Studies in biology from the Biological Laboratories of the Owens College

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THE AMERICAN MUSEUM

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NATURAL HISTORY

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BIOLOGICAL LABORATORIES

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Tier OWENS COLLEGE

VOLUME I.

PUBLISHED BY THE COUNCIL OF THE COLLEGE
AND EDITED BY

PROFESSOR MILNES MARSHALL.

MANCHESTER:
J. KE. Cornisu,
1886.

PRICE TEN SHILLINGS.

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MANCUNIENSIS.

PREFACE.

The present volume, the first of a series which the Council propose
to issue from time to time, consists of papers containing the results
of investigations conducted in the Biological Laboratories of the
College during the past two or three years. Two papers of earlier
date have been included because they deal with a problem that has
attracted much attention of late years, and one of the latest phases
of which is discussed in Dr. Beard’s essay on the “ Branchial Sense
Organs of Vertebrates.”

Mr. Marshall-Ward’s paper on “An Aquatic Myxomycete,”’ and
the contributions of Mr. Dendy and Mr. C. F. Marshall appear here
for the first time. The remaining papers are reprinted from the
Transactions of Scientific Societies, or from Scientific Journals, the
editors of which have courteously granted permission to republish
them and to reproduce the plates by which they are illustrated.

Special interest attaches to the two earlier papers of Mr. Marshall
Ward, and to those of Dr. Beard and Mr. Fowler, as the investiga-
tions the results of which they record were carried on by these
gentlemen during their tenure of Bishop Berkeley Research Fellow-
‘ships. To the generous encouragement afforded by the founder of

these fellowships is due the opportunity for the completion of these
researches, and to him this volume is respectfully dedicated.

Much of the work here recorded was performed under the difficul-
ties of cramped accommodation and limited appliances, and it may
confidently be hoped that the material for future volumes will show
corresponding improvement when, in the immediate future, the
completion of the new Morphological Laboratories shall have placed
the College in a position of exceptional advantage for the promotion
of Biological research.

Owens College,
March, 1886.

CONTENTS.

Mr. MarsHatt Warp.—On the Morphology and the Development of
Menioua, a genus of tropical epiphyllous Fungi. Plates I.
and II. Reprinted from the “Philosophical Transactions of the
OY Ol sSOCtetYy meme OTE LINN BSo pir atstnieritedsae cass eveslseleseiiesesanesiactetes

Mr.. MarsHatn Warp.—On the Sexuality of the Fungi. Reprinted
From the “ Quarterly Journal of Microscopical Science,’ 1884 ......

Mr. Nees Warp.—An Aquatic Myxomycete, Plates JIT. and

ere ee eee eee ee ee ee eer ee ee ek er rrr

Prof. Marsuatt and Mr. Sprencer.—Observations on the Cranial
Nerves of Scyllium, Plates V. and VI. Reprinted from the
“ Quarterly Journal of Microscopical Science,” 1881 ............00000s
Prof. MarsHaun.—The Segmental Value of the Cranial Nerves.
Reprinted from the “ Journal of Anatony and Physiology,’ 1882..
Dr. Joun Brarp,—The System of Branchial Sense Organs and their
Associated Ganglia in Ichthyopsida. A contribution to the
Ancestral History of Vertebrates. Plates VII, VIII, and IX.
Reprinted from the ‘‘ Quarterly Journal of Microscopical Science,”
BB Binion in Seas gage aiainn cist seca tan vaca aisle caiamt cia slac it soa Meee Romeo
Mr. B. Menitanp.—A simplified view of the Histology of the Striped
Muscle-fibre. Plate X. Reprinted from the “ Quarterly Journal
Of MicroscopicaliScvence.wanl SS om mnscepenonaate core eeceh cenit erate
Mr. G. H. Fowzirer.—The Anatomy of the Madreporaria. Part I.
Plates XI., XII., and XIII. Reprinted from the ‘ Quarterly
Journal of Microscopical Science,’ V885..........e0.0s.ccscensccseces see
Prof. MarsHaLu.—On the Nervous System of Antedon rosaceus. Plate
XIV. Reprinted from the “Quarterly Journal of Microscopical
Science,” 1884:
Mr. A. Denpy.—On the Regeneration of the Visceral Mass in Ante-
COM TOSACCUS 325, ciies Geielsclsnetenecaciecaltemace setaile Monee cee acre eee Te ooet
Mr. C. f. MarsHatt.—Some Investigations on the Physiology of the
Nervousisystemsok themlobstereesccescecosetnscheeoaeecee cece
Prof. MarsHaLi.—The Morphology of the Sexual Organs of Hydra.
Reprinted from the “Memoirs of the Manchester Literary and
Philosophical Society,” 1885 ......s.c.0es Rico eauMasrasiamuaeaae Ga6¢600900006

PAGE

23

64

243

313

324

ON THE MORPHOLOGY AND THE DEVELOPMENT OF THE
PERITHECIUM OF MZLIOLA, A GENUS OF TROPICAL
EPIPHYLLOUS FUNGI.

By H. MarsHatt Warp, B.A., Berkeley Fellow of Owens College,
Victoria University ; late Cryptogamist to the Ceylon Government.

[Prates I—IT.]-

During the course of recent researches into the nature of parasitic
fungi, my attention was arrested for some time by several forms
of epiphytal growths which occupy a sort of half-way position between
the more pronounced endophyllous parasites and those fungi which
cannot be looked upon as requiring more than a hold-fast or shelter
from their hosts. Among these are the Meliolas, a group established
by Fries in 1825 to receive certain tropical fungi! In the ‘ Annales
des Sciences Naturelles’ for 1851? is a memoir by Bornet on the
species constituting the genus J/eliola, in which the characters of
these remarkable epiphytes are enumerated and examined, and a
classification of the known forms proposed: this paper is a standing
authority on the subject, and I shall have occasion to refer to it at
intervals subsequently, partly to confirm some of Boruet’s work, partly
to add new observations and correct older views as to the nature or
significance of various points.

The Meliolas are minute epiphyllous fungi, belonging to the
Pyrenomycetes, the deep-brown or black mycelium of which appears as

2 Systema Orbis Vegetabilium.’
2 Ser, iii., Bot., t. xvi., pp. 257, &e,

2 H. MARSHALL WARD.

sooty patches on many and various plants in the tropics, and presents,
roughly, a similar appearance to the masses of Capnodium or Fumago
sometimes observed in European woods on the leaves of living plants.

Though, according to Bornet, several species must have been known
under different names to the earlier botanists, the name established
by Fries, and published in his revised system, was accepted by Mon-
tagne and Léveillé and has persisted since: Berkeley, in England, has
referred to the group in his ‘ Cryptogamic Botany,’ and has described
several species from the tropics in various papers.

The habit of these fungi, and the fact that no true Hrysiphe had
been found among the collections of travellers in the tropics, led Fries*
to insist strongly on the known or supposed analogies between the
two genera, and, Bornet following Fries, the J/eliolas have thus come
to be regarded as replacing the Hrysiphes in tropical countries—as,
in fact, “ representative species.” Bornet added several facts to those
already known concerning the coarser anatomy of the group ; but even
his excellent and systematic memoir left large gaps in the knowledge
of important details, and practically nothing was known of their
development or of the formation of their “fruit-bodies.” These and
other gaps I hope to fill up to at least a large extent in the present
essay.

The appearance of this fungus, as presented to the unaided eye,
is much the same as that offered by Asterina and similar forms, and
the reader may be referred to a recently published drawing of that
fungus for a tolerably accurate idea of it? The chief difference is
that the black maculze presented by well developed plants of Meliola
are more decided and thicker than those of Asterina; all transitions
are found, however, and, as with many other forms of epiphyllous
Pyrenomycetes, it is impossible to detect exactly what fungus is present
by a superficial examination.

The fungus Meliola may be conveniently considered as composed
of a mycelium, which supports appendages and perithecia, and which
arises from spores developed within the asci of the latter. Bornet
considered the “receptacle” as an equally important and distinct con-
stituent, but this is perhaps unnecessary, since, as will be shown, the
so-called receptacle can only be looked upon as a more or less acci-

1 ‘Summa Vegetabilium,’ p. 406: ‘‘ Genus in foliis tropicis vulgatissimum ut Hrysiphes
in terris temperatis.’

2 Quar. Journ. Micr. Sc., October, 1882, plate 27, figs. 1 and g. See also Bornet’s
beautiful figures, Ann, des Sc, Nat,, ser, iii, t. xvi., plates 21 and 22.

THE PERITHECIUM OF MELIOLA. 3

dental development, so to speak, depending and following upon the
formation of the perithecium.

The mycelium, forming the chief part of the black patches found
on the surface of the affected leaves, petioles, &c., spreads in an
irregularly stellate manner from a common centre or centres (see
fig. 1). It is detached with comparative ease from the epidermis of
the leaf, and bristles with fine, simple or branched, pointed appen-
dages, of a black colour, which spring from the main hyphe, and
from around the subglobular perithecia which are irregularly scattered
over the surface.

The main hyphe constituting this vegetative part of the fungus,
are irregularly radiating, sinuous or zigzag filaments, closely appressed
to the epidermis of the leaf, &c., and composed of cylindrical joints or
cells placed end to end, and branching at angles of about 45 degrees
(of. fig. 2, and fig. 5), Their stiff and even brittle walls are deeply
coloured brown or biack, and thus obscure the view of their contents:
sections and reagents prove these to be finely grained protoplasm,
with or without oily drops in the interior. The diameter of the hypha
is equal throughout, the apex being, as a rule, evenly rounded: the
cross-septa dividing the hyphe into cells are firmly marked, thick and
dark-coloured like the outer walls.

The main branches of the mycelium all present the same general
characters described above. In many cases, however, the blunt apices
of the larger hyphee, instead of being evenly rounded, become curiously
deformed by an accumulation of abnormal growths, of the nature of
caps (see 43, fig. 7) fitting roughly one over the other: these consist
of swollen, more or less cuticularised thickenings of the cell-wall,
with or without granular débris between the layers. They are evi-
dently produced by irregularities in the forward growth of the hypha :
in the moist intervals the growing apex, more delicate than the
older portions of the hypha, creeps along the surface of the leaf in
the normal manner; during recurrent dry and hot unfavourable
periods, however, sudden hardening and stoppage of growth causes
the accumulation of the caps. That unfavourable intervals in out-
_ ward circumstances may produce such abnormalities is well seen in
the Saprolegnie, and I have in these observed the formation of suc-
cessive shell-like caps of dense cellulose, more or less altered, and
enclosing granular matter between the layers: the caps are coloured
blue by solution of ginc-chloride and iodine, the granular débris yellow,

4 H, MARSHALL WARD.

These phenomena were by no means uncommon with the hyphe of
Achlya and Saprolegnia grown, in summer, too long in the same
water ; of course the pathological changes are produced by different
causes in the two cases.’

Besides the main branches of the mycelium, certain small pyriform
or flask-shaped “outgrowths are given off at pretty regular intervals
from the cylindrical cells of the larger hyphe (see figs. 2, 3, and 4):
in some cases each cell or joint gives off such a short branchlet from
each side, in others from alternate sides. More rarely they are absent
altogether. In all cases examined the short lateral branchlet arises
as a simple bulging out of the lateral wall of the cell: as this proceeds,
the bud (as it may be considered) swells out, and its cavity finally
becomes separated from that of the parent branch by a firm septum.
The long axis of the bud-like protuberance is very generally, though
not always directed at an angle of nearly 45 degrees to that of the
portion of the main hypha lying nearer the growing point (see fig.
2, &c.): its walls are similarly dark coloured and firm, and it contains
fine grained protoplasm much as the cells of the main hyphe. Mor-
phologically considered, the short lateral outgrowths are undoubtedly
of the nature of arrested branches.

In one form of JMeliola, growing on the leaves of a species of Con-
volvulus, I have observed a second form of the lateral branchlet (see
fig. 4), co-existing with the commoner pyriform type. In this case
the outgrowth was longer, narrowed into a sort of neck, and presented
the general shape of a Florence flask, seated with its bulged out body
on the parent branch. In some specimens, each cell of the latter
supported two opposite flask-shaped branchlets : in others only one,
with or without a pyriform body in addition. Sometimes one or the
other type occurred singly and irregularly (fig. 4).

‘The flask-shaped body is sometimes open at the apex, though I have
never succeeded in observing anything emitted from the pore. These
flask-shaped appendages recall to mind the peculiar bodies figured by
Woronin in another group of the Pyrenomycetes (Sordaria),? and
although no grounds exist for correlating the two phenomena in detail,
the fact is at least worth recording that the lateral pyriform bodies in
Meliola are capable of subserving reproduction, as will be shown here-

after.

1 There seems reason to believe that further investigation may throw light on this subject
in connection with the apposition of the cell-wall.

- 2 “Beitrige zur Morph. u. Phys. d. Pilze,” De Bary and Woronin, ser, iii., plate 5,

THE PERITHECIUM OF MELIOLA. 5

When the hyphe or branchlets of this fungus are looked upon from
above, and a strong light passes through from below, one often observes
a minute, circular, bright spot, which appears to shine through the
upper wall like a very small oil-drop ; on reversing the object, so
that the lower side of the hypha comes uppermost, this brilliant pore-
like spot appears much more evident, and is clearly due to a thinning
in the wall of the under side of the hypha, at a spot where no colouring
matter is deposited in the cell-walls, and where the contained proto-
plasm is placed more nearly in connection with the outside (see
figs. 7, 21, and 40).

Bornet apparently refers to these bright spots when he speaks of
oily globules in the interior of the hyphe,' though he may have been
speaking of actual oil-drops developed in the dried specimens with
which he chiefly worked. If Bornet’s remarks refer to the bright
spots here described, the facts of their appearing only on the lower
wall and not being altered by alcohol, &c., remain to be explained.

Taking all the facts into account, the view seems to recommend
itself that these bright spots are the points of attachment of the hyphee
to the epidermis ; if so, they are to be regarded as haustoria of a very
rudimentary nature. The mycelium certainly is attached to the sur-
face of the leaf, though but feebly, and it appears suggestive that
alcohol specimens are more easily detached than fresh ones, possibly
because the protoplasm becomes contracted and rendered brittle. No
other anchoring bodies have been observed, and one notes that the
position of these brilliant spots accords with that of the well-developed
haustoria in Asterina,* a genus of fungi at least allied to the Meliolas.
These bright points are not always present, and in some cases seem
to be normally absent. They are very generally formed at once
on germination, appearing on the first short tubes put forth by the
spore (fig. 40), a condition of things which may again be compared to
what occurs in Asterina,’ and also in Hrysiphe and allied forms.* Still
another point reminding us of Asterzma and the LHrystiphee is the
function of the pyriform branchlets ; in some cases at least, they become
detached, and act as vegetative reproductive organs or conidia, each
putting forth bud-like processes which develop into new hyphe.
Bornet remarked the separation of these buds in Meliola amphitricha,

1 Op. cit., p. 260, and plate 21, fig. 3.

2 See my description in Quar. Journ. Micr. Sc., October, 1882.

3 Bornet, op. cit., plate 28, fig. 5.

* De Bary, “‘ Beitrige zur Morph. u. Phys. d. Pilze,” 1870, ser. iii., plate 12, figs. 1 and 2.

6 H, MARSHALL WARD.

and hints at their possibly serving as reproductive bodies much as the
Ordium forms of Erysiphee: since he worked with dried specimens,
however, this question could not be decided.

Bornet remarks that the mycelium on the upper side of many leaves
are sterile, while those below and protected from the direct rays of
the sun alone support perithecia: this is certainly not true for the
Species examined by me, and, indeed, I cannot determine any difference
between the upper and lower mycelia in this respect. Those on the
upper surface seem quite as productive of spores, &c., as those below,
and in many cases—e. g., those Meliole so common on Memecylon—
the mycelium vegetates almost exclusively on the upper surface, and
is quite fertile there.

Besides the short pyriform and flask-shaped branchlets described
above, the mycelium bears certain stiff, upright, appendages of the
nature of sete (see figs. 1, 41, and 8): these sete spring from the
cells of the hyphe at various points in their course, and, from their
position and mode of origin, are probably to be regarded, morphologi-
cally speaking, as lateral branchlets which become elongated in a
direction more or less perpendicular to the plane of the leaf. Such a
seta grows very rapidly and soon reaches its limit: the cylindrical
cells composing it are relatively longer than those of the hyphee, but
resemble them in other respects (the walls being, perhaps, somewhat
stiffer and more deeply coloured), and taper above, in the simple types,
or become variously branched.

In most Deliolas the sete are especially aggregated around the
perithecia, forming circles of stiff radii springing from what Bornet
terms the “receptacle” : they are also developed, however, from various
isolated points of the mycelium bearing no direct relation to the fruit-
bodies.

The forms of the sete vary from a simple, upright or curved filament,
to structures branched like antlers, trifurcate, twisted, &c., at the tip
(cf. fig. 8 and Bornet’s figures?) : Bornet has made use of these details
in classifying the formal species, and although it is doubtful
whether the more similar types are constant, there can be no objec-
tion to their use much in the same manner that the appendages of
Erysiphee, &c., are used to distinguish the forms of that group.
Bornet regards the origin of the sete at points on the mycelium as
marking out places where new perithecia are to be developed : I cannot

1 Loe, cit., plates 21 and 22, figs, 6, 15, 16, &c.

THE PERITHECIUM OF MELIOLA. 7

say that this idea is altogether a false one, but investigation of the
development of the fruit-bodies seems to show that at least no neces-
sary connexion exists between the two phenomena,

Ag to the function of the sete, little or nothing can be stated. The
earlier suggestions of Sprengel and Fries (as quoted by Bornet) that
they may be organs for the exit of the spores cannot be accepted: not
only on the ground of the disproportion between the numbers, but
also because the spores are too large to pass through the setw, even
supposing the cavity continuous and ending in an ostiolum, which is
not the case. I have often tried to discover conidia or other bodies
in connexion with the setw, but have been forced to the belief that
they have no function whatever connected with spore-production.
One is not now impressed with the necessity for assigning any special
function to such structures: if the sete are merely free-growing
branches of the otherwise appressed, creeping mycelium, there is
nothing surprising in the fact that some differences in form and con-
sistency are correlated with their sub-iierial habit. This is at least no
more remarkable than that the looser branches of an alga, like Coleo-
cheete, should have a facies slightly differing from that of the cell
series comprising the lower, creeping, appressed parts of the thallus.

The collection of sete immediately around the “ fruit-body” simply
results, immediately, from the vigorous development of hyphz which
accompanies the later stages of formation of the perithecium: this
mass of setigerous hyphze, which seems comparable with the formation
of haustoria and such-like organs in other fungi during the fruit de-
velopment, was called the “véceptacle” by Bornet. As to a possible
protective influence of the circles of sete, the question must be left open
until we know more of the conditions: in some cases, at any rate, the
sete do not arise until the perithecium is completely formed, and the
spores nearly ripe.

The perithecium, when completely developed, is a globular or
sub-globular body, consisting of a shining black or brown external
case, the outer thick walls of which appear regularly embossed, and
an internal mass composed of asci and spores, &c. The embossed
pattern on the outer walls results from the thick-walled cells, of which
it is composed, projecting at their free surfaces: where the cells join
each other, forming polygonal figures, they do not so project.1_ What
may be termed the base of the perithecium is sessile on the mycelium :

2 See Bornet’s figures, loc. cit., plates 21 and 22.

8 H. MARSHALL WARD.

at the opposite pole, or apex, is frequently a slight papilla, not ob-
viously pierced by any pore. Bornet, noting this fact, imagines that
the dehiscence takes place below, the whole upper part of the peri-
thecium becoming broken away by a circular rupture at the base. In
some forms, at least, the spores escape through an opening at the
apex: how far this is general I do not know (fig. 43), but facts exist
to render it probable that a minute and dilatable pore occurs in
others.

Vertical sections of the mature perithecium show that within the
firm, deep-coloured, external wall is a lining of softer cells, with swollen
envelopes and of a more or less flattened form: this inner lining of
the perithecium extends two or three cell-series deep, and is slightly
yellow or pale-brown in colour (see figs. 33 and 34). In the cavity
thus enclosed are the groups of asci in various stages of development :
these delicate, clavate sacs contain spores, or have emptied them into
the semi-gelatinous, granular matrix around.

With these preliminaries, I may pass on to consider and describe the
development of the perithecium, as followed step by step on a species
of Meliola which I have investigated with no slight success :! this
will be found to throw light on the morphology of these fungi from
the best of sources—development—and aid in a more critical estima-
tion of their proposed systematic position. After describing in detail
the origin, mode of development and fate of the fruit and spores, I
propose, therefore, to examine the relations of the Afeliolas to Hrysiphe
and other fungi.

On examining portions of the epiphyllous mycelium bearing the
short pyriform, lateral branchlets so often referred to above, one fre-
quently discovers specimens presenting the appearances depicted at
figs. 9, 10, 11, &c. The simple pyriform body, after becoming more
swollen, has suffered division into two portions or cells by a septum,
usually vertical to the plane of the mycelium and leaf, and passing
diagonally across the cavity with a slight curve, so as to abut on the
outer walls at right angles, or nearly so. The originally unicellular
protuberance becomes in this manner divided into two more or less
unequal cells, and it will be shown in the sequel that these two cells
have, from the first, each a different destiny in the formation of the
fruit. For this reason I have indicated in the drawings, by shading,

1 I must take this opportunity of thanking Professor De Bary for kind suggestions with
respect to this work,

THE PERITHECIUM OF MELIOLA. 9

a difference which does not present itself in the natural object at this
stage. The more apical cell, which is smaller and shaded darker in
fig. 9, may be indicated throughout by the letter A: it will be found
that this cell produces the central ascogenous tissue of the young
perithecium, while the other, which will be referred to as cell B,
originates the outer portions of the case or perithecium wall.

Following close upon the preliminary division above described, a
septum appears across the larger of the two cells, cutting the first-
formed division wall at right angles, or nearly so: this is rapidly
followed by another septum (fig. 10), and so the larger cell (B) becomes
cut up into three. Following upon these, a number of further divi-
sions in planes at right angles to the preceding are soon established
(figs. 11 to 17), and at the same time, though much more slowly, one
or two more division walls are formed in the cell A, thus cutting it up
into a short series of about three cells (figs. 14, 15).

If the above description has been followed, it becomes clear that the
division of the more rapidly growing cell, B, results in the production
of a sheet of cells affixed, so to speak, to the few-celled mass resulting
from the slow division of A: such being the case, and the sheet ex-
tending as new divisions are formed, the cells resulting from A become
gradually enveloped more and more in those resulting from B. A
comparison of the figs. 9 to 17 will facilitate matters here, and for
convenience of description hereafter, and in consideration of its destiny,
we may term the mass of cells produced from A the ascogenous core—
or simply the core.

At a stage which may conveniently be considered the next one to
fig. 11, the cells resulting from the division of B are observed to be
extending as a curved layer over the ‘“ core” of cells formed by A.
If, at this stage, the young fruit-body is cut off, and allowed to roll
over in fluid under the microscope, the form and arrangement are
found to be somewhat as sketched in fig. 12, where a represents the
view from below, 6 that from the side, and ¢ an end elevation of the
structure. The cell A, in fact, is becoming gradually enfolded by the
layer of cells derived from B, a process which results, at a later period
(fig. 17), in the almost complete tucking in of the “ core” as the centre
of a sub-globular mass of cells,

As this process of “invagination by epibole” (as it would be termed
in the case of an embryo) goes on, the “core” has been more slowly
cut up into cells—at first by walls perpendicular to its long axis, and

10 ‘H. MARSHALL WARD.

then by septa in other planes at right angles—and the sub-globular
body thus produced lies with the open part towards the epidermis.

After this period, two events occur: Ist, the cells of the “core,”
possessing very thin walls, acquire a different aspect from those of the
outer shell; their finely granular protoplasm makes them appear
denser and more opaque, shining through the latter until this becomes
too thick to be transparent ; 2nd, the open part of the growing peri-
thecium becomes closed over, and the internal structures can no longer
be made out without the aid of actual sections. At this point my
observations have failed to decide which of two possible modes of
growth take place: Is the covering in of the “core” completed simply
by the extension and closing in of the edges of the outer layer ; or are
cells, cut off from the “ core” below, intercalated, so to speak, into the
open gap? One is led to expect by analogy that the former process
takes place, but some events lead to the suspicion that such may not
be the case.

At the stage corresponding to fig. 19 the young perithecium
appears almost opaque, very little light passing through the dark-
coloured and thick outer walls; from below, however, the larger cells
composing the “core” can be readily seen in the optical section,
shining by means of their dense, fine-grained contents through the
shell. In the next stages, the “core” can only be seen dimly through
the outer envelope (fig. 20), even after treatment with reagents, or, as
in figs. 21 and 22, after cutting or tearing off some of the outer cells.

Nothing but a fortunate vertical section through the young fruit
at or near this stage will decide finally whether the lower side is
covered in by the meeting of the outer shell edges, or by partial
“delamination” from the lower side of the “core,” and this I have
not succeeded in obtaining. The thick, dark outer walls have now
become so opaque, that optical sections fail to determine the course of
events; and treatment with reagents does not atford evidence suffi-
ciently satisfactory to decide the questions, since it seems impossible
to remove the colouring matter, Potassic hydrate or weak acids do,
it is true, render the structures a little more translucent after some
time ; but even the extreme resort of heating in weak chromic acid
has only yielded partial results, and with this slight information on
the point I have reluctantly been compelled to content myself for the
present. A comparison of figs. 17 to 21 certainly suggests that the
process of envelopment is completed by the outer layer of cells derived

THE PERITHECIUM OF MELIOLA. iL

from the repeated and rapid division of the cell B, and this view may
be recommended on the ground of analogies with the Lrysiphee, to be
examined hereafter; but, while figs. 19 and 20 by no means decide
the point, we shall find that in the perithecium of another species of
Meliola (or an allied form) the construction almost certainly proceeds
by continued cutting up and “ delamination ” of the results of division
of one cell.

Be this as it may, the young perithecium now consists of the follow-
ing parts :—A central “ core ” of delicate-walled colourless or yellowish
cells, very rich in finely granular protoplasm, and, surrounding this
completely, a single layer of cells with thick, hard, dark-coloured walls
(especially those on the exterior surface) ; the whole mass is attached
to the hypha from which it originated by a very short pedicle or joint
(see figs. 19-24).

At a period slightly later than the above, the cells of the outer
layer are becoming multiplied by tangential walls, and those of the
inner core by radial and horizontal divisions : these processes go on for
some time until the whole perithecium is a complex of many small
cells, the outer of which become firmer and darker-coloured, the inner
delicate and full of fine-grained protoplasm as described.

No trace of the internal structure is, however, visible now from the
outside. On isolating a perithecium at this stage—a matter of no
slight difficulty, but practicable with a slender knife used under a low
power of the microscope—it presents the forms shown in fig. 25
on being rolled over. Above, the outer surface curves equally away
from the centre, and the slightly projecting walls of the cells give it
an appearance of being embossed (fig. 25, x). From below (fig. 25, y),
the object looks very different; the surface is much flattened and
nearly circular, and from many of the cells are processes developing as
hyphee in all directions. These radiating processes creep close along
the surface of the leaf, to which the fruit-body is also appressed, and
no doubt serve to give a much firmer hold for the fruit ; at first their
thin walls are only of a pale brown hue, but rapidly acquire the
thickness and deep colour of the fruit and mycelium. Seen from the
side, the young perithecium presents the appearance sketched at fig.
25, z. It is these radiating anchoring hyphe which form collectively
what Bornet terms the “réceptacle,’ and from them, at a later period,
the bristling seée found around the mature fruit are developed.

From the stage just described the development of the fruit-body

12 H. MARSHALL WARD.

proceeds rapidly ; but, since the objects now become of a more manage-
able size, I have been able, by actual sections through the perithecium
embedded in spermaceti or gum, or, better still, in elder pith, to
obtain some insight into the processes going on even in the centre of
the mass of cells.

At stages just prior to the one last described, the central core of
thin walled cells—which it will be remembered has been derived from
continuous divisions of the cell A—is commencing to divide up by
septa in several directions (figs. 23, 24), while the outer layers sur-
rounding this—derived primitively from B, and, possibly, in part
from A—are divided more regularly by tangential walls, followed by
radial ones at right angles as the area enlarges. As the increasing
small and delicate cells of the core become formed more rapidly, a
certain tendency at least to a regular arrangement can be recognised
in the later stages, as shown in such sections as figs. 28 and 29 and
fig. 27: this regularity becomes interfered with by the mutual pres-
sure of the cells, and the outer ones, of which the walls are especially
soft and swollen, become flattened and pulled in the tangential direc-
tion, and only marked by the very granular yellowish protoplasm in
their diminishing cavities. In the central lower part of the core,
vertical sections at this, and slightly later stages, show that certain
cells, with very delicate outlines and finely granular refractive con-
tents, maintain their larger size and upright arrangement, and are by
these peculiarities well distinguished as a special group or tuft of cells
(see fig. 28, and fig. 31). In oblique (fig. 29) and horizontal (fig. 30)
sections passing through the lower third of the developing perithecium,
they can also be readily distinguished by their special peculiarities,
and no question can be entertained as to their signiticance in the for-
mation of the essential parts of the fruit-body. This group of cells is
the forerunner of the young asci, and may be termed the Ascogonium.

As development proceeds continuously, the outermost layers
acquiring thicker and more deeply coloured walls, the above named
group of upright cells become relatively larger, increasing slowly in
number by a few divisions, while the diffluent, compressed cells between
them and the outermost layers slowly give up their contents, and
become reduced to mere granular streaks embedded in a jelly-like
mass of swollen and fused cell-walls (see fig. 31). This process is
exactly. comparable to what takes place in the developing embryo-sac

THE PERITHECIUM OF MELIOLA. 13

of certain phanerogams,! or of the pollen mother cells in the anther,?
in so far as the larger cells clearly develop at the expense of material
derived from those around.

The tuft of successful cells thus nourished is, in fact, the “ ascogo-
nium” of this fungus. At a slightly later stage than the one last
figured, the space formerly occupied by the deliquescent remains of
small cells is filled with an almost transparent semi-fluid mucus, in
which a few bright granules are embedded; while the lower part of
the perithecium contains a tuft of asci in various stages of develop-
ment (see fig. 33), and which have evidently proceeded from the large
cells of figs. 28 and 31, which have devoured all, or nearly all, the
smaller soft cells surrounding them.

Sections of perithecia at a stage between those shown in figs. 31
and 33, have not been obtained, but enough evidence has been secured
to enable me to conclude that the asci are the direct result of the
transformation of the elongated upright cells of fig. 31, which are
nourished at the expense of the cells of the inner layers. Partly from
the brittle nature of the outer walls, enclosing a space filled with
almost fluid contents, and partly from the extreme delicacy of the
young asci, I have been unable to decide whether any distinct branch-
ing of the ascogenous cells precedes the formation of the definite asci:
probably such is the case. We have now followed the development of
the perithecium to the period when it may be considered ripe: a period
of some duration, since the asci are continually and successively formed
in the tuft for some time.

Fortunate sections of the perithecium wall at this stage have yielded
the following information. In the centre of the apical wall, where a
slight protuberance sometimes occurs, the cells of the inner wall are
found to radiate towards a pale translucent spot or pore (see fig. 36),
and although I have not been able to obtain sections exactly through
this, and am therefore unable to affirm positively that it is an actual
pore, there seems little doubt that this is at least the weak point
through which the spores escape from the ripe perithecium, no doubt
forced through by the swelling of the materials around. Bornet®
believes that the perithecium opens by a circular rupture at the base :
I have tried to confirm this, but failed, and am strongly persuaded
that the apical spot figured is the point of exit for the spores. That a

1 Cf., amongst others, Strasburger, ‘ Angiospermen und Gymnospermen.’
_ 2 Of., Strasburger, ‘Bau und Wachsthum der Zellhaute,’ 1882,
® Op. cit., p. 261.

14 H, MARSHALL WARD.

minute pore should escape observation from without is not remark-
able: the reflection of the light from the black shining outer cells
might easily obscure it. The general structure of these walls has
already been described, and figure 34, drawn from an extremely for-
tunate and very thin section, shows the details.

The very young ascus presents no features of importance to distin-
guish it from that of many other pyrenomycetous fungi. In its earliest
state it is recognisable as a single thin-walled, club-shaped cell, taper-
ing to a point at the lower attached end, and filled with finely granular,
yellowish protoplasm (see fig. 37 a.): sometimes a small pale, refrac-
tive nucleus-like point is seen in the protoplasm. .As the young ascus
grows longer, and its protoplasm increases in quantity, a fine, sharp

division line makes its appearance somewhat oblique to the long axis -

of the whole (fig. 37, c.) ; this is soon followed by a second, similar
longitudinal division, in a plane at right angles to the former (fig.
37, d.), and four well-defined masses are thus marked out. These, the
young spores, do not include the whole of the protoplasm (fig. 37,
dand f.), but lie in a scanty matrix of granular matter, closely ap-
posed face to face, and following the curve of the wall of the enlarging
ascus on their outer walls.

As the four, almost fusiform young spores increase in size, and
acquire more distinct membranous envelopes, they come to lie some-
what more loosely in the cavity of the ascus, and may cross one another
in accommodation to the space at disposal. Then appear cross-septa
(fig. 37, ¢., f.), dividing the material of the spore into a number of
compartments varying from three to five—or, in one case, a single
septum only is formed—and vacuoles and granules appear in the
hitherto almost homogeneous contents. As the spores ripen, their
cross-septa become more firmly marked, their outer walls thicker, and
gradually brown or nearly black in colour, like the hyphee of the de-
veloped mycelium; the side walls of the separate compartments also
become bulged out slightly, giving the mature spore thé appearance of
a long oval body, constricted at intervals (see fig. 39). Very com-
monly one or two oily-looking drops accumulate in the compartments *
of the ripe spore.

Such is the typical mode of development of the perithecium, asci
and spores. I have found no modifications of importance from a
morphological point of view ; it should be recorded, however, that the
number of spores in the agcus varies from two to eight. Sometimes

a ee

THE PERITHECIUM OF MELIOLA. 15

in the same perithecium one finds asci in which one, two, or three
spores develop at the expense of their presumably weaker neighbours
(fig. 38), in other cases the number two appears constant, only one
complete division occurring in the ascus (fig. 38), while in one case to
be referred to later, the asci normally produce eight two-chambered
spores (fig. 43).

On germination, which may take place soon after the emission from
the ripe perithecium, the spores seem to behave generally in the same
manner ; one or several simple protuberances emerge from any of the
partitioned chambers (see fig. 40), and proceed to develop into a
typical mycelium, often with a preliminary formation of the rudi-
mentary haustoria referred to in an earlier part of this paper. This
mycelium grows rapidly in moist weather, forming branches, sete
and fruit-bodies as before. In some seasons the leaves of various
plants may be seen covered with hundreds of these young mycelia,
which dry up when the atmosphere does, only to renew their growth
with the rains.

Before passing on to the consideration of the pathological influence
of these fungi, and of their systematic position, I will record a few
details concerning a form of Jfeliola which varies somewhat from the
typical cases hitherto considered ; at any rate, it seems to differ more
from the six or eight forms to which the above description refers, than
they do among themselves.

The species to be examined has only been found on the leaves of
Pavetta indica, and its mycelium forms more spreading and less de-
fined patches on the leaves of that plant than the easily recognisable
sooty patches of the other Meliolas. The main features of its myce-
lium, &c., are shown in fig. 41, and differ chiefly in the delicate
straggling hyphee, with a paler brown colour and no trace of haustoria.
The branching is very irregular, and somewhat like that of the form
figured at fig. 3, but the short, lateral branchlets are not always ovoid,
but often have sinuous, almost angular outlines, reminding one of the
similar structures in Asterina, except that the latter bear distinct
haustoria, The sete are here quite simple, short, and not so hard and
brittle as usual; they are also produced in smaller numbers than in
the more typical species.

The greatest peculiarities, however, are offered by the fruit-bodies,
or perithecia. Each of these arises as before by the successive dividing
up of a short, lateral branchlet (plate 44, fig. 42), with this difference,

16 H. MARSHALL WARD.

that the rapidly following septa permit no recognition of primitive
cells destined to form the outer walls, ascogonium, &c., as before.

After a few radial, vertical, and horizontal walls have been formed,
tangential septa (fig. 42, d.) make their appearance, cutting out series
of cells which are to form the outer wall, and which become firmer
and more deeply coloured, from an inner cell mass which gives rise to
the ascogonium much as before. Only a few asci are formed, in each
of which arise eight small oval uniseptate spores, which acquire a pale
brown colour as they ripen (fig. 42, f, and fig. 43).

The mature perithecium is shaped like a pear or top, the broad end
attached to the hypha by a short pedicei, the narrow free end, or apex,
becoming thin and diffluent in order to allow of the escape of the
spores (fig. 43). Very few or no sete are formed around the peri-
thecium, and these of the same simple type as those scattered on the
mycelium (fig. 41). The whole structure of the fruit-body is, there-
fore, much simpler than that of the above described forms, and, from
the semi-translucent characters of the thinner cell-walls, allows the
main details to be made out by optical sections only. In some of the
dark-coloured cells of freshly prepared specimens, a bluish tint is often
observable ; I have not seen this in any other similar form.

In no case have I succeeded in tracing a distinct alterative or de-
structive action of the Melolas on the cells of leaves to which they
are attached. In many instances, as, for example, thick leathery
leaves like those of Wemecylon capitellatum, &c., the haustoria seem to
have no function beyond that of holdfasts ; in others, such as Pavetta,
Triumfetta, &c., attacked leaves certainly suffer from the presence of
the fungus. Nevertheless, I cannot trace this to any direct action of
the mycelium ; the contents of the cells show no effects which can be
regarded as due to the fungus mycelium directly. We must conclude,
therefore, that where the life of the leaf is interfered with at all, it is
indirectly ; the dense crust of a well-developed Meliola no doubt ob-
structs the play of physiological functions in an obvious manner by
obscuring it from light, blocking up stomata, We.

It is now possible to consider the question of the systematic position
of these remarkable and interesting fungi. Bornet,’ following Fries
and Léveillé, places Meliola near the old group of Sphaerias with
especial reference to Hrysiphe.. I have already quoted the view of
Fries that the Meliolas may be considered tropical representatives of

1 Qp, cit., p. 266.

THE PERITHECIUM OF MELIOLA. 17

our Lrysiphee, and Berkeley? takes the same position, These opinions
appear to have been based simply on the resemblance in habit and
the more obvious anatomical characters, and on the fact that no Hry-
siphe is known in the tropics.

The details of structure, and especially of the development of the
fruit-bodies above described, enable us to criticise these views from a
somewhat firmer standpoint.

Apart from minor points of resemblance between MJeliola and the
typical Hrysiphee, such as the haustoria (not well developed in
Meliola), the asci, &c., there can be no question as to certain points of
agreement in the structure and development of the perithecia;
nevertheless, the origin of the fruit-body in the two groups is not
obviously similar, and at first sight the differences may seem greater
than they really are.

In the typical simpler Lrysiphee, such as Podosphaera, as is well
known from De Bary’s classical researches,” the carpogonium and anthe-
ridium arise each as a short lateral branch from separate hyphe, at
the point where two hyphe cross: each becomes cut off by a septum,
which is formed close to the parent hypha in the case of the pyriform
carpogonium, and about half way up the curved antheridium branch.
The free end of the latter becomes closely applied to the top of the
carpogonum, and fertilisation—possibly not complete in a physio-
logical sense, however—is said to be complete. After this process
numerous branchlets arise from the base of the antheridium filament
(and also from the base of the carpogonium), grow rapidly and with
numerous segments, and invest the carpogonium, which meanwhile
begins to be (more slowly) cut up into cells.

In Zurotium*® we have an essentially similar process, except in
minute details, and the antheridiwm is a branch springing from the
same hypha which bears the carpogonium, and arises just beneath the
latter. Here, as before, the perithecium envelope is formed chiefly by
the rapid overgrowth of cells derived from the antheridium branch.
It is quite conceivable that a form allied to Hrysiphe and Zurotium, &c.,
might have the unicellular carpogoniwm and antheridium arising quite
in contact at their bases from the same branch.

If we now compare the above with the succession of events in the

2 Introd. to ‘Crypt. Bot.,’ p. 275.

2 ‘Beitr. z. Morph. u. Phys. d. Pilze,’ R. iii., 1870
3 Cf., De Bary, loc. cit.

18 H, MARSHALL WARD.

development of Meliola, the following points of analogy seem to me
sound. ‘The original pyriform branchlet—containing in itself, so to
speak, the elements of the fruit-body—after the first division (fig. yy
may be considered as establishing morphologically an archecarpium !
and an antheridial branch—or the latter may be considered as contain-
ing in itself the antheridium, plus the elements of the perithecium wall.

If the cells A and B (fig. 9) became further developed, and diverged
at their apices, we should have no difficulty in seeing these points of
homology.

Thus much cannot but be allowed. The cell A resembles a true
archecarpium in so far that it slowly produces the ascogonium and
asci ; the homology will not be weakened, but the contrary, if further
research shows that part of the perithecium wall results from cells
derived from A. The cell B so far acts as an anthertdium branch in
that it is closely applied to A, divides up more rapidly, and thus pro-
duces most—perhaps all—of the perithecium wall.

The above may possibly suggest some difficulties to those who have
not followed the recent progress in our knowledge of sexual organs
and their homologies in the lower fungi. It has of late been shown
to be not improbable, but on the contrary very likely, that we should
view the Hrysiphee as a group connecting the higher Ascomycetes, on
the one hand, and the Phycomycetes* (Mucor, Peronosporec, and Sapro-
legnice) on the other: the evolution of the latter group seems un-
doubtedly attended by a fusion of parts before separated—a withdrawal
of the sexual organs, so to speak, into one another,—and De Bary has
followed this out with marvellous skill and success in a number of
forms passing from Pythiwm, through the Peronosporee, to certain
Saprolegnace, in which the male sexual organ (antheridium, pollinodium)
is normally suppressed. Whether or not we suppose, with De Bary,
that the Lrysiphee took origin from some Peronospora-like form, it
seems reasonable to look upon Meliola and its immediate allies as a
branch group derived from the Zrysiphe stem, either from the ancestor
of Hrysiphe itself or from ancestors which gave rise to Hurotewm and
Lrysiphe, and that this group has become developed in tropical lands
along lines more or less parallel to those along which the Kuropean
forms have proceeded in temperate climates, being, in fact—though
not in the strictest sense perhaps—“ representative species.” Be this

1 De Bary, Beitrige IV., proposes to use this word as denoting that part of the body
which becomes the ascus and pedicel in Podosphaera.
2 Vide De Bary, ‘‘ Beitr. z. Morph. u. Phys. d. Pilze,” R, IV., 1881.

THE PERITHECIUM OF MELIOLA. 19

view entertained or rejected, I am strongly impressed with the neces-
sity for further and closer investigation of the very remarkable group

of fungi centering around or near the Melole, since they will probably
fill up yet more completely the gap—partially bridged over, it is
true—between the lower and higher Ascomycetes.

Fig.

Vig.

bo

DESCRIPTION OF PLATES I & II.

Meliola sp. with portion of epidermis of Memecylon. On the
mycelium are setw, branchlets, and fruit-bodies in various
stages of development.—Zeiss D.

Mycelium of another species of the same, found on the Teavee

of Schutereia (Conv.), with portion more highly magnified.
—Gundl. 4 and Zeiss D.

Portion of me ees of a species of Meliola on Triumfetta
(Tiliacew).—Zeiss D.

Portions of more advanced mycelium of fig. 2 more highly
magnified, and showing various forms of lateral branch-
lets.— Zeiss J.

Portions of mycelium on JMemecylon showing mode of
branching and young fruit-bodies—Gundl. 4 and Zeiss D.

Vertical section through portion of mycelium where fruit-
body is being formed. The section is not median,—
Zeiss J.

End of hypha with three cap-like thickenings and pore-like
spot (haustorium ?) seen from below.—Zeiss J.

Various forms of sete in plan and elevation.—Zeiss D, and J.

End of hypha (with one cap-like thickening) bearing lateral
pyriform branchlet which is to become a Perithecium.
The first oblique septum has already appeared, the
smaller cell (A) represents the ascogonium, &e., and is
shaded darker; the larger one (B) will divide up more
rapidly, and enclose the cell A and its progeny.

Figs, 10 and 11. Further stages in the development of the young

Perithecium. The cell B is becoming divided.—Zeiss J.

20

Fig. 12.

Fig.

ig, 18.
igen:

29.

H. MARSHALL WARD.

Young Perithecium seen from below (a), from the side (6),
and from one end (c). In all, the dark cell is the one
marked (A) in fig. 9; the remainder have resulted from
the growth and division of the cell (B).—Zeiss J.

and 14. Slightly later stages:)seen from below. ‘The cell
(A) has become divided by a cross septum.—Zeiss J.

and 16. Similar preparations seen from above and below.—
Zeiss J.

. Somewhat more advanced Perithecium seen from the side.

The cells resulting from the division of A (“ascogenous
core”) are seen through those formed by B, which are
growing over them.—Zeiss J.

Somewhat more advanced stage.—Zeiss J.

Slightly later stage. The upper figure is seen from above,
the lower from below: the latter shows the “ ascogenous
core.”—Zeiss J.

. Similar preparations seen from above (lower figure) and

below (upper figure).—Zeiss J.
and 22. Slightly advanced Perithecia cut by the razor. The
‘‘ ascogenous core” is exposed at the cut parts.—Zeiss E.
and 24. Similar preparations treated with chromic acid.
The ‘“ascogenous core” is seen enveloped by the cells
forming the Perithecium-wall: all much swollen, and
fig. 23 slightly crushed.—Zeiss J.

. More advanced Perithecium seen from outside and above

(x), below (y), and from the side (z). The radiating

hyphee (réceptacle) spring from the external walls below.
—Zeiss E.

ig, 26. Portion of mycelium with young Perithecium seen from above

and below.—Zeiss E.

. Somewhat older Perithecium. The razor has cut off one

side obliquely.—Zeiss J. (camera).

. Vertical section through young Perithecium about this stage.

The ascogenous cells in the middle are distinguished by
their larger size and arrangement.—Zeigs J.

Oblique (nearly horizontal and median) section through the
same.—Zeiss J.

THE PERITHECIUM OF MELIOLA. 21

Fig. 30. Horizontal section above the base of same.—Zeiss J.

Fig. 31. Somewhat older stage in vertical section. The ascogenous
cells in the centre are enlarging at the expense of those
around.—Zeiss J.

Fig. 32. Portion of outer wall with disorganised cells lining it—
Zeiss J.

Fig. 33. Vertical section through nearly ripe Perithecium, showing
asci and spores embedded in the gelatinous mass produced
by the disorganisation of the unemployed cells.—Zeiss D.

Fig. 34. Portion of outer wall of latter in vertical section.—Zeiss J.

Fig. 35. Vertical—not median—section through ripe Perithecium
(and portion of epidermis of host-plant), showing crowds of
spores.

Fig. 36. Thin slice from top of similar Perithecium. A pore-like
spot is seen in the centre of the radial marking.

Figs. 37, 38 and 39. Various stages in the development of the asci
and spores.—Zeiss J.

Fig. 40. Germinating spores.—Zeiss D. and E.

Fig. 41. Portion of mycelium of a species of Meliola found on Pavetta,
showing mycelium, sete, and young Perithecia.—Zeiss J.
and D

Figs. 42 and 43, Development of Perithecia and extrusion of spores.

seis
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Plate ath

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F. Huth, Lith? Edin?

ON THE SEXUALITY OF THE FUNGI.

By H. Marsnatt Warp, M.A., Fellow of Christ’s College, Cambridge ;
Assistant Lecturer in Botany at the Owens College, Manchester.

The fruitfulness and stimulus of the theory of descent have pro-
bably been felt in no province of biology with more effect than in the
investigation of the more minute forms of plants; and the results
obtained from the study of microscopic fungi have absorbed attention
and interest of late years to an extent which, whether commensurate
or not with their importance, promises even more in the future than
has been attained in the past.

Not only with respect to the economic aspect of a thorough know-
ledge of Fungi inimical to the animal and vegetable world, but also
as regards the real position of these remarkable organisms in nature,
it is of the greatest importance that investigations should proceed and
multiply. For we have learned in this as in other departments of
science, that the results of thorough and accurate knowledge cannot
really be foreseen, and that new side lights are thrown on other mat-
ters by every acquisition of facts and principles.

Apart from their interests more directly affecting mankind, the
fungi have seemed to present problems of life in some respects simpler
than other forms, and have thus in a manner promised a solution of
phylogenetic and physiological questions more nearly approaching the
ideal of the evolutionist. As research progressed, however, and the
methods of observation were improved, experience showed that the
study of the Fungi—though yielding results much beyond rather

24 H, MARSHALL WARD.

than below what was expected—is attended with unlooked for diffi-
culties. Not only is the isolation and cultivation of any given fungus
an extremely difficult matter, but the following it through all the
phases of its life-history brings the observer face to face with problems
of quite a special nature.

As time progressed and observations multiplied, it became clear
that the fungi are by no means so simple as they perhaps appeared.
Apart from practical difficulties of manipulation, consequent on their
minuteness, number and intermixture with other forms, it soon became
evident that special conditions of various kinds affect their develop-
ment, and that the complete life-cycle of any one fungus—and evi-
dence based on a thorough knowledge of this is alone admissible for
the purposes of science—may present various forms of complexity.

Even to-day, notwithstanding the considerable additions to our
knowledge derived from the study of development, and notwith-
standing that we possess several comprehensive generalisations as to
the curious changes undergone by typical forms in their development,
we are far from possessing sufficient knowledge of these matters to
enable us to group the fungi satisfactorily from a phylogenetic point
of view. This, however, is a distinct aim of biology, and every addi-
tion to knowledge in this direction is to be welcomed.

In the present essay it is proposed to describe some of the more
recent and most suggestive observations on fungi; and especially on
their reproductive organs, since it is in these that the most important
phenomena (from the phylogenetic point of view) are centred. We
shall ‘have occasion to refer to, and in part to trace certain processes
connected with their development ; and finally to see how far it may
be possible to generalise from the facts now known.

In s0 far as this paper simply recounts observations—for the most
part made by others—it cannot claim scientific merit; but if, after
condensing and arranging the facts, and stating the condition of our
present knowledge of the subject, the attempt to bring this knowledge
under a more general statement succeeds, it may be that we have
helped to advance matters after all.

If, however, further criticism results in the overthrow of the hypo-
thesis brought forward at the conclusion, we may nevertheless hope
that some service is rendered in arranging the facts, and drawing
attention to the necessity of employing physiological as well as mor-
phological considerations in the attempt to construct a phylogenetic

THE SEXUALITY OF THE FUNGI. 95

system of the fungi. In the last case we may at least succeed in
attracting more attention to the direction in which modern research
in this region is impelling the more thoughtful biologists, and go call
forth confirmatory evidence or criticism of unseen fallacies,

If we neglect a few isolated observations as having led to no general
views on the subject, we may regard Pringsheim’s discovery of the
sexual organs in the Saprolegnie in 1858* as the starting point of our
knowledge of the sexuality of the fungi. This observation was made
at a time when attention was being drawn particularly to the sexes of
the Algee by the researches of Thuret, De Bary, Cohn, Nigeli and
others; and the Saprolegnice were then, and fora long time afterwards,
regarded as Algze.

Since that time numerous cases of the occurrence of sexual organs
have been described among other fungi, chiefly by the labours of De
Bary and the school of cryptogamic morphology practically established
by him and his pupils.

With De Bary’s brilliant researches on the Ascomycetes,? and
especially the Lrysiphee,® a point was reached where a definite opinion
on the sexuality of the fungi became accepted; and the conclusions
drawn from the study of Spherotheca, Eurotium, and Peziza led to the
view that the fruit-body of a higher fungus results from a process of
fertilisation preceding the development of asci, and that Hofmeister’s
supposition that the asci are sexual organs was to be abandoned.

In 1871 Janczewski* described the sexual organs in Ascobolus fur-
furaceus. Other researches by Baranetski, Gilkinet, Woronin, Van
Tieghem, and Brefeld were considered to support the then generally
received opinion that the fungi, while differing considerably as to
their forms and mode of producing spores and fructification, probably
all develope their chief reproductive bodies as the consequence of a
sexual process.

When in 1874 Stahl demonstrated the sexuality of a Lichen,* the
matter seemed to be placed beyond doubt ; and it was freely admitted
that in the cases—now somewhat numerous—where a definite union
of sexual organs could not be established, that the failure was largely
due to the extraordinary difficulties attending the investigation. In

1 Sachs, ‘Geschichte der Botanik.’
2 ‘Ueber die Fruchtentwickelung der Ascomyceten,’ 1863,
8 ‘Beitraige zur Morph., &c., der Pilze,’ R. iii., 1870.

+ © Bot. Zeitg.’ 1871.
5 ‘Bot. Zeitg.,’ 1874,

26 H. MARSHALL WARD.

this manner, apparently, it came to be widely believed that in such
cases as Sordaria,’ Penicillium, Sphaeria lemanece,? and. Chaetomium
the sexual process is essentially the same as that described for the
simpler Lrysiphee.

All the observers agreed in the main that the asci are either parts
of the Ascogonium, or female sexual organ (Spherotheca, Podosphera),
or are developed by budding from it, in each case no doubt after the
Ascogonium had received something from the male organ (Pollinodium)
attached on its surface. Meanwhile, investigation was not confined to
the Ascomycetes.

The researches of Brefeld,* Van Tieghem,° and others’ demonstrated
a simple form of sexual reproduction in the Mucorini, now so well
known that we need not dwell upon it. It is interesting to note in
passing, however, that Ehrenberg had discovered the conjugation of
Syzigites so long ago as 1820."

We now pass to the discovery of the true nature and sexual organs
of the Peronosporece by De Bary, who, in a series of masterly memoirs,®
has made this group and its allies a special study. De Bary showed
that in certain members of this group an antheridium bores into the
oogonium, sending a “fertilising tube” into the oosphere contained
therein ; the oosphere then becomes an oospore, and capable of germi-
nation.

A considerable amount of labour had been devoted to the study of
the Saprolegnie since Pringsheim’s first publication, much of it by this
investigator himself? and among other remarkable discoveries his
observation that, in certain cases, the oospores become normally
developed and capable of germination without any male organs being
formed at all, is to be noted. Pringsheim himself termed these oos-
pores parthenogenetic.” We may pass over the controversy between

1 Gilkinet, ‘‘ Recherches Morphologiques,” ‘Bull. Acad. r. de Belg.,’ ser. 2, 1874. Woronin,
‘Beitr. z. Morph., &c.,’ R. iii.

2 Brefeld, ‘ Schimmelpilze,”’ ii.

3 Woronin, ‘ Beitr. zur Morph., &c.,’ R. iii.

* *Schimmelpilze,’ H. i.

5 ¢ Ann. des Se. Nat.,’ ser. 6., t. i.

® De Bary, ‘ Beitr. z. Morph.,’ i.

7 This ig the date in Sachs’ ‘ Geschichte der Botanik,’ p. 473. De Bary, ‘Beitr.,’i., p. 74,
gives the date 1829.

8 ‘Ann, des Sc. Nat.,’ ser. 4, vol. xx, and later, ‘ Beitrige zur Morph., R. ii; ditto, R. iv.
© Bot. Zeit.,’ 1881.

9 ‘Jahrb. fiir wiss. Bot.’

10 © Jahrb, fiir wiss. Bot.’ ix.

s

THE SEXUALITY OF THE FUNGI. aN

Cornu! and Pringsheim as to certain details in the manner of fecun-
dation of the oospheres of Saprolegnie. It,is sufficient to note that
in 1880, or thereabouts, the matter appeared to stand thus: While
the typical Saprolegnice possess oospheres in an oogonium, and anthe-
ridia as simple or branched structures which send “ fertilising tubes ”
through the walls of the oogonia as far as the oospheres, which they
appeared to fertilise ; there are others in which the oospheres develope
into fertile oospores without contact of the antheridia.

If we now turn aside from the fungi referred to in the preceding
sketch, we find a vast number of forms comprehended under the
Ustilaginew, Uredinece (dicidiomycetes), and the larger Basidiomycetes.
The parasitic Ustilaginee have received much attention since Tulasne?
and De Bary® brought them together and led the way to a more
scientific knowledge of their nature. Much has been done since, and
much opinion has been expressed as to the signification of the cross
unions made by the “sporidia” developed from the promycelium of
the germinating spores* in some cases. We must regard the view as
to its supposed sexual character with grave suspicion.

