilia. 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 (2 427) 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 (53 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. 456). 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.

Piauber. 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 ^^7 3 p. 464.)

Koch. An area pellucida carrying two embryos, one of which shows anterior duplicity.
(Quoted from 2 4? 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 (28% 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 (2 47). 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 (#-£7 3 p. 450-460). More recent
instances of this kind are given by (a) Walla, (&) 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 (24.7 3 p. 465 et seq.). 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 Dreifachbildungen, Jenai&che Zeitschr. Natw. 7 (142-175).

3 Drei abnorm. gebildete Eier von Tropidonotus natrix, Anat. Am. Jena, 18 (425-440).

4 Report on Teratological Literature, J. Anat. Physiol. London, 40 p. 296.

5 (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.

I. All the bodies separate.
(Trisomi dieriti of
Taruffi, 847 III. p. 458)

Three embryos
from one ovum.

1 — The two united bodies
showing axial union.

A Only two of the

II. The bodies not

all separate.

(Trisomi sinerili of
Taruffi, III. p. 464)

B

bodies united.

2 — 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 (^^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 (847 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 Fernandez l on the
embryology of the Edentate Tatusia hybrida Desm. Confirming and greatly extending previous
observations by v. Kolliker, 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 Gnrtelthiere, I. Aforph. 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 (21+7 III. 466 et 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 (2 4? 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 Eeina.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.

lAtl\ dell. Accad. Gioen. 8 p. 203. 2Gurlt'a Lehrbuch der Pathologischen Anatomie, II. p. 201, Berlin 1832.

'Quoted from 2^7 III. p. 465. 'Quoted from Bruch, Jenaische Ztitschr. Natw. 1 p. 158.

CHAPTER III.
CYCLOPIA.

PL 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. Retina, 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 supra-orbitals 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 (1'3 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 are 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 oculi 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 be 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 (143) 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 (81) also figures a typical cyclopic salmonid embryo well advanced in development, and
resembling the specimen I have illustrated in PI. XXIV. fig. 100.

The remarkable instance of survival referred to above was recorded by Paolucci (181). 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 F. 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, 9 \ inches ; breadth between tips of pectoral fins 1 7 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 (337) on the
production of cyclopia in the eggs of Fundulus heteroditus by the addition of magnesium
chloride to the sea-water in which the eggs were being hatched. With certain strengths
of solutions (i m. sea-water solutions of MgCl2), one-eyed embryos occurred with surprising
regularity 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.

Semicydopia 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-cyclopia, is illustrated in PI. I. fig. 3 and XXVI. figs. 109,
110. Similar specimens are figured by Girdwoyn (81 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 awanting, 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.

MINOK 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

(£) 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

(J) 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
Miillerian duct of Ml 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
be noted in this connection that the male Raia clavata is occasionally found with teeth as much
blunted as those of the female (Day 34 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 (239 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 lay 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
(polycyclic 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; Iwanzoff 105; Jiickel 107;
Jiirvi 113; Johnstone 118; Kyle 135; Lowne 146, pp. 164-5; Luther 147; Martens, 149;
Masterman 150 ; Newman 167; Orlandi 177; Patterson 184; Poey 193; Sandman 214;
Schneider 218-9; Simpson 226a; Smith 228-30; Smitt 231; Southwell 232; Stewart 235;
VogtVOS; Weber 268; Williamson 269-2 70.

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 resum6 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, Squamipinncs, Sparidae, Scombridae,
Mugilidae, Gasterosteidae, Ldbridae, Gadidae, Pleuroncctidae, Cyprinidae, Esocidae, Salmonidae, Clupeidae.
The condition seems also to have been recorded in Acipenser, and among the Selachii (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 frequently 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 Atuitomy 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, Serramis, 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's1 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 CURVATUEE OF THE SNOUT.
(Pug-head ; Bulldog-head, etc.)

KEFERENCES :

Salmonidae. — Carlet 36; Couch 45; Day 54 II- p. 102; Lidth de Jeude 114\ Lowne
146 p. 55; Malloch 14?a; Saint Hilaire SIS 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. (Salnw 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 (198, PL VII. Fig. 2).

1 "Hermaphroditism in Mollusoa," Q.J. Microsc. Society, London, 37 (19-46).

MINOR ABNORMALITIES— PUG-HEAD 49

Other Fishes:

Examples in Cyprinus are furnished by Knauthe (1*25-6), Lowne (146 P- 55), Otto (178
thirteen instances), St. Hilaire (213 PI. I. figs. 4-6), Tornier (350) ; in Abramis vimba by Leon-
hardt (140) ; in Esox by Otto (178), and Vrolik (266); in Anguilla by Lowne (146 p. 55), and
Tornier (250); in Lumpenus by Pappenheim (183); in Perca by Pellegrin (189); in Mitgil chelo,
M. capita, Gobius ophiocephalus, Labrax lupus, Gadus minutus, Pleuronectes italicus, Merluccius vulgaris,
by Ninni (1 68) ; in Cottus by Nystrom (1 74), and in Mugil capita by Canestrini (35). Leonhardt
(140) refers to records in Cyprinus by Neydeck, Steindachner, and Hofer; in Lumpenus lampetrae-
formis, Gadus mcrlangus, G. morrhua, and an eel, by Lonneberg ; and in Salmo irideus and Esox
Indus by Hofer. Freund (70) gives a record, having reference to Phoxinus laevis.

In typical examples the snout is markedly reduced in size, and curved in such a way that the
anterior frontal region arches rapidly downwards just in front of the eyes. The latter are lessened
in horizontal diameter and become oval in shape, having the long axis vertical. The lower jaw
retains its usual size, and accordingly projects a considerable distance in front of the rest of the
head. The incurving of the tip of the snout may be so great that what should be its dorsal surface
comes to look downwards into the floor of the mouth. The deformity varies greatly in amount in
different specimens, and also passes without very abrupt separation into the " round-head " condition
to be afterwards described (p. 50).

Skeleton. The skull is practically normal as far forward as the anterior border of the eyes.
In front of this region the usual skeletal elements tend to occur, but they are reduced in size,
altered in shape, and curved downwards, while still retaining as far as possible their ordinary
connections with one another. In the region affected, the axial stem of the skull consists of vomer
and parasphenoid. Sometimes the stem suffers simple shortening, below and in front of the eyes,
and sometimes, in addition to being shortened, it becomes sharply bent on itself in this region, so as
to form a V, the apex of which points upwards. These two varieties are connected by inter-
mediate forms.

The frontals curve downwards so that the mesethmoid lies below instead of anterior to them ;
the lateral ethmoids and the nasals are similarly displaced, while the premaxillaries are so far curved
round that they lie behind instead of anterior to the mesethmoid.

The intermaxillary and maxillary are smaller, and carry fewer teeth, than in the normal
condition. Thus the former has two teeth in the instance described by Tornier (S50), and four in
that described by Lidth de Jeude (114), the normal number being about six. In an excellent
example from the eel, Tornier (250 p. 307) notes that neither the pterygoids nor the maxillaries
nor the vomer are provided with teeth, while all these bones have many rows of teeth in the normal
condition. The shortened maxillaries lie transversely, or, in extreme cases, are directed backwards.
The skeletal elements which go to form the anterior or palato-pterygoid limb of the suspensorial arch
are smaller than normal, particularly those near the ethmo-palatine extremity of the limb, where,
indeed, some of them may be no longer recognisable. On the other hand, those which form the
posterior (hyomandibular-quadrate) limb of the arch in question tend to be of normal size.

A number of these points are brought out in the following description from Carlet (36 Salmo).
The frontals curve downwards at the level of their orbital processes. The mesethmoid lies below
the frontals. The lateral ethmoids and the openings of the nostrils are also displaced downwards.
The pterygo-palatine is reduced in length, but the shortening affects the ento- and ecto-pterygoids less
than the palatines, which are short and almost transverse. The vomer is slightly shortened, and the
parasphenoid is very short, especially in its anterior part. The orbito- and ali-sphenoids are similarly
reduced in length. The descriptions by Hofer (96a) and Freund (70) may also be consulted.

Leonhardt (140) gives a careful series of measurements of different parts of the head
in his Abramis vimba specimens, together with a fairly complete account of their osteology.

Mention should be made of an example from the eel in the Eoyal College of Surgeons'
Museum (Lowne 146 p. 55). The mandibular extends forwards beyond the anterior extremity

50 MINOR ABNORMALITIES— ROUND-HEAD

of the truncated skull. As regards the brain, "the four anterior ganglia are much compressed
from before backwards. The anterior pair of ganglia (olfactory) have displaced the second pair,
and lie practically between, instead of entirely in front of, them."

Causation. In the main, the defect seems to be a developmental one of germinal origin,
and not acquired through later injury. However, in the Salmonidae, apart from the instances
figured by Girdwoyn (81) and de Quatrefages (198), the occurrence of pug-headed embryos does
not seem to have been actually put on record.

It is always possible that, as in cyclopia, the defect may at times be due to pressure on the
embryo (Leonhardt) or some other factor depending on environment. Tornier (249) attempts to show
that the condition in question, as well as the other striking variations found in different goldfish races,
are due to interference with development through swelling up of yolk elements by undue inhibition
of water. This may conceivably explain the occurrence of isolated and irregular instances, but cannot
possibly account for the strictly hereditary qualities which the condition has been stated to exhibit.

Knauthe (125-126) allowed breeding between a male and female Lettcaspius delineatus v. Sieb.
which, though apparently normal, were derived from " puggy " parents, with the result that thirty
out of 250 in the offspring resembled their grandparents (i.e. a proportion of 12'5 °/Q). A second
brood from the same parents only gave a proportion of 8 °/Q.

Another male and female of Leucaspius, normal themselves though derived from " puggy "
parents, provided in the offspring the large number of fifty out of 180 which "reverted" to the
grandparental form.

The same author, experimenting with a different species (Oyprinus apliya ?), obtained breeding
between two pairs of males and females, apparently normal and derived from apparently normal
parents, but having " puggy " grandparents. The result was that twenty out of a total of 210 in
the offspring resembled their great-grandparents.

In the succeeding year Knauthe was able to obtain breeding between some of the seemingly
normal examples of the latest generation noted above. Six of the progeny out of a total of 168
were " puggy," that is to say, resembled their great-great-grandparents, and yet, as has been detailed
above, the intervening ancestors were all apparently normal. A control brood in a kindred species
of similar age reared under exactly the same conditions provided not a single example of " pugginess."
The repetition and extension of Knauthe's experiments are greatly to be desired.

THE BOUND-HEADED CONDITION.

The skull is more or less sharply humped upwards in the frontal region so as to present a
somewhat prominent brow, or forehead. The eyes are compressed antero-posteriorly, forming ovals
with the long axis vertical. The anterior part of the snout is not ventrally incurved, nor does the
lower jaw project markedly beyond the upper one. Tornier (250) seems to be the only author
who gives structural details of this condition, which he describes as having been observed by him
in a haddock, a carp, and a bleak. The basal axis of the skull is again always shortened. In the
bleak this axis showed a single sharp A-s^aPe(i curve opposite the middle of the orbit. In the
haddock there were two sharp curves, one opposite the orbit and the other in front of that. Lastly,
in the carp the basal axis of the skull remained straight, but was extremely shortened in the region
underlying the brain-box and behind the eyes.

The skeletal elements forming the posterior (hyomandibular-quadrate) limb of the suspensorial
arch are markedly reduced in size. At the same time, the bones forming the anterior limb of the arch,
as well as the other bones of the head in front of the orbital region, tend to be smaller than normal.
The mouth usually looks upwards as well as forwards, so that the condition is not unlike what is
commonly described as pug-headedness, e.g. in dogs. The lower jaw, however, does not project
freely beyond the upper one.

Causation. The causation is probably of the same nature as that on which the pug-headed
condition depends.

MINOR ABNORMALITIES— DEFECTIVE LOWER JAW, ETC. 51

DEFECTS OF THE LOWER, JAW.

Grave defects of the lower jaw are less common than pug-noses or round-heads. The two
best examples I have come across are provided by the gurnard (Triyla gurnardus).

One is a jar specimen in the Glasgow Corporation Museum showing a good large head of this
fish severed from the body and mounted whole. The mandibular arch is extremely small, lying
entirely under cover of the upper jaw and hardly reaching forward to the middle of the roof of the
mouth. The chief prominence below the mouth is formed by the mesial portion of the hyoid arch.
The other is a similar specimen described in some detail by Johnstone (118). Regarding the
osteology of the lower jaw, this author notes that the angulars are not recognisable as separate
elements, while the articulars are short and placed almost vertically. Joining the distal ends of the
two articulars is a hoop-shaped element, which was taken as representing the united and ossified
Meckelian bars.

Profound defects of the lower jaw sometimes accompany cyclopia (pp. 42, 44), but here survival
will be impossible. Donnadieu (58), however, gives an account of an adult carp, showing what may
be described as semi-cyclopia, accompanied by malformations of both upper and lower jaws. The
muzzle is shortened ; the eyes approach the middle line ; the skull ends abruptly above the eyes ;
olfactory openings are wanting ; the whole face looks downwards and to the right ; the shortened
upper and lower jaws are in great part united together, and the mouth is reduced to a small,
somewhat circular, opening 3 mm. in diameter, lying below the right eye. Lowne (146 p. 55)
also notes an example in the carp in which there is deficiency of the face in front of the eyes, the
lower jaw being defective. (See also p. 62.)

Carnation. It seems likely that defective conditions of the lower jaw may arise in the two
different ways already described in connection with cyclopia, pug-head, etc.— (1) as autogenetic
developmental aberrations (probably the two gurnard specimens and certain examples of higher
animals, including mammals, showing similar malformations), or (2) as the result of interference with
normal development through pressure or some other factor external to the embryo itself.

HUMP-BACK AND ASSOCIATED CONDITIONS.
(Hog-back ; shortening of body or of tail ; twisting of body or tail.)

This deformity has been noted time and again, even in large-sized fish, not only among the
Salmonidae, but also in many other families.

Salmonidae (adult). Barrington 10; Couch 45; Day 54 II. 98, 102; Giinther 86; Hofer
(96a p. 307); Lowne 14-6 p. 94; Malloch 147 a; Eitchie 207; Stoddart £39; Yarrell 275.

Specimens have been sent to the author by Henry Lamond, Esq., and others, and probably
every angler of experience has had examples to his catch. As regards early stages, nearly every one
who has paid attention to fish hatcheries, notes the occurrence of embryos, the bodies of which show
practically every form, degree, and locality of curvature (Bugnion 34)-

Other Fishes:

Of other adult fishes the perch provides a number of instances, e.g. Day (54 I- 4), Lowne (146
p. 94), Houghton (99 p. 4), Howes (102a). Examples in Mugil ckelo, M. capita, Anguilla and Labrax
lupus, are put on record by Ninni (1 68). A very striking example in the cod is described by Storrow
(241)- It has long been known that in certain localities, e.g. the Firth of Forth, the cod sometimes
shows a particular deformation of the body due to shortening and coalescence of vertebrae (Cobbold
41a), Dyce (61a), Smith (228a), etc. This is the so-called Lord Fish of Yarrell, and seems to have
been counted by some naturalists as a separate species (see Smith loc. cit. and also Day 54 276-8).
In the sole, two instances are recorded by Howes (102a), while Lowne (146 p. 94) has also a note
on an example from this species. The former calls attention to Hyrtl's important paper (102V) on
Synostoses in the vertebral column of fishes, and notes that examples were also described by

52

MINOR ABNORMALITIES— HUMP-BACK, ETC.

Erdl and Stannius. The papers by Hofer (9 6 a), Freund (70)1 and Pellegrin (188) should also
be consulted.

Not many data are available regarding twisting of the body in the early stages of fishes other
than the Salmonidae, but attention may be called to notes by Holt (9 7) on this condition as occurring
in embryos of Zoarces, and by Williamson (2 69 a) in those of the herring. Both these authors refer
to observations on the influence of cold in producing the condition in question.

Structure. Hump-back and allied deformations of body or tail may be accompanied by the
following modifications of the vertebral column.

(a) Coalescence or synostosis of the vertebral centra in groups of several (e.g. 2 to 6) segments,
while the neural and haemal spines, the muscle masses, and the spinal nerves retain their normal
number. An excellent example is described by Ritchie (207), and another cited by Howes (102a)
from Hyrtl (10%b). The specimens described by Smith (2%8a), Cobbold (41 a), and Dyce (6 la),
referred to above, also belong to this type. The condition is noted by Tornier (%49) as being

characteristic of some of the races of goldfish. It is usually,
if not always, accompanied by increase in the transverse
diameter of the affected centra, which may be so great that
the weight of the fused centra exceeds that of an equal
number of normal ones (Howes 10%a). There is also
shortening in the antero-posterior direction, which may be
so extreme that four or five coalesced centra occupy less
than the length of two normal ones.

(b) Simple shortening of the length of the vertebrae
with increase in their transverse measurements, but without
fusion of centra. This is said to occur in goldfish races
(Tornier ^49). Mr. J. Eitchie, in a note to me, states that
he has found a similar condition in certain fish from the
Firth of Forth. Lowne (IJ^G p. 122) catalogues a specimen
of the sole in which there is arrested development of
1 2 post-anal vertebrae, " the bodies of which have only
about half their usual length and look as if they had been
pressed together." Probably the condition in question
grades into that described under (a).

(c) The backbone may show twisting or curvature
without coalescence of vertebrae. Perhaps the most striking
examples are those described by Howes (10£a) in a sole, and

by Storrow (%41) iQ a codfish. In both there are five sharp sinuosities. The vertebral bodies are
complete in number, independent, and only modified in shape so as to suit their position in the
curves. Howes notes the interesting point that the neural and haemal spines are altered (by
shortening, change of slope, etc.) so as to conform as far as possible to a normal body outline. The
length of the body is, however, very distinctly lessened and its dorso-ventral diameter increased.
A second sole described by Howes (10%a) has its vertebral column curved only in the anterior
region. Three distinct sinuosities occur in the backbone of a perch referred to in the same paper.

(d) In a specimen of the sea trout (Salmo trutta) sent to the author by Henry Lamond, Esq.,
a shallow notch in the contour of the dorsal surface of the fish behind the dorsal fin was marked
internally by an entire absence of one of the neural arches, and a semi-fibrous condition of the dorsal
portion of the succeeding myomere. None of the spinal nerves was missing.

1 This author notices a number of other records, chiefly from papers to which I have not had access, and thereby adds
to the above list examples from Acanthias, Amia, Grenildbrus, Esox, Gymnotus, Leuciscus, Mormyrus, Mulhis, Phoxinus,
Polypterus, Tinea, and Trigla. From Pellegrin (188) we have further to add Oobitis and Scomber. Hofer quotes the
statement from Hubrecht (Klassen u. Ordn. 6. Abt. 1-3, p. 60), that in shark-like fishes, pathological coalescence of vertebrae
is commonest at the places of connection of the paired fins with the vertebral column.

7!f-

FIG. 6.— Transverse section through posterior part
of body of an advanced Salmo fario embryo, showing
curvature and deformation due to the condition
described under (/). be, body cavity; df, dorsal fin ;
hoc, haemal arch cartilage ; int, rectum ; Ipf, left
pelvic fin ; mdl, dorso lateral muscle mass ; mdf,
small muscles connected with the dorsal fin ; nch,
notochord ; pfc, pelvic fin cartilage ; rpf, right pelvic
fin ; sc, supporting cartilage of fin ray ; sfc, spinal
cord ; wi, Wolffian body.

MINOR ABNORMALITIES— HUMP-BACK, ETC. 53

(e) Some of the sharpest twistings are exhibited by hemididymous or mesodidymous forms, after
exhaustion of the yolk store.

(/) Irregular curvature may be associated with absence, more or less complete, of the great
lateral muscle masses on one side. In a crooked specimen examined by the method of serial sections,
I found that a short distance in front of the vent the muscle masses vanish quickly and completely
on the left side (Text-fig. 6).

The blank thus left is filled up in small part by loose connective tissue, but is chiefly
compensated for by the mid-dorsal and mid-ventral structures curving round so as to encroach on
the left side. In this manner the dorsal edge membrane and the left pelvic fin approach each other
until they are separated by less than a fourth part of the circumference of the body. All the other
organs tend to be displaced or modified in form so as to suit the new shape. A transverse section
in front of the vent is illustrated in the accompanying figure. Just behind the vent the proper
muscle tissue of the left side reappears, but in an exceedingly incomplete manner, and this condition
persists to the end of the tail, which never fully recovers its bilateral symmetry. The tail fin is
bunched together so that at first sight it appears to be absent. I would refer the origin of this
deformity to an early katadidymous condition, in which, later, the left component became completely
lost.

Caiisation. We are still in the dark as regards the general question of the causation of
"twisted bodies" and the allied deformations. No doubt they are mainly congenital, and it is probable
that their production depends on some very early developmental aberration capable of being induced
by the action of external factors. But I believe the corollary is that (as in pug-head, cyclopia, etc.),
certain forms of this defect are likely to appear from time to time spontaneously, by abrupt
autogenetic variation. In the latter case they may be transmitted to descendants provided they
permit growth and survival into adult life. Thus the goldfish races referred to by Tornier (%4&)
indicate that in some of its forms the condition is hereditary, and the same thing is very strongly
suggested by various facts relating to the widely distributed and constantly recurring Lord-fish
type of cod, although Day and others class this type among " diseased " forms.

Mechanical injury has naturally been thought of as a possible cause. Thus Day (54) suggests
that the hump-backed condition of many trout in certain hill streams might be due to violence
through rolling over small cascades and falls at an early stage. Experimentally it is stated that
twisted body is one of the defects that tend to appear if developing eggs are subjected to mechanical
disturbance, or to unfavourable influences of other kinds. Holt (97) quotes Ryder as giving
some data on the influence of cold on its production, while Williamson (%69a) refers to similar data
noted by Meyer for the herring.

In certain twisted trout embryos examined by the method of serial sections I noted the
presence of premature calcifications within the sheath of the notochord. Conceivably, localised
conditions of this kind might, by preventing normal axial elongation, lead in some cases to curvature
of the backbone and in others to shortening without curvature. The membranous character of the
ossification in the formation of vertebral centra seems highly favourable to coalescence of centrum-
units wherever very marked antero-posterior shortening occurs, especially if the shortening be
accompanied by tissue irritation such as pressure might produce. Something of the kind is
perhaps indicated by the fact (Howes 103a) that the weight of the shortened coalesced centra may
be greater than that of an equal number of normal ones.

It has been suggested (Bugnion 34, Howes 102a) that the body muscles may be primarily at
fault, through undue rigidity, contraction, or failure in growth. There is no convincing evidence
for this view, however, except in the type described under (/) above, and here the original defect
must date as far back as the period of actual formation of the muscle plates.

It need hardly be added that the condition is likely to be due sometimes to ordinary
pathological conditions, for example, to inflammation, or to some other form of bone disease.

54 MINOR ABNORMALITIES— RELATING TO FINS

LOCAL DEFICIENCY OR REDUPLICATION OF THE NOTOCHORD.

In examining serial sections of trout embryos I have come across three cases of local reduplica-
tion of the notochord. In two of them the notochord is bifid at its anterior extremity, becoming
single while still in the intra-cranial region. The parachordal cartilages are broad in front and
enclose both ends of the notochord. There is no duplicity of any other structure. It is perhaps
remarkable that one of these embryos was a cy clops of type B (pp. 41-42).

The third example of reduplication of the notochord was observed in sections which had been
cut from an apparently normal embryo for the purpose of serving as a typical series. In the
middle abdominal region the notochord divides into two limbs which lie adjacent to, but quite
separate from, one another for four or five segments, and then unite again. Where they are widest
apart each has a separate sheath and separate sets of neural and haemal arch cartilages. The
adjacent cartilages are disposed exactly as in double monstrosities at the region of transition from
the double to the single condition. These cases seem to be examples of local fission affecting a single
axial organ, rather than examples of true axial duplicity.

Local deficiency of the notochord occurred in one specimen. Here the notochord, which is
normal in the cranial and cervical regions, ceases abruptly just behind the level of the pectoral fins.
After being absent for six somites, it reappears and runs backwards normally along the rest of the
trunk. Plate XXVI. fig. 113 illustrates a transverse section in the defective region. The neural
and haemal arch cartilages have fused together to form a series of half-rings below the cord.
Ventral to these the lateral muscle masses meet one another in a mesial raphe above the dorsal
aorta, forming a strong support and sling for the vertebral column and the cord.

ABNORMALITIES OF FINS.
(a) EXCESS. Examples:

Salmo irideus, a supernumerary pair of pectorals (Mazzarelli 154)',

Salmo fario, a supernumerary dorsal (Seligmann %%6);

Salmo fario, two dorsals side by side (observed by the author in a recently hatched embryo) ;

Siluris glanis, a small supernumerary pelvic almost median in position, but belonging to the
right side (Warpachowski 267);

Pleuronectes platessa, a well-formed four-rayed fin near the middle line just behind the two
normal six-rayed pelvic fins (Duncker 59 376-8);

Pleuronectes platessa, an abnormal precaudal fin frill on the left (i.e. under) side (Boulenger 30)
and M'Intosh (155);

Acanthias vulgaris, an almost median accessory fin on the top of the head (Grosser and
Przibram 85) ;

Haia clavata, an upright, oval-shaped dorsal fin in middle of back (Day 5Jf II. p. 344); a pair
of rudimentary pectoral fins on the back (Gervais 80) ;

Eaia latis, two examples of accessory fins, both being of the paired-fin type (Eennie 203).

In various goldfish races there occur: (1) excessive growth of a fin or portion of a fin, and (2)
partial or complete doubling of certain fins, e.g. of the caudals and anals. The latter condition may
be accompanied by doubling of the fin-ray supports and of the inferior spinous processes of the last
two vertebrae (see Bateson 19 pp. 451-454). 0. Storch (%40a), who describes in detail the
muscular and skeletal elements of the paired anal fins in the "Einkin " variety of goldfish,
argues that we have here the key to the evolution of the true paired fins. In this view these
fins have not taken origin from lateral fin membranes, but have arisen by the splitting of
portions of a median fin, while the limb-girdles have been evolved in response to the new
dynamical conditions at the bases of these fins.

The lung-fishes supply a number of examples, among which may be noted :

Protopterus, a bifid right pectoral (Albrecht 1) ; a trifid left pectoral (Boulenger 28, Hopley 98) ;

MINOR ABNORMALITIES— RELATING TO FINS 55

Lepidosiren, an instance in which the extremities of both pectorals underwent some degree of
branching (Goeldi 82).

Barfurth's Petromyzon larva with trifid tail should again be noted (see p. 36 of this work).

Causation. Certain cases of supernumerary fins are due to the persistence of an embryonic
twin rudiment in this form. The instances recorded by Mazzarelli (154) and Gervais (80) appear
to be of this kind, and possibly also those given by Eennie (203). It is by no means uncommon to
find among broods of young trout towards the end of the embryonic stage examples of parasitic twins,
in which one or both of the pectoral fins are the chief remaining structures. Lereboullet's (143)
work on the pike provides an occasional example of this kind observed during actual development.

The doubled dorsal fin which I observed in a recently hatched trout embryo could best be
accounted for as being due to a healed-up partial mesodidymus.

For other cases the somewhat theoretical explanation may be advanced (Eennie 203), that there
has been an attempt at independent development on the part of germ-cells that have somehow
wandered or become separated from their proper locality. Or one may speak simply of hypertrophy
with excess of the particular fin-forming embryonic material, and draw comparisons with examples of
malformation by excess in other structures, e.g. digits (M'lntosh 155).

Certain instances are clearly due to supra-regeneration after removal or to undue growth
following injury or irritation. This applies particularly to the Dipnoan cases and, according to
Boulenger (30), to that recorded by him in the plaice.

The goldfish races provide examples of great germinal variation, the extreme forms of which
have been preserved and accentuated by selection.

(b) DEFECT. Examples:

(1) Absence. The following cases may be noted: Cyprinus, with tail fin awanting, Otto (178
p. 133), Fiebiger (68), Nusbaum (173) ; Esox lucius, with false tail fin (Hofer 96); Abramis Uicca,
without pelvic fins (Brindley 32 and probably same specimen Lowne 146 p. 112); Acanthias mdgaris,
without second dorsal and caudal fins and with tail deformed (Grosser u. Przibram 85) ; goldfish
races with dorsals awanting (Tornier 249) ; Ameiurus natalis without trace of pelvic fins (Eigenmann
and Cox 62). See also p. 61.

(2) Defective development. Goldfish races with various fins extremely reduced, Tornier (249) ;
Cyprinus with reduced dorsal, Otto (178 p. 266) ; Eaia clavata, showing notch between the head and
the anterior edge of the pectoral fins, so that the latter sticks out as a horn on either side of the
former, Day (54 II. PL CLXXI. 2), Traquair (253), Johnstone,1 Vaillant (262), Williamson (270
p. 54); cyclopic Mylidbatis, with similar horns, Paolucci (181); misshapen spines and rays in
the anal fin of Sebastes, Jaquet (111) ; deformed caudal fin in Motella, Jaquet (112).

Duncker (69 375-378) refers to a number of minor abnormalities in the fins of the genus
Pleuronectes, e.g. reduction in size of the pelvic fins ; blanks in the series of dorsal and anal fin rays ;
rays developed only in their proximal or distal parts ; doubled rays ; fusion of distal ends of fin
rays ; fusion of edge of right pectoral fin with the adjacent skin.

Numerous instances are on record of flatfish with the anterior extremity of the dorsal fin
ending abruptly, so that a notch occurs between this fin and the head. The condition is usually
associated with abnormal pigmentation of the under side, and often also with incomplete migration
of the eye belonging to the side in question (see p. 56).

Causation. No single factor is sufficient to explain all cases of defective or absent fins. It
seems necessary to assume the following as possible causes : (1) injury or removal followed by
healing, and in some cases by partial regeneration ; (2) arrest of development due to external
causes acting during embryonic life, or in the early stages of growth ; (3) spontaneous germinal
variation, or reversion. Probably the first four cases cited under Absence belong to the category of
injuries, while the Acanthias provides an example of induced arrest, and the remainder are mainly
due to the third of the causes named above, though some of them may be referable to the others.

1 Report, 1905, Lancashire Sea Fisheries Committee, p. 188.

56 MINOR ABNORMALITIES— AMBICOLORATION

(c) THE SO-CALLED "TAILLESS" TROUT.

Eeference is made to this condition by Couch (4.5), Day (54 II. p. 102), Stoddart1 (239),
Peach (185), and Traquair (351-2). The latter gives a full account of the general appearance and
the structure of the fins affected, as well as a discussion of the probable causes, and his papers may
with advantage be consulted for fuller details. In these trout, neither tail nor tail-fin is awanting,
but the rays of the latter are abnormally shortened and thickened at their extremities, which tend
to be bent upwards or downwards towards a median posterior angular projection. In the specimens
described by Traquair from Islay and Kirkcudbrightshire, the extremities of the rays, besides showing
a tendency to coalesce, were deficient as regards dichotomisation and also as regards the number of
their transverse joints. Specimens from the Eiver Carron showed the same general features, except
that the amount of dichotomisation at the ends of the fin rays was not reduced.

It is important to note that the caudals are not the only fins affected. The anals usually
share in the malformation, the pectorals often, and more rarely the pelvics. The dorsal fins seem
never to suffer.

The causation is unknown. Traquair (254) gives an excellent analysis of the available
evidence on the subject.

ABNORMALITIES OF COLORATION.

(a) In Fishes generally.

Ordinary variations and abnormalities of coloration hardly fall within the scope of this work.
The larger systematic books on fishes contain mention of such variations in numerous species, and
reference may be made to the literature index (pp. xi-xvii) for the following papers or notices
dealing with the subject: Bateson (19 p. 466); Bellotti (21) ; Berg (22); Bird (24); Bolan (26);
Dean (55, albinism) ; Giinther (87) ; Kershaw (.?##); Ninni(.770); Panceri (17 9, albinism); Pettis
(191, albinism); Schneider (1880); Suomalainen (£43); Toruier (849); Traquair (255); Trois
(257-60). Attention should also be called to the important work of Cunningham and MacMunn
(50a) on the coloration of the skins of fishes, and to that of Schondorff (221) and Wagner (Intern.
Rev. ffydrobiol. Leipzig 4 1911 pp. 1-33) on the normal pigmentation of the Salmonidae.

(b) In flat Fishes.

This group requires separate mention. To begin with, its members are subject to the same
minor colour variations as occur in other fishes, for example to albinism, usually partial in character
and patchy or local in distribution.

Of much greater importance is the so-called ambicoloured condition, in which the whole or part
of the under side is more or less deeply pigmented. If not very pronounced or complete, the
condition may be unassociated with any other abnormality. Instances of the kind have been
recorded in Pleuronectes flesus, PI. oblongus, PL microcephalus, PI. j^tessa, PL limanda, Solea mdgaris,
Rhombus maxim-us, and R. laevis, while PL cynoglossus normally has many chromatophores (50 per
sq. cm. (Elmhirst 64)) on the under side. Bateson calls attention to the fact that in the better
marked examples of this and the succeeding sets of cases there is often correspondence in character
and position between the abnormal colour markings on the under side and the normal ones on
the upper side. This correspondence seems to depend primarily on the persistence and further
development of chromatophore groupings normally present in the larva. It does not accordingly
furnish a pure illustration of Homoeosis, i.e. the acquirement of secondary or " imitative " bilateral
symmetry (see Bateson, 17, 18), although this principle must be in operation in regard to minor and
specific details.

Pronounced and widespread pigmentation of the under side, particularly when the head is
wholly or partly involved, is found to be associated :

1 See footnote on p. 46.

MINOR ABNORMALITIES— AMBICOLORATION 57

(a) with shortening of the cranial attachment of the dorsal fin, so that the tip of this fin
reaches forward for some distance as a free hook-shaped process ;

(&) with the presence of spines, tubercles, or rough scales on the under side simulating in some
degree those which are normally found on the upper side. In addition, there may be unusual
thickness of the muscular layers, and undue pigmentation of the peritoneal lining, on the under side ;

(c) often also with abnormality in the position of the eye belonging to the under side, which
looks as if it had not completed its migration. In well-marked instances the eye in question may
not have travelled further round than the (apparent) dorsal edge of the head and may accordingly
be partly visible from the blind side of the fish, as in the so-called " cyclopean " condition. Or more
commonly, both eyes may be on upper side of the head, but lying a little wider apart than normal
and approximately in the same transverse line.

References to the earlier literature on the subject of these abnormalities are given by Bateson
(19, 466-473), and Cunningham and MacMunn (50a). The former quotes Steenstrup as stating
that hooking of the dorsal fin has been observed in all flat-fish, except the halibut (Hippoglossus).
For the turbot (Rhombus maximus) Cunningham makes the generalisation that hooking of the dorsal
fin is present in all ambicoloured specimens which show pigmentation on the under side as far
forward as the preopercular region, the tip of the dorsal fin, and the lower jaw. In this species
the rest of the under side of the head seems hardly ever to be pigmented. In ambicoloured
flounders, however, the whole of the under side may show pronounced pigmentation.

Of instances later than those which are noticed by Bateson (loc. cit.), we may quote the
following :

(1) Cases in which the ambicoloured condition is associated both with hooking of the dorsal fin
and with abnormality in the position of the eye :

Plcurontctes flesus, Cunningham (50 a), Elmhirst (64), M'Intosh (155, two specimens) ;

Pleuronectes platessa, Cunningham (50 a, noted by Dr. Brandt, Kiel) ;

Pleuronectes italicus,1 Ninni (1 69) ;

Pleuronectes limanda, Elmhirst (64) ', Vaillant (361);

Rhombus maximus, Cunningham (50 a), Ritchie (206, eyes approximately normal in

position); Elmhirst (64), Sacchi (211), M'Intosh (155, four

specimens) ;

Rhombus laevis, Filhol (69), Johnstone (119);

Solea vulgaris, Ninni (169), Cue"uot (46).

(2) Cases in which the ambicoloured condition is associated with a fin abnormality, but
apparently not with any marked abnormality in the position of the eye : Rhombus maximus,
Rh. laevis, Pleuronectes flesus, PL platessa Cunningham (50a, 50), PL platessa, Elmhirst (64).

The Rh. laevis mentioned under (2) above as having been described by Cunningham is of
particular interest. In the first place it is a reversed specimen, and in the second place the upper
surface of the body is extremely deficient in pigment, except anteriorly, where a certain amount occurs
in the head region, especially between the eyes and around the dorsal eye. The lower side, on the
other hand, is deeply, but not uniformly, coloured, pigmentation being absent from the head and
from the dorsal region above the head. The specimen was very young (4 '4 cm. in length).

In this connection it may be noted that one of the ambicoloured plaice recorded by Elmhirst
(64) was almost white on the upper side in the posterior quarter of the body, and had the dorsal fin
not reaching to the head but ending over the origin of the pectorals.

Mode of Origin and Carnation. It seems to be certain that in most flat-fishes (Plagusia being

1 A careful analysis of the differences between this specimen and a normal PI. itcdiclis is given by Ninni (/6'6), from which
the following points may be noted, in addition to the three principal ones which have been named : The form of the body is
elongated instead of being oval ; the month opening is almost vertical instead of being oblique ; there are well-marked
tubercles on the under side ; the interorbital space measures 4 mm. instead of 1 mm. The lateral line makes a very slight
bend above the pectorals instead of a somewhat pronounced curve. The contour of the preopercular is well marked instead of
being hardly distinguishable. The pelvic fins are equal, and the pectoral fin on the upper side is as large as that on the blind
side, instead of being somewhat smaller.
H

58 MINOR ABNORMALITIES— AMBICOLORATION

an exception) the migration of the eye from the blind side takes place prior to the forward extension
of the dorsal fin. Speaking in mechanical terms, one may say that marked incompleteness of migra-
tion is able to interpose a direct obstacle in the way of this extension, e.g. in the " cyclopean " types.
On the other hand, simple delay in the migration may hold back the extension in question till the
fin- and cranial-tissues have lost their early powers of mutual union and growth. The delayed
migration of the eye may in the end be fully carried out, e.g. in the instances mentioned under (2)
above, or it may remain somewhat incomplete, as in the numerous instances where the eyes are set
slightly further apart than in the normal condition.

Probably, however, the arrested migration of the eye is significant, less from actually
obstructing the forward growth of the dorsal fin, than in being an expression of a general aberration
of development involving other departures from the normal moulding of the head, some of which fall
under the category of excess, and others under that of defect. As Traquair (BSla) pointed out, the
dorsal fin of a flat-fish does not extend forward in the morphological middle line of the head, but is
distinctly to the blind side of this line. One of the defects may well be that a new mid dorsal edge
of the head is never prepared at all or never twisted round sufficiently to enable it to unite with
the dorsal fin.

Among characters indicating excess, we may place the undue development of spines, tubercles,
etc., on the under side. This can occur without pigmentation of the side in question. As Bateson
points out, it must be put down in the main not to reversion or atavism, but to the principle of
Homoeosis, i.e. to the tendency towards secondary or " imitative " bilateral symmetry.

An interesting attempt at analysis of the germinal factors involved has been made by Cunning-
ham in connection with his description of the brill (R. laevis), to which attention was previously
drawn. He suggests that the determinant system of the ovum may have been altered in such a way
as to match a reversed flatfish type of body, with a normal flatfish type of head, the resulting
misfit being evidenced by the condition of the dorsal fin.

This might account for the presence of pigment on the under side of the body and its absence
from the under side of the head. The upper side of the body, though white at first, would darken
later, since, as Cunningham had previously shown (50a), pigmentation can be induced even on the
under side of a flat-fish, through exposure to light. The theory fails when applied to ambicoloration
in the flounder, since specimens not infrequently occur in which the under side of the head is
pigmented as well as the under side of the body. Again, as Cunningham himself points out,
it does not explain the presence of the characteristic spines and tubercles which occur on the upper
side of ambicoloured as of normal flat-fish. The arrested migration of the eye is also not
satisfactorily accounted for, and on the whole the explanation besides depending on theoretical
conditions does not carry us as far as the facts themselves.

As Cunningham rightly emphasised, the point which needs explanation is that in ambicoloured
flat-fish the under side of the head is usually either unpigmented or has less pigment than the rest of
the under side. One might suggest the following consideration as supplying a not too far-fetched
reason. Whiteness of the under side is not merely a reduction phenomenon, but is of importance to
fishes in general by rendering them less conspicuous in the water, and thus enabling them more
readily to obtain food or escape from enemies. This does not apply in the ordinary way to flat-fish,
but even to these it is likely to be of some importance in feeding,1 that the head as seen from below
should not appear so conspicuously black as the presence of pigment, together with the want of
incident light, would make it. Whiteness of the under side of the head (and in a less degree of the
parts adjacent) might thus come to be a character of positive importance, and accordingly less liable
to variation, than the condition as regards colour of the rest of the under surface, a condition which
is traceable more directly to simple reduction. Even in Cunningham's own experiments, it was only
slowly and with difficulty that pigment could be induced to appear on the blind side of the head.

That in most cases the origin of the complex of abnormalities under discussion is a germinal

1 Crustacea and Polychaeta seem to be staple articles of diet with certain flat-fish, e.g. the plaice (Masterman in Vol. XIII.
Des Rapports et Proc&s- Verbaux du Council International pour I' Exploration de la Mer, Jan. 1911, p. 17).

REDUCTION AND PARASITISM 59

one there can be little doubt. I believe, on general grounds, that it will also be found capable
of being produced by the action of external factors, but meantime direct experimental evidence on
the question is awanting. M'Intosh (155) has noted two points of great interest, (a) that in Pleuro-
nectids which are metamorphosing normally, the future under side is paler than the other, even
before the young fishes begin to swim obliquely or to leave their pelagic habit of life ; (&) that in
particular instances where the pelagic life was unduly prolonged, the same lessening of colour
occurred.

REDUCTION AND PARASITISM.

This condition occurs in each of the classes of double monsters. All degrees of reduction on
the part of the minor twin may have been attained even before the end of embryonic life. In the
adult, supernumerary paired fins (p. 54) are probably the structures which most readily give evidence
of an originally double condition, obscured later by almost complete degeneration of one of the
components. Lereboullet's work provides cases in which the process of reduction of a twin rudiment
was actually observed in the embryo. In the trout, out of many examples, I shall only select two
for particular description, and both of them belong to Class I. (p. 12).

The first specimen is in some respects unique. My attention was directed to it by the
presence of an interruption or cleft in the right upper jaw, producing the appearance of a right-
sided harelip in what seemed to be, in other respects, a normal newly-hatched trout embryo.
On cutting serial sections, I found that a small additional eye lies at the bottom of this cleft,
in the roof of the mouth, to the right side of the middle line, and in the same transverse plane
as the normal eyes. This eye is embedded in confused muscular tissue, has a well-developed
lens, a small retina, no choroidal fissure and no choroidal gland. Its optic nerve is represented
by a small bundle of fibres which sweep over the edge of the retina to join the right normal
optic nerve (PI. XXIV. fig. 105). The retina is small and elongated antero-posteriorly. The
pigment-layer is present as such only in the posterior half of the retina. Anteriorly, the corresponding
layer is non-pigmented, richly cellular, and becomes continuous with the brain-wall just in front of
the optic recess, in such a way that the central cavity of the brain is prolonged into the space
between the retina and the pigment-layer. An optic stalk, embryonic in condition, is thus present.

Two deep grooves are found in the floor of the third ventricle and the mid-brain, each leading
down into a separate infuudibulum and hypophysis. The grooves are separated by a considerable
ridge of brain-tissue. The right hypophysis and its hypoaria are somewhat compressed ; the rest of
the brain is normal. The right palato-quadrate bar is absent and the trabeculae cranii are displaced
to the left.

Taken by itself, the supernumerary eye might seem to be simply a case of repetition, since its
nerve is derived from the right optic nerve. But the persistence of an embryonic optic stalk,
together with the presence of a double hypophysis in the brain, would seem to indicate that the
explanation is to be found in an extremely local degree of axial duplicity. It is not clear,
however, whether the accessory parts are to be looked upon as being the remains of inner
(adjacent) structures in a double-head, or as representing a reduced second head. A somewhat
analogous case is described by Gurlt (Lekrbuck der pathologischen Anatomic, ii. Theil, p. 221,
Berlin 1832). In this instance the right ramus of the lower jaw in a lamb has an accessory ramus
on its inner side with an accessory set of molar teeth. The tongue is double anteriorly. There
are two pituitary glands and two infundibula arising from a single large tuber cinereum, two
pineal glands, three pairs of corpora quadrigemina, and two aqueducts of Sylvius. Three accessory
nerves, arising from the mid-ventral line of the brain, go to an " ocular rudiment in the sphenoid."
This account is quoted from Taruffi (Storia della Teratologia, vol. iii. p. 115). For other examples
of duplicity of the hypophysis see Ahlfeld (Die Missbttdungen der Menschen, p. 73, Leipzig 1880)
and Bland Sutton (Transactions of the Odontological Society (Dental Record}, 1888, pp. 73-78).

In the trout, if new and more definite cases of the kind described above become available
to found on, it may be necessary to institute a Class of anadidymi which will come in front of

60 PATHOLOGICAL CONDITIONS OF OVUM, ETC.

my Class I. (p. 3) as showing duplicity in a still less marked degree. Meantime we do not
need to do more than call attention to this possibility, pointing out in its favour that Barbieri's (7)
early double trout embryo exhibited much greater doubling in the floor than in the roof of the
brain, while Lereboullet (see p. 11) showed in general that a very considerable degree of early
axial duplicity can afterwards become completely or almost completely lost, through the working
of secondary fusion. Schmitt's Group 7 makes provision for some such Class. At the same
time, the little that is known on the subject, renders the alternative possible, that obscure
and limited cases of axial duplicity may have had their origin in union of the longitudinal or
parallel type (p. 29).