The Uredinee, apart from their interest as parasites on economic
and other plants, have absorbed much attention from the point of view
we are concerned with. It was natural to look for sexual organs in
them, especially after the successes met with elsewhere, Neverthe-
less, from Tulasne’s® and De Bary’s ° earlier investigations, more than
thirty years ago, down to the present time, no one has succeeded in
demonstrating even a trace of any intelligible sexual process or organs.
This is the more remarkable since many of the Meidiomycetes produce
no less than four forms of reproductive bodies. Moreover, the group
has been studied with extraordinary success, and our knowledge of
the nature of parasitism and heteroecism is largely if not chiefly due
to this success. The best views as to the reproduction of these fungi
held up to 1880 may be fairly stated thus. They form at most two
kinds of asexual spores (Uredospores and Teleutospores) and Avcidia and
Spermogonia ; the latter were regarded as probably the bodies con-
cerned in sexual reproduction, the Spermatia emitted by the Spermo-

1 * Ann. des Sc. Nat.,’ ser. 5, t. xv, &.

2 *Ann. des Sc. Nat.,’ ser. 3, t. vii, and ser. 4, t. ii.
5 “Die Brandpilze,’ 1853.

* Tulasne, op. cit.

5 Op. cit.

® Op. cit,

28 H. MARSHALL WARD.

gonia being the male organs, and the “cidium fruit” probably
resulting from a fertilised body equivalent to the ascogonium of the
Ascomycetes. This view was strengthened and supported by Stahl’s
discovery of the sexual process in Lichens; but no organs like the
ascogonium or trichogyne have yet been discovered in spite of much
labour. Finally, we may dismiss the larger Basidiomycetes by refer-
ring to Brefeld’s magnificent research * on certain types, and particu-
larly on Coprinus.

Brefeld placed beyond all reasonable doubt that the stalked pileus
arises from the mycelium, and completes its development without the
intervention of any sexual process, or the appearance of any sexual
organs; and since no one has succeeded in rendering it probable that
sexual organs occur later, we may probably accept Brefeld’s view that
no sexes exist in the Agarics as we know them, but that they are
large aggregations of hyphe producing asexual spores. Whether we
really know the whole life history of any of these forms is a question
which cannot be raised with much advantage just now.

It thus appears that while the discoveries of Pringsheim, Tulasne,
and De Bary led, on the one hand, to numerous other observations of
sexual organs in the fungi, and seemed to show that a sexual process
is nearly universal with them as with other groups of living beings
equally complex in organisation ; on the other hand, there were nume-
rous cases where room for serious doubts existed—doubts not dispelled
by the recognition of the difficulty of the research. As time passed,
moreover, the suspicion that certain groups of fungi are really devoid
of sexual organs (although analogy would lead us to expect them)
increased, and in some cases reached conviction. Of course, we are
not referring to the very obscure lower groups—the Schizomycetes,
Saccharomycetes, and Myxomycetes, &e.—which we shall leave out of
account altogether in this survey.

It is not to be forgotten that much more was known about the
physiology of the fungi by this time, and that the recognition of sapro-
phytic and parasitic forms implied considerable advance in our know-
ledge of their modes of life, changes of habit, and so forth. The
progress made in the study of fermentation, moreover, had its effect
on the study of mycology generally ; and the progress of biology as a
whole—so particularly active during this period—had, in 1880, left
its mark on this specialised branch of research.

2 *Schimmelpilze,’ H. iii,

THE SEXUALITY OF THE FUNGI. 29

It is not necessary to enter into all the systems of classification
proposed for the fungi during this period. The well-known grouping
of Sachs and Cohn, presented to English readers in the “ Text-book”
of the former, was admitted to be in great measure artificial, and those
proposed by Van Tieghem* and by Winter* appear to answer their
purpose only temporarily, and certainly need not occupy us here. The
same is true for other classifications up to 1880.

It was just prior to this period that the very important memoir by
Brefeld® was published, in which he detailed the results of his investi-
gations into the nature of the Basidiomycetes.

By cultivation in nutritive media, Brefeld succeeded in tracing the
whole cycle of Coprinus from the basidio-spore to the formation of a
mycelium and fructification. He shows that the latter arises by a
purely vegetative process from the dense mass of interwoven hyphee
(Sclerotium) budded off from the mycelium, and that no trace of a
sexual process or of the formation of sexual organs can be detected
either previously to the development of the Sclerotiwm or afterwards.
The pileus with its hymenium are produced simply by a budding off
of numerous hyphe growing up together, either directly from the
mycelium, or with the intervention of the Sclerotium. Brefeld regards
it as certain that these fungi are entirely without sexual organs.

It is impossible to go into the details of this voluminous memoir ;
but it is important to notice the results embodied in a scheme of a
proposed classification of the fungi which Brefeld tabulates at the end
of his valuable paper, since we have here a comprehensive view of the
direction in which modern speculations in mycology were tending.

In the accompanying diagram I have slightly condensed Brefeld’s
scheme, since the original contains details of little importance for our
present purpose.

i ©Ann. des Sc. Nat.,’ ser. 6, t. iv, 1878. :
2 ‘Hedwigia,’ 1879—see also Rabenhorst’s ‘ Kryptogamen Flora.’
8 *Schimmelpilze,’ Heft iii, 1877.

MARSHALL WARD.

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THE SEXUALITY OF THE FUNGI. 31

There is little need to dwell on this scheme, since its chief interest
for us is in its being an intelligible attempt to classify the fungi from
the point of view of the theory of descent. A point of some impor-
tance, however, may be referred to, as we shall have occasion to speak
of it later. Brefeld indicates the possibility that the Oosporew (typi-
fied by Peronosporee) may be allied to the “true fungi” otherwise
than by a common descent from some Alga-like ancestor. He also
recognises a common origin for the Oosporew and the Zygomycetes. In
other respects the system is chiefly remarkable for the peculiar views
taken of the descent of the two great groups, the typical Basidiomycetes
(Gasteromycetes and Hymenomycetes, &c.) and the Ascomycetes, which
he regards as having long ago diverged from a common point, at a
time when the ancestral forms commenced to specialise their repro-
ductive organs, While on the one hand asci arose as specialised forms
of sporangia—complications resulting from the development of peri-
thecia, &c., being considered unimportant—on the other hand, the
sporangia became degraded to conidia, and the Basediomycetes came to
be merely highly developed tufts of conidiophores.

In a later memoir! Brefeld insists on regarding the so-called Polli-
nodia of the Ascomycetes as simply tubes for enveloping the ascogenous
cell or filament ; and it is interesting to note that he quotes Melano-
spora as acase where the non-sexual relation of the ascogenous cell
and the filaments which envelop it may be clearly observed. Brefeld
also points out that in the Ascomycetes we can trace gradual degra-
dations of the various forms of fructification, with a disappearance of
sexuality at the same time. He supposes that all the fungi arose
from an ancestral form containing chlorophyll and possessing sporangia,
and that the variations met with are derived by modifications of this
sporangium, as already indicated.

It seems unnecessary to criticise these views in detail, since it is
obvious that no decision can be arrived at apart from the considera-
tion of numerous facts. It will be noticed, however, that Brefeld’s
hypothesis assumes that, in addition to purely vegetative modes of
multiplication (e. g. the breaking up of filaments, c&c.), certain fungi
must have acquired other forms of reproduction than those inherited
and specialised—some Wcidiomycetes, for instance, with their four
kinds of spores or spore-like bodies (secidospores, spermatia, uredos-
pores, and teleutospores) must have acquired at least one of these spores.

1 6Schimmelpilze,’ iv, 1881,

BD) H. MARSHALL WARD.

At this point we may leave this short survey of Brefeld’s important
work, and turn to the consideration of a memoir! published by De
Bary about the same time as the last one quoted. In this—probably
the most important contribution to mycology yet made—the author
describes his observations on the Peronosporee and Saprolegnie ; and
bases upon these and other previous observations a classification of
the fungi which is in large measure new, and certainly promises to be
more fruitful than any yet proposed.

De Bary finds that, passing from the typical Peronosporee (Pythium,
&c.) to the Hrysiphee on the one hand, and to the Saprolegnie on
the other, the sexual process is gradually eliminated, and the sexual
organs become at first functionless and then disappear altogether. In
Pythium itself, the antheridium pierces the oogonium wall and ferti-
lises the oosphere by pouring protoplasm into it.2. In Phytophthora
and Peronospora the process is essentially similar, but the quantity
of protoplasm passed over from the antheridium is smaller.

In the Saprolegnie—which differ from Pythiwm and other Pero-
nosporee in forming several oospheres in each oogonium—the fertilis-
ing tubes do not open, and no protoplasm can be observed to pass
over from the antheridium to the oospheres.* Or, in some forms, no
antheridia are present at all—a fact already recognised by Pringsheim
—and the parthenogenetic spores are nevertheless capable of germi-
nating.

Now if the typical and thoroughly investigated case of Podosphera*
be compared with the Peronosporece, it is, as De Bary states, evident
that the antheridia correspond in both cases; while the “archecarp-
ium” (z. e the cell which produces the ascus, and to which the
autheridium applies itself) of Podosphwra is homologous with the
oogonium of the Peronosporee. It is a remarkable fact that, as De
Bary noticed long ago,’ the antheridium of Podosphera only applies
itself closely to the archecarpium, and does not pierce it ; it appears
highly probable, moreover, that no passage of substance from one to
the other takes place—that the ascus, in fact, arises without a sexual
process, though the sexual organs are present.

1 ‘Beitriige zur Morph. and Phys. der Pilze, &c.’ R. iv, 1881.
2 Cf. Quart. Journ. Mic. Se.,’ October, 1883.

5 Cf. also Quart. Journ. Mic. Sc.,’ July, 1883,

* De Bary, ‘Beitr. zur Morph. und Phys.,’ R. ili.

5 De Bary, op. cit.

vi - a ee

THE SEXUALITY OF THE FUNGI. 33

Now a whole series of forms are known leading us up from Podo-
sphera and the other Hrisyphece through the Pyrenomycetes and Disco-
mycetes, and it is remarkable that (apart from some peculiar forms to
be referred to shortly) the best investigations lead us to conclude that
while the sexual organs are present, but functionless, in the lowest
forms, they disappear entirely in the higher Ascomycetes.

These facts may be put shortly in the form of a diagram as annexed,
where attention is only paid to the points referred to.

Other Ascomycetes

Erysiphez
Podosphera
Other Peronospores
Phytophthora

Pythium.

If we now turn to the Saprolegnic, we may note that De Bary finds
that, between the typical cases where the antheridia pierce the oogo-
mium wall, but do not empty any protoplasm into the oospheres (Achlya
and some Saprolegnias) and the extreme parthogenetic forms of Sapro-
legnia where no antheridia are formed at all, there exist cases where
the antheridia apply themselves to the outside of the oogonia, but
either form no antheridial tubes at all or only rudimentary ones (3,
torulosa, S. asterophora). These facts may also be diagrammatically

represented as follows.

Parthenogenetic
forms of Saprolegnia

8. torulosa, S. Asterophora

Achlya, Typical
Saprolegnia

Peronosporec.

If now the various cases are duly considered, De Bary thinks that
we may probably regard the Peronosporece as phylogenetically impor-
tant in two senses :

1. Their general biology strongly suggests that they are derived
from Algal ancestors, possibly not very unlike Vaucheria and its allies. |

D

34 H. MARSHALL WARD.

2. That they are the progenitors (or the near relations of progeni-
tors) of a few chief series of true fungi—on the one hand the main
series of Ascomycetes and allies; on the other the Saprolegme and
forms derived from them, and allied to them.

If we now regard these forms more closely, it is not difficult to
agree with many of De Bary’s conclusions. It will be clear, in fact,
that some of them are not new, though they are stated in a much
clearer form than by Brefeld and others who have helped to systema-
tise the chief groups already. We will first shortly consider the main
subdivisions themselves.

The Zygomycetes are regarded as branching off from the Perono-
sporee. In this group De Bary arranges the Mucorini and the Ento-
mophthorece, basing the conclusion that Lntomophthora is a Zygomycete
chiefly on Nowakowski’s description of the zygospores.t It should be
remembered that Brefeld considers the resting spores of this genus as
arising asexually ; but that he, too, indicates the possible alliance of
the Entomophthorece with the Oosporec, and therefore, indirectly, with
Zygomycetes.

Pythium seems closely allied to the Ancylistee of Pfitzer,? which
lead us on to the Chytridece, in which we meet with forms which con-
jugate (and are therefore sexually simpler) as well as purely apoga-
mous genera. It is not improbable that among these latter the
asexually produced resting spores are really oogonia, a view already
held by Brefeld in other cases.

De Bary raises the point of the possible alliance of these simple
parasitic Chytridiacee with the lower Algze (e. g. Protococcacee), and
decides that it is, on the whole, more probable that they have been
derived from the higher fungi, as indicated, by degeneration.

The Ostilaginee are next dealt with. The author expresses himself
cautiously, but points out that this important and very natural group
may be looked upon as a series, beginning with the simpler Zntyloma,
Tilletia, &c., and rising to more complex forms, such as Sorisporiwm
and Urocystis, on the one hand, and Ustilago on the other.

There are many difficult points to consider in classifying the Uséz-
laginee. Their asexually produced resting-spores generally form a
“ promycelium” on germination, from which “sporidia” arise ; and,
as is known, these “sporidia” commonly become united by cross-

1 © Bot, Zeitung,’ 1877.
2 © Monatsber. der Berling Akad.,’ May, 1872)

THE SEXUALITY OF THE FUNGI. 85

unions. But whether this is to be regarded as a “ copulation” or
not, the sporidia often germinate without it. It should be noted that
Woronin? points out this significant fact also, and it is all but certain
that the so-called “copulation” is not of a sexual nature at all. If
the lower Ustilaginew are allied to the Chytridece by means of Enty-
loma and Protomyces, we seem to have a satisfactory position for the
former group. Of course, if this view be accepted, the resting spores
of the Ustilaginew are the homologues of oogonia, which become
developed apogamously. The preceding facts may be summarised in
the following diagram :
Ustilaginese

Ascomycetes, «ce. Protomyces, &c.

Olpidiopsis, &e.

N.B.—If these forms produce
i zygospores, they may be allied
Erysiphew Polyphagus, &c. to the Zygomycetes also; this

could not be shown on one plane.

Ancylistez
Peronosporese

Zygomycetes Saprolegniz.

Continuing our survey of De Bary’s memoir, we may pass over the
opinion respecting the Saccharomyces, and proceed to the part dealing
with a much more difficult and important series of forms. As the
author showed in 1879,? the Zremellint may well be looked upon as
derived from Uredinew and allies; while those Uredinew which form
eecidia resemble the Ascomycetes in so many points of structure and
development, that we may regard them as closely allied. The for-
mation of three forms of conidia (Uredospores, Teleutospores, and
Sporidia) may be in part due to specialisation ; but it must be remem-
bered that the Ascomycetes are also in the habit of forming many and
various conidia. It is, however, in the many points of resemblance
between the eecidia and perithecia, and the spermagonia and spermatia
of both groups, that the alliance appears most clearly. True, no
observer has found a trace of sexual organs in the young ecidia ; but
the same is certainly true for the perithecia of many Ascomycetes. In
structure, apart from peculiarities in the development of the spores in

1 *Beitr zur Morph.’ R. v., 1882
2 * Bot. Zeit,, 1879, p 825 &e.

36 H. MARSHALL WARD.

the two cases—ecidia and perithecia present many points of agree-
ment, while the ‘spermogonia and spermatia of both groups are quite
alike. It cannot be overlooked, however, that the peculiar develop-
ment of the ecidiospores affects the question of this alliance. De
Bary does not even allude to the similarity between these and the
“eonidia” of Cystopus ; and it is of course obvious that the asexually-
produced spores of the latter are rather to be regarded as homologous
with one of the conidial forms of the Wecdiomycetes ; for if the eecidium
fruit is homologous with the perithecium of Podosphera, its further
homologies are with the oogonium, &e., of Cystopus. It has been
suggested, however, that a solution of these difficulties should be
sought in the direction hinted at here.

De Bary thus considers the Zeidiomycetes as a group allied to the
Ascomycetes genetically, though we have not sufficient knowledge as
yet to enable us to place them at any particular spot in the scheme
of that series. The Zremellint are Basidiomycetes, with basidiospores
so suggestive of the teleutospores of Uredinece that De Bary does not
hesitate to place them as derived—with considerable reduction and
simplification—from those of Uredinece which possess no ecidia (e. g.
Chrysomyxa). This is regarded as no more extraordinary than the
peculiar simplification of phanerogamous water-plants, &e., or of
Saccharomycetes, if they are reduced Ascomycetes. The Tremellini
would then lead us to the Hymenomycetes and G'astromycetes, though it
is by no means clear how this came about. We are here plunged into
the greatest difficulties, because the development and life-history of
these groups are so little known ; and we may thus leave the discussion
of their phylogeny for the present. It must suffice to add that De
Bary believes it possible that the Tremellint having arisen by degene-
ration of uredinous forms, the other Basidiomycetes developed anew
progressively as forms adapted to special modes of life.

The annexed scheme sums up the whole of the preceding.

THE SEXUALITY OF THE FUNGI. Bi /

— Tremellini — Ge

Saccharomycetes— Gasteromycetes.

Bourpoad

— Chytridee — Protomycetes — Ustilaginee.
Zygomycetes —

— Saprolegnie.

esty — xezodsouo1sg — svoydishig — sojyooftmoosy

The details may be filled in according to what has been said.

On comparing this diagram with that offered by Brefeld, it will be
noticed that there is some agreement in general between them ; it is
chiefly in points of detail that the differences appear. Both the
authorities agree as to the serial arrangements (on the whole) of the
main groups ; but Brefeld, while also placing the Zremellini as derived

"from Uredinez, seeks another origin for the other Basidiomycetes.
When we notice De Bary’s cautionin not deriving the Uredinew from
any particular point in the huge ascomycetous series, we may allow
that he and Brefeld do not differ much in opinion as to their origin
—the latter simply places their origin more definitely lower down in
the main series, a fact which would possibly be of significance if we
were inclined, after all, to regard the similarities between Meczdvo-
mycetes and Cystopus.

The chief motive in Brefeld’s scheme is afforded by his peculiar
views on Pycinidia, and on the relationships of the Hntomophthorew.
He regards the point where the main series of higher fungi developed
pycinidia, as the point whence the Basidiomycetes (excepting Tremel-
lint and allies) and Ascomycetes diverge; the former then became
specialised as conidium-bearing fungi, the latter as modified sporan-
gium-bearing forms—z. e. as ascus-bearing fungi.

The great motive of De Bary’s views, as already shown, depends

38 H. MARSHALL WARD.

upon the value of the new knowledge obtained of the Peronssporee,
and the meaning of the oogonia (or ascogonia) and antheridial fila-
ments respectively.

Tf, in Brefeld’s scheme, we tack the remaining Basidiomycetes on to
the Zremellini, &c., and bring his ‘ Oosporew” to the base of the main
trunk of the phylogenetic tree, slight though not unimportant altera-
tions in detail are necessary to make the two diagrams agree.

If we now turn our attention to the investigations of the last two
or three years, it is suggestive that the best results have been won
among the Ascomycetes. The influence of De Bary’s memoir shows itself
repeatedly in these, and they must be regarded as tending on the
whole to strengthen his conclusions. No object is to be served by
taking the various memoirs in strict chronological order, and I may
therefore commence by reviewing an important contribution by Kihl-
mann.? It may first be stated that Pyronema confluens, a Peziza, is
one of the Ascomycetes which has been particularly well studied ; and
is classical in that De Bary’ first described the sexual organs in it in
1863, and that Tulasne’s celebrated figure * of these organs has been
so much copied. In 1866 De Bary, having devoted much attention
to the development of the fructification of this fungus, wrote as fol-
lows concerning the pairs of peculiar organs assumed to be sexual :
“Ob und wie sie einer Befruchtung dienen ist eine durchaus unent-
schiedene Frage.” *

These sexual organs consist of pairs of swollen branchlets arranged
in groups. Each pair consists of a macrocyst and a so-called paracyst.
The macrocyst is an ovoid vesicle, filled with protoplasm, &c., and
provided with a hook-like tubular prolongation. The paracyst is a
club-shaped branchlet close to the macrocyst ; the apex of the paracyst
and the hook-like prolongation become united. After this—the “ pro-
cess of fertilisation ”—branches spring from below, envelope the sexual
organs, and form the perithecium, in which the asci arise.

Tulasne® described an open communication through the hook-like
process, placing the protoplasm of the macrocyst and paracyst in
connection. De Bary, much later,’ speaks with great caution, and

1 * Acta Soc. Scient. Fenn.,’ t. xiii,

2 © Ueber the Fruchtentw. der Ascomyceten.’

3 *Ann. des Sc. Nat., 8.5, t. vi; cf. Sachs’ ‘Textbook, &c.’
* ©Morph. und Phys. der Pilze, &c.,’ p. 164.

5 Op. cit.

® * Beitr. zur Morph., &c.,’ R. iv.

THE SEXUALITY OF THE FUNGI, 39

considers it undecided that a true sexual process occurs. Fisch?
doubts whether these organs have anything to do with the formation
of the asci. Sachs” and Goebel® describe the organs and the sexual
process in terms which Kihlmann thinks too confident and premature
at the time.

I now pass to the observations made by Kihlmann himself. The
macrocysts and paracysts arise together in pairs as terminal swellings
of the branches. The hook-like prolongation of the macrocyst becomes
fused to the apex of the paracyst, much as described by previous
writers ; but before the apex of the hook-like process meets the paracyst,
a solid septum cuts off its communication with the macrocyst at the
point where it leaves the latter.

Hence a mingling of the protoplasm in the two “sexual” organs is
impossible, and the open communication described by Tulasne never
occurs. From certain changes in the appearance of the protoplasm of
the paracyst, at the time when the hook-like prolongation fuses with
it, and from the peculiar refractive appearance of the septum, the
author is compelled to ask, May not diffusion occur? But, as a matter
of observation, the protoplasm in the paracyst does not perceptibly
diminish in quantity, and soon regains its former appearance, like that
of the macrocyst.

The paracyst and macrocyst both become enlarged, and hyphee bad
out from below, enclosing them as described before. Meanwhile, buds
appear on the paracyst, which are from the first much thicker than
the paraphyses, and are evidently the ascogenous hyphee.

The macrocyst is therefore an Ascogonvwm, and the paracyst is mor-
phologically an Antheridium. Whether these sexual organs are so
physiologically is very doubtful.

Comparison with the Collemacew* suggests that the hook-like pro-
longation from the macrocyst is really a Zrichogyne; and it is not
impossible that here, as in the Collemacec, an extremely small quantity
of material may pass into the ascogonium, and the sexual act be
physiologically complete also.

Proceeding to compare the foregoing with what we know of other
Discomycetes, Kihlmann thinks that the sexual process in Ascobolus

1 * Bot. Zeit.,’ 1882; ‘ Beitr. zur Entw. einiger Ascomyceten.’
2 € Textbook,’ p. 309.

3 * Grundziige der Systematik, &c.,’ p. 123.

* Stahl, ‘Beitrage zur Entw. der Flechten,’ H. i.

Ee ee

40 H. MARSHALL WARD.

furfuraceus* is still less established than in Pyronema. The ascogo-
nium in Ascobolus is large and well marked, it is true, but the so-called
pollinodium is very little, if at all, different from the ordinary mycelium,
Probably in this, as in other Ascoboli,? the male organ is degenerated.
Reference is then made to recent researches on other Discomycetes.
Mattirolo’s® and Brefeld’s* investigations of Peziza sclerotiorum show
that the process of reduction has gone still further in this case; even
the ascogonium seems to have disappeared. Other Pezizae,° so far as
the researches allow us to judge, seem to present similar degenerations.

Kihlmann thinks we may probably say that Pyronema conjluens
possesses sharply distinguished sexual organs—at any rate, morpho-
logically. Ascobolus furfuraceus probably produces its fructification
parthenogenetically, while Pezza sclerotiorum forms its asci and para-
physes in a purely vegetative manner. At any rate, apogamy must be
regarded as occurring in these Déscomycetes, and as being attained
gradually through a series of forms.

Before referring to other work of Kihlmann’s I wish to review an
important paper by Fisch,° published in 1882. In this are detailed
the development of the fructification of several Ascomycetes which form
a stroma, in which the proper perithecia are buried, more or less. He
is clearly acquainted with the recent researches and speculations of De
Bary, and, in fact, worked in his laboratory. Very little has been
done with regard to the Fungi mentioned, and so careful a paper as
this is especially welcome. The Fungi examined by Fisch are Poly-
stigma, Xylaria, Olaviceps, and Cordiceps.

Polystigma occurs in the leaves of Prunus,’ forming swollen masses.
The formation of the ascospores takes place some months after the
fall of the leaf. ‘The ascospores, sown in water, produce secondary
spores. These send hyphe through the epidermis of the living leaf,
and a mycelium is formed in the intercellular spaces. This breaks
down the cells in part, or stimulates them to hypertrophy, and thus
the stroma is formed, partly of mycelium, partly of hypertrophied
leaf-tissue.® Hight weeks after infection, the young spermagonia

Janczewski, ‘ Bot. Zeit.,’ 1871.
Ei. g. A. puleherrimus ; Woronin, ‘ Beitr. zur Morph.,’ R. ii.
‘Nuovo Giorn. Bot. Ital.,’ vol. xiv.
§ Schimmelpilze,’ iv.
5 Cf. Woronin, op. cit., and Tulasne, ‘Ann, des Se. Nat.,’ ser. 5, t. vis
6 « Bntw. GENO E Ascomyceten,’’ ‘ Bot. Zeit.,’ No. 49, 1882.
7 Cf. Frank, ‘ Krankheiten der Pflanzen,’ p. 632.
8 Of. Tulasne, ‘Selecta Fung. Carp.,’ iii, and De Bary, ‘ Morph. und Phys, der Pilze,’ &c.,
P 8.; also Frank, loc. cit.

1
2
5
a

THE SEXUALITY OF THE FUNGI. 4]

appear as knots of hyphe, which become hollow and abstrict the
spermatia.

The young perithecia now arise as small clumps of fine hyphe,
which soon form a sub-globular mass, and in the interior of which a
spirally-coiled group of cells represents the Ascogonium, and reminds
the observer of the ascogonium of the Collemacee.1 This body is
somewhat irregular, not evidently attached to a particular part of the
mass enveloping it, and it slowly grows as the surrounding perithecium
cells multiply.

One end of the spiral grows out straight, passes through a stoma,
and is clearly of the nature of a ‘‘trichogyne.” This was frequently
seen, and is figured several times. Though spermatia were seen to
adhere firmly to the end of the trichogyne, the author could not con-
vince himself that fertilisation took place.

It requires two or three months to complete these processes and the
formation of the ripe perithecia. Meanwhile, the trichogyne begins to
be disorganised from its free apex inwards. This was confirmed in
both the species examined, and the author thinks it is a more pro-
nounced degeneration than the change induced in the trichogyne of
Collema on fertilisation.

The paraphyses now bud from the base of the perithecium—not
from the ascogonium—and soon fill up the space formerly occupied by
the dense tissue surrounding the coiled portion of the Ascogoniwm.
This tissue meanwhile becomes resorbed, and the few remaining basal
cells of the ascogonium—the trichogyne and upper part have dis-
appeared—give rise to asci by budding. All the stages of develop-
ment are clearly described.

With Xylaria polymorpha Fisch was able to clear up the points
left undecided by De Bary? and Fuisting.? The young perithecia
arise in the dense stroma as clumps of interwoven hyphee, in the midst
of which a mass of paler. cell-rows arises, which are coiled and inter-
woven into a core or “‘nucleus.” These are the “ Woronin’s hyphe ”
of Fuisting. While these are developing, the outer walls of the perithe-
cium become differentiated. The ‘‘ Woronin’s hyph” now break up,
first into pieces of one or two cells, and then into a disorganised mass,
which soon becomes gelatinous and amorphous.

1 Stahl, op. cit.
2 Morph. und Phys.,’ pp. 97—99.
8 ‘Bot. Zeit.,’ 1867, pp. 303—310.

49 H. MARSHALL WARD.

The paraphyses now spring from the dense mass forming the inner
wall of the perithecium. The asci arise from among these, and have
therefore nothing to do with the “ Woronin’s hyphe,” which have
disappeared in a slimy mass, Further details offer nothing new.

In Claviceps, the young perithecium arises as a mass of small cells,
which rapidly divide and form a parenchyma-like mass.: A hollow
then appears in the interior (reminding one in many respects of the
cleavage cavity in some animal embryos) by the separation of the cells.
The mass now consists of a thick basal portion, above which is a hollow
space roofed over in a dome-like manner by the upper cells. There is
no trace of an ascogonium or of “ Woronin’s hyphe” at any time ;
the asci arise by budding from the cells forming the floor of the cavity.

Cordiceps and other species of both genera agree in the main with
what has been described.

In summing up the foregoing, the author points out that while in
Polystigma we have morphologically the same organs as occur in the
Collemacez (viz. spermatia, trichogyne and ascogonium), no sexual
process could be demonstrated. In Xylaria the sexual organ—at any
rate the ascogonium-—is represented morphologically, but has become
functionless—it deliquesces and is absorbed before the asci arise ;
these spring in a purely vegetative manner from the lining walls of
the perithecium. It is possible that certain facts observed in Cucur-
bitaria point to the same end.

In Claviceps the perithecium is purely apogamous—no trace of an
ascogonium occurs, and the asci are produced by a vegetative process
of budding from the floor of the perithecium cavity.

Putting together the foregoing facts, and what is known otherwise
of the allies of these Fungi, Fisch shows that the compound Pyreno-
mycetes present a series of forms which commence with a complete
differentiation of sexual organs (ascogyne, trichogyne, and spermatia),
and end in forms which are quite apogamous, and with no trace of
sexuality. We may go further than this.

Beginning with Podosphera, in which a sexual process is possibly
still recognisable, we trace a series through the simple Pyrenomycetes
and the Discomycetes branching off from these, ending with the com-
pletely apogamous Chetomiwm’ and Pleospora.? In this series there
is no place for the composite Ascomycetes and Lichens—for although

1 Van Tiechem, ‘Compt. rend.,’ 1875; and ‘Bull. Soc. Bot. Fr.,’ 1876,
2 Zopf, ‘Bot, Zeit.,’ 1879, p. 73. Bauke, ‘Bot, Zeit.,’ 1879,

THE SEXUALITY OF THE FUNGI. 43

the sexual process by means of spermatia is only an adaptive form,
the difference is too great to fit into the main series. How then do
these forms abut on to the main series? Has a sexual process arisen
in the Ascomycetes a second time; or did these forms branch off early,
and evolve and specialise their peculiar mode of fertilisation from the
original type? The first hypothesis cannot be maintained ; the second
seems highly probable.

We must regard the separation of the sexual organs in the compo-
site Ascomycetes and Lichens as adaptation, though we cannot say
how it came about or served the organisms concerned. It is remark-
able that this only .ovcurs in forms which develope a stroma.

The composite Ascomycetes, therefore, branched off before the
sexual process was lost ; whether the Lichens came off at the same time
is not clear—the latter possibly form more than one series, moreover.

The Discomycetes must also have branched off early from the main
series ; they form a series in the following forms, gradually culminating
in apogamy—Ascobolus furfuraceus, A. pulcherrimus, &e., Pyronema
confluens, Peziza tuberosa, Fuckeliana Willkommi (the latter examined
by Fisch).

The Uredineze must also have come off very early from forms in
which sexuality still existed.

As I understand the foregoing, the following scheme fairly expresses
the views ; it being borne in mind, however, that the lines are not in-
tended to indicate more than the general directions in which descent
may be traced. Of course, such a diagram suffers from being drawn
on a plane surface. No doubt more than one line should be drawn
towards the Lichens.

44 H. MARSHALL WARD.

Pleospora, &c.

Uredinze Lichens
2
‘avi 22
Claviceps Peziza a8
a a on
n ie) jj
o
~
: 3 Oo
Xylaria > Apo
ee a 5 Pyronema 28
| £S
3 & &
>
Pa Sb
Polystigma 2 sad
@ [or a8 eS
x Ascobolus 55
[or g mG i
wv - ° No
% ae eee
Podosphera and =.
Erysiphez Sa
S 5
Peronosporee. Pen

We may now shortly consider the chief results obtained by Kihl-
mann from cultivations of Melanospora parasitica,’ a pyrenomycetous
fungus found associated with, and parasitic upon /sarza.

Lsaria grows upon and destroys insect larvee, and Kihlmann ob-
served that large quantities of the perithecia of J/elanospora soon
appear with it; the same is true for otrytis—another insect killing
fungus. In both cases the sowing of Melanospora spores on these fungi
soon resulted in the formation of abundant perithecia.

This, of course, only suggests, but does not prove, the parasitism of
the Melanospora on the Lsaria, or Botrytis.

Spores of Melanospora—whether conidia or ascospores—if sown in
water only swell and throw out short tubes, which invariably die off
soon. ‘The same happened with all the numerous nutritive solutions
tried. These solutions were varied not only as to composition, but
also as to concentration, &c.

If a spore germinates in the neighbourhood of a living hypha of
Isaria, however, the germinal tube fixes upon the Jsarza hypha and at
once emits more tubes, which are thicker and more vigorous than before.
If the germinal tube from the Melanospora spore comes within a cer-
tain maximum distance from a branch of /saria its apex grows directly

2 Op, cits

THE SEXUALITY OF THE FUNGI. 45

towards the latter until a union is effected. This was observed and
confirmed several times. All the above facts are generally true of the
conidia also; and Botrytis may be substituted for Zsaria as the host
plant.

After about eight days the above processes have resulted in the
formation of a vigorous mycelium and the formation of young peri-
thecia. The perithecium commences by the development of a lateral
branchlet, which becomes coiled two or three times, and divided by a
few septa; this is the ascogonium. It frequently resembles that of
Ascobolus.

Thinner hyphal branches now spring from below the ascogonium,
and envelope it by applying themselves closely to it, and branching and
dividing ; although one of these may grow out more rapidly at first, it
does not seem to more than hint at an antheridial branch. But very
often two or more arise together, and others soon follow in all cases.

None of these branches copulate with the ascogonium. Although
Tsaria branches may be close to and serve to nourish the hyphe pro-
ducing the fructification, there is no doubt whatever that only the
hyphee from the M/elanospora enter into the constitution of the fructifi-
cation.

The details of the development of the perithecium wall from the
enveloping hyphe are interesting, but present nothing essentially new,
and need not be described here.

Of the four or five cells into which the coiled ascogonium is divided,
the cell below the apex forms the ascogenous tissue. The terminal
cell above it becomes disorganised ; it is sterile, and soon disappears.
Its immediate neighbour (2. e. the cell below the apex) becomes cut
up by numerous septa in all planes, and forms an ascogenous core of
parenchymatous tissue. As this occurs, the internal layers of the now
dense envelope—produced by repeated ramifications and divisions of
the interweaving hyphe of the enveloping branches-—become dis-
organised, deliquescent, and evidently then serve as nourishment for
the cells of the ascogenous core.*? The proximal cells (2. ¢. those cells
between the ascogenous cells and the mycelium) of the ascogonium
also disappear, and the enveloping layers around them become elon-

1 Cf. my description of the behaviour of a hypha of Pythiwm gracile, ‘Quart. Journ. Mic.

Sc.,’ October, 1883, p. 504; and also the remarks below on the behaviour of Spirogyra in con-
jugation.

2 Cf. my description of the development of the perithecium in Meliola, ‘Phil, Trans.,’ 1883,
p. 592, &e.

46 H. MARSHALL WARD.

gated to form the long neck of the perithecium. It thus comes about
that the free apex of the ripe perithecium corresponds to what was
the attached end of the ascogonium.

The walls meanwhile become coloured deep brown, and rudimentary
paraphyses spring from their internal layers. The asci arise from the
colourless cells of the ascogenous core, in which a cavity is produced
by the tangential growth of its peripheral cells.

In the concluding remarks stress is again laid upon the fact that
we cannot speak of an antheridium here ; the antheridia have degene-
rated to mere vegetative hyphe, and the ascogenous core produces its
asci without any sexual process whatever. A single cell produces the
asci. In most respects this agrees with Gilkinet’s description of the
analogous processes in Sordara fimicola.1 But in Sordaria, Gilkinet
finds one enveloping hypha apply itself to the ascogonium before the
rest, though he could not decide that copulation took place.? Kihl-
mann denies that this can be termed an “ antheridium.” He regards
Melanospora as somewhat midway between those Hrysiphee which,
like Hurotium and Podosphera, have the sexual organs at least mor-
phologically present, and the truly apogamous Pyrenomycetes, Cheto-
mium* and Pleospora.* We may no doubt fairly represent these views
in such a diagram as the following :

Pleospora, &c. (completely apogamous)
Melanospora (no antheridium)

Podosphera (sexual organs well developed).

I now proceed to notice a further contribution to our knowledge of
the Ascomycetes, by Eidam.’ Passing over his description of Hre-
mascus albus, a new species and genus (in which the process of con-
jugation is, however, strongly suggestive of the Zygosporew), we may
briefly notice the general absence of any recognisable process of fertili-
sation, though an ascogonium is always present, and Kidam seems to
regard one of the enveloping branches as an antheridium—morpho-
logically at least.

This observer has studied the development of the perithecium in

1 *Bull. Acad. r. de Belgique,’ ser. 2, t. xxxvii.

2 Of. also De Bary and Woronin, ‘ Beitr. zur Morph. und Phys. der Pilze, &c.,’ 1870, R. iii
3 Zopf, ‘ Nova Acta, Leop. Car. Akad.,’ B. xlii.

* Bauke, ‘ Bot. Zeit.,’ 1877.

5 Cohn’s ‘ Beitr, zur Biologie, &c.,’ B. iii, H. iii.

THE SEXUALITY OF THE FUNGI. 47

Chetomium. The ascogonium arises as an isolated coiled branch ;
fine branched hyphee then envelope it. The tufts of fine hyphe may,
however, arise independently of ascogonia also. Other cases occur
where the ascogonia show no traces of anything but vegetative bud-
ding from the hyphe. He regards it as possible that a sexual process
occurs in the cases first described, and that all stages of degeneration
to complete apogamy occur. Questions of nutrition seem to affect
this matter.

It must be allowed that the figures do not establish this, however,
and it seems very questionable if any antheridial branch whatever can
be distinguished.

Sterigmatocystis nidulans is a new species of fungi allied to Asper-
gillus and Eurotium. An interesting description of its capability of
producing pathological changes when injected into the blood of animals
can only be adverted to here. The fructification occurs embedded in
a sort of stroma of hyphe, interwoven into dense cushions, the pecu-
liarities of which need not be detailed.

The simple ascogonium is enveloped by a hypha (‘“antheridial
branch ”), which soon becomes septate and branched, and forms the
perithecium wall. The ascogonium forms a multicellular core, from
which the asci arise. No fertilisation is shown to occur.

Felicosporangium parasiticum was also thoroughly studied. Here,
again, we have simple ascogonia enveloped gradually by so-called
antheridial branches. The author does not make quite clear, however,
what are the ultimate fates of the several parts. One central cell
becomes filled with spores, but Eidam differs from Karsten as to the
meaning of this. He also denies that Helicosporangium is parasitic.

A closely-allied form is Papulospora, which agrees with the latter
in forming the peculiar mases of cells which seem to represent young
perithecia. It is difficult to avoid the conclusion that Hidam has
either figured ill-nourished specimens—which appears unlikely—or
that some unknown conditions would have caused the bulbil-like
bodies to form perithecia.1 Be this as it may, the bodies in question
form no asci, but “germinate” like compound spores. The great
variability in the formation of the spores and fructification in these
fungi supports the suggestion ventured above, and there can be little

» Thave drawings of somewhat similar bodies from an unknown fungus, which cannot as

yet be made to develope further: they appear to be young porithecia, but they germinate
directly, like gemme, when the conditions are favourable.

48 H. MARSHALL WARD.

doubt that Eidam has opened a question of great importance, and
succeeded in showing that variability occurs in these processes—
whether due to conditions of temperature, nutrition, moisture, &c., or
not, cannot yet be determined. There are facts to support this, and
indeed Eidam has shown this in some examples.

Concerning Hremascus, where a true conjugation takes place between
the apices of two similar hyphee coiled round one another like a double
screw, it is not easy to see why the product of the sexual act (a glo-
bular body situated between the conjugating apices) should not rather
be termed a zygospore than an ascus. The fact of its containing eight
“spores” instead of one, is no more peculiar than in the case of the
Saprolegnize, where an oogonium may contain one to twelve or many
more oospheres. Hidam recognises the general similarity to Zygo-
spore, but gives no adequate reasons for choosing the name “ascus”
in preference to “zygospore.” The eight-spored body would be an
extremely anomalous ascus; but it is impossible to decide the matter
until the asexual spores are discovered. It is interesting to note,
however, that the so-called “asci” arise parthenogenetically in rare
cases.

The main results of Eidam’s observations go to prove that in apo-
gamous forms there may be more or less indications of certain rudi-
mentary organs—antheridial branches (?)—but they do not seem to
establish his conclusions that sexuality exists in these forms. Of
course it is open to imagine that the sexual act comes in now and
again, as Hidam suggests, but no one acquainted with the facts will
lay stress on this supposition.

If we now turn from the Ascomycetes to the other groups of fungi,
the chief papers published lately are not very numerous.

The most important, probably, is Woronin’s memoir on the Ustila-
ginee, and his description of the hitherto little-understood Tubercinia
trientalis. Woronin devoted much time to this investigation, com-
menced sixteen years ago. We may shortly summarise the life-history
as follows :

In May and June the under side of 7rientalis leaves are apt to be
covered with white patches. These consist of the colourless conidia,”
supported on long hyphee, much like those of Ramularia, Peronospora,

1 ‘Abhandl. Senk. Nat. f. Gesellschaft,’ B. xii, H. iv, 1881.

2 These are true conidia, homologous with those of Ascomycetes, and have nothing to do
with the ordinary spores and ‘‘ sporidia.”’

THE SEXUALITY OF THE FUNGI. 49

and others. These conidia-bearing hyphee spring from a mycelium in
the leaves. In the mesophyll are abundance of the usual brown usti-
laginous spores—very like those of Sorosporiwm, &c.—in dense clusters.

In the autumn the Zrientalis plants are found spotted with black
patches. These are due to the densely-clustered brown compacted
spores, as before, but no conidia occur now. The pyriform colourless
conidium germinates on the leaf surface; the germinal tube bores its
way in, grows to a mycelium which ramifies between the cells, and
sends branched haustoria into their cavities.

At certain points on the mycelium arise lateral branchlets, which
superficially resemble ascogonia, at least in some cases ; these—single
or several together—become enveloped by fine hyphee, and soon pre-
sent the appearance of a dense grape-like cluster of spores embedded
in the interwoven mass of fine hyphe. The investment becomes dis-
organised as the clusters of spores turn yellow, and then brown, and
acquire thick coats. The cluster of spores germinates as a whole,
putting out tubes (“promycelia”’) at the end of which the crown of
“sporidia” appear according to the type of Z%lletva. These oval
sporidia may algo “copulate” in pairs in the well-known manner ; but
this often does not occur, and it seems to be an unimportant point,
not affecting the future of the sporidia or their progeny at all. Secon-
dary and tertiary sporidia may arise from the primary sporidia by
budding.

After sowing the brown spores on young plants of Zrventalis, still
level with the ground or nearly so, the mycelium arises in the seed-
ling, and, as soon as the leaves unfold, produces the white conidia
externally and the brown compound spores internally. This is no
doubt the best established case of the existence of two generations
(producing conidia and spores respectively) that we as yet know of in
this group; it is true, it is not the only case, for we have the same
thing in Lntyloma.’ Tubercinia also agrees with the others in being
anteecious, 2c. in passing its whole life on the same host plant.
Woronin is of opinion, however, that “a whole series of hetercecious
forms ” will be discovered among Ustilaginew ; whether this remark is
inspired by facts not yet published does not appear.

Of the remainder of Woronin’s magnificent paper nothing need be
said here; and space does not admit of our referring more in detail

1 De Bary, ‘ Bot. Zeit., 1874 p 81

50 H. MARSHALL WARD.
to the other papers lately published on the Ustilaginew by Brefeld?
and Max Cornu,” which, moreover, contain little of importance for our
present purpose.

In the foregoing part of this essay I have collected a mass of evi-
dence tending to support the view definitely stated by De Bary, to
the effect that, as we proceed along the main lines from the lower to
the higher fungi, the sexual process and sexual organs gradually
become less and less evident, and at length disappear altogether, and
the fructification arises by apogamy,

If we try to follow the various groups of fungi phylogenetically,
there can be no doubt that they may be placed, on morphological
grounds, much as De Bary has grouped them ; and if, taking into
account what has been said above, we attempt to arrange the smaller
groups as branches of the larger ones, we shall, I think, arrive at a
scheme not very different from that annexed,

Uredineze
Peziza
Pleospora
Claviceps ILielirem
nD Pyronema
2 2
2 8
% & Melanospora S$
@ Xylaria z5 Pee: &
(S) BS S
4 5 Ascobolus .2
= A oS)
2 3
2 Polystigma 2
a ae is Erysiphez
=) rh
a A=|
7)
Ustilaginez
Podosphera
Zygomycetes
Peronospore

eee

Alge

2 © Schimmelpilze,’ iv,
2 fAnn. des Sc. Nat,’

THE SEXUALITY OF THE FUNGI 51

Tt must be borne in mind that we confine ourselves strictly to the
evidence derived from the study of living forms—it may or may not
be that numerous primary or ancestral forms, long since disappeared,
would lead us to different conclusions, as the imagination of such led
Brefeld to different views ; but the only true method is to adhere to
what we know as the basis of our plans for knowledge to come.

It must be allowed at the outset that we know very few forms
accurately or thoroughly, and there are therefore almost endless possi-
bilities in the future. Keeping these cautions in mind, however, we
need not fear to point out whatever points of general significance can
be obtained from our present knowledge.

The first and most important fact with regard to the scheme is that
if we pay regard to the terminal members of most of the main branches,
we notice that- they are all, or nearly all, parasitic forms, or, at any
rate, include such forms.

The higher Ascomycetes offer us the following examples from
different branches, the Lichens, Pleospora, &c., Claviceps, &e., and
Pexiza sclerotiordes.

Then come the Uredinee (we need not necessarily imagine the Tre-
mellas and Basidiomycetes as derived from the highest Uredinee; the
evidence does not decide this), all strongly parasitic.

The Ustilaginee of course are parasites par excellence, and they
terminate a side series.

We have still two main groups to deal with—the Saprolegnie
(which, so far as known, are mostly saprophytes) and the Zygosporee,
which are also generally saprophytes. Our very imperfect knowledge
of the Basidiomycetes will be cited as an excuse for putting them aside
in what follows : I do not for a moment under-value what we do know
of them, but, as the sequel will show, their present position becomes
more and more anomalous, if we really know the entire life-history.
Of course we have no right to quarrel with the evidence, but the story
of these fungi, as told at present, completely negatives their being
included in the scheme to follow, and we must therefore neglect them
for the moment, merely reminding the reader that some of them are
parasitic.

Neglecting the Basidiomycetes, then, we may proceed to note that
not only are the terminal groups of the series named usually parasites,
but that it is just in those groups which are most intensely parasitic
that least hope of our discovering sexual organs exists. In the Zygo-

59 H. MARSHALL WARD.

mycetes, on the other hand, we have the sexual process and typical
saprophytic habits together, while in the Saprolegnic the case seems
doubtful.

Looking still more closely into the matter, it appears as if the
absence or presence of sexual organs (or their rudiments) rises or falls
with the nature of the parasitism or saprophytism displayed. In the
Saprolegnie, for instance, the Fungi may probably be looked upon as
very highly nourished by the decomposing proteids of animals.1. Their
sexual organs seem to be present in most cases, but functionless.

In the Zygomycetes, which are essentially saprophytes on decaying
vegetable matter, &c., or parasitic on one another—and may probably
be regarded as not so highly nourished—we find the sexual organs
functionally perfect, though very simple in character.

In the Ustelaginee we meet with parasitism of a peculiarly high
order, so to speak. 'The fungus not only robs its host, but has in most
cases curiously adapted its life to the habits of the latter, using it
rather as a slave than as a victim to be destroyed forthwith.

The same is true for the highly organised Uredinew (d'cidiomycetes),

and we here meet with the highest adaptation of all, hetercecism. But
in these two groups the search for sexual organs has proved utterly
futile (if we except the so-called “copulation” of the “sporidia” in
Ustilaginee, which cannot be regarded as an essential process, or as
sexual in the above meaning).

Again, if we proceed upwards from the Evysiphew, which are epi-
phytes—adapting themselves to parasitic habits of that special kind
which leads to life in the interior of temporary organs like leaves—
through the Ascomycetes, we find, speaking generally, more and more
tendency towards close and specially adapted parasitism, ending in the
Lichens, the parasitic Pezizas, forms like the Pleosporas, &c., and
especially Claviceps.

Now it is at least remarkable that no trace of sexual organs has yet
been found in the higher Lichens—z.e. in those forms in which the
fungus makes a particularly well-regulated use of its slave-like host,
which is an Alga containing chlorophyll. Krabbe’ considers that in
Sphyridvum the fructification arises independently of any process of
fertilisation, and my own observations on Strigula complanata® lead to
the same conclusion. It will be noted that in the beautiful case de-

1 Tf not, indeed, by living flesh. Cf. Prof. Huxley, ‘Quart, Journ. Mic. Se.,’ 1882.
2 ‘Bot. Zeitg.,’ February, 1882, No. 5.
3 SZinn. Trans., ser: 2, Bot.,’ vol ii, 1884,

THE SEXUALITY OF THE FUNGI. 53

monstrated by Stahl,' the host is a blue-green Alga, and the parasitism
may well be considered as lower in many respects. Moreover, it is by
no means certain that the Lichens represent one group.

In Claviceps purpurea we have an excellent example of the highly-
developed parasitism referred to. The ravages of the parasitic my-
celium seem to be confined to one organ of the host—the young fruit—
and we have seen from Fisch’s researches that the asci arise in the
stromata, developed later, in a purely vegetative manner.

Our knowledge of the large group of the simpler Pyrenomycetes
does not enable us to make a generalisation of very much value ; but
it is significant for our present purpose that the apogamous Pleospora,
for instance, is parasitic during the early stages of its life, and, like so
many of its allies, adapts its cycles to those of its host, producing a
large stock of asexual conidia on the living leaves, and using up their
contents before falling, to complete the development of the asci, &c.,
on the ground. It is scarcely necessary to remind the reader how
great an advantage accrues to these higher parasites, when they scatter
immense quantities of spores from leaf to leaf of the living tree. That
their perfect “fruits” should be formed later, when the mycelium
has gathered up all the material possible, is quite in accordance with
what occurs in the formation of stromata, sclerotia, and masses of
hyphee (often with haustoria) around the young perithecia in other
cases.

The same is generally true for such Discomycetes as Peziza sclero-
tioides,? P. Fuckeliana, and other parasitic Pezizee; and it will be
remembered that it is in these forms that De Bary and others failed
to find any traces of sexuality, thus placing them in strong contrast
to such as Ascobolus (according to Janckzewski’s researches), unless
intermediate forms like Pyronema and the saprophytic Pezizas are
compared also.

Enough has now been said to show that there is at least strong
reason for believing that a connection exists between the mode of life
of a given fungus and the extent to which it is apogamous. It will
no doubt be suggested that there are still cases where this view seems
at variance with the facts. Without wishing in any way to strain
matters at this point, it may be noted that we really know very little
of the mode of life of very many fungi, and that the terms saprophyte

1 Op.cit
2 Frank, ‘ Krankheiten der Pflanzen,’ p, 531, &c.

54 H. MARSHALL WARD.

and parasite are used somewhat loosely. This being admitted, it
may happen that further knowledge will strengthen the connection
spoken of.

We are at least assured that profound differences exist—in degree,
at any rate—between the saprophytism of a JJucor growing in a
solution of horse-dung, and of a Pythium developing its fructification
in the rotting parenchyma of a plant which it has previously killed.

There is also an equally striking difference between the parasitism
of an epiphyte like Zrysiphe and that of a highly-specialised Aicidio-
mycete like Puccinia. But I would insist upon more than this. It
is not only in the mode of attacking or living upon the substratum
that one fungus differs from another ; differences as to the kinds and
quantities of the various matters absorbed must also exist, and a
Uredine in a leaf no doubt obtains different food (and in a different
way) from that taken by Claviceps in a grain of rye, or Ustilago in a
hypertrophied swollen stem of Zea Mays. That these differences may
be very important—though we do not know exactly in what they con-
sist—is fully demonstrated in cases of hetercecism.

I have already pointed out that the coexistence of apogamy (or the
total suppression of sexual organs) and parasitism is noticeable espe-
cially in the highly specialised parasites. In forms which, like the
majority of the parasitic Peronosporee and Zygomycetes (e.g. Pepto-
cephalis), are nevertheless provided with sexual organs, which, so far as
we can see, are quite like those found in the saprophytic forms, we
have two points to notice. First, these forms are close to the parent
stock in phylogeny—z.e. they are not much modified from the type
of Pythium itself, which (as a comparison with Vaucheria shows), is
no doubt derived from algal ancestors, and with strongly inherited
sexuality. Secondly, such forms are probably not so highly parasitic
as is commonly supposed. I do not mean to say that their living
hosts are not robbed by them; but it is significant that the Peronos-
porece are often saprophytes, and that even the most parasitic forms
break down the parenchyma of the hosts to a rotting, fetid mass, on
which they then flourish. Moreover they are aided by bacteria in
this process. In addition to this they are apt to be omnivorous. I
have cultivated Pythium De Baryanum' on the most various sub-
stances, as well as on more than a dozen widely different living plants,

In all these cases the parasite appears to flourish in a variety of

1 Of, also De Bary op. cit., and ‘Quart. Journ, Mic. Sc.,’ 1883.

THE SEXUALITY OF THE FUNGI. 55

substrata, and it has not got over the clumsy habit of destroying its
host forthwith. If we compare the highly developed, almost intelli-
gent, parasitism of a higher Ascomycete or Uredine with this, it will be
understood what I mean by specialised parasitism. Instead of clum-
sily destroying its host (like Phytophtora infestans does the potato), a
Puccinia is adapted to live in isolated patches of carefully-sheltered
leaf tissue, ramifying in the lacune filled with oxygenated air and
aqueous vapour. Here it taps the cells as they manufacture organised
substances in the sunlight, taxing them not too much for their strength,?
and its mycelium keeping near the stomata. Its spores are then pro-
truded in centrifugal succession, and shaken off from their advan-
tageously high position on to other leaves, &c. All such adaptations
must imply long periods of descent (and the fungus is therefore much
further from the parent stock in the phylogenetic scheme), during
which even the strong hereditary tendency to produce sexual organs,
&c., might become lost, if such organs for any reason became super-
fluous.

This, however, brings us at once to the last object of the present
essay ; and I propose to show that it is probable that the sexuality of
the higher fungi has disappeared, because its purpose has been equally
well or better attained otherwise than by means of sexual organs.

Preliminary to this it will be necessary to be quite clear as to what
sexual organs and the sexual process essentially are.

The two points common to all the cases of sexual reproduction
which have been directly observed are the following :

1, A larger or smaller quantity of protoplasmic material passes
from one portion (the male organ) of the same or another individual,
into the protoplasm contained in another portion (the female organ).

2. The protoplasm contained in the female organ therefore becomes
capable of further development ; either at once, or, more generally,
after undergoing a period of rest.

It is not necessary to quote the numerous cases of observed ana-
logies between the sexual reproduction of animals and plants; but
will suffice to note that the essential in the sexual process is always
the addition of a portion of protoplasm from the male, to the proto-
plasm of the female.

But this is not all. It is now well established in embryology that

» Many Uredinew appear to do no injury at all, unless in large amount and for a long
time—i.¢. the host can pay the tax easily.

|

56 H. MARSHALL WARD.

the normal ovum, or female mass of protoplasm, is incapable of further
development until it has received the protoplasm of the male; that
the latter, in fact, incites the former to further development. In
many cases, indeed, the protoplasm of the egg or ovum gets rid of a
small portion of its substance, as the “ polar bodies,” as if to make
room (so to speak) for the substance coming to it from the male.?

While in the higher organisms we can distinguish the male elements
—spermatozoa, antherozoids, &c., only in so far that they are much
smaller and more numerous than those of the female organs ; we find
that in the lower forms of life even this difference in size is absent,
and there is absolutely no safe criterion to determine which of the
two conjugating masses of protoplasm is to be regarded as male and
which as female.

Nevertheless, if we consider cases such as are afforded by the fungi,
we are certainly on safe ground when we call the antheridium of
Pythium a male organ, and the oogonium of the same a female organ.
The protoplasm contained in the former is itself incapable of further
development, but normally passes over into the protoplasm (oosphere)
contained in the latter ; the oosphere is then—z.e. after fertilisation—
capable of further development.

This ‘“ further development,” however, is nothing more than growth ;
and, what is more, growth according to the same laws as affected the
parent plant which produced the sexual organs. In cases where the
plant is divided into cells, this growth or germination of the oospore
commences with division into a number of cells.

The outcome of all we know of these matters leads to the conviction
that we have in the germination or development of an oospore—and
the same is true for an egg, &c., the terms being different—simply a
renewal of the growth of the organism; and from this and other con-
victions follows the result that the formation of an oosphere, although
it may take place after an accumulation of large quantities of food,
implies a condition of weariness—if the term may be allowed—on the
part of the protoplasm for the time being. No doubt the molecular
energy of the protoplasm forming the oosphere, is less than that of
the rest of the plant for the time being ; the access of the antherozoid
or male protoplasm, however, reinvigorates the sluggish mass, and
renewed life ensues. This may require some time, however, and we

1 That something of the same kind takes place in the Saprolegnie is suggested in my paper
on this group, ‘Quarterly Journ. Mic. Sc.,’ 1883,

THE SEXUALITY OF THE FUNGI. 57

may possibly not be far wrong if we imagine that interval to be occu-
pied in molecular rearrangements in the mass.

But, although we can sum up the foregoing by saying that, after a
time, protoplasm requires re-invigorating by the addition of fresh
protoplasm from another source, it is extremely improbable that the
protoplasm of the male and female organs is at all similar.

While we have reasons for believing that the mass of an oosphere
consists in the main of protoplasm such as occurs in any cell capable
of growth, it would be absurd to suppose that the protoplasm of the
male element is of the same nature. There is, moreover, strong evi-
dence to support the opposite view, that the protoplasm of the male
and the essential protoplasm of the female differ extremely.

Anyone who reads Strasburger’s description of the process of fertili-
sation in the ferns,’ cannot fail to be struck with the peculiar beha-
viour of the antherozoids as soon as they come within a certain distance
of the oosphere. It seems impossible to avoid the inference that the
oosphere in some way attracts the spermatozoids. A similar pheno-
menon is described by Juranyi in the fertilisation of Qdogonium,?
where the relatively large antherozoid forces its way through an aper-
ture too small for it, in order to reach the attracting oosphere.

With such phenomena may be compared the case of Spirogyra and
other Conjugate, where, as is well known, the cells of filaments which
are laid parallel to one another, and within a certain distance of one
another, put forth conjugating tubes which meet in the middle; or
neighbouring cells conjugate.

In the Peronosporeze, again, the oosphere appears not only to
attract the antheridium, but even to induce its formation from a
neighbouring hypha ;* and other cases may be cited, all tending to
show that some important difference exists between the protoplasm of
the two sexual organs.

It does not concern us here to give any opinion on De Bary’s sug-
gestion that profound chemical differences exist, and affect the environ-
ment; or on Sachs’ recently expressed view* as to the analogies
between ferment actions and fertilisation.

Enough for our purpose that the knowledge we possess goes to show
that sexual reproduction essentially consists in the reinvigoration of a

1 ‘Jahrb. f. wiss. Bot., vil.

2 * Jahrb. f. wiss. Bot., ix.

3 De Bary, ‘ Beitr. zur Morph. und Phys.,’ iv.

* *Vorlesungen iiber Pflanzenphysiologie,’ p. 491,

58 H, MARSHALL WARD.

sluggish mass of protoplasm, by the addition of another and different
mass of protoplasm. That an advantage is often attained by the latter
mass coming from a distant source, is sufficiently evident from what
we know of cross fertilisation generally.

It now remains to be seen if we can throw any light on the curious
disappearance of sexual organs and sexuality in the fungi—curious,
because the sexual process appears to be all but universal in all organ-
isms excepting the very lowest.

A hypothesis which suggests itself, and which Eidam favours, and
which is certainly supported by some analogies, is to the effect that
the apogamous fungi are not always apogamous. We know that many
forms only produce their sexual organs at comparatively long and rare
intervals. The JZucors, for instance, may be propagated through
numerous generations by means of the asexual spores; the sexual
organs only arising now and again under favourable conditions.

Accepting that the sexual process consists essentially in a re-invigo-
ration of the protoplasm of the organism, may it not be that one sexual
act is effective through long periods and many generations? Such a
view is supported by the known cases of parthenogenesis in other
plants, and would explain such cases as the Saprolegnie, if it were
placed beyond doubt that protoplasm does occasionally pass through
the ‘“fertilising tubes” to the oospheres.

Moreover, the cases of polyembryony—where several embryos arise
in an embryo sac, although only one oosphere is fertilised—favour the
view that the effect of fertilisation may be extensive; and we cannot
doubt that such is the case where adventitious covering branches arise
after the conjugation of certain I/ucorini (e.g. Mortierella), and in the
Orchidew, where fertilisation or even the mere growth of the pollen
tube affects the whole flower.

In other cases, however, great difficulties are experienced. It is
not easy to conceive how fertilisation in a distant past has transmitted
its effects through countless generations to the individual plants of
Chara crinita which now reproduce without any sexual act at all.
And the same is true for other cases.

There is one fact apparently universal in sexual reproduction ; it
does not take place until a large quantity of material is either accumu-
lated, or is in some way placed at the disposal of the sexual organs.
If these sexual organs are to be looked upon as specialised to secrete

THE SEXUALITY OF THE FUNGI. 59

the sexual elements, or to sort the substances of which they consist,
as it were, this may be of importance.

It must be allowed that no satisfactory theory exists, however, to
account for the gradual disappearance, first of sexuality, and then of
even the morphologically represented sexual organs in the fungi; and
any attempt to explain the matter seems to involve more than one
vicious assumption.

The sexual act, however, consisting simply or mainly in the re-invigo-
ration of protoplasm by the addition of protoplasm of a different
nature (though we do not know the kind or limit of difference) from
a distance, it may be that an explanation of what occurs in the fungi
is afforded by their mode of life. I have already pointed out that the
fungi in which sexual organs seem to be most certainly absent are
those which are most highly specialised as parasites. Now, we have
every reason to believe, first that parasitism is a matter of degree, and
secondly that the most highly specialised form of parasitism consists
in directly obtaining those contents of the cells of the host which are
chemically most complex, and therefore contain most energy.

I need not dwell on the degrees of parasitism exemplified by plants
which merely rob their hosts of space or moisture, or which have ob-
tained a hold so intimate that they break it up and feed on the rotting
débris, but may at once pass on to consider a few vonsequences which
follow from the mode of life of those highly specialised parasites which
have become so closely adapted to their host, that they exist for a
time as all but an organic part of its tissues and substance.

It can scarcely be doubted that the protoplasm of a higher plant,
such as a phanerogam, differs from that of a lower cryptogam in being
capable of doing more work ; and that the great advantage derived by
a parasitic fungus which has its life so adapted that it can tax the
cells of a phanerogamous host plant, is that it obtains its food
materials in a condition more nearly approaching that of its own
substance, than would be the case if it had to work these materials
up from inorganic matters.

Now it seems not improbable that the protoplasmic substance of a
higher phanerogam may contain so much energy, that it can not only
supply the vegetative mycelium of a parasitic fungus with all that it
requires. for its immediate growth, but also suffices to enable that
fungus to store up enough energy in its asexual or apogamous spores

60 H. MARSHALL WARD.

to last until the next generation of the fungus gains its hold-fast on
another (and it may be distant) source of life-giving substance.

Let us take the case of a Uredinous fungus parasitic in the leaves of
a phanerogam. We know that the substances necessary for the whole
growth of the phanerogam are formed in the cells of the leaf ; not only
so, the matters which eventually find their place in the reproductive
organs must be formed there also, potentially at least. The leaf of a
phanerogam so attacked, moreover, is able to support the parasitic
fungus for a long time uninjured, as I have convinced myself by ex-
periment, and there can be no doubt that substances pass into the
fungus which would normally have passed into other parts of the host
plant itself. Since these substances serve to support the compara-
tively enormous display of energy evinced in the growth, &e., of the
phanerogam ; we need not be surprised if they can also provide in
addition for the parasite for the time being.

But we may imagine even this to fail after a time. Without specu-
lating as to the possible differences effective to a mycelium which
obtains enough to produce spores on one leaf, which, germinating on
another, produce a mycelium which derives an advantage correspond-
ing to that obtained by plants when cross fertilised —we may suppose
that at length the fungus derives too little benefit to be able to go on,
or the season during which the host plant flourishes is drawing to
an end.

No doubt we have in hetercecism the salvation of such a fungus.
Not only is it carried through a dangerous period, by seeking relief at
the hands of a second host, but-—and which I believe to be far more
important—it obtains re-invigoration by the new protoplasm with
which it comes in contact. We may not inaptly compare the sojourn
of the fungus on its second host, to a trip to the sea-side, where the
weary and enfeebled organism enjoys fresh diet and associations for a
time, which in their turn pall and prepare the recipients to renew old
modes of life.

We have seen that the disappearance of the sexual organs, leading
to apogamy, commences especially in the lower Ascomycetes, and it
may be more than a coincidence that epiphytic forms, which show a
tendency to produce one kind of spore while on the living leaf and
develope their asci on the fallen leaf are common here; such forms
suggest how the parasitism and hetercecism of higher forms may have

THE SEXUALITY OF THE FUNGI. 61

begun, and it is remarkable that the apogamy becomes more and more
complete as we ascend through the latter.

It is not pretended that the hypothesis embodied above at once
explains all the cases possible, and it will be well to state a few of the
' greater difficulties at once. The Basidiomycetes I shall not dwell upon,
since our knowledge of them is still very imperfect. The few cases of
parasitic Basidiomycetes known can hardly be cited as supporting the
view adduced; and if it turns out that all the forms are as utterly
devoid of sexuality as Brefeld’s Coprinus, and that no other generation
exists than the one now known, it will be difficult or impossible to
reconcile the facts, and the coincidences referred to in this essay may
have to be accepted as coincidences only.

Apart from this, the difficulty must suggest itself to many that
there are parasitic fungi—such -.as the Peronosporee—which neverthe-
less develop the sexual organs in the condition typical and perfect for
the group to which they belong. I. have already referred to the fact
that many of these forms are really saprophytes, and that others break
down and destroy the tissues of their hosts—clumsily killing their
prey aud then feeding on the rotten mass—and have pointed out that
this is a much less specialised form of parasitism than that of the
higher fungi and Ustilaginee. It is true we do not know much about
the nature of the food which these fungi take from the host; but there
is evidence to show that it is of the nature of fermenting sap, and
therefore possibly contains far less energy than the substances absorbed
by the higher parasitic fungi. There are two other points which may
also be of importance.

The Peronosporee are almost certainly descended directly from Algee
which had already won and established strongly marked sexuality.
This would probably be lost only after a long time; for we have every
reason to suppose that inherited sexual tendencies are among the last
to disappear in the modified descendants of organisms.

Nevertheless, and this is the second point, the sexuality shows signs
of disappearance in extreme members, even within the groups of the
Peronosporee. De Bary * shows that in Phytophthora and Peronospora
there is a less evident passage over of protoplasm from the antheridium
to the oosphere than in Pythium ; and that in some cases, indeed, the
quantity passing over is too small to be observed. I will not attempt
to lay stress on the coincidence that in Phytophthora infestans (the

2 ‘Beitr.,’ iv, p. 72:

62 H. MARSHALL WARD.

fungus of the potato disease) no sexual act has as yet been discovered.

Another obvious objection may be raised as follows: The Sapro-
legnice are in the main saprophytes, and yet they are said to be ad-
vanced towards apogamy—parthenogenetic, at any rate. The answer
may be that they are saprophytic chiefly on animal protoplasm which
“contains more potential energy than does vegetable protoplasm. At
the same time, some Saprolegnie are parasitic on plants, and S. ferax
now appears to be parasitic on fish.?

Among the Zygomycetes, again, we meet with parasitic forms in
which the very simple sexual organs and process are, so far as we
know, as typically perfect as in the other members of the group.
The reply here is the same as in the case of the Peronosporee. The
Mucors must be an older group than Piptocephalis and others which
are parasitic upon them. Hence we may assume that the inherited
sexuality is too strong to have been replaced by the effects of admix-
ture of the protoplasm of the Mucor, which, moreover, is probably not
very different, and can scarcely be considered as provided with more
energy. A similar argument may apply to the Lichens. The higher
forms are specialised parasites on green Alove, which must be able to
supply substances containing great potential energy, and no traces of
sexuality are found in them. In the Collemacew, however, where sexual
organs occur, the fungus is associated with a very low form of Alga,
one of the Cyanophycew, and appears rather to be feeding upon the
diffluent matters around the algal cells, than strictly parasitic on the
Alga proper. This is so much the case that, as is well known, some
lichenologists have doubted whether to rank the Collemacece with
Lichens at all; and all observers must agree that it is difficult to
decide when a mass of Wostoc is to be regarded as all Alga or passing
into the state of Collema. I remember cases in my time at Cambridge,
when I observed patches of Mostoc on the roadside at Shelford, and
patches of Collema some distance away. At points between, there
were patches of Vostoc in various stages of transition between the two,
In Ceylon, again, I have observed masses of Aivularia with fungoid
hyphee associated at least as definitely as in these cases, and the same
occurs in masses of Glaocapsa in greenhouses. I do not attempt or
wish to cast a doubt on the lichen character of the Collemacee; I
merely point out that, as in.the case of other parasitic Fungi, the
Ascomycetes of the Lichens exhibit gradations of parasitism from mere

1 Prof. Huxley, loc. cit.

THE SEXUALITY OF THE FUNGI. 63

association to highly adaptive parasitism, where the fungus has learned
(so to speak) to use its host as a slave.

The most serious objections to the above hypothesis will probably
occur to those who draw conclusions from the life-history of imperfectly
known forms. Without wishing to disarm any criticism whatever, I
would mention two points to be borne in mind in this connection.

Many fungi are known to be capable of adapting themselves to
widely different modes of life, and it is extremely difficult to say how
far they are parasitic or saprophytic. Leaving the Bacteria alone, I
need only mention Koch’s experiments with species of Mucor and
Aspergillus, and Eidam’s observations on Sterigmatocystis:+ these
fungi were found to be pathogenic to a disastrous extent when in-
jected into the blood-vessels of living animals. Again, Kihlmann’s
brilliant research on MJelanospora, embodied in an earlier portion of
this essay, brings to light an extraordinary case of parasitism and
adaptation.

Secondly, we really know very little of the mode of life of many
fungi in their earlier stages ; we assume, rather than know, that many
forms of Pyrenomycetes, for instance, are Saprophytes. However, less
is to be gained by dwelling upon these doubtful matters than by court-
ing criticism of the main point at issue.

I may say, in conclusion, that it was during the study of the
parasitic fungus of the Coffee disease (Hemileia vastatrix) in Ceylon
that I was first led to speculate on the enormous amount of energy
displayed by an organism which shows not the remotest satisfactory
trace of sexuality, but which reproduces itself through many genera-
tions exclusively by means of asexual spores. That this energy of
reproduction is derived from the Coffee tree there can be no doubt,
and that it is at the cost of the reproduction of the host is sadly
evident ; the clear inference from the fact that the Coffee leaf supplies
substance for the reproduction, &c., of a fungus at the expense of its
own fruit, is that the fungus takes matters which are very rich in
energy, so rich, indeed, that the fungus is not necessitated to sort
these substances in special reproductive organs, and to secrete sexual
elements, one of which would then re-invigorate the other, but may
employ them forthwith for the purposes of its own relatively simpler
existence and reproduction.

1 Cohn’s ‘ Beitrige,’ B, ii, H. iii., p. 397 ff

64 H. MARSHALL WARD.

THE MORPHOLOGY AND PHYSIOLOGY OF AN AQUATIC
MYXOMYCETE.

By H. Marsuatt Warp, M.A, Fellow of Christ’s College, Cambridge;
Assistant Lecturer in Botany, Owens College, and Lecturer in the
Victoria University, Manchester.

[Prates ITI—IV.]

During the progress of certain experiments which I instituted during
the course of the past winter and spring, in order to obtain information
as to the feasibility of employing the electric light in the botanical
laboratory, so that demonstrations on the assimilation of green plants
might be made independently of the sunlight, of which we obtain so
little in Manchester, it became necessary to employ among other
plants actively growing Hyacinths ; in order the better to study the
effect of the electric light on these, I grew them in nutritive solutions
consisting of minerals dissolved in pure water, according to a method
devised some years ago by Sachs, and now much used in my laboratory
course for teaching purposes. A few fractions of a gram of phosphates,
sulphates, and nitrates of calcium, magnesium, and potassium are
dissolved in one litre of distilled water, and a trace of common salt is
added. All the salts should be chemically pure, and plants may be
easily cultivated in the solution for weeks and months, as is now
abundantly proved (fig. 1).

The Hyacinths in question were grown with their bulbs resting on
the necks of tall glass jars, cylindrical in shape, and the roots of the

Sr i Nt

AN AQUATIC MYXOMYCETE. 65

plant hung perpendicularly into the solution mentioned above ; in
this they developed vigorously, and were to all appearance quite
normal. During the course of the experiments, and while the plants
were exposed to the influence of the electric light emitted from incan-
descent Swann’s lamps, certain minute black spots were several times
observed on the white roots hanging down into the liquid; these spots
were all very small, but varied in size, and evidently increased in
number day after day and week after week, and beyond making a few
notes as to their occurrence and increase I paid little attention to them
at first, my interest being aroused in the progress of the more special
investigation to which my experiments were directed. Very soon,
however, the time came when it was necessary to examine the roots,
and their tiny black spots, and I then discovered that the little specks
were really the sporangia of a microscopic Myxomycete. Fig. 1 shows
the appearance to the unaided eye of the roots and the black spots
referred to, drawn to their natural size.

In the first place their order of appearance showed no obvious
regularity with respect to either space or time. Sometimes one of
the tiny sporangia would arise at or near the tip of the root, at others
it would appear near the basal portion, or irregularly at any spot
between the base and apex. Again, while three or four might be
formed during a given 24 hours, none or several might make their
appearance during the next day, and so on. Moreover, I could detect
no obvious relationship between the order (or want of order) of suc-
cession, and any changes in the light (electric light) and temperature
which were noted with reference to my other investigations. All that
I was able to establish in this connection was that the black sporangia
became developed on the roots, both in the glare of the electric light
used, and in the diffuse daylight of January and February ; subsequent
investigation showed that they also became developed on the roots
when kept in the dark. The temperatures varied somewhat con-
siderably, and the sporangia were formed in some cases at 18° C,,
during the periods when the electric light was turned on, and even at
14° or 15°C. during the night, and at those periods when the roots
were in the dark.

Another point of interest is that by far the majority of the sporan-
gia referred to were developed in the nutritive solution—7.e. on parts
of the roots entirely submerged—though, as I shall show later, they
may also be formed on parts of the roots which are in the damp

B

66 H. MARSHALL WARD.

atmosphere outside the liquid. On the sides of the bulbs, or on the
leaves, and generally on any part of the Hyacinth exposed to the dry
air outside, the sporangia are not developed, and every attempt to
cultivate them there has so far failed utterly.

There can be no doubt that the life-history of this Myxomycete then
is passed through in the aqueous solution, and it seems to me a point
of no small interest to have found what may justly be described as an
aquatic Myxomycete.

There is still another point of general importance to be mentioned.
The sporangia arise on what are to all intents and purposes perfectly
healthy, living, and intact roots, and there are no observations to show
that the organism causes any injury whatever to the plant on which
itis found. It is not intended to deny that this Myxomycete may
produce its sporangia on dead or dying roots—a point which I have
not directly investigated, though some experiments to be described
later show that it may well be so—but all investigation places beyond
doubt that no interference with the normal relations of the root are
necessary for the well-being of the Myxomycete.

The black sporangia referred to contain spores ; these spores ger-
minate readily and emit “myxamoebe,’ which pass rapidly into
“‘myxozoospores” ; after a varied term of life, during which nutrition
is carried on, and various phenomena connected with reproduction are
observed, these “myxameebe” unite into plasmodia ; which, in their
turn, again form sporangia. The above may be regarded as a cursory
sketch of the life-history which I have observed point for point, and
partly because the above headings agree broadly with the order of inves-
tigation, and partly for other reasons, I shall adopt the simple plan of
describing the subsequent details in the following order. (1) The
anatomy of the sporangium and its contents ; (2) the germination of
the spore, and the production of myxamecbee ; (3) the changes under-
gone by the myxamosbee—their nutrition, changes of form, encystment
and division; and (4) the formation of the plasmodium, and the
changes which it undergoes. Other remarks, on the physiology of
the myxameebee chiefly, may find place under one or more of the above
headings.

THe SPoRANGIUM.

As shown in fig. 1, the sporangium is a minute black body, of
globular or ovoid form, or occasionally less regular and shaped like a

oe
Se Oe el ee

AN AQUATIC MYXOMYCETE. 67

slug or planarian: it varies in diameter from $ a millimeter to 12
millimeters or more, and stands up from the epidermis of the root, to
which it is attached by a slightly flattened base. As shown both by
sections and by the ease with which the sporangium can be detached
from the root by means of a razor or even the point of a needle, the
sporangium is only fixed on to the outside of the epidermal layer, or
to the outer cells of the root-cap, and is in no way connected anatomi-
cally with the interior of the cells or tissues: in some cases, however,
it can be shown that the base of the sporangium is actually in contact
with the external cell-walls of the root, and not merely adherent to
the slime, &c., found on the surface.

When detached, examination fails to show any aperture in the
Sporangium, or any special organs of adhesion, except in so far as the
thin network to be referred to shortly, and which passes off from the
surface of the sporangium to that of the root may be looked upon as
a hold-fast.

On slightly magnifying the sporangium (figs. 2, 3, and 4) it is at
once noticed that the smooth hard black appearance is due chiefly to
two causes. In the first place the thin outer covering or shell reflects
the light in various ways from its rounded surfaces, which are, more-
over, wet and bright, and evidently coloured with a dark pigment ;
and secondly the densely packed, minute, purplish-brown spores in
the interior render the whole mass perfectly opaque, except at the
edges where they may be seen shining through with more or less dis-
tinctness (figs. 3 and 4).

Closer examination now shows also that the deep black mass of
Sporangium and contents proper is fringed as it were (at any rate in
most cases) with a bright canary yellow border, and, as will be shown
below, this extends over the sporangium proper as a sort of outer coat
or network.

The whole sporangium, then, is composed of two coats—an inner coat
composing the wall of the sporangium proper, is a thin, tough, very
elastic and almost horny homogeneous membrane, which is quite
smooth or nearly so, being occasionally marked with almost impercep-
tible rugosities. Its colour is, as already stated, dark, and evidently
due to a purple-brown pigment usually evenly distributed throughout
its substance. In comparatively rare cases the pigment is nearly absent
in places, and the depth of its hue varies somewhat : these variations

68 H. MARSHALL WARD.

seem to depend on the age and relative thickness of the membrane,
but may be in part due to other causes.

External to this, the proper shell of the sporangium, is a yellow net-
work or coarser membraue somewhat less constant in character. In
some cases, at any rate, it can be traced all over the surface of the
sporangium, but in other cases it appears to be incomplete at places.
The meshes of this layer vary considerably in size, and the whole net-
work is thicker or thinner at some spots than at others: to these
peculiarities may be referred the different shades of the yellow colour.
It is where this yellow meshwork passes off from the surface of the
sporangium to become extended irregularly on to the surface of the sub-
stratum that the yellow fringe or border (figs. 3 and 4) is observed, and
in a certain sense this “outer peridium,” as the network might be
termed, may be looked upon as fixing the sporangium which it im-
prisons to the substratum. The relative positions of the two struc-
tures are no doubt due to the fact that the network is a mass of coarse
granular material excreted by the protoplasm of the plasmodium when
it is forming the spores and sporangium.

In the large number of cases where the sporangia present the
typical bright black colour to the unaided eye, or when but slightly
magnified, this thin yellow network offers nothing further worthy of
note. It occasionally happens, however, that certain of the sporangia
referred to above as being developed not in the nutritive solution
but on parts of the roots which are in the damp atmosphere just
above the surface of the liquid, present a dull greyish appearance, in
place of the bright black one described in detail above: in these cases
the thin outer network is infiltrated with carbonate of lime in
varying degrees and may be closer in texture accordingly. This
seems to mask the yellow colour, and the network becomes a coarsely
granular and, it may be, brittle greyish membrane. There is much
variability in this connection however.

Two points of interest present themselves here. ‘First, as already
said, the external granular membrane or network is evidently a
remnant or excretion from the plasmodium, left over in the process
of formation of the sporangium proper from its protoplasm; and
secondly, it depends on circumstances whether it contains much or
little calcium carbonate. My observations on the subject lead to
the following conclusions.

AN AQUATIC MYXOMYCETE, 69

The bright black sporangia developed on the completely submerged
roots have no perceptible amount of lime on their surfaces ; they are
smooth and bright, simply because the yellow network which passes over
them is too fine and translucent to materially affect the appearance of
the pigmented proper membrane of the sporangium (c f. figs. 3—6).
But in the case of the dull greyish sporangia developed on the roots
outside the liquid, the dullness is clearly due to the large quantities of
calcium carbonate deposited as fine granules and crystalline masses in
the outer coat. In some cases the infiltration with lime goes so far
that the membrane is quite brittle and cracks into angular pieces (fig. 5),
and on the addition of dilute acetic acid the crystalline needles (fig. 9)
or granules rapidly dissolve, copious effervescence of carbon dioxide
taking place at the same time. These facts suggest the question
whether calcium carbonate is not perhaps formed in abundance on all
the developing sporangia, but in the case of those submerged in
the water it is possibly carried off dissolved in the carbonic acid
diffused throughout the solution, whereas in the case of those
sporangia developed out of the solution, the surplus carbon dioxide
escapes into the damp air around leaving the crystalline solid calcium
carbonate behind.

That calcium carbonate is actually produced in the interior of the
submerged sporangia also is abundantly proved by the excreted nodules
of that mineral which are contained in the nodes of the capillitium to
be referred to shortly ; of course the conditions under which clusters
of needles or granules of calcium carbonate are excreted in the cavity
of the sporangium might be very different from those referred to above.
The capillitium, as shown in fig. 8, is constituted simply of a loose
irregular network of delicate horny fibres, which may be either colour-
less and nearly transparent, or tinged with the purple-brown pigment
so common in other parts of the sporangium (fig. 5). What may be
termed the nodes of this irregular meshwork are usually enlarged, and
in the triangular or irregularly angular enlargements are deposited the
concretions of granules or crystals of calcium carbonate referred to
above (fig. 8). The fibres or rods forming the capillitium are apparently
solid, and spring from the inside of the wall of the sporangium, their
bases being dilated at the areas of attachment (figs. 5 and 8). The
fibres pass directly across the cavity of the sporangium in some cases,
while in others they seem to terminate freely in it; or they are
connected together, at various angles, with other fibres which cross

70 _ HH, MARSHALL WARD.

them. The meshwork of this rudimentary capillitium reminds the
observer of the ceratose skeleton of some sponges, a similarity which
is perhaps not lessened on examining the texture and composition of
the fibres.

In concluding these remarks on the sporangium, a few words may
be added as to the action of reagents on the structures described.
Even in extreme cases it is possible, after removing all the calcium
carbonate by means of dilute acetic acid, to separate more or less
completely the outer network or membrane from the thin pigmented
proper wall of the sporangium (fig. 5): the latter retains its even
colour and tough elastic character, but the external layer, if much
hardened by calcium carbonate, may be cracked in all directions under
the pressure of the cover slip.

Solutions of iodine alone colour both the decalcified outer membrane
and the horny internal one yellow or brown, and the addition of
sulphuric acid enabled me to detect the blue colouration indicative of
cellulose in some parts at least of the inner membrane. This reaction
is by no means satisfactory however, and the rule is that when it
does make its appearance the blue colour is only in patches bere and
there. The same ig true of the reaction with carefully prepared
chlor. zine iodine (Schulze’s solution): very often the only indication
of cellulose is a pale bluish-green patch here and there in the
membrane. Sulphuric acid causes partial solution of the test and
swelling of the fibres of the capillitium; and the latter are coloured
bright golden brown by iodine, the colour only being rendered deeper
on the addition of sulphuric acid.

Certain variability in these reactions of the walls and capillitium is
no doubt due, in part at least, to the age of the sporangium, as is the
depth of colour of the natural pigment : of course the depth of colour
is affected by the thickness of the membrane also, but this could
scarcely be a serious difficulty. In all the cases referred to the
spores, the only remaining contents of the sporangium, were ripe ; and
I now proceed to describe them.

The ripe spore (figs. 5, 6, 7, and 10) in sporangia which have
become normally mature on roots in the solution, is a spherical cell,
sdovin. in diameter, with a tough homogeneous cell wall, which is
coloured purple-brown, and appears to resemble the proper membrane
of the sporangium in all essential respects. With the usual reagents
the spore membrane yields better cellulose reactions, however, though

AN AQUATIC MYXOMYCETE. 71

even here it evidently does not consist entirely of that substance, but
appears to contain a body (possibly approaching cutin in composition)
of some more resistant nature. In most cases the spores are so nearly
smooth that the extremely fine punctations on their exterior may
easily be overlooked or neglected ; but in other examples the fine
punctation is quite noticeable, and it seemed to me that these more
evidently marked spores were smaller than the average size, though
no differences could be detected in their behaviour. They certainly
germinate as readily as do the smoother spores, and behave in exactly
the same manner during the process. But, as I shall show directly,
there are much greater variations in the size of the spores than these,
none of which seem to affect the purpose or behaviour of the spore in
the slightest degree.

Each normally ripe spore contains a quantity of very pale translucent
protoplasm in or near the centre of which is a paler and brighter round
spot : this is the nucleus shining through the hyaline protoplasm, The
nucleus is particularly clear at the moment when germination com-
mences (fig. 11, 14), but can always be detected in the resting con-
ditions, and also, as will appear subsequently, in other conditions of
the contents of the spore.

With respect to the other varieties of spore referred to above, these
are much rarer than what I have described as the normal form. The
specimens drawn in fig. 13, however, all came from the same sporan-
gium:; they are clearly of the nature of double or triple, &c., giant
spores, or are malformed specimens, the ripening of which is perhaps
completed before the protoplasm has finished dividing up in the young
sporangium. In some cases it is almost certain that they simply result
from an imperfect separation of the formative masses set apart for the
production of spores; an assumption which is supported by the fact
that these malformed or giant spores (or complexes of spores) germi-
nate quite normally, simply giving rise to a larger number of indi-
vidual myxamoebee than do the perfectly isolated spherical spores,
selected, on account of their being the more common, as the typical
spores. Such abnormal spores are by no means confined to this Myxo-
mycete*; apart from the variations in the sizes of the spores of nume-
rous species,” I have myself observed equally striking differences in the
spores of a Myxomycete in the tropics. There can be no doubt, I think,

1 Cf. De Bary, Vergt. Morphol., der Pilze, &c., p. 452.
2 Zopt{, Die Pilz-Thievre, p. 52.

72 H, MARSHALL WARD.

that the differences referred to are of no morphological importance,
and only depend on obscure conditions which make themselves felt
during the development of the spores in the sporangium.

In the case of the typical spores formed in the sporangia on the
submerged roots, the spherical shape is maintained until the moment
of germination ; but in those sporangia which ripen outside the liquid,
it often happens that the ripe spores present the appearance repre-
sented in fig. 12. This, however, is simply due to an infolding of the
spore membrane, consequent on the loss of water, a fact which may
easily be proved by partially drying any of the spores slowly and at a
low temperature ; or a similar result follows if the water is extracted
by means of alcohol or glycerine. In both cases the plump spherical
shape is immediately restored on the addition of water, but of course
the spores long treated with the reagents are killed. Spores contracted
in the former manner, however, germinate at once and normally
exactly as do those taken from the submerged sporangia: if dried very
thoroughly, however, it may require several days before the swollen
Spore germinates in water, and of course it is possible to carry the
desiccation too far unless very great care is taken.

GERMINATION.

The normally ripened spore is at once capable of germination, and
the process is usually completed in from 12 to 24 hours, at ordinary
temperatures. 3

The germination is easily observed as follows. A sporangium
ripened in or out of the water (there are no further differences to be
noted, and the following description applies to spores obtained from
both sources) is broken open by means of a sharp clean needle, and
the point of the instrument is then immersed in a small drop of water
on a perfectly clean glass slide: some of the spores attached to the
needle are left behind in the drop of water, and the whole is then
placed in a moist chamber, and kept at the ordinary temperature of
the laboratory. In a few hours germination commences, and the
escape of the contents of the spores as myxamoebze is completed in
from 12—24 hours, even when the temperature has varied between
20° C. and 15° C.

In order to study the details of this process, and to cultivate the
myxameebee further for even long periods (several weeks) I have
successfully employed a method well known in Germany but still far

AN AQUATIC MYXOMYCETE, 73

too little used in England, and it may be worth while to give a
description of the process, which can of course be made use of for the
cultivation of almost any minute organisms.

A moist chamber is made by cutting a piece of thick cardboard,
or several thicknesses of filter paper, to the size of an ordinary glass
slide (3in. by lin.) and cutting a hole through the centre: this hole
may be about fin. in diameter, large enough to let all necessary light
pass through, but not too large to be efficiently covered by means of a
square or circular thin cover-slip, which will be held close down to the
perforated board or paper when the latter is saturated with distilled
water by the capillary attraction at the edges. The cardboard or
paper pad is then saturated with water, and can obviously be boiled
or heated if necessary, for a short time ; its wet surface adheres firmly
to the glass slip on which it is now placed, and which supplies a
transparent floor to the small cylindrical chamber, the walls of which
are formed by the saturated board or paper, and the roof by the thin
cover-slip. It is obvious that a small drop of water suspended from
the under side of the thin cover-slip, will evaporate very slowly, so
long as the cardboard or filter-paper is kept thoroughly wet, because
the air in the chamber will be nearly saturated with water ; hence
any spores &c. suspended in the drop will run no risk of drying up
for a considerable number of hours. Moreover, it is clear that light can
pass through the whole chamber, and its suspended drop, in sufficient
quantity to enable us to examine what is going on in the drop even
with fairly high powers, provided the cover-slip is sufficiently thin,
and the drop of water not so large that objects falling to the lower
surface of it are out of focus. At any rate it is easy to work with
Zeiss D and E in this way. Another great advantage to be claimed
for these damp cells is the ease with which atmospheric oxygen can
gain access to the interior: the comparatively trivial practical difficul-
ties need not be entered upon here.

It is, of course, difficult to keep Bacteria, &c., out of such damp
cells—heated needles, pure water, treating all the glass parts with
acids, absolute alcohol or heat, &c., are all without avail unless the
water saturating the parts is kept clean, and, of course, unless spores
and foreign bodies are absent from the material sown, &c. Some
remarks on this subject will be made shortly, however, in connection
with certain experiments of physiological importance.

74 H. MARSHALL WARD.

Observed in such a suspended drop of water as that described above,
the first changes noticed in the germinating spore are as follows. The
‘ spore slowly swells, the nucleus being very distinct, and a slight pro-
tuberance makes its appearance at one side: over this protuberance it
is easy to see that the cell wall is distinctly thinner, and hence this
portion of the spore looks paler, from the two facts that the same
amount of pigment is distributed over a larger area of thinner mem-
brane, and the translucent refractive protoplasm shines through with
a sort of pearl-like lustre.

Meanwhile the protoplasm in the interior begins to move very
slowly, and in some cases it is certain that a vacuole is already formed
which begins to pulsate feebly at long intervals. As the protuberance
becomes more pronounced, the movements become a little more active,
and the vacuole pulsates at intervals of from one to two minutes:
minute brilliant granules can now be distinctly observed to change their
positions in the hyaline protoplasm. Very soon after this the lateral
protuberance ruptures, the crack slowly extending (figs. 11 and 14)
and the contents becoming exposed: all these phenomena may occur
in from 4—12 hours after sowing the spores.

In favorably situated spores the contents may now be seen to divide
into two, and two nuclei (in one case two vacuoles also were seen)
make their appearance. In some examples the following phenomenon
was observed : the protoplasm thus exposed to direct contact with the
water suddenly retreated into the spore again, then slowly commenced
to emerge as before and shrank back suddenly again. This was re-
peated several times before the protoplasm escaped as two myxameebee.

In other cases, it may be observed that, in a few minutes after the
appearance of the two nuclei, there are two amoeboid masses of proto-
plasm in the spore, one of which slowly emerges, followed soon after-
wards by the second one. Thus, in fig. 15 the spore had developed
the protuberance at 11.40: this ruptured at 12.10, and the naked
protoplasm rapidly withdrew as if it had sustained some shock on con-
tact with the environment: at 12.22 one of the myxamcebe was seen
slowly emerging (the other one of the two having escaped about one
minute previously was now slowly creeping about on the surface of
the spore) and at 12.24, when nearly completely emerged, it suddenly
withdrew into the spore, as if some shock had irritated it. At 12.26
it protruded again, and in the course of the next minute escaped as a
free myxamoeba. During the course of the next few minutes both

AN AQUATIC MYXOMYCETE, 75

the myxameebee slowly moved about the field, each with a clearly
marked nucleus and a slowly pulsating vacuole, and at 12.32 each of
the active myxamoebz began to come slowly to rest, and rounded
itself off as a cyst. At 1.37 the two spherical cysts were lying in the
same places and their vacuoles only making extremely feeble pul-
sations at very long intervals : shortly afterwards they had completely
come to rest—a condition I shall have more to say about shortly.

It occasionally happens that the process of germination is even less
normal than in the case mentioned above. The drawings in fig. 16
will illustrate this, and I may describe these before proceeding to offer
some explanation of both sets of phenomena. In this case the spores
were sown at 4 p.m., and left to germinate during the night: at 10-30
next morning many of them were germinating, and the figures were
drawn from one that was watched without intermission during the
next two hours. At 10.46 one of the spores (fig. 16, a) emitted its
contents bodily as a spherical mass of hyaline naked protoplasm, in
which were numerous very fine bright granules, and a large well-formed
nucleus: during the course of the next quarter of an hour several
(2 to 4) small vacuoles kept making their appearance and very slowly
pulsating (fig, 16, 6), and a feeble “frothing,” so to speak, of the
protoplasm went on (fig. 16, c) until about 11.20, the granules and
nucleus slowly changing their positions at the same time. At 11.26
(compare the figures bracketted fig. 16, d) three faint lines made their
appearance in the mass of protoplasm, and these disappeared in about
half a minute as quickly as they had come, the slow movements and
“frothing” still continuing: at 11.33 a feebly marked line appeared
half way across the protoplasm, and this, too, only lasted for about
half a minute. Five minutes later (e, fig. 16) not a trace of these
lines could be detected ; but at 11.45, in place of the one not very
conspicuous nucleus seen shortly before, there appeared two nuclei
imperfectly separated by a line passing about half way across the
protoplasm (fig. 16, 7): this line also disappeared during the next three
minutes, but at 12.1 (fig. 16, g) the sphere of protoplasm was unmis-
takably divided across by a line passing between the two nuclei, and
two minutes later these two nucleated portions commenced to separate.
It now became evident that the protoplasmic mass had formed a very
thin membranous envelope on its exterior, and, in fact, the two masses
of nucleated protoplasm escaped one after the other (through separate

76 H. MARSHALL WARD.

apertures) from their envelope, and moved away very slowly as
myxameebee (fig. 16, 7).

In the drop of water containing the spores, the germination of which
has just been described, there were relatively large quantities of organic
debris, as well as Bacteria and numerous Infusoria and other oxygen
consumers, and I have very little doubt that what was abnormal in the
process was due to the want of oxygen caused by these organisms ;
for although it was impossible to show this directly, subsequent
experiments proved that an additional supply of fresh water caused
the dormant myxamoebee to become active again, and I found that
the more free the cultivations were from Infusoria and Monads
especially, the longer the myxamoebze tended to be active. Bacteria
need not be absent, though of course there may be too many of them:
there are even facts pointing to the conclusion that within certain
limits the Bacteria not only do no harm, but even serve as food. The
deprivation of oxygen, then, no doubt accounts for the tardiness of
some of the processes described; while it seems not improbable
that the curious phenomena which took place during the germination
of the specimen to which fig. 16 refers, were connected with nuclear
division, however difficult it may be to follow the connection in detail.

It now remains to describe what may be termed the normal process
of germination which occurs when sufficient precautions are taken to
avoid an accumulation of oxygen-consuming objects in the cultivation
and, it may also be added, when the temperature is a little higher than
before ; I have satisfied myself, however, that the spores can germinate
normally when the thermometer in the laboratory stands at 18° C, and
even lower. I will describe the germination first, and the methods
employed afterwards. i

On the rupture of the thin-walled papilla (¢. f. figs. 11 and 14) the
two myxameebee escape very rapidly, and at once begin to move about
actively, and close examination now shows that each possesses a single
flagellum—a long, very thin, but tapering, stiff cilium, the free end of
which is exceedingly flexible—in addition to the evident nucleus, and
now actively pulsating vacuole. The vacuole slowly expands, taking
about # of a minute to fill, and then closes with a rapid wink-like action,
to repeat the process almost immediately. The nucleus offers nothing
remarkable, so far as I could determine ; it and the fine granules are
in the more internal portions, and are constantly changing their posi-
tions as the amceboid movements proceed. The flagellum stands out

AN AQUATIC MYXOMYCETR, 77

straight and stiff from the scarcely distinguishable ectoplasm, its very
fine flexible tip swaying to and fro with an irregular swinging move-
ment, The pseudopodia are small, numerous, and very irregular (fig.
17a and fig. 18). Very often the myxamceba may be seen to creep
forward, the flagellum in front, for a short time, and then rather
quickly to leave go, as it were, of the glass and subside in the drop,
and at once assume an elongated shape, like that of a club, and steadily
glide forward, suspended in the water as a “‘zoospore” (fig. 170). This
zoospore moves with the flagellum projecting stiffly from the front
pointed end, where the nucleus also is situated ; the pulsating vacuole
appears to be always at the rounded dilated hinder end. The free swim-
ming zoospore-form may then come in contact with the glass surface,
and either assume the ameeboid form and movement at once, or wriggle
about with quick jerking movements (fig. 17¢d) for a time, and then
pass again into either the zoospore or the amceboid form. The out-
lines figured at d for instance are sketches of the forms assumed within
3 minutes by the same zoospore; while those shown in fig. 18 sufii-
ciently illustrate the passage to the amceboid condition.

After a certain time, however, it is noticed that a number of per-
fectly spherical colourless bodies (fig. 17e) are scattered about the
field, and as these increase in number the amcebe and zoospores
diminish : these spheres are encysted myxamcebe, and indeed fig. 17e
was drawn from a specimen observed to pass over from the zoospore
to the amoeboid conditions (d), the latter of which then slowly rounded
off and assumed the encysted state.

Before describing this and other phenomena, however, I may say a
few words as to the methods of culture referred to above. It is obvious
that absolute immunity from foreign organisms cannot be expected
when the sowing is made from sporangia on the submerged roots of
Hyacinth, and moreover it could not be supposed that the results of
germination could be cultivated for any length of time in distilled
water : nor was such the case in the examples cited above, for small
quantities of organic matter from the roots and of minerals from the
solution in which they grew were certainly added during the process
of sowing. .

Tn order to obtain as few foreign organisms as possible, and to be at
least fuirly certain whence they were derived, I employed the following
precautions. A nutritive solution (that used for growing the Hya-
cinths in) was prepared in a large test-tube, and a piece of fresh Hya-

78 H. MARSHALL WARD.

cinth root added: the whole was then boiled. A new close fitting
cork was fitted and provided with a short bent tube and a long one as
for an ordinary wash-bottle, but with the following differences. The
long exit tube dipping below into the solution in the test-tube was
drawn to a fine capillary end, and the free end of the same tube was
drawn out very fine indeed and its end hermetically sealed in a flame.
The short tube had the limb which is placed in the mouth.constricted
in the middle to a narrow capillary, between which and the mouth a
plug of cotton wool was fitted. The fused tip was then broken off and
the whole was ready for use. On blowing gently down the mouthpiece,
the cotton-wool prevents the passage of spores into the liquid, and the
fusion of the exit. end, after a minute drop of the nutritive fluid has
been expelled, effectually prevents spores obtaining access that way.
The other advantages are obvious, and the highly putrescible nutri-
tive solution can be employed day after day for the supply of minute
drops of nutriment to the cultures. While it is not intended to imply
that such a vessel is absolutely proof against bacteria, it is certainly
useful in preventing their being added in serious quantities, and can
easily be quite freed from other spores.

The next precaution was to sow as few spores of the Myxomycete
as possible at a time, and this I accomplished by the well known
method of dipping the needle point, on which the spores to be sown
adhered, into a drop of pure water, a drop from which was then placed
in the culture drop suspended in the damp-cell. Certain other more
obvious precautions may be passed over. The result was that in a
fair proportion of sowings germination proceeded normally and rapidly,
and very few bacteria (and no larger organisms) made their appearance
during the earlier stages; hence the objectionable turbidity was ob-
viated, and the struggle for existence was considerably in favour of
the Myxomycete.

A number of experiments were next made in which the nature of the
fluid was varied: the following sums up the chief results. Germination
takes place normally in pure water (unless the temperature is too low,
or other unfavourable conditions prevail), but the myxamoebee rapidly
pass over into the dormant condition and encyst. In a purely mineral
nutritive solution the same results are obtained. If an infusion of
Hyacinth roots is added to the mineral solution, however, the myxa-
moebee most evidently flourish, and I have kept them and their
progeny alive for as long as 16 days in such a solution. Bacteria

AN AQUATIC MYXOMYCETE. 79

always make their appearance from spores, &c., introduced with those
of the culture; but, as already said, it is not difficult to exclude
Infusoria and other relatively large oxygen-consumers, and the myxa-
moebee seem to flourish so well in the presence of a certain quantity of
bacteria, that the question naturally suggests itself whether the latter
do not favour their growth and nutrition, either by breaking up the
organic matter, or by serving directly as food. This point is not easy
to decide: in cases where the myxamoebee pass over rapidly into a
dormant condition in a solution of nutritive substances, it is difficult
to determine whether a want of oxygen (due to the long-continued
boiling, &c.) or some other cause is acting, and any manipulation
introduces risks which only increase the final difficulty.

For instance, having satisfied myself that something other than pure
water and minerals was absolutely necessary for the nutrition of the
myxameosbe, the next question was, is organic matter (derived from
boiled Hyacinth roots, e.g.) in solution the only other ingredient neces-
sary? and in the successful cultures one always felt that the answer
was obscured because there were always bacteria present, and three
obvious possibilities suggested themselves: (1) the myxameebee might be
feeding simply and directly on the dissolved substances in the original
solution ; (2) they might be absorbing substances that only arose after
the action of the bacteria on the original solution ; and (3) they might
be chiefly nourished by the bacteria themselves, since they certainly
envelope bacteria after the usual manner of amoebee. It thus follows
that my experiments only prove that organic matter in some form is
necessary for the nutrition and growth of the myxameebze, but do not
decide in what form it is absorbed. I regard the last question as
important with respect to any hypothesis as to the animal or vegetable
characteristics of the ameeboid stage, though a positive answer even to
this question could by no means be regarded as conclusive of the
animal nature of the organism, in view of the other facts to hand.
It may be shortly remarked, in this connection, that the conditions
of cultivation in my damp-cells must have been remarkably similar
to those which the myxameebee meet with in the roots on the glass

vessels. JI may now pass on to the other phenomena observed in the
life of the myxameebee.

|

80 H. MARSHALL WARD.

Division."

It is easy to convince one’s self of the fact that the myxamoebee divide.
I have several times observed this process directly and in detail ; but
it might also be safely concluded from the fact that in sowings of from
8 or 9 to 20 or 30 spores, which yielded twice that number of myxa-
moebee on germination, it was frequently observed that the number of
myxameebee increased several fold in the course of two or three days.
In figs. 19 and 20, however, are drawings of the process as actually
observed-in successful cultures. A myxameeba was detected in the con-
dition at a (fig. 19) at 10.48, having become extended and slightly
constricted in the middle, where it was relatively at rest, though
several small pseudopodia were being protruded and withdrawn at the
ends. No nucleus was detected, but two pulsating vacuoles were
clearly visible. Two minutes later the constriction had deepened con-
siderably, and at 10.52 (two minutes later still) it had cut the mass
in two (fig. 196 and c), and a nucleus as well as a contractile vacuole
was observed in each. The two daughter amcebee remained quiescent
and in contact for a few minutes, and at 11 o’clock were again active
and moving away. In some cases (fig. 20) the process was varied to
the following unimportant extent. The dividing myxameeba (fig. 20a)
was extremely sluggish, and then elongated very slowly as in the
figure: two nuclei were now observed in it. This was at 1.17 o'clock.
Division then followed quickly, the nuclei not being noticed during
the process. The stage d was attained at 1.20, ¢ at 1.22. The two
separated daughter amoebee remained quiescent for several minutes,
then slowly moved apart (d) to a very little distance, and then
encysted. Here again I have reasons for thinking that deficient
oxygen had to do with the sluggishness noticed in some cases.

Enoystep SracE.?
As already stated, the myxamecebee become dormant, round off into
a sphere, and form a delicate colourless membrane when oxygen is”
deficient in the culture, or the temperature is very low, or from other
causes. This deficiency of oxygen may arise simply from the fact that
the myxamoebze have themselves increased so rapidly by division that
the supply of oxygen is unequal to their demand ; or it may be due
to other causes, among which the presence of Infusoria and other

1 Cf. De Bary, Vergleichende Morphol. und Biol. der Pilze, &c., p. 455.—Zopf, Die Pilz-
Thiere, p. 10.
2 Cf, De Bary, op. cit., p. 46, and Zopf, p. 99

AN AQUATIC MYXOMYCETE, 81

oxygen-consumers is the chief. While not denying that the myxa-
moebee may be impelled to encyst from lack of food, or when the
temperature is too low, there is reason for ascribing these dormant
states chiefly to a lack of oxygen, since the myxamoebe may be re-
awakened to activity on the mere addition of pure water containing
oxygen, though the temperature (not too low) remains the same.
Beyond the fact that such cysts may lie unchanged for 12 or 14 days,
and then be restored to activity, I do not know how long they retain
their vitality.

In fig. 21 (a) is a cyst kept under observation (with seven others)
for some time: after resting for two days, a minute drop of fresh
nutritive solution was added, and three hours later it and its com-
panions were beginning to germinate. The process (¢f. fig, 21 ¢ and d)
is very simple, the extremely thin membrane opens by a minute pore,
evidently formed by the myxameeba itself, through which the whole of
the protoplasmic contents pass out as an extremely active myxameeba,
which soon shows the presence of a cilium. The pulsating vacuole is
formed before its escape ; the nucleus is evident all through the dor-
mant state: the flagellum seems to be protruded after the escape of
the myxameeba. In some cases the extremely fine colourless envelope
was dragged for a short distance by the myxameeba, but it soon fell
off, and the tiny round hole through which the contents had escaped
could then be detected. Such empty cysts were often to be seen in
certain cultures which had been allowed to dry up and were then
wetted, and as they increased the number of encysted myxamoebee
decreased. Moreover, it was by no means difficult to see the process
of escape of the contents, as in the specimen figured ; I have watched
the process from beginning to end at least five times. It only remains
to add that the free myxamoeba may feed, grow and divide, or may
again encyst, according to circumstances ; or, finally, under certain con-
ditions, which I have failed to refer to anything external, the myxa-
moebee may commence to congregate in masses and subsequently form
plasmodia. I have failed to obtain satisfactory evidence as to the con-
stitution of the extremely thin wall of the cyst: it is, presumably,
cellulose, but its delicacy is such that reagents destroy every trace of it
before one can be-assured of a colour reaction. It is not improbable
that this encysted stage is far commoner in myxameebe than has
hitherto been supposed, for the very delicate transparent empty cases
are almost sure to be overlooked at first.

G

82 H. MARSHALL WARD.

THe Puasmopium.'

On the roots of the Hyacinths on which the sporangia were situated,
it was not difficult to find larger and more complex amceboid masses
of protoplasm (fig. 24a) and among the sediment at the bottom of the
glass cylinder occurred very similar specimens (fig. 246) which, how-
ever, seemed as a rule less active. Since those on the roots were
mingled with myxamecebie, bacteria, spores of the myxomycete, and
sporangia in all stages, it may no doubt be inferred that they are
the plasmodium produced by the fusion of the myxamobe. The
inference becomes very near certainty after watching the specimens
under cultivation. In fig. 22 are drawings of myxamcebe from a
much larger aggregate which were found in a culture kept under
observation for 11 days. These myxamocebee were cultivated from
the spores, and had passed through the stages of division and encyst-
ment, and by this time outnumbered the original spores at least
eighty or a hundred to one. They were now moving sluggishly about
on the glass, and also on the lower surface of the suspended drop of
nutrient fluid: the nuclei were quite distinct, the vacuoles pulsating
slowly and at long intervals, and the flagellum appeared to have
been withdrawn in all cases. It was not until I had seen such
cases, where the sluggish myxamcebze glide slowly over one another
for some time, that it became clear that this is a preparatory
stage to the formation of plasmodia. I then had opportunities of
examining the process more in detail: as the myxamcebee approach
more closely to one another, they slowly glide over one another, in
undoubted contact, but without sticking or blending together in the
least. It appeared at times as if an extremely delicate investment
separated them—if such an investment exists, however, it must
follow every movement of the protoplasm. As fig. 25 shows, some of
them may still retain the flagellum, but the majority have lost it, and
all do so eventually: the conditions figured at a to ce (fig. 25) were
drawn at intervals of 2 minutes, and d 5 minutes after c, to show
this slow “swarming” process, as it might be termed.

Later on, after from 12 to 30 hours, several relatively large plas-
modia were found on the slide, and that these were formed by the
fusion of the myxameebee was evident from the fact that the process
was still going on. As shown in fig. 23, larger masses of plasmodium
were being added to by smaller ones and by myxamebe, which now,

2 Of, De Bary (op. cit., p. 455), and Zopf, p. 22, for a general description of Plasmodia.

AN AQUATIC MYXOMYCETE, 83

instead of gliding over the surface stuck to it and were either at once
drawn in, as it were, or partially separated off and then again became
part of the larger mass. In some cultures I had as many as eight or
nine separate plasmodia slowly moving on the glass for two or three
days.

One of the largest (more than a millimeter in length) is drawn at
fig. 26, and a description of it will serve once for all. Its changing
outlines were very irregular: short and long pseudopodia would be
put forth together or separately, and withdrawn or extended, the fine
granules (as well as fewer larger ones and nuclei) flowing quickly
down the central portion. The general hue was a pale dirty yellow,
the very clear ectoplasm being colourless, in thin parts at any rate.
There were several or even many contractile vacuoles varying in size
and activity. Bacteria, spores, empty cysts and other objects were
frequently observed in the plasmodium, and were also seen to be en-
veloped by its pseudopodia ; some to be carried into the interior, others
to become free again as the pseudopodia withdrew. The following
curious phenomenon was observed more than once. The actively
moving plasmodium would come to rest in irregular or rounded clumps
during the night (fig. 27), possibly on account of the lowered tempera-
ture, and would then spread out again next morning, and move as
before: this would be repeated—even several times. In other cases
this dormant condition would last a longer time, and a distinct invest-
ment (the character of which was not made out exactly) would be
excreted (fig. 27a and 6) suggesting an effort on the part of the plasmo-
dium to produce a sporangium.* No undoubted sporangia were formed
in the suspended drops, however.

There can be little doubt that external conditions have to do with
these changes ; but although I have notes on the subject which show
that at certain stages the myxamoebee collect at the lighted side or on
the free surface of the drop, while at others they choose the surface
of the glass generally, and that the plasmodia seem to favour the
upper regions of the drop, they are too scanty and not sufficiently
decisive for the purpose of determining the questions which arise.

The failure to produce sporangia in the above cultures, and the
observation that the plasmodia tend to the upper regions of the drop,
however, led me to try the following experiment, which was at any

» Or they may be the “ macro-cysts” described by Cienkowski in Pericheenia.—Ct. Zopf,
p. 92.