The second specimen I shall describe under the heading of Eeduction was quite normal in
appearance, except for the presence of a tiny refractive knob behind the left eye. Examination
of serial sections shows the knob to contain a lens of considerable size, enveloped in muscle-fibre,
but unaccompanied by any other eye-structure, and lying in front of an exceedingly minute fore-
brain and third ventricle. The cavity of this third ventricle communicates with the mid-brain cavity
of the normal head (PL XXIV. fig. 106). The embryo was quite lively when obtained, and its
chances of survival would probably not have been appreciably diminished by the small tumour
in question. It will be seen from fig. 2 that the functional eyes and fore-brain belong to a
predominant right twin head, as also do the olfactory organs, the mouth, and the anterior cranial
cartilages generally. The back part of the brain and the whole of the body are, however, composite,
since their left moiety represents structures which are continuous with the left side of the
left (aborted) twin, while their right side is a continuation backwards of the right side of the
right twin. This gives an even more complex mixture of " individualities " than is found in
ordinary cases of symmetrical double monstrosity.

PATHOLOGICAL CONDITIONS OF THE OVUM OK EAELY EMBKYO.

Some of these have been referred to already, for example under the last two of Lereboullet 's
types (p. 10). Numerous single instances may be found throughout this author's account of his
observations. Attention should also be drawn to Kauber's description of the following anomalous
conditions in the development of pike and salmonid eggs (3025 (668-702)).

(a) Examples (salmonid) just at the end of segmentation, of blastoderms showing irregularities
of outline or surface, and want of uniformity in the size of the segmentation cells.

(5) Instances (salmonid) in which the margin of the blastoderm has failed completely, or almost
completely, to become thickened in order to give rise to the embryo-rudiment. In such -cases the
blastoderm may extend as far down as the equator or even the lower pole of the egg. The growing
margin tends to become irregular, showing projecting angles and lobes.

(c) Defective formation of the anterior, middle, or posterior portions of the embryo. Five
examples of salmonid eggs are given in which the anterior part of the embryo is defective ; three of
the pike in which the anterior and middle portions are wanting ; one (salmonid) in which the
middle and posterior portions are defective, and another salmonid in which the embryonic rudiment
has not been formed at all.

(d) Partial or total dehiscence of the body halves. This condition is referred to under
Mesodidymus (p. 25).

(e) Oblique position of the embryonic rudiment. Several examples were noted in the salmon.

Girdwoyn (81) makes reference to embryos without eyes, with both eyes present but rudi-
mentary, with only one eye and that in its normal place, with well developed head but rest of body
rudimentary, without head but with developed body, etc. Loeb (14&) describes fish embryos without
circulation, while Stockard (1236) and others refer to various pathological conditions produced in the
course of their experiments.

ADDITIONAL MINOR ABNORMALITIES 61

ADDITIONAL ABNORMALITIES.

Having reference for the most part to single organs.

Barbels. Additional barbels in Ameiurus natalis (Eigenmann and Cox 62}. Doubling of a
regenerated barbel in Silurus (Eoth 209). »

Brain. Anomalous cerebellum of Alopias (Leger 138).

Torpedo embryo with a neuro-epithelial organ in relation with the cerebral ganglia
(Coggi 40).

Eyes.

Absence of one eye. Giraldus Cambrensis (III. c. x.) speaks of eels, trout and perch existing in the
lakes of Snowdon which only possessed the right eye but were invariably blind with the
left (quoted from Day 54 H. P- 102). It is stated somewhere that fish with only one
eye are still from time to time caught in the district named.

Lowne (146 525-6, pp. 124-5) catalogues the head of a large carp with entire
deficiency of the eye on the left side, and the head of a barbel with absence of the right
eye.

Williamson (270 p. 53) describes an angler-fish with only one eye.
See also Stoddart (239) quoted in footnote on p. 46 of this work.

Atrophy. Mencl (157) records the presence of two lenses without other eye structures. Compare
the case of parasitism described on page 60. A somewhat similar condition in which the
lens alone of all the eye structures has survived is shown in PI. XXVI, fig. 112, of this
work, which illustrates a transverse section of a single atrophic head. The mouth, the
lower jaw, the trabeculae, and the palato- quadrates are absent. One large lens, clothed
with muscle-fibres, is present on the right side ventrally and compresses the lower part
of the brain. A second smaller hour-glass-shaped lens lies beside it, all other ocular
structures being deficient.

Egg, double. Vayssiere (264) describes an egg capsule of the porbeagle shark, which contained two
separate ova, each with a growing blastoderm. A somewhat similar instance in Scyllium
is recorded by Joseph (120).

Fins, absent or defective. The following instances, collated by Freund (70), fall to be added to
the list on p. 55 : a Plwxinus laevis with defective pelvic bones (Fatio), and another with
an absent pectoral (Lunel) ; Silurus glanis -with atrophic pectoral fin (Jaquet); Leudscus
dobula without pelvic fins (Martens) ; Sqtuilius cavedanus without pelvic fins and with
reduced pelvic girdle, carps without pelvic fins but with pelvic girdle and musculature
(Hofer) ; goldfish without dorsal fins (Hofer) ; goldfish without left pelvic fin (Day) ;
thirteen examples of Gasterosteus pungitius without pelvic fins or girdles (Day) ; goldfish
with various other abnormalities of fins, including triple or even quadruple lobing of the
caudal fin (Pavesi, Watase, Cori, Kishinouye). See Freund, loc. cit. p. 726-7. For other
papers on goldfish, or references to goldfish literature, see Bateson (19 pp. 309, 451, 453),
Pouchet '(194), Eyder (210), Stoll (240), Storch (240a), Tornier (249).

Fin rays. Unusual elongation of second anal fin ray in Protcracanthus (Pellegrin 186). Various
minor abnormalities in Cyprinidae (Knauthe 125). See also p. 55.

Gill absent in trout (Stoddart 239) quoted in footnote on p. 46.

Gill-cleft. Reduction of the last gill-cleft in Squalus acanthias L. (Ekman 63b).

Gill-tufts present instead of teeth (see under Teeth).

Gill-covers. Day (54 II. 102) mentions trout from Maltham Tarn, Yorkshire, with imperfectly
developed gill-covers, and Lowne (146 p. 55) catalogues a similar specimen from Loch
Assynt, Sutherlandshire. See also Miall (158a). Some further instances are given by
Hofer (96a).

62

ADDITIONAL MINOR ABNORMALITIES

Gonads. Williamson (370 p. 64) describes a partly hardened tough tuberculated ovary in the
haddock, and Hefford (91) notes arrest of development in one gonad of a male conger.

Hernia. Mudge (166) describes an adult female dogfish with hernia of part of the stomach and
certain other organs into the lumen of the pharynx.

Kidney. Howes (100) gives an account of variations in the kidney of Eaia clavata which point in
his opinion to a process of adaptative evolutionary change.

Lateral line. Supplementary in Acipenser (Jaquet 110).

Do. Interrupted in a haddock (Mr. J. Eitchie, in a note to the author).

Mouth. Several instances have been put on record of fair-sized fish in which the mouth opening
was either absent or minute, the condition being associated with other cranio-facial
defects. Cases of the kind are noted by Laurence (186a) in Sderognathus, and ,by
Hofer (96a), St. Hilaire (213 I. p. 285), and Steindachner (234a) in Cyprinus. Freund
(70 p. 719) also cites records from Cyprinus by Schiemenz and Bruyant.

The bones of the upper and lower jaw exhibit various degrees of defect or malfor-
mation, the actual closure or narrowing of the mouth being effected by skin or membrane
extending over their representatives. Food must have entered the pharynx through the
gill slits with the respiratory current.

It will be remembered that the specimen described by Donnadieu (see p. 51 of this
work) had an extremely small mouth opening, and that a similar condition (p. 42) was
found in one of my cyclopic trout embryos.

No doubt in its best marked forms the condition is a congenital one, although
healing after wounds certainly accounts for many minor cranio-facial defects. A number
of instances of this kind have been brought together by Hofer (96a) and Freund (70),
while Garstang (75) notes a remarkable malformation in the mouth of the sea-bream,
which he thinks was probably the result of injury.

Muscle. An abnormal superior oblique in Carcharias (Allis 3).

Nerves. Variations in the pelvic plexus of Acanthias (Punnet 196).

Notched body. Johnstone (116) describes a plaice with a large notch on the ventral edge of the
posterior part of the body, and thinks that injury was the probable cause.

Ovaries. Three in a sturgeon (Grimm 83). See also under Gonad.s.

Oviduct. Present in an adult male skate (Matthews 151). See also Borcea (27).

Pyloric caeca. Branching in Centrolophus (Biggio ^OJf).

Respiratory organs. Variations in the lamprey and hag (Howes 1 08).

Reversal (Metastrophy, Cunningham, 50) seems to occur with varying frequency in all, or
practically all, the species of flatfish. In this condition the side which is normally the
under or blind side lies uppermost, and there is a corresponding interchange between
the two sides as regards their external features and their ocular and cranio-facial
characteristics. The viscera, however, retain their usual disposition. Thus alike in
normal and in reversed flatfish the liver hangs down to the left side of the abdominal
cavity and the coils of intestine are directed to the right (Cunningham, 50a p. 861).

Scales. Increased number on one side in the pilchard (Bateson 15-16 and Lowne 146 p. 119).

Snout. Bifid tip of snout in Acipenser ruthenus (Jaquet, quoted in Freund 70 p. 719).

Spiral valve of Eaia. Exhibiting a large degree of variation (Parker 183a).

Teeth. Male Raia with blunted teeth (Day 54 II. p. 344).

Acanthias with gills instead of teeth in the upper jaw (Grosser 85).
Rhinoptera, with abnormal dentition (Woodward 273).

Telescope eyes of goldfish (Tornier 2J/.9, and others, see above under fins).

INDEX TO STRUCTURES AND SUBJECTS.

The italic numbers refer to pages in the text, while the erect Roman and ordinary numerals refer to Plates and Figures.
01. I., 01. ii., etc. indicate different Classes of double monstrosity, while a., trans., norm., horiz., etc. are abbreviations
respectively for section, transverse, normal, horizontal, etc.

Absence, 52 (of lateral muscles) : 54 (of notochord) : 55, 61 (of
certain fins) : 61 (of eye) : XXVI. 113 (absent notochord).

Acardiac condition in triple monster, 34, 39.

Acquired, abnormalities, xi. See also under environment.

Air-ducts or air-sacs, 13 (normal): 15 (01. I.): IS (01. II.) :
XII. 45 (a. horiz. 01. 1.) : XXII. 95, XXIII. 97, 98 (triple
monster).

Albinism, 56.

Alimentary canal. See under intestinal canal and stomach.

Alternation of generations, xi, 5.

Ambicoloration, 56-57.

Amphibia, 5 (twinning) : 25 (hemididytnus) : 37 (super-
numerary parts).

Anadidymus, 3 (classification): 6-7 (mode of origin, frequency,
etc.) : 9-23, 30-33 (structure).

Anakatadidymus, 3 (classification) : 7 (mode of origin, etc.) :
23-24 (structure, etc.). See also 30-32.

Anal fin. See under ventral edge membrane.

Aorta (dorsal), 14 (01. i.) : 18 (01. n.) : IV. 18-19, VI. 24-26
(s. trans, norm.): VI. 27-29 (s. trans. 01. vi.): VII. 32
(triple monster) : IX. 38 (s. trans. 01. i.): XVI. 55 (s.
horiz. 01. in.): Ao in XVIII. 67 (01. I.), 68 (01. n.):
XXIII. 69 (triple monster) : XXVI. 113 (s. trans, absent
notochord).

Aorta (ventral). See under truncus arteriosus.

Aortic arch, or root, IS (normal): 14 (01. I.) : 17 (01. n.):
IV. 18 (s. trans, norm.): XI. 42 (s. horiz. 01. i.): XIV.
51 (s. trans. 01. n.) : XVIII. 67 (01. i.), 68 (01. n.).

Apical growth, 28.

Arrest of development. See under defect and parasitism.

Artery. See under carotid, caudal, choroidal, hyoid, pseudo-
branch.

Artificial breeding of trout, 2 (first account of).

Ascidia, 48 (hermaphroditism).

Atavism, 5 (in relation to twinning, etc.): 45, 48 (herma-
phroditism) : 50 (pug-head).

Atrophy. See under defect and parasitism.

Auditory cartilage. See under labyrinth and periotic.

Auditory organs, inner, 10 (Moser's specimen) : 17 (01. II.) :
26 (hemididymus). See also under otocyst, saccule,
semicircular canal, utricle.

Auricle of heart, 13 (normal): 19 (01. m.): 20 (01. iv.):
IV. 18 (s. trans, norm.) : IX. 38 (s. trans. 01. i.) : XVI.
54 (s. horiz. 01. HI.): Au in XIX. 73 (normal), 74
(01. in.), 75(01. IV.).

Atitogenetic origin of various abnormalities, xi, 5, 45, 51, 55.
Axial organs, 8, 29 (in parallel sets) : 25 (in hemididymus) :

38 (in triplicity). See also under notochord, spinal cord,

etc.

Basihyal (or hypohyal) : 11 (normal): 16 (01. II.): XVI.

54 (s. horiz. Cl. in.): bh in XVII. 58 (01. I.): Bff in

XVIII. 64 (01. i.), 65 (01. n.).
Barbels, additional, 61.
Bile ducts, 19 (01. in.): 20 (01. iv.): XVI. 54 (s. horiz.

01. HI.) : XXII. 95 (triple monster).
Birds, 1, 5, 8, 29, S3, 37 (double or multiple forms) : 38-39

(triplicity).

Bladder. See under air and urinary.
Blastocyst, 8 (mammalian) : 38 (of sheep) : 38 (of Tatusia).
Blastoderm, 6, 37 (extension over yolk) : 6, 8-10, 35-37 (two

or more embryos on one blastoderm).
Body cavity, 24 (anakatadidymus) : 25 (mesodidymus) : 48

(hermaphroditism): V. 23 (s. horiz. norm.): XII. 46

(s. horiz. 01. I.).
Brain cavity, in parallel union, 29 : diagrams of, XVIII. 69

(normal), 70 (01. i.), 71 (01. n.), 72 (01. i.): XXI. 90,

93 (showing planes of sections).
Branchial cartilages. See under visceral arch.
Bulldog-head, 48 et seq.

Calcification, premature, of notochord, 53.

Canal of spinal cord, V. 21 (a. horiz. norm.) : XV. 52 (s.

horiz. 01. in.): Spc in XVIII. 69-71. See also under

spinal cord.
Cardinal vein, 12 (normal): 19 (01. m.): 20 (01. iv.): IV.

19-20 (s. trans, norm.): V. 23 (s. horiz. norm.): XVI.

54 (s. horiz. 01. in.).

Carotid artery, 13 (normal) : 14 (01. i.) : 17 (01. II.).
Carron river, " tailless " trout from, 56.
Cartilage. See under basihyal, ceratohyal, copula, glosso-

hyal, haemal arch, hyoid, hyomaudibular, hypohyal,

Meckel's, neural arch, olfactory, palato-pterygoid, para-

chordal, pectoral girdle, periotic, tegminal, trabeculae

cranii, visceral arch.
Caudal artery, 20 (01. v.) : 23 (01. vi.).
Caudal fin, 13 (normal): 22 (Cl. vi.): 34 (triple monster):

53 (hemididymns) : 54-55 (various abnormalities).
Caudal vein, 20 (Cl. v.) : 21, 23 (Cl. vi.) : VI. 26 (s. trans.

norm.) : VII. 31 (s. trans. Cl. VI.), 32 (triple monster).

64

INDEX TO STRUCTURES AND SUBJECTS

Causation, 4 (of double monstrosity) : 33 (triple monstrosity) :

44 (of cyclopia) : 48 (of pug-head) : 52-53 (of twisted

body) : 55 (of fin abnormalities).
Cavity, cerebral lobes, 41-43 (eyclopia) : III. 13 (a. trans.

norm.) : VIII. 35 (Cl. I.) : X. 39-40, XII. 43, 44 (s. horiz.

Cl. I.) : XIII. 47 (s. trans. Cl. II.) : GFB in XVIII. 69

(normal), 70 (Cl. I.), 71 (Cl. 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. Cl. I.) : X. 39-40

(s. horiz. Cl. I.): XV. 52-53 (s. horiz. Cl. in.): COL in

XVIII. 69 (normal), 70 (Cl. I.), 71 (Cl. n.), 72 (Cl. i.):

XXI. 91-93 (diagrams) : CO in XXIV. 106 (parasitism) :

XXV. 109-110 (semi-cyclopia).
Ceratohyal, 11 (normal): 16 (Cl. n.): XVI. 54 (s. horiz.

Cl. in.).
Cerebellum, 61 (anomaly of) : IV. 17 (s. trans, norm.) : V. 21

(s. horiz. norm.) : XV. 52 (a. horiz. Cl. in.) : XXVI. Ill

(semi-cyclopia).
Cerebral lobes, 13 (01. I.): 41-42 (cyclopia): neural epithelial

organ connected with, 61 : III. 13 (s. trans, norm.) : V.

21-22 (s. horiz. norm.) : VIII. 35 (s. trans. Cl. I.) : X. 39

(s. horiz. Cl. I.) : XIII. 48 (s. trans. Cl. n.) : XXV. 107-

108 (cyclopia) : XXVI. 112 (s. trans, atrophic head).
Chick. See under birds.
Chordate, 48 (ancestry).
Choroidal arteries, ^(normal): 14 (Cl. I.): 41 (cyclopia):

AChor in XVIII. 67 (Cl. I.), 68 (Cl. II.): XXVI. Ill

(cyclopia).
Choroidal fissure of eye, 14 (Cl. I.): 41-42 (cyclopia): XI.

41-42 (s. horiz. Cl. I.): XXIV. 104 (cyclopia), 105

(supernumerary eye) : XXV. 110 (semi-cyclopia).
Choroidal gland, 113 (normal) : 14 (Cl. I.) : 41, 4% (cyclopia) :

XXV. 108 (semi-cyclopia).
Circulation, 34 (in anakatadidymus) : GO (embryos without).

See also heart, etc.
Cirripedes, 48 (hermaphroditism).
Classification, 3-4, 31-32 (in double monstrosity): 38 (in

triple monstrosity) : 40 (in cyclopia) : 47 (in herma-
phroditism).

Coalescence of vertebrae. See under synostosis.
Cold, influence of, in producing twisted body, SSt.
Coloration, 56.

Concrescence, theory, 6, 10, 25-28.
Copula visceral arch II. (glossohyal), 11 (normal): 16 (Cl. n.):

III. 13 (s. trans, norm.): XVI. 54 (Cl. in.): OH in

XVIII. 64 (Cl. i.), 65 (Cl. n.).
Copula visceral arch III., VIII. 36 (s. trans. Cl. I.) : XVI.

54 (s. horiz. Cl. III.): Cop in XVIII. 64 (Cl. I.), 65

(Cl. II.).
Cranial skeleton, 11 (normal) : 13 (Cl. I.) : 15 (Cl. II.) : 40, 43

(cyclopia) : 44 (semi-cyclopia) : 4S (pug-head) : 49 (round-
head). See also XVII., XXI.
Cranio-facial defects, 40-44 (cyclopia) : 48-51 (pug-head, etc.) :

59-60 (parasitism) : 61 (atrophy) : 62 (absence of mouth,

etc.).

Crustacea, 58 (as food of flatfish).
Curvature, 25, 53 (of body and tail in hemididymus) ; 61-53

(do. in hump-back, etc. ) : 48-50 (of snout in pug-head

and round-head).
Cnvier, duct of, 18 (Cl. in.) : 20 (Cl. IV.) : XIX. 73 (normal),

75 (Cl. in.), 74 (Cl. iv.).
"Cyclopean " position of eye in ambicoloured flatfish, 57.

Cyclopia, 7 (in double monsters) : 40-45 (classification, struc-
ture, comparison, etc.).
Cyclopic ray, ,?, 43.
Cyclops, 1, 43.

Defects, 10, 60 (in formation of embryonic rudiment) : 25-29 (of
inner structures in hemididymus) : 33-34 (in triple mon-
ster) : 40-42 (in cyclopia) : 48-51 (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, GO (in parasitism, etc.).

Dehiscence, of body-halves (Kauber), 26 : of ova into body-
cavity, 47.

Development, 6, 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. (Cl. I.) : XIII. -XIV. (Cl. n.).

Dichotomy or fission, 8, 27, 28.

Dioecious condition in fishes, etc., 46-48.

Distance between centres of embryo-formation on blastoderm,
6, 7, 30, 35, 36.

Dog, 37 (triplicity) : 50 (pug-head).

Dorsal edge membrane, incl. dorsal fins, 12 (normal) : 20
(Cl. v.) : SI, 23 (Cl. VI.) : 34 (triple monster) : 5;? (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. Cl. VI.) : VII. 32-34 (triple monster).

Double eggs, 61.

Double monstrosity in fishes, passim, in Chap. I. pp. 1-32 (see
Contents, vii-viii) : S, 28 (in vertebrates generally).

Dropsy of central nervous system and cyclopia, 40.

Duct. See under air, bile, Cuvier, Wolflmn.

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) : GO
(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, 28.

Environment, and causation of abnormalities, xi, 5, 43, 44,
50, 51, 53, 55, 58. 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, 30, 43, 44, 53. See also environ-
ment.

External factors. See under environment.

Eyes, 3, 11 : 14 (Cl. I.) : 26 (hemididymus) : 36 (quadruplicity) :
40-45 (cyclopia) : 49 (pug-head) : 50 (round-head) : 56
(in ambicoloration) : 59 (supernumerary) : 60, 61 (absence) :
60, 61 (represented by lens) : III. 13-la (s. trans, norm.) :
V. 21-23 (s. horiz. norm.): VIII. 36, IX. 37 (s. trans.

INDEX TO STRUCTURES AND SUBJECTS

65

Cl. i.): X. 39-40, XI. 41-42, XII. 43-46 (s. horiz. Cl. I.) :

XXIV. 100 (oyclopia), 105, 106 (supernumerary eyes) :

XXV. 107-110 (oyclopia and semi-cyclopia).
Eyes, muscles. See under obliquus and rectus.

Fins. See under anal, caudal, membrane dorsal and ventral
edge, pectoral, pelvic. 5.4-56', 61 (abnormalities of) : 55,
56, 61 (fin rays).

Fishes. See special Index, 68,

Fission, S (of embryonic rudiment) : 27, 38 (of embryonic axis
in hemididymus) : 54 (of single axial structure).

Fontanelles, 11 (normal) : 13 (Cl. I.) : 16 (Cl. 11.) : /l,/2,/3
in XVII. 57 (normal), 59 (Cl. I.), 61 (Cl. n.) : XXI. 89,
92 (diagrams).