84 H. MARSHALL WARD.

rate partially successful. Having cultivated several fairly large plas-
modia, I inverted the cover-slip (fig. 28c) and suspended vertically
from a glass filament (/) supported by a cork (e) a piece of fresh clean
Hyacinth root (4) so that its tip plunged into the drop (d) containing
the plasmodia. The whole was then placed within a larger damp
chamber, and the root kept thoroughly wet: in several cases the plas-
modia crept on to the root, and in one example I believe that a plas-
modium commenced to form a sporangium. Further than this I was
not able to go, and so the matter rests for a time, my investigations
being brought to an end by the pressure of other business.

It now remains to say what can be decided as to the systematic

position of this aquatic myxomycete.

CLASSIFICATION.

Rostafinski’ divides the more typical Myxomycetes (‘‘Endosporese”)
into seven groups or “orders,” characterised chiefly by the presence
or absence of lime in the fructification, the colour of the spores,
presence or absence of a capillitium and columella, &c. According
to this classification the present Myxomycete would be included under
Rostafinski’s order VI. Calcaree, and of the four “tribes” into which
these are divided, we may at once discard the fourth, Spwmariacee, on
account of the columella there present. If the nature of the deposits
of calcium carbonate—in crystals or amorphous—is really to be
regarded as important, which may well be doubted, then the tribes
Cienkowskiacee and Physaracee may be also put aside as not
including the present species, and we are limited to the Didymiacee.
But here there is also a columella, and so on; the fact being that the
aquatic myxomycete I have described would, if certain characters were
insisted on, impel us to connect two or more of Rostafinski’s “ tribes.”

If we now follow the classification proposed by Zopf,? we may at
once discard the first group of his extended system—the Monadinew —
for, although they are usually aquatic, they form no true plasmodia.
Turning to the Humycetozoa, which Zopf characterises as “ Lafthewohner”
however, there are three sub-groups to notice ; the first (Sorophorec)
have no zoospore stage, the plasmodia are not typical, and there are
other characters which at once exclude the present Myxomycete : the
third group, comprising the one genus Ceratium, may also be forthwith

1 Versuch eines Systems der Mycetozoa. (Inaug. diss., Strassburg, 1873.)
2 Zopf, op. cit., p. 95,

AN AQUATIC MYXOMYCETE, 85

discarded. Under the second group (Hndosporec) it is not difficult to
refer our species to the Hndotrichece, characterised by possessing a
eapillitium which traverses the lumen of the sporangium, and consists
of solid fibres (“stereonemata”). Here we are limited to two of
Rostafinski’s “ tribes ””—the Calcariacece and the Amaurocheetacece, and
a comparison of their characters at once places the present form among
the Calcariacee. Of the three ‘ families” into which Zopf distributes
these genera, the second (Didymiacew) seems to embrace the characters
of our form, and it seems very likely to belong to the genus Diderma,
having several of the characters of that genus well marked. It is,
indeed, not improbable that we have here an aquatic form of D. difforme,
one of the commonest of our Myxomycetes, and if so, we have another
proof of the all but uselessness of attempting to classify the lower
organisms, until we know more of their habits under varying con-
ditions. In any case, some of the facts here described show that
characters for the description of “genera” and “species” of Myxo-
mycetes should be chosen very carefully.

86 H, MARSHALL WARD.

DESCRIPTION OF PLATES III—IV.

Fig. 1. Sporangia of Myxomycete on the roots of Hyacinth. Natural
size.

Figs. 2, 3 & 4. Sporangia magnified. o, the yellow border. Figs,
3 & 4 are attached to the epidermis of the root: the
spores can be seen within.

Fig. 5. Portion of wall of sporangium, seen from within, and showing
places of origin of capillitium.

Fig. 6. Portion of wail seen from outside, with looser yellow net-
work.

Fig. 7. Sporangium bursting, and liberating spores. >

Fig. 8. Burst sporangia, showing capillitium i situ.

Fig. 9. Spicules of calcium carbonate from walls of sporangium.

Figs. 10 & 11. Spores commencing to germinate in water.

Fig. 12. Spores after desiccation, after lying in alcohol.

Fig. 13. Abnormal and giant spores, drawn in outline only.

Figs. 14, 15 & 16. Spores germinating, in various stages (wde pp.
74 seq.)

Fig. 17. Myxamcebee and zoospores which have emerged from the
spores. Each has a cilium, contractile vacuole, and
nucleus.

Fig. 18. A myxameeba sketched at short intervals.

Figs. 19-20. Division of myxamceba: stages at short intervals.

Fig. 21. Germination of an encysted myxameeba.

Fig. 22. A group of myxameebee.

Figs, 23—24. Plasmodia resulting from the fusion of myxameebee.

Fig. 25. Myxameebee prior to their fusion into plasmodia.

Fig. 26. A large plasmodium.

Fig. 27. Resting condition of plasmodia previous to the formation of
sporangia. ;

Fig. 28. Apparatus used for cultivation of plasmodia (vide p. 84).
a, glass slip ; 4, fibulous paper cell; c, cover slip; d, drop
of water containing myxameebee and plasmodia ; e, a cork,
supporting (h) a piece of Hyacinth root by means of the
glass rod f.

H.Marshall Ward, del.

Plate Ill.

= F. Huth Lith? Edin?

Ni

ON,

OBSERVATIONS ON THE CRANIAL NERVES OF SCYLLIUM.

By A. Mityzs Marsuatt, M.A., D.Se., Professor of Zoology in Owens
College ; and W. Baupwin SPENCER, of Owens College.

[Prates V—VI.|

The investigations recorded here were undertaken in the first instance
for the purpose of controlling certain determinations published by one
of us in a previous number of this journal’ concerning the cranial
nerves of Elasmobranchs. To this end we have carefully re-examined
the specimens upon the investigation of which the former account
was based, and have, in addition, made a large number of new
preparations, illustrating more especially the later stages of develop-
ment—stages m to Q of Balfour’s nomenclature.”

During the course of our work so many altogether new and unex-
pected points were brought to light, that we soon found it necessary
to widen considerably the scope and limits of our investigations, and
have finally been led to attempt a complete account of the development
of the cranial nerves from stage K to the adult form, and to endeavour
in this way to connect together directly the accounts previously given
of the early stages® with the descriptions of the nerves of adult Elas-
mobranchs published by Stannius,* Gegenbaur,’ and other anatomists.°

1 Marshall, ‘‘On the Head Cavities and Associated Nerves of Elasmobranchs,” ‘ Quart.
Journ. Micr. Sc., Jan., 1881, pp. 71 seg. Future references will be to this paper unless
otherwise specified.

® Elasmobranch fishes, pp. 79 and 80.

5 Balfour, op. cit. Marshall, loc. cit.

* Stannius, ‘Das peripherische Nervensystem der Fische,’ Rostock, 1849.

5 Gegenbaur, “Die Kopfnerven von Hexanchus,” ‘Jenaische Zeitschrift,’ Bd. vi.

. © Hsp. Jackson and Clarke, ‘‘ The Cranial Nerves of Echinorhinus spinosus,” ‘Journal 0
Anatomy,’ vol, x.

88 PROF. MARSHALL AND MR. SPENCER.

Owing to defective supply of materials, our observations on the
stages earlier than K are too fragmentary to be relied on ; this we greatly
regret, inasmuch as many features in the early stages are of extreme
importance, and would well repay thorough investigation.

In the present paper we propose to confine ourselves to the
consideration of the preauditory nerves, reserving the postauditory,
which present many features of peculiar interest, for a future occasion.

Our investigations have been conducted almost exclusively by means
of sections of hardened embryos of Scylliwm, and, as on former occasions,
we have found a mixture of chromic and osmic acids superior to any
other hardening agent. For the specimens from which our best
preparations have been made we are indebted to the courtesy of the
managers of the Southport Aquarium ; our thanks are also due to
Mr. A. J. Moss, of Owens College, for his gift of a fine specimen of
Mustelus, as well as for valuable assistance in connection with the
literature of our subject.

Tue THirD (OcuLomoror) Nerve.—We do not propose to deal in the
present paper with either the olfactory or optic nerves, inasmuch as
the former has been already fully described,t while concerning the
latter we have nothing new to communicate ; we therefore commence
with the third or oculomotor nerve.

Concerning the development of this nerve we have very little to add
to the account given in the paper already referred to.2 At stage L it
arises from the base of the mid-brain, not far from the mid-ventral
line, by a large posterior ganglionic root and by several smaller anterior
ones clearly distinguished from the former by possessing no ganglion
cells. The nerve itself runs backwards as a long slender stem to the
interval between the first and second head cavities (fig. 10, 1—2),
where it expands into a ganglionic swelling (fig. 10, cg.) wedged
in between the tops of the two cavities. From this ganglion the two
main branches of the third arise ; of these the upper one (fig. 15, m1 4),
at a rather later stage, supplies the rectus superior and rectus internus
muscles, whilst the lower one (fig. 15, m1 ¢c), runs down behind the
rectus inferior, and ends in the obliquus inferior muscle (fig. 15, 0.2.).
At stage K, at which our observations commence, the third nerve has
the same point of origin and the same relation to the head cavities ;

1 Marshall, ‘‘ Morphology of Vertebrate Olfactory Organ,” ‘Quart. Journ, Micr. Sc.,’
July, 1879, pp. 300 seq. :
2 Marshall, loc. cit., pp. 78 seq.

THE CRANIAL NERVES OF SCYLLIUM. 89

it differs from the condition described above chiefly in not possessing
anterior non-ganglionic roots, and in not having its terminal branches
fully developed. At stages later than N the nerve seems to arise from
the base of the mid brain by a single large ganglionic root, no smaller
non-ganglionic ones being visible (fig. 14, 11), though at stage n itself
they are very prominent (fig. 13, 11).

Besides the branches of the third nerve, mentioned above, there are
two others in direct connection with the ganglion cg.: of these the
first, at stage L (fig. 10, W.c.), is a short nerve, which lies along the
top of the second head cavity and serves to connect directly the
ganglion c.g. of the third nerve with the Gasserian ganglion at the
root of the fifth.

The second (figs. 10, 11, and 12, 7) passes straight forward from the
ganglion e.g. running through the walls of the first head cavity under
the rectus superior and rectus internus, and through the substance of
the sclerotic on the inner surface of the eyeball. Passing out from the
orbit, immediately above the obliquus inferior, it still pursues its
course straight forward, but becomes more superficial. In the earlier
stages it crosses the root of the olfactory nerve, with which it lies in
very close contact : in the later stages (figs. 12 and 15, w) it is rather
more dorsally situated, and crosses the ophthalmic branches of the
fifth and seventh nerves at a considerable angle (fig. 12, n), ending
in the skin at the extreme fore part of the head. After careful and
repeated examination we have failed to detect any branch given off
from the nerve at any point of its length.

At stage K all the above-mentioned branches of the third nerve are
developed, except the upper branch, to the rectus superior and rectus
mmternus, which we have failed to detect: the ganglion c.g. is very
conspicuous, and the nerves V.c. and WV. have the same structure and
connections as at stage n, the latter of the two stretching forward to
the extreme anterior part of the head, in the skin at which it ends.

In the later stages the modifications which the third nerve
undergoes are merely ones of detail, all the principal branches of the
nerve being already established, and maintaining their relations prac-
tically unaltered in the adult. The most important changes concern
the ganglion c.g.; this, which at stages K and L is a large prominent
swelling (fig. 10, ¢.g.), in the later stages becomes far less conspicuous,
and the ganglion cells, instead of being concentrated at one spot,
occur in small scattered patches at different parts of the nerve. This

90 PROF. MARSHALL AND MR. SPENCER.

change is seen commencing at stage N (fig. 11, ¢.g.), where the ganglion
has divided into two main portions, one part retaining its original
position, whilst the other becomes connected with the nerve J. at
some little distance from the third nerve; at stage o-p (figs. 14 and 15)
ganglion cells appear to be constantly present at two well-marked
points in the course of the third, (1) where the nerve JM. is given off,
and (2) immediately above the rectus superior. .

At stages K and t (fig. 10) the angle between the nerves Wc. and JW.
is very considerable, and this increases in the later stages so much
that at stage n (fig. 11) the two nerves are almost at right angles to
one another; at stages later than this the nerve WV. is much more
difficult to define, whilst owing to the close proximity of the third to
the fifth nerve it is only with extreme difficulty that the nerve . c.
can be distinguished at all.

We find, therefore, that the main stem and the branches 1 6 and
111 ¢ of stage N become directly the nerves which have the same course
and relations in the adult. The ganglion c.g. becomes the ciliary
ganglion of the adult. The nerves WV. and V.c. become directly
ccentinuous with one another, and together form the nerve known as
the Ramus ophthalmicus profundus. The discussion of the morphological
import of these two very remarkable nerves we postpone till after the
description of the fifth and seventh nerves.

Tue Fourtu (Patuetic) NeRvE.—Concerning the development of the
fourth nerve no description has yet appeared, and though our
observations do not enable us to give a complete account, still, so
far as they go, they are of so definite a character that we think it well
to record them here rather than wait for the possibility of completing
them at some future time.

The condition of the fourth nerve at stage n is well shown in figures
11 and 13, of which the former shows the greater part of its course,
and the latter its root of origin. ‘The nerve arises’ (fig. 13, 1v) from
the dorsal surface of the extreme hinder border of the mid-brain, so
far back indeed that very careful examination is necessary to
determine that its origin is really from mid and not from hind-
brain. The roots of the two nerves are in close contact on the
dorsal surface of the brain.» From its root each nerve runs at

+ Marshall, loc. cit., p. 87, and Schwalbe, ‘Das Ganglion Qculomotorii,” ‘den. Zeit,’
Ba, xiii,

THE CRANIAL NERVES OF SCYLLIUM. : 91

first almost directly outwards, following the curvature of the brain,
until it comes to lie a very short distance below the surface ; it then
runs backwards and downwards as a long, straight, and very slender
nerve (fig. 11, 1v), which very commonly branches early in its course,
and terminates in the obl/iquus superior muscle. Just before reaching
the muscle (fig. 11) the fourth nerve crosses the ophthalmic branches
of the fifth and seventh nerves at right angles, lying at a slightly
deeper level than these nerves. It also divides into a number of
branches, of which one or more appear to become connected with
similar branches from the ophthalmic branch of the fifth, the remainder
entering the obliquus superior, whilst in the later stages (fig. 16) the
main nerve is seen to come into very close contact with the two
ophthalmic branches themselves of the fifth and seventh nerves.

It will be seen from the above description that the fourth nerve has
by stage N acquired its adult relations and distribution, and at stages
o-P (fig. 15) and Q (fig. 16), where it is shown in transverse section,
it is still the same. In the adult its course, like that of the other
nerves, becomes altered owing to the varying rates of growth of the
brain, the skull, and the face; the rapid growth of the skull relatively
to that of the brain causing the fourth nerve to run some distance
forward within the brain case before passing out, when, as before
stated, it runs immediately beneath and in very close contact with
the ophthalmic branches of the fifth and seventh nerves.

In attempting to trace the fourth nerve in stages earlier than N we
have met with considerable difficulties, and have hitherto obtained
only a moderate amount of success. Atm the relations are the same as
at N, the sole difference being that the nerve rs more slender than at the
later stage. At stage L, though we have examined a very considerable
number of specimens in excellent histological preservation, we have
as yet recognised the fourth nerve in one specimen only, and even in
that one not with absolute certainty. In the specimen in question
the nerve has the same position and relations as at N, but is very
much more slender, so thin, in fact, as to be almost unrecognisable.
Earlier than t we have failed, after the most careful search, to find
any trace of the nerve.

Though our observations are imperfect they yet seem to point te
certain conclusions of importance touching the morphology of this
important nerve. In the first place the fourth nerve is the only one
in the body which, in the adult, arises from the dorsal surface of the

92 : PROF. MARSHALL AND MR. SPENCER.

brain; it is, therefore, of great importance to notice that from the
very earliest stage at which we have seen it the point of origin is that
of the adult nerve.t Inasmuch as the majority of the cranial nerves,
as well as the dorsal roots of the spinal nerves, arise at jirst from the
dorsal surface of the brain or spinal cord, it seems natural to suggest
that the fourth nerve differs from all the rest, not in its mode of
origin, but in the fact that, whilst all the other nerves shift their
attachment to a greater or less extent, it alone preserves the primitive
position of its roots of origin. This shifting of the roots is, in part,’
due to the rapid growth of the dorsal part of the brain forcing the
roots of the two sides from each other; and it becomes of interest to
notice that the fourth nerve arises from a portion of the brain where
this rapid growth of the roof does not occur, and where, consequently,
one cause of the change in the other nerves is absent.

The fact that the direction of the fourth nerve is at first at right
angles, or nearly so, to the axis of the part of the head from which it
arises is of importance, as showing that the fourth nerve comes under
the category of segmental nerves ;* and inasmuch as there is no room
as regards visceral arches and clefts for a segmental nerve between the
third and fifth the fourth would probably be rightly viewed as a
separated branch of the third—the only other nerve arising from the
mid-brain.

On the other hand, certain other facts in connection with the fourth
nerve cannot be explained so easily. Thus, segmental nerves not only
arise from the neural crest ; they also arise early, are from the first
large, and have ganglia at or near their roots. Now, so far as our
observations go, the fourth nerve, though it may possibly arise from
the neural crest, would not appear to arise early, and certainly is not
at first a large nerve, being of much greater size at stage o-P (fig. 15)
than at stage N (fig. 11); whilst at stage x, in the single specimen in
which it has been detected, it is a nerve of extreme slenderness ;
moreover, at no part of its length have ganglion cells been found—an
important point of difference from segmental nerves.

Another curious feature concerning the fourth nerve is that, in our
embryos, it appears almost constantly to divide close to its root into
two or even more branches, as is well shown in fig. 11, 1v. These

1 Balfour, ‘Elasmobranch Fishes;’ pp. 156 and 191.

2 We say ‘‘in part,” because it will be shown further on in this paper that another process
contributes greatly to this shifting.

8 Marshall, ‘‘ Morphology of Olf, Organ,” p. 318, ‘Quart. Journ, Micr, Se,,’ July, 1879,

THE CRANIAL NERVES OF SCYLLIUM. 93

again subdivide near their terminations (figs. 11 and 15), but all the
branches, whether primary or secondary, are distributed to the superior
oblique muscle, with the possible exception of a few of the smaller
ones, which appear to join the ophthalmic branch of the fifth: We
would suggest that this branching may possibly be an indication of
the fourth nerve having previously had a more extended distribution
than its present very limited one.

On the whole, our observations lead us to believe that the fourth
nerve is to be regarded as a separated part of that segmental nerve of
which the third nerve forms the main portion. A further suggestion
concerning the fourth nerve will be made after the seventh nerve hag
been considered.

Tue Firrn (TriceminaL) Nerve. —We propose to consider separately
the roots and the branches.

A. The roots of origin.—The earliest stage in the development of the
fifth nerve that we propose to treat of in the present paper is that
represented in fig. 1, taken from a transverse section through the
hind-brain of an embryo at a stage intermediate between 1 and x, the
plane of section passing on the left side through the roots of the
trigeminal (v).

As shown in the figure, the roof of the hind-brain is very thin, and
passes rather abruptly into the much thickened sides. To the top of
the thickened portion the fifth nerve is attached by a long slender
root (fig. 1, va). This root passes down alongside the brain, but not
in actual connection with it, widening considerably as it does so. Its
inner border follows the curvature of the brain until reaching a point
about half way down the sides of the neural canal; it then turns
suddenly outwards, leaving the brain altogether, and forming, as it
does so, a very conspicuous blunt projection (fig. 1, v 8), which is in
rather closer proximity to the brain than is the part of the nerve
immediately above it. The trunk of the nerve then passes outwards
and downwards, lying just beneath the superficial epiblast, between it
and the outer wall of the second or mandibular head cavity (fig. 1, 2).
The whole of the nerve, including its root, consist of closely-packed
Spherical or polygonal cells, which, like all nerve cells in the early
stages, stain very deeply with osmic acid, and differ materially in
appearance from the much less closely arranged mesoblast cells.

By stage x the root of the fifth nerve has undergone very remarkable

94 PROF, MARSHALL AND MR. SPENCER,

changes ; as shown in fig. 4, the dorsal attachment (fig. 1, v a) to the
top of the thickened side of the brain has disappeared completely, and
the nerve is now attached to the brain at a point about half way down
the side (fig. 4, v 8), te. at a point exactly corresponding to the
conspicuous projection (fig. 1, v 2) already described at the earlier stage.
Immediately beyond the root of origin the nerve enlarges, suddenly,
and presents a distinct dorsal projection at the base of the secondary
root of attachment. Although hitherto we have not succeeded in
following all the intermediate stages, and have not yet obtained
satisfactory preparations of the stages earlier than that drawn in fig. 1,
yet we feel justified in putting forward the following explanation of
the appearances we have just described, relying for our justification
partly upon the description given by Balfour, and still more on our
own observations on the development of the roots of the seventh nerve,
which will be described immediately.

Balfour has described and figured the fifth nerve as arising at “stage
a, near the anterior end of the hind-brain, as an outgrowth from the
extreme dorsal summit of the brain, in identically the same way as
the dorsal root of a spinal nerve.”*

He has further described? how, by the growth of the roof of the
brain, the nerves of the two sides, which at first are in contact dorsally,
shift their position and become widely separate. His descriptions and
our own somewhat fragmentary observations on these stages, when
considered in connection with our much more complete series of
observations on the seventh nerve, leave no room for doubt that the
root of origin (v a) shown in fig. 1 is the primary root, the one which
at stage G was situated at the top of the brain, and which has acquired
its present position merely in consequence of the rapid growth of the
roof of the brain pushing its two lateral halves apart, and so separating
the roots of the nerves.

Concerning the root of attachment (V @) shown at stage Kk in fig. 4
there is more room for dispute. Balfour appears to hold® that this
further change in position is due to the same cause as the former one,
2.e. to growth of the roof of the brain ; but this explanation, while it
would fully account for the first change, would in no way explain such
a shifting of the root down the thickened side of the brain, ag is

1 Op. cit., p. 191, and Pl. XIV.,, fig. 3.
2 Op. cit., p. 196.
3 Op. cit., p. 196.

THE CRANIAL NERVES OF SCYLLIUM. 95

clearly seen to have occurred on comparing fig. 4 with fig. 1. We
believe that what really happens is that about the commencement of
stage K the nerve acquires a new and secondary connection with the
brain at the point (v 8) opposite the projection already noticed ; that
the primary attachment (v a) is lost ; and that the part of the nerve
left above the secondary root rapidly diminishes and ultimately
disappears altogether, the slight dorsal projection already noticed in
fig. 4 being the last rudiment of it. As our arguments in support of
this view depend almost entirely on our own observations on the
development of the roots of the seventh nerve, we postpone further
considersetion of the point till a later portion of this paper.

At the commencement of stage K then, the fifth nerve arises about
half way up the sides of the hind-brain by a single large root in which
ganglion cells are abundant, and opposite to which there is a well-
marked external bulging of the walls of the brain ;' beyond this root
the nerve expands suddenly into a very large ganglionic swelling, the
future Gasserian ganglion.

Before the close of stage K additional roots appear; a long, slender
process runs forward from the antero-superior angle of the Gasserian
ganglion, and becomes connected with the brain some distance in front
of the main root; in addition to which one, two, or more roots of a
similar kind appear in intermediate situations.

These anterior roots of the fifth nerve are well shown at the next
stage (L) in fig 10, v y, which shows that the fifth nerve at this time
arises from the brain by three distinct roots, of which the posterior
one is much the largest, and is the ganglionic root (v /) of fig. 4, while
the two anterior slender non-ganglionic roots are the new ones. We
have studied these roots very carefully, but have been unable to
determine with certainty whether they are outgrowths from the brain
to meet the ganglion, or from the ganglion towards the brain; dur
cbservations, however, though inconclusive, tend very strongly towards
the latter of the two alternatives. It has also occurred to us that
these new roots may possibly be the original primary root of origin
(v a, fig. 1), which, after losing its original attachment, has acquired
a new one lower down ; however, though the dates of the disappearance
of va and appearance of v y agree fairly well with this hypothesis,
we have yet no actual observations in its favour, and do not wish to
lay stress upon it.

: 1 Marshall, loc. cit., p. 84.

96 PROF. MARSHALL AND MR. SPENCER.

These anterior roots which, during stage L, may be three or more
in number, appear in the later stages to be very constantly reduced to
two, one of which is the most anterior of the original roots while the
other appears to be formed by the fusion of the intermediate ones.

This condition at stage N is well shown in fig. 11, vy. Ata stage
between o and P (fig. 14, vy) they are rather less conspicuous owing
to the interval between them and the secondary root (v 6) being
filled up by dense tissue. They are clearly recognisable in the adult,
and form, as will be noticed more fully further on, the first or anterior
root of the fifth nerve of zootomists.

B. The branches of the fifth nerve-—The Gasserian ganglion is, from
its first appearance, wedged in between the dorsal ends of the second
and third head cavities in the same manner as is the ciliary ganglion
between the first and second (fig. 10). From the Gasserian ganglion,
at stage K, two nerves arise ; of these, one, which runs straight down
between the second and third head cavities, and then along the
anterior border of the mandibular arch in front of the ventral portion
of the cavity in the latter, is the mandibular branch ; and from this,
a considerable distance below the ganglion, a small anterior branch—
the maxillary nerve—is given off. The second branch from the
Gasserian ganglion arises from its anterior inferior angle, runs along
the top of the first head cavity and joins the ciliary ganglion ; it is
the communicating branch between the fifth and third nerves already
mentioned, and forms the proximal part of the ramus ophthalmicus
profundus of zootomists.

At stage L a slender branch arises from the anterior superior angle
of the Gasserian ganglion ; this is the ophthalmic branch of the fifth
(fig. 10, v a) which runs forward through the orbit dorsal to all the
eye muscles, giving off branches to the neighbouring parts in its
course and terminating in the skin of the fore part of the head. One
other nerve in connection with the fifth remains to be noticed ; this is
the connecting branch (.c’., fig. 10) between the fifth and seventh
nerves: this is present at K, at which stage as well as at L it forms a
very stout though short nerve, running forwards and downwards from
the seventh nerve, over the top of the third head cavity, to join the
lower part of the Gasserian ganglion.

The branches of the fifth nerve at stage N are well seen in figs. 11
and 12, which have been constructed so that each of them may show
the whole course and distribution of certain nerves; the’ outlines of

THE CRANIAL NERVES OF SCYLLIUM. 97

the figures were drawn, with the camera, from individual sections and
the branches of the several nerves carefully filled in, again by the aid
of the camera, from other sections of the same series. In this way
such a view of the nerve is obtained as might be got from a transpa-
rent embryo in which the nerves alone stood out as opaque objects.
To prevent confusion, from the overlapping of different nerves, two
figures have been given, of which the first (fig. 11) shows the roots of
the fifth and seventh, the connections of these with one another and
with the third nerve, the branches of the latter and of the fifth, the
fourth nerve, and some of the branches of the seventh ; in the second
(fig. 12) the remaining branches of the seventh, with certain branches
of the glossopharyngeal are shown, and in addition to these, the whole
course of the ophthalmic branches of both fifth and seventh nerves.

The branches of the fifth nerve are seen to be the following:

1. The ophthalmic branch (v a), which arises by a slightly ganglionic
root, runs forward over the obliqguus superior (0.s.), crossing, as it
does so, the fourth nerve at right angles and giving off branches, some
of which appear to be connected with the fourth.

2. The communicating branch (M.c.) between the Gasserian and
ciliary ganglia, the position and relations of which nerve are sufiiciently
well shown in the figure. We reserve the discussion of this branch,
merely noticing here that, though we describe it with the fifth nerve,
it appears to belong to the third quite as much as to the latter.

3. The main stem of the fifth, running down behind the first head
cavity (1) and the rectus externus, receiving the communicating branch
(W.c’.) from the seventh, and after passing downwards and forwards
for some distance, dividing into two branches, (a) an anterior or
maxillary nerve (v >) which again gives off numerous branches to the
skin of the upper jaw; and (6) a posterior or mandibular (v c) which
runs backwards and downwards, lying in close contact with the outer
wall of the lower part of the mandibular head cavity (2) and supplying
it with branches. The distribution of ganglion cells is sufficiently
shown in the figure; there is a small ganglion at the base of the
ophthalmic nerve, and the ganglion cells of the main stem extend
some little distance beyond the point of junction with the communi-
cating branch from the seventh.

In figs. 14 and 15 some of these branches (v a, v 6, Vv c) are seen at
a stage between 0 and P: except that the roots of v and vi are much
more closely approximated, there is no difference of importance between

H

98 PROF. MARSHALL AND MR. SPENCER.

this stage and the earlier one which we have more fully described.
We have traced all these nerves up to what is practically the adult
condition, and have identified them with the branches bearing the
same names in the adult. Our observations show that in the fifth, as
in the third nerve, all the main branches of the adult nerve are fully
established by stage L, and that the after changes are comparatively
unimportant.
THE SEVENTH, oR Factan NERVE.

A. The roots of origin.—Fig. 2 represents a transverse section
through the roots of origin of the seventh nerve of an embryo between
stages 1 and kK, the same, in fact, of which fig. 1 represents the roots
of the fifth nerve. The nerves (vil) are seen to arise from the
extreme dorsal summit of the hind-brain, the roots of origin of the
two (vil a) being continuous with one another across the top of the
brain. It will also be noticed, as contrasted with figure 1, that
the two sides of the hind-brain are close together, both above and
below, and that the cavity of the hind-brain is a mere vertical slit ;
that, in fact, the growth of the roof of the brain, which we have seen
is the first cause of the separation of the roots of the fifth, has not yet
commenced in the part of the brain from which the seventh nerves
arise. The section further shows that the nerve on either side extends
down as a somewhat club-shaped mass of compactly arranged polygonal
cells lying between the external epiblast and the neural canal, but
distinct from both, its ventral end having a tendency to pass to the
outer side of the third head cavity (fig. 2, 3), just as the fifth nerve
passed to the outer side of the second cavity (fig. 1, 2).

The next stage is represented in fig. 8, a section through the hind
brain and roots of the seventh nerve of an embryo of stage K, of the
same age though not from the same specimen as fig. 4. The figure
shows that very important changes have occurred; the roof of the
hind-brain has grown rapidly and considerably, so as to separate
widely the two primary roots of the seventh nerves (vil a). On the
right side this dorsal primary root is alone seen, but on the left side a
considerable portion of the nerve is shown, and it is seen that, im
addition to the primary root (vil a), which is still present, the nerve has
acquired a new or secondary root (vil 3) about half way down the side
of the brain. Both roots of attachment are perfectly clear and unmis-
takeable, while between them the nerve and brain are quite distinet
from one another, and separated by an appreciable interval.

THE CRANIAL NERVES OF SOYLLIUM, 99

If fig. 3, showing the condition of the root of the seventh nerve at
stage K, be compared with fig. 1, showing the root of the fifth nerve
at a rather earlier stage, it will be seen at once that there is a very
close resemblance between the two; the sole point of difference being
that in fig. 1, though the nerve still retains its primary attachment,
the secondary has not yet been actually acquired. Balfour’s figures
and description, already referred to, show that at a still earlier stage
the fifth nerve has exactly the same appearance and relations which
the seventh has in fig. 2; and it is mainly on this fact, coupled with
the close similarity between such specimens as those represented in
figs. 1 and 3, that we rely in support of the explanation we have
given above of the root of the fifth nerve.

Inasmuch as figs. 1 and 2 are taken from the same embryo, it would
seem that the fifth nerve appears before the seventh, and is, during
the earlier phases of its formation, just one stage ahead of it in-develop-
ment, At a time (fig. 1) when the primary roots of the fifth have
already become widely separated by growth of the brain-roof, and the
secondary attachment (vy /3) is on the point of being acquired, the two
seventh nerves (fig. 2) are still in contact with one another across the
top of the unexpanded brain-roof; and at stage x the seventh nerve
(fig. 3) is in exactly the same condition as the fifth is at the end of
stage 1 (fig. 1).

Our observations appear, therefore, to prove conclusively that as
concerns the seventh nerve, while the change of position of the dorsal
or primary root (vi a) is due solely to rapid growth of the roof of the
brain, the lower or ventral root (vi /5) is a new and purely secondary
attachment.

Whilst these results concerning the roots of the seventh are, we
believe, new as applied to Elasmobranchs, they are in perfect accord-
ance with the account previously given by one of us of the develop-
ment of the seventh nerve in the chick, in which the very same series
of changes--the separation of the primary roots by growth of the
brain-roof, and the acquiring of new or secondary roots—are shown to
occur ina precisely similar manner.’ The close correspondence between
these two very different types of vertebrates is of much interest, partly

+ Marshall, ‘“‘ Develop. of Cranial Nerves in Chick,” ‘ Quart. Journ. Micr. Se.,’ Jan. 1878,
pp. 34 and 36.

The prediction there made, that the secondary attachment of the nerves in Elasmobranchs

would prove on further investigation to be acquired in exactly the same manner as in the
chick, is now completely verified,

100 PROF. MARSHALL AND MR. SPENCER.

as tending to confirm the correctness of the account, and partly as
showing that this curious shifting of the nerve roots, though clearly a
change of a secondary nature, must yet have been acquired very early
by Vertebrates.

The later stages of development of the roots of the seventh also pre-
sent points of great interest. Fig. 6 represents a transverse section
through the roots of the seventh nerve in the same embryo, at stage N,
of which fig. 5 shows the roots of the fifth nerve. The seventh nerve is
seen to arise on either side by two roots, one (vil a) from the top of the
side of the brain at the junction of the thickened part with the thin roof,
while the other (vu (3) arises about half way down the side of the brain.
Between the two roots the nerve is in contact with the brain, but
apparently not connected with it. We have traced the intermediate steps
between figs. 3 and 6, and find that the upper root (v1 a) of fig. 6 is the
original dorsal or primary root, and the lower one (vir /3) the secondary
root of fig. 3. In other words, there is an important difference between
the fifth and the seventh nerves, inasmuch as in the former the primary
root ts lost and the secondary alone retained, whilst in the latter both primary
and secondary roots are retained up to stage N, and indeed, as we shall see
emmediately, throughout life. The difference between the roots of the
fifth and seventh nerves just noticed does not occur in the case of the
chick, in which the primary root of the seventh is lost as completely
as is that of the fifth in Hlasmobranchs.!

This shifting of the roots of origin and acquiring of a secondary
connection with the sides of the brain is not confined to the cranial
nerves. It has already been shown to occur in the posterior roots of
the spinal nerves of the chick,? and occurs also in the posterior roots
of the spinal nerves of Elasmobranchs. ‘Tt isa point of much interest
to note that the seventh nerve, in the retention of its primary as well
as its secondary root, ts not only more primitive than the fifth, but more
primitive even than the spinal nerves. ;

The condition of the roots of the seventh at stage 0 is shown in the
left-hand side of fig. 9, representing half of a transverse section through
the hind-brain and roots of origin of this nerve. The two roots, the
primary (vil a) and the secondary (vii /3), are even more distinct than
at the earlier stages. The primary root (vil a) arises as before from

1 Marshall, § Quart. Journ. Micr. Sc.,’ Jan., 1878, pp. 24 and 25.
? Marshall, ‘On the Early Stages of Development of the Nerves in Birds,” ’ Journal of
Anatomy,’ vol xi., 1877.”

THE CRANIAL NERVES OF SCYLLIUM. 101

the top of the thickened side of the hind-brain just before its junction
with the thin roof; from this origin the root runs downward, alongside
of and closely applied to the brain, but unconnected with it, to join
the secondary root (vu §). This latter is now situated still nearer
to the ventral surface than at its first appearance, the distance between
the two roots being considerably greater than at the earlier stages, as
is evident from a comparison of fig. 9 with fig. 6. The two roots also
differ histologically, the dorsal or primary root consisting almost entirely
of elongated fusiform cells, whilst the ventral root (vit 3) is mainly
composed of spherical ganglion cells.

This ventral root, at stage 0, has, as shown in the figure, two
distinct attachments to the brain, one just below the other. It is
shown in longitudinal and vertical section at stage Nn in fig. 11, where
the brain presents opposite to its point of origin an external bulging
precisely similar to that opposite to the secondary root of the fifth
(v 6). The dorsal or primary root (vil a) is shown at the same stage
in fig. 12.

In fig. 14 the two roots of the seventh are seen in longitudinal and
vertical section, at an age intermediate between stages o and p. The
dorsal root (vit a) arises very far up the side of the brain, in fact, as
in the earlier stages, from the junction of side and roof; it is of con-
siderable length, is widely separated from the secondary root, and still
consists mainly of fusiform cells ; the secondary or ventral root, which
is overlaid and almost concealed by the primary root, is only seen
in part, its most anterior portion alone being visible.

The dorsal or primary root is also well shown at the same stage in

fic. 15.

B. Comparison of the embryonic roots of the Jifth and seventh nerves
with those of the adult.—It will be convenient here to briefly summarise
our results concerning the roots of origin of the fifth and seventh
nerves, and to trace their changes up to the adult form.

About the close of stage 1 the fifth nerve (fig. 1) still retains its
primary attachment (v a) to the brain, and is on the point of acquiring
its secondary one (v 3) ; owing to the growth of the roof of the brain
the two primary roots, which were at first continuous across the top
of the brain, are now widely separate. The seventh nerve (fig. 2)
arises by its primary root from the dorsal summit of the brain, whose
roof at this point has not yet commenced its rapid growth, so that the

102 PROF. MARSHALL AND MR. SPENCER.

nerves of the two sides are still directly continuous with one another ;
there is as yet no trace of the secondary root of the seventh.

At stage x the fifth nerve (fig. 4) has completely lost its primary
root and is now attached to the brain by the secondary root alone
(a B); a slight trace of the former is still present as a small dorsal
projection on the nerve just beyond the root of attachment. Towards
the close of stage K, the tertiary or anterior roots have appeared,
arising almost certainly as outgrowths of the ganglion towards the
brain ; but whether these are altogether new developments or merely
new attachments of the primary root is uncertain. The seventh nerve
(fig, 3) is in the same condition as the fifth at the preceding stage ; it
is now attached by both primary and secondary roots, the former,
owing to the growth of the brain-roof, being widely separate from one
another.

At stage n the condition of the roots is much the same as at the
end of stage K. The fifth nerve (figs. 5 and 11) is attached by its
secondary and tertiary roots, the latter being very constantly two in
number, of which the anterior is the larger and attached to the brain
some distance in front of the secondary root (v 3). The seventh nerve
(figs 6, 11 and 12) is attached by both primary and secondary roots,
the nerve between the two being in contact, but not in connection,
with the brain ; the secondary root (fig. 11) is divided into an anterior
or facial, and a posterior or auditory division.

At stage o (figs 8 and 9) the chief differences are—firstly, that, owing
to increased growth of the brain, the distance between the primary and
secondary roots of the seventh nerve is much greater than before ;
secondly, that the roots of the fifth and seventh nerves, which from
the first have been quite independent of one another, are now situated
much closer together than they were at the earlier stages.

At stage o-P (fig. 14) the two roots of the seventh (vi a and vi f)
are still further apart from one another, but are now very close to
those of the fifth, The connection between the two nerves which we
have already seen is fully established at stage K, and which is shown
at stage L in fig. 10 (W.c’.) and at stage wn in fig. 11 (Mc’.) is, by stage
o-P, very much more extensive and intimate than previously. The
roots of the nerves are still quite distinct from one another (fig. 14),
but immediately beyond these roots the two nerves become so closely
and extensively united together that it is impossible to draw a line of
separation between them, The connection is rendered still more

THE CRANIAL NERVES OF SCYLLIUM. 103

intimate by the crossing of one of the branches of the seventh, as will
be described fully later on, over the main stem of the fifth, so as to lie
in front of the branches of this nerve.

The condition of the roots, as of the branches, of the fifth and
seventh nerves at stage o-p differs but little from that of the adult,
the sole change of importance as concerns the roots being that the
ventral roots (v 6 and vit (£) approach still closer together, and come
into actual contact.

The primitive distinctness, gradual approximation, and ultimate
more or less complete fusion of the roots of the fifth and seventh are of
great interest, as proving that the fusion of these two nerves, so
characteristic of Pisces and Amphibia, is a purely secondary feature ;
and that the two are at first as independent of one another in these
forms as they are throughout life in the higher Vertebrates.

In adult Elasmobranchs the combined roots of the fifth and seventh
nerves are usually described together, and the descriptions of different
observers, though not quite in harmony with one another in certain
details, yet agree fairly well on the main points. Stannius, whose
descriptions are the most precise, describes the combined fifth and
seventh nerves as arising in Plagiostomes by three roots,’ of which one
is seen on closer examination to be double, giving four roots in all ;
of these the first, or most anterior one, arises from the ventral surface
of the medulla by two short non-ganglionic roots, which unite together
shortly after leaving the brain. This root is in Raja, according to
Stannius, mainly motor, supplying the muscles by which the respira-
tory movements of the anterior wall of the spiracle are effected, and
also certain others in connection with the jaws. The second root of
Stannius is large, lies posterior to the first, and is in close proximity
behind with the auditory nerve; it may be distinguished into an
anterior part which belongs to the trigeminal, and a posterior, more
ventrally situated portion, which belongs to the facial. The third
root is very large and much more dorsally situated than the others ; it
is connected by its deeper fibres with the second root, whilst from its
superficial fibres are derived, according to Stannius, the ramus ophthal-
mMACUs superficialis of the fifth, and also, in part, the maxillary and
buccal nerves.

Gegenbaur,” in his account of the cranial nerves of Hexanchus, dis-

1 Stannius, “Das peripherische Nervensystem der Fische.” Rostock, 1849, pp. 29 and 30.
* “Ueber die Kopfnerven von Hexanchus,” ‘Jenaische Zeitschrift,’ Bd. vi, 1871, pp.
501, 502, and 513, 514.

104 PROF. MARSHALL AND MR. SPENCER.

tinguishes between the roots of the fifth and the seventh. He describes
the fifth as arising by the union of two trunks of about equal size, an
anterior and a posterior; of these the anterior (a) arises from the
ventral surface of the medulla by two roots situated very close together ;
the posterior (>) has also two distinct roots, a dorsal one (a) arising
from the side of the medulla by a large swelling projecting into the
fourth ventricle, and a ventral one (3) situated immediately above the
root of the facial, and in front of, and above that of the auditory.

The seventh nerve in Hexanchus is described as arising by two
roots, a larger one immediately in front of the auditory, and a smaller
one passing to it from the fifth.

Jackson and Clarke! describe the combined fifth and seventh nerves
in Lchinorhinus as arising by three roots; an anterior inferior root
(v a), itself with two well-marked rootlets, a second root (Vv /) arising
by a well-marked superior rootlet from the lobus trigeminus and by a
smaller inferior one, and a third root (v y and vit) closely connected
with the second one,

Balfour? describes the fifth nerve in Scylliwm stellare as arising by
three roots :—(1) an anterior more or less ventral root; (2) a root
rather behind this arising by two strands, a dorsal and a ventral, and
closely connected behind with the root of the seventh ; and (3) a quite
distinct dorsal and posterior root situated slightly behind the dorsal
strand of the second root. The seventh nerve is described as arising
by a single root close to, and behind, the second root of the fifth.

On comparing these descriptions of adult Elasmobranchs with our
own observations on embryos and adults we are led to the following
conclusions.

The fifth nerve in the adult arises by two roots:

a. An anterior one arising from the ventral surface of the medulla
by two non-ganglionic rootlets, whose distinctness varies much in
different adult Elasmobranchs. These rootlets are the tertiary or
anterior roots of our embryos (figs. 10, 11, and 14, v y). This root
corresponds to the first root of Stannius; the anterior root (a) of
Gegenbaur ; the anterior root (v a) of Jackson and Clarke ; and the
anterior root (1) of Balfour.

6. A posterior larger ganglionic root, the ventral or secondary root
of our embryos (figs. 10, 11, and 14, v 6). This is at first quite

1 “The Brain and Cranial Nerves of Echinorhinus spinosus.” ‘Journal of Anat. and
Phys.,’ vol. x, p. 81,

2 Op, cit., pp. 194 and 195,

THE CRANIAL NERVES OF SCYLLIUM. 105

distinct from the root of the seventh, but during the later stages of
development gradually approaches this latter, and in the adult cannot
be clearly distinguished from it. —

This root is the anterior part of the second root of Stannius; the
ventral division (3) of the posterior root (4) of the fifth of Gegenbaur ;
apparently the inferior rootlet of the second root (v (3) and possibly
part of the third root (v y and viz) as well, of Jackson and Clarke ;
and the second root (2) of the fifth of Balfour.

The seventh nerve in the adult arises by two roots :

a. A dorsal root arising far up the side of the medulla, at the
junction of the thickened side and thin roof of the fourth ventricle.
This root is the primary or dorsal root of the seventh nerve of our
embryos (figs. 2, 3, 6, 9, 12, 14, and 15, vir a). It has by previous
observers been almost invariably described as a root of the fifth, and
never as a true root of the seventh ; owr description and figures here
given leave no room for doubt that it belongs to the seventh. As already
noticed, the retention of this root marks the seventh as being more
primitive than the spinal, and possibly more so than any of the other
cranial nerves, all the other nerves apparently retaining their secondary
roots only.

This root is the third or dorsal root of Stannius ; the dorsal rootlet
(a) of the posterior trunk (0) of the fifth of Gegenbaur ; the superior
rootlet of the second root (v 3) of Jackson and Clarke ; and the dorsal
and posterior root (3) of the fifth of Balfour.

6. A ventral root arising from the side of the medulla at a rather
lower level than the posterior root of the fifth. This is the secondary
or ventral root of the seventh of our embryos (figs. 3, 6, 9, 10, 11, and
15, vir 6). The auditory nerve is at first derived from this root, but in
the adult appears to be more distinct from the facial than is the case
in the embryo. This root is, at first, some little distance behind the
secondary one (v 5) of the fifth nerve (figs. 10 and 11), from which it
is perfectly distinct; in the later stages the two roots gradually
approach one another (fig. 14), and in the adult are usually in close
contact.

This root is the posterior part of the second root of Stannius; the
root of the seventh of Gegenbaur ; part or possibly the whole of the
third root (v y and viz) of Jackson and Clarke; and the single root of
the seventh of Balfour.

It would appear, therefore, that the fifth nerve loses its primary

106 PROF, MARSHALL AND MR. SPENCER.

root, retains its secondary, and acquires tertiary roots, while the
seventh retains both primary and secondary. Concerning the fourth
nerve we would suggest the possibility that it may prove to be the
primary root of a nerve of which the third nerve is the secondary root,
which has, in this case, acquired complete independence.

c. The branches of the seventh nerve-—At stage K the seventh nerve,
which, as we have already seen, has acquired its secondary as well as
its primary roots of origin (fig. 3), expands below the secondary root
into a large ganglionic swelling lying immediately behind the third
head cavity. From this enlargement three branches arise: (1) from
the anterior and superior angle of the ganglion a large nerve with a
ganglionic base arises, and runs forward along the dorsal surface of the
head, lying just beneath the superficial epiblast ; this is the ophthalmic
branch of the seventh, and is referred to in our figures as VII a.

(2) The second branch, which is also large, and has a ganglionic
base, arises from the front part of the ganglion immediately below the
root of the ophthalmic ; its deeper portion runs forwards and slightly
downwards over the top of the third head cavity and becomes con-
nected with the main stem of the fifth; it is referred to in the figures
as Vic’. The more superficial portion passes on further forward in
the same direction, crosses the mandibular arch, and enters the
maxillary process, lying immediately superficial to the maxillary nerve
and just beneath the external epiblast ; it is referred to in the figures
as VII d.

(3) The third branch is the direct continuation of the main stem
of the facial nerve; it runs downwards and backwards along the
anterior border of the hyoidean arch, and is the rudiment of the
posterior or hyoidean branch of the seventh in the adult ; it is referred
to in the figures as VII ¢.

Of these branches the first, or ophthalmic, is from its earliest
appearance connected with the dorsal or primary root of the seventh
rather than with the ventral root. The second branch is the most
remarkable of the three ; its deeper portion forms, as we have seen, a
direct connection between the fifth and seventh nerves, a communi-
cation which appears to be very early established, inasmuch as by
stage K the connecting branch is already a nerve of considerable size ;
the superficial portion of this branch (vm d@) is noteworthy, mainly on

THE CRANIAL NERVES OF SCYLLIUM. 107

account of its very close relation with the maxillary division of the
fifth nerve.

At stage L the only changes of importance are, (1) that the several
branches have increased in size, and, excepting the branch vir d, which
has a very straight course and ends abruptly in the skin, have
divided into secondary branches near their terminations ; and (2), that
a small anterior branch has arisen from the hyoidean nerve (vii c),
some distance from the brain, which runs forward over the top of the
spiracular or hyomandibular cleft, and then down in the anterior wall
of the spiracle, «e. in the posterior portion of the mandibular arch ;
this branch will be referred to as vir 0.

The several branches of the seventh nerve at stage N are well shown
in the diagrammatic figures 11 and 12. The ophthalmic branch (v1 a)
is seen in fig. 12 arising from the base of the primary or dorsal root
(vir a) as a stout nerve, which expands very shortly after its origin
into a large somewhat fusiform ganglion, beyond which the nerve rung
forward as a stout trunk to the extreme anterior part of the head.
Throughout its course it lies just beneath the external epiblast, and
immediately dorsal to the ophthalmic branch of the fifth (v a), with
which it is in very close relation ; like this latter nerve it gives off
branches along its whole course to the integument of the neighbouring
parts, the branches being few in number at the proximal end, and
much more numerous distally. A short distance before its termination
this nerve, like the ophthalmic branch of the fifth, is crossed at a
considerable angle by the nerve J (fig. 12).

The connecting branch (N.c’.) between the seventh and fifth nerves
is well seen in figs. 11 and 12; it is now shorter and wider than at
stage u (fig. 10), and contains very numerous ganglion cells along its
whole length.

The superficial portion of this nerve (vir d) is not shown in fig. 11,
but is represented along its whole length in fig. 12; it is a stout nerve
with a remarkably straight course; it gives off no branches at all
along the greater part of its length, but near its distal termination
divides rather suddenly into a number of branches, which end in the
integument of the maxillary process, the most anterior of them
extending forwards almost as far as the hinder border of the olfactory
pit. The relations of this nerve to the maxillary branch of the fifth
are very curious ; the two nerves are very close together, the branch
of the seventh lying, as at the earlier stage, immediately superficial

108 PROF. MARSHALL AND MR, SPENCER.

to-that of the fifth. These relations are well seen in the transverse
section drawn in fig. 6. This shows, as already noticed, the primary
and secondary roots of the seventh, and also the proximal portion of
the nerve vir d. This nerve is seen to be a direct continuation of the
primary root ; its inner, or deeper, portion is seen on both sides to
become continuous with the maxillary branch of the fifth (v 6), the
junction of the two forming the connection between the fifth and
seventh nerves already noticed. Beyond this point of union the nerve
vu d is continued downwards, lying immediately superficial to the
maxillary nerve (v b). The two nerves preserve this relation up to
their terminal distribution, two of the ultimate branches being
represented in figure 5 (v 6 and vid). This very remarkable branch
of the seventh puzzled us greatly for a long time, and it was only
after working out the whole history of its development up to stage q
that we succeeded in determining its import. The nerve has already
been described and figured by one of us,! and named tentatively the
palatine. This determination now proves to be erroneous ; the palatine
is a deeply placed nerve, whereas the nerve vu d retains its superficial
position in the adult.

This nerve (vi d) we have now identified as the buccal nerve, the
proof of this determination, consisting in our having traced the nerve
directly up to the adult. The buccal nerve has always hitherto been
regarded as a branch of the fifth, and is described as such by Stannius,*
Gegenbaur,? Jackson and Clarke,‘ Balfour,> and others. Stannius®
points out that the buccal nerve in fishes is very variable ; that it may
either arise from the main stem of the fifth, from the maxillary or the
mandibular, or from both these nerves, or, finally, may be absent
altogether.

Up to stage n, as shown in figures 6, 11, and 12, the buccal nerve
is clearly a branch of the seventh, and could not possibly be taken for
a branch of the fifth. After stage nN, however, the connection between
the roots of the fifth and seventh nerves becomes, as we have seen,
very much more intimate; and at the stage between o and P,
represented in fig. 14, the buccal nerve (vit d), which is now situated
completely in front of the maxillary (v 6), might very easily be taken

1 Marshall, loc. cit., pp. 86, 87; and Pls. V, fig. 15, and VI, figs. 28 and 29.
2 * Handbuch der Zootomie,’ p. 158.

3 Loc. cit., p. 509.

* Loc cit., p 86.

5 Op. cit., p. 195.

® “Das Peripherische Nervensystem,’ pp. 41 and 42.

THE CRANIAL NERVES OF SCYLLIUM. 109

for a branch of the fifth rather than of the seventh ; careful examina-
tion shows, however, that the buccal, which is, as before, the most
superficial of all the ventral branches, can be traced up to the dorsal
root of the seventh from which it arises, as in the earlier stages.

This origin of the buccal nerve from the root vir a has already been
noticed by Stannius,? who, however, as we have seen, did not refer the
root in question to the seventh. Stannius’ figure of the nerves in
Chimera? shows clearly the very superficial position of the buccal
nerve and its independence of both maxillary and mandibular nerves.

Of the remaining branches of the seventh the anterior one (v1 6) is
shown in fig. 11 at its origin, arising from a large ganglionic swelling
on the main or hyoidean branch of the seventh, and running forward
in close contact with the top of the spiracle (sp.), in front of which it
divides almost at once into two branches, the distribution of which is
shown in fig. 12; of these the anterior one (vii pa) runs downwards,
forwards, and inwards, giving off numerous branches to the roof of the
mouth, In fig. 12 the anterior branches of this nerve appear to cross
the posterior branches of the buccal, but it must be borne in mind
that at this point the two nerves are at very different levels, the
buccal being very superficial and the nerve (vi pa) lying very deep.
This latter is seen in transverse section in fig 7, which shows, on the
right side, its origin from the ganglion, and, on the left, its distribution
to the mucous membrane of the mouth. By comparing this figure with
fig. 6, the difference of levels between the two nerves will be at once
apparent. This anterior branch (vit pa) is the palatine nerve ; it has
already acquired by stage wn its characteristic distribution, and under-
goes no further change of importance from this period up to the adult
Stage.

The second or posterior division (fig. 12, vu sp) of the nerve (vi 6)
runs downwards and slightly backwards along the anterior border of
the spiracular cleft ; it gives off branches along the whole of its length,
the great majority of which run backwards to the mucous membrane

of the border of the cleft and to the spiracular branchia. This nerve,

which at this stage is of about equal size with the palatine, is the
spiracular or pree-spiracular nerve of zootomists.

The only branch of the seventh still left for description is the main
trunk or hyoidean branch (fig. 11, vi c), which forms the direct

1 Loc. cit., p. 30.
® Loe. cit., Taf. 1, fig,

110 PROF. MARSHALL AND MR. SPENCER.

continuation of the main stem of the nerve. This, as is seen from
fic. 11, arises from the ventral or secondary root of the seventh, and
is at its origin closely connected with the auditory nerve (viii).
Immediately after the auditory nerve leaves it, the facial forms a
ganglionic swelling from which the communicating branch (JV.c’.) to
the fifth nerve is given off; beyond this point it is continued for a
short distance as a stout nerve with comparatively few ganglion cells ;
this very speedily dilates into the large ganglionic swelling on the
top of the spiracular cleft, from which the anterior branch (vu |) is
given off. The main stem of the seventh (vi ¢) continues its course
downwards, running along the anterior border of the hyoid arch and
very close to the posterior border of the spiracular cleft; during this
part of its course it contains few or no ganglion cells, but gives off a
number of branches, of which the first is the largest, from its posterior
border, which supply the muscles derived from the wall of the third
head cavity (3). A short distance below the lower edge of the
spiracular cleft the nerve divides into two branches, of which the
anterior (vi c’.) runs forward along the lower border of the mandibular
arch, sending numerous branches to the integument of this part and
extending forward so as to come into very close relation with the
posterior branches of the maxillary division of the fifth (vb). The
posterior of the two branches (vii ¢, 2) into which the seventh divides
continues the direction of the main stem, and runs down in the hyoid
arch just in front of the third head cavity, in the terminal dilatation
of which it ends. Of these two terminal branches of the seventh,
the anterior, sensory, and superficial one is the ramus mandibularis
externus of Stanniust and Gegenbaur,? while the posterior, muscular,
and deep branch is the vamus mandibularis internus v. profundus of
the same authors. Both these branches have already acquired, by
stage N, not only the characteristic distribution of the adult nerves,
but nearly all the minor branches as well.

To recapitulate ; we find that by stage n the seventh nerve has
acquired all the important branches of the adult nerve, the main
trunks and many of the branches being fully developed at a much
earlier period—stage t. The seventh nerve at stage n has two roots,
a dorsal or primary, and a ventral or secondary. From the dorsal root
(vil a) arise two branches: (1) the ophthalmic (vi a) and (2) the

1 Loc. cit., p. 65.
® Loc, cits, p. 514,

THE CRANIAL NERVES OF SCYLLIUM. 111

buccal (vir d), both of which appear to be purely sensory nerves,
The connecting branch (J.c’.) to the fifth nerve, though it appears
in longitudinal section (fig. 11) to be a distinct branch, in transverse
sections (fig. 6) seems to be only the deeper portion of the buccal
nerve. From the ventral root arises the main or hyoidean branch
(vit c), from which the branch vir 6 runs forward over the top of the
spiracle, dividing, almost immediately, into the palatine (pa) and
spiracular (sp) nerves, whilst the hyoidean itself divides distally into
the sensory ramus mandibularis externus (v ¢, 1), and the motor ramus
mandibularis internus (Vv ¢, 2).

Tue SrxtH (Aspucens) Nerve.—Concerning the development of the
sixth nerve our observations simply confirm the account already given
by one of us."

The whole length of the nerve is shown in longitudinal and vertical
section in fig. 13 (v1), where it is seen arising from the base of the
brain by a number of small non-ganglionic roots which unite to form
a slender nerve ; this nerve runs forwards a short distance, then turns
downwards, pierces the investing mass (7.v.), and ends in the posterior
extremity of the rectus externus muscle (7.¢.).

Fig. 7 shows the sixth nerve in transverse section at the same stage

(x): on the left side of the figure the termination of the nerve in the

rectus externus is seen; while on the right side, which is taken from a
more posterior section, one of the roots of origin isshown. This figure
shows that the roots of the sixth are considerably nearer the mid-
ventral line than are the secondary roots of the seventh nerves, and
also that the sixth and seventh nerves are quite independent of one
another. At stage n the sixth nerve appears to be altogether behind
the seventh, but in stage o it is situated rather further forwards, so
that the same section may pass through the roots of both nerves.

We have not yet detected the sixth nerve in embryos younger than
stage L: concerning the morphological value of this nerve we adhere
to the opinion already expressed that it is to be viewed as bearing the
same relation to the seventh that the anterior root of a spinal nerve
does to its posterior root.

Tue Ercutu (Aupitory) Nerve.—This nerve also we can dispose of
briefly : at stage K it appears as a large ganglionic posterior branch of
1 Marshall, loc: cit,, pp: 89=—93

112 PROF. MARSHALL AND MR. SPENCER.

the seventh nerve, given off immediately beyond the root of origin. It
is from the first connected with the ventral or secondary root (vu /3).
The condition at stage L is shown in fig. 10 (viz). At stage Nn (fig. 11)
its root, though still intimately connected with that of the facial,
shows a very evident line of separation from it; the ganglionic
character of the auditory nerve placing it in marked contrast with
the non-ganglionic root of the facial. This distinction between the
two roots becomes more marked in the later stages.

GENERAL CoNSIDERATIONS.— Several questions of a more general
character arise out of the facts we have recorded above, and we pro-
pose to conclude the present paper with a brief notice of the more
important of these. The problems in connection with the roots of
origin of the nerves have been already sufficiently discussed, so that
we at once turn to the consideration of their branches, concerning
which the most important points are the determinations of the equiva
lence of the branches of the different nerves to one another.

We commence with the ophthalmic branches of the fifth and seventh
nerves, the branches named v a and vil a in our figures. These two
nerves, whose course and relations are well shown in figs. 11 and 12,
accompany one another very closely along their whole length; they
appear to be both sensory nerves, their branches being distributed
exclusively to the skin of the top and front of the head, and more
especially to the mucous canals of these parts. Of the two nerves the
branch of the fifth nerve (v a) is the smaller and the more ventrally
placed of the two: though the smaller, its branches are, especially in
the earlier part of its course, more numerous than those of the seventh.
The two nerves in their course through the orbit le dorsal to alt
the other contents of the orbit. They are at first quite distinct from
one another (figs. 11 and 12) and lie close beneath the external
epiblast (fig. 5, vit a); the branch of the seventh being the more
superficial of the two. In the later stages of develépment, as in the
adult, the two nerves lie in very close contact with one another
(fig. 16, v a and vi a), the branch of the seventh lying immediately
dorsal to the branch of the fifth ; they also, as shown in fig. 16, lie at
a deeper level than at the earlier stages.

The fourth nerve bears, as already noticed, a very close relation to
these branches. As shown in figs. 11 (iv) and 16 (iv) it crosses the
ophthalmic branches at right angles, lying at a slightly deeper level

THE CRANIAL NERVES OF SCYLLIUM. 113

but turning outwards immediately beneath them, to end in the
superior oblique muscle (0.s.). At the point of crossing the branches
of the two nerves are in very close relation with one another, and we
are inclined to believe that a communication exists between the fourth
nerve and the ophthalmic branch of the fifth at this point, though we
have failed to determine this with certainty.

In determining the morphological value of these ophthalmic branches
of the fifth and seventh nerves, very valuable evidence, by which we have
been much influenced, is afforded by the condition of the glossopharyn-
geal nerve. This nerve, at stage L, gives off, just beyond its root of
origin, a slender dorsal branch (fig. 10, 1x a), which, at first passing
upwards and backwards, soon curves round the hinder end of the
auditory vesicle (awd.), and reaching the dorsal surface of the head, runs
forward a short distance, lying immediately beneath the superficial
epiblast. It gives off branches along its whole course, which are
distributed, as shown in the figure, to the integument of the top of the
head. At stage N this nerve (fig 12, 1x @) has the same course and
appearance, the only differences being that it extends rather further
forwards than at the earlier stage, so as to reach some distance
in front of the middle of the auditory vesicle; and, secondly,
that its branches are now seen to be in connection with the
commencing mucous canals of this region. This branch of the
ninth nerve is clearly the ramus dorsalis, and an examination of
the figures 11 and 12 will, we think, leave no doubt that the nerves
vu a and va, which have a similarly superficial course and a like
distribution to mucous canals, must be viewed as the equivalent
branches of the seventh and fifth nerves. We are, therefore, led
to adopt the view put forward by Balfour, that the ophthalmic
branches va and vita of the fifth and seventh nerves are the rami
dorsales of these nerves.1 Stannius and Gegenbaur speak of the ophthal-
mics as ram dorsales, but refer them entirely to the fifth.

What the causes are which have led to the very marked extension
forwards of the ramz dorsales of these nerves is not very evident; we

1 In my paper on the head cavities of Elasmobranchs I abandoned the view previously put
forward (‘ Quart. Journ. Micr. Sc.,’ Jan., 1878, p. 30), that the ophthalmics were persistent
remains of the commissure connecting together the roots of the nerves at their first appearance,
but did not expressly adopt the view that they were rami dorsales. I have been led to adopt
this view mainly because it now appears that, instead of being perfectly exceptional in
their course, as I had previously supposed them to be, the ophthalmics merely express an

exaggerated condition of a state of things shown ina less extreme form by the ramus dorsalis
of the glossopharyngeal.—A. M. M,

I

114 PROF, MARSHALL AND MR. SPENCER.

would suggest that it is due mainly to an extension forwards, and
accumulation at the anterior end of the head, of the special tegumentary
sense organs— the mucous canals—,this extension forwards involving
a corresponding extension of the nerves supplying these organs; in
connection with this suggestion it is of interest to note that no one of
the nerves in front of the fifth sends any branches to these organs.
Whether there is any trace of a ramus dorsalis to the third is very
doubtful ; at any rate the fourth nerve cannot be the ramus dorsalis
of the third, as its course is at right angles (fig. 11) to the rami dorsales
of the fifth and seventh nerves ; and, secondly, it is a motor and not a
sensory nerve.

We now come toa far more intricate problem, viz, the import of
the connecting branches between the third, fifth, and seventh nerves,
with which it will be convenient to consider the nerve J. (figs. 10, 11,
12, and 15).

These three nerves, V.c., /.c.’, and JV, all appear very early ; we
have failed to determine the date of their first origin, but by stage kK
they are fully established. The posterior one (WV.c’.), connecting the
fifth and seventh nerves together, is the most difficult to investigate,
owing to its appearing from the first as merely the deeper portion of
the buccal nerve (fig. 6); in longitudinal sections, however, it appears
very distinct (fig. 10). It is from the first much shorter than
either of the other two nerves we are considering, and in the later
stages (fig. 14) and the adult condition, owing to the close approxi-
mation of the fifth and seventh nerves, ceases to be visible as a dis-
tinct trunk,

The second of the three nerves .(N.c., figs. 10 and 11) forms, as
already noticed, a direct connection between the Gasserian ganglion of
the fifth and the ciliary ganglion (c.g.) of the third nerve, and is much
more slender than W.c’. Concerning the nerve in question, it is of the
utmost importance to notice that not only is it fully established at the
stave at which our observations commence, but that it ts from the very
Jirst a connecting nerve, and that there is no reason whatever in the early
stages for considering it as belonging to the fifth rather than to the third
nerve. We have, therefore, in this paper given it a perfectly neutral
name.

The last of these nerves, JV.,.is still more remarkable ; like the others
it is present at K. Starting at this stage from the ciliary ganglion it
runs in an almost perfectly straight course to the anterior end of the head,

THE CRANIAL NERVES OF SCYLLIUM. 115

ending abruptly in the external epiblast, and gwing off no branches
whatever. At stage L it is in very close relation with the olfactory
nerve, and in some specimens seems to be connected with it, though
we cannot speak with certainty on this point.

Ag soon as the eye muscles are established they have very definite
relations to this nerve; the rectus superior and internus, and the
obliquus superior lying above it, and the remaining three muscles
below it (fig. 11). It also passes through the substance of the sclerotic,
as noticed in a former portion of this paper.

Concerning the morphological significance of these nerves, bearing
in mind their very early appearance and the total absence of evidence
for regarding them as branches of either of the nerves they serve to
connect, we are disposed to view the nerves WV.c. and JV. c’. as per-
sistent portions of the neural ridge between the outgrowths to form
the third, fifth, and seventh nerves, and as being, therefore, homologous
with the primitive commissure connecting the posterior roots of the
spinal nerves together.!. As to the nerve WV. we are in much more
doubt; its apparent connection with the olfactory nerve at L, if
confirmed, would tell in favour of its being regarded as a similar com-
missure between the third and olfactory nerves, and would greatly
support views previously advanced by one of us concerning the mor-
phological value of the olfactory nerve.? On the other hand, the
extension forwards of the nerve JV. beyond the olfactory nerve to the
extreme anterior end of the head must, for the present, be regarded
as almost conclusive against its commissural nature. Jn this case tt
can only bea branch of the third nerve, for the only other nerve with
which it is in direct, or indirect, connection is the connecting nerve
(N.c.) between the third and fifth, which, if it does not belong to the
third, there is at any rate no reason for referring to the fifth.

In the adult Scyldium this nerve retains the relations to other nerves
which it has clearly acquired by stage N ; it is described in the adult
by Schwalbe® as “dieser scheinbare Zweig des Oculomotorius.” In
Mustelus Schwalbe -describes it as a branch of the fifth, We much
regret that we have had no opportunity of studying the development of

1 For these commissures in Elasmobranchs, vide Balfour, op. cit., pp. 158—160, and
Pl. XI, fig. 18, and Pl. XIV, fig. 15b. In the chick, Marshall, ‘Quart. Journ. Mic. Sc.’, Jan.,
1878, Pl. III, figs. 27 and 28.

? Marshall, ‘ Quart. Journ. Mic. Sc.,’ vol. xix, pp. 300 seq.

5 Schwalbe, ‘Das Ganglion Oculomotorius,’ p. 16,

116 PROF, MARSHALL AND MR. SPENCER.

this nerve in Mustelus: should it prove to arise as in Scyllium, then
it must definitely be regarded as a branch of the third.

As we have already pointed out, the nerves Vic and J. together
make up the ramus ophthalmicus profundus of zootomists, a nerve
which seems to have escaped Balfour’s notice both in the adult and in
the embryo. Balfour does, indeed, in his description of the nerves of
the adult Scylliwm, speak of a ramus ophthalmicus profundus, but
inasmuch as he says concerning it that “this latter nerve arises from
the anterior root of the fifth, separately pierces the wall of the orbit,
and takes a course slightly ventral to the superior ophthalmic nerve,
but does not (as rs usual in Hlasmobranchs) run below the superior rectus
”1 it is clear that he is describing
the ophthalmic branch of the fifth and not the true profundus, whose
existence he has overlooked. There appears to be considerable con-
fusion in the use of the terms ramus ophthalmicus superficialis and
ramus ophthalmicus profundus by different writers, a confusion
which our observations on Scyllium may help to remove. We find, as
already stated, three perfectly distinct nerves to which the term
ophthalmic nerve can be, and is, applied ; of these the two dorsal ones
(v a and vit a of our figures) are the rami dorsales of the fifth and
seventh nerves, and may be spoken of as the ophthalmic branches of
the fifth and seventh nerves respectively. Both these nerves are very
superficial along their whole course, and both lie dorsal to all the eye

and superior oblique muscles of the eye,

muscles and other contents of the orbit. The two nerves are at first
perfectly distinct, but in the adult unite more or less closely together,
the extent of their union varying much in different Elasmobranchs,
The two together constitute the ramus ophthalmicus superficialis.

The third of the ophthalmic nerves, the ramus ophthalmicus pro-
fundus, has a very different course, and is of a totally different nature ;
it is formed in Seyllxwm by the union of the connecting branch between
the fifth and third nerves (WV.c.) with the branch n of the third nerve.
It is very definitely characterised by its course ventral to the superior
rectus, superior oblique, and internal rectus muscles ; by its close relation
with the inner wall of the eyeball ; by the fact that the ciliary ganglion
is either in its trunk or is connected with it directly ; by its having at
first no branches, and by its close connection with the olfactory nerve.

We believe that the ophthalmicus superficialis and ophthalmicus
profundus always maintain these relations ; that the profundus, which

1 Op. cit., p. 194: the italics are our own.

THE CRANIAL NERVES OF SCYLLIUM. 117

is clearly the nasal nerve of Mammalia, is a primitive and very con-
stant nerve, and that it never shifts its position so as to lie dorsal to
all the eye muscles, as supposed by Balfour.

The two divisions of the ophthalmicus superficialis, on the other
hand, appear to be very variable indeed in different Vertebrates ;
they attain their maximum development in the Elasmobranchs, pro-
bably on account of the great development and importance of the
organs they supply—the mucous canals. In Mammalia the ophthalmic
branch of the fifth is represented by the frontal and lachrymal nerves,
while the seventh nerve has no ophthalmic branch.

It would hardly be profitable to discuss the various descriptions of
these nerves by different writers; we will merely point out here that
Schwalbe! clearly distinguishes the three ophthalmic nerves. He calls,
as we have done, the ophthalmic branches of the seventh and fifth
nerves together the ramus ophthalmicus superficialis, distinguishing the
component parts as portio major (VII a) and portio minor (Vv a) respec-
tively. He also employs the term ramus ophthalmicus profundus in the
Same sense ay we have done. Balfour, who was the first to clearly
recognise the double nature of the ophthalimcus superficialis, is in error
in calling the lower portion of it (v a) the ophthalmicus profundus,

Concerning the other branches of the nerves in question, there can
be little doubt that the hyoidean branch (vir c) of the seventh and the
mandibular branch (v c) of the fifth are homologous nerves, supplying
respectively the anterior walls of the hyoidean and mandibular arches ;
and there appear to be good reasons for viewing the nerve (111 c) as the
corresponding branch of the third.? All the three nerves in question
are either mainly or exclusively motor in function.

The seventh, like the hinder cranial nerves, forks over a visceral
cleft—the spiracle. As shown in figs. 11 and 12, there are two
branches of the seventh which run down in front of the spiracular
cleft, viz., the buccal (vit d) and the mandibular (vi 6), which latter
divides almost at once into the palatine and spiracular nerves. The
history of their development and a comparison of the branches of
the seventh with those of the glossopharyngeal (1x. fig. 12), leave no
possible room for doubt that of these the mandibular branch (vir 6) is
the homologue of the anterior branch (1x 6) of the glossopharyngeal.

1 Das Ganglion Oculomotorius,” ‘ Jenaische Zeitschrift,’ Bd, xiii., pp. 11 seq.
2 Marshall, loc. cit., p. 88.

118 PROF. MARSHALL AND MR. SPENCER.

This latter nerve (1x 0, fig. 12) extends very far forwards in the
hyoidean arch, being in this respect very closely imitated by the
palatine nerve (vil. pa), so that we are disposed to regard the whole
of the mandibular division (vir 6) of the seventh, ¢.e. both palatine
and spiracular nerves, as together equivalent to the anterior or
hyoidean branch (1x 0) of the glossopharyngeal. .

Balfour’ describes the mandibular branch of the seventh as being
large in the embryo, so large, in fact, that he feels difficulty about
identifying it with the adult spiracular nerve. His figures, however,
show perfectly clearly that what he describes as the mandibular branch
of the seventh is really the nerve we have shown to be the buccal.?

The maxillary nerve (v 6) is, from its time and mode of development,
almost certainly to be regarded as the true anterior branch of the
fifth corresponding to the mandibular branch of the seventh, although
in the absence of a visceral cleft in this region this determination can-
not be considered absolutely proved. Whether there is any equivalent
branch of the third nerve is very doubtful; at any rate no such branch
can be pointed out with certainty.

There now remains for consideration the buccal nerve, the deter-
mination of which, as a branch of the seventh, is one of the most
striking points we have brought to light. Whether this remarkable
nerve has any homologue among the branches of the fifth is a point
our investigations have not yet enabled us to determine. It is,
perhaps, worth while pointing out that there are many points of
resemblance between this nerve and the ramus ophthalmicus profundus,
points of sufficient importance to render a comparison between the
two nerves at any rate a possible and suggestive one. In each case
the proximal portion of the nerves in question connects together
directly the ganglion of one segmental nerve with that of the nerve
next in front, while the distal portion passes forward into the segment
anterior to that in which the main branches of the nerve are con-
tained. The early origin, the curiously straight course, and the
absence of branches until close to their termination, are features
common to the two nerves, and ones in which they stand in marked
contrast to most other branches. The deep course of the profundus
as contrasted with the very superficial one of the buccal nerve may
perhaps be attributed to the great development of the eye: in front

1 Op. cit., p. 202.
2 Op, cit,, Pl. XIV, fig. 2 and fig. 15 a.

THE CRANIAL NERVES OF SCYLLIUM. 119

of the orbit the profundus is a superficial nerve, and, like the buccal,
is purely sensory in its distribution.

On the other hand, it must be noticed that, as already pointed out,
the evidence is distinctly in favour of the distal portion of the profun-
dus (beyond the ciliary ganglion) being a branch of the third rather
than of the fifth nerve. Another point of distinction between them
lies in the fact that the former (the profundus) is distributed to what
is morphologically the dorsal surface, while the buccal supplies the
ventral surface.

In the present paper we have purposely refrained from attempting
to determine the homologies between the nerves of Scyl/iwm and those
of other vertebrates, preferring to wait until, by the study of the
development of other types, we are enabled to bring forward positive
evidence in support of our determinations.

In conclusion we would express the hope that, by working out the
development of the roots and branches of the cranial nerves in a very
typical vertebrate, and following these roots and branches through
their subsequent changes up to their adult condition, we have rendered
more practicable than has hitherto been the case comparisons between
the descriptions of embryos and of the corresponding adult animals, and
have contributed something towards the establishment of comparative
neurology upon a firm and satisfactory basis.

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120 PROF. MARSHALL AND MR. SPENCER.

DESCRIPTION OF PLATES V—VI.

All the figures are taken from sections of Scyllium embryos.
Figures 1, 2, 3, 4, 5, 6, 8, and 13, were drawn from single sections.
Figures 7 and 9 each represent two sections, the right hand half of
the figure being in both cases taken from a section a short distance
posterior to that represented in the left hand half. The remaining
figures, viz. 10, 11, 12, 14, 15, 16, and 17, are of a diagrammatic nature,
each being constructed by combining a number of sections from the
same embryo; in these, as in the other figures, the outlines were
drawn by means of a Hartnack camera.

The numbers attached indicate, in diameters, the magnifying power
employed.

Alphabetical List of References.

aud., auditory vesicle; Br. 1, first branchial arch; c¢g., ciliary
ganglion ; er., cerebellum; 4.0., hind-brain; Hy., hyoid arch; znf.,
infundibulum ; 2., lens; m., muscle; m.6., mid-brain ; m.c., mucous
canal ; J/n., mandibular arch ; n., notochord; J., nerve from ciliary
ganglion to fore part of head, forming the distal portion of the ramus
ophthalmicus profundus; N.c., communicating branch between third
and fifth nerves, forming the proximal part of the ramus ophthalmicus
profundus; N.c’., communicating branch between fifth and seventh
nerves; 0.¢., optic cup; 0.2, obliquus inferior; olf., olfactory pit ;
0.8., obliquus superior; pit., pituitary involution from mouth; me.
rectus externus; 1r.2., rectus inferior; r.int., rectus imternus; 1.s.,
rectus superior ; spr., spiracle, or hyomandibular cleft ; 1, first head
cavity ; 2, second head cavity; 3, third head cavity; 1, olfactory
nerve ; I, optic nerve; 111, third, or oculomotor nerve; 11 0, branch
of third nerve to rectus superior; 1 c, branch of third nerve to
obliquus inferior ; 111 B, 1 secondary root of 11; 11 y, anterior roots
of 11; Iv, fourth nerve, or patheticus ; V, fifth, or trigeminal nerve ;
va, ophthalmic branch of fifth nerve ; vd, maxillary branch of fifth ;
vc, mandibular branch of fifth ; v a, primary dorsal root of fifth ; v ,
secondary root of fifth ; v y, anterior, or tertiary roots of fifth ; v1,
sixth, or abducens nerve ; vil, seventh, or facial nerve ; vil a, oph-
thalmic branch of seventh nerve; vir 6, mandibular branch of seventh ;
vil pa, palatine division of mandibular branch ; vit sp, spiracular

THE CRANIAL NERVES OF SCYLLIUM. 121

division of mandibular branch; vit c, hyoidean branch of seventh ;
viel, ramus mandibularis externus, or branch of hyoidean to
mandibular arch ; vic 2, ramus mandibularis internus, or terminal
branch of hyoidean to walls of third head cavity ; vm d, buccal branch
of seventh ; vila, primary, or dorsal root of seventh; vit SB, secondary
root of seventh ; vii, eighth, or auditory nerve ; 1x, ninth, or glosso-
pharyngeal nerve ; 1x a, ramus dorsalis of ninth nerve ; 1x 6, hyoidean
branch of ninth; 1x ¢, branch of ninth to first branchial arch ; x,
tenth, or pneumogastric nerve; x 0, anterior branch of tenth nerve
in first branchial arch.

Figs. 1 & 2. Transverse sections through the hind-brain of a Seyllium
embryo, intermediate between stagestandK. x 90.

Fig. 1, showing dorsal (primary) and ventral (secondary) roots

of fifth nerve ; also the second or mandibular head cavity,

Fig, 2, showing origin of dorsal (primary) roots of seventh
nerve from neural crest; also the third or hyoidean head
cavity.

Figs. 3 & 4, Transverse sections through the hind-brain of an embryo
of stage K. x 90.

Fig. 4, showing the fifth nerve, arising by its ventral or secon-
dary root alone.

Fig. 3, showing the dorsal and ventral roots of the seventh
nerve.

Figs. 5 to 7. Transverse sections through the hind-brain of an
embryo of stage n. x 20.

Fig. 5, showing the ventral roots of the fifth nerves, the
ophthalmic branches of the seventh nerves, and the ter-
minations of the maxillary branch of the fifth and the
buccal branch of the seventh nerve ; also the first head
cavity.

Fig. 6, showing the dorsal and ventral roots of the seventh
nerve, the buccal branch of the seventh aud maxillary
branch of the fifth, and the connection between these two
nerves ; also the second head cavity.

Fig. 7 shows on the left side the ventral root of the seventh
nerve, the termination of the sixth nerve in the rectus
externus, the mandibular branch of the fifth nerve, the

122 PROF. MARSHALL AND MR. SPENCER.

second head cavity, and the termination of the palatine
branch of the seventh nerve. On the right side, which
represents a more posterior section, are shown the origin
of the sixth nerve, the auditory nerve and vesicle, the
palatine branch of the seventh nerve, and the second
head cavity.

Figs. 8 & 9. Transverse sections through the hind-brain of an embryo
of stageo. x 17.

Fig. 8, showing the ventral root of the fifth nerve; also the
optic nerves and infundibulum.

Fig. 9, on the left side shows the dorsal and ventral roots
of the seventh nerve, and the terminal branches of the
buccal and maxillary nerves ; on the right side the audi-
tory vesicle, the seventh nerve with its hyoidean branch,
and the termination of the palatine nerve.

Fig. 10. Longitudinal and vertical section through the head
of an embryo of stage u. The outline is from one section,
while the details have been filled in from several consecu-
tive sections in order to show the roots of the fifth and
seventh nerves, the relations of the third, fifth, and
seventh nerves to the head cavities, and the connections
of these three nerves with one another. The figure also
shows the glossopharyngeal nerve with its ramus dorsalis,
and its hyoidean branch. x 20.

Figs. 11 & 12. Diagrammatic longitudinal and vertical sections
through the head of an embryo of stage N; as in the
preceding figure the outline was drawn from one section,
and the details from other sections of the same embryo.

The two figures together show the roots and all the branches,
with their terminal distribution, of the fifth and seventh
nerves, the connections of these nerves ‘with one another
and with the third nerve, the principal branches of the
third nerve, and the course and distribution of the fourth
and ninth nerves, at stage n. No single section could
show all or even the greater number of the parts repre-
sented in either of these figures, as they lie at very
different levels. x 25.

THE CRANIAL NERVES OF SCYLLIUM. 123

Fig. 11, drawn to show the whole course and relations of the
fifth nerve and its connections with the third and seventh
nerves. The figure also shows the fourth nerve, and the
terminal distribution of the hyoidean branch of the seventh.

Fig. 12 shows the whole course of the seventh nerve and its
branches, with the exception of the hyoidean branch
(shown in the preceding figure); also the course and
distribution of the ninth nerve, and certain branches of
the fifth and tenth nerves.

Fig. 13. Longitudinal and vertical section through the head of an
embryo of stage n; the figure, which is taken from two
consecutive sections, shows the origin and main trunk of
the third nerve, the root of the fourth, and the roots,
course, and distribution to the rectus externus of the sixth
nerve. x 20.

Figs. 14 & 15. Diagrammatic longitudinal and vertical sections
through the head of an embryo of an age intermediate
between stages o and Pp. x 15.

Fig. 14 shows the roots of origin of the third, fifth, and
seventh nerves together with the course and distribution
of certain of their branches and the mutual connections
between these three nerves.

Fig. 15 shows the dorsal root of the seventh nerve, the
ophthalmic branches of the fifth and seventh nerves, the
ramus ophthalmicus profundus, the fourth nerve, the
maxillo-mandibular branch of the fifth, and the buccal and
palatine branches of the seventh.

Fig. 16. Transverse section through the head of an embryo of stage
Q, shortly before the period of hatching. Shows the origin,
course, and distribution of the fourth nerve, and its close
proximity to the ophthalmic branches of the fifth and
seventh nerves. x 10.

Fig. 17. Diagrammatic longitudinal and vertical section through the
head of an embryo of stage n. Shows the position and
arrangement of the segmental cranial nerves. x 20.

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THE SEGMENTAL VALUE OF THE CRANIAL NERVES.

By A. Mines Marspatt, I.A., D.Sc, Fellow of St. John’s College,
Cambridge ; Beyer Professor of Zoology in Owens College.

[The references are to Plates V & VI of the preceding paper.]

Whether the nerves arising from the brain are directly comparable to
those taking their origin from the spinal cord, and, if so, to hoy many
pairs of the more symmetrically arranged spinal nerves the cranial
ones are equivalent, are questions which have attracted the attention
and exercised the ingenuity of many of the greatest anatomists,
and which have been answered in the most varied senses by the
different writers who have attempted to grapple with their difficulties,
So long as the problems were attacked from the morphological side
alone, as was the case with all the earlier attempts to solve them, the
answers obtained were vague, inconclusive, and mutually contra-
dictory ; but since the clear light of embryology has been directed
upon them, the clouds of uncertainty have been to a very considerable
extent dispersed, and there is now, especially amongst those who have
most recently dealt with these questions, a very considerable and
satisfactory agreement as to the main outlines of the answers to be
given, although in many points of detail there is still much dis-
crepancy between the several accounts.

The present paper is an attempt to set forth the actual position of
these problems, and the leading phases through which they have
passed in their gradual maturation. In preparing it I have made

126 PROFESSOR MARSHALL

use of the investigations of others, so far as known to me, as well as
of my own published in this Journal and elsewhere."

HistoricAL SKetoH.—The older writers relied exclusively on
anatomical evidence in dealing with the problems before us, and
their determinations were rather of the nature of guesses than logical
endeavours to grapple seriously with the difficulties encountered.
Moreover, in the great majority of cases their judgment was influenced
in a very prejudicial manner by preconceived ideas on the morpho-
logical constitution of the skull.

Inasmuch as these older theories are all based on the same
arguments, and differ from one another only in points of minor
importance, it will be sufficient to take one of them and examine it
critically. For this purpose I select the theory advanced by Stieda,
the most recent, indeed the only recent, advocate of the views in
question.

Stieda,? in attempting to solve the problem of the segmental value
of the cranial nerves, commences by stating that as he accepts Oken’s
theory that the skull consists of three vertebree, the number of pairs
of segmental cranial nerves must necessarily be two; viz. a pair
leaving the skull between the first and second skull-vertebre on either
side, and a pair emerging between the second and third skull-vertebre,
the nerves passing out between the skull and the first cervical
vertebra being universally considered, when present, the first pair of
spinal nerves.

Having in this very summary manner determined the number of
segmental cranial nerves, Stieda proceeds to divide the nerves actually
present into two groups in accordance with this determination. He
first rejects the nerves of special sensation, z.e. the olfactory, optic,
and auditory, on the ground that embryology shows them to be really
parts of the brain, and therefore not directly comparable with the other
nerves.

Concerning the remaining nine pairs of nérves still left for
consideration, he holds that the most reliable evidence is afforded
by the fact that zm certain groups of animals some of these nerves do
not arise independently from the brain, but are represented by branches of
other nerves.

1 A list of the works consulted is given at the end of this paper.
2 Stieda, Studien tiber das centrale Nervensystem der Wirbelthiere, Leipzig, 1870. Separate
Abdruck wus der Zeitschrift fiir wissenschaftliche Zoologic, Bd. Xx,, p. 166, seq.

THE. SEGMENTAL VALUE OF THE CRANIAL NERVES, 127

Reasoning from these data, Stieda comes to the conclusion that the
component factors of his first cranial segmental nerve are the third
or oculomotor, the fourth or trochlear, the fifth or trigeminal, the
sixth or abducent, and the seventh or facial nerves ; and that of these
the third, fourth, sixth, and seventh nerves, and the motor root of
the fifth together represent the anterior or motor root, while the
sensory portion of the fifth nerve is the representative of the posterior
or sensory root. In support of these conclusions he adduces the
following arguments :—

1. That the three eye-muscle nerves and the facial nerve may
sometimes be replaced by branches of the trigeminal,’ and therefore
may be considered as belonging primarily to that nerve.

2. That the three eye-muscle nerves, the facial nerve, and the portio
minor of the trigeminal behave with reference to their origin from the
brain like the anterior roots of the spinal nerves; the portio major
of the trigeminal, on the contrary, like a posterior root: meaning by
this, the relations of the nerves in question to the nuclei of origin
within the substance of the brain.

The second or posterior cranial segmental nerve he considers to be
made up of the ninth or glossopharyngeal, the tenth or vagus, the
anterior roots of the eleventh or spinal accessory, and the twelfth
or hypoglossal nerves; the ninth, tenth, and anterior roots of the
eleventh making up the posterior root, and the twelfth nerve represent-
ing the anterior or motor root, the main grounds of determination being
the same as those relied on in the case of the supposed first nerve.

I have quoted Stieda at some length mainly in order to direct
attention to the nature of the evidence on which he attempts to solve
the question. The main points on which he relies are contained in
the passages I have italicised above, viz., (1) that the nerves of special
sense are contrasted with the other cranial nerves as being, properly
speaking, parts of the brain and not nerves in the strict sense of the
word; and (2) that in certain groups of animals one or more of the
cranial nerves may lose their more uswal independent character and
appear as, or be replaced by, branches of some other nerve; and
further, that this is to be taken as indicating that the nerves in
question were originally branches of this other nerve, and that thew
independent origin from the brain, when it does occur, is a secondarily
acquired feature.

* All the cases in which this replacement is alleged to occur will be discussed later on in
this paper;

128 PROFESSOR MARSHALL,

Now these two points are of primary importance, forming, as is at
once seen, the whole basis of Stieda’s argument; and in relying on
them he is very far from standing alone. Indeed, until some five or
six years ago, their correctness has been assumed, either tacitly or
explicitly, by the great majority of those who have dealt with the
question, including some of the most eminent anatomists of the time,
such as J. Miiller,! Arnold,*? Langer,’ Gegenbaur,* and, though in a
somewhat less positive manner, Huxley.’ I direct attention to this at
once, because we shall find further on that there are very strong
reasons for holding that neither of the points in question is really
correct. Ihave taken Stieda as the most recent representative of a
school to which C. V. Carus, Arnold, Buchner, J. Miiller, Langer,® and
many other prominent anatomists belonged, a school which attacked
the problem of the segmental value of the cranial nerves by first
determining perfectly independently the number of segments or
skull-vertebree in the head, a determination made as a rule on very
insufficient and often purely fanciful grounds, and having thus
decided the number of segments, and therefore of segmental nerves,
proceeding to apportion the several nerves to these segments, usually
in a very arbitrary manner. The writers named above differ, indeed,
in the number of head-segments they respectively adopt, but agree in
the principle on which they work, viz., determining the number of
segmental nerves from that of the supposed segments or vertebrae composing
the skull.

Stannius was the first to deal with the question in a more philo-
sophical spirit, and to attempt to determine the number of segmental
nerves by a direct study of the nerves themselves. The results of his
investigations’ are contained in his invaluable treatise on the Peripheral
Nervous System of Fishes published in 1849. He leaves the three
nerves of special sense out of consideration for the same reason aé
Stieda and the other anatomists we have mentioned, «¢, that they
are rather parts of the brain than true nerves. He also omits the

1 Joh, Miller, Handbuch der Physiologie des Menschen, 1844, p. 631.

2 Arnold, Handbuch der Anatomie des Menschen, 1851, Bd. ii. pp. 880-834,

+ Langer, Lehrbuch der Anatomie des Menschen, 1865, p. 429.

4 Gegenbaur, ‘‘ Uber die Kopfnerven yon Hexanchus,” Jendisene Zeitschrift, Bd. vi. 1871,
pp. 648-551.

6 Huxley, Vhe Anatomy of Vertebrated Animals, 1871, pp. 71-74.

© An excellent summary of the views of these and other writers on the segmental value
of the cranial nerves will be found in Stieda’s paper already quoted. They all agree in
principle with the account given above, the differences being merely in points of detail.

7 Stannius, Das peripherische Nervensystem dev Fische, Rostock, 1849, pp, 125-131,

Ay

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 129

eye-muscle nerves, remarking that any attempts to homologise them
with spinal nerves “encounter insuperable difficulties on account of
their peculiar origin, their absence of ganglia, and their exclusive
distribution to the muscles of a sensory apparatus, which are in no way
comparable with the muscles of the vertebra.” The remaining nerves,
however, Stannius deals with in a very complete and masterly manner.
He considers that the fifth, seventh, ninth, and tenth nerves are each
equivalent to a spinal nerve, and compares in detail both the roots of
origin and the branches of these nerves with those of the spinal nerves.

Stannius was also the first to point out the very important relations
of the ventral branches of these segmental cranial nerves to the
visceral arches. In the essay quoted above he shows how each visceral
arch is supplied by two branches belonging to different nerves, one
running along its anterior border, and one along the posterior. He
points out how the first branchial arch is supplied along its anterior
border by the glossopharyngeal nerve, and along its posterior by the
vagus ; how the remaining branchial arches are supplied by the vagus,
each arch by branches from separate stems; how the hyoid arch is
supplied in front by the hyoidean branch of the facial nerve, and
behind by the anterior branch of the glossopharyngeal; how the
mandibular arch has the mandibular branch of the trigeminal nerve
along its anterior border, and along its posterior the anterior branch
of the facial, which he identifies as the chorda tympani of Aves and
Mammalia; and finally, how the upper jaw is supplied by the
ophthalmic and maxillary divisions of the fifth nerve.

He concludes this portion of his treatise with the following very
suggestive sentence -—“Hence it follows that the number of the
ventral branches of each cranial nerve, and the number of the
spinal-like (segmental) cranial nerves is not determined so much by
the number of the skull-vertebre as by that of the visceral arches.”

In thus stating that the number of segmental cranial nerves was
no longer to be determined by preconceived ideas concerning the
composition of the skull, but by direct examination of the nerves
themselves, Stannius rendered an invaluable service to morphology.
Had he, indeed, gone one step further; had he been able to completely
disabuse his mind of this notion of skull-vertebree which was exercising
so pernicious an influence on the zoologists of the day, he would have

1 Stannius, op, zt. p, 181

—_—

130 PROFESSOR MARSHALL.

anticipated by more than twenty years Gegenbaur’s announcement?
of that theory of the vertebrate skull which has since, with some
slight modifications, been accepted almost universally.

While the school of morphologists we first dealt with determined
the number of the segmental nerves by that of the skull-segments,
Stannius showed conclusively that there was no relation whatever
between the two, but that ‘there was a very definite and remarkable
one between the segmental nerves and the visceral arches. Gegenbaur
went a step further, and, starting with the segmental nerves and
visceral arches, determined from them the number of head-segments,
thus completely reversing the order of proceeding of the older school.

Gegenbaur is sometimes credited with being the first to establish
the relations of the cranial nerves to the visceral arches, a determination
which, as we have seen, had been already made by Stannius. The
often quoted table of the cranial nerves given by Gegenbaur,? contains,
in fact, nothing that had not been already pointed out by Stannius,
except an attempt to rank the labial cartilages as visceral arches, an
attempt which has not met with general acceptance. Gegenbaur’s
real merit consisted in pointing out that the ideal number of skull-
vertebree, as determined by Oken and other “transcendental anato-
mists,” was to he left out of consideration altogether ; that the evidence
offered by the cranial nerves and visceral arches was to be accepted in
full, and was to be taken as the basis for determining the number of
segments in the head; and that the vagus nerve was, from the fact
of its supplying more than one visceral cleft, to be considered as
equivalent to more than one segmental nerve, and to be regarded as
formed by the fusion of a certain number of primitively distinct nerves,

Thus it has come to pass that the cranial nerves, while formerly
considered of very subordinate importance, are now recognised as
affording a very valuable and reliable clue to the solution of that
favourite morphological problem—the segmentation of the vertebrate
head ; and Gegenbaur’s paper, which was undoubtedly the chief means
by which the cranial nerves were rescued from their former dependent
position, must be viewed as marking a most important era in its
history.

Attention being thus pointedly directed to the cranial nerves, their
comparative anatomy and embryology quickly engaged the attention

1 Gegenbaur, ‘‘ Ueber die Kopfnerven yon Hexanchus,” Jéndische Zeitschrift, 1871.
2 Gegenbaur, loc. cit. p. 552:

THE SEGMENTAL VALUE OF THE CRANIAL NERVES, Uk

of zoologists ; and during the last five or six years our knowledge on
these points has received very material additions, additions which
have, on the whole, tended strongly to confirm Gegenbaur’s views,
while causing modification of them in many secondary points.

The most important of these more recent contributions is un-
doubtedly the series of facts brought to light by Balfour concerning
the early stages of development of the spinal and cranial nerves in
Elasmobranch fishes. Balfour showed that,’ contrary to the generally
accepted theory, the nerves are outgrowths from the central nervous
system, and therefore of epiblastic origin, instead of being, as formerly
supposed, structures arising independently in the mesoblast and only
acquiring a secondary connection with the brain and cord. .

In the case of the spinal nerves, he showed that the two roots,
anterior and posterior, arise separately and independently; that the
posterior roots are local outgrowths of a continuous longitudinal band
—the neural crest—which grows out along the mid-dorsal line of the
spinal cord. By lateral growth of the dorsal summit of the cord the
nerve roots of the two sides, which are at first directly continuous with
one another across the top of the cord, become separated to a certain
extent. The nerve root on either side grows downwards, closely
applied to the side of the cord, it then acquires a new or secondary
attachment? to the side of the cord, some little distance below the
primary one. A little later the primary attachment disappears, and
the secondary alone remains as the permanent attachment of the
posterior root to the cord.

The anterior roots arise later than the posterior, each as an indepen-
dent conical outgrowth from the latero-ventral angle of the cord.
The roots grow rapidly, and soon form elongated bands of fusiform
cells, which retain their original points of origin from the cord. Each
is at first, and for some time, quite distinct from the posterior root,
with which, however, it subsequently unites to form the adult spinal
nerve.

Further differences between the anterior and posterior roots
are afforded by the fact that the posterior develops at a very
early period a large ganglionic swelling—the future spinal ganglion—

1 Balfour, ‘‘On the Development of the Spinal Nerves in Elasmobranch Fishes,” Phit.
Trans. vol. clxvi: pt, 1, 1875; and A Monograph of the Development of Hlasmobranch Fishes,
1878, pp. 156-161 and 191-205.

£ The account of this shifting is based on my own observations. Balfour expresses
himself as “ inclined to adopt this view” (Comparative Embryology, vol. ii. p. 872), but does
not definitely do so.

132 PROFESSOR MARSHALL.

while the anterior root is devoid of ganglion cells. The roots of origin
of the anterior root are also very generally multiple, while those of
the posterior roots, whether primary or secondary, are apparently
invariably single.

Balfour’s observations were soon extended to birds and mammals,
and the description given above is now recognised as that of the
general and typical mode of development of the vertebrate spinal
nerves. It was further found that the neural crest is not confined to
the spinal cord, but extends forwards along the top of the brain,
and that certain of the cranial nerves are developed from it in the
same way as the posterior roots of the spinal nerves. By this
discovery a new and very reliable clue to the segmental value of the
cranial nerves is obtained, for it is clear that if certain of the cranial
nerves do, and others do not, conform to the mode of development of
the typically segmental spinal nerves, there is strong reason for
regarding the former as being of segmental value, and the latter
as not.

Embryology has furnished us with one further test of the segmental
value of cranial nerves, for which again we are indebted to Mr. Balfour,
who has shown that in Elasmobranchs (and the observation has since
been extended to other groups) the two halves of the ccelom or body-
cavity at an early period extend forward on either side of the neck
into the head, and that on the appearance of the visceral clefts each
of the halves becomes cut up in a series of isolated compartments,
one in each visceral arch.1 If the visceral clefts and arches are
segmental, it is clear that these “ head-cavities,” as they are called,
must be also, and that they will therefore afford an additional clue
to determining the segmental value of the nerves associated with them.

SUMMARY OF EVIDENCE OF SEGMENTAL VALUE OF CRANIAL NERVES.—
From what has been said above it will be evident that we have now
several independent tests of the metameric or segmental value of the
cranial nerves,—tests with all of which a nerve ought to comply to
entitle it to rank as segmental. For convenience of reference, these
tests, the majority of which have already been discussed, may be
enumerated here :2—

1 Balfour, Hlasmobranch Fishes, pp. 206-209; also Marshall, ‘‘Head-Cavities and Asso<
ciated Nerves of Elasmobranchs,” Quart. Journ. of Micros. Science, January 1881.

2 Cf. Marshall, ‘‘ The Morphology of the Vertebrate Olfactory Organ,” Quart. Journ. of
Micros. Science, July 1879, p. 317.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES, 133

1. Segmental nerves develop at a very early stage as outgrowths
from the neural ridge on the dorsal surface of the brain,

2. At an early period they shift downwards, and acquire new or
secondary roots of attachment to the sides of the brain.

3. The general course of the main stem of a segmental nerve is at
right angles, or nearly so, to the axis of the head at the point of
origin of the nerve. This feature, which is explained more fully in
the paper quoted above, is evident from an inspection of fig. 17, in
which the directions of the segmental nerves are shown, and from the
consideration that the course of segmental nerves must be approxi-
mately parallel to the boundaries of the segments to which they
belong : a segmental nerve could not run transversely across a number
of segments.

4, Segmental nerves have the characteristic relations to the visceral
clefts and arches, and, therefore, also to the head cavities in these
arches, first pointed out by Stannius as noticed above, each nerve
supplying the borders of one cleft, and therefore of two arches.
Concerning this test, it may be noted that, although from the
constancy of the relations of visceral clefts to other structures in all
vertebrates above Amphioxus, there can be no doubt that Gegenbaur,
Huxley, Semper, and others are correct in maintaining the segmental
value of these clefts, yet that the total absence of any correspondence
between the visceral clefts and the body segments in Amphioxus, and
still more in the Ascidians, makes it very doubtful whether this
segmental character is a primitive one.

5. Segmental nerves very constantly present ganglionic enlarge-
ments, either at or near their points of division into their two main
ventral branches.

Having thus cleared the ground, and explained what we mean by a
segmental nerve, and why it is of importance to determine which of
the cranial nerves are of segmental value, and which are not, I propose
to consider these nerves and discuss their claims in order, beginning
with the most anterior ones, and taking them in the sequence usually
adopted by anatomists.

I. Tue First or Otractory Nerve,—This nerve was until recently
supposed, by reason of its development, to stand quite apart from the
rest of the cranial nerves, and to be, properly speaking, a part of the

134 PROFESSOR MARSHALL,

brain rather than a nerve in the strict sense of the word.! Instead of
developing like the other nerves, the olfactory was stated to arise as
a hollow outgrowth from the anterior part of the cerebral hemisphere
—the so-called olfactory lobe or vesicle: it was also stated to arise
considerably later than the posterior cranial nerves,

It is now known that these supposed distinctions between the
olfactory and the other nerves do not really obtain,? but, on the
contrary, that the olfactory nerves develope in precisely the same
way as the other cranial nerves; that they arise at first from the
-upper part of the fore-brain and gradually shift downwards, acquiring
by so doing a secondary connection with the cerebral hemispheres, of
which they are at first completely independent ; and, finally, that the
olfactory lobe or vesicle so far from being the earliest part to be
developed is actually the last, no vestige of it appearing in the chick
until the seventh day of incubation, in the salmon till long after
hatching, or in dogfish until stage O of Balfour’s nomenclature.

If, then, the olfactory nerve agrees in all important features of its
development with the other cranial, and the spinal nerves, the further
question at once suggests itself,—has it segmental value ® An exami-
nation of the evidence at our disposal, which is unfortunately far from
complete, shows that there is much to be said in favour of such a
view ; thus, applying to the olfactory nerve the several tests of the
metameric value of cranial nerves in the order given above, we obtain
the following results :—

1. The olfactory nerve develops very early: the actual date of its
first appearance is very difficult to determine, and has not yet been
ascertained with certainty in any case, but in both the chick and
the dogfish it appears at a very early stage of development, and in
the chick, indeed, an attempt has been made to show that the
olfactory nerve is ‘fone of the first nerves in the body to appear,”*
arising before any of the spinal nerves. There is also evidence,
though as yet inconclusive, in favour of the origin of the olfactory
nerve in the chick from the neural crest. 4

1 Vide, ¢.g., Huxley, Anatomy of Vertebrated Animals, p. 71; and Gegenbaur, Elements
of Comparative Anatomy, English Translation, p. 515.

2 Marshall. ‘‘ Morphology of Vertebrate Olfactory Organ,” Quart. Journ. of Micros.
Science,” July 1879; and Balfour, Comparative Embryology, vol. ii, 1881, pp. 336 and 382.

3 T have dealt with this question at some length in a former paper on ‘‘ The Morphology
of the Vertebrate Olfactory Organ,” Quart. Journ. of Micros. Science, July 1879; to which
I would beg to refer the reader who may desire further details than I can give here.

* Marshall, “The Development of the Cranial Nerves in the Chick,” Quart, Journ. of
Micros. Science, Jan. 1878, p. 23.

~

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 135

2. The olfactory nerve resembles the segmental nerves in undergoing
during the earlier stages of its development a very considerable
displacement of its root of attachment to the brain, and as this
feature is one of the most remarkable characters of these segmental
nerves, and is, so far as we know, confined to them, its occurrence in
the olfactory nerve must be admitted to be of much weight.

In both the dogfish and chick the olfactory nerves are clearly
recognisable before the cerebral hemispheres have commenced to
develop, the nerves at this stage arising from the dorsal part of the
sides of the original fore-brain or anterior cerebral vesicle. The
hemispheres in the chick are lateral outgrowths of the fore-brain, and
are from the first situated above, z.e. on the dorsal side of, the roots
of the olfactory nerves ; they grow forwards and upwards with great
rapidity, driving the olfactory nerves down to the base of the brain,
and so causing these nerves to appear to arise from their under and
anterior part. Whether the root of the olfactory nerve undergoes
any change comparable to the secondary attachment described above
as occurring in the spinal nerves, has, however, not yet been ascer-
tained.

3. The general course of the olfactory nerve in the early stages of
development is, like the segmental nerves, at right angles to the axis
of the head at the point of origin of the nerve, although, owing to
cranial flexure, it is very far from being parallel to the hinder
seomental nerves. This feature is shown in fig. 17,1. In the later
stages of development, owing to the forward growth of the nasal
region this relation becomes completely lost.

4, Concerning the relations of the olfactory nerve to visceral arches
and clefts, I must beg to refer the reader to the paper quoted above,
in which I have drawn attention to “the very close resemblance as to
form, structure, general relations, time of appearance, &c., existing
between the olfactory organ and the gill clefts,” and have adduced
other arguments on which I have attempted to establish the following
conclusions :—“ That the olfactory organ is the most anterior visceral
cleft; that the olfactory nerve is the segmental nerve supplying the
two sides of that cleft in a manner precisely similar to that in which
the hinder clefts are supplied by their respective nerves ; and that the
Schneiderian folds are homologues of gills.”

+ Morphology of Vertebrate Olfactory Organ,” Quart. Journ, of Micros. Science, July
1879, p. 330.

136 PROFESSOR MARSHALL.

5. The olfactory nerve is distinctly ganglionic near its root of origin
from the brain in Elasmobranchs and in the chick.

It would thus appear that although the evidence is at present far
from conclusive, and although further information is needed on many
points, notably concerning the earliest stages of development of the
olfactory nerve, yet that the nerve answers fairly well to the tests of
segmental value as defined above; and it is important to note that
the points in which it responds incompletely are precisely those on
which our knowledge of the nerve is avowedly imperfect, and that in
no case is a test directly contradicted. I am therefore disposed, while
fully admitting the need for further investigation, to rank the olfactory
nerve as the most anterior of the cranial segmental nerves, the nerve
belonging to the first head-segment.

The segmental value of the olfactory nerve has recently been
advocated by Wiedersheim, who draws attention to the fact that in
Epicrium, and probably in other Gymnophiona as well, there are on
either side two olfactory nerves, one dorsal and one ventral, the roots
of the two being perfectly independent, and some little distance apart.!
Wiedersheim considers that these two roots are homologues of the
dorsal and ventral roots of a spinal nerve, and that by their discovery
the segmental rank of the olfactory nerve may be considered to be
established.

A similar condition of the olfactory nerve in Pipa dorsigera has
been figured, though not described, by Fischer.*

These two cases, in both of which the additional root is the dorsal
one, tend strongly to confirm the view taken above of the primitive
connection of the olfactory nerve with the dorsal surface of the brain,
and therefore presumably with the neural crest ; but in the absence
of any observations on either the development or the physiological
properties of the two rovts in question, I do not think that much
weight can be attached to Wiedersheim’s suggestion of their homology
with the roots of a spinal nerve.

Balfour argues against the segmental value of the olfactory nerve,
on the ground that it is incompatible with the views which he holds
concerning the primitive vertebrate mouth, and concerning the relations
between the nervous systems of vertebrates and invertebrates. His

1 Wiedersheim, Die Anatomie der Gymnophionen, Jena, 1879, pp. 59, 60, and pl. iv. fig.
35, pl. vi. fig. 62.

2 Fischer, Amphibiorum nudorum neurologice specimen primum, 1843, Tab. ii. fig. 1,

& Balfour; Comparative Embryology, vol, ii. 1881, pp. 260-265 and 383,

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 137

views on these points are of very great interest and importance ; but
inasmuch as they involve the descent of Chetopods and Vertebrates,
not from a common segmented ancestral type, but from a common
unsegmented type, and also the existence of a group of segmented
animals, which “appears now to have perished” without leaving any
trace behind, it would clearly be impossible to discuss them here in
full. His theory that the vertebrate fore-brain is the homologue of the
supra-cesophageal ganglia of Arthropods and Cheeotopods is, however,
to my mind open to very serious objections, some of the more weighty
of which he has himself mentioned, viz., (1) that there is no actual
anatomical or embryological break between the fore-brain and the
hinder portion of the central nervous system, such as one might
reasonably expect to find on his hypothesis ; (2) that the lowest known
vertebrate, Amphioxus, instead of lending any support to this view,
distinctly contradicts it, the fore-brain being less differentiated from
the hinder portion than in any other vertebrate, while ‘‘the termina-
tion of the notochord immediately behind the fore-brain ”—almost
the only direct evidence he adduces in favour of the “ morphological
distinctness ” of the fore-brain—again fails completely, the notochord
in Amphioxus, as is well known, extending to the extreme anterior end
of the head, some distance beyond the front end of the brain.

IJ, Tue Seconp or Optic Nerve.—Although, as we have just seen,
the statement that the olfactory nerve is rather a part of the brain
than a nerve in the strict sense of the word, is found on examination
not to hold good, yet, as regards the optic nerve, it is certainly
correct ; the mede of development of the optic nerve, which is too
well known to require a detailed description here, placing it in this
respect in marked contrast to every other nerve in the body.

From the fore-brain or anterior cerebral vesicle two hollow lateral
outgrowths arise—the optic vesicles. These become constricted at
their origin from the brain, the constricted portions or optic stalks
becoming ultimately the optic nerves. By a process of unequal growth
or the different parts, coupled with a direct pushing in of the outer
wall by the formation of the lens, each vesicle becomes doubled up
on itself, the outer wall being pushed back into the inner, and so
giving rise to the double-walled “optic cup” or secondary optic
vesicle,

| ! This mode of development, which, with secondary modifications,

138 PROFESSOR MARSHALL.

applies to all vertebrates except Amphioxus, and must therefore be
considered as primitive so far as vertebrates are concerned, differs go
fundamentally from. the development of the hinder cranial or spinal
nerves that no comparison whatever is possible between thera. The
optic nerve must therefore be regarded as one suc generis, and
which can accordingly have no claim to be considered of segmental
value.

The existence of this clearly non-segmental nerve between the
olfactory and the hinder nerves is undoubtedly an objection to the
view advocated above concerning the segmental value of the olfactory
nerve; but until we otain a clearer light than we are at present able
to throw on the phylogenetic history of the vertebrate eye, and indeed
of the vertebrate race altogether, it is difficult to gauge properly the
weight of the objection.

Tue Eys-Muscite Nerves.—Concerning the morphological value of
these three nerves—the third, fourth, and sixth pairs—opinions have
perhaps differed more than in the case of any of the other cranial
nerves.