Fowl. See under birds.

Fragmentation of ova, 30.

Frequency, 1 (of double monstrosity in fishes and birds) : 33
(of triple monstrosity in fishes and birds) : 7, 28 (of differ-
ent types of double monstrosity) : 41 (of cyclopia).

Frog larvae, 44-

Fusion, 4, 6 (primary) : 4, 11, 15, 30 (secondary) : 40-44 (of
cerebral lobes, mid-brain, eyes, etc., in cyclopia) : 12-29
(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., 17 (01. II.) : IV. 17 (s. trans, norm.) :

XIV. 51 (s. trans. Cl. n.).
Ganglion, vagus, 17 (Cl. n.) : IV. 18 (s. trans, norm.) : IX.

38 (s. trans. 01. I.).
Gastrulation, 1, 30 (double in Petromyzon) : 6 (centres in

double monstrosity).

Germ cell, predisposition in, 5, 33. See also under autogenetic.
Germinal area, mammalian, S, 3S.
Gill. See also under pseudobranch and visceral arches. 34

(triple monster) : 61 (absent) : 62 (variation in lamprey) :

62 (replacing teeth) : XI. 42, XII. 45, 46 (s. horiz. 01. I.) :

XVI. 54 (s. horiz. 01. ill.).
Gill cover, 61 (absent or deformed) : V. 23 (s. horiz. norm.) :

XVI. 54 (s. horiz. Cl. HI.).
Glomerulus of head-kidney, 10 (absence) : 12 (normal) : 14

(01. i.) : 18 (Cl. n.) : 19 (01. in.) : V. 23 (s. horiz. norm.) :

XI. 41 (s. horiz. 01. I.) : Ol in XX. 83 (normal), 84, 85

(01. i.): 86 (Cl. iv.): 87 (01. in.).
Glossohyal. See copula visceral arch II.
Gonads, 47, 48 (in hermaphroditism) : 62 (defects).
Grooves, 24 (on surface of yolk) : 41, 42 (in floor of brain in

cyclopia) : 59 (in supernumerary eye).

Haemal arch cartilage, 12 (normal) : 13 (01. I.) : 17 (01. II.) :

52 (hemididymus): IV. 19 (s. trans, norm.) : XII. 43 (a.

horiz. Cl. i.) : XVI. 55 (a. horiz. Cl. in.).
Harelip, apparent, in a trout embryo, 59.
Head-kidney or pronephros, 12 (normal): 14 (Cl. I.): 18

(01. II.): 19 (01. in.): 20 (Cl. IV.): IV. 19 (s. trans.

norm.): V. 23 (s. horiz. norm.): XI. 41, XII. 45 (s.

horiz. 01. i.): XXII. 96, XXIII. 97 (triple monster):

HK in the last five Figs, cited under glomerulus.
Hemididymus, 25-29 (in Salmonidae) : 26 (in Esox) : 55

(doubling of dorsal fin) : 52 (twisting of body). See also

under katadidymus and mesodidymus.

Hereditary nature, of abnormalities, xi, 50. Sec also 53.

Hermaphroditism, 46-48.

Hernia in a dogfish, 62.

" Heterocercal," portion of tail, 12 (normal): 31, 22 (01.

VI.).

Hiatus or cleft, in mesodidymus, 25, 26, 28.

Hog-back. See under hump-back.

Homoeosis, 56, 58.

Homologous or identical twins, 7, 37 : triplets, 38.

Hooking of dorsal fin in flat fishes, 55, 57.

Hump-back, 51-53.

Hyoid artery, 12 (normal) : 14 (01. I.) : 17 (01. n.).

Hyoid cartilage. See under visceral arch II.

Hyomandibular, 11 (normal): 13 (Cl. i.): 16 (Cl. n.): 49
(pug-head): 50 (round-head): IV. 17 (s. trans, norm.):
V. 23 (s. horiz. norm.) : XL 41, 42, XII. 45, 46 (s. horiz.
CL i.): XIII. 48, XIV. 49, 51 (s. trans. 01. n.): » in
XVII. 56 (normal), 58 (01. i.), 60 (01. ir.): XXI. 88, 89
(diagrams): XXVI. Ill (cyclopia).

Hypoaria, 13 (01. I.): 41, 42 (cyclopia): IX. 37 (s. trans.
01. i.): XI. 41, 42, XII. 45 (s. horiz. Cl. I.): XIV. 50
(s. trans. Cl. n.) : Hya in XVIII. 72 (01. I.).

Hypohyal. See under basihyal.

Hypophysis, 13 (Cl. I.) : 41, 42 (cyclopia) : 50 (double) : V. 23
(s. horiz. norm.) : XI. 41, 42, XII. 45, 46 (s. horiz. 01. i.):
Hyp in XVIII. 72 (01. 1.).

Induced, abnormalities. See under acquired.

"Individualities," mixing of, in double monster, 60.

Infundibulum, 13 (01. i.) : III. 16 (s. trans, norm.) : V. 23 (s.
horiz. norm.): VIII. 36 (s. trans. Cl. i.) : XII. 45, 46 (s.
horiz. 01. i.) : XIV. 49 (s. trans. Cl. n.) : Inf in XVIII.
72 (Cl. i.): XXV. 109, 110 (semi-cyclopia).

Injury, mechanical, 53, 62. See also under experimental.

Intestinal canal, 12 (normal): 15 (01. I.): 18 (01. n.): 19
(01. m.) : SO (01. iv.) : 21 (Cl. v.) : 23 (Cl. VI.) : 25 (hemi-
didymus) : 34 (triple monster) : 62 (hernia) : IV. 20, VI.
24 (s. trans, norm.) : VI. 27, XXVI. 114 (s. trans. Cls. v.,
vi.): XVI. 54 (s. horiz. 01. in.): XXVI. 113 (absent
notochord) : XXII. 95, XXIII. 97-99 (triple monster).

Invertebrates, 5 (ova).

Islay, tailless trout from, 56.

Iter or Sylvian aqueduct, III. 16 (s. trans, norm.): IX. 37
(s. trans, a. I.) : XIV. 50 (s. trans. Cl. n.) : XV. 53 (s.
horiz. 01. in.): Git in XVIII. 69-71, XXV. 110 (semi-
cyclopia).

Jaws. See under visceral arches. Also 41< 4% (cyclopia) : 4$
(pug-head) : 50 (round-head) : 51 (lower absent) : 59
(accessory) : 62 (gill tufts instead of teeth) : 62 (mouth
absent): VIII. 35 (s. trans. Cl. I.): XVI. 54 (s. horiz.
01. in.): XXV. 107, 108 (cyclopia).

Jugular vein, 18 (01. n.): 19 (01. in.): IV. 18 (s. trans,
norm.): IX. 38 (s. trans. 01. I.): XIV. 51 (s. trans.
01. n.): VI in XIX 73 (normal), 74 (01. HI.), 75
(01. iv.).

Katadidymus, 3, 5, 6, S, 25. See also under hemididymus.
Kidney, 62 (variations in). See also head-kidney, Wolffian

body.

Kirkcudbright, tailless trout from, 56.
Kupffer, two vesicles of, 29.

66

INDEX TO STRUCTURES AND SUBJECTS

Labyrinth, V. 22 (a. horiz. norm.): XII. 43-44 (s. horiz.
01. I.): XIV. 51 (s. trans. 01. n.): XXVI. Ill (semi-
cyclopia). See also under auditory and periotic.

Lacerta, S (double blastoderm) : 2!) (double primitive streak) :
37 (trifid tail).

Laminae, paraehordal, 11 (normal) : 13 (01. I.).

Lateral line, 57 (curvature of, in ambieoloured flat fish) : 63
(interrupted) : 62 (supernumerary).

Lateral muscles, 25, 52 (hemididymus). See also under
muscle plates.

Lateral union, 3, 6, IS.

Lens, H (01. I.): 41-44 (cyclopia and semi-cyclopia) : 44
(ectoderm for) : 60, 61 (without other eye structures) : III.
14 (s. trans norm.) : V. 22 (s. horiz. norm.) : VIII. 36, IX.
37 (s. trans. 01. I.) : X. 40, XII. 44 (s. horiz. 01. I.) : L in
XXIV. 106 (parasitism) : XXV. 107, 110 (cyclopia and
semi-cyclopia) : XXVI. 112 (s. trans, atrophic head).

Light, influence of exposure to, 58.

Limpet, 47.

Lip, lower. See under jaw. Also XIII. 47-48 (s. trans. 01. n. ).

Literature, general index of, xi-xvii. See also under the
various chapters and sections on particular malforma-
tions.

Lithium chloride, .'/'/.

Liver, 19 (01. ill.): 20 (01. iv.): 34 (triple monster): 62
(in reversed flatfish): XII. 46 (s. horiz. 01. I.): XXII.
95, XXIII. 97-98 (triple monster) : XXVI. 114 (s. trans.
01. v.).

Lizard. See under Lacerta.

Longitudinal or parallel union, 4, 29, 30.

Magnesium chloride, 43, 44-

Maltham Tarn, trout from, 61.

Mammal, 5, 8 (double and multiple forms) : 35, 36, 37

(triplicity) : 39, 43 (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, S, 4, n (union at) : IV. 18 (s. trans.

norm.): V. 21-22 (s. horiz. norm.): IX. 38 (s. trans.

01. I.) : X. 39-40, XII. 43-44 (s. horiz. 01. i.) : XIV. 51

(s. trans. 01. n.) : XXIII. 97-98 (triple monster) : XXVI.

Ill (semi-cyclopia).
Membrane. See under dorsal edge, ventral edge. 35, 26

(intermediate, in hemididymus) : 23 (preanal edge, in

anakatadidymus).
Meroblastic ova, xi, 5.
Mesentery, SO, 21, 22 (double in Cls. iv., v., vi.) : VI. 28-29

(a. trans. 01. via.).

Mesial muscular mass. See under muscle segments.
Mesodidymus, 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-f>%.
Mode of origin, of double and multiple monstrosity, xi, 6-8 :

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, 48.

Mouth, 15 (01. i.): 16 (01. n.): 41-4,2 (cyclopia): 44 (semi-
cyclopia) : 40 (pug-head) : 50 (round-head) : 51 (defective) :
61, 62 (absence of mouth, etc.) : III. 15 (s. trans norm.):

XIV. 49, 51 (s. trans. 01. 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. 01. I.): XIII. 47
(s. trans. 01. n.): 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 (01. I.): 17 (01. H.): 19 (01. m.): 35-27
(hemididymus) : 20 (parallel union) : 5,2 (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. 01. i. ) : X. 39-40, XI. 41,
XII. 43-44 (s. horiz. 01. I.): XV. 52-53, XVI. 54-55 (s.
horiz. 01. in.) : XXIII. 97-98 (triple monster).

Muscular raphe, 54, XXVI. 113 (s. trans, absence of noto-
ehord).

Mythology and teratology, 1.

Nasal cartilage. See under olfactory.

Nerve II., 13-14 (01. I.): 41, 42, 44 (cyclopia): 59 (super-
numerary eye): III. 15 (s. trans, norm.): VIII. 35 (s.
trans. 01. I.) : XI. 41-42, XII. 46 (s. horiz. 01. I.) : XXV.
108 (cyclopia) : XXV. 110 (semi-cyclopia).

Nerve III., 13 (01. I.) : III. 16 (s. trans, norm.).

Nerves of pelvic plexus, 62 (variations).

Nerves, sensory, and growth, 15.

Neural arch cartilage, 12 (normal) : 13 (01. i.) : 17 (01. n.) :
19 (01. in.) : 52 (twisted body, etc.) : 54 (absent note-
chord) : IV. 19 (s. trans, norm. ) : X. 39-40 (s. horiz.
01. i.) : XV. 52, XVI. 55 (s. horiz. 01. in.) : No, in XVII.
62 (normal), 63 (duplicity) : XXVI. 113 (s. trans, 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 (01. 1.) : 17 (01. n.): 18 (01. in.):

19 (01. iv.): 20 (01. v.): 21-22 (01. vi.): 25 (hemididy-
mus) : 29 (parallel union) : 34-36 (triple monster) : 54
(absent) : 54 (double) : IV. 17-19, VI. 24-26 (s. trans,
norm.): V. 22 (s. horiz. norm.): VI. 27-29 (s. trans.
01. via.): VII. 30 (s. trans. 01. VI b.), 31 (s. trans. 01.
via.): XXVI. 114 (s. trans. Cl. iv.): VII. 32-34 (triple
monster): IX. 38 (s. trans. 01. I.): X. 40, XI. 41-42,
XH. 44-46 (s. horiz. 01. I.) : XIV. 51 (s. trans. 01. II.) :

XV. 53, XVI. 54-55 (s. horiz. 01. m.): XXVI. 113
(absence of) : XXIII. 97, 98-99 (triple monster) : XXI.
89-90 (diagrams) : n in XVII. 56 (normal), 58 (01. I.), 60
(01. II.): nch in XVII. 62 (normal), 63 (duplicity): IV.

20 (s. trans. , reduplication).

Obliquus oc., inferior, 14 (01. I.): 41-42 (cyclopia, etc.): V.

23 (s. horiz. norm.) : VIII. 35 (s. trans. 01. i.) : XII. 46

(s. horiz. 01. I.) : XIII. 47-48 (s. trans. 01. n.).
Obliquus oc., superior, 14 (01. I.): 41-42 (cyclopia, etc.): 62

(abnormality of): III. 13 (s. trans, norm.): V. 22 (s.

horiz. norm.): VIII. 35 (s. trans. Cl. I.): X. 39-40 (s.

horiz. 01. i.).

INDEX TO STRUCTURES AND SUBJECTS

Oesophagus, 16 (01. n.) : 19 (Cl. HI.) : 34 (triple monster) : 62
(hernia into) : IV. 19 (a. trans, norm.) : XII. 45 (s. horiz.
Cl. I.) : XXin. 97 (triple monster).

Olfactory capsular cartilage, 11 (normal): 13 (Cl. I.): 3^
(triple monster): 40, 41, 42 (cyclopia) : V. 22-23 (s.
horiz. norm.): XI. 41-42, XII. 45-46 (s. horiz. Cl. I.) :
XIII. 47-48 (s. trans. Cl. n. ) : XXI. 88-89 (diagrams) :
O in XXIV. 101 (cyclopia).

Olfactory nerve, 41-44 (cyclopia): XIII. 47 (s. trans. Cl. u.).

Olfactory pit or organ, 40 (pug-head) : XII. 45 (s. horiz.
Cl. I.): XIII. 47-48 (s. trans. Cl. n.) : XXIV. 100-101
(cyclopia).

Optic chiasma, V. 23 (s. horiz. norm.).

Optic lobes, S (union): 13(Cl.i.): 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. Cl. I.) :
X. 39-40, XII. 43-44 (s. horiz. Cl. I.): XIII. 48, XIV.
49-50 (s. trans. Cl. n.): XV. 52-53 (s. horiz. Cl. m.):
XXV. 109-110 (semi-cyclopia) : Ol in XXIV. 103, 106
(cyclopia).

Optic nerve. See under nerve II.

Optic vesicles, 6 (in young double monsters) : S6 (hemididy-
mus): 29 (parallel union) : 43-44 (cyclopia) : 59, 60 (super-
numerary eye).

Osmotic pressure, 6, 27, SO, 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-S9.

Oviducal, 46 (gland) : 47 (slits).

Oviduct, 46 (in male skate) : 47 (in hermaphroditic conditions) :
62 (abnormal).

Paired fins, possible origin of, 54. See also pectoral, pelvic.

Palato-pterygoid, or palate-quadrate cartilage, 11 (normal) :
13 (Cl. I.) : 16 (Cl. II.) : 40-41 (cyclopia) : 49 (pug-head) :
59 (supernumerary eye): III. 13-16 (s. trans, norm.):
VIII. -IX. 35-37 (s. trans. Cl. I.): XIII. 47-48, XIV.
49-50 (s. trans. Cl. n.): XXVI. Ill (cyclopia): XXV.
108 (cyclopia) : XXI. 88-89 (diagrams) : /and/' in XVII.
56 (normal), 58 (01. I.), 60 (Cl. n.): PI in XXIV. 105
(cyclopia).

Pancreatic tissue, IV. 20 (s. trans, norm. ) : XVI. 54 (s.
horiz. Cl. in.).

Parachordal cartilage, 11 (normal) : 13 (Cl. I.) : 16 (01. n.) :
63 (doubled notochord) : IV. 18 (s. trans, norm.) : V. 22
(s. horiz. norm.): IX. 38 (s. trans. Cl. I.): XI. 41-42,
XII. 44-46 (s. horiz. Cl. I.): XIV. 51 (s. trans. Cl. n.):
XXI. 88-89 (diagrams): G in XVII. 56 (normal), 58
(Cl. I.), 60 (Cl. n.).

Parasitic forms, 3, 4 (classification) : 54 (as accessory fins) :
69, 60 (certain examples of).

Parasphenoid in pug-head and round-head, 49, SO.

Pathological conditions of ovum or early embryo, 60.

Pectoral fin, 4, 18 (Cl. in.) : 4, 19 (Cl. IV.) : 54 (supernumer-
ary): 55 (notched, fused with skin): 56 (in "tailless
tront"): 61 (defective): IV. 19 (s. trans, norm.):
XVI. 54 (s. horiz. Cl. in.): XXIII. 97-98 (triple
monster).

Pectoral girdle, 11 (normal): 19 (Cl. in.): 19 (Cl. iv.) :
IV. 18 (s. trans, norm.) : IX. 38 (s. trans. Cl. i.). See
also XVIII. 66.

Pelvic fin, 19 (Cl. iv.): 20 (Cl. v.): 21-22 (Cl. VI.): 34

(triple monster) : 52 (displaced) : 54 (supernumerary) :

65, 61 (absence or defect) : 56 (in " tailless trout ") :

VI. 25 (s. trans, norm.): VI. 28 (s. trans. Cl. vi.):

XXIII. 97-98 (triple monster).
Periotic cartilage, 11 (normal): 13 (Cl. I.): 16 (Cl. 11.): S4

(triple monster) : IV. 17 (s. trans, norm.) : V. 21-23 (s.

horiz. norm.): IX. 38 (s. trans. 01. I.): X. 39, 40, XI.

41, 42, XII. 43, 44 (s. horiz. Cl. I.) : XIV. 49, 50, 51 (s.

trans. Cl. n.) : XV. 52, 53 (s. horiz. Cl. HI.) : XXV. 110,

XXVI. Ill (semi-cyclopia) : XXI. 88-92 (diagrams) : k in

XVII. 56 (norm.) : 58 (Cl. I.), 60 (Cl. n.).
Peritoneal lining, pigmentation of, 57.
Pharynx, 15 (01. I.): 16 (Cl. n.): IV. 17 (s. trans, norm.):

IX. 38(s. trans. Cl. I.).
Pigment layer of retina, 43 (cyclopia) : 59 (supernumerary

eye) : XXIV. 104 (cyclopia) : XXV. 109 (semi-cyclopia).
Pigmentation, abnormalities of, 56-58.
Pineal body, or sac or diverticulum, 13 (Cl. I.): 41, 4%

(cyclopia): III. 13 (s. trans, norm.): VIII. 35 (s.

trans. Cl. I.): XIII. 48 (s. trans. Cl. n.): XV. 53

(s. horiz. Cl. m.) : Pin in XXIV. 102 (cyclopia) : XXV.

107, 108 (cyclopia).
Pituitary space, 11 (normal) : 13 (Cl. I.) : 41 (cyclopia) : IX.

37 (s. trans. Cl. I.) : XXI. 88, 90 (diagrams) : m in XVII.

56 (normal), 58 (Cl. I.), 60 (Cl. n.).
Planes of certain sections. See under diagrams.
Pneumatic ducts. See under air-ducts.
Polychaeta, 58 (as food of flatfish).
Polycylic hermaphroditism, 47.
Polyembryony. See under multiple forms.
Pons, 14 (Cl. I.) : V. 22 (s. horiz. norm.).
Potentiality in teratology, xi, 5, S8, 48.
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, 47.
Pseudobranch and vessels, 12 (normal) : 14 (Cl. I) : 17 (Cl. II. ) :

41 (cyclopia).
Pug-head, 48.

Quadruplicity, 36, 38.

Radiation, posterior (Rauber), S8.

Records, of double monstrosity in osseous fishes 1, in carti-
laginous fishes 30-31, and in other fishes 30 ; of survival of
monstrosities S, 43. See also sections dealing with parti-
cular abnormalities.

Rectum, VI. 25, 26 (s. trans, norm.): VI. 28, 29 (s. trans.
Cl. via.). See also under vent.

Rectus abdominis, @4 ('n anakatadidymus).

Rectus oculi, externus, 11 (normal) : 14 (Cl. i.) : 17 (Cl. II.) :
41, 42 (cyclopia) : V. 22, 23 (a. horiz. norm.) : XI. 41, 42,
XII. 45, 46 (s. horiz. Cl. I.) : XIII. 48 (s. trans. Cl. n.) :
XXI. 88, 89 (diagrams) : XXVI. Ill (semi-cyclopia).

Rectus ocnli, inferior, 14 (Cl. I.) : 41, 42 (cyclopia): III. 15,
16 (s. trans, norm.): VIII. -IX. 35, 36, IX. 37 (s. trans.
Cl. i.) : XIII. 47, XIV. 49, 50 (s. trans. Cl. n.): XXV.
108 (cyclopia) : XXV. 110 (semi-cyclopia).

68

INDEX TO STRUCTURES AND SUBJECTS

Rectus oouli, interims, 14 (Cl. I.) : 41, 43 (cyclopia) : III. 13-
16 (s. trans, norm.) : V. 23 (a. horiz. norm.) : VIII. 35-36
(s. trans. Cl. I.) : XI. 41, 42, XII. 44 (s. horiz. Cl. I.):

XIII. 48, XIV. 49 (s. trans. Cl. n.).

Rectus oculi, superior, 14 (Cl. I.): 41 -42 (cyclopia) : III. 14-
16 (s. trans, norm.): VIII. 35-37 (s. trans. Cl. I.): X.
39-40, XL 41-42, XII. 45-46 (s. horiz. Cl. I.) : XIII. 47-48,

XIV. 49-50 (s. horiz. Cl. n.) : XXV. 107-108 (cyclopia) :
XXV. 109-110 (semi-cyclopia).

Reduplication, local, of notochord, S3 : IV. 20.

Reptiles, 8, 29, 37 (double and multiple forms). See also
Lacerta, snake, Tropidonotus.

Retina, 14 (Cl. I.): 41-43 (cyclopia): 59 (supernumerary eye):
III. 14-15 (s. trans, norm.): V. 21-23 (s. horiz. norm.):
VIII. 36, IX. 37 (s. trans. Cl. I.) : X. 39, 40, XI. 41, 42,
XII. 43-46 (s. horiz. Cl. I.) : XXIV. 104 (cyclopia) : XXV.
107-108 (cyclopia) : XXV. 109-110 (semi-cyclopia).