The nerves in question are small, with a singularly limited and
constant distribution to the muscles moving the eyeball, and to certain
other parts in connection with the eye, the third nerve supplying the
rectus superior, rectus internus, rectus wmferior, and obliquus inferior
muscles of the eye ball, also the levator palpebre superioris and the
circular muscle of the iris; the fourth nerve supplying the obliquus
superior muscle, and in many vertebrates sending sensory branches to
the conjunctiva and the skin of the upper eyelid ; and the sixth nerve
supplying the rectus externus muscle, and in many cases the suspensory
muscle of the bulb of the eye and the muscles of the nictitating
membrane. In dealing with these it will be convenient to consider
them at first collectively, inasmuch as many points of importance
concern them all alike, and afterwards to consider briefly the several
points of individual interest which they present respectively.

Until very recently it was the almost universal custom amongst
anatomists, when discussing the segmental value of the cranial nerves,
to exclude the eye-muscle nerves altogether from consideration, on the
ground that they were not constant in their distribution, but that one or
more of the muscles normally supplied by them might under special
circumstances be supplied by branches of the fifth nerve, the further

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 139

inference being drawn from these special cases that the eye-muscle
nerves were primitively branches of the fifth nerve, which have in the
majority of existing vertebrates attained independence and acquired the
appearance of distinct nerves, a title to which they have really no claim.

This view has very recently been advocated by Wiedersheim,
whom I quote in order to illustrate my statements. In dealing with
the fourth nerve in the frog he notices that it usually forms anastomotic
communications with the ophthalmic branch of the fifth nerve as it
crosses it, and that the number of these communicating branches is
very variable. He then says:—“ Dies eben beschriebene Verhalten
sowie auch dasjenige des Adbducens und des spiiter abzuhandelnden
Oculomotorius liefert eine hiibsche lustration zu der in hoheren
Thiergruppen in immer stirkerer Weise hervortretenden Tendenz der
Augenmuskelnerven, sich von ihrem Stammboden, der Trigeminus-
eruppe, zu emancipiren, um endlich eine gut individualisirte Selbst-
stiindigkeit zu erlangen.” ?

As this view, so definitely expressed by Wiedersheim in the above
passage, appears to have met with very general acceptance, and as it
very seriously affects and concerns the subject of the present paper, I
have taken some trouble to collect all the recorded cases in which the
distribution of the eye-muscle nerves or the supply of the eye-muscles
in vertebrates is said to present any constant deviation from the nor-
mal arrangement as noticed above; and I propose now to examine
critically these alleged exceptions to the general rule.

1. Amphiovus.’—The azygos character of the eye and its extreme
simplicity of structure render any comparison with the eyes of higher
and more typical vertebrates perfectly futile.

2. Marsipobranchir.

(a) Hyperotreti.itAmong the myxinoid fishes, according to Stan-
nius', J. Miiller,? Huxley,® Gegenbaur,’ and others, the eye-muscle
nerves are completely absent. Here again we are dealing with animals

1 Vide, e.g., Gegenbaur, ‘‘ Ueber die Kopfnerven von Hexanchus,” Jenaische Zeitschrift,
1871, pp. 548, 549; Huxley, Anatomy of Vertebrated Animals, 1871, p. 73; also Stieda and
the various authors quoted by him in his ‘ Studien ueber das centrale Nervensystem der
Wirbelthiere,” Zeitschrift fiir wissenschaftliche Zoologie, Bd xx. 1870.

2 Wiedersheim, in Ecker’s Anatomie des Frosches, Zweite Abtheilung, 1881, p. 24, note 1.

$ Stannius, in his Handbuch der Anatomie der Wirbelthiere, Zweite Auflage, 1854, p. 161,
notices the absence of the eye-muscle nerves in Amphioxus.

* Stannius, op. cit.; p. 161.

5 J. Miiller, Vergleichende Neurologie der Myxinoiden, p. 49.

®° Huxley, Vertebrates, p. 73.

7 Hexanchus, p. 549.

140 PROFESSOR MARSHALL.

in which the eyes are in a very rudimentary condition, and the eye-
muscles either absent or extremely imperfectly developed ; so that, as
pointed out by Schwalbe,’ no importance can be attached to them in
determining the question of the primitive independence of the eye-
muscle nerves, and this consideration is much strengthened by the
strong evidence we possess of the Myxinoids being degenerate or de-
eraded forms.*

(6) Hyperoartiz.—Attention has been directed to the condition of the
eye-muscle nerves in the lampreys by a number of writers. According
to Schlemm and d’Alton,°® the lampreys have independent eye-muscle
nerves, but their number is diminished, and some of the muscles are
supplied by the fifth nerve. The fourth nerve is described as having
its usual origin behind the optic lobes and entering the orbit in com-
pany with the third, which has an independent origin in front of that
of the fifth, The combined nerve, formed by the union of the third
and fourth, divides into two main branches, an upper one supplying
the rectus superior, and a lower one supplying the rectus internus and
obliquus superior. The three other muscles, viz., rectus inferior, rectus
externus, and obliquus inferior, are said to receive their nerves from
the trunk of the fifth nerve.

Fischer* and Stieda’ also refer to the peculiar distribution of the eye-
muscle nerves in the lampreys, but avowedly draw their information
from Schlemm and d’Alton’s paper, from which it would appear that
Huxley,® and probably Owen’ and Giinther® also, derive their ac-
counts.

Gegenbaur® gives a slightly different account. He says that in
Petromyzon there is an independent fourth nerve, but that the sixth
is a branch of the fifth, which supplies the rectus inferior as well as
the rectus externus, while the third nerve is limited in its distribution,
supplying the rectus superior, rectus internus, and obliquus imferior.
He gives no reference in support of his statement, and must therefore
be supposed to make it on his own authority, especially as it differs

1 Schwalbe, ‘‘ Das Ganglion Oculomotorii, Jenazsche Zeitsehrift, Bd. xiii., p. 71.
2 Cf. Balfour, Comparative Embryology, vol. ii., 1881, p. 268, note 2.

8 Schlemm u. D’Alton, ‘“‘ Ueber das Nervensystem der Petromyzon,” Miiller’s Archiv, 1838

* Fischer, Amphibiorum nudorum Neurologic: Specimen Primum, 1843, p. 47.

5 Stieda, loc. cit., p. 174.

6 Huxley, Anatomy of Vertebrated Animals, p. 73.

7 Owen, Anatomy of Vertebrates, 1866, vol. i., p. 300.

8 Giinther, Introduction to the Study of Fishes, 1880, p. 105.
® Gegenbaur, Hexanchus, p. 549, note 1,

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 141

notably from the accounts of all other writers whom I have been able
to consult.

Concerning the above accounts, it appears that they can be reduced
to two sources—(1) the description given in 1838 by Schlemm and
@Alton, which I have assumed to be the source from which Owen,
Huxley, and Giinther obtain the accounts given in their text-books
quoted above, because their descriptions, which are very brief, agree
exactly with that of Schlemm and d’Alton, and add nothing to it; and
(2) Gegenbaur’s description in 1871, which must be independent, in-
asmuch as it does not quite agree with Schlemm and d’Alton’s.
According to Gegenbaur, the only peculiarity is that the sixth nerve is
not independent but a branch of the fifth, which supplies the rectus
inferior as well as the rectus externus ; while, according to Schlemm
and d’Alton, three of the muscles—the rectus inferior, rectus externus,
and obliquus inferior—are supplied by the fifth nerve ; and, in addition
to this, the third and fourth nerves unite together, a point which
Gegenbaur does not notice.

The dissection is a difficult one, on account of the small size of the
nerves concerned ; and additional evidence from direct observation is
necessary before we can decide whether either of the above descrip-
tions is perfectly correct.

There are, however, certain points of considerable importance which
concern not only Petromyzon, but many other forms as well, and may
be conveniently dealt with here,

Both the third and fourth nerves are distinctly stated to have in-
dependent roots of origin, and to arise from the normal situations in
the brain; and this being the case, I wish to point out that the ana-
tomical arrangement of the nerves would probably be more correctly
described by saying that the third nerve, though having a separate
root of origin, becomes connected with the fifth, so that in the adult
some of its branches appear to be derived from the fifth ; than by
saying, with Huxley, Stieda, and Giinther, that the muscles in question
are supplied by branches of the fifth.

Fischer long ago adopted this view. He describes, on Schlemm and
d’Alton’s authority, the condition of the nerves in Petromyzon in these
words: — “Genus Petromyzon duas Oculomotorii ostendit partes,
alteram liberam, parisque quarti quoque continentem fibras, alteram
cum Trigemino conjunctam ;”? and Stannius gives still clearer ex-

1 Fischer, op. cit., p. 47, note 1,

142 PROFESSOR MARSHALL.

pression to it ; for after referring to Schlemm and d’Alton’s observa-
tions, he says :—‘ Offenbar ist hier ein Theil der Wurzelelemente des
N. oculorum motorius, so wie auch die Wurzel des N. abducens, in
die Bahn des N. trigeminus, tibergetreten,”1 and remarks that it is
quite possible that a very careful examination of the nerve-roots would
show that the abducens has really an independent root of origin.

The point at issue is an important one, and must be clearly stated.
When we find two nerves—the third and fifth, which in the great
majority of vertebrates are independent of one another both in origin
and distribution—in certain forms, as the lampreys, arising from the
brain independently and normally, but becoming united together at
some point or other of their course, so that it is no longer possible
from mere anatomical observation to say with certainty to which of
the two a given branch belongs, are we to infer, as is done tacitly or
explicitly by many writers,’ that the condition shown by the lamprey
is the more primitive one, and represents an intermediate stage in the
process by which the eye-muscle nerves gradually emancipated them-
selves from their parent nerve—the fifth—and attained ultimately
the complete independence they show in the great majority of existing
vertebrates? Or, on the other hand, are we to infer that the indepen-
dent origin of the third nerve is primitive, and that its connection
with the fifth, when, as in the lamprey, it does occur, is a secondarily
acquired one? ‘To my mind there can be no doubt whatever that the
latter is the correct explanation; and the chief reasons that lead me
to think so are the following :—

(2) Though we know of instances—notably in the case of the vagus
—of nerves originally distinct and independent gradually becoming
fused, and then this fusion getting thrown back to a very early de-
velopmental stage; yet we know of no established case of a branch
attaining independence, and acquiring the character of a distinet
nerve.

(b) Supposing it were possible for such a process to occur, it would
certainly be very surprising if, as in the supposed case of the third nerve,
the process of differentiation should commence at the proximal end,
and that there should be a stage in which the roots were independent
and the two nerves still fused distally.

(c) There are very strong reasons, which we shall discuss later on,

2 Stannius, Das peripherische Nervensystem der Fische, p. 18.
a Cf. the authors mentioned above, and especially the passage quoted from Wiedersheita
on p. 189 above,

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. _ 143

for viewing both the third and fifth nerves as segmental, and there-
fore primitively independent of one another.

(d) If Wiedersheim’s view were correct, we should certainly expect
the third nerve of higher vertebrates in its early stages of develop-
ment to show some indication of its supposed primitive connection
with the fifth. So far, however, is this from being the case, that in
all cases where the development of the third nerve has yet been
traced, it is a perfectly independent nerve from the start.1

(ec) A crucial test is afforded by the fact that other nerves—e.g.
the fifth and seventh—though, as a rule, separate from one another
throughout the vertebrate series, may in some forms become more or
less closely united together, so that it is impossible by mere ana-
tomical evidence to distinguish branches of the one from those of the
other; the forms in which this fusion of the fifth and seventh nerves
occurs being, as we shall see more fully later on, in many cases the
same as those in which the fifth and the eye-muscle nerves tend to
fuse. In the case of the dog-fish, in which this fusion of the fifth and
seventh nerves is a marked feature of the adult state, all the stages
of development are now known,’ and it is found that, so far from the
state of fusion being a primitive one, the two nerves are in their early
stages quite independent and some distance apart, as in other verte-
brates, and that their subsequent gradual approximation and fusion
are purely secondary characters.

The above arguments appear to me to establish the proposition that
the third nerve is primitively an independent one,’ and that its partial
fusion with the fifth, when it occurs, is a purely secondary and not a
primary character.

If they prove the case for the third nerve, so also for the fourth
and sixth nerves. The presence of independent roots of origin from
the brain must be held to establish that, however close may be the
connection of their trunks with the fifth nerve, they are really in-
dependent nerves, and are not to be described as being “ replaced by
branches of the fifth nerve.”

1 Marshall, ‘On the Development of the Cranial Nerves in the Chick,” Quart. Jowrn. of
Micros. Science, Jan. 1878, pp. 23—27; and ‘‘On the Head Cavities and Associated Nerves of
Elasmobranchs,” Quart. Journ. of Micros. Science, Jan. 1881, pp. 783—88.

2 Marshall and Spencer, ‘‘ Observations on the Cranial Nerves of Scyllium,” Quart. Jowin.
of Micros. Science, July 1881, pp. 482—486.

5 The independence of the third nerve has recently been upheld on anatomical grounds
by Schwalbe—Das Ganglion Oculomotorit; and by Balfour, on embryclogical ones—Com:
parative Embryology, vol. ii.

144 PROFESSOR MARSHALL.

In the case of the lampreys, then, I hold that we have no reliable
evidence of the third or fourth nerves being in any way abnormal in
their distribution to the eye-muscles; while, as regards the sixth
nerve, although no distinct root of origin has yet been seen, I hold,
with Stannius, that a much more careful and searching investigation
must be made for it before any statement as to its absence can be
accepted.

3. Ganoide.—In the majority of Ganoids the nerves of the eye-
muscles have the normal arrangement, and are completely indepen-
dent of the fifth, except where the third unites with the ophthalmic
branch of the fifth at the ciliary ganglion. Only one exception is
known.

In Lepidosteus according to J. Miiller,’ the arrangement is abnormal,
the third and fourth nerves entering the orbit closely united with the
ophthalmic division of the fifth, of which they appear as branches.
The sixth nerve is described and figured as accompanying the main
trunk of the fifth, but distinct from it.

Stieda, in his essay® before referred to, quotes Miiller’s account, but
does so incorrectly, making Miiller say that there is a distinct fourth
nerve, but that the third and sixth are replaced by the branches of
the fifth ; whereas Miiller really says that the sixth is a distinct nerve,
and that the third and fourth are, not “replaced by branches of the
fifth,” but contained in the ophthalmic nerve.

Stannius,® referring to Miiller’s account, observes that it is pro-
bably merely another instance of juxtaposition of originally distinct
nerves.

Concerning this alleged exception, we notice in the first case that
it rests on a solitary description, which has not yet been confirmed, and
that confirmation is needed is evident from the figures referred to.
Miiller gives two figures of the cranial nerves of Lepidosteus, which
do not agree in all points; indeed, the points of difference are so
marked that the two figures are by no means €asy to reconcile
with one another. Miiller’s figure 3 appears to me to present nothing
exceptional, except that the third and fourth nerves enter the orbit as
one trunk, and that the fourth nerve at the point where it crosses the

1 J. Miiller, Ueber den Buu und die Grenzen der Ganoiden, 1846, p. 97, and plate iv. figs. 2
and 3.

2 Stieda, loc. ctt., p.174.

3 Stannius, Das peripherische Nevvensystem der Fische, p. 19.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES, 145

portio minor of the ramus ophthalmicus superficialis' is rather more ex-
tensively connected with this nerve than is usually the case. The
nerve which Miller marks y, and calls the “ ophthalmic branch of the
fifth,” but which he does not seem to have followed to the brain, I
see no reason for considering as other than what one would naturally
suppose it to be from its distribution to all the eye-muscles except the
rectus externus, t.e. the combined third and fourth nerves. In Miiller’s
ficure 2 there is a remarkable point of difference, inasmuch as the
nerve which I have considered in the former figure to be the proximal
part of the portio minor, or trigeminal portion of the ramus ophthal-
micus superficialis, is entirely omitted. No mention is made either in
the text or in the description of the figures of this very important
difference. I would further notice that, although the two figures in
question are drawn of the same size and to the same scale, yet that
the relative proportions of the several nerves, and more especially the
extent to which they are fused with one another, are so very different
in the two cases that one is driven to suppose either that the figures
are taken from different specimens, in which case there must be con-
siderable individual variability in the very points alleged to be excep-
tional, or else that one or others of the figures is taken from an
incomplete dissection.

The above considerations lead to the conclusion that, in the absence
of direct confirmation, Miiller’s account of the eye-muscle nerves in
Lepidosteus does not prove that they are in any way exceptional,
except in the fact of the third and fourth nerves entering the orbit as
one trunk.

Very important information concerning these nerves in Lepidosteus
has recently been afforded by Schwalbe, who finds, from a careful
examination of the nerves and brain, that both the third and fourth
nerves have independent origins from the brain; a fact which, as in
the case of Petromyzon, must be held to conclusively prove that such
connection as may actually occur between the fifth nerve on the one
hand, and the third and fourth on the other, beyond their roots of
origin, is of a purely secondary character, and that it does not in the
very slightest degree militate against the claims of the third and
fourth to rank as independent cranial nerves.

1 For the nomenclature of these ophthalmic nerves, vide Marshall and Spencer, ‘‘ Obser-

vations on the Cranial Nerves of Scyllium,” part i., Quart. Jowrn. of Micros. Science,
July 1881.

2 Schwalbe, Das Ganglion Oculomotorit, pp. 23, 72, and 73.
ae

a

146 PROFESSOR MARSHALL.

4, Teleoste.—The only recorded instances that I can find of devia-
tion from the normal arrangement of the eye-muscle nerves among
osseous fish are :—

(a) Amblyopsis,' the blind fish of the Mammoth cave of Kentucky,
in which the eyes are rudimentary and functionless, and the eye-
muscle nerves, as might be expected, absent.

(0) Silurus glanis, in which, according to Stannius,? the eyes are
small, the eye-muscles very slender, and the eye-muscle nerves outside
the skull closely united with the ophthalmic branch of the fifth. Stan-
nius points out, however, that careful examination shows that all three
nerves arise independently from the brain at the normal situations,
and expressly notices that, but for the discovery of these extremely
slender roots, the eye-muscle nerves of Szlwrus would have been
beyond all doubt described as branches of the fifth nerve. It is of
course probable that in the other species of blind fish, whether living
in caves, as Zyphlichthys, Stygicola, Gronias, Ailia, &e., or living at
ereat Ocean depths, as the Scopelide, the eye-muscle nerves are, as in
Amblyopsis spelceus, rudimentary or absent; but it will be sufficiently
evident, from what has been already said, that neither these blind
fish nor such cases as Silurus tell in any way against the independent
rank of the eye-muscle nerves.

5, Dipnot.—In his account of the African Lepidosiren (Protopterus)
annectens, Professor Owen’ notices that the optic nerves “ are remark-
ably small, in correspondence with the feebly-developed organs of
vision ;” also that the eye-ball ‘‘has no special muscles, whence the
absence of the third, fourth and sixth cerebral nerves.”

)

According to Hyrtl* in the South American form, Lepidosiren para-
doxa, in which also the eyes are very small, the four recte muscles are
present, but the two obliqgui not represented. The eye-muscle nerves
were not found, but were believed to be replaced by two fine branches
of the ophthalmic division of the sixth nerve, which branches, how-
ever, were not traced into the rect: muscles. ,

Professor Humphry’s® description of Lepidosiren (Protopterus) annec-

1 Noticed by Stannius, Das pevipherische Nervensystem, p. 18; and Schwalbe, Joc, cit.,
p. 71.

2 Stannius, op. cié., pp. 18, 19.

3 Owen, “Description of the Lepidostren annectens,” Trans. Linnean Soc., vol. xviii.,
1839, p. 340.

4 Hyrtl, ‘' Lepidostren paradoxa,” Prag. 1845, p. 44,

5 Humphry, Observations in Myology, 1872; The Muscles of Lepidosiren annectens with
the Cranial Nerves, pp. 77 and 79.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 147

tens very closely agrees with Hyrtl’s of Z. paradoxa in the points with
which we are now concerned. He finds, contrary to Owen, that the
four rectt muscles “may clearly be distinguished,” though there are
no obliqui. “Special nerves to these muscles (the third, fourth, and
sixth) were not found ;” but the ophthalmic division of the fifth is
described as giving off in the orbit “ ciliary and oculo-motor nerves,”
which, however, do not appear to have been traced to their distri-
bution.

Gegenbaur’? simply quotes Hyrtl to the effect that all three eye-
muscle nerves are represented by branches of the fifth; which, how-
ever, is a wider and more positive statement than Hyrtl really made.

Stannius? also states, on Hyrtl’s authority, that the eye-muscle
nerves have no independent roots; and Huxley* notes that in Lepido-
siren “the three motor nerves of the eyeball are completely fused
with the ophthalmic division of the fifth,” a condition which he is dis-
posed to view as the most primitive arrangement met with among
vertebrates.

In considering what importance is to be attached to this often-
quoted exception to the general rule, we have first to notice that we
are dealing with animals in which the eyes are “ very small” and
“‘feebly developed ;” secondly, that the eye-muscles are so small that
their very existence was not only overlooked, but expressly denied, by so
competent an anatomist as Professor Owen; thirdly that the two anato-
mists, Hyrtl and Humphry, who have described these muscles, agree
in saying that the rectz muscles are alone present, a condition clearly
not fully realised by those who state, on Hyrtl’s authority, that the
fourth nerve is, like the third and sixth, represented by a branch of
the fifth ; fourthly, that in neither of the cases mentioned were the
nerves actually traced into the muscles in question.

To these points we must add one, urged with great force by
Schwalbe,* and which acquires much weight from the cases of Petro-
myzon and Lepidosteus already considered, viz. that a sufficiently care-
ful examination of the brain has not been made to render us certain
as to the alleged absence of independent roots of origin for such of
the eye-muscle nerves as may be present.

The importance of Schwalbe’s warning is strikingly exemplified by

1 Gegenbaur, Hexanchus, p. 549.

2 Stannius, Das peripherische Nervensystem, p. 18.

8 Huxley, Anatomy of Vertebrated Animals, p. 78, note.
* Schwalbe, Das Ganglion Oculomotorti, p. 72.

148 PROFESSOR MARSHALL.

the recent observations of Wiedersheim' on the nervous system of
Lepidosiren (Protopterus) annectens. Wiedersheim describes a mode-
rately long but exceedingly slender nerve which leaves the skull
through a special foramen in front of that of the fifth, and loses itself
in the eye-muscles in a manner which he was unable to determine with
certainty. In spite, however, of taking “all conceivable pains,” he was
unable to ascertain whether this hitherto overlooked eye-muscle nerve
arises independently from the brain, or is a mere branch of the fifth,
though he is inclined himself to regard it as an independently arising
third nerve.

Under these circumstances, and especially when we consider Wieder-
sheim’s discovery of a distinct eye-muscle nerve, and his statement of
the extreme difficulty he experienced in tracing this nerve even to the
limited extent which he succeeded in doing, we must, I think, con-
clude that, whatever subsequent investigation may tell us, Lepedosuren
at present offers no definite or reliable evidence against the statement
that the eye-muscle nerves are independently arising nerves in all
vertebrates in which the eye-muscles themselves are present.

6. Amphibia.—Statements of exceptional innervation of one or more
of the eye-muscles among Amphibia are by no means uncommon ; and
though I have devoted some time to making my list as complete as
possible, I am far from certain that I have succeeded in collecting all
the alleged cases. The following list includes all I have been able to
refer to, and certainly all that are mentioned in the standard works
and papers on the subject :—

A. Apoda (Gymnophiona).—-Wiedersheim, in his monograph on this
group,” mentions that in Cecilia the eye-muscles are present, but of
exceedingly small size, so small indeed that he could not make out
either their number or arrangement ; neither was he able to ascertain
anything concerning their innervation ; indeed, he makes no mention
whatever of the eye-muscle nerves. Fischer’ also failed, from his dis-
section of a single specimen, to make out anything definite concerning
the eye-muscle nerves. Inasmuch as the eyes of Cwcdlia are very
small, it would seem probable that we have here another instance of
rudimentary eyes, accompanied very possibly by a reduction in the
number of eye-muscles ; and we have already seen that the evidence

1 Wiedersheim, Morphologische Studien, Heft 1; III. Das Shkelet und Nervensystem von
Lepidosiren annectens, 1880.

2 Wiedersheim, Die Anatomic der Gynutophionen. Jena, 1879, pp. 55, 56, and 61.

8 Fischer, op. cit., p. 47.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES, 149

yielded by such cases cannot be accepted as in any way affecting the
question of the primitive independence of the eye-muscle nerves.
B. Caudata (Urodela).

(a) Proteus.—The specimen of Proteus dissected by Fischer" was, like
that of Cecilia, too imperfectly preserved to permit him to make any
positive statement concerning the eye-muscle nerves ; indeed, he calls
attention to and expressly regrets his inability to determine whether
these nerves are present or absent. The eyes of this cave-dwelling
amphibian are situated beneath the skin, and are of very rudimentary
structure, being arrested at what is in other vertebrates a very early
embryonic condition.2 As has been pointed out by Schwalbe,’ Fischer
does not in any way deny the existence of eye-muscle nerves, but
merely records his inability to find them in a very imperfectly pre-
served specimen.

(6) Salamandra and Triton.—I take these two genera somewhat
out of their proper zoological order, because they afford perhaps the
most widely-known and frequently-quoted examples of abnormal in-
nervation of the eye-muscles—instances which must accordingly be
carefully considered.

Fischer, who was the first to draw attention to the point,* states
that in Salamandra and 7riton the third nerve, though rising indepen-
dently from the brain, only supplies three of the eye-muscles—the
rectus internus, rectus inferior, and obliquus inferior—the rectus superior
receiving a special branch from the ‘nasal division” of the fifth,
which branch is absent in Anwra in which the innervation is normal.
In discussing the importance of this, he says :—“ Quid igitur vert
possit esse similius, quam quod partium duarum, in quas penes Salaman-
drina divisum sit oculomotorius, altera eandem, quam im Ecaudatis
retinuertt formam, altera cum Trigemino se conjunxerit?” The fourth
nerve in the same two genera, according to Fischer, “seems to have
coalesced with the fifth pair ;” at any rate, he was unable to discover
any independent nerve, and the obliquus superior muscle is supplied
by the “nasal branch” of the fifth. The sixth nerve is perfectly
normal both in its origin and distribution ; it passes very close to the
Gasserian ganglion, but is really distinct from it, and leaves the skull
by an aperture distinct from that of the fifth.

1 Fischer, op. cit., pp. 35 and 47.

* For a description and figure of the eye of Proteus, vide Semper, Animal Life, Inter
national Science Series, pp. 78, 79.

* Schwalbe, Das Ganglion Oculomotorii, p. 72.

* Fischer, op. cit., pp. 24, 25, 32, and 47.

150 PROFESSOR MARSHALL.

Fischer’s careful descriptions, which have the great advantage of
being illustrated by as careful figures,* have been referred to by many
writers—Stannius,? Gegenbaur,® Hoffmann,’ Stieda,® &c.—who, how-
ever, have added nothing to our knowledge on the subject from direct
observations of their own.

Schwalbe,® who appears to be the only anatomist since Fischer’s
time who has directly investigated this interesting point, has fur-
nished additional information of great value concerning it. He finds,
in confirmation of Fischer’s statement, that the nerve to the rectus
superior muscle is derived, not from the third nerve, but from the
“nasal branch” of the fifth; but points out that before this nerve is
given off the third and nasal nerves cross and lie in very close contact
with one another. He considers it probable that at this point there
is direct connection between the two, although he was unable to prove
it; and he accordingly supports the view, held also by Fischer and
Stannius, that the supply of the rectus superior by the fifth is only
apparent and due to the close connection and partial fusion of the
third and fifth nerves at this point of crossing.

Concerning the fourth nerve, Schwalbe’s results are more positive,
and of great importance. He finds that although in the majority of
specimens of Salamandra maculosa he dissected, the arrangement de-
scribed by Fischer obtained, the nerve to the obliquus superior appear-
ing as a branch of the nasal nerve, yet that in some cases, one of which
he figures,” the fourth may be a completely independent nerve, arising
from the brain in the normal position.

Reviewing, then, these much-quoted cases of Salamandra and Triton,
we find that Fischer’s account of the anatomical arrangement of the
nerve is confirmed by Schwalbe. We find that the sixth nerve is per-
fectly independent both at its root and along its whole course—is, in
fact, in every way normal. That the fourth nerve is, as a rule, an
apparent branch of the “nasal branch” of the fifth, but, at least in
Salamandra, may be not uncommonly an independent nerve, normal
in every respect. That the third nerve always arises independently
from the brain ; that it crosses the “nasal branch” of the fifth, lying

1 Fischer, op. cit., tab. ii., fig. 2 (Salamandra), and fig. 3 (Triton).
2 Stannius, Das peripherische Nervensystem, p. 19.

3 Gegenbaur, Hexanchus, p. 549, note 1.

4 Hoffmann, Bronn’s Thierreich, Bd. vi, Heft ii, Amphibvia, p. 204.
5 Stieda, loc. cit., p. 174.

® Schwalbe, Das Ganglion Oculomotorit, pp. 25—27.

7 Schwalbe, Das Ganglion Oculomotorti, Tah. xiii., fig. 13.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 151

in close contact with it as it does so; and that it supplies only three
muscles—the rectus internus, rectus inferior, and obliquus inferior —
the rectus superior receiving its branch from the “ nasal nerve,” and
this branch coming off beyond the point of crossing of the third and nasal
nerves; and that this condition of things is interpreted by both the
writers who have investigated it directly—Fischer and Schwalbe—as
merely implying that the third nerve has become partially fused with
the fifth.

Concerning this “nasal nerve,” from which, in the two genera in
question, the branch to the rectus superior always, and that to the
obliquus superior usually, arises, there is a further point of importance,
Schwalbe! has attempted to prove that this “nasal nerve” really cor-
responds, in part at least, to the ramus ophthalmicus profundus of
Selachians. The point could only be decided by a study of the de-
velopment of this nerve in Urodela, of which at present we know
nothing ; but should Schwalbe prove to be correct, the very slight
amount of deviation from the normal condition which we have found
to be all that really occurs in Salamandra and Triton would be still
further reduced ; for embryology teaches us that the ramus ophthal-
micus profundus of Selachians is really a connecting branch between
the third and fifth nerves, which cannot be said to belong distinctly
to either the one or the other, and that the portion of this nerve
beyond the point at which it crosses the third nerve, from which por-
tion we have seen that the branch to the rectus superior arises, has
nothing whatever to do with the fifth, but belongs really to the third
nerve.”

From what has been said above, I think that no other conclusion
can be drawn than that the cases of Salamandra and Triton do not
afford any reason for regarding the eye-muscle nerves as other than
independent and constant nerves.

(c) Menobranchus.—Gegenbaur’ states, on Fischer’s authority, that
in Menobranchus, as in Salamandra and Triton, the fourth nerve is
replaced by a branch of the fifth. I have been unable to refer to
Fischer’s account, so that any discussion of the case would be unprofit-
able. It is, however, very possible that the condition is really what
Schwalbe has shown to occur in Salamandra.

1 Schwalbe, Das Ganglion Oculomotorii, p. 26.

2 Marshall, ‘Head Cavities and Associated Nerves of Elasmobranchs,” Quart. Journ, of
Micros. Science, January 1881, p. 89; and Marshall and Spencer, ‘* Cranial Nerves of Scyl-
lium,” Qaart. Journ. Micros. Sctence, July 1881, pp. 494 seq.

3 Gegenbaur, Hexanchus, p. 549, note 1,

152 PROFESSCR MARSHALL

(2) Stredon.—Fischer’ has established that the third and fourth
nerves are normal in origin and distribution, but was unable to make
out anything definite concerning the sixth nerve.

(ce) Cryptobranchus. —Schmidt, Goddard, and V. d, Hoeven are
quoted by Hoffmann? as stating that in the Cryptobranch the third
and fourth are independent nerves, but that the sixth is a branch of
the nasal division of the fifth. .

Professor Humphry® remarks that the dissection of the cranial
nerves is difficult, on account of the “tough areolar tissue of the
animal and the numerous accompanying veins.” He was unable to
‘discover the third, fourth or sixth nerves in the orbit.” The third
and fourth were, however, found in the cranial cavity, but not the sixth.

Here, again, our information is too imperfect to allow definite conclu-
sions to be drawn. Ifthe sixth nerve really appears as a branch of the
fifth, it is of importance to note that, as is evident from Professor Hum-
phry’s figure, the fifth and seventh nerves are quite distinct from one
another—a point to which we shall refer when considering the Anura.

C. Anura.—The condition of the eye-muscle nerves in Anwra has
been carefully investigated by a number of anatomists, notably by
Fischert and Schwalbe. The results of these investigations are as
follows :—In all Anwra that have been examined, the third and fourth
are distinct and independent nerves, with normal origin and distri-
bution. In Pelobates and Bombinator the third leaves the skull by
the same foramen as the fifth, with which it is in very close contact,
though the two nerves are really distinct.

The sixth nerve in all cases has an independent origin from the
brain in the normal position. In Bufo,° the sixth nerve preserves its
independence along its whole course, and is in all respects perfectly
normal. In the other Anura examined—viz. Pipa, Rana, Pelobates,
Bombinator, and Hyla—the sixth nerve, though arising independently,
unites with the Gasserian ganglion, and the branch to the rectus ex-
ternus is derived from the ‘nasal branch” of the fifth.’

1 Fischer, Anatomische Abhandlungen tiber die Perennibranchiaten und Derotremen,
Hamburg, 1864, p. 127.

2 Bronn’s Thierreich, Bd. vi.

3 Humphry, Observations in Myology, p. 45, and pl. iv. fig. 22.

4 Fischer, Amphibiorum nudorum Neurologic specimen primum, pp. 3—22 and 45—48,

5 Schwalbe, Das Ganglion Oculomotorii, pp. 28—31.

® Fischer, op. cit., p. 5, and tab. ii fig. 1;

7 Cf. Fischer, op. cit., pp. 3—22, and tab. i. fig. 2 (Hyla), fig. 3 (Bombinator), fig. 4 (Pelo-
bates) ; and tab. ii. fig. 1 (Pipa), fig. 4 (Rana); also Wyman, Anatomy of the Nervous System
of Rana pipiens, New York, 1853, pp. 26—28; also Wiedersheim in Ecker’s Anatomie des
Frosches, Zweite Abtheilung, 1881, pp. 20—21,

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 153

But little criticism is called for by the above account. As was
urged in the case of Lepidosiren, the presence of a distinct root of
origin in the normal position must be held to prove that the sixth
nerve is in the cases quoted above really an independent nerve, in
spite of its apparent fusion with the fifth at the Gasserian ganglion.
The fact that the sixth in an allied genus (Su/fo) retains its indepen-
dence, is an additional argument in favour of the fusion being secon-
darily acquired ; and this view must be considered to be established
by the statement made by Stannius,® on Fischer’s authority, that the
sixth nerve is independent of the fifth in the larval stages of those
forms which, when adult, have the two nerves fused.

This concludes the list of recorded instances of exceptional inner-
vation of the eye-muscles. Leaving out, as we are fairly entitled to,
the cases of Amphioxus and of those forms in which, as in Amblyopsis,
the eyes are rudimentary and functionless ; the results of an examina-
tion of the remaining instances may be stated thus :—

1. That in no single instance has it been established that any one
of the eye-muscle nerves is replaced by a branch of the fifth, or of
any other nerve—the cases in which this is alleged to occur being far
more naturally explained by supposing partial fusion between the
nerves concerned to have occurred.

2, That in the alleged cases of replacement of one or more of the
eye-muscle nerves by a branch of the fifth nerve, the “branch of the
fifth” in question is very probably the ramus ophthalmicus profundus,
which is really a communicating nerve between the third and fifth,
belonging as much to one as to the other in its posterior portion, and
in its anterior part belonging exclusively to the third.

3. That the instances in which the absence of one or other of the
eye-muscle nerves has been alleged are either, as in Petromyzon, Lept-
dosteus, Pipa, Hyla, &c., cases in which the nerves in question arise
from the brain in a perfectly normal manner, and after running a
certain distance within the skull become connected more or less inti-
mately with the fifth nerve; or else cases in which, as in Lepidosiren,
the eyes are small, the eye-muscles imperfectly developed, and the
descriptions of their anatomy incomplete and unsatisfactory.

4. That such cases do not in any way invalidate the proposition
that the third, fourth and sixth are independent nerves throughout
the vertebrate sub-kingdom.

* Stannius, Handbuch der Zootomie, Zweites Buch, Die Amphibien, 1856, p. 150, note 3,

154 PROFESSOR MARSHALL.

I propose now to consider briefly the leading features exhibited by
the eye-muscle nerves individually.

III. Tur Turrp, orn Ocutomotor Nerve.—Since the third nerve
is found to be an independent nerve throughout the vertebrate series,
it becomes of interest to inquire whether or not it possessés segmental
value.

Observations by different investigators during the last few years
have tended very strongly to support, if, indeed, they may not be
said to have established, the claim of the third nerve to rank among’
segmental nerves. Inasmuch as this point has been very fully dis-
cussed recently’ I do not propose to go over the whole of the evidence
here, but shall merely apply, in a somewhat summary manner, the
several tests of segmental value in the order given on a previous page.”

1. Though the earliest stages of development of the third nerve
have not yet been ascertained with precision in any case, yet there is
very strong reason for thinking that in the chick, at any rate, the
third nerve develops, like the hinder cranial nerves and the posterior
roots of the spinal nerves, as an outgrowth from the neural crest on
the top of the mid-brain.®

2. Inasmuch as, at a rather later, though still early period—about
the sixtieth hour in the chick, and stage K of Balfour’s nomenclature
in the dog-fish—the third nerve arises from the base of the mid-brain,
very near the mid-ventral line, it is clear that, if the observations on
the earlier stages are correct, the roots must shift downwards at an
early period, and to an extent unequalled by any other nerve.

Kolliker has described the later stages of this shifting, as seen in
rabbit embryos, as follows :*—In an embryo 12 days 5 hours old, and
7 mm. long, the third nerve arose from the mid-brain, not from its
ventral surface, but about half-way up its side; later on it shifts
ventralwards, “‘like the ganglionated cranial nerves and the sensory
spinal roots,” being found on the ventral surface of the mid-brain in
an embryo of the 14th day, and 15 mm. long. ;

1 Marshall, ‘‘ Development of Cranial Nerves in Chick,” Quart. Jown, Micros. Science,
January 1878, pp. 283—27; and ‘‘ Head Cavities and Associated Nerves of Elasmobranchs,”
Quart. Journ. Micros. Science, January 1881, pp. 783—83 ; also Schwalbe, Das Ganglion Oculo-
motorit.

2 Supra, p. 11.

3 Cf. Balfour, Comparative Embryology, vol. ii., p. 379.

4 Kolliker, Entwickelungsgeschichte des Menschen und der hoheren Thiere, Zweite Auflage,

1879, p. 6130

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 155

3. The course of the main stem of the nerve is (fig. 17, IV) at right
angles to the axis of the head at the point of origin of the nerve.

4, Morphologists are very far from agreeing as to the existence of a
visceral cleft in front of the mouth, so that it would be premature to
discuss the relations of the third nerve to this “lachrymal cleft,” for
whose existence there is, however, much to be said. Concerning the
head cavities, however, the evidence yielded by the third nerve is of a
perfectly definite and convincing character. The nerve in Elasmo-
branchs passes downwards and backwards from its root of origin to
the interval between the dorsal ends of the first and second head-
cavities, where it expands into a ganglionic swelling—the ciliary
ganglion. Beyond this point the main trunk of the nerve passes
down between the two cavities, the relations of the third nerve to the
first and second canities being precisely the same as those of the fifth nerve
to the second and third cavities.’

5. As just noticed, there is a very evident ganglionic swelling at
the point of division of the third nerve into its two main branches.

These considerations are, I think, when taken in conjunction with
its previously established constancy throughout the vertebrate series,
sufficient to establish the proposition that the third nerve is of seg-
mental nature. The further question, whether the third represents
ap entire segmental nerve, or only a portion of one, will be best
answered by considering the fourth nerve.

TV. Tut Fourtu, or TRocoHLEAR Nerve.—Having established the
constancy of this nerve, we have now to consider its morphological
import. Concerning its development we know very little, but that
little is of importance. In the dog-fish it has been shown? that the
fourth nerve, at the earliest period at which it has been recognised,
arises from the brain at the same spot as in the adult, ze, the dorsal
surface of the hinder end of the mid-brain; further, that its course is
from the first that of a segmental nerve.

Now, if the visceral clefts and arches, and the head-cavities give us,
as they most certainly do, reliable clues as to the segmentation of the
head, then it is seen at once that there is no room for a segmental nerve

1 Marshall, ‘‘Head Cavities and Associated Nerves of Elasmobranchs,” Quart. Jown. of
Micros. Science, Jan. 1881, pp. 78 seq.

2 Marshall and Spencer, ‘‘ Cranial Nerves of Scyllium,” Quart. Journ. of Micros. Science,
July 1881, pp. 672—674.

156 PROFESSOR MARSHALL.

between the third and fifth nerves ; and therefore, if the fourth is of
segmental nature, it must belong to one or other of these nerves.

The following considerations seem to point very strongly to the
third and fourth nerves being connected together, and favour the view
that they are together equivalent to a segmental nerve.

1, The two nerves in question, the third and fourth, both arise
from the mid-brain or middle cerebral vesicle. Furthermore, they
are the only nerves that arise from this division of the brain, either
in the embryo or the adult. There are independent reasons for
thinking that these brain-vesicles have segmental value ;1 and though
these reasons may not be considered conclusive on the point, they
nevertheless lend some support to the view that the two nerves arising
from one of these vesicles belong to the same segment.

2. The third and fourth nerves, though arising separately from the
brain, may be connected together more or less intimately beyond their
roots of origin. This, for instance, is a marked feature both in Petro-
myzon and Lepidosteus, also in Salamandra and Triton, if Schwalbe is
correct in identifying the “ nasal branch” of the fifth with which both
the third and fourth nerves are connected as the ramus ophthalmicus
profundus.

3. According to Meynert,? the third and fourth nerves arise in the
adult from a common nucleus. This has, however, been denied by
Forel,® though supported by other investigators, and probably requires
confirmation,

4, The fourth, though chiefly known as a motor nerve, is really in
many animals a nerve of mixed function, giving off in Selachians and
Amphibians* sensory branches to the conjunctiva and skin of the upper
eyelid. This point is of importance, because if the third and fourth
are together equivalent to a segmental nerve, it would be only reason-
able to expect that certain of its fibres should be sensory ; and analogy
would certainly lead us to look for sensory branches in the portion
with the more dorsally situated root, 7.e. the fourth nerve, which,
as we have just seen, does actually present such sensory fibres.

1 Vide Foster and Balfour, Elements of Embryology, part i., p. 188; and Marshall, De-
velopment of Nerves in Birds, Journ. Anat. Phys., vol. Xi., p. 510.

2 Meynert, ‘The Brain of Mammals,” Stricker’s Histology, New Sydenham Society’s
Translation, vol. ii., pp. 444, 445.

3 Forel, Haubenregionen.

* Schwalbe, Das Ganglion Oculomotorii, p. 14; Wiedersheim, Morphologische Studien,
p. 21; and in Ecker’s Anatomie des Frosches, p, 24; also Hoffmann, Bronn’s Thierreich,
Bad. vi., p. 203.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 157

5, That the fourth nerve is itself not an entire segmental nerve is
rendered probable by the fact, noticed by Schwalbe, that it has no
ganglion, and is strongly supported by the further fact that the third
nerve almost certainly arises at first from the dorsal surface of the
brain, and beyond all doubt is, during its early stages, attached much
higher up the side of the brain than it is at a later stage, ae. that
the third nerve behaves like a posterior spinal root.

Since, as we have seen, there is no room for a separate segmental
nerve between the third and the fifth, I am inclined to view the third
and fourth nerves as together equivalent to a segmental nerve, which
has divided into two portions, whereof one—the fourth
in its primitive position on the top of the brain, while the other—the

has remained

third—has, like the other cranial nerves and the posterior spinal roots,
shifted downwards, the extent of the shifting being greater than that
of any of the other nerves, but the several steps of the process pro-
bably the same as in these, This view will be found to he very closely
in accordance with that advocated by Schwalbe.*

V. Tue Firts, on TriceminaL Nerve.—It will be convenient to
continue the consideration of the cranial nerves in the usual sequence,
and to take the remaining eye-muscle nerve—the sixth—after the tri-
geminal,

The fifth nerve completely fulfils all the conditions of a true seg-
mental nerve.” It appears very early as an outgrowth from the neural
crest. The root of origin from the brain shifts down at an early
period, acquires a secondary attachment to the side of the brain, and
loses its primary attachment completely. The direction of the stem
is at right angles to the axis of the head at the point of origin of the
nerve. The maxillary and mandibular branches are related to the
maxillo-mandibular or buccal cleft in the manner characteristic of the
posterior segmental nerves, as was first pointed out by Stannius. The
relations of the fifth nerve to the second and third head cavities are of
a perfectly typical nature ; and finally a ganglionic enlargement—the
Gasserian ganglion—is present in the nerve a short distance above
its division into the two main branches.

1 Schwalbe, Das Ganglion Oculomotorii, pp. 77, 78.

2 For the development of the fifth nerve in Elasmobranchs, vide Balfour, Hlasmobranch
Fishes, 1878, pp. 196—19S ; also Marshall and Spencer, ‘‘ Observations on the Cranial Nerves
of Scyllium,” Quart. Journ. of Micros. Science, July 1881, pp. 474—479; in the Chick, vide
Marshall, Quart. Journ. of Micros. Science, Jan. 1878, pp. 23—32; and in the Rabbit, Kél-
liker, Entwickelungsgeschichte, 1879, pp. 610—712,

158 PROFESSOR MARSHALL,

The only possible doubt as to the independent segmental value of
the fifth nerve hinges on the fact that in the two lower classes of
vertebrates—Pisces and Amphibia—the fifth is very generally fused
more or less completely with the seventh in the adult condition ; the
fusion sometimes, as in most fishes, involving the roots to a greater
or less extent, sometimes, as usually in Amphibians, occurring a short
distance beyond the roots and close to the Gasserian ganglion.

This approximation or fusion of the fifth and seventh nerves has, as
mentioned above, been employed by J. Miiller, Stieda, and others, as
an argument against the two nerves being of independent segmental
value.

A crucial test of the force of this argument is afforded by a study
of the development of the roots of the two nerves in Elasmobranchs,
in which the fusion of the roots in the adult is so complete that what
is really one of the roots of the seventh has hitherto been almost in-
variably described by anatomists as a root of the fifth. In the dog-
fish it has been shown that the two nerves, though so intimately
connected in the adult, are in the early embryonic stages perfectly
distinct from one another, and some distance apart, as far from one
another, indeed, as they are in corresponding stages of such forms as
the chick or lizard in which they remain completely separate through-
out life; and that the gradual approximation and fusion of the two
nerves, which occur during the later developmental stages, and all the
steps of which have been traced, must, like the partial fusion which
we have seen may occur in some forms between the third and fifth
nerves, be viewed as purely secondary features.

In early stages of both Zeleosteans and Amphibians, I have also
noticed that the roots of the fifth and seventh nerves are perfectly
distinct from one another, and some distance apart, and that their sub-
sequent approximation must accordingly be, as in Elasmobranchs, of
a purely secondary nature.

The claim of the fifth nerve to rank as an independent segmental
nerve must, I think, from what has been said above, be considered as
definitely established.

VI. Tue Sixru, or Appucent Nerve.—The proper morphological
position of this nerve is by no means easy to determine with any

1 A full account of the development of the roots of the fifth and seventh nerves in the
dog-fish, and of the relation of the embryonic to the adult roots, will be found in the paper
by Mr. Spencer and myself quoted above, Quart. Jowrn. of Micros. Science, July 1881, pp.
482—486.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES, 159

degree of certainty; and the views of different writers on the point
are far from being in harmony with one another.

In a former section of this paper we have established the fact that
the sixth is an independent nerve throughout the vertebrate sub-
kingdom. It always supplies the rectws externus muscle of the eyeball,
and may supply other parts as well; thus, in reptiles it supplies the
retractor muscle of the bulb of the eye, and in Batrachia the suspensor
muscle of the bulb and the muscles of the nictitating membrane. In
all cases it isa purely motor nerve. Indeed, if we omit the eleventh
and twelfth pairs, which are not constant cranial nerves, the sixth is
not only the most purely motor cranial nerve, but the only exclusively
motor one throughout the vertebrate series.

Its point of origin from the brain in adult vertebrates is also a
remarkable and constant one. It arises from the under surface of the
medulla, very close to the mid-ventral line, and vertically below, or
more usually slightly posterior to the common root of origin of the
seventh and eighth nerves. In some cases the root may be in front
of that of the seventh nerve. The root is always slender, and devoid
of ganglion cells.

Concerning the development of the sixth nerve, we unfortunately
know but little. At the fifth day in the chick,’ and at a correspond-
ing stage in the dog-fish’, it has been detected and described, its
appearance and relations being practically identical in the two cases.
It is a slender nerve, with no ganglion cells at any point in its length,
arising from the ventral surface of the hind-brain, below the seventh
nerve, by a number of small slender roots, and running forward to the
rectus externus muscle, in which it ends. The roots are, from the
earliest period at which the nerve can be recognised, close to the median
ventral line (fig. 7, VI), and some distance below the root of the seventh
(fig. 7, VII), from which they are from the start perfectly distinct. So
far as can be inferred from negative evidence, the sixth nerve appears
to develop later than the seventh and other segmental nerves.

From the above account it is clear that the sixth has no claim
whatever to segmental rank, inasmuch as it distinctly fails to answer
any one of the tests of such rank laid down on page 133. It does

1 Stannius, Handbuch der Zootomie, Zweite Aufiage, Zootomie der Amphibien, 1856, p. 150.

2 Marshall, ‘‘ Development of Cranial Nerves of Chick,” Quart. Journ. of Micros. Science,
Jan. 1878, pp. 23—25.

5 Marshall, ‘““Head Cavities and Associated Nerves of Elasmobranchs,” Quart, Journ, of
Micros, Science, Jan, 1881, pp. 89—93,

160 PROFESSOR MARSHALL,

not develop from the neural crest. The roots of origin do noé shift
downwards, but are from their first appearance in the adult position.
The course of the nerve is nearly parallel to, and certainly not perpen-
dicular to the axis of the head. It has not the definite relations to
the visceral clefts and arches, and to the head cavities, characteristic
of a segmental nerve. And it has no ganglion cells at any point in
its length.

As the nerve is not an independent segmental nerve, it must either
belong to one of the segmental nerves or else be a nerve of altogether
exceptional nature. The latter supposition should, I think, only be
adopted as a last resource if all the other attempts at explanation fail,
and I therefore propose now to consider the relations of the sixth to
the segmental nerves, or rather to the fifth and seventh nerves, which
are clearly the only ones which could claim it.

By the majority of writers who have discussed this point, the sixth
is referred to the fifth. Thus, Gegenbaur considers the sixth to be an
independently arising motor root of the fifth, a view which Schwalbe?
also adopts. Wiedersheim speaks of the fifth and sixth nerves as to-
gether making up a segmental nerve; while Huxley’ is disposed to
view the sixth as primarily part of the fifth.

Notwithstanding the weight of authority against me, I think that
the sixth nerve should be grouped with the seventh, and not with the
fifth, for the following reasons :—

1. In the early stages of both chick and dog-fish the roots of the
sixth are completely behind those of the fifth nerve. Indeed, the
majority of the roots are even behind the roots of the seventh; and
although a transverse section may, as in fig. 7, pass through the roots
of both sixth and seventh nerves, yet the root of the sixth in such a
section is the most anterior of the series, the other roots being further
back, and completely behind the seventh root.

2. In adult vertebrates, also the sixth nerve usually arises beneath
or slightly behind the seventh, very rarely in front of it.

3. Though the sixth nerve may, beyond its root, be closely con-
nected with the fifth, yet it is important to notice that all the cases—
Petromyzon, Lepidosiren, Pipa, Rana, Anura—in which it is described
as fusing with the fifth, are also cases in which the seventh and fifth
nerves are very closely connected together, so that the connection between

1 Schwalbe, Das Ganglion Oculomotorit, p. 74.
2 Wiedersheim, Morphologische Studien, p. 23.
3 Huxley, Anatomy of Vertebrated Animals, p. 73, note,

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 161

the sixth and fifth in these instances by no means proves that the
sixth belongs to the fifth, but is more probably due to the same cause
—whatever it may be—that determines the approximation or fusion
of the seventh and fifth nerves.

Concerning the actual value of the sixth nerve, I see no reason to
alter the opinion I have previously expressed, that the sixth nerve
may be regarded as having the same relation to the seventh that the
anterior root of a spinal nerve has to its posterior root. Ishall return
to this point when considering the seventh nerve.

VII. Tue Sevents, or Factan Nervse.—As to the segmental value
of the seventh nerve there can be no doubt whatever ; for, like the
fifth, it completely and indisputably fulfils all the conditions of a seg-
mental nerve laid down on page 133.

It develops' very early as an outgrowth from the neural crest on
the dorsal surface of the hind-brain (fig. 2). At an early stage the
nerve acquires a new or secondary attachment to the side of the brain
(fig. 3); but, unlike all the other nerves, cranial or spinal, the original
or primary root is retained as well as the secondary root, whereas
in all the other nerves the primary attachment appears to be lost.
The general course of the nerve is at right angles to the axis of
the head at its point of origin. The relations of its branches to the
hyo-mandibular cleft, first pointed out by Stannius, and afterwards
insisted on by Gegenbaur, are those of a typical segmental nerve, as are
also its relations to the head-cavities ; whilst, finally, it is ganglionic at
its division into the two main ventral branches.

As to the independent rank of the seventh nerve, 1 have already
discussed fully the theory that the seventh and fifth nerves are con-
nected together primarily, and have stated the arguments leading to
the conclusion, that although in many vertebrates—fish and am-
phibians—the two nerves are more or less closely fused together, yet
that embryology shows that this fusion is a secondarily acquired
character.

The relation between the sixth and seventh nerves is of still greater
importance, from its bearing on the disputed question of whether there

1 For an account of the development of the seventh nerve in Elasmobranchs, vide Balfour,
Elasmobranch Fishes, 1878, pp. 198—202; and Marshall and Spencer, Quart. Jowin. Micros.
Science, July 1881, pp. 679—691 ; in the Chick, vide Marshall, Quart. Jowin. Mieros. Science,
dan. 1878, pp. 34—86 ; and in Mammals, Kolliker, Entwicklungsgeschichte.

M

4

————————— ee —S————C™!TC—COCO OT COU

162 PROFESSOR MARSHALL.

are to be found in any of the cranial nerves roots strictly comparable
to the anterior roots of the spinal nerves.

In dealing with this question, it is first necessary to establish cer-
tain general conclusions concerning the cranio-spinal nerves. As was
first pointed out by Balfour, the posterior roots of the spinal nerves
must be regarded as of a more primitive nature than the anterior
roots, the grounds on which this conclusion is based being the fol-
lowing :—

1. The actual mode of development of the two kinds of roots in the
spinal nerves. As noticed in a previous page, the posterior roots
appear before the anterior ones, and are also in their mode of develop-
ment of a more primitive character than these latter, the posterior
roots consisting at first entirely of undifferentiated spherical or poly-
gonal cells, while the anterior roots are almost from their first ap-
pearance fibrillar.

2, The condition of the nervous system iv Amphioxus, in which,
according to Balfour, all the nerves arise by single roots, which
roots correspond to the dorsal or posterior roots of other vertebrates,
and must clearly in Amphiowus be of mixed motor and sensory
function.

From these facts the further conclusion is drawn “ that primitively
the cranio-spinal nerves of vertebrates were nerves of mixed function
with one root only, and that root a dorsal one; and that the present
anterior or ventral root is a secondary acquisition.” }

Concerning the several steps by which these anterior roots have
been acquired, the evidence at our disposal is of an imperfect, and in
ereat part merely conjectural character. Still I think that, although
we may not be able to solve the problem completely, we can at any
rate define its limits fairly accurately, and perhaps indicate the path
along which the solution will ultimately be found.

The problem is how, from animals resembling Amphioxus in pos-
sessing only dorsal roots to the nerves, and these dorsal roots con-
sequently of mixed function, has the type of spinal nerve met with
among existing vertebrates, with two distinct roots, dorsal or sensory
and ventral or motor, been derived ?

It appears to me that there are two ways in which we can conceive
this change as having come about :—

1 Balfour, Elasmobranch Fishes, p. 193:

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 163

Firstly, we might suppose that in some way, and for some reason,
the sensory and motor portions of the originally single root became
completely separated from one another, and that while the sensory
portion of the nerve retained the primitive mode of development and
position of attachment of the root, the motor portion acquired a new
mode of development and a new position of attachment, and then
united farther on with the posterior root to form a spinal nerve. On
this view the motor and sensory roots of a spinal nerve correspond to
the motor and sensory portions of the singie root of Amphioxus.