Reversed flatfish, 57, 58, G2.

Reversion, in descendants of pug-headed fish, 50. See also
atavism.

Rhythm, of respiration and circulation in auakatadidymus, Sfy.

Round-head, 49-50.

Rudiment, embryonic, 6, 10 (in double monstrosity) : 37 (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 and tubercles on under side in ambicolonred flatfish, 57 :

increased number, 63.
Sea urchin, ^7 (apparent protaudry in).
Secondary fusion. See under fusion.
Segmentation, syncytial in Salmonidae, 5 ; more than one

centre of, 8, 37.
Segments, normal number, etc, 12, See also under muscle

plates.
Semicircular canal, 17 (Cl. II.) : 34 (triple monster) : IV. 17

(s. trans, norm.): V. 21 (s. horiz. norm.): X. 39-40 (s.

horiz. Cl. I.): XIV. 49-51 (s. trans. Cl. n.): XV. 52-53

(s. horiz. Cl. m.): ASc, PSc in XX. 82 (Cl. n.): XXV.

110 (semi-cyclopia).
Semi-cyclopia, 24 (in anakatadidymus) : 44 (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 (Cl. III.): HO (Cl. rv.): IX. 38 (s. trans.

CL I.) : SFin XIX. 73 (normal), 75 (Cl. iv.).
Skeleton, cranial. See under cranial, neural and haemal

arch, notochord, etc.
Snake, 37 (three headed?).

Snout, 40-43 (in cyclopia) : 40-51 (in pug-head, etc.) : 62 (bifid).
Specific action of salts in the production of cyclopia, etc., 43.
Spinal cord, 14 (Cl. I.) : 17 (Cl. n.) : 18 (Cl. in.) : 19 (Cl. IV.):

HO (Cl. v.) : SI (Cl. VI.) : 35 (hemididymus) : 39 (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. Cl. I.) : XV. 53, XVI. 55 (s. horiz. Cl. HI.) : Spc

in XVIII. 69 (normal), 70(01. I.), 71 (Cl. n.): XXIII.

97-99 (triple monster): XXVI. 113 (s. trans, absence of

notochord).

Spines. See under tubercles.

Spinous processes of vertebrae in twisted body, 53 : absence

of, in a vertebra, 53.
Spurious hermaphroditism, 46.
Stimulation, electrical, producing hemididymus, 26. See

also under experimental.
Stomach, IB (Cl. in.) : 34 (triple monster) : 62 (hernia) : XII.

46 (s. horiz, Cl. I.) : 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, 62 : scales, 62.
Supraorbital cartilage or bar, 11 (normal): 13 (Cl. I.): 1G

(Cl. II.): 41-42 (cyclopia): III. 13-16 (s. trans, norm.):

V. 21 (s. horiz. norm.): VIII. 35-36, IX. 37 (s. trans.

01. I.): X. 39-40, XII. 43-44 (s. horiz. Cl. I.): XIII. 47-48,

XIV. 49-50 (s. trans. Cl. n.): s and s' in XVII. 57

(normal), 59 (01. I.), 61 (01. 11.): XXI. 90-91 (diagrams) :

XXV. 109-110 (semi-cyclopia).
Supra-regeneration, 36, 37, 55, 61.
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, 38. See also under classification.
Tail bud in mesodidymus, 26.
Tail, 3G (in Petromyzon, trifid) : 37 (in Pelobates larva, do.) :

37 (in lizards, do.): 3, 4, 20-23 (union by): 31, 52

(abnormalities). See caudal.
Tatusia, xi, 5, 8, 38 (polyembryony in).
Teeth, 46 (blunted in male ray) : 62 (abnormal).
Tegminal cartilage, 11 (normal) : VIII. 35 (s. trans. 01. I.) :

X. 39 (s. horiz. Cl. I.) : X in XVII. 57 (normal) : XXI.

90-92 (diagrams) : XXV. 107-108 (cyclopia) : XXVI. 112

(s. trans, atrophic head).
Telescopic eyes of goldfish, 63.
Testes in hermaphroditism, 47.
Trabeculae cranii, 11 (normal) : 13 (01. 1.) : 16 (01. n.) : 40-43

(cyclopia) : 44 (semi-cyclopia) : 5,9 (supernumerary eye) :

III. 13-16 (s. trans, norm.): VIII. -IX. 35-37 (s. trans.

Cl. I.) : XI. 41-42 (s. horiz. 01. I.) : XIII. 47-48, XIV.

49-50 (s. trans. Cl. n.) : 6 to d in XVII. 56 (normal), 58

(Cl. I.), 60 (Cl. n.) : XXI. 88, 91 (diagrams) : XXV. 108,

XXVI. Ill (semi-cyclopia).

Transition, region of, from the double to single condition.
Sec under the various classes of anadidymus.

Transmission, of abnormalities, see under hereditary.

Tripartite division of embryonic axis, 36, 39.

Triple monstrosity, 8 (general) : 33 (description of specimen) :
35 (other instances in fishes) : 36 (mode of origin in fishes
and other vertebrates) : 38 (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 (Cl. I.) : 17 (Cl. n.): IV. 17 (s. trans.
norm.): XIV. 51 (s. trans. Cl. n.): XVI. 54 (s. horiz.
Cl. in.) : TA in XVIII. 68 (Cl. n.).

Tubercle, posterior, in mesodidymus, 25, 28.

INDEX TO STRUCTURES AND SUBJECTS

69

Tubercles, spines, and rough scales present on underside of

ambicoloured flatfish, 58,
Twinning, xi, 5, 7, 30, 37.

Twisting of body or tail. See under hump-back.
Types of double monstrosity, 1, 3, G, 8, 28 : of anomalous

development in eggs of pike, 10.

Unioval, duplicity, xi, 7, S : triplicity, 33-S9.

Urinary bladder or opening, 12 (normal) : 21 (Cl. v.) : 32-23

(Cl. VI.): S3 (anakatadidymus) : 34, 48 (hermaphroditic

trout) : Bl and BV in XIX. 77-81 : XXII. 94, 96 (triple

monster).
Utricle, 17 (Cl. n.): 34 (triple monster): IV. 17 (s. trans.

norm.): V. 21 (s. horiz. norm.): XIV. 50 (s. trans. Cl.

ii.): UtrinXX. 82 (Cl. n.).

Valve, spiral, variation, 62.

Vein. See under cardinal, caudal, jugular, yolk-sac.

Vent, SO-23 (Cls. v.-vi.): S3 (anakatadidymus): Sit (triple

monster): XIX. 81 (01. vi.b): XXII. 94-95 (triple

monster).
Ventral edge membrane (incl. anal fin), IS (normal) : 20 (Cl. v.) :

81-$% (Cl. vi.) : 23 (anakatadidymus) : 34 (triple monster) :

54 (doubling in goldfish) : 55 (absence or malformation) :

VI. 26 (s. trans, norm.): VI. 29, VII. 30, 31 (s. trans.

Cl. VI.) : VII. 32-34 (triple monster).
Ventral union, S, 7, S3, S3.
Ventricle III., 41-42 (cyclopia) : III. 14-15 (s. trans, norm.) :

V. 21-22 (s. horiz. norm.) : X. 39-40, XII. 43-44 (s. horiz.

Cl. i.) : C3rd Fin XVIII. 69-71 (normal, Cl. I., Cl. n.)»

XXI. 91-93 (diagrams) : 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.

Cl. I.) : XV. 52-53 (s. horiz. Cl. in.) : C4th V in XVIII.

69-71 : XXI. 90, 93 (diagrams) : XXVI. Ill (semi-

cyclopia).
Ventricle of heart, 19-SO (Cls. in.-iv.): IV. 18 (B. trans.

norm.): XVI. 54 (s. horiz. Cl. ill.): V in XIX. 73

(normal), 74 (Cl. in.), 75 (Cl. n.).
Vertebral column, in twisted body, etc., 51-53. See also

notochord, etc.

Vertebrate, 5 (ova) : 8, 28, 31, 38 (double and multiple mon-
strosity).

Visceral arch I. (incl. Meckel's cartilage), 11 (normal) : 13
(Cl. I.): 15 (Cl. II.): 41-43 (cyclopia): III. 13-16 (s.
trans, norm.): VIII.-IX. 35-37 (s. trans. 01. I.): XIII.
47-48, XIV. 49-50 (s. trans. Cl. n.): XVI. 54 (s. horiz.
Cl. HI.) : g and g' in XVII. 56 (normal), 58 (Cl. I.), 60
(Cl. II.) : Meek in XVIII. 64 (Cl. I.), 65 (Cl. n.) : XXI.
88, 91 (diagrams). See also under jaw.

Visceral arch II., hyoid, 11 (normal) : 13 (Cl. I.) : 15 (Cl. n.):
41-42 (cyclopia) : 51 (defective lower jaw) : III. 15-16
(a. trans, norm.): VIII.-IX. 36-37 (s. trans. Cl. I.):
XIV. 49-51 (s. trans. 01. U.) : h in XVII. 56 (normal),
58 (01. i.), 60 (01. n.). See also XVIII. 64-65: XXI.
88, 91 (diagrams).

Visceral arch III., 11 (normal) : 13 (01. i.) : 17 (01. n.) : III.
16, IV. 17-18 (s. trans, norm.) : VIII.-IX. 36-37 (s. trans.
Cl. I.): XIV. 51 (s. trans. Cl. n). See also XVIII. 64
(01. i.), 65 (Cl. ii.).

Visceral arch IV., IV. 17-18 (s. trans, norm.): XIV. 51 (s.
trans. 01. II.).

Visceral arch V., as under above.

Visceral arch VI., IV. 18 (s. trans, norm.).

Vomer, in pug-head, 49.

Weight of centra in twisted body, 53.

Wolffian body, IS (normal): VI. 25 (s. trans, norm.): XVI.

54, 55 (s. horiz. Cl. m.): HK in XX. 83 (normal), 84

(01. i.), 86 (Cl. IV.) : XXIII. 99 (triple monster) : XXVI.

1 13 (s. trans, absent notochord).
Wolffiau ducts, IS (normal) : 14 (01. I.) : 18 (Cl. n.) : 19-S3

(Cls. m. -vi.): SB (hemididyinns) : 34 (triple monster):

IV. 19, VI. 24-25 (s. trans, norm.) : VI. 27-29 (s. trans.

Cl. VI.): WD or CWD 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.
Cl. vi.): IX. 38 (s. trans. Cl. i.) : XVI. 54 (s. trans.
01. in.): XXIII. 97-99 (triple monster): XXVI. 114 (s.
trans. 01. v.).

Yolk -sac, S (exhaustion of yolk) : S-4 (union by yolk-sac) :
6-7 (overgrowth of blastoderm) : S (more than one blasto-
derm) : S3-S4 (in anakatadidymus) : 25 (in hemididymus) :
SO (yolk and osmosis) : 61 (double-yolked eggs).

Zig-zag fission, 27 (supposed in katadidymus).

INDEX OF FISHES.

Abramis blicca, 55.
Abramis vimba, 49.

Acanthias, 31, 51, 54, 55, 61, 62. See
also under Selachii and Selachoidei.
Acipenser, 47, 62.
Alopias, 61.

Ameiurus natal it, 55, 61.
Amia, 52.

Am/ihioxux, 48 (unisexuality).
Anarrichas, 1, 2, 30.
Angler-fish, 61.
Anguilla, 49, 51, 61.

Bleak, 50. See also Abramis and Cypri-
nus blicca.
Blennim, 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.
Centrolophus, 62.

Clupeidae, 47. See Herring, Pilchard.
Oobitis, 47, 52.

Cod, 51, 52, 53. See Oadus morrhua.
Conger, 62.
Coitus, 49.
Orenilabrus, 52.
Cyprinidae, 47, 61.
Cyprinus, 49, 50, 51, 55, 61, 62. See

also Goldfish.
Cyprinus aphya ? 50.
Cyprinus blicca, 1, 30.

I lipnoan. 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 Jlems.

Fundulus heteroclitus, 43.

Gadidae, 47.

Oadus aeglefinus. See Haddock.

Oadiis merlangtts, 49.

Oadus min u tun, 49.

Oadus morrhua, 49. See Cod.

Oasterosteidae, 47.

Oasterosteus, 61.

Qirardinus, 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 Petromyzon.

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 vulgaris, 49.

Mormyrus, 52.

Motdla, 55.

Mugilidae, 47.

Afugil capita, 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 fiesus, 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.
Protoplerus, 54.

Raia alba, 5.

Itain batis, 54.

/in in davata, 5, 46, 54, 55, 62. See

under Skate.
Bay (Myliobatis), 2.
Bhinoptera, 62.

Rhombus laevis, 56, 57, 58. See Brill.
Rhombus maximus, 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

Salmo fario, 2, 9, 25, 26, 35, 36, 48, 54.

See Trout.

Salmo irideus, 9, 29, 49, 54.
Salmo lacuslris, 2.
Salmo solar, 9, 48. See Salmon.
Salmo salvelinua, 25, 35.
Salmo trutla, 52.
Sderognathus, 62.
Scomber, 1, 30, 52. See Mackerel.
Scombridae, 47.
Scyllium, 61. See Dogfish.
Sea-bream, 62.

Sea-trout. See Salmo trutta.
Sebastes, 55.

Selachii. See under Cartilaginous fishes.

Stlachoidei, 1, 2, 31.

Serranw, 48.

Shark (Porbeagle), Lamnacomwhica, 61.

Shark-like fishes. See under Selachoidei.

Silurus glanis, 54, 61.

Skate, 32, 46. See under Raia clavala.

Solea vulgaris, 51, 52, 56, 57.

Sparidae, 47.

Sparus centrodontus. See Sea-bream.

Squalius cavedanus, 61.

Squalua acanthias, 61. See under Acan-
thiai.

Squamipinties, 47.

Sturgeon, 62. See Acipenser.

Tdeostei. See under Osseous fishes.

Tinea, 52.

Torpedo, 1, 29, 31, 61.

Trigla gumardus, 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 maximum.

Zoarces, 52.

INDEX OF AUTHORS.

Ahlfeld, 59.
Albrecht, 54.
Aldrovandi, 1, 2, 31, 37.
Allis, 62.
Ambrosini, 2.
Asshcton, 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.

Boulenger, 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.

Con, 61.

Coste, 1, 2, 24.

Conch, 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.
Donuadieu, 51, 62.
Duncker, 54, 55.
Dyce, 51, 52.

Edwards, Milne, 38.
Eigenmann, 55, 61.
Eismond, 5.
Ekman, 61.
Elmhirst, 56, 57.
Emeljanov, 1.
Erdl, 51.

Fabricius ab Acquapendente, 8.

Fattori, 39.

Fernandez, 5, 38.

Fiebiger, 55.

Fiedler, 39.

Filhol,57.

Forster, 3, 25.

Freund, 49, 51, 61, 62.

v. Froriep, 39.

Fuhrmann, 47.

Gadeau de Kerville, 1, 31.

Garman, 1.

Garstang, 62.

Geramill, 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. Kiilliker, 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.
1

INDEX OF AUTHORS

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.
Neydeok, 49.
Ninni, 49, 51, 56, 57.
Nusbaum, 55.
Nystrom, 49.

Oellaoher, 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.

Eathke, 1, 30.

Rauber, 1, 2, 3, 5, 7, 8, 9, 10, 25, 26, 28,

33, 35, 36, 37, 39, 60.
Reina, 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.
Button, 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 (6) ).

PI. VII. figs. 32-34.

Transverse sections through the triple monster described
on pp. 33-34 ; see also Pis. XXII.-XXI

IH. figs. 94-99.

Pis. VIII. -IX. figs. 35-38.
Transverse section throu
with union of the twin
lobes (Class I.).

h the head of a double monster
rains in the region of the optic

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).

PL 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.
PL 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 VI. (union
by the body or tail).

PL 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.

PL 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.

PL 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 PL VII. figs. 32-34.

PL XXIV. figs. 100-106.

The figures illustrate cyclopia (pp. 40-45) and super-
numerary eyes (pp. 59, 60).

PL XXV. figs. 107-110.

Transverse sections through the ocular region in cyclopia
and semicyclopia (p. 44).

PL 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 section showing absence of notochord ; and
Fig. 114 a section behind the pectoral fins in a double
monster belonging to Class IV.

AC oi a <io i
of A iimil

< >f crania! si
III. figs. 64,67. 70,72.
•'5- (*3->

PL. I.

i

OF co us^r .— <

PI. XXII. figs. 94-96, PI. XXIV. I : • ,im drawings made

l>y the atith.-: and »re cased on the study of numerous series of section*. All the otln ma figures

are from phnfnniarographs of specimens or sections.

trout emljrj c.. and of I
of double i

Ph. !

Jkfl^** IIMM-* t*i

QWWMV »•*••« "f

hwiaotfiMI to v«r

«.. PI. VI. figs. 24-26.

•"•rue outturn "f • normal trout embryo at the Mint
tm# m U .-(.-.,- ««t M UM monstrosities doocribod.

f the Utter, shown in

•<«•. 21-91.
BsMBBtal oouticsM of nnmul trout embryo, M above.

a^. 87-1W; •loo PI. VII. «*• *>-3l

TntMVOTM OMtiMO Of d«hbfc IMMtan Vtlfc MMOB to

pastoriuf pon

girdle and fin cartilages in an e> imjile from Class
(union in pectoral region, inner pect 'vg present

but united). Figs. 69-72 are diagrams to iUootnte the
cavities of the brain in a normal embryo aswi
Mojotor* belonging to Classes I. and II.

••if*. 78-81.

Figs. 7-%- 75 illustrate the heart in a normal embryo and
in double monsters hrtnaglag to Class III.
pectoral region, adjacent peetora] tins not !>•
sented) and Class IV. (uniou in pectoral region, adjacent
pectoral fins being present but nulUd). Mas. '
Ulu.tr.te the posterior ends of the Wolffian Jucta, the
blxidon »nd urinary pores in a normal embryo .<.-.

H VII rtg. ».

Ti

OB pp. 33 34 i so* alac Pta. X.xil. XX111. fig*, in W
PU. VIII. -IX. tigs. 36^38.

v Transverse section through the heatKof a doable monster
union of the twin brains >n theVegion of the optic

lead of a double monster
>rains in the region of the optic

PI. XII. figs. 43^46.

m i» *

u uf tb« twin brain* in the region of the medulla
(CW-

Pls. XV.-XVI. figs. 52-55.

Horizontal sections of a dn«bl
the pectoral region, inner Doctoral
(Claw III.).

PI. XVI 1. tigs. 56-CS.

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 65jilustrate the visceral arch cartilage*, and
Fins. 67_anA-<8tlie 'vascular system in double moo

Fig. 66 show* th« i

belonging to Classes V. and VI. (union

Tt.tm.n

fl saowi

Figs. 83-8'
yan

•
U. and IV

re» of Ik* tripl* OMHMttor

alimentary and urinary stnctn

,

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 cyclofOa
and semicyclopia (p. 44).

PL XXVI. figs. 111-114.

Fig. Ill follows Fig. 110 in illustrating semicyclopia:
Fig. 112 shows the atropjuc head referred-^ on p. 61 .
Fig. 113 is a seeti"n aj«<»"flj nliosa7ir1ifTintcu\liriiif . and
Fig. 1 14«», secflSnbehind the pectoral fin* i* a double

FIG. i. — Side view of a normal trout embryo of same age as> the
monsters described. Seclions of similar normal specimens are
shown in PI. III. figs. 13-16, IV. figs. 17-20, VI. figs. 24-26 (trans-
verse); and in PI. V. fig?. 21-23 (horizontal). Diagrams of skeleton,
etc., in PI. XVII. figs. 56, 57, 62, XVIII. fig. 69, XIX. figs. 73, 76,
XX. fig. 83. Description on pp. 11-12. Contrast with Figs. 2-12.
(X3.)

FIG. 4. — View from above of a double monster belonging to
Class I., i.e. with union of the twin brains in the region of the optic
lobes. Transverse sections of a. similar specimen are shown in
Pis. VI 1 1. -IX., figs. 35-38, horizontal sections in Pis. X.-XII.
figs. 39-46, and diagrams of cranial skeleton, etc., in PI. XVI I.
figs. 58, 59 ; XVI II. figs. 64, 67, 70, 72. Description under Class I.
on pp. 12-15. (><3-)

FIG. 2. — Side view of a double trout embryo, w ith 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 Meckelian bars.
For references, see under Fig. 4. ( x 3.)

FIG. 3. -Side view of a specimen similar to the last, but with one

See pp. 7, 44.
sections of this

"*->• ,}• .^Illt. *n_v» Ul tl afdlUldl J'""1<4I V1-" UIC ill

of the heads showing a condition of semi-cyclopia. See pp. 7, 44.
Figs. 109-111, in Pis. XXV.-XXVI., are from

specimen. (x3.)

FIG. 6. — View from below of a double monster belonging to
Class II., i.e. with union of the twin brains in the region of the
medulla oblongata. The two adjacent eyes are seen ; also the two
mouth-openings, separated by a thick septum containing the
adjacent hyomandibular and Meckelian cartilages. Sections are
shown in PI. XIII. figs. 47, 48, and XIV. figs. 49-51. Diagrams of
cranial skeleton, etc., in PI. XVII. figs. 60, 61, XVIII. figs. 65, 68.
71. Description under Class II. on pp. 15-18. (X3.)

PL. I.

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 1JI. XV. figs. 52,
53, XVI. figs. 54, 55. Diagrams of vertebral column, heart, etc., in
I'l. XVII. fig. 63," XIX. fig. 75, XX. fig. 84. Description under
Class III. on pp. 18-19. ' ne body wall over the yolk is ruptured
along the middle line. (*3.)

FIG. 10. — Double monster with union in posterior part of body.
The union is obliquely by the ventral aspect of each twin, and the
tail has three angles. See pp. 21-22. Compare PI. VI. tigs. 27 29,
and PI. VII. fig. 31. (X3.)

FIG. 8. — View from above of double monster, with union in
pectoral region, the adjacent pectoral fins not being represented.
For references see under Fig. 7. ( x 3.)

FIG. ii. — 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. ( x3.)

FIG. 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 m
PI VII. fig 30. Description under Class VI. (0) on pp. 22-23.

PL. II.

FIG. 7. —View fro
to Class III., <>.wil
fins not being ii'pres
53. XVI. figi. 54, «
PI. XVII. fig. *}.
Class III. on pp i
along the mi

r.irjuMcT be!-
>n. the ,-ulj.icnu |"
nons in I'l. XV. figs. 52,
•,i! column, heart, etc.. in
X. fig. 84. Inscription

• i'T thr yolk is rupturt-ii

on is obliq'. :
tail h;i- i
and PI. VII. fijf. 31. (X3.)

--t'.'l 1O!

eavh twin, and

7 29.

KiG

i'Ki. 9.— View from Ih« iide of a double in, union

behind the |» » One of the r

size. The posti ti the body is etrical.

Fordescripi -^ V. on pp. 20-21. (x3.)

Km. 1 1. — Double monster \\ ith

of the twin

••nt, each pair Vicing r.
' 'i nt twins. The t:
PI. VII. fig. 30.

Fri;. 13. — Trausvir-
through head of normal trout
(PI. I. fig. i), sho,

>(M .

PI. VIII !li

,»nd P>. M r ("»'°

meili." cc

I. I. fig. i. ( -->5-)

Mucous canal
Pineal body
Supraorbital bar
Cavity cerebral lobes

• Cerebral lobes

Obliquus oculi superior
.' Retina and lens
•Rectus oculi internus

Trabeculae cranii

Palato-pterygoid bar
'Copula visceral arch II

Visceral arch I

Cavity optic lobes . ^

Optic lobes
Supraorbital bar . . TJ

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 -

KlO. 14. —Transverse section
through head of normal embryo (as
in last figure), across anterior part of
optic lobes and entrances of optic
nerves. 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. '. <x'5->

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

Supraorbital bar

Nerve III

Infundibulum

Recti oculi intern, super, and infer.

Trabeculae cranii

Palato-pterygoid bar

Visceral arch 1 1 1

,, II

I

Copula visceral arch IV-

! !•*. I. I • «

]. F. G.

PL.

(hod !,-i<:;l(

• , -•i/tiO
eaal bnr. btiilyil-

''idfiiT

II tlllE JB1902iv BluqoO'

asdol oilqo \irt.

iBlidiOBiqi/S
.00 au)o3

. . 889D91 'ji JqO

V-; snsi fanfi Bfiug^
1 jnmjin iluoo au):

(rnulodibniilni) III sbhlnaV
sn'Dsil

II

aornslni iluso

riJuoM
II rtoi
I ilaiB IsiaoziV

-••- II •
" ' - - bir<

•

-n ..

Ill

FIG. 13. — Transverse section
through head of normal trout embryo
(PI. I. fig. i), showing eyes, cerebral
lobes, and pineal sac. Compare with
PI. VIII. fig. 35 (union at optic lobes)
and PI. XIII. fig. 47 (union at
medulla). For other references see
under PI. I. fig. i. (xi5-)

FIG. 14. — Transverse section
through head of normal embryo (as
in last figure), across anterior part of
optic lobes and entrances of optic
nerves. Compare with PI. VIII. fig.
36 (union at optic lobes), and with
PI. XIV. fig. 50 (union at medulla).
For other references see under I 'I. I.
fig. i. (x 15.)

FIG.

-Transverse section as

above, through middle of optic lobes,
and through optic commissure. For
other references see under PI. I. fig. i.
(xis-)

FIG. 16. — Transverse section as
above, through back part of optic
lobes and infundibulum. Compare
with PI. IX. fig. 37 (union at optic
lobes), and with PI. XIV, fig. 50(11111011
at medulla). For other references see
under PI. I. fig. i. ( x 15. )

PL. III.

FIG. 17. — Transverse section
through head of normal trout embryo,
showing cerebellum, auditory organ?,
medulla, pharynx, gill chambers, etc.
Compare with PI. XIV. fig. 51 (union
at medulla). For other references see
under PI. I. tig. i. ( x 15.)

FIG. 18. — Transverse section
through head of normal trout embryo,
showing 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 see under
PI. 1. fig. i. (xi5.)

FIG. 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. i.

FIG. 20. — Transverse section of nor-
mal trout embryo, across middle of
body, showing swim-biadder, intestine,
and pancreatic tissue. The specimen,
which is otherwise normal, shows local
reduplication of the notochord. See
p. 54. Other references under PI. I.
fig. i. (xis.)

PL. IV.

VI afohlnsv

HIV svi3n noi

bnfi biorfooJoVI
(nigho) 2mm)y.3 if/jiio gu
III rbiK
VI ,,
V ,,

ivity optic I..
_____ . Supraorbital

•

.. .-Cerebellum
..-• Ventricle IV
.... Semicircular c
Utricle

i'eriotic carl:
"'• Medulla •
Spinal rord

71

faiorlooJoVlj

III ibiB oinoA •
III dole 'a
V ,VI
IV

biO3 teniqS

• rto'iB I/nuaM

tnorfooloM

rt3~u3 l£iri9£H •

•<bo<f nBmio
Jaub iiBiftloV

nft

jIIoY ..

bloo
• (slduo

-- ni'i/ ,n

. XIoY

71

KlG. 17. — Transverse
i head of normal I
is; cerebellum
medulla, pharynx.
Compare with PI. .\
at medulla). Kor oih--i
under I'l. I. fif{. i. { » ij.)

Cerebellum
Cavity ventricle IV
Periotic cartilage
Semi-circular canal
Utricle

Ganglion nerve VIII
Saccule

Hyomandibular
- Notochord and
Rectus oculi externus (origin)
Visceral arch III

,, IV

,, V

Pharynx
Truncus 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

rAuricle

Cartilage pectoral^girdle..
Ventricle

KlG. 18. Transverse section

through head ot norms! trout embryo,

>ij the posterior part of the

e gill chaml*-rs, and the

if the heart. Compare

11 at optic

i- I'. ... : '.. .r ;; .•

nml trout embryo

The section passe*, through the head-
kidney, oesophagus, and pectoral fin.
Other references under PI. I. fig. i.

Spinal cord
Neural arch cartilage
Notochord

- Haemal arch cartilage
. Aorta

• Wolffian body
Wolffian duct
Posterior cardinal vein

. Oesophagus

- Pectoral fin
. . Yolk

Spinal cord -••
Notochord (double) •••

Wolffian duct - •

Posterior cardinal vein - -

Swim-bladder ••

Pancreas • •

Intestine -

Yolk • -

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, shou
reduplication of the notochon I
p. 54. Other references mid
fig. i. (xi5.)

J. F. G.

PI.. IV.

IV

Fio. 21.-

nornuii t: > •'

\ I .- !

various

I tliL- central
>1 cord. Compart:
i, 40 (union at optic
•ii;. 4j'( union at medulla).
and with I'l. XV. tig. 53 (in
pectoral region). For other references
see under PI. I. fig. i. (X13-)

Mucous canal
Supraorbital cartilage
Cerebral lobes
Ventricle III
Retina

Cavity optic lobes
Supraorbital cartilage
Optic lobes
Cerebellum
Ventricle IV
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 externus
. Ganglion nerve V
. Pons
. Periotic cartilage

- Parts of labyrinth
..Medulla

. Parachordal cartilages

. Notochord

.- Muscle segments

i ' I

FK.. 23. <

and the dorsal [
pare »

medulla
PI. I. fig. i.

'

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. F. G.

PL. V.

v

Ill sbrmiaV... '"•••

aadol Dili|o \3i'tK'J . .

godot DiJqO- *

VI gbn

ahiti'J

bioa Ir.niqz lo IEHEO ""

brao lEiiiqg

CereU

Cavitv -

Periotic can

Semi-cirenilai

Utricle

Ganglion nerve VIII

Saccule

•ius (origin)
• HI
IV

Pharynx

oiisqus iiu->o aoilpiidO
aadol IB i

III

t

znol .

bioriooloH -

ziirralni iluoo aW»5J
lohslni iluso auijpildO

jsnrJs^I -
GtmEifo oiJqO
muliidibnu^nl

8om9Jx.3 iluoo

Spina:

9f*f)l

Oesophagus
Pectoral fin

Yolk

V

FIG. 21. — Horizontal section of
normal trout embryo (PI. I. fig. i).
The section passes through the upper
parts of the eyes, and the auditory
organs, as well as through the various
cavities of the brain, and the central
canal of the spinal cord. Compare
with PI. X. figs. 39, 40 (union at optic
lobes), XII. fig. 43 (union at medulla),
and with PI. XV. fig. 53 (union in
pectoral region). For other references
see under PI. I. fig. i. ( x 13. )

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
eyes and the auditory organs. Com-
pare with PI XII. figs. 44, 45 (union
at medulla). Other references under
PI. I. fig. i. (xi3.)

FIG. 23. — Horizontal section as
above, but passing through the lower
parts of the eyes, brain, and auditory
organs, as well as through the head-
kidney, the duct of the swim-bladder,
and the dorsal portions of the gill
arches, and of the body cavity. Com-
pare with Pis. XI., XII. figs. 42, 45,
46 (union at optic lobes, and at
medulla). Other references under
PI. I. fig. i. (xi3.)

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. i. (xi4.)

$•&•-.; *..'.i>; ^;-;

FIG. 27. — Transverse section corresponding with Fig. 24,
but through a double monster of the type illustrated in
PL II. fig. 10. Figs. 28, 29, and 31 are from the same
series. At this level the two intestinal canals are widely
separated. For relations of Wolffian ducts, etc., see pp.
22, 23 and PI. XIX. figs. 76-79. (xi4.)

FIG. 25. — Transverse section as in
Fig. 24, but a little further back.
Compare with Fig. 28 (union by pos-
terior part of body). Other references
under PI. I. fig. i. (xi4.)

FIG. 28. — Transverse section of the same double monster
as in Fig. 27, but a little further back in the series. The
intestine is now single, but has two mesenteries. The Wolftian
bodies and ducts are coming together by their inner, or
adjacent, sides. Compare with Fig. 25 (normal). Other
references under Fig. 27. (xi-|.)

FIG. 26. — Transverse section, as in
the two preceding figures, but passing
just in 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. i. (xi4.)

FIG. 29. — Transverse section through same double mon-
ster as in Figs. 27 and 28, but further back in the series.
Compare with Fig. 26 (normal). The adjacent Wolffinn
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.

BHoA

biorboJoX
[BaioQ
bio-j :i,niq8 ••-

uub nsrftloW

dloY

b'too I

oub nj i

V4-: ' snbaaJnl

•ai6?

biodDoioVI
bios Ij2niq8

nigh) BJDiib /i;,ift!o'//

(llal hnr.

(sldnob) vTjJirjealiC

muloa^

oft Qivlg*?

' snBidmsrn iBSioQ
yj" i biool

- bioriooJoVl
fiJioA

HRfftloW

' alaob

. ...nft oi
>r&ai meiubrftnc of defective t

biodDoJoVI"

bioo lj;(nc!^-

on/ridrnam I^s'ioQ-.

ErtoA

^loub r

g ji part

ta*t fipni-.

ili'.r.-

.-J8 '

IV

Dorsal membrane
Spinal cord

Wolffian ducts

Yolk

FIG. 2.1

through nor
s'derable d:r
Compare wi
tei ior part o
under PI. 1.

ruimcrse section

l>ryo, a con-

i of the vent.

i. on by pos-

: references

Muscle segments
Aorta

Notochord
Dorsal membrane
Spinal cord

..Wolffian duct

•Intestine

•Yolk

FK;. 27. — Transverse section corresponding wi
but through a double monster of the type illusn.
PI. II. fig. 10. Figs. 28, 29, and 31 are fro;
series. At this level the two intestinal canals are
separated. For relations of Wolffian ducts, etc., set.- pp".
22. 23 and I'!. XIX. figs 76-79. (xr.|.)

Dorsal membrane

Spinal cord •• •

Notochord • • •

Aorta . . .

Wolffian body ..;.

Wolffian ducts.--'
Rectum

Pelvic fin....

.. Notochord
Spinal cord

Dorsal membrane
- Aorta

Wolffian ducts (right

and left)

Mesentery (double)
Rectum
Pelvic fin

FlC. 35. --Trim*"
frig. 34, but a little furlh
Compare wtth Fig. 28 (unio
terior part of body). Other <
under PI. I. fig. i. (.14.)

by pos-
Vrences

a« in Fig. 97, but a thtlr furttw- \a<\ in ihr ••••••.--. 1 1,,.

intc<line is ii '. Irh.in

.»t»«l ducts HC»' c(;?iinii; tr.^riiifr U) their inner, or
adjacent. >ide». C'ompjirt- with Fig. 25 (normal). Other
reference* under Fig. 27. (xi.f.)

Muscle segments
Spinal cord .

Notochord -

Aorta
Caudal vein

Bladder
Rectum .
Ventral membrane •

Flo. at. — Transverse section, as in
the two preceding figures, but passing
just in front ol the vent. The Wolffian
ducts ha\e opemil into the bladder,
which is seen lying dorsal to the
rectum. Compart (union

by posterior [xirt

.......... Notochord

•Spinal cord
.Dorsal membrane
Aorta

•Wolffian ducts
1 Bladder

Mesentery

Rectum

Ventral membrane

references under I'l. I. fig.

I >

FIG. 29.- Transfers*- section through
sler as in Figs. 27 und 28, but fv
Compare with Fig. 26 (norh
ducts have opened into a first
have not yet reached t'n
the other. These relations a
references uiiiln :

J. F. G.

PL

VI.

VI

Ventral membrane (composite)

Dorsal membrane

Spinal cord

Notochord

Spinal cord

Notochord - - • :,
Dorsal membrane '"

Ventral membrane (composite) ~

FIG. 30. — Transverse section through tail
of embryo belonging to the type illustrated
in PI. II. fig. 12 (ventral union by posterior
part of body). There are two spinal cords,
notochords, and dorsal edge membranes.
Two ventral edge membranes are also
present, but these are composite, being
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. (xi4.)

Dorsal membrane
Muscle segments
Spinal cord
Notochord (composite)

Aorta

Caudal vein
Ventral membrane
Spinal cord

FIG. 31. — Transverse section through
tail of an embryo belonging to the
general type illustrated in PI. VI. figs.
27-29. The two notochords have fused
together, and the ventral edge membrane
is composite and single. Compare with
Fig. 30. See also under Fig. 27. (x 14.)

l». Tran>versr section through com-
: tile tail of the triple monster
trout described on pp. 33-34 See also PI.
XX11. figs. oj-96and 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
aspect, the two dorsal fins of the defective em-
bryos. A little further up on either side is a
composite ventral edge membrane (anal fin),
belonging in part to the principal embryo avid
10 one or other of the defect! ve embryos.

Muscle segments of principal embryo
Spinal cord ,, ,,

Notochord
...... Aorta

Caudal vein

Muscular tissue of defective embryos

$£•-••• Ventral membrane (composite)

Dorsal membrane of defective embryo B

A

Dorsal membrane
Muscle segments

Spinal cord
Notochord

Ventral membrane (composite)

Ventral membrane (composite) _

Dorsal membranes of \
embryos A and B /

Naming of parts same as in
last figure. The dorsal mem-
branes of A and B are now
united.

l'n;. 33. — Transverse section through tail of
same specimen as in last figure, but considerably
further back. All the fin membranes are still
present, but the two dorsals of the defective em-
bryo 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 hnve now
united. (xi8. )

J. F. G.

PL. VII.

vn

Dorsai membrain

Dorsal membrane

Spinal cord.

Notochord,

Aorta, .
Wolffian body

Wolffian ducts ••

wpdn

Rectum .

Pelvic fin
H oxidrns avil
A

. Spinal cord

K)> Dorsal membrane
^•- Aorta

n ducts (right
and

ni ir, 3int,i sneq \o g
-rnsm Ifisiob sriT /nugft
won STB 8 boe A 10

.balino
Muscle segment

Spinal cord

Notochord •

Caudal vein
Hladder .
Rectum
Ventral membrane •

IcaioU

biodaoJoU

(sliaoqmoo

(9»«Q<jnioa> t

Aorta

f lo asnEidm-sm I
'

I IV

J. F. G.

•I Vi

FIG. 30. — Transverse section through tail
of embryo belonging to the type illustrated
in PI. II. fig. 12 (ventral union by posterior
part of body). There are two spinal cords,
notochords, and dorsal edge membranes.
Two ventral edge membranes are also
present, but these are composite, being
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-J.)

FIG. 31. — Transverse section through
tail of an embryo belonging to the
general type illustrated in PI. VI. figs.
27-29. The two notochords have fused
together, and the ventral edge membrane
is composite and single. Compare with
Fig. 30. See also under Fig. 27. (xi4.)

FIG. 32. — Transverse section through com-
mencement of the 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
aspect, the two dorsal fins of the defective em-
bryos. A little further up on either side is a
composite ventral edge membrane (anal fin),
belonging in part to the principal embryo and
in part to one or other of the defective embryos.

FIG. 33. — Transverse section through tail of
same specimen as in last figure, but considerably
further back. All the fin membranes are still
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 of the fused adjacent hyomandi-
bulars, with which articulates a small bar representing the adjacent
Meckelian cartilages. Compare with PL III. fig. 13 (normal) and with
PI. XIII. fig. 48 (union at medulla). Pis. X.-XI1. 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. (x2o. )

FIG. 36. — 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 cianii 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. III. figs. 14
and 15 (normal). For other references see under PI. I. fig. 4, and for plane
of section see PI. XXI. figs. 88-90, line 36. (xao.)

PL. VIII.

lonni ari) lo ano ^o) 1 1
i ''ib lo ano loj ."ini
'!(!<< 4/1] "io H

e.bio '! "i'j{ini bajir

aagfilijifio •- :M byjinu ariT

MM»

(Me^
1 1 (hyoid)

29dol uijqr

.qoe .00 2i'

,"jfl •

Jn:
.Ini .

sdJ ^o sno 1o .que hnr> .lui gi;J:r
asv; >

(I iBd Uioaeiv)

: l

III
III

HIV

• •* '

:• :

Tegminal cartilage
Composite optic lobes
Pineal body
Cavity of cerebral lobes
Supraorbital bar
Cerebral lobes
Obliquus sup. of outer eye
Recti sup. of inner eyes
Rectus int. of inner eyes
Obliquus inf. of outer eves.
Palato-pterygoid cart.
Trabeculae cranii

Nerve II (of one of the inner eyes)
Rectus inf. (of one of the inner eyes)
Nerve II of the other inner eye
The united inner palato-pterygoids
The united inner Meckel's cartilages
Soft tissues of the composite lower jaw

PI. XIII. rig. 48 mnion at medulla). IT.

sections of a double monster similar to this

lobes. For plane of section see PI. XXI. figs. 88-90. I )tln-r references are

given under PI. I. fig. 4. (x2O.)

Optic lobes
Cavity of optic lobes
Mucous groove
Supraorbital bar

Infundibulum

Rectus oc. sup.

Retina

Lens

Rectus oc. int.

Rectus oc. inf.

Rectus inf. and sup. of one of the

inner eyes

Palato-pterygoid bar
Trabeculae cranii
Meckel's cartilage (visceral bar I)
Hyoid cartilage (visceral bar II)
Visceral bar III
Copula bar III
FIG. 36. section from san

across the n:iddle of the outer eyes. Ti < \ the

composite optic lobe region li;i . : are

doubled. A few of the muscles bel<>
near their origins. In the fie

ind the copula of the tl; • • h I']. III. figs, r |

; (normal). For othei < . 4, and for pi

••>• PI. XXI. figs. 88-90, line 36 I y 20.)

J. F. G.

PI.. VIII.

VIII

Optic lobes

Cavity of optic lobes

Supraorbital bar

Commencement of Her

Hypoaria

Rectus sup. and inf.

. Lens and retina
Outer trabecular unit

• Pituitary space of right twin head

Palato-pterygoid bar
Inner trabecular units, fused
•Visceral arch cartilage III

I (Meckel's)

'.', ,, ., II (hyoid)
Copular cartilage IV
17. -Tranav series (do"'

with union at optic lobesi utrated in the t».

figures. The section is further back than in Fig. 36. Evidences ol
duplicity are disappearing, but some are still to be made out, e.g.
two sets of infundibular downgrowths in the floor of the brain, and
two pairs of trabeculae, of which the inner, or adjacent, units have
united to form a mesial bar separating the two pituitary spaces.
Compare with PI. III. fig. 16 (normal) and with PI. XIV. fig. 50
(union at medulla). Other references under PI. I. fig. 4 and plane
of section in PI. XXI. figs. 88-00, line 37. ( X2O.)

Periotic cartilage
(posterior end

and
inni- >
line 38. (

Posterior end of composite medulla

•• Lateral muscle segments

Outer parachordal bar

•Inner parachordal bar

- The two notochords

• The two aortae
Ganglion of vagus
Jugular vein
Pharynx

The last visceral arch cartilage

Sinus venosus

Cartilage of pectoral girdle

Auricle
Yolk

J. F. 0.

PL.

asdol oiJqO

l oijqo lo vJJvjiO

l cartilage

.mi lull: .<]!;•>. «M >•>»>! --~f^

1 brir. arj4 J

I .^uiu ifiluMMaf 9ii;O - -.
br.irl njivl ton lo^iafiqe yiElkiJiM -

-r/i .-i, :: ,o ..',: ..

]^) I ,, S

(bio-(ri)ll ,
VI

(of one

_ - - .... •
"Nerve II of iheoth.
•The united inner ;
The united inner Meckt:
Soft tissues of the composite lower jaw

fcllufaam 9)iaoqrnOD 1o

alnarngaa abaurn

( b

-p..

Kectusbcffnt,
,us (

RectusSuf. :
inn """
Palato-pW;
Trabeculae crariTi
Meckel's cartilage (visceral bar 1)
Hyoid cartilage (visceral bar II;
Visceral bar I ! I
Copula bar III

d sup. of one of the
bar

XI

J. F. G.

FIG. 37.— Transverse section from same series (double monster
with union at optic lobes) as that illustrated in the two preceding
figures. The section is further back than in Fig. 36. Evidences of
duplicity are disappearing, but some are still to be made out, e.g.
two sets of infundibular downgrowths in the floor of the brain, and
two pairs of trabeculae, of which the inner, or adjacent, units have
united to form a mesial bar separating the two pituitary spaces.
Compare with PI. III. fig. 16 (normal) and with PI. XIV. fig. 50
(union at medulla). Other references under PL I. fig. 4 and plane
of section in PI. XXI. figs. 88-90, line 37. ( x 20.)

FIG. 38. — Transverse section from same series as in the last
three figures, but passing through the back part of the head. It
is remarkable that the evidence of duplicity here is greater than
in the preceding section. The two notochorcls are widely separated,
and are surrounded by a mass of cartilage representing both pairs
of parachordals. There is a median series of muscle segments
representing the fused adjacent somites. The heart cavities, how-
ever, form a single series. Compare with PI. IV. fig. 18 (normal)
and with PI. XVII. fig. 57 (union at medulla). Other references
under PI. I. fig. 4, and plane of section in PI. XXI. figs. 88-90,
line 38. (xjo.)

PL. IX.

FIG. 39. — Horizontal section through a double monster with
union in region of optic lobes, and belonging to Class I., i.e.
the type illustrated in PI. I. figs. 4-5 (surface views), and in
Pis. VIII. -IX. figs. 35-38 (transverse sections). Compare with
PI. V. fig. 21 (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 and the medulla oblongata. Other refer-
ences are given under PI. I. fig. 4. ( x 16.)

FIG. 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 pee
under PI. I. fig. 4. ( x 16.)

PL. X.