Or, secondly, we might imagine the anterior root to be, not the
motor portion of the original root, but an altogether new development,
an independent outgrowth from the spinal cord to supply the more
complicated system of muscles that would necessarily accompany the
gradual perfection and complication of the internal skeleton ; that this
new root was at first completely independent of the original or dorsal
root, and for a time coexisted with a dorsal root of mixed function ;
that in the case of the spinal nerves the whole motor function gradually
got transferred to, or usurped by, the new root; while the two
roots, originally separate along their whole length, became united to
form the mixed trunk of the spinal nerve.

Now, although there are very considerable and obvious difficulties
in the way of accepting either of these alternatives, yet it appears to
me that the second is far more in accordance with the actual facts
than the first, and that it offers a ready explanation of many points
unintelligible on the first hypothesis. Thus, the second view explains
why in actual development the anterior spinal roots appear later than
the posterior, and why they are for some time quite distinct from
these latter; it also explains such cases as Petromyzon, in which the
anterior and posterior roots of the spinal nerves are said to remain
distinct from one another throughout life.

By far the most important argument, however, in favour of the
second hypothesis is afforded by the explanation it yields of the con-
dition of the cranial nerves as compared with the spinal; and in
connection with this pomt I would direct special attention to the
statements already made concerning the sixth and seventh nerves.

It has been shown above that the seventh nerve in Elasmobranchs
develops in a manner precisely similar to the posterior roots of the
spinal nerves; that it arises as an outgrowth from the neural crest
(fig. 2, VII), the nerves of the two sides being at first directly and

164 PROFESSOR MARSHALL.

widely continuous with one another across the top of the brain;
that by growth of the mid-dorsal roof of the brain the two nerves
get separated from one another; that the root acquires a secon-
dary attachment to the side of the brain (fig. 3, VII), but that,
unlike the other cranial or spinal nerves, it retains the primary as well
as the secondary root throughout life. In this respect the seventh is, with
the possible exception of the fourth, the most primitive nerve in the
body, inasmuch as it exists throughout life in a condition which is
only a transitory one in all the other nerves. However unexpected
this point may be, I cannot but think that it is one of the greatest
importance in the determination of any question concerning the mor-
phology of the cranial and spinal nerves.

The seventh being a very primitive nerve, there is strong a@ priors
reason for thinking that the sixth nerve, which we have seen reason
for grouping with the seventh, is also of a primitive nature, and it is
clear that on the second hypothesis such is the case, the complete
independence of the sixth nerve being merely the retention of a primi-
tive character, while its limited and special distribution to muscles
not present in Amphioxus affords a very possible explanation of its
appearance in higher vertebrates. On the first hypothesis, on the
other hand, the sixth nerve would be, not a root which had retained
us promitive independence of the seventh, but a root which had as a per-
fectly exceptional occurrence acquired independence, a view directly
contradicted by the primitive condition of the seventh itself.

It must surely be regarded as a very significant fact that a trans-
verse section through the hind-brain of ether an embryo or adult Elas-
mobranch passing through the roots of the sixth and seventh nerves
(fig. 7) agrees absolutely in all essential points with a section at an
early embryonic stage through the roots of a spinal nerve in the same
animal, ze. that a condition which is transitory in the case of
the spinal nerves is permanently retained in the case of the sixth
and seventh nerves. This fact, which is the strongest possible argument
in favour of the second hypothesis, clearly directly contradicts the first.

If the doctrine that the cranial nerves are more primitive than the
spinal appear at first sight paradoxical,! I would point out that there

1 I have myself on a former occasion both felt and urged this objection (‘‘ Head Cavities
of Elasmobranchs,” Quart. Journ. of Miscros. Science, Jan. 1881, p. 91). Further investi-
gation has convinced me that I was then wrong, and that Balfour was right in considering

(Elasmobranch Fishes, p. 193) the cranial nerves as more primitive than the spinal, though
do not agree with his con¢lusion that the cranial nerves have no anterior roots.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 165

is independent evidence-in favour of the head retaining a more primi-
tive condition than the body. Thus the skull, though subjected to
very extensive secondary modifications, is really in a more primitive
state than the vertebral column, for the skull represents the per-
manent retention of a condition, that of a continuous unsegmented
cartilaginous tube, which is only transitory in the case of the vertebral
column except in the lowest vertebrates ; the division of the cartilagi-
nous tube into segments or vertebree never occurring, and in all pro-
bability never having occurred in the skull, though so constantly
present in the vertebral column. The fact that it is in the lowest
vertebrates alone that this unsegmented condition is retained in the
trunk as well as in the head is a strong argument in favour of the
view that the head is really in a more primitive condition than the
trunk as regards skeletal elements.?

On the second hypothesis, the mixed—motor and sensory—nature
of the seventh nerve is explained as due, like the persistence of the
primary root and the independence of the sixth nerve, to retention of
the primitive condition, and the extreme variability presented by the
relative importance of the sensory and motor functions of the seventh
nerve in different vertebrates may help to render intelligible how the
posterior spinal roots, which were originally of mixed function, have
become converted into purely sensory roots.

If the hypothesis advanced above should prove correct, it would be
only reasonable to expect that the posterior roots of the spinal nerves

‘should in some exceptional cases be found to retain in part their primi-

tive mixed character, and to co-exist as mixed posterior roots with
exclusively motor anterior roots. I am not aware of any such cases,
or of the existence of any residual physiological phenomena that would
support such a view, but would suggest that a direct investigation of
the functions of the spinal roots in the lampreys, where the two roots
are stated to remain distinct from one another throughout life, might
conceivably lead to interesting results.

The application of the hypothesis to the remaining cranial nerves
is sufficiently obvious from the accounts given of these. The main
point of: difficulty concerns the determination of the presence or ab-

* The following quotation from Balfour, which I only became acquainted with after the
above passage was written, strongly confirms this view :—‘ This development (of the skull)
probably indicates that the basilar plate contains in itself the same elements as those from
which the neural arches and the centra of the vertebral column are formed, but that it never
passes beyond the unsegmented stage at first characteristic of the vertebral column,”—Com-
parative Embryology, vol. ii., p. 467.

166 PROFESSOR MARSHALL.

“gence of anterior motor roots to these nerves; and on this point I

have no additional evidence beyond what I have already stated else-
where.?

VIII. Tue Ercuty, or Avupitory Nerve.—ZIn all the forms in
which the development of the auditory nerve has been ascertained, it

arises as part of the seventh nerve. Neither its development: nor its

anatomical relations afford the slightest ground for thinking it to be
of segmental rank.’

IX. Tue NINTH, oR GLOSsOPHARYNGEAL Nerve.—Like the audi-

tory, the ninth nerve can be disposed of very briefly, but for a directly

opposite reason. Since Gegenbaur confirmed Stannius’ account of its
relations to the first branchial cleft, the claim of the glossopharyngeal
to rank as an independent segmental nerve has been very generally
admitted; and as the history of its development’ shows that it con-
forms in all respects to the characters of a segmental nerve as defined
on page 133, it would be superfluous to discuss in detail its now uni-
versally recognised claims to segmental value.

X. Toe TENTH, on Vacus Nerve.—The tenth nerve stands in much
the same position as the ninth, with the exception that while the

‘glossopharyngeal is a single segmental nerve, the vagus, from its

relations to a number of visceral clefts, must be considered as equi-
valent to an equal number of segmental nerves fused together. This
was first pointed out by Stannius, and subsequently developed in
much more detail by Gegenbaur; and since the publication by the
latter of his memorable essay on the cranial nerves of Hexanchus, has
been accepted almost universally as the true theory of the morpho-
logical value of the tenth nerve. It is only necessary to add here that
the study of its development shows that it completely fulfils all the
conditions required of segmental nerves. y

Concerning the number of primitively separate segmental nerves
fused together to form the vagus, we cannot speak positively. The

1 Marshall, “Head Gavities and Associated Nerves of Elasmobranchs,” Quart. Journ. of
Micros. Science, Jan. 1881, pp. 91—93.

2 For the development of this nerve in Elasmobranchs, vide Balfour, Zlasmobranch Fishes,
p. 198; in the Chick, Marshall, ‘‘ Development of Cranial Nerves in the Chick,” Quart. Journ.
of Micros. Science, Jan. 1878, pp. 34—36.

3 For the development of the glossopharyngeal and vagus nerves in Elasmobranchs, vide
Balfour, Hlasmobranch Fishes, pp. 202, seq.; in the Chick, Marshall, loc, cit., pp. 36—89,

THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 167

ereatest number of clefts supplied by it in vertebrates above Am-
phioxus is met with among the Marsepobranchii and in Notidanus,
where it supplies the six posterior branchial clefts, and must therefore
be equivalent to at least six segmental nerves. Whether this is the
full number, however, is a point not yet decided.

XI. anp XIJ. Tae ELeventu, or Spinan ACCESSORY, AND TWELFTH,
or Hyrociossat Nerves.—Neither of these nerves is constant as a
cranial nerve througout the vertebrate series, a fact which renders
it very doubtful whether the claim of either of them to segmental
value could be entertained. For this reason, and partly because I am
at present engaged in investigating their development, about which
we know as yet very little, I do not propose to deal further with them
in the present paper. Forming, as they do, the connecting links
between cranial and spinal nerves, they may be expected to yield
valuable evidence concerning the validity of the hypothesis propounded
above concerning the relations between these two groups of nerves.

Summary.—The conclusions arrived at concerning the segmental
value of the cranial nerves may be expressed in a tabular form thus
(c.f, fig. 17) :—

eT ee

Segment. Nerve. Visceral Cleft. Visceral Arch.
1. Preoral. I, Olfactory. Olfactory.
IIT. Oculomotor, ;
2 DO: ; Teves Trochiesa i Lachrymal.
SS aaeata aoe aS a See ES oe Maxillary.
3. Oral. V. Trigeminal. Buccal. ——
An Tae veel { VIL. Facial, Spiracular or Mandibular.
; : VI. Abducent. hyomandibular, ee
SS SSS Hyoid.
5. Do. IX. Glossopharyngeal.| 1st Branchial. |——
DSSS — 1st Branchial.
Gy Do: X. Vagus,Istbranch.| 2nd ,, a
————$ —___—__—_—_— —_——_——| 2nd ,,
(seo: 9) FRB! - “6p 3rd 5 —_ —___.
—————— | —— — —_— | —_—_— 3rd 5
8. Do. SE ROLC ss 4th FH
ae ae ae ee a ah
Ss IDG: op) Sees gy 5th 5 —_——_-——_—_
——$§,« ——_— —— — —— th oy
10. Do. a OLDS 55 6th
pisisisaeewee — —| 6th 1)

11. Do. ” 6th 9 7th 9 FOREN Er eee

168 PROFESSOR MARSHALL.

List of chief Works and Papers referred to, arranged according to

date of publication.

1838. Scutemm v. pD’Atton.—Ueber das Nervensystem der Petro-
myzon. Miiller’s Archiv, p. 262.

1839. OwEn.—Description of the Lepidosiren annectens. Trans.
Linnean Society, vol, xviii.

1840. J. Mitiur.— Vergleich. Neurologie d. Myxinoiden.
1843. Fiscoer.—Amphibiorum nudorum Neurologic specimen primum.
1844. J. Mitter.— Handbuch der Physiologie des Menschen.

| 1845. Hyrri.—Lepidosiren paradoxa.

1846. J. Mituer.— Ueber den Bau und die Grenzen der Ganoiden.
1849. Srannius.—Das peripherische Nervensystem der Fische.
1851. Arnotp.—Handbuch der Anatomie des Menschen.

1855. Wyman.—Anatomy of the Nervous System of Rana pipiens.

1856. Stannius.—Handbuch der Anatomie der Wirbelthiere. Zweite
Auflage.

1864, Fiscurr.—Die Gehirnnerven der Saurier.

1866. Ownn.—Anatomy of Vertebrates, vol. i.

1870. StrepaA.—Studien tiber das centrale Nervensystem der Wirbel-
thiere. Zeitschrift f. wissenschaftliche Zoologie, Bd. xx.
1871. GaaEnBAuR.—Ueber die Kopfnerven von Hexanchus u. ihr
Verhiltniss zur Wirbeltheorie d. Schidels. Jenaische Zeit-
schrift, Bd. vi. |
Huxtey.—Manual of the Anatomy of Vertebrated Animals.

1872. Meynert.—The Brain of Mammals. Stricker’s Histology, vol. ii.
New Sydenham Society’s Translation.

... Humpury.—Observations in Myology.
1875. Batrour.—On the Development of the Spinal Nerves in Elasmo-
branch Fishes. Phil. Trans., vol. cxxxvi.
1877. MarsHatu.—On the early Stages of Development of the Nerves
in Birds. Journal of Anatomy and Physiology, vol. xi,

1878.

1879.

THE SEGMENTAL VALUE OF THE CRANIAL NERVES, 169

Batrour.—Monograph on the Development of Hlasmobranch
Fishes.

GrecENnBAUR.—Llements of Comparative Anatomy, English Trans-
lation.

MarsHatu.—The Development of the Cranial Nerves in the
Chick. Quart. Journ. of Micros. Science, vol. xviii.

K6uuiKker.—Lntwickelungsgeschichte des Menschen und der
hiheren Thiere. Zweite Auflage.

MarsHaLt.—The Morphology of the Vertebrate Olfactory
Organ. Quart. Journ. of Micros. Science, vol. xix.
ScHWALBE.—Das Ganglion Oculomotorii. Jenaische Zeitschrift,
Bd. xiii.
WIEDERSHEIM.—Die Anatomie der Gymnophionen.
GintHer.—ntroduction to the Study of Fishes.
WirpErsHeim.—Dorphologische Studien, Heft i.
BatFrour.—Tveatise on Comparative Embryology, vol. ii.
MarsHaLlt.—On the Head-Cavities and Associated Nerves of
Elasmobranchs. Quart. Journ. of Micros. Science, vol, xxi.
MarsHALL and SPENCER.—Observations on the Cranial Nerves
of Seyllium. Quart. Journ. of Micros. Science, vol. xxi.

WIEDERSHEIM.—Die Anatomie des Frosches, von Dr. Alexander
Ecker. Zweite Abtheilung.

170 JOHN BEARD

THE SYSTEM OF BRANCHIAL SENSE ORGANS AND
THEIR ASSOCIATED GANGLIA IN ICHTHYOPSIDA. A
CONTRIBUTION TO THE ANCESTRAL HISTORY OF
VERTEBRATES.

By Joun Buard, Pz.D., B.Sc., Berkeley Fellow of Owens College, Victoria
University, Manchester.

[Puates VII, VIII, & IX.]

INTRODUCTION.

Among the many weighty questions which have arisen with the rise
and progress of comparative embryology, that of the origin and an-
cestral history of Vertebrates has occupied, and still occupies, an
important place.

That the question, if capable of solution at all, would be solved by
the discoveries of embryology, is now, and has been for the last ten
years, a general opinion among zoologists. So much for a general
agreement. But as to the particular line of descent one might recall
half a dozen different theories supported by different schools of workers.

The impulse to these speculations was first given by the discovery
of the tadpole-like larva of Ascidians, and the opinion that Vertebrates
were derived from Ascidians we owe to Kowalewski and Kupffer. This
view has had its day, and is now only a reminiscence.

Another important theory, important because clothed with the
authority attached to the name of Balfour, is the theory that Verte-

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. ib 7Al

brates arose from unsegmented worms, in which two lateral nerve-
cords were supposed to have coalesced dorsally instead of ventrally, as
in Annelida.

Following this, one is reminded of Hubrecht’s theory, which allies
Vertebrates with Nemertines, and sees the Vertebrate notochord
reflected in the Nemetine proboscis sheath.

By no means least important is the celebrated Annelidan theory of
the origin of Vertebrates first originated by Dohrn? and Semper. A
theory which, in spite of all attacks, still survives, and at present
seems to be more probable than any other.

Finally, the alliance of Balanoglossus with Ascidians, Amphioxus,
and Vertebrates, recently advocated by Bateson,’ must be mentioned.
Interesting though this is, it cannot yet be considered as sufficiently
established to be accepted without reserve ; but if more evidence for
it be forthcoming, it is a moot point whether our existing notions of
the relations of Vertebrates and Annelida will not have to be modified,
for we know of no existing Annelid which has relationships with Bala-
noglossus. And here I would point out that my own researches on
the cranial nervous system and sense organs of Vertebrates, instead
of supporting the alliance of Balanoglossus with Vertebrates as high
as fishes, present rather a hindrance in the way of such alliance, whilst
they are still more opposed to the alliance of Vertebrates with existing
Annelida.

That Vertebrates have their nearest allies, except Balanoglossus, in
the group of Annelida, is becoming more and more obvious from recent
researches, especially from those of Dohrn; but the links of such an
alliance seem to have been rather in long extinct Annelida than in
any at present existing.

In the following pages an account will be given of the morphology
and development of the branchial sense organs and associated ganglia
in Amphibians and Fishes, chiefly in Elasmobranchs. The branchial
sense organs are those sense organs which have usually been called
organs of the lateral line, and were formerly called “‘ segmental sense
organs” by me. The name “organs of the lateral line” is bad, be-
cause it chiefly refers to those sense organs along the lateral line of

2 Dohrn, ‘ Ursprung der Wirbelthiere,’ 1875.

2 Semper, ‘‘ Verwandschaftsbeziehungen der gegliederten Thiere,” ‘ Arbeiten a. d. Zool.
Institut zu Wiirzburg,’ 1875.

5 Bateson, W., ‘‘ Development of Balanoglossus,” ‘Quart. Journ. Micro. Sc.,’ Supple-
ment, July 1885,

172 JOHN BEARD.

the trunk, which morphologically form only a small portion of the
sense organs. I have myself seen reason to reject the name segmental
sense organs, because although originally they are segmental, and in
later life may occur one in each segment of the trunk, still at first they
are confined to one region only of the body, the gill-bearing region, and
only extend into the trunk much later. Originally they are seated
one above each gill-cleft or over the site of each cleft, and may, there-
fore, be called branchial sense organs.’

The so-called ganglia of the posterior roots of the cranial nerves
arise in connection with them, and must be regarded as originally
special ganglia of these sense organs.”

One general conclusion may be referred to here, and that is, that
at present we are acquainted with no invertebrate nervous system which
ts built upon the same plan as that of Vertebrates.

The matter will be discussed later on, and I only refer to it here in
order that from the outset the branchial sense organs may be raised
from their present position of neglect and obscurity, and may be given
that important morphological (and physiological) place which their
relationships to the gill-clefts on the one hand, and to the ganglia of
the posterior roots of cranial nerves on the other, most certainly entitle
them to,

Unlike many previous observers, I have found that it is absolutely
impossible to study the branchial sense organs of fishes without at the
same time dealing with the posterior roots of the cranial nerves, which
are morphologically as well as physiologically inseparably connected
with the former.

It would take up too much time and space to give here a history of
all the researches on these two sets of organs, which have hitherto
been usually treated apart from each other as if they had no con-
nection.

The work has been mainly carried out on embryos of Torpedo ocel-
lata, for which I have to thank the Zoological Station at Naples. But
I have also studied Teleostei and Amphibians, and have had a few
embryos of Mustelus and Pristiurus. However, in the descriptions in
the following pages, unless otherwise stated, the condition of affairs in
Torpedo will be understood to be under discussion,

1 Beard, ‘‘ Cranial Ganglia and Segmental Sense Organs,” ‘ Zool. Anzeig.,’ 192, 1885 ; also
Froriep, ‘‘Ueber Anlagen von Sinnesorganen am Facialis, &c.,” ‘Archiv fiir Anat. and
Physiol.,’ 1885.

2 Beard, op. cit.,; Froriep, op. cit.; and Spencer, ‘‘ Notes on the Early Development of
Rana temporaria,” ‘Quart, Journ, Micro. Sc.,’ Supplement, J uly 1885,

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. lis

In the first place, I think it will be of great advantage and will tend
to simplify matters very much if the general schema of the develop-
ment of a cranial nerve (dorsal root) of an Elasmobranch, such as
Torpedo, be given.

Then those cranial nerves, which I regard as segmental, will be dis-
cussed: olfactory, nerve of ciliary segment, trigeminal, facial, auditory,
glossopharyngeal, and vagus.

The optic nerve is left entirely out of consideration. Firstly,
because I have made no investigations, and hence have no new facts
about it to record ; and secondly, as is well known, its whole develop-
ment is different from that of the other cranial nerves ; and I can only
agree with those zoologists who class the optic nerve entirely apart
from the other cranial nerves.

Not so, however, with the olfactory and auditory nerves and organs.
Partly following Marshall, I feel bound to place these nerves in the
category of cranial segmental nerves, and to class the olfactory and
auditory organs! as specialised branchial sense organs.

Finally, after the account of the various nerves, the bearing of the
facts described on the morphology and ancestral history of Vertebrates
will be discussed.

GENERAL SCHEMA OF THE DEVELOPMENT OF A Dorsat RooT oF A
Cranial NERVE.

According to the existing views of the development of a dorsal root
of a cranial nerve in Elasmobranchii, based mainly on the researches
of Balfour,? Marshall,’ and Van Wijhe,* the nerve, soon after its
development from the neural ridge, divides into two main branches, a
dorsal one and a ventral one. The dorsal branch is sensory, and
supplies the so-called organs of the lateral line. The ventral one is
mainly motor ; it soon divides again into two branches, which, as
Stannius’ first showed, pass one on each side of a visceral cleft. The
posterior branch is mainly concerned with the innervation of the

1 Beard, ‘On the Segmental Sense Organs, &c.,” ‘Zool. ae 161, 162, 1884; ‘‘On
Cranial Ganglia, &c.,” ‘Zool. Anzeiger,’ 192, 1885.

? Balfour, ‘‘ Elasmobranch Fishes.”

* Marshall, ‘‘The Development of the Cranial Nerves in the Chick,” ‘Quart. Journ. Micr.
Sc.,’ 1878; Marshall, ‘‘On the Head Cavities, &c.,” ‘Quart. Journ. Micr. Sc.,’ 1880; Marshall,

** On the Segmental Value of the Cranial Nerves, &c.,” ‘Journ. of Anat. and Physiol.,’ 1882 ;
also separate.

* Van Wijhe, ‘Ueber die Mesodermsegmente und die Entwickelung der Nerven des
Selachierkopfes,’ Amsterdam, 1882.

* Stannius, ‘Das Peripherische Nervensystem det Fische,’ 1849,

174 JOHN BEARD.

oill muscles. According to Van Wijhe, the dorsal branch becomes
intimately connected with the skin, and is there in connection with
the rudiments of the so-called sense organs of the lateral line. He
further holds that the sensory epithelium takes part in the formation
of the nerve. In this respect the dorsal branch differs from the ventral
one, which does not, according to any writer, arise either wholly or
partially from the skin, but is a direct outgrowth of the neural crest
(Marshall). The branch in front of the cleft is developed later than the
other branches, but how is still uncertain. Atany rate, both Professor
Froriep and I have failed to gather from Van Wijhe, who alone has
studied the development of this branch, how this branch and the
Ramus pharyngeus are developed. In Amphibians, Gotte’ long ago
held that the so-called dorsal branches were split off from the skin.

These various branches have all received general names, some of
which require alteration in view of the researches contained in this
paper. ‘The branch posterior to the cleft is called the main or posterior
branch (Balfour), and post-trematic by Van Wijhe ; in this paper it
will be spoken of as the post-branchial nerve. The branch in front of
the cleft, viz. the pree-trematic of Van Wijhe, I shall call the pre-
branchial nerve.

The Ramus pharyngeus of Van Wijhe will retain the same name
when spoken of here. But now for the so-called dorsal branches ; of
all the general names this is by far the worst. It is true that the name
has been employed by many distinguished zoologists, Stannius, Gegen-
baur, Balfour, Marshall, and Van Wijhe, and that therefore to propose
a change, except for very weighty reasons, would be a very high-
handed and arbitrary proceeding. However, it must be done, and on
grounds to be afterwards stated.

Though some of these various so-called dorsal nerves may come te
occupy a dorsal position, still, as was first mentioned to me by Pro-
fessor Dohrn, it is morphologically wrong to regard them as dorsal.
Of the truth of this I have fully convinced myself, and hope soon to
convince the reader also. I have, however, no means of knowing
whether my reasons for rejecting the name are the same as Professor
Dohrn’s. These branches will be described by the general name of
supra-branchial.

So much for a general view of the adult condition. A schema of
the development in Elasmobranchii would be as follows. (This

1 A, Gotte, ‘Entwickelungsgesch. d. Unke,’ 1875,

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 175

account is in accordance with my own researches, and contains some
additions to the accounts given by my predecessors.)

The nerve grows outwards and downwards from the neural ridge
towards the lateral surface of the head. In its course it lies directly
under, but unconnected with, the epiblast. In the case of those nerves
which are connected with gill-clefts, and are therefore typical, the
nerve lies just over the cleft (fig. 50). All this is well known, and
has been described by Balfour, Marshall, Van Wijhe, &e.

The subsequent events are as follows 3?

1. When the nerve reaches the level of the notochord, or a little
below that level, it fuses with the epiblast (fig. 34).

2. Part of the nerve, however, passes on to the lateral muscle-plates
of the segment (figs. 34, 50).

3. At the point of fusion mentioned in 1. a local thickening of
epiblast has previously taken place (fig. 14).

4, After the fusion has taken place a proliferation of some of the
cells composing the thickening ensues. The proliferated cells form a
mass of actively dividing elements still connected with the skin and
fused with the dorsal root (fig. 16).

5, This mass of cells is the rudiment of the ganglion of the dorsal root,
and externally to it is situated the rudiment of the primitive branchial
sense organ of that root (figs. 12 and 13).

6. For some time cells continue to be given off from the thickened
epiblast, and of those already given off many show nuclear figures
(fig. 8) indicating rapid division.

7. While the ganglion is still fused with the epiblastic thickening
the latter begins to grow in length, and to push its way either forwards
or backwards, as the case may be, between the general epiblast cells
(figs. 40 and 41).

8. The general epiblast cells thus pushed away are probably lost
(figs. 40 and 41, z.e.).

9. Concomitantly with this growth of the sensory thickening, the
ganglion begins to separate from the skin, and so comes to lie deeper
in the mesoblast (fig. 35). As it separates there arises a nerve from
the sensory thickening (figs. 11, 13, &c.). This nerve grows centri-
fugally from the ganglion, arising from the elements of the thickening,
and being in fact split off from the latter along its whole length. It

+ Beard, ‘‘On the Cranial Ganglia and Segmental Sense Organs,” ‘ Zool. Anzeig.,’ 192,
1885 ; also, on some points, Spencer, ‘‘Notes on the Development of Rana temporaria,”
‘Quart. Journ. Micr. Sc.,’ Supplement, July 1885.

176 JOHN BEARD,

is the so-called dorsal branch, and, as previously stated, will be here
called the supra-branchial branch.

10. The sensory thickening of a segment, which gives rise to the
branchial sense organs of that segment, may remain very small or may
increase to a very considerable length, but in any case the nerve con-
necting the whole length of the thickening with its ganglion is split
off from the thickening, and split off simultaneously with the growth
of the latter.

11. The pree-branchial nerve is also formed as the ganglion separates
from the skin, and is probably in all cases also split off from the
epiblast in front of each cleft.

12. Of the development of the F. pharyngeus nothing can be here
recorded ; but I think, from the nature of the case, that this nerve also
probably arises from the cells on the upper wall of the cleft.

Thus, as the general result of these observations, the existing views
of the development of the dorsal root of a cranial nerve will have to
undergo some modification, ‘That in Elasmobranchs the main root of
the nerve is a direct outgrowth from the neural ridge, as stated by
Balfour and Marshall, is certainly true. The shifting and acquisition
of a secondary point of attachment described by Marshall also seem to
take place. The post-branchial branch also appears to arise from the
direct outgrowth from the neural ridge, but in the formation of the
rest the epiblast probably plays a part. In the case of the supra-
branchial branches this is certain, and it is highly probable in the
case of the ganglion. That the other branches, viz. the pree-branchial
and R. pharyngeus of Van Wijhe, are derived from the skin is pro-
bable, and in one case it can be proved, viz. the pree-branchial nerve
of the hyoid.

Having now a general view of the development of a typical cranial
nerve, the various nerves may be considered. In the above schema
we have the key to all the cranial nerves. Some, such as the ninth,
or glossopharyngeal, we shall find to fit in pretty exactly with the
schema. But in others the story that ontogeny often omits or distorts
ancestral history is also repeated.

Some of the branches may be absent even in the ontogeny, while
others may be abnormally developed. Others, again, may be partially
fused with neighbouring nerves, as has been abundantly demonstrated
by previous writers. But whatever the adult condition of any of the
dorsal roots of the cranial nerves, whatever the actual condition of

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. LG

olfactory nerve, nerve of the ciliary ganglion, fifth, seventh, eighth,
ninth, and vagus complex, all can, by the consideration of their actual
development, and of the condition of the various organs which are, or
would be if present, related to them, be reduced to the general schema.

The divergencies between the various nerves are, as might be
suspected, naturally dependent on the presence or absence of gill-
clefts in connection with the segment to which the nerve belongs.

For this reason I shall consider the nerves out of their natural order,
taking those of the true gill-clefts first. Their order of treatment will
thus be as follows:

Nerve. Cleft. Segment. +
Seventh. Spiracle, and one absent. Fourth and fifth.
Ninth. First branchial. Seventh,
Vagus. Second, third, fourth,and Lighth, ninth, tenth, and
fifth branchial. eleventh.
Fifth. Mouth. Third.
Ciliary. Hypophysis. (?) Second.
Olfactory. Absent. First.
Auditory. Absent. Sixth.

In the above list it will be noticed that the cleft of the fifth nerve
is described as the mouth. This view, which we owe to Prof. Dohrn,
seems to me to receive very considerable support from my researches.
I shall refer to the matter subsequently.

For the ciliary, olfactory, and auditory nerves I have hesitated to
assign clefts, because the evidence for their existence is uncertain, and
the nature of the three nerves is more easily explicable if we regard
the clefts as absent or metamorphosed. Here it will suffice to say that
clefts have been assigned to these nerves by various zoologists, with
what justification we shall see later on.

DorsaL Root oF THE FourRTH AND Firta SiGMEnts, SeventH Nerve
OR FactAatis.
As already described by Balfour? and Marshall,’ the seventh nerve
arises from the neural crest in the region of the hind-brain and just
in front of the auditory capsule.

1 The numbering of the segments is in accordance with those conclusions from my
researches which appear to me to be fairly certain. Probably the facial nerve is a complex of
two segmental nerves, apart from the auditory segmental nerve. If this be the case, then

there are eleven segments at least from the olfactory nerve to the fourth root of the vagus
inclusive.

2 Balfour, ‘Comp. Embryol.,’ vol. ii, p. 377.
5 Milnes Marshall, “‘Head Cavities and Associated Nerves in Hlasmobranchii,” ‘Quart.
Journ. Micr, Sc.,’ 1880; also, “‘ Nervous System of Chick,” ‘Quart, Journ. Micr, Se,,’ 1878,

N

178 JOHN BEARD,

These authors further agree in assigning a common root of origin
to the seventh and auditory nerves. Marshall has, however, in one
of his early works, drawn attention to a line of division between the
ganglia of the auditory and facial nerves in the chick. Now,
although the rudiments of the facial and auditory nerves lie very
closely together, I consider that at first the two are really distinct.
The facial grows downwards and outwards from the neural crest, and
just under the epiblast. When it reaches the level of the notochord
part of it fuses with the sensory thickening above the hyoid arch, and
just above the future hyoid cleft. The rest passes on (fig. 20) to the
lateral muscle plates of the hyoid arch. At the point of fusion with
the sensory thickening the ganglion is formed. Of this, one stage is
figured in fig. 20. In this condition the nerve is to be regarded as
passing through an ancestral stage. Its condition is then figured in
the diagram of a typical dorsal root (fig. 50), which passes from the
brain to the primitive branchial sense organ and its associated ganglion
above a gill-cleft, and from which ganglion a nerve passes along the
posterior side of the cleft to the muscles of the gill.

In later stages the ganglion is still partly fused with the skin, but
it soon separates, leaving behind it the rudiments of several branches.

These branches are the supra-branchial, the pree-branchial, and the
pharyngeal. The development of the pharyngeal branch has not yet
been traced, The other branches are split off from the epiblast. The
supra-branchial (figs. 21 and 22) is formed at the expense of the deeper
portion of the sensory thickening, which has begun to grow forwards
over the face.

Very soon this nerve divides into two branches; that is, the sensory
thickening grows forwards as two divergent thickenings, from each of
which nerve-fibres are split off, and thus two branches are formed (fig.
51, p.b.n.). This development from the dichotomously dividing
rudiment has been described by Van Wijhe.t These two branches
have been described by Marshall and Spencer. The upper one is the
portio facialis of the oph. superficialis (Marshall), the lower one the
ramus buccalis (Marshall and Spencer). The upper one Balfour, Mar-
shall, and Spencer classed as a ramus dorsalis of the seventh. As stated
by Van Wijhe,’ they are concerned in the innervation of the supra- and

1 Van Wijhe, op. cit., pp. 26, 27.

2 Marshall and Spencer, ‘‘On the Cranial Nerves of Scyllium,” ‘Quart. Journ, of Micr.
Sc.,’ 1881.

3 Op. cit., p. 27.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. t79

infra-orbital sense organs respectively (branchial sense organs). These
branchial sense organs, it is hardly necessary to state, are developed
from the dichotomously divided sensory thickening mentioned above.
The portio facialis of the ophth. superficialis (fig. 51, p.f.), is obviously
enough, as pointed out by Marshall, Balfour, and Van Wijhe, a
so-called dorsal branch ; that is, what we have here called a supra-
branchial. Van Wijhe has, and I fully agree with him, classed the
r. buccalis (fig. 51, 7.0.) as a ‘dorsal branch,” and gives these reasons:
(1) Its origin from the same rudiment as the former nerve ; (2) its
simultaneous appearance with that nerve ; (3) its similar development
and distribution to (branchial) sense organs. Van Wijhe, indeed,
regards the two as branches of one nerve, and as therefore equivalent
to one so-called dorsal branch. Dohrn’ has advanced very weighty
reasons for the existence of a hyomandibular segment in front of the
hyoid and behind the mouth, but has not adduced the cranial nerves
in support of his view. I would here venture to suggest that an
additional ground for his view is to be seen in the existence of two
supra-branchial nerves in the facial. It would indeed be remarkable
if Van Wijhe were correct in regarding these two nerves as merely
branches of one nerve, for in no other single and simple cranial nerve
do we meet with more than one supra-branchial nerve. To my mind
the best explanation of the presence of these two branches is that the
facial is composed of the fusion of two cranial segmental nerves, and
this apart from its fusion with the auditory. The reader may compare
Dohrn’s views on the nature of the hyomandibular with this explana-
tion. Except for this the facial seems to be a fairly typical cranial
nerve, and agrees well with the general schema. It should be noticed
that the supra-branchial branches grow forwards, for this point will be
referred to in discussing the vagus. «Though I agree fully with Van
Wijhe’s* view that there are two segments in the hyoid arch, and this
apart from the hyomandibular portion, I cannot treat the auditory
nerve here. ‘The special modifications it has undergone will be best
considered after some of the other nerves have been discussed. In
their earliest appearance I believe the auditory and facial nerves are
not fused, and even in the later stages (figs. 21, 42), as already noticed
by Marshall in the chick, the ganglia of the two nerves are partially
separated, and the line of division is easily recognisable. For the

2 Dohrn, ‘‘Studien zur Urgeschichte des Wirbelthier-Kérpers,” No. vii, ‘ Mittheil. a. d.
Zool. Stat. zu Neapel,’ vol. vi, part i.

2 Op, cit., pp. 9 and 28,

180 JOHN BEARD.

later stages of the facial the reader is referred to Marshall’s works and
to the paper by Marshall and Spencer.

NERVE OF THE SEVENTH SEGMENT—GLOSSOPHARYNGEAL.

This nerve arises from the neural ridge (Balfour) immediately
behind the auditory organ, and grows down the lateral wall of the
body to just above the point of origin of the first true branchial cleft.
Its fusion with the skin is represented in fig. 32, and the origin of its
ganglion from the skin and in connection with the branchial sense
organ of this segment in fig. 42. The main portion of the nerve grows
downwards behind the cleft, and proceeds to the lateral muscle plates
of the first branchial arch.

Later, as the ganglion separates from the skin, the supra-branchial
nerve is developed. Like other supra-branchial nerves it splits off
from the skin in connection with a sensory thickening which gives rise
to the supra-temporal sense organs.

Marshall described the course but not the development of this
branch in the embryo.

The direction of growth of this nerve is somewhat different from
that of the corresponding branches of the seventh. It grows dorsally
and forwards (fig. 51, s.é.g.)

In late stages pree-branchial and pharyngeal nerves are developed,
but I have no observations as to their mode of origin to record.

It is obvious that the glossopharyngeal agrees exactly with the
general schema. ‘The sole peculiarity to be noticed is the direction
of growth of its supra-branchial branch. As in the cases of other
nerves, the shifting and secondary attachment described by Marshall
probably occur ; I have, however, not studied them.

Nerves oF THE E1cutu, Ninta, Tanto, AND ELEVENTH SEGMENTS—
VAGUS COMPLEX.?

The actual development of this complex has been fairly accurately
described by Van Wijhe. However, as in the cases of other nerves, he
omitted to record some steps in the process of development, and referred
the actual connection of the complex with the skin to a later stage than
that in which it first arises.

He further, though describing the connection of the supra-branchial

1 For the vagus the condition in Torpedo is taken, in which there are at least four nerves
concerned ; in Hexanchus the vagus has five elements, in Heptanchus six (Gegenbaur).

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 181

branches with the skin, and though figuring the actual fusion of the
vagus ganglia with the sensory thickening, does not ascribe to the skin
any part in the formation of the ganglia.

Like Van Wijhe, I cannot find in the vagus outgrowth itself any
real segmentation in its earliest stages. The first outgrowth from the
neural crest (fig. 33) is a broad uninterrupted band stretching from
just behind the glossopharyngeal, which it almost joins, to a consider-
able distance backwards.

Like other posterior roots, this outgrowth grows outwards and
downwards towards the portion of epiblast just above the second, third,
fourth, and fifth branchial clefts, which are now just forming (fig. 33).
Here the epiblast forms a longish sensory thickening, with which the
vagus fuses.

Portions of the vagus pass on (fig. 34) behind the rudiments of each
of the above-mentioned clefts, and form, as in other cases, the post-
branchial nerves.

At the point of fusion with the skin, cells are proliferated from the
epiblast to form the ganglia.

Soon, as pointed out by Van Wijhe, we get the ganglion of the first
vagus cleft separated from the rest of the mass and fused with an
isolated thickening above the second true branchial cleft.

For the rest of the vagus there is usually only one ganglionic mass,
which, however, ventrally, and by its post-branchial branches, shows
a division into three portions. This mass lies over the last three
clefts, and is to be regarded as made up of the fused ganglia of the
three branchial sense organs of these clefts, with the addition, how-
ever, of rudiments of nerve elements of a certain number of clefts
which have disappeared, and even in the ontogeny hardly present
traces of their former existence. In Torpedo, however, as first noticed
by Wyman,! there is a rudiment of one cleft which never breaks
through to the surface, and which is therefore never functional.* The
rudiment of this cleft is very obvious in horizontal longitudinal
sections of certain stages, and is represented in fig. 47. Here there is
a considerable hypoblastic depression (c/. v1) of the pharynx just
behind the last or fifth branchial cleft.

Corresponding to it is a shallower but still marked epiblastic

2 Wyman, “ Observations on the Development of Raja batis,” ‘Mem. Amer. Acad. of Arts
and Sciences,’ vol. ix, 1864.

? This paper of Wyman’s was not accessible, and the statement in the text is given from
Balfour’s ‘ Embryology,’ vol. ii,

182 JOHN BEARD.

involution. Along the posterior side of this hypoblastic depression
the intestinal branch of the vagus runs. Gegenbaur has regarded this
branch of the vagus as containing rudiments of post-branchial branches
of aborted clefts ; and I think that in the relationship of this intestinal
branch in Torpedo to rudiments of a sixth cleft we have a new support
for this view.

The ramus intestinalis is, as Van Wijhe states, mainly made up of
the post-branchial branch of the last true visceral arch ; but, as just
stated, it must also contain portions of the post-branchial branches of
one or more aborted clefts. Certainly this is the case in Torpedo.

In the question of the homology of this nerve I can only agree with
Van Wijhe in rejecting Balfour’s view that the ramus intestinalis is a
commissure.

The statement just made concerning aborted clefts is also in accord-
ance with Van Bemmelen’s researches on the thymus. His discovery
of thymus elements behind the vagus is mentioned by Dohrn’ in his
last great work, as supporting his view that Vertebrates formerly
possessed many more gill-clefts than they do at present. The question
will be returned to later on.

It is thus seen that in Torpedo at any rate the vagus contains the
elements of at least four segmental nerves and the rudimentary
portion of a fifth.

The first one of the lot is, shortly after its first development, slightly
separated from the fused mass which contains the sense organs and
ganglionic portions of the rest.

Hence vagus I. can be treated alone. As mentioned before, its
post-branchial branch passes along the posterior wall of the second
branchial cleft to the musculature of the cleft. The skin takes no
part in its formation. Above the cleft the main nerve fuses with the
skin, and there, as in other cases, the ganglion and primitive branchial
sense organ are formed. In this case too—and fig. 34 shows it fairly
well—the sensory thickening must be considered as taking part in the
formation of the ganglion.

Later, the ganglion separates from the skin, and, along with this
separation, the sensory thickening grows forwards and takes also a
dorsal direction, a supra-branchial nerve splits off, and the sense organs
formed are part of the supra-temporal branchial sense organs (fig 51,

1 Dohrn, “Studien zur Urgeschichte, &c., No. vii, ‘Mittheil. a. d. Zool. Stat. zu Neapel,’
Bd. vi, Heft 1,

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA, 183

st.v.). Here, as in the glossopharyngeal, the supra-branchial branch
has a dorso-anterior direction.

Vagus I. also fits into the schema very well. It is formed just in
the way described in the schema, has the same relation to a cleft,
develops a primitive branchial sense organ and associated ganglion,
&e. In fact, its development might have been taken in giving the
schema.

For the rest of the vagus there is only one ganglionic mass, and one
long, broadish thickening with which the ganglionic mass is associated.

When the common nerve rudiment grows from the neural ridge and
fuses with the epiblast, at the point of fusion the ganglionic mass is
proliferated, probably entirely from the skin. From the ganglionic
mass branches are sent off along the posterior sides of each of the
three last clefts to the musculature of the clefts. They are the
post-branchial branches, and are not developed from the skin. The
last of the three is the so-called intestinal branch of the vagus.
Along with the separation of the ganglion from the skin, the sensory
thickening begins to grow backwards along the lateral surface of the
trunk (fig. 39), This thickening is the rudiment of the so-called
lateral line. The description of its development to be given here is
in the main identical with that given by Van Wijhe.* It agrees with
Gotte’s? and Semper’s® researches in so far as it describes the origin
from the skin of the so-called lateral nerve, and in this point it differs
from Balfour’s account.* It is, as Semper stated, very easy in Elas-
mobranchs, though by no means so in Teleostei, to follow the whole
development of the lateral line and nerve.

In horizontal longitudinal sections the whole process is obvious
enough, and I can fully endorse Van Wijhe in the opinion that
Balfour would have had no doubt about the matter had he studied
the point with horizontal sections instead of with transverse ones.
The question of the direction of sections is here a vital one. In fig. 39,
(vg.gl.) the compound vagus ganglion is represented as fused with the
skin, and the lateral line, /./., has commenced to grow backwards.

It is an interesting and by no means an unimportant point that the
lateral line increases in length not by the actual conversion of the
epiblast cells behind the growing point of the line into sensory cells

1 Op. cit., pp. 34, 35.

2 Gotte, ‘Entwickelungsgesch. d. Unke,’ p. 672.
3 Op. cit., p. 256.

+ Balfour, ‘ Elasmobranch Fishes,’ p. 141,

184 JOHN BEARD.

similar to those already present in the line, but that there is an actual
growth backwards of the lateral line itself (figs. 40 and 41). That is,
the sensory cells which compose the rudiments of the “line,” and
which anteriorly give rise to the compound vagus ganglion (vg. 2, 3,
and 4), repeatedly and rapidly divide, and in such a manner that
the “line” is increased in length and pushes its way between the indif-
ferent epiblastic cells behind it (fig. 40). These indifferent epiblastic
cells (figs. 40 and 41, ze.) are actually thrust aside and probably lost
along the whole course of the “lateral line” and concomitantly with its
growth.

Part of the epiblast which is cast off is figured in figs. 40, 41, ze.
It is possibly this temporary epiblast seen in transverse section which
led to Balfour’s view of a special origin of the canals of the sense organs
in the trunk of Elasmobranchs.

As in other cases the nerve of the sense organs, the so-called lateral
nerve, is formed from the deeper portion of the sensory thickening.
This mode of origin of the lateral nerve was first described by Semper,
and afterwards more fully by Van Wijhe in Elasmobranchs.

The point is far easier to determine here than in the case of other
supra-branchial nerves ; indeed, it attracts the eye with startling dis-
tinctness in horizontal longitudinal sections of embryos of the proper
age. The nerve is formed as the sensory thickening grows backwards
along the body. It is well shown in figs. 40 and 41, U.m., and can be
traced from the vagus ganglion (vg.g/.) backwards along the thickening,
eradually becoming thinner and Jess differentiated until finally it
ceases in the cells of the sensory thickening.

That here there is no actual growth backwards of the nerve is
obvious enough, for when the development has taken place for some
length, then near the ganglion the nerve is fibrillar and has few nuclei,
these latter increasing as the nerve proceeds backwards, and the fibres
becoming, part passu, fewer, and ending gradually in the protoplasm
of the sensory thickening. :

Where the compound vagus ganglion (wg.gl. 2, 3, 4) separates from
the skin (fig. 36) it is easily seen that above each of the three
branchial clefts fibres are given off from the separating ganglion to
the sensory thickening. In fact, each of the elementary nerves making
up the vagus compound, viz. xg. 2 and 3, and the intestinal branch,
vg. 4 and 5, takes part in the formation of the so-called “lateral line.”
In other words, the lateral line is made up of supra-branchial branches

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 185

of at least four segmental nerves, probably of more than four, viz.
vagus 2, 3, 4,5. The fifth root is the rudiment of the nerve of the
rudimentary cleft mentioned before.

We have seen that the facial, which is probably a compound nerve,
has a large forked supra-branchial branch, and we shall find that the
fifth and ciliary also, as already well known, have each a very long
supra-branchial nerve, extending over the snout (fig. 51, ops. and
oph. pro.), and hence we need not be much surprised that a supra-
branchial nerve, which is made up of the elements of at least four
Supra-branchial branches, should grow right away to the tail, and
supply a very long series of branchial sense organs.

In a former note! I put forward certain hypotheses concerning the
posterior roots of spinal nerves to account for the apparently abnormal
innervation by the vagus, that is by a cranial nerve complex, of a
region extending right to the tail. These hypotheses I now see reason
to reject, and after a study of the actual facts of development in Elas-
mobranchs, as now recorded, I can only conclude that the so-called
lateral line only differs in length and direction of growth from the
other branchial sense organs. Its length is sufficiently accounted for
by its containing the elements of at least four supra-branchial nerves,
and its direction offers in itself nothing really remarkable, for the
direction of growth of the other supra-branchial branches is not always
the same. Those of the fifth, seventh, and ciliary grow forwards ; those
of the glossopharyngeal and vagus I. grow dorso-anteriorly, and that
of the rest of the vagus grows backwards (figs. 46 and 51).

In fact, the direction of growth of sense organs and nerves would
seem to be determined by the usefulness or need of having branchial
sense organs in regions of the body other than the region just above
the gill-clefts where they primitively occur.

Judging by the great variations one meets with in the arrangement
of these branchial sense organs in Ichthyopsida it would seem as
though different families of fishes and Amphibians had independently
solved the matter for themselves. Zhe great morphological point to be
noticed, and I shall lay great stress on it later, 2s that at first there is
the rudiment of one branchial sense organ with its associated ganglion
over each gill-cleft or over the site of a potential gill-cleft.

With reference to the hypotheses about spinal nerves mentioned
above, I may here state that I see no reason now for assuming that

* Beard, ‘On Segmental Sense Organs, &¢.” ‘Zool. Anz.,” 161, 162, 1884.

186 JOHN BEARD.

true spinal nerves were ever connected with branchial sense organs.
So far as my researches go there is a wide difference both in mor-
phology and development between the cranial and spinal nerves.

The mode of development of the lateral nerve here described is, as
previously mentioned, in the main the same as that ascribed to it by
Van Wijhe. The only author who has assigned to it a different origin
in Elasmobranchs is Balfour, who was inclined to the view that the
nerve really grows backwards from the vagus ganglion. 7

My own researches on Teleostei' led me to accept Balfour’s view, but
since I have had the opportunity of investigating the matter in Elas-
mobranchii I conclude that my interpretation of the matter in Teleostei
was erroneous.

No doubt the account given by Hoffmann? of the development in
Teleostei is correct. It accords well with the facts as recorded for
Elasmobranchs here and by Van Wijhe.

But none the less it may not be superfluous to point out that the
existing accounts of the development of what I have called supra-
branchial nerves in Teleostei, Elasmobranchii, and Amphibians—that
is, the accounts given by Semper, Gétte, Hoffmann, and Van Wijhe—
contain in them one element of uncertainty. That is, as to how the
nerve thus developed from the skin acquires its connection with the
appropriate ganglion.

Most of the accounts are quite silent on this point. Gdtte, it is
true, recognised the importance of the matter, and stated that the
nerve in any particular case separates from the skin along part of its
length and grows to its ganglion, This view, however, is not in
accordance with the facts, and I have reason to believe that Prof.
Gétte has now himself ceased to hold it.

The apparent absence of connection between the nervous structure
of the brain and the branchial sense organs of the head was to Balfour
a great objection to Gotte’s and Semper’s view. He said, and to a
certain extent he was right, that at first there is no nerve in connec-
tion with the developing sensory thickening.

This is right so far as its growing point is concerned, for there the
nerve has not developed.

But, as Van Wijhe has pointed out, it is not really the case so far

as relates to entire absence of nerve in connection with the sensory
1 Op. cit.
2 Hoffmann, ‘‘Zur Ontogenie der Knochenfische,” ‘Archiv fiir Micr. Anat.,’ Bd. xxiii,
p. 45,

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 187

thickening, and further, the connection between sense-thickening and
nerve is best made out in early stages, and is afterwards not so easy
to trace.

Van Wijhe himself, though he has given a true, accurate, but some-
what incomplete account of the development of these supra-branchial
branches to the sense organs, cannot be said to have solved the
difficulty under discussion. He has rather ignored it, and though
possessing the material for its solution, has not mentioned the matter.

It is very curious that, although he has figured the fusion of various
ganglia with the skin, he has apparently not noticed that the supra-
branchial branches grow in the various cases out of the various ganglia
so fused, and therefore are in connection with their appropriate ganglia
from the first.

In fact the whole rationale of the formation of supra-branchial
nerves is to be seen in the deploying of the branchial sense organs,
and in the connection of these organs with the ganglionic centre by
longer or shorter conducting fibres—the supra-branchial nerves.
Originally the sense organs were restricted to one over each gill-cleft,
with an associated ganglion. This increased, and gave rise to two by
division, and so on. ‘This is the more certain when we remember that
even in late stages, according to Malbranc,* the sense organs of
Amphibia increase by division. I have myself noticed and recorded
this mode of increase in embryonic Teleostei.*

It is hardly necessary to repeat that Gegenbaur’s view of the com-
position of the vagus out of a number of typical posterior roots is
quite true. We have seen that it really contains rudiments of at
least five such elements in Torpedo.

It follows from this that the vagus agrees with the schema given in
the preceding pages. It is equivalent to and shows the development
of, at least four such schematic nerves, True, there is only one supra-
branchial branch,* the lateral nerve, for all the elements of the vagus
except the first. But thisis probably secondary, and due to the fusion

2 Beard, “‘ Segmental Sense Organs and Associated Ganglia,” ‘Zool. Anz.,’ 192, 1885; also
Froriep, ‘‘ Ueber Anlagen von Sinnesorganen am Facialis, &c.,” ‘Archiv fiir Anat. und
Physiol.,” 1885.

? Malbranc, ‘“ Von der Seitenlinie u. ihren Sinnesorganen bei Amphibien,” ‘ Zeit. f. wiss.
Zool.’ vol. xxvi, 1876.

* Beard, ‘Segmental Sense Organs of Lateral Line,” ‘Zool. Anzeiger,’ Nos. 161, 162,
1884.

* In Torpedo and many other forms. In other cases the “lateral line” is more compli-
cated ; especially is this the case in Amphibia, vide Malbrance, op. cit.

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188 JOHN BEARD.

of the posterior elements of the vagus, and, as stated before, vg. 2, 3,
and 4, all give fibres to the lateral line.

It is worth mentioning here, because these researches confirm one
of Balfour’s views, that the “lateral line” was originally, as he
believed, restricted to the anterior part of the body. The whole
development of all these branchial sense organs shows the truth of
this. But it is, at the same time, a very curious fact that these sense
orgaus along the trunk of Teleostei are segmental (fig. 44, b7.0.).
This is well known, and is figured in the above figure, which is part
of a horizontal section of a salmon hatched about six weeks.

At one time I believed, with Eisig and others, that great morpho-
logical importance could be attached to these facts; but I feel now
compelled to adopt Balfour’s view, and in discussing the morphology
of these sense organs I shall strongly urge that in face of the facts of
development here recorded, the morphological connection between
these branchial sense organs of Vertebrates and the “ Seitenorgane”
of Capitellide, first suggested by Hisig,' becomes of a very doubtful
nature. And here again I may be permitted to remind the reader
that Balfour? long ago rejected the existence of any homology between
these two sets of organs.

Vacus IN AMPHIBIA.

Mr. Spencer has recorded in this Journal’ certain observations on
the nerves of Amphibians. He has found that not merely the ganglia
of the dorsal roots of cranial nerves of Amphibians, but that the whole
of the nerves themselves are split off from the skin. I have figured
the origin of the vagus nerve and ganglion in the frog in fig. 27. I
have investigated the facts in Amphibians, and can fully confirm Mr.
Spencer in most points. The development as seen in Amphibians is
interesting, as in some respects showing a very primitive condition of
the nervous system, viz. a nerve sheath or part of one; in other
respects it is impossible in them to get as good a view of the primitive
nerve composition of the head as in Elasmobranchs.

In Amphibians a considerable amount of fusion of once separate
nerves has taken place, not only behind the auditory organ, but also
in front of it. As an instance, it may be mentioned that the ciliary

1 Hisig, ‘‘Die Seitenorgane der Capitelliden,” ‘ Mittheil. a, d. Zool. Stat. z. Neapel,’
vol. i., 142.

2 Balfour, ‘Comp. Embryol.,’ vol. ii., p. 142.

2 ‘Quart. Journ, Micr. Sc.,’ Supplement, July 1885.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 189

ganglion, which in Elasmobranchs, and even in birds, is quite distinct
in its development, is in the Amphibians fused with the Gasserian, and
the two arise together as one fused mass.

Vagus 1, 2, 3, and 4 are also all fused into one mass in Amphibia;
the figure (27) is a transverse section through this mass. In it the
nerve has not separated from the skin, and the ganglionic portion is
readily recognisable as a mass of yolk-filled cells on the level of the
lateral line. Later, both ganglion and nerve leave the skin as in
Elasmobranchs. :

NERVE OF THE THIRD SEGMENT—TRIGEMINAL LESS OPHTHALMICUS
PROFUNDUS.

The fifth nerve is well suited for studying the development of the
ganglion of a dorsal root.

It is well known, from Balfour’s and Marshall’s researches (opera
cit.), that it arises from the third of the brain vesicles. In fact, from
their researches and those of Van Wijhe, the development of the fifth
is fairly well known with the exception of three stages. These are the
fusion with the skin, the formation of the Gasserian ganglion, and the
mode of development of the supra-branchial nerve (portio minor of the
ophthalmicus superficialis, Schwalbe).

To explain these stages it will be necessary to repeat some facts
which are already known.