The i.<

vodfi 1o e^ioiiaque iJoa
•ijjRlittRO Ifinirnga

89dol Ifild3190 to '{Jivjs'J

i iluao au

o-{idm9 Jfigii to 3^a ^
' [ oijqo to '{Ji

S9dol '>il4tt*
BBliMBO "jitoi i

VI slohtnsV
(sliaoqmoo

gjnarngse
s^BfiJXBD rioi£ I
o) bioo

eye
ontrr

fiimissure

Trir /.l.rcri

The two ao'

Inner nr
Hcad-kidnei

831(9 renni sriT
vodfi lo a9ioii9qi;a i

(i9nn

biotl'i-
(slieoqrnoo) bn. , 1, ic'

The inner eyes
Recti superiores of above
Tegminal cartilage
Cavity of cerebral lobes
Supraorbital cartilage

Optic lobes
Ventricle III
Oblic|uus 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)

tht-ir central cavity are seen in hori/ontal section, as also are
the fourth ventricle and the medulla oblougata. Other refer-
ences are given under PI. I. fig. 4. ( x 16.)

FIG. 40. — I! -ame double monster as

in last figure, but at a I he section passes

through t! but only through the

upper segments of thf < niposite cnvitv of
the optic lobes will be seen to bifurcate in front, and ]«>d into

the cavities of the two forebrains. For other references see
under PI. I. fig. 4. (xi6.)

The inner eyes
Recti superiores of above
Supraorbital cartilage
Cavity of cerebral lobes
Obliquus superior
Ventricle III

Optic lobes

Outer eye of right twin

Lens

Retina

Rectus superior

Cavity of optic lobes

Optic lobes

Periotic cartilage

Semicircular canals

Medulla

Muscle segments

Neural arch cartilages

Muscle segments (inner)

Notochord

Spinal cord (composite)

J. F. G.

PL. X.

The two inner or adjacent eyes

I Choroidal fissures

.. Obliq. inf. of an inner eye
_.•• Nerve 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

Kir,. 41 — Hon in timiugh same ii-

,.'Uion is at a still < . uul passes l>elo\\ tin- 11

!he Itnsnl parts of the brain and !

nerves and fissurr >sl to the cur «><ly is

cut through somewhat obliquely, IB -ml- and head-kidneys,

as well as th. composite spinal coi iin-dian mii'scular mass. TV-re is

greater apparent duplicity in the (a. •• body than in the posterior cerebral

region. For other references see under I'l. I. Fig. i. ( x 12. )

The two inner or adjacent eyes

Choroidal fissures

Obliq. inf. of an inner eye

Nerve 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

- Choroidal fissure

- 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

The two aortae

Fir,, 42. — The anterior pi

(X2I.)

An aortic root (third)

of last specimen shown in higher magnification.

J. F. G.

PL. XI.

XI

9Blin9d£-u lo gninnigsd

9iu2airnrnoo

/siieoq^ri^o else owl

Tcginir
Supraorbital cartil

. 'mieir'
/entricle I V

Medull

33J11U lt>J J'J^Jii"' * -^^_^_y IA * VI M li'-< I*' ^<- iiii I : t>

briE abiodaolon b9'iiBq 3riT ^o 'W \^ Neural arch cartilages

/- — N v \ Spinal cord (composite)

3EJTOB OWJ 9riT- :1

rfjiw

ebiodoolon ov/j 3t(T

aliramjaa abaurn IBISJB^!

bioo Isniqa slisoqrnoD sriT- V--

asya JnaoBJbB TO i-jruii ow) adT

(BbioioriO
31(9 I9nni nfito .1ni .pildO .

379 isnni .qua euJo9H ••••-./,
i9Juo .Ini .pildO -

3'(9 131UO .Ini

oo bnB 37isn DiJqO

[KbioioriD , ,

9* owl sriT ..... X

o .JX

Ilij lo abtra isqqU-.,

• Cavity of cerehrai lobes

- Obliquus superior

Ventricle III

— Optic lobes

_il^ ' " - Outer eye of right twin

superior

>t optic lobes
>bes

artilage

rnicirci^lar canals
(^Medulla

ments
cartilages

;ments

(conipo-sitc)

bnr. abioriaolon baiifiq 9rlT /

9BJTOB O'Nl 3dT..

(biirij) jooi ainor. nA

IX

J. F. G.

FIG. 41. —Horizontal section through same double monster as in last two figures.
The section is at a still deeper level, and passes below the auditory organs, through
the basal parts of the brain and the lower segments of the eyes. All four-optic
nerves and fissures 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. i. (xi2.)

FIG. 42. — The anterior part of last specimen shown in higher magnification.
(xai.)

PL. XI.

•"•FlGSt 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. ( x 14.)

FIG. 44. — Horizontal section of the embryo described above,
passing through the middle of the outer eyes and of the
auditory 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.)

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. (xi4.)

FlG. 46.— Horizontal section of the embryo described above,
at a deeper level than in Fig. 45. The globe made up of the
fused adjacent eyes is cut in median section, while the outer
eyes show their choroidal fissure and places of entrance of the
optic nerves, (x 14.)

PL. XII.

ilBlidioBiqua T>mti balin'J

ujBiqua i9)uO

, iEid'j-193 to x)iv<5'0

.1111 .

•Diqua nanni balin'J

Ifilidiofiique 'isJuO
asdol oiJqO

injio oilohaS

r.a ri3iB IsinsBd bslinU

inoo Isiiiqa -jliaoqinc'J •

IOI3E1IO

Ini .00 euupildO

• .00 2UJ39H

.1X9 .30 ZU13

hnfi Bludibfiulni owl a

'InodDolod ow) srlT

: >ril
lliglo an/;q isqq'J

ibfimolg lo gninnig
lo Ji

eefirn 'iBlij'jau
ibiorloolon owl sdT

bioa Iiiniqe 3)ieoqmoo ariT

anBgio ipoJc

9gBlillB3 '

n9l bnB a9'{9 lanni balinU

. . 9^9 19)00 n A
.. .qua .30 aulosH

.. .1/9 .OO

..• bar, Bludibnulni o'wl sdT

. ssdoiB Higlo ansq isqqU

---- sbiorisolon owl sriT
..... alab'iorbBiBq lisril bn«

aloub VK out srlJ "io gnif

abiorbolon owl srfT
"•---.. bioo Ifiniqa 9Jieoqmo3 SfIT

IIX

- — Horizontal !ouble monster (union

illustrated in V\. I. figs. ; X. figs. 35-38, and Pis. X., XI. figs.

iis, however, sonic iuipoitant differences when compared with the two l.<
one another, so that the common lens lies buried below the surface in th':

greater evidence of duii! i > do the parts round th< ,, -fourth vc;, ng to

the body is cut through • >s in Ms. X., XI. figs. , hows a dou! . of th

other structures. The '.• rest of the alimentary < njle, but there are two swim-t>!

general references see tin ;». 4.

»e type as the sp.-<

e adjacent eyes and brain

ret. The ey'

e optic lobes show
^^^Hfe'OI the specimen,
1 of certain
der» «r :

United inner supraorbitals

Outer supraorbital

Cavity of cerebral lobes

An outer eye

Ventricle III

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
Outer supraorbilal
Cavity of cerebral lobes
Rectus oc. int.

Ventricle III
Optic lobes

Periotic cartilage
Ventricle IV
Parts of labyrinth

Medulla

' Parachordals

The two notochords

Mesial muscular mass

Outer series of muscle segments

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. (xi4.)

FIG. 44. — Horizontal section of the embryo described above,
passing through the middle of the outer eyes and of the
auditory organs. The mesial optic lobe mass, referred to
under Fig. 43, is now larger. The mesi.il muscular mass
between the two notochords is also to be noted. ( x 14.)

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 I.

and their parachordals - -

Oesophagus
teginning of the two air ducts

Head-kidney .

The two notochords ni'Z<

The composite spinal cord

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 mns^
The two notochords

— • The composite spinal cord

FIG. 45.— Horizontal section of the emb

v.-: ihnn in the last two tigr
••« two adjacent c>

seen, as also arc the two »••

fundibulai imiHpm»lh« and the origins of the two swim-
bladders. (

t.il section <

, - , -,:i Fig. 45.

v '•:> is cut in me*i

id pb
nerves. ( x 14. )

]. F. G.

PI, XII.

XII

Mucous groove

Inner eye of right twin bead

Supraorbital bar

Rectus oc. ext.

Cavity of cerebral lobes

Olfactory cartilage

Olfactory nerve and pit
Rectus oc. internus
Obliq. inf. of outer eye
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

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

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 PI. I. fig. 6. ( x 18.)

^%

"Commencement of optic lobes

Supraorbital bars

Pineal body

Rectus oc. ext.

Rectus oc. sup.

Cerebral lobes

Olfactory cartilage

Rectus oc. int.

Olfactory organ
-•Obliq. oc. inf. of outer eye
••-Trabeculae cranii
•••• Palato-pterygoid cartilage

•- An inner lower lip

• Anterior end of united inner hyomandibulars
Upper end of united inner Meckel's cartilages
Inward dip of surface epithelium

Fu;. 48. — Transverse section from same series as last figure, but a short
distance further back. Compare with PI. 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 PI. XXI. figs. 91-93.
Other references are given under PI. I. fig. 6. ( x 18. )

J. F. G.

PL. XIII.

XIII

wooig auoouM

I niwl jrigii 1o 9y9 "rarinl

United inner suprWSH

.JX9

Jguill

"-> ^

'»9H p.,

,,, Wo.

\"entricle III-

Outer supraorln

Opti-
Pen

Ventricle IV - -

Parts of labyrinth •

iMiji'-ir!;iq"> ajr.Viue 'to qib faisv/nl

• i9/rni ME 1o InsmssngrnmoO

ai bglinulo ebna loiisinA

Mesial OHiacdv OHM-
Compowte »p;n.-.

" Parachordals

•

•

• Composite spinal cord

Inner olfacton

asdol oiJqo 10 )

Oil;]

• o iu\-M9i
qui .30 auJDsM

'.i

. I !

Kectus oc. ext. .. t/V^-./ fay)

>u\a and .an
tbeii .

Hyomandibular . •_,;

; "&* W?J

Upper parts of gill arches . ^ /

r -A

The two notochords --

and their parachordal-

Oesophagus K.

Beginning 01 the two air (hats

Head-kidi
The two notocho

The composite tpina! cr>

1o

li b9Jirm 1o
)iq9 9DBhuz 1o qib

• Obliquus <K
Rectus oc. sup.

• Optic nerve
Rectus oc. ext.
The two infundibi:

'

Beginning of stomach
Part ol

• Lateral mu:

.j.'

III

~,

.O .1 .1

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 vaiious 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.)

FIG. 48. — Transverse section from same series as last figure, but a short
distance further back. Compare with PI. 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 byline 48 in PL XXI. figs. 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 hyomandibular 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. (xiS.)

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 51. ( x 18.)

PL. XIV.

isniii 1o alfinfio
Javooio siduob

.qua .00 ai;!39JI- •
anigho ii9ril 16911 ilo9i istuil ./.; ../^~\....:

asdol oiiq<
inno ailohsl

iBd IfiJidioBiq
muludibrrutnl-

"I9JUO .
19IUO .111! .

__._ b^lin'J
abioyri )n

iliBO a'b^o9t/ J(i9or,(_bf> owj 9ril
rijuom owl

fd in I

lavooloo alduob li^sasiqrnoo ignm
aadol DiJqO1"— ;

1IJ OWJ

isd loidTOK
.qua .00 auJ39H

IuaqEO oilohsq to

iinBiD MbnnMT-4 ........... ••

•I3JIJO

biogvi9lq-olBlES — -\
abioyri ln9DB[bE bglinU ' '
19)00 nA

Optic lobe*
Cavity of optic lobe*

III «W8ST90ZIV ,DH9

2JOOT bnfi rijiB 'jirio/

A-.-^-^^T^ ^«r

*•!( "Optic lobes

ty of optic lobes
Cerebellum
Periolic capsule

vujar canal

tr mass
IV

- Dila)ed ) er.ua! caiial of upper
.,nal cord

ifi 191UO nfi) ggBlinco bio'(H -

ei/zo
egninsqo rljuotn c>

n the

VT/

FIG. 49. — Transverse section from the same series as Figs. 47, 48. hut through
the outer eye>, tin- optic lobes, and the hyoinandibular suss a the inner

or adjacent sides of the twin heads. Below, in the middle line, is the v
bf-tw > mouth-openings, and in it the inner or adjacen

cartil i moulding of the adjacent united periotie capsules and 01

otocysts is extremely interesting. See also the two following figures. (
references are given under PI. I. fig. 6, and the plane of section is indicat.
line 49 in PI. XXI. figs. 91-93. ( X 18.)

The

Semicircular canals of inner
compressed double otocyst
"" Optic lobes
•••-Periotie cartilage
— • -Supraorbital bar

-Infundibulum
J','"—Rectus oc. sup.

,i\- Inner recti near their origins

I-IJ+- Rect. int. outer eye

'•I- Rect. inf. outer eye

"Palato-pterygoid

"'• Trabeculae cranii

Periotie cartilage (composite)
United adjacent hyomandibulars
United adjacent hyoids

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.
.-Cavity of hypoarium

•|- Floor of periotie capsule
••*•!•- Trabeculae cranii
W--)-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.

FIG. y Transverse

fr. ••

through the posterior part ni the "

inner hyixwna, the entrance of the optic nerve on one side, a
the mesial oral septum containing the united adjacent hyoi
und-r PI. I. fig. 6. (x 18.)

r ittr, the
ck part of
leferences

FIG. 51. — TransvetS'1
through the outer auditory organs .
obviously made up of two converg
the inner parachordals i 'I mesia

them. Compare this section .•
(union at optic lobes) Other lefcieiiuts unc

I". XXI. !i£!v ',1-9:

Mesial muscular mass

Medulla (almost double)

Parts of inner composite labyrinth
• • United inner jugular veins

Periotie 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
rh* »< but

i, which is

•• notochord is double, and
ass of muscle wedged in between
ial), and with PI. IX. fig. 38
'I. I. fig. o, and plane of sectio?

J. F. G.

PL. XIV.

XIV

Optic lobes

• Cavity of optic lobes

• Cerebellum
Periotic capsule

• Semicircular canal
-Mesial muscular mass

--•Ventricle IV

• Dilated central canal of upper

part of spinal cord
Lateral muscles
Neural arch cartilages

FIG. 52.— Horizontal sectio-
to the type illusti
Adjacent pectoral fins «rc not pr. -

•

11 IK v*lM:

Pineal divertic.

Optic lobes
Cavity of optic lobes

Her
Ventricle TV

Mesial muscular mass
Medulla

Periotic cartilage

Semicircular canal

Lateral muscles

Notochords

Mesial muscular mass

Spinal cord (composite)

ay. The action hare
uter s*rt«s ^f

30 i

J. F. G.

PL.

asdo! jjj.;>

esdol Dilqo lo yjiveJ •••^~
d^ia'J- --^

'ibim-j^

isqqu lo lenso lEUn

bio • : iiKq

•HiJ.— ---•

;!

' their origins

Trabecuiae cranii
Periotic cartilage (composite)
1 adjacent hyomandibulars
ed adjacent hyoids

Fhe two adjacent Meckel's cartilages in the septum between
the two mouth openings

pi esied double octocyst
>ptic lobes

Supraorbital bar
Rectus oc. sup.
9Hkr- Cavity of hypoarium

••-^Floor of periotic capsule
^K-Trabeculae rranii
^jJiH-Rea. inf. outer eye

W^ff*"~ Palato-pterygoid

N The i
( j

...\.../.. eadol ail.

asdol oilqo lo <()ivB'J

ppsite

er jugular veins

.... Her;

.•—•—.. - Ii.aK-j i/iluoiioitnug

\U9SUfn I/jiial/

101) Ii,oiq£ ,
u arcnTII

T ^'oots

Visceral arch IV

Visiural a.-ch V

"•••- Copula arch V

i.artilage (an outer

i arter:

m mouth op<

YX

J. F. G.

.0 /-I .i

XIV

FIG. 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. The section passes through
the optic lobe masses, the cerebellum, and the upper parts of the
periotic cartilages in either head, while, further back, 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.)

FIG. 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. As in Pis. X.-XII. figs. 39-46, owing to the curvature of
the specimen, the trunk is cut through obliquely. The section here
shows the composite spinal cord, the two outer series of muscle
segments, and a mesial similar series between the two notochords.
For other references see under PI. II. fig. 7. ( x 20.)

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. (xao.)

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.

imsa ilji" v/;.i rav/ollo aauaail llo?.
I/.vrloaaolO

.

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(I rbir, Ir.-maiv) ag/siinco i'hd

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. , • -. .. • >\

nr-3t\ :.
JiE3rl

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lay, •

vfJ

(jfl I/noJosq to s^/Jirt/O
' illoY.

bus sueeij DiJB3ipn/;'i

"• ...

•:"J8 9.1) buovsd Jeui eanilasjni ov/j -jrlT

noilonu( yd Ivtfmol ^nheslni '!• ..-IMS

o/;l aril lo

vbod rii.lilioV/,.

• b;ort*W»<nhir-HlT. /

"/•• .

^Efri ^fi^IJ^^I/r^ IfiiR^I/ i .. . ''••

M! (;;VJH,.I --J

>,- '(q JMp^pl 9 ' '

ortt

,1 1 .q so-f

, ,

'• \\
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/
J

'

.. .•:• .. la .<•<

I b»M>qqij< 311.

luo nwf.ib rl-juin ,bioo .'rniqe sjieoqfrioL") ...
gb'iorioolon ov/3 orlT '"

MU

,

afaioriooJon ov/i srIT ••'""..

bioo [B/riq8 '"'

rwi* ,di .q

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IVX

Soft tissues of upper jaw

- Soft tissues of lower jaw with germs of teeth

Glossohynl

Hypohyal

" " 'Copula, visceral arch III

^'Meckel's cartilage (visceral arch 1)
Ceratohyal

•• Meckel's cartilage
..- Ceratohyal

, I, II, III
" Truncus arteriosus
Ventricle of heart
Auricle of heart

Inferior cardinal veins

Gill cover

Cartilage of pectoral fin

Yolk

Pancreatic tissue and bile ducts

........ The two intestines just beyond the stomachs

....-- Single portion of intestine formed by junction
of the two former

.Wolffian body
. .The two notochords

^^\ Mesial muscular mass

• Lateral muscles

FIG. 54. — Horizontal section belonging to the same s'-ta-s as the hi^t,
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 pericardia! 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. (X20.)

•X.

FIG. 55. — Horizontal section furthe
Tlie section passes through tlv
the place of union. The spii.
section is drawn out trnnw-i
distal section is a!>
The mesial m

and is seen to be < ; • tenet

two outer series of mu-
neural arch cartilages

(X28.)

... Composite spinal cord, much drawn out

transversely
"" The two notochords

— • Inner neural arch cartilages

Haemal arch cartilages

......... Aortae

......... Wolffian bodies

""Mesial muscular mas-,
" Lateral muscles

•-...."'••The two notochords

• Spinal cord

rs the last.

body behind

!s proximal

while the

1 i itochords.

here,

ing with the
ie adjacent
' II. fig. 7-

J. F. G.

PL. XVI.

XVI

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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. i.

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 th6 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. 1 6, and for other references PI. I. fig. 6.

FIG. 61. — Diagram showing from above the roof carti-
lages of the skull of the double monster illustrated in
Fig. 60. For general references sec under Fig. 60.

FIG. 62. — Diagram of neural and haemal arch cartilages
in a normal trout embryo, six weeks after hatching.
See p. 12.

o, sue we

FIG. 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.

-•• *_ariiiagt ot pectorai

tissue and bile ducts
LKTTERIXU IN Pi.. XVII. FIGS. 56-63.

and the stomachs

• i. Olfactory capsular cartilage.

b. Trabeculae cranii.

M. Hypohyal cartilage. (The glossohyal is omitted ; see
PI. XVIII. fig. 64.)

c. Parachorclal cartilages.

d. Is placed on the trabeculae cranii just in front of the

pituitary space in Fig. 56, or the two pituitary spaces
in Figs. 58, 60.

f. Is placed on the parachordals just behind the pituitary-
space in Fig. 56, or the two pituitary spaces in
Figs. 58, 60.

f. Palato-pterygoid bar ( /', inner or adjacent palato-

pterygoid bar).
fiifvfz- Anterior, middle, and posterior fontanelles.

g. Meckel's cartilage of lower jaw (g1 inner or adjacent

Meckel's cartilage of lower jaw).
//. Hyoid bar.
ha. Haemal arch cartilage (ha1 inner or adjacent haemal arch

cartilage).

/. Hyomandibular (;'', inner or adjacent hyomandibular).
k. On floor of periotic capsule (k1, on floor of inner or ad-
jacent periotic capsule).
m. Pituitary space with external recti muscles passing

through it.
No. Neural arch cartilage (Na', inner or adjacent neural

arch cartilage).
Ncli. Notochord.

;-. Roof part of olfactory capsular cartilage.

s. Supraorbital bars (/. inner or adjacettt supraorbital

bars).

spf. Spinal cord.
/. Cartilage of roof of periotic cap
•W. Laminar part of parachordals.

\-y. Tcgminal cartilages above the third ventricle and cere-
helium respectively. ,
i! muscular

PI

ffcC

58.

63.

). V. G.

PL. XVII.

Meek

Meek

VII

64-.

Meek,

Meek

III

>, Cop.

Cop.

Scap.

66

A-CHor.

A-Chor J/ tO -^ A.chor

A.Chor.' A a,'

Chor

AChor

67

C.F.B.

C.F.B-.

C.O.L.

}. V. G.

PL. XVIII.

PLATE XVIII

FIG. 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
PI. II. fig. 6.

Meek. Outer Meckelian bars.
Meek'. Inner or adjacent Meckelian bars united and greatly

reduced.

GH. Glossohyal.
BH. Hypohyal.
LH. Ceratohyal.

BH'. Inner or adjacent hypohyals partly united.
LH'. ,, ,, ceratohyals unit' (!.

Ill, IV, V, VI, VII. The branchial cartilages.

('of. Copular pieces, the anterior one being bind.

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.

Scaf., Cor. Scapular and coracoid parts respectively of the coraco-
scapular bars. The ventral ends of the coracoids
are united.
CI.. The limb-cartilages united for the greater part of their

length.
K. 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 PI. I. fig. 4.

Ao' '. 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.
Ps/>r. Pseudobranch.
A . Char. Choroidal artery.

A.Chor1. 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.

FIG. 68.— Diagram illustrating the ventral aorta, the
dorsal aorta, and the aortic roots in the double monster
belonging to Class II., from which PI. XVII. fig. 60 was
taken. For references see under PI. II. fig. 6. Letter-
ing as in Fig. 67 with additional :

TA. Ventral aorta and its branches i, 2, 3, 4.
Car1. Inner or adjacent carotids, giving off Aa' . arteries to the
inner or adjacent pseudobranchs fsir1.

•j-r-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.

FIG. 69. — Outline of central cavity of brain of normal
trout embryo. Lettering as in Fig. 71. For references
see under PI. I. fig. I.

FIG. 70. — Outline of central cavity of brain of the double
monster (Class I.) illustrated in PI. XVI I. fig. 58. Letter-
ing as in Fig. 71. For references see under PI. I. fig. 4.

FIG. 71. — 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 hemispheres.
Cyd V. ,, 3rd ventricle.

COL. ,, optic lobes.

CIt. ,, iter a lertio ad quartum ventriculum.

C^tk V. „ 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 (q.v.). There are two infunclibula and two sets
of hypoarial cavities, while the main optic lobe cavity is
single.

COL. Cavity of optic lobes.
Inf. Infundibulum.
Hyp. Hypophysis sac.
Hya. One of the hypoarial cavities.

Meek.

Meek-

GH

BH

LH

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69.

72.

PL. XVII

V.I.

75.

XIX 3TAJ4

bnB JDtib rifirftloV/ 9ri) gnijfiijaulli mBi^BiQ — .d\ .Ol'i
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81.

PL. XIX.

PLATE XIX

8ft .!

FIG. 73. — Diagram of heart, etc., of normal trout embryo
(p. 12). Lettering as in Fig. 75. Above s.v. is the
common opening of the anterior jugular veins. For
general references see under PI. I. fig. i.

FIG. 74.-- Diagram of the heart, etc., of the double
monster (Class III.) illustrated in PI. II. fig. 7, which see
for general references. Lettering as in Fig. 75.

FIG. 75. — Diagram of the heart, etc., of a double monster
belonging to Class IV., i.e. with union in pectoral region,
the adjacent pectoral fins being present but united (p. 19).

V. Ventricle.
An. Auricle.
SV. Sinus venosus.
£>6". Duct of Cuvier.
DC'. Inner or adjacent ducts of Cuvier.
VJ. Jugular vein.
VC. Cardinal vein (posterior).
VC'. Inner or adjacent cardinal veins.

ijou. I

FIG. 76. — Diagram illustiating the Wolffian duct and
urinary bladder in a normal trout embryo (p. 12). Letter-
ing as in Figs. 77-81. For references see under PI. I. fig. i.

FlGS. 77-81. — Diagrams illustrating the arrangement of
the Wolffian ducts and bladders according to the different
types described on pp. 21-23. It w'" be noted that the
ducts which associate themselves in pairs with the bladders
nearly always consist of the right duct of one embryo and
the left of the other. Fig. So is an exception, and Fig. 81
an example of pure ventral union (p. 23).
HL. Bladder.
/>'/.'. Bladder formed by the inner or adjacent pair of

Wolffian ducts.
WD. Wolffian ducts.
WDb, \VDt'. Wolffian ducts belonging to one of the twins in each

of the figures.
\VDa, WDa'. Wolffian ducts belonging to the other twin in each'

of the figures.

Int. Intestine. :m 3,

39, K. Rectum.

K Vent.

P. Urinary pore.

•"A. .\»H

i ••

V.I. S AIL.

V.I.

D.C.

D.C.

V.C.

73.

75.

77.

WDb

W.DOL

78.

W.Da -.

wpfc =:

80

wot.

I. F. G.

PL. XIX.

H.K.

A.Sc.

H.K

WD

WD.

84.

WD.

C.WD

CWL>

WD.

86.

87.

J. F. G.

PL. XX.

FIG. 86.

FIG. 90.

IX fig.s 35 36, 37, 38. Fig. 8q represents the"roof pans of the skull with transverse lines having the same meaning as in Fig. 88 (see explanation under
I'l. XVli. fig. 59*. Pic. 82.— Lateral viexvdalstbwiediDfiositeuartjditttr^. 8gC4 ( xapjKjs. 184. — Composite head-kidney in a double mon-
FIG. 90.—!' present in the angle between the twin heads of a cloubieiouble sitwsity, of the'rfyp.ei<ie'S<h<^a«<»dpjter4V'<;Ld
monster belo^g'ing to Class II.,1^.^'^^ of the i^Txv^ftft^jT^ glomerura^t^t's1"^' pr'ese'ii
brains at the medulla oblongata. Description on p. 17, divided into three chambers.

brains at the medulla oblongata. Description on p.
and other references under PI. I. fig. 6.

Utricle.

An anteriorly placed semicircular canal.

A posteriorly ,^

heail-lydney of a normal trou»

it rf v— at!

,:.- '

Glomerular tuft of ve:
Afferent/Vessel,
- Effere

Capsul^of glomerulus.l
Corunjencement of VVoilj
i of Wolffian duofll

divided into three chambers.

O. Mesial chamber.

FIG. 85. — Head-kidney in a double monstrosity of same
general type as last, but here the glomeruli have not united
ether, though the adjacent halves of each give origin to
n-dycts-or only te a short Wind one-; -tittering

/ Fits. 86 AT^B-S?. — ft«ad-kidney glomer^ii of other two

double monstrositie* showing a decree of duplicity greater
nlhat illustrated in Fig. 85. . A blind1 sacculated tube
off from the middle compartment of the composite
lus and represents united adjacent Wolffian ducts.

KlG. 93.

H

FIG. 91.

FIG. 93.

,. 91-93.— Diagrams of the cartilagin ton and of the brain cavities in a double monster belonging to Class II., i.e. with union in the

of the medulla. The numbered 'ti < nes are for the same purpose, and have the same meaning as in PI. XXI. hgs. 88-90. The general

'lass II. lire given under PI. I. lip a under PI. XVII. fig. 61. XVIII. fig. 71. (xapprox. 15.)

olfactory capsular can;
trabeculae cranii.
parachovdal cartilages.

I.KTTKRING IN ALL THE FIGURES.

//'
'

d on trabeculae in front of the two pituitary spaces.
t on parachordals behind the pituitary spaces and the tendon of
reel. oc. ext.

^

outer and inner palato-ptei ygoids.

,, ,, hyomandibulars.

,, ,, periotic cartilages,

in the two pituitary spaces,
outer and inner Medici's bars.

„ ,, hyoid arches.

J. F. G.

PL. XXI.

82

83

-norn slduob B ni varibiji-bfisrl
ni as 3nn3l)3j .(_i .q no
B ni

i:»j .(.i .q no ug\irrja3D 3q<i SQJ -I
Jnsasiq si iuiinsrnols 0;/T

-frnEfbf:-

XX

093

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owJ tsrfjo 'io iluismofg

Tiic<>tT) v<r-,r(nn()'j0 33-1^35 J; yniworfs z*ftiaoi}8noffi 3ldi/ob
,^^JD^ bnild A .j8 .gi'i ni bslBiJauIIi JBftl nfiri)
' aril 'io JrrsrrmBqrnoo sibbim 3t(} moilTfto asmoo
i nBfflloV/ jn3Dfi[bB balinu ajnsaaiqsi bns aulwornolji

Dfie yiolibufi siiaoqrnoD sri) to 7/91 / ljn-3ji;.l — .£<:
slduob B lo ebfisrf ntwi arfj n33v/jad algrtB ad) ni
niwj sril lo noinu
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>\"U

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•li/. \

: >¥).'•[ .r>

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oV/ lo

A'-N 'i

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J. F. G.

IM, XX.

35

36 -

35

. /
X 5 \i e e

FIG. 89.

Kio. 90.

FJGS. 88, 89. — Reconstruction diagrams of the cranial cartilaginous skeleton in a double monster belonging to Class I., i.e. with union in the region of
the optic lobes. Fig. 88 is a view from above of the skeletal parts after removal of the roof cartilages by a section passing horizontally through the olfactory
and the auditory capsular cartilages. Fig. 88 also includes the mandibular and hyoid arches, which for diagrammatic purposes are represented as wider
than normal. The transverse lines numbered 35, 36, 37, 38, indicate respectively the planes and levels of the various transverse sections 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 as in Fig. 88 (see explanation under
PI. XVII. fig. 59). The general references to Class I. will be found under PI. I. fig. 4. ( xapprox. 15.)

FIG. 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 lines have the same meaning as in these figures. The large composite cavity of the mid-brain (optic lobes) is seen to lead, in front, into the
cavities of the two forebrains, and, behind, into that of the single medulla (see under PI. XVIII. fig. 70).

.

Fie;. 92.

FIGS. 91-93. — Diagrams of the cartilaginous cranial skeleton and of the brain cavities in a double monster belonging to Class II., i.e. with union in the
region of the medulla. The numbered transverse lines are for the same purpose, and have the same meaning as in PI. XXI. figs. 88-90. The general
references to Class II. are given under PI. I. fig. 6. See also under PL XVII. fig. 61, XVIII. fig. 71. (xapprox. 15.)

LETTERING IN ALL THE FIGURES.

a olfactory capsular cartilage.
b trabeculae cranii.
c parachordal cartilages.

d on trabeculae in front of the two pituitary spaces.
e on parachordals behind the pituitary spaces and the tendon of
rect. oc. ext.

ff outer and inner palato-pterygoids.
ii' ,, ,, hyomandibulars.

kk' ,, ,, periotic cartilages.

in in the two pituitary spaces.

gg' outer and inner Meckel's bars.
h h ' , , , , hyoid arches.

). V. G.

PL. XXI.

FIG. 94. — Sketch of triple monster trout embryo described on pp. 33, 34, as
seen from the left side of the principal embryo. Between the latter and
embryo A are two vents close together, while the single urinary pore is
behind the vents. A marked keel along the left side of the composite tail is
formed by fusion of the ventral edge membranes of these two embryos. A
similar keel occurs along the right side, and is derived from the fusion of
corresponding elements belonging to the principal embryo and to embryo B.
The lower angle of the composite tail is formed by the fusion of the dorsal
edge membranes of A and B. The specimen is also illustrated in the two
succeeding figures and in PI. VII. figs. 32-^4, and PI. XXIII. figs. 97-09.
(x6.)

'An/i it W/io /

FIG. 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.

JluJ.-mJ»

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 . /, unite before opening
into this bladder, as also do the left duct of.-/, and the right duct of B. The
left duct of B ends blindly. Other references are given under Fig. 94.

J. F. G.

PI,. XXII.

Lateral muscle mass of principal embryo
Pelvic fin ,, ,,

Note-chord ,, ,,

Spinal cord ,, „

Intestine „

Head-kidney of embryo A
Brain tissue ,,

Pectoral fin (left) „
Oesophagus , ,

Air-duct ,,

The common yolk mass
Right pectoral fin of embryo K
Oesophagus „

Head-kidney „

Liver tissue ,,

Spinal cord „

Medulla
Otocysts

Pectoral fin (left)

KH;. i

while the v.\

obii([Ue!y t!n

;_;ll (hi: hill.-

three oilmen:

those of A fin

the oesophagus, an-

pneumatic ducts. In .)'

develop'

principal embryo the section paw

distance behind' this organ. C"

are given under Fig. 94. ( x 10.)

FIG. 98.— Transvi i

to the sami-

jjure, but a

• • inbryo
'• tissue
Mile II
ntricle
ies of ./ ,unl
n ducts, air-sacs,
iiic tissue belong-

.

A pelvic fin of the
principal embryo comes into the section
on its : 'i her references are given

Lateral muscle mass of principal embryo
Spinal cord ,, ,,

Notochord ,, „

Wolftian 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-sac ,,

• Wolftian ducts ,,

- Spinal cord ,,
,. Medulla

Stomach of embryo A

Air-sac ,,

Medulla

Left pectoral fin of embryo A

Lateral muscle mass of principal embryo
Spinal cord ,, ,,

Notochord ,, . •

Aorta > i » »

Wolffian body

Intestine ,, ,,

Common yolk mass

Intestine of embryo A
B

Commencement of the portion of intestine
common to A and B

Bodies of A and B uniting

Flu. 99. — Transverse section from same
as the last two figures, but a con-
siderable distance further back. The
, of the two defrctivr embryos are
are seen to be

made up 01 -• 'he

spinal cords beii 'sable.

The intotin.il on «e embryos

part seen towards

uddle lieing the commencement of
.union terminal (wrtion. References
undiT tig. 94. ( x 17.)

J. F. G.

PI, XXIII.

xxin

ov/idrnf> Isqbnhq to zzsm sbaum

-- .«' ',', biorboloM

• < «

ma to nft IjnoJosq JrigiH

Flo. 94
seen froi

[sqbnitq 1o eesrn sfoaurn [B'
,, bioo [

p.Jouh nfifflloW

ov nornrnoO

fjiqioniiq to nrt oivfaq itej
^ oyidma ^o auaaij Di

K oyidrns lo suaeil oiJcqaH
ri3Emol2

bioo Ijin

K o^idras 1o rio

,, OBE-ljA

, , sIlubsM
o^idtna 1o nft IfnoJsaq IlsJ

„

fe-v

•iftp- iiy

•f triple monster tro
i the left side of the principa
1 an two vents close togeth

•\ marked keel along — —

I the ventral edge membranes of tl

.f > Moit£ the right side, and is d* . me msiun ui

' principal emluyo and to embryo B.

.. <:!•• • !' ihr composite tail is formed by the lu-ion of the dorsal
•f ,4 and II. The- specimen is also illustrated in the two
•ml in PI. VII. figs. 32-34, and PI. XXI II. figs. 97-99.

suaaij
bioo [

A

fusion of

»ii'l Ui! part of the intestine in
K>rtions of intestine, and liver out-
• given under Fig. 94.

KIG.

of the

into ti

.' 'tpffdifd [jjqbahq to iitun sb^iini In
bTOD UniqS

,, biorfooloK

,, -{bod nfirftloW

nomrnoO

V-. ov/idrns to snijaslnl

stihaslni to noinoq sri) to i
,,„„,, Tne & brifi K OJ nomrnoa

f- \\Olfnan ducts are
«>ie. The left duct

•^"- <snl)lnu '.\ bn

1IIXX

J. F. G.

PL. XXII.

FIG. 97. — Transverse section through the
triple monster illustrated in the three pre-
ceding figures. The section passes across the
middle of the body in the principal embryo,
while the two defective embryos are cut
obliquely through the pectoral region, and
through the hinder part of the brain. All
three alimentary canals are here separate,
those of A and B being cut in the region of
the oesophagus, and having beside them the
pneumatic ducts. In A and B also a well-
developed head-kidney is seen, but in the
principal embryo the section passes some
distance behind this organ. Other references
are given under Fig. 94. ( x 10.)

Fir;. 98. — Transverse section belonging
to the same series as the last figure, but a
little further back. The body of embryo
A is taken up chiefly with the nerve tissue
of the medulla and spinal cord, while B
shows a section of the tiny fourth ventricle
and medulla. Below the' bodies of .-/ and
B are seen the Wolffian ducts, air-sacs,
stomachs, and the hepatic tissue belong-
ing to these embryos. A pelvic fin of the
principal embryo comes into the section
on its left side. Other references are given
under Fig. 94. ( x 14.)

Fi<;. 99. — Transverse section from same
series as the last two figures, but a con-
siderable distance further back. The
bodies of the two defective embryos are
now uniting, and they are seen to be
made up of very irregular tissues, the
spinal cords being hardly recognisable.
The intestinal canals of these embryos
are joining together, the part seen towards
the middle being the commencement of
the common terminal portion. References
under fig. 94. ( x 17.)

PI.. XXIII.

100.

103.

101.

--AV.

Tr. PL

F.B.

102.

104.

0— -//^

-H — I- -^u .

106.

105.

J. F. (i.

PL. XXIV.

PLATE XXIV

FlG. 100. i 'yclopic trout embryo of type A (p. 40),
seen from leli sule and showing the large single eye
overarched l.y the mesial frontal 'process.
.

Kn;. 101. -Transverse section of the frontal process
referred to abmr, showing the Small approximated
olfactory pits (('), and the reduced olfactory capsular
cartilage (C').i.

FlG. 102. Transverse- section through the posterior
parts of the i i rebral lobes in the same 'spec! men, showing
tin- pineal di\ ertic ultim (/V/7.), and the cerebral lobes
(/•'./A), the latter being intimately united along their
inner surfaces. Tht central cavity of -the brain (C. l-~.fi.)
is not markedly dropsical, and the cerebral lobes are
almost normal in their transverse measurements (p. 41).

_ . f . .

FIG. 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 infimdibulum. Infundi-
bulum and hypophysis are absent.

FlG. 104. — Section through back part of retina of
cyclopic eye (type 15) described on p. 42, showing failure
of development on the part of optic nerve and choroidal
fissure. Pg. . . . hexagonal pigment layer ; Ret. . . .
retina ; «... a few nerve fibres not perforating the
pigment layer.

FIG. 105.- Transverse section of head of trout embryo
with supernumerary eye (see p. 59).

/,. Lens of supernumerary eye.
A'. Retina

R.O. Right normal eye.

N. Right optic nerve receiving fibres from retina of super-
numerary eye.

C. Central cavity of brain with two deep grooves in its floor,
each of which leads downwards into an infundibulum
and a hypophysis.

Ti: Train i displaced towards left.

P. /.. Palalo-quadrate bar on left side ; the right bar is absent.
S.o. Supra-orbital bar.
Av. Anterior corner of auditory capsular cartilage.

FlG. 106.— Horizontal section of head of trout embryo
with supernumerary eye (see p. 60).

L. Lens telonging to the left (aborted) twin head.

CL'. Cerebral lobes belonging to ditto.

jrd 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.

;p. ,. _
. -us),.

Retinae

Lenses,..

tatf inferior

VIXX 3TAJS

oyidms tuoii Io bfiarilo noilaaa saigvanBiT — .jot .
•(9? -1 9

."A.

ays iBrmon irigiSI .O.c
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muludibaulni n* oini ebiBwnwob ebEil rioidw Io li-jKt

.atayriqoqyd R bns

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H\ .O

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olo xlivJiO LoO
.,lol oilqO .AO
libiiA/ .aK

103

--AV.

Tr. PL

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106.

XXIV.

I 'meal divertic.

Tegminal cartilage ^

Central cavity of brain •"'

Fused cerebral lobes
Recti oc. sup.

Retina

Lens.

Sott tissues of upper jaw

Ti L;ininal t:ntilage
I'mivil diverticuluni
Mucous canal
Ventricle III
Fused cerebral lobes
Recti oc. sup.

Choroidal glands
Retina
Optic nerve
Choroidal fissure

.Recti oc. inf.

Tissue of upper jaw with

anterior ends of
.Palato-quadrates and
Trabeculae

FIGS. 107 and 108. -S
and its lens, and tlu- latter throu^l
'ies for the tv.

ii lure see pp. 40 4.

Optic lobes-
Oavity of optic lobes • .

Infundibuluni'. . .

Supraorbital bar^

Recti oc. sup... _
I'i^nient layer (continuous),.

Retinae.-

Lenses

FK;S. 109

s, but they
re absent.
i>r einl- of w

Optic lobes
Cavity of optic lobes
Her (beginning)
Periotic cartilage
Supraorbital bar
Semicircular canal
Infundibuluni

Recti oc. sup.

Retinae
Optic nerve

Choroidal fissures

Lenses
Recti inferior

• .

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vxx

F'lGS. 107 and 108. — Sections through head of a cyclopio trout embryo, the former through the middle of the median eye
and its lens, and the latter through the entrance of the single optic nerve. This nerve is seen to divide at once into two
branches for the two partly separate retinae. The fused condition of the cerebral lobes will be noted, particularly in Fig. 107.
For details of structure see pp. 40-42. Compare also with the two succeeding figures, and with PI. XXVI. fig. in. (xa-j. )

FIGS. 109 and no. — Transverse sections of the head of a trout embryo showing the condition of semi-cyclopia. There are
two eyes, but they are close together, and the optic nerve is single, although there are two choroidal fissures. The internal
recti are absent. As in typical cyclopia, the eyes intervene between the base of the brain and the trabeculae cranii, the
anterior ends of which are separated by a wide gap from the rudimentary olfactory capsular cartilages. The latter are not
shown in this section. Compare with the two preceding figures, and with PL XXVI. fig. in. See also p. 44. ( X24.)

PL. XXV.

FIG. in. — Section through head of a semi-cyclopic
trout embryo, from same series as PI. XXV. figs. 109,
no, but further back in the series. The section passes
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 sagittal
plane. (x24.)

I-'IG. 113. — Transverse section of body
of trout embryo just behind the pectoral
tins, showing local absence of the noto-
chord. The adjacent neural and haemal
aid) 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. ( x 20. )

'

FIG. 112. — Transverse section through
atrophic head of an embryo trout, in
w hich the lenses alone of all 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.)

FIG. 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 body, and shows the twin sets of structures coming together to
unite in lateral union. The intestinal canals are here separate, as also
are the livers, air-sacs, Wolfh'an bodies, etc. ( x 16. )

PL. XXVI.

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Ijihi'nori'i t'lu; .)/-•>

ifiliidihdr.nioyH

inn l»;i'>li:l lii-i'n.Mii; '

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.•[r.iuri toniMMai owl -HIT

?.r>vil o
jlloy aoinmoo sdT .

ll)<

-'

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Cerebellum..

Ventricle IV-

Periotic capsule .

Parts of labyrinth .

Medulla

Ganglion nerve V.,
Recti. oc. ext. and choroidal
arteries
Hyomandibular

Trabeculae cr.

Palato-pterygoids.

FIG. m.

trout embryo, In,

no, but further back in i fhe section

almost horizontally along the trabeculae cranii, which
here form a single mesial rod. See pp. 40, -14.
Anteriorly, the trabeculae are seen to articulate with
the palato-pterygoids ; behind them the two extern, il
recti of the eye are passing through the pituitary
space in company with the two choroidal arteries.
The head is somewhat compressed in the sagittal
plane. ( x 24. )

. ... Spinal cord

Neural archcartilage
Half rings of cartilage below
the spinal cord

Muscular raphe

Aorta
Wolffian body

Intestine

i lion "i b

uf tn, ui embiuj just behind the pec
fins, showing local absence of tin-
chord. The adjacent neural and li
arch cartilages (use together to form a
series of half rings below the spinal cord.
The lateral muscles meet in a m
raphe above the dorsal aorta and I
the spinal cord. See p. 54. ( >. 20.)

Tegminal cartilage

Cerebral lobes J

Muscular tissue

Lens .

The adjacent lateral muscular

masses uniting

_ The two spinal cords

. The two notochords

. . • The two Wolffian Iwdies
_. The two intestinal canals

The two livers
... The common yolk mass

I i<;. H2. — Transverse section thr<Mii;!i
atrophic head of an embryo trout, in
» Inch the lenses alone of al' the eye struc-
ttnes have survived. K\en the mouth,
the Mi-ekel's cartilages, the trab>
and i
See pp. 60, 61

Fie. 114. — Trans\ cr-" muster In-longing to tin

described under Class V. <>n ] •;> 20-21. The section passes acro-
mi'MI structures coming togei

here separate, as also

['I. XXVI.

J. F. G.

XXVI

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