The outgrowth from the neural ridge, which forms the rudiment of
the fifth, is broad and extends backwards almost to the region of the
seventh. Anteriorly it stretches forwards almost to the roots of the
ciliary, to be hereafter mentioned.

But the region between the two ganglia is well defined in the earliest
stages by the indifferent epithelium between them, and by the position
of the second head cavity which lies between them (fig. 11, h.c. 2).

The nerve rudiment grows down to the level of the notochord (fig. 14)
and fuses with an epiblastic thickening, just as the other nerves do.
Here cells can be seen leaving the thickening to form the ganglion
(fig. 15).

In this case and in that of the ciliary there can be little doubt as to
the actual mode of formation of the ganglion. The thickening which
gives rise to the ganglion is situated just dorsal to the mouth, and in
fact has just the position of a branchial sense organ.

The ganglion is figured in fig. 17, still connected with the skin, and

190 JOHN BEARD.

possessing then what we may regard as its primitive branchial sense
organ.

Later, the sensory thickening grows in an anterior direction, and as
it does so the ganglion separates from the skin, leaving behind it, as
in other cases, a nerve which is split off from the sensory thickening,
and which is the supra-branchial branch of the fifth (fig. 51, op.s.).
Its course, &c., have been described by Marshall and Spencer, and it
is usually called the portio minor of the ophthal. superfic. It was first
classed as the r. dorsalis of the fifth by Balfour, and Marshall and
Spencer afterwards expressed their agreement with this view. Where
the main nerve fuses with the skin its course is continued along the
mandibular arch by a number of cells of the nerve. These form the
post-branchial branch, and innervate the musculature of the man-
dibular arch. Later, a pree-branchial nerve is developed (Van Wijhe
and others), which hooks over the angle of the mouth in the way that
other pree-branchial branches hook over gill-clefts.

Another apparent branch of the fifth is the nerve which Marshall
has called a communicating nerve between the ciliary and Gasserian
ganglia (fig. 51, ¢.b.). Its true nature has been worked out by Van
Wijhe, who has shown that it really belongs to the ciliary ganglion.
As I accept this statement I shall describe the nerve, as Van Wijhe
has done, as part of the nerve of the second segment.

The ophthalmicus profundus (fig. 51, oph. pro.) is also a part of the
nerve of the second segment. This has been recognised by Marshall
and Spencer, and also by Van Wijhe.

The later fusions which occur between the fifth and seventh and the
fifth and ciliary are in the early stagesabsent. In fact, in its develop-
ment the fifth has the typical characters of the posterior root of a gill-
bearing segment. It fulfils in every way, as Marshall found, the
requirements of a segmental nerve as laid down by him, and it accords
with our schema. It possesses a primitive branchial sense organ and
an associated ganglion just above a cleft, the mouth, It has the homo-
logues of post-branchial and pre-branchial branches, and it develops a
supra-branchial nerve in connection with the branchial sense organs
over the snout (fig. 51, op.s.).

The new additional light thrown on the nature of the mouth will be
referred to in discussing the general morphological considerations
arising out of these researches. Suffice it here to say that the facts

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 191

given above seem to me to confirm Dohrn’s' conclusion that the mouth
arose from a pair of coalesced gill-clefts.

SeconD SEGMENTAL NERVE—OPHTHALMICUS PROFUNDUS, CILIARY
GANGLION, AND Rapix Lonea. ,

A good deal of confusion exists as to the actual nerve components
of this segment.

Marshall? regards the motor oculi as the main stem of the ciliary
ganglion, and attributes to it the character of an anterior and posterior
root. In Marshall and Spencer’s* paper the ophthalmicus profundus is
also classed as part of this segment. Schwalbe* had previously shown
that the ciliary ganglion was really the ganglion of the posterior root
of this segment, a demonstration which Marshall confirmed embryo-
logically. Following on and extending these discoveries, Van Wijhe
recognised the most important component of this segment in the
ophthalmicus profundus, which he classed as the posterior root of the
seement. While accepting to a certain extent Van Wijhe’s view, I
feel bound to admit that from Van Wijhe’s researches alone, the
matter does not stand in a very clear light.

Here, as in other cases, Van Wijhe’s preconceived notions as to the
correspondence of the roots of cranial nerves to those of the spinal
nerves, interfered with the proper interpretation. Marshall’ first gave
an account of the development of the ciliary ganglion ; this account
Van Wijhe added to, but it is still by no means complete. And
although the development of no cranial ganglion is easier to follow,
and no fusion of the epiblast more obvious than the development and
fusion of the ciliary ganglion, this fusion has never before been figured,
and Van Wijhe’s earliest stage figured (fig. 31, g/.c., op. cit.) is a stage
at which the ganglion is in great part separated from the skin, and in
which the ophthalmicus profundus which runs from the ganglion along
the snout and forms the supra-branchial branch, has just begun to
develope. ;

A glance at the diagrams (figs. 45 and 46) of the cranial nerves,
according to my views, will simplify matters and pave the way for the
account shortly to be given.

1 Dohrn, ‘‘Studien, &e.,” No. 1, ‘Mittheil. a. d. Zool. Station zu Neapel,’ Bd. iii., p. 252,

: Marshall, ‘‘ Segmental Value of Cranial Nerves,” ‘ Journ. of Anat. and Physiol.,’ 18382.

5 Op. cit., p. 29.

+ Schwalbe, ‘ Das Ganglion Oculomotorii.’
5 Marshall, ‘‘ Head Cavities and Associated Nerves, &c.,” ‘Quart. Journ. Micr. Se.,’ 1880

192 JOHN BEARD.

Taking the ninth nerve, or glossopharyngeal, as a type of a cranial
nerve to a true gill-cleft, we see that there is a main stem (p.r.), a
ganglion with associated sense organ, and then three other branches.
These are a post-branchial (p..), a pree-branchial (p.b.n.), and a
supra-branchial (s.0.n.). As their names imply, the post-branchial
and préee-branchial run behind and in front of the cleft respectively.
The supra-branchial nerve is the nerve connected with the later
developed additional branchial sense organs.

Now we may turn to the nerve of the second segment. The first
thing noticeable is that the cleft is absent,1 or at any rate the gill
muscles are not present even in the ontogeny.

As a natural corollary to the absence or metamorphosis of the cleft,
and absence of its muscles, the post-branchial and pre-branchial nerves
are also aborted.

In the diagram this abortion is represented by dotted lines (fig. 46).
Hence all that we can expect to find of the posterior root of this
Segment is a supra-branchial branch to the branchial sense organs, the
ganglion of the branchial sense organs, and the main stem connecting
the ganglion with the brain. The ganglion is the ciliary, the main
stem is the radix longa, connecting the ciliary and Gasserian ganglia,
and the supra-branchial branch is the ophthalmicus profundus.

This identification is very similar to that given by Van Wijhe, but
the matter is approached from an entirely different point of view.

The actual development is as follows: From the neural crest of the
mid-brain, just before the closure of the neural folds, cells grow out-
wards and downwards to a thickened patch of epiblast just above and
behind the eye (fig. 7).

This outgrowth has been seen and described by Marshall and Van
Wijhe. But Marshall recognised in it the first rudiment of the motor-
oculi, and Van Wijhe that of the ophthalmicus profundus. Neither
observer saw the skin fusion or the development of the ganglion.
When the outgrowth reaches the thickened patch of epiblast it fuses
with it (fig. 6). Cells are then proliferated off from the skin to form
the ganglion, and the outer portion of the thickening begins to form
the primitive branchial sense organ (figs. 8 and 9). From the thicken-

1 Or metamorphosed. Dohrn has recognised what he believes to be a cleft behind the
nose and in front of the mouth in the hupophysis. He does not say that it is the cleft of the
ciliary ganglion, but this would seem to follow if Dohrn’s view were accepted. As at
present, though possible, no relationship of this supposed cleft to the ciliary ganglion has
yet been demonstrated, Dohrn’s view must be accepted with reserve.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 193

ing, cells are given off for some time, until a large ganglionic mass is
formed, which still for some time remains fused with the skin.

In fact, in the case of the ciliary ganglion the mode of development
is well marked and very easy to study. The sensory thickening soon
begins to grow forwards over the snout, and as it does so the ganglion
begins to leave the skin. As this takes place a nerve is developed from
the thickening, and connects the ganglion with its branchial sense
organs.

From its course, relations, &c., this nerve is seen to be the ophthal-
micus profundus.? It is morphologically the supra-branchial nerve of the
second segment.

The distance between the ciliary and Gasserian ganglia, even in
early stages, is very short. The outgrowth from the neural ridge
which forms the main stem of the ciliary ganglion is practically con-
tinuous with the outgrowth which forms the main stem of the fifth.
Van Wijhe has also drawn attention to this.

Hence it can hardly be wondered at that the connection of the two
ganglia with the brain soon becomes a common one, which distally
divides into two portions, one of which is continued to the Gasserian
ganglion, while the other goes somewhat obliquely to the ciliary, and
forms its so-called radix longa (fig. 51, ¢.0.).

Although I have no observations to record as to the development of
the third or motor-oculi nerve, still Marshall’s opinions on the nature
of the nerve must be discussed, and as his views are inconsistent with
the other facts as recorded in this paper, I shall state what seem to be
urgent reasons for modifying them.

Marshall has advanced the suggestion that the third and fourth
nerves together make up a segmental nerve. Hesays:? “There is very
strong reason for thinking that, in the chick at any rate, the third
nerve develops, like the hinder cranial nerves and the posterior roots
of spinal nerves, as an outgrowth from the neural crest on the top of
the mid-brain.” Since the third nerve later on arises from the base of
the mid-brain, “very near the mid-ventral line,” he infers that the
nerve must shift downwards, and to an extent unequalled by any other
neve.

Now, leaving aside the fact that the shifting in the case of the third

1 Apparently also Van Wijhe’s identification, but not very obvious from his description.

2 Marshall, ‘Segmental Value of Cranial Nerves,” ‘Journ, of Anat. and Physiol.,’ p, 35
1882, :

Q

194 JOHN BEARD.

nerve, if it does take place, occurs, by Marshall’s admission, to a
greater extent than in the case of the other cranial nerves, a point
which is surely of some importance, there are other objections which
cannot, I think, be ignored. Marshall’s views have also been contested
by Van Wijhe, for whose reasons the reader is referred to his oft-
quoted work on the nerves of the Elasmobranchii.

In any discussion as to the nature of the third nerve the morpho-
logy of the head cavities is bound to have an important place. The
second or mandibular head cavity undoubtedly gives rise to the
superior oblique muscle (fig. 12, h.c.). On this point I can fully
confirm Van Wijhe.

This fact alone ought to dispose of the fourth nerve, which Marshall
considers as part of the nerve of the second segment—that is, as part
of the third nerve. The mandibular head cavity arises from the meso-
blast plate of the mandibular arch, according to Balfour, Marshall, and
Van Wijhe. It gives rise to the superior oblique muscle, therefore
the nerve of this muscle, the fourth nerve, must also belong to the
mandibular segment, as Van Wijhe insists.

Further, if the first head cavity is morphologically of the same
nature as the second and third head cavities, then the third nerve,
which innervates the muscles derived from the third head cavity, is,
a priori, of the same nature as the fourth and sixth nerves.

Marshall himself regards the sixth nerve as a ventral root of the
seventh nerve,! and says: ‘‘ Concerning the actual value of the sixth
nerve, I see no reason to alter the opinion I previously expressed, that
the sixth nerve may be regarded as having the same relation to the
seventh that the anterior root of a spinal nerve has to its posterior
root.”

We have also seen reason to believe that the fourth is a ventral root
of the trigeminal nerve. And from all these facts we might fairly
regard the third as also a ventral root.

But further, the dorsal root of no other cranial nerve, if we except
the third, innervates the structures arising out of a head cavity. The
dorsal roots, so far as they are motor, only innervate those structures
derived from the lateral muscle plates (Van Wijhe).

According to Van Wijhe, the third nerve develops after the ciliary
ganglion, and hence could not be its dorsal root. The third, at any
rate, is an exceedingly fine nerve, and is much thinner than the ophthal-

1 ‘Segmental Value, &c.,’ pp. 42—44.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 195

micus profundus ; hence, if the third nerve be the dorsal root of the
second segment, then the proximal stem of the nerve is thinner than
one of its distal branches. Hence there seems to be no avoiding the
conclusion, in which I agree with Krause and Van Wijhe, that the
third is not the dorsal root of the ciliary ganglion, but is the ventral
root of the second segment.

Returning to the general schema of the development of the dorsal
root of a cranial nerve, it is found that, so far as its development goes,
the nerve of the second segment agrees with the schema, In this
instance allowance has to be made for the absence of a gill-cleft, and
more especially, of a gill-musculature. In this the absence even in
the ontogeny of post-branchial and pree-branchial branches is accounted
for. Otherwise the development is normal. ‘There is a main stem
with primitive branchial sense organs and an associated ganglion, the
ciliary. There are no other branches except the later developing
supra-branchial nerve (ophth. profund.). This nerve, as elsewhere, is
developed in connection with the extension forwards of the branchial
sense organs (fig. 51, oph. pro.). The reduction which has probably
taken place in the nerve of the second segment prepares the way for
the recognition and interpretation of the still greater specialisation
which the two remaining cranial segmental nerves have undergone ;
and affords a better insight into the true nature of the olfactory and
auditory nerves.

First SegmMentaL Nerve—Ouractory NERVE.

The olfactory nerve has usually been classed with the auditory and
optic nerves apart from the true segmental cranial nerves, Dohrn,
in his essay on “ Die Ursprung der Wirbelthiere,” first suggested that
the nose was a gill-cleft, and Marshall? very strongly advocated this
view as the result of his researches on the chick and in Elasmo-
branchii. He insisted, and as I believe with justice, on the segmental
nature of the olfactory nerve. His reasons for this view were
based on the actual development of the olfactory nerve; and he
states—and so far as my researches go they only confirm his state-
ment—that “the olfactory nerves develope in precisely the same way

’ Huxley, ‘ Anat. of Vertebrates,’ p. 71; Gegenbaur, ‘Elements of Comp. Anat.,’ English
trans., p: 515 ; Gétte, Entwickelungsgesch. d. Unke, &c.’

2 Marshall, A.M., “The Development of the Cranial Nerves in the Chick,” ‘ Quart. Journ.
Micr, Sc., 1878, p. 23; and also, Morphology of the Vertebrate Olfactory Organ,” ‘ Quart,
Journ, Micr, Sc.,’ 1879,

196 JOHN BEARD.

as the other cranial (segmental) nerves: they arise at first from the upper
part of the fore-brain and gradually shift downwards, acquiring by so
doing a secondary connection with the cerebral hemispheres, of which
they are at first completely independent ; and finally the olfactory
lobe or vesicle, so far from being the earliest part to be developed, is
actually the last, no vestige of it appearing in the chick until the
seventh day of incubation, in the salmon till long after hatching, or
in the dogfish until stage O. of Balfour’s nomenclature,”

For the rest it is hardly necessary to repeat here the evidence
advanced by Marshall of the segmental nature of the olfactory nerve.
Though in my opinion not quite conclusive, it is of value so far as it
goes, and it will be summarised later on after additional evidence
has been adduced in favour of the segmental nature of the olfactory
nerve.

But Marshall recognises in the olfactory organ the rudiment of a
gill-cleft, and, as I am led to a somewhat different view, it may be of
advantage to give a summary of Marshall’s reasons for this opinion.

For the detailed account the reader is referred to the paper on
“The Morphology of the Vertebrate Olfactory Organ.” The following
abstract is taken from Wiedersheim’s ‘Lehrbuch der Vergleichenden
Anatomie,’ p. 375. The epitome there given is so concise and clear
that I do not feel it necessary to offer any excuse for reproducing it
here.

Starting from the fact that the olfactory nerve agrees in its develop-
ment with the other cranial nerves—that is, that it represents a spinal-
like nerve which springs from the neural ridge—Marshall regards the
olfactory groove as a primitive gill-cleft, which in exactly an analogous
position to that in which the true gill-clefts are supplied by branches
of the glossopharyngeal and vagus, has an anterior (upper) and a
posterior (lower) branch of the olfactory nerve, these branches being
respectively in front of and behind the supposed olfactory cleft. The
Schneiderian folds of the nasal mucous membrane are comparable to
the gill-filaments of fishes. As a consequence of the above view a com-
munication between the nasal and oral cavities must once have existed
in all Vertebrates, including fishes. Leaving aside the fact that such
a condition is still present in Myxinoids, traces of it are to be seen in
the naso-oral groove of Selachians, and also in the development of other
fishes. Thus Marshall found in salmon embryos obvious diverticula of

1 Marshall, ‘Segmental Value of Cranial Nerves,’ p. 13.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 197

the oral mucous membrane, which stretched towards the nasal groove,
but which later in the development disappeared. Smelling, argued
Marshall, is only a modified breathing, and thus no violent physio-
logical change is necessary to convert a gill into a smelling organ.

Wiedersheim! himself formerly supported Marshall’s view, and
pointed out that in Epicrium, and probably in other Gymnophiona as
well, there are on either side two olfactory nerves, one dorsal and one
ventral, the roots of the two being perfectly independent and some little
distance apart. He considered these roots to be homologous with the
dorsal and ventral roots of a spinal nerve, and that by their discovery
the segmental rank of the olfactory nerve was established. But, as
Prof. Wiedersheim has kindly informed me by letter, he has, since the
appearance of Blaue’s paper (“Ueber Bau der Nasenschleimhaut bei
Fischen und Amphibien,” ‘Archiv fiir Anat.,’ 1884), seen reason to
change his views on this subject.

The contents of this really important paper will be referred to
shortly, and here I need only express my conviction that the results of
Blaue’s work, taken in conjunction with the light which I hope to
throw on the development of the nose and its relationship to the other
branchial sense organs, settle in a very definite and satisfactory manner
the true homology of the nose.

What has now to be demonstrated is that the nose 2s really a
branchial sense organ, that is, the sense organ of a non-existent gill-
cleft, and not a gill-cleft itself.

It ought here to be mentioned that Hoffmann has already expressed
a very similar view of the nature of the nose.? That is, he compares
its whole development to that of the ear and of the so-called organs of
the lateral line, and rejects Marshall’s view entirely.

Although I have very little that is new to add concerning the
development of the olfactory nerve, still the novel way in which its
development will be regarded is not without importance.

It was seen in discussing the nerve of the second segment—the root
of the ciliary ganglion—that the whole nature of the nerve of this
segment was obvious enough when it was noticed that the musculature
of the lateral plates, that 1s, the gill-musculature, was absent, even in the
ontogeny.

2 Wiedersheim, ‘ Anatomie der Gymnophionen,’ 1879, pp. 59, 60.
® Hoffmann, ‘‘ Zur Ontogenie der Knochenfische,” ‘Archiv f. Micros. Anat.,’ Bd. xxiii.,
p. 88.

198 JOHN BEARD,

As a consequence, post-branchial and pre-branchial nerves were absent,
and the whole segmental nerve was reduced to a ganglion and a supra-
branchial sensory nerve, this nerve, as its name implies, being con-
nected with the innervation of the still-existing branchial sense organs.
Of course the main stem of the nerve connecting ganglion and brain
was also present.

A very similar condition of things exists in the nose. The early
development has its exact parallel in the development of the nerve of
the second segment. The sole difference is that the sense organs of
the nose have not, as in the case of those of the second segment,
undergone further development in a linear direction (fig. 46), but have
confined that development to a somewhat circular area; that is, they
have developed in many directions, but to a limited extent in each.
A change of function has also probably occurred. In higher forms
this, of course, is certain.

A glance at the diagram (fig. 46) will illustrate the meaning of the
above remarks. The supra-branchial nerve of the second segment
(s.b.n.) is represented by a line. In the nose (o/f.o.) a supra-branchial
nerve can hardly be said to be present. The sense organs have
developed within an enclosed figure.

For the rest, the development of the nerve of the first segment is
practically that of a typical segmental nerve in which post- and pree-
branchial branches are aborted.

The nerve grows down from the brain to a thickening of epiblast, it
fuses with this thickening (fig. 1), anda ganglion is formed at the
point of fusion (figs. 2, 3, and 4). Even with the limited amount of
material at my disposal, it can fairly well be shown that the ganglion
is formed from the skin.

When the nerve first fuses with the skin, just as in other cases, no
ganglion is present (fig. 1).

The ganglion first develops after the fusion, and from the inspection
of figs. 2, 3, and 4, which are camera drawings of actual sections, it
will be plain that there are strong reasons for believing that, as in
other cases, the ganglion is proliferated from the sensory thickening.
At any rate, in a later stage, which has also been figured by
Marshall (fig. 5), it is seen that the state of affairs exactly resembles
that in the ciliary ganglion and thickening (fig. 8), Gasserian
ganglion and thickening (fig. 17), &c. The only difference between
the olfactory ganglion and thickening and the complete segmental

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 199

nerve, ganglion, and thickening of a gill-bearing segment, is the
absence in the olfactory segment of any pre- or post-branchial nerves.

Fig. 2 shows us a ganglion fused with an epiblastic sensory thicken-
ing and connected with the brain by a short nerve stalk. In fact, it
is the picture of a branchial sense organ and its associated ganglion.

The facts of development here given, which accord so marvellously
with the development of the other cranial segmental nerves, certainly
render necessary a modification of Marshall’s view as to the nature of
the olfactory organ, and in fact a modification in the sense of the
above passage, in which the nose is regarded not as a gill-cleft, but as
the sense organ of a gill-cleft.

Marshall based his views firstly on the correspondence in anatomical
and histological structure between the nose and other gill-clefts ;
secondly, on the frequent occurrence of two branches of the olfactory
nerve, one on each side of the supposed cleft ; and he further compared
the Schneiderian folds of the nasal mucous membrane, as Stannius!
had previously done, to the folds of a gill.

The facts of development, as stated by Marshall, have been here
admitted, but at the same time slightly extended, and in such a wise
that the development of the olfactory nerve and organ is shown to
agree very closely with that of the nerve, ganglion, and branchial sense
organs of any other cranial segmental nerve.

But now as to the relationships of the branches of the olfactory
nerve to the supposed cleft, and as to the nature of the branches them-
selves.

In its earliest development the olfactory nerve shows nothing that
can really be homologised with the post-branchial branch of a cranial
nerve. Such a resemblance, when present at all, is only existent in
much later stages,

But the post-branchial branch of a cranial nerve, whenever developed,
is, par excellence, concerned with the innervation of the gill muscula-
ture, and if it contains sensory fibres its main portion ismotor. There
is nothing like a gill-musculature, even in early stages, connected with
the olfactory organ.

No one has yet described an arterial arch, gill-cartilage, or muscula-
ture in connection with the supposed nasal visceral arch. The
Schneiderian folds have indeed, in Elasmobranchii and other forms, a
certain resemblance to gill-folds, but. this alone would not be sufficient

2 Stannius, ‘Lehrbuch der Vergleichenden Anatomie,’ ii. Theil

200 JOHN BEARD.

to homologise the two structures, and the folding could be more easily-
explained as brought about by the mere physiological need of increased
surface. But surely it is a great change from a respiratory structure
and function to a sensory structure and function ; a change which, in
spite of the basis of truth in Dohrn’s law of change of function, has
not, so far as I am aware, been shown to have occurred in any
other case. True, Dohrn has recognised a gill-cleft in the hypophysis,
but he has declined to ascribe a sensory function to that structure.

Froriep,” also, in discussing my views as to the nature of the
Vertebrate auditory organ, has suggested that the ear is really a
a modified gill-cleft. But, as I shall presently show, this suggestion
cannot be accepted, or even be held with any amount of reserve, for it
is based on erroneous ideas of the primitive nature of the dorsal roots
of cranial nerves.

If my discoveries stood alone, I should conceive it as highly
probable, if not certain, that the nose is really a branchial sense organ.
But this view of its nature is confirmed in a most striking manner,
and rendered as certain as anything can possibly be by the researches
of Blaue.®

These researches have been carried out on a considerable series of
fish and Amphibians, and have led to the conclusion that in the lowest
form of adult nose met with, viz. the nose of some fishes and Amphi-
bians (Belone, the herring, and Proteus), the structure of the nasal
membrane is essentially made up of a series of “smell buds” (Riech-
knospen), and between these an indifferent stratified epithelium.
These smell buds are identical in structure with the so-called taste-
buds of the papilla foliata of the tongue, say of a rabbit, and are also
identical with the structures in the skin of fishes, which are here called
branchial sense organs, and which are usually known as sense organs
of the lateral line.

In the common Triton those structures described by Blaue are
readily found in transverse sections passing through the nasal cavities.
One such section is figured in outline in fig. 48, and a part of the
section, showing two sense bulbs of the nose, or smell buds, is figured
under high magnifying power in fig. 49.

1 Dohrn, ‘‘ Studien, &c.,” No. 2, ‘ Mittheil. a. d. Zool. Stat. zu Neapel,’ Bd. ii.

2 Froriep, ‘‘Ueber Anlagen von Sinnesorganen am Facialis,” ‘Archiv fiir Anat. und
Physiol.,’ 1885.

% Blaue, ‘‘Ueber Bau der Nasenschleimhaut bei Fischen und Amphibien,” ‘ Archiv fiir
Anat. und Physiol.,’ 1884,

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 201

In Triton I have fully convinced myself by actual investigation that
Blaue’s results are true and accurate. And I have also somewhat
examined the state of things in a few fishes. There can really be no
doubt as to the accuracy of Blaue’s results; and here it only reniains
to give a very short résumé of the paper, referring the reader who
desires further detail to the original, which is illustrated by a number
of very beautiful drawings.

In many Amphibians and fishes the nasal membrane has the struc-
ture mentioned above, but in others the indifferent epithelium becomes
reduced, so that the bulbs come to lie nearer together. This reduction
of the indifferent epithelium begins around the bases of the buds. The
basal epithelium is pushed away, and in such a fashion that the bulbs
are in contact basally, but are separated distally by indifferent epithe-
lium (Exocoetus),

In Trigla typical smell buds are found along with others that have
increased in width and pushed the indifferent epithelium away.

In Cottus the smell buds are almost completely fused together, but
there is still a little indifferent epithelium, and a few buds still remain
isolated.

Lastly, in Fierasfer and others the indifferent epithelium has dis-
appeared entirely from the folds of the nasal membrane, and a con-
tinuous sensory epithelium is present.

Thus Blaue has furnished very valuable evidence, from which, in
conjunction with our knowledge of the development in Elasmobranchii
the nature of the nose can be decided with greater probability than
hitherto.

In Elasmobranchii separate bulbs are not present even in the
embryo. The indifferent epithelium has disappeared even in the
ontogeny ; but from Blaue’s researches on the structure of the nasal
membrane in adult fishes generally, and from the mode of development
of the nose, its ganglion and nerve, there can really be no hesitation
about classing the nose with the branchial sense organs, and hence we
are justified in calling it the modified sense organ of a gill-cleft.}
F. E. Schultze* had previously stated his conviction that the “Gesch-
mackorgane ” of taste buds were the last remains of the skin sense
bulbs of fishes, and Blaue now homologises the smell buds and the
sense bulbs of the skin of fishes.

1 Beard, ‘‘Cranial Ganglia and Segmental Sense Organs,” ‘Zool. Anz.,’ 192, 1885.
? F. E. Schultze, “Ueber die becherférmigen Organe der Fische,” ‘ Zeit. f. wiss. Zool,’
Bd. xii, 1863,

202 JOHN BEARD.

But though he is convinced of this homology, he nowhere hints that
the nose is to be regarded as a specialised portion of the so-called
organs of the lateral line, and in fact accepts and supports Marshall’s
gill theory of the nature of the nose, and derives his smell buds from
skin sense bulbs which, originally present on the nasal visceral arch,
as in other cases, have wandered into the nasal-cleft.

Now, although sense bulbs are present on and along the visceral
arches of many fishes, they are not primitively there, their primitive
position being above the cleft, not along it. Their presence along the
arch is a later development. This fact and the facts of development
as given before are entirely opposed to Blaue’s supposition.

It is a curious commentary on the influence of the same set of facts
on the views of different zoologists, that while Blaue, as the result of
his researches, advocates the gill nature of the nose, Prof. Wieders-
heim, as he has kindly informed me by letter, since reading Blaue’s
paper, considers it necessary, as most morphologists would, to give up
entirely the notion that the nose is a gill-cleft.

My own opinion does not rest on the researches of Blaue alone.
Apart from those discoveries, I should believe myself justified in hold-
ing, as against the views of Prof. Dohrn and of my own teacher, Prof.
Marshall, that the nose is the modified sense organ of a gill-cleft
rather than a gill-cleft itself.

But though maintaining that Blaue’s results are not necessary to
support this view, yet, blending together those results and the facts
recorded in this paper as to the development, &c., of the supra-branchial
sense organs and of the nose itself, I believe that my view of the
nature of the nose has so solid a foundation in facts that even the
most sceptical zoologist can have little hesitation in accepting it.

Shortly stated, the olfactory organ is a branchial sense organ, and
the olfactory nerve is a segmental nerve, the post-branchial and pre-
branchial branches of which, in consequence of the absence of a nasal-
cleft, are not developed. In fact, the olfactory nerve is the sensory
remnant of the most anterior segmental nerve.

DEVELOPMENT OF THE Nose IN AMPHIBIA AND TELEOSTEI.
Hoffmann has described the development in Salmo, but has not
ascribed an epiblastic origin to the nerve ; this, however, is the case
in both Teleostei and Amphibians. In Amphibia, Gétte held that
the olfactory nerve was developed in mesoblast. In fig. 4, the develop-

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 203

ing olfactory nerve and organ of a Teleostean, Rhodeus amarus, is
figured, and in fig. 3a similar stage in Rana temporaria. In both
cases there is an epiblastic thickening, with which is united the
rudiment of a ganglion, and there is also the rudiment of a nerve,
the future. olfactory nerve (o/f.n), just splitting off from the skin.
The development here is precisely similar to the development of the
fifth nerve in the frog as described by Spencer, or to that of the vagus
in the same animal as described in the preceding pages.

It is hardly necessary to say that these facts confirm what has been
said of the nature of the nose in Klasmobranchii.

NERVE OF THE SIxTH Secment—AvpiTory NERVE.

In a former paper’ I suggested the homology of the auditory organ
with the so-called organs of the lateral line or branchial sense organs.
Subsequent investigation has only confirmed this suggestion,

Gegenbaur originally ranked the auditory nerve as a dorsal branch
of the seventh. On embryological grounds Marshall and Balfour had
also been led to the conclusion that the auditory nerve was not in itself
entitled to segmental rank, but was in its development only a dorsal
sensory branch of the seventh. Marshall, indeed, held that there
was not room for another segmental nerve between the seventh and
ninth.

Recent researches have led different zoologists to the opinion that
the hyoid arch is composed of two originally distinct arches.

Van Wijhe considers that the obliterated cleft was behind the facial
nerve, while Dohrn holds that it was in front of the hyoid cleft. The
possibility that both are right appears to me not unlikely. Dohrn
sees remains of a former cleft in the hyo-mandibular and in the
thyroid body. The only evidence afforded by the nerves in support
of this appears to be the existence of two supra-branchial nerves for
the seventh. Alone it is not convincing evidence, but taken in con-
nection with Dohrn’s facts? it is, I think, of importance.

That a cleft formerly existed behind the hyoid cleft and in front of
the first branchial is not admitted by Dohrn, and he has declined to

1 Beard, “On the Segmental Sense Organs, &c.,” ‘Zool. Anzeig.,’ Nos. 161, 162, 1884.

2 Dohrn even goes further, and postulates a separate spiracular visceral arch just behind
the mandibular arch. Thus, according to Dohrn, there are four arches included between
the fifth nerve and the seventh nerve, viz. mandibular, spiracular, hyomandibular, and
hyoid. So far as my researches extend, I have found nothing in the nerves that would

suggest a spiracular arch. However, bearing in mind what has taken place in the case of
the vagus, I should hesitate to cast even a doubt on the truth of his view.

204 JOHN BEARD.

attach any weight to the reasons which Van Wijhe urged for this
opinion, which was based on the presence of two head cavities in the
hyoid arch. Van Wijhe does not appear to have attached much
importance to the evidence offered by the nerves, for he did not regard
the auditory nerve as in itself of segmental value, and he never
suggested the homology of the auditory organ with the branchial sense
organs.

DEVELOPMENT OF THE AuDITORY NERVE.

In Elasmobranchii the facts of development for this segment are
exactly comparable to those described for the olfactory segment. The
arrangement is here the same. There is no gill-cleft, and of course,
as a consequence of the absence of that, we cannot expect to find a
post-branchial nerve.

The following line of argument may, as in the case of the olfactory,
be used for the auditory segment. The sense organs and ganglion
connected with the ciliary segment are without doubt homologous with
the sense organs and ganglion of a cleft-bearing segment such as the
glossopharyngeal. The ciliary has no pre- or post-branchial nerves
because there is no gill-musculature or cleft. The auditory segment
has no pree- or post-branchial branch just as the ciliary, but its sense
organs, ganglion, and nerve are exactly like, and have the same structure
as the sense organ, ganglion, and nerve of the ciliary segment. There-
fore the auditory nerve, organ, and ganglion are homologous with the
nerve, sense organ, and ganglion of the ciliary segment, and therefore
are also the homologues of the nerve, sense organ, and ganglion of the
glossopharyngeal segment. But the sense organ and ganglion of the
latter are a branchial sense organ and its ganglion, therefore the
auditory organ is also a branchial sense organ, and the auditory nerve
the remnant of a segmental nerve.

Immediately behind and somewhat overlapping the sensory thicken-
ing which gives rise to the facial branchial sense organ is a long and
broad auditory thickening (fig. 23). Behind the outgrowth of the
neural crest which forms the facial nerve there is at a certain stage a
small short outgrowth. ‘This is the rudiment of the auditory nerve
(fig. 23). It soon reaches the auditory thickening, fuses with it (figs.
24 and 25), and the ganglion begins to be formed at the point of
fusion, and probably from the thickening itself as a proliferation, just
as in other cases.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 205

Before the auditory involution has proceeded very far there is a
considerable ganglion formed, and fused with the auditory thickening
(fig. 29). At this stage the whole nerve, sense organ, and ganglion
correspond exactly with the nerve, sense organ, and ganglion of the
ciliary segment (fig. 8).

Soon the involution is carried to such an extent that the auditory
organ forms a sac, but it still opens on to the surface, and in Elasmo-
branchs does so throughout life. Even after the formation of the
sac, cells continue to be given off from the thickening to form the
ganglion (fig. 31). The later-formed semicircular canals, &ec., are
obviously secondary complications, which have as their motive the
extension aud perfection of the sensory surface, and which resemble
somewhat the formation of a supra-branchial nerve and its sense
organs.

The resemblance in structure between the sensory cells of the ear
and those of the branchial sense organs is obvious enough, and need
not be dilated upon here.

In Amphibia (Rana temporaria) the auditory organ, nerve, &c., are
formed just like the sense organ, nerve, &c., of the trigeminus of the
same animal. The nerve is split off from the epiblast, the auditory
thickening is developed from the deeper layer of the epiblast opposite
the notochord, and, as in the stage figured (fig 28), there is no
auditory ganglion, it is fair to assume that it is formed just as in
other cranial posterior nerves in Amphibia in connection with the
auditory thickening.

In Elasmobranchii, &c., the auditory ganglion and nerve become so
fused with the facial that the nerve has usually been described as a
branch of the facial. We have seen that it developes separately from
the facial, and even when partially fused (fig. 21), the line dividing
the two nerves is readily seen (cf. Marshall).

GENERAL CONSIDERATIONS.

Morphology of the branchial sense organs.—It is pretty clear from
the facts recorded in the preceding pages that the so-called organs of
the lateral line have some physiological relationship with the gill-clefts.
They arise at the same time as the latter, are originally seated one
over each gill-cleft, and have each a ganglion of a dorsal root of a
cranial nerve arising with and attached to them. From the ganglion
nerve-fibres pass to the gill-musculature on the one hand, and to the

206 JOHN BEARD.

brain on the other. In fact, these sense organs may very well be
regarded as special sense organs of the gill-clefts or as branchial sense
organs. ‘This conclusion Prof. Froriep and I have independently
arrived at.

From the above considerations, and from the facts of development
recorded here, it also follows that the ganglia of the posterior roots are
primitively ganglia of these branchial sense organs. Originally con-
nected directly with this branchial sense organ, the ganglion of the
posterior root has now left its primitive position and has come to lie in
the mesoblast, being only connected with its sense organ by nerve-
fibres. In this conclusion as to the nature of the ganglion I am again
independently in agreement with Froriep and Spencer.

In describing the schematic development of a dorsal root I have, I
think, sufficiently emphasised its true nature. Primitively, a dorsal
root of a cranial nerve is the nerve of a gill-cleft, and is apparently
only connected with the innervation of its cleft. It sends fibres from
the brain to the sense organ and ganglion above the cleft, thence other
fibres pass to the musculature and walls of the cleft (fig. 50).

It is not without importance to notice that any division of the dorsal
root of a cranial nerve into so-called dorsal and ventral branches is
primitively absent (fig 50). Such divisions only occur in the later
development in consequence of the separation of the ganglion from the
skin, and of the formation of a greater number of branchial sense
organs. Of course the ventral branch is there from the start, but in
itself it is mainly motor and gives rise to no ganglion, and probably
never has sense organs in connection with it. It certainly is not
directly concerned in the innervation of a primitive branchial sense
organ. Through a misunderstanding of this point Prof. Froriep has
been led into rather serious errors as to the nature of the dorsal roots.
He concluded from Van Wijhe’s researches, and, I must admit, not
without reason, for the matter is there very vaguely stated, that the
branchial sense organ and ganglion could occur on the ventral branch
of a cranial nerve as well as on a dorsal. This conclusion led him to
the opinion that the auditory nerve is a ventral branch. The blame
of the matter lies very much at the door of Van Wijhe, for he
described a cranial nerve (dorsal root) as typically possessing two
branches, a dorsal and a ventral one, both of which could possess a
ganglion. Now, we have seen in the development that the so-called
dorsal branch (supra-branchial nerve) forms late in the development,

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 207

and arises simply from the necessity of extension and increase of the
branchial sense organs, with which it is solely concerned, the ventral
branch as such being probably solely concerned with the innervation of
the gill-clefts.

A few words may be devoted to the researches of Bodenstein' and
Solger,? which have led to the conclusion that in the sense organs of
the lateral line in Teleostei nerve strands connecting the various sense
organs together are present. From the account of the development
given here such a connection might be expected to occur, for I have
shown that the “lateral line” has arisen solely by the extension and
multiplication of the primitive branchial sense organs of the vagus:
these are, as we have seen, connected in development, being formed
from one continuous sensory rudiment, and as they form one physio-
logical whole, we could expect a connection in the adult. Although I
have not attempted here to give an account of the development of the
“lateral line” in Teleostei, I may perhaps be allowed a few words on
it, as it seems to confirm the researches under discussion.

In this case in the growth backwards of the sensory rudiment there
are found thicker portions, which are segmental, and thinner portions
connecting them. ‘The nerve is split off along the whole length, just
as in Elasmobranchs. The thicker portions give rise to the sense
organs, the thinner portions only to nerve structures, and probably to
those connecting strands described by Bodenstein and Solger.

REMAINS OF BRANCHIAL SENSE ORGANS IN HicgHER VERTEBRATES.

Prof. Froriep’s paper, leaving aside the small error just mentioned,
is a very interesting and very important addition to our knowledge of
the ancestry of Mammalia. It is mainly concerned with the descrip-
tion of rudiments of these branchial sense organs of the facial, glosso-
pharyngeal, and vagus in Mammalia, viz. cow and sheep embryos.
These rudiments are only found in certain stages, and disappear later.
When they still exist the corresponding ganglia of these cranial nerves,
viz. the ganglia of facial, glossopharyngeal, and vagus, are fused with
the skin ; indeed, the conditions seem to be much the same as in
Elasmobranchii. That the ganglia are wholly or partly derived from
the skin in Mammalia, Prof. Froriep hesitates to decide. It is some-

> Bodenstein, E., ‘‘ Der Seitencanal von Cottus Gobio,” ‘Zeit. f. wiss. Zool.,’ Bd. xxxvii,
Heft 1.

* Solger,“* Ueber die Seitenorganen Ketten der Fische,” ‘Zool. Anzeig.,’ 1882, No. 127,
p 660

208 JOHN BEARD.

what remarkable that Prof. Froriep should have failed to find rudiments
of such sense organs in connection with the Gasserian and ciliary
ganglia, and I cannot help expressing a firm conviction that such
rudiments exist at some stage or other in Mammalian development.
This conviction rests on a twofold basis—an a priori one, that in
Elasmobranchii the sense organs of the ciliary and Gasserian ganglia
are very well developed ; and, secondly, on the discovery, of which I
hope soon to give a full account, that such rudiments occur, and are
very obvious in embryo chicks. They are in the chick especially obvious
in the cases of the ciliary and trigeminal segments, but they also occur
in the segments of the facial, glossopharyngeal, and vagus.

Of course here, as in Mammalia, they disappear after the fish stage
has been passed through, but when they attain the maximum of their
development one could almost fancy, in studying them, that it was
an Elasmobranch embryo which was under examination, the state of
affairs in both cases being so alike that one can only marvel that these
rudiments have hitherto escaped notice in the chick. So much for the

present.

THE Nose anpD Ear as Brancuian SENSE Orcans.

In the preceding pages abundant evidence has, I think, been adduced
to show that the nose and ear are specialised branchial sense organs.
Whether they ever had gill-clefts in connection with them is a point
which, from the evidence at present at our disposal, we cannot decide,
and can only suspect that such was once the case from the relationship
of the other branchial sense organs to gill-clefts, and from the known
facts that certainly Vertebrates once possessed more clefts than at
present. At any rate, at present the thymus or thyroid of the nose
and ear, or their equivalents, have still to be found.

The only zoologists who have suggested a different view of their
nature are Froriep and Blaue, who have suggested that the ear is a
gill-cleft. Apart from the evidence given in the preceding pages,
which is inconsistent with this view, one may reasonably ask that the
supporters of such a view shall give us more evidence than that afforded
by an epiblastic depression that an organ is a gill-cleft.

In this matter the nose and ear stand on equal terms, and until we
have a few more of the structures which compose a gill-cleft and visceral
arch, such as arterial arch, cartilage, &c., assigned to them, we can
reasonably regard the matter with a certain amount of reserve.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 209

It is interesting to notice that if my views be correct the nose and
ear are the only remains of the branchial sense organs’ in the adults
of higher Vertebrates. They have survived with a possible change of
function, while the other branchial sense organs have disappeared
except in the first stages of the embryo, and are then only transitory
structures.

THE MORPHOLOGY OF THE SUPRA-BRANCHIAL NERVES.

This point has, I think, been sufficiently demonstrated in the general
part of this work. The supra-branchial nerves are merely concerned
in extensions of the branchial sense organs to a distance from the
ganglia. They are erroneously called dorsal, for this condition when
acquired is purely secondary.

Any commissural nature of some of these branches, as suggested by
Marshall and Speucer, is out of question. None of them are remains
of the neural ridge. Still less can I accept Spencer’s recent suggestion,”
that “the two curious branches which unite respectively the fifth and
seventh and fifth and third cranial nerves ..... may be regarded
as persistent parts of the lateral nerve which united the ganglia of the
sense organs along the lateral line in the head, and which, separating
from the skin, have come in the course of development to occupy a
much deeper position, together with the ganglia, with which they
preserve their primitive connection.”

These “curious branches” are portions of fused supra-branchial
nerves, as a glance at the diagrams (figs. 46 and 51) will show.

THe RELATIONS oF THE Heap aND TRUNK IN VERTEBRATES.

Many attempts have been made to homologise the components of
the segments of the head and trunk, and naturally such attempts have
extended to the nerves. The spinal nerves, it is hardly necessary to
Say, present anterior and posterior roots, the latter of which are
ganglionated. Such a state of affairs has been sought for also in the
head, but in face of the facts previously recorded it is at least doubtful,
even if the existence of cranial anterior and posterior roots be granted,
whether these can be homologised with those of the spinal nerves.
The posterior roots of cranial and spinal nerves develop differently, for

* Professor F. E. Schultze notwithstanding, the possibility that the taste buds of the
tongue of higher Vertebrates are also to be referred to those sense organs must be borne in

mind, Their innervation by the glossopharyngeal is, in this connection, very suggestive,
® Spencer, ‘ Notes on the Early Development of Rana temporaria,’ p. 12.

ie

210 JOHN BEARD.

the spinal have no connection with the skin in early stages; that is,
the ganglion is never fused with the skin, and their roots are never
connected with gill-clefts or with special sense organs.

One of the most striking results of these researches is the great dis-
tinction of the body of Vertebrates into a gill-bearing region and a
non-gill-bearing region; and at present, with the sharply-defined
differences which obtain in the development of the organs of these two
regions, attempts to homologise organs in the two different regions
would seem to meet with indifferent success. That Balfour was right
in regarding the cranial nerves as more primitive than the spinal is
probable enough, but at the same time it is very questionable whether
the spinal nerves ever had the same primitive characters as the cranial.

Dohrn’s idea that the anus arose from a pair of coalesced gill-
clefts may be rejected without more ado, for there seems to be
no evidence for it. Not so, however, his mode of regarding the
mouth as a pair of coalesced gill-clefts; that is probably true. In
dealing with the relations of head and trunk, the vexed question
of anterior roots of cranial nerves crops up, and with it the
nature of the head cavities. I have no observations to record on the
so-called anterior roots of cranial nerves except on the hypoglossus,
which has certainly nothing to do with the cranial nerves, as Dohrn
has pointed out. Van Wijhe regarded the hypoglossus as made up in
Elasmobranchs of three anterior roots of the vagus, In this point my
researches agree with those of Dohrn and Froriep. The hypoglossus
has nothing to do with the vagus.

Froriep’s' account of the development of the former in Mammalia
seems to hold good also for Elasmobranchs. As in Mammalia, the
hypoglossus of Elasmobranchs is derived from the anterior roots of the
first three spinal nerves. The posterior roots are developed in the
embryo, but afterwards abort. I have not figured them, because the
spinal nerves really lay beyond the scope of this work.

As to the head cavities themselves, their persistence in the anterior
part of the head may, as other observers have stated, be due to their
functional connection with the eyes. That they once occurred in all
the segments of the head is probable enough, though with what organs
they were originally connected is not so plain. Possibly from their
muscular nature, and the apparent absence of sensory elements, even
in development, in their nerves, they may have been the muscles of

2 Op. cit., pp. 5 and 48.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. Dit

neural parapodia. That they had nothing to do with the gill-clefts
themselves is pretty certain.

Nature oF THE Movru.

A few words may be here said on the bearing of these researches on
the nature of the mouth.

Dohrn! first suggested that the mouth was primitively a pair of gill-
clefts, which have coalesced and come to open mesially. He after-
wards showed? that it arises in Teleostei as two lateral depressions just
like gill-clefts. In the preceding pages I have shown that in Elasmo-
branchs there is a primitive branchial sense organ over the angle of the
mouth, and with this sense organ an associated ganglion, the Gasserian ;
and also that, just as in the nerves of other gill-clefts, a supra-branchial
nerve is afterwards developed from this ganglion in connection with
the extension of the branchial sense organs of the mouth cleft. I need
hardly say that I see in these facts a strong additional support for
Dohrn’s view.

SEGMENTATION OF THE HEAD.

Admittedly this is one of the most difficult problems in Vertebrate
morphology, and I cannot flatter myself that I am nearer a solution of
it than other zoologists. But it may be remarked that the tendency
of recent researches has been to increase the number of segments
recognisable in the Vertebrate head. In ordinary sharks with five
true gill-clefts, Marshall and Van Wijhe recognised nine segments, but
Van Wijhe rejected Marshall’s olfactory segment, and Marshall did not
regard the hyoid as composed of two segments. I should increase the
number to at least eleven in sharks, with four roots to the vagus, and
apparently Dohrn would agree with this number, but his segments
might not be quite the same.

Indeed, at present it is impossible to solve the problem with any
degree of probability, and it is a question whether it ever will be solved.
Hence the following table is only a tentative one, and is only meant
to give a general view of the results of the researches recorded here.
In passing I may remark that Dohrn’s recent criticism of Ahlborn’s’

1 Dohrn, ‘ Ursprung der Wirbelthiere.’

2 Dohrn, “Studien, &c,” ‘Mittheil. a. d. Zool. Station zu Neapel.’ Bd. iii I. ‘Der
Mund der Knochenfische.”

8 Ahlborn, ‘‘ Ueber die Segmentation des Wirbelthier-Kérpers,” ‘Zeit. fiir wiss. Zool
Bd. xl., p. 309. ;

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212

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 213

paper on this point seems to me to meet the objectionable points so
successfully that further criticism is unnecessary.

In connection with this table the reader would do well to consult
the three diagrammatic figures (figs. 43, 46, 51). The same results
are there shown. Fig. 43 is a diagrammatic horizontal section through
the various sense organs and ganglia, and with fig. 45, which is a side
view of the same structures, shows the primitive condition, Fig, 45
shows the primitive position of these sense organs over the gill-clefts :
in it, for simplicity, the post-branchial nerves are left out; but in
fig. 46 these and the pre-branchial nerves are shown. The closed
gill-clefts are also given, with the absorbed branches, in dotted lines.
Finally, fig. 51 is meant to show the adult condition of the supra-
branchial nerves, which are very diagrammatically given in fig. 46.

Tue RELATIONS OF THE BRANCHIAL SENSE ORGANS TO THE “SEITEN-
ORGANE” OF CAPITELLIDA

Fisig? first suggested that these two sets of organs were homologous.
Since then no one has added anything to the grounds for this
homology furnished by Hisig. Until now it may truly be said that we
knew nothing of the morphology of these branchial sense organs of
Vertebrates. Now we do know a little, and this appears to me to place
the homology of the “ Seitenorgane” of Capitellids with the branchial
sense organs in a very doubtful light. We have seen that primitively
these branchial sense organs are not found in all segments of the
body, but are limited to the head, that they have special ganglia, and
are special sense organs of the gill-clefts.

In all these points they differ from the Seitenorgane of the Capitel-
lidee, and, interesting and important as Hisig’s researches are, we must
at present, I think, hesitate to accept the proposed homology.

PHYSIOLOGY OF THE BRANCHIAL SENSE ORGANS,

Of this we really know nothing. Leydig, who has the honour of
having first described these sense organs, thought they were organs of
a sixth sense. By others they have been regarded as touch organs,
and as organs for testing the water breathed. Lastly, Mayser?
suggested that they were a low form of auditory organ, and Emery?

1 Risig, ‘* Die Seitenorgane und beckerformigen Organe der Capitelliden,” Mittheil. a. d.
Zool. Station zu Neapel,’ Bd. i.

2 Mayser, “ Studien tiber das Gehirn der Knochenfische, ‘Zeit. f. wiss. Zool.,’ vol. xxxvii,

1881.
5 Emery, ‘‘ Fierasfer,” p. 48, ‘Fauna and Flora of the Bay of Naples,’

914 JOHN BEARD.

instituted a comparison between the auditory labyrinth and branchial
sense organs, and concluded that the two sets of organs have an
analogous function. That this is the case seems now very possible ;
that they are concerned in the perception of wave motion is obvious
enough from their structure.

T have here shown, and Professor Froriep’ has also come to the same
conclusion, that they are the special sense organs of the gill-clefts.
On this view we may assume that they give notice of impending
danger to the gill-clefts, and so enable the latter to be closed. Of
course they were existent long before an operculum was developed in
any fish.

After this demonstration that these sense organs stand in some
important relationship to the gill-clefts, it may reasonably be expected
that experimental evidence of their real nature will shortly be forth-
coming. Here a valuable field of research is open for the physiologist,
and a very important one too, for researches in it may lead to a better
knowledge of other Vertebrate sense organs, such as the nose and ear,
which appear to have been primitively of the same nature as these
branchial sense organs.

If the researches recorded here should give any impulse to the

physiological study of these organs, they will have done a great deal ;

for in spite of the many brilliant researches on the structure of these
branchial sense organs, which have undoubtedly told us much about
their structure and distribution, we cannot till now be said to have
gained a clearer insight into their true nature than we possessed after
Leydig’s researches. This honoured histologist and zoologist showed
that they were really sense organs, but there the matter has remained
for thirty-five years.

My researches on the lateral line were commenced over two years
ago in Professor Semper’s laboratory at Wiirzburg. In consequence of
difficulties with the only material I then had, viz. embryos of
Teleostei, they led to very little result. Afterwards they were for a
time laid aside for other work. Although the results of the work in
Wiirzburg were very barren, being made in what appeared to be a
dreary and empty field, still my gratitude is none the less due, and
is here expressed, to Professor Semper for his untiring advice and
assistance.

1 This was stated by Professor Froriep and myself independently.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA, 215

To Professor Milnes Marshall, in whose laboratory the later
researches on Elasmobranchs were made, my acknowledgments are
due not only for the privilege of the use of his library of zoological
works, but also for his valuable assistance, criticism, and advice. I
also wish to express my best thanks to Professor Wiedersheim for
good counsel, and to my friend Dr. L. Will, who very kindly made a
number of useful extracts from Gotte’s great ‘“ Unke” work, a work
which was inaccessible to me in Manchester.

216

16.

17.

JOHN BEARD.

LITERATURE OF THE BRANCHIAL SENSE ORGANS,

. Batrour, F. M.—‘A Monograph of the Development of Elasmo-

branch Fishes,’ 1878.

. Baurour, F. M.—‘ Comparative Embryology,’ vol. ii.

. Bearp, J.—‘‘On the Segmental Sense Organs, and on the Mor-
phology of the Vertebrate Auditory Organ,” ‘Zool. Anzeig.,’
Nos. 161, 162, 1884.

. BearD, J.—‘‘On the Cranial Ganglia and Segmental Sense
Organs,” ‘Zool. Anzeig.,’ 192, 1885.

. Buave, J.—“ Ueber den Bau der Nasenschleimhaut bei Fischen

und Amphibien,” ‘ Archiv fiir Anat. und Physiol.,’ 1884.

. Boprnstetn, E.—“ Der Seitencanal von Cottus Gobio,” ‘ Zeitschr.
f. wiss. Zool.,’ Bd. xxxvii.
. Deroum, F.—“ The Lateral Sensory Apparatus of Fishes,” ‘ Proc.

Acad. Nat. Sci.,’ Philadelphia, 1879.

. Dourn, A.—‘ Ursprung der Wirbelthiere,’ 1875.
. Exsic, H.—“ Die Seitenorgane und beckerformigen Organen der

Capitelliden,” ‘ Mittheil. a. d. Zool. Station zu Neapel,’ Bd.i.,
1879.

. Emery, C.—“ Fierasfer,” ‘Fauna and Flora of the Bay of Naples,’

vol. ii.

. Froriep, A.—‘‘ Ueber Anlagen von Sinnesorgane am Facialis,

&e.,” ‘Archiv fiir Anat. und Physiol.’ 1885.

. GecEnBAUR, C.—‘ Elements of Comparative Anatomy,’ English

translation.

. Gorn, A.‘ Entwickelungsgeschichte der Unke,’ Leipzig, 1875.
. HorrmMann, C. K.—“ Zur Ontogenie der Knochenfische,” ‘ Archiv

fiir Micros. Anat.,’ vol. xxiii.

. Kyox, —.—‘‘On the Theory of the Existence of a Sixth Sense in

Fishes,” ‘Edinburgh Journ. of Sci., vol. ii, 1825. (Quoted in
Emery, ‘‘ Fierasfer.”)

LANGERHANS, P.—‘ Untersuchungen iiber Petromyzon planeri,”
Freiburg i/B., 1873.

LanceruHans, P.—“ Ueber die Haut der Larve von Salamandra
maculosa,” ‘ Archiv fiir Micros. Anat.,’ Bd, ix, 1873,

18.

19:

29.

30.

31.

32.

33.

34.

35.

36,

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 217

Leypic, F.—‘‘Ueber die Schleimcanile der Knochenfischen,”
‘Miiller’s Archiv,’ 1850.

Leypic, F.—‘‘ Ueber die Haut einiger Stisswasserfische,” ‘ Zeitsch.
fiir wiss. Zool.,’ Bd. ii, 1851.

. Leypic, F.—“ Zur Anat. und Histologie der Chimaera monstrosa,”

‘Archiv fiir Anat. und Physiol.’ 1851.

. Leypic, F.—‘ Rochen und Haie,’ Leipzig, 1852,

. Leypic, F,—Anatomische -histologische Untersuchungen iiber

Fische und Reptilien,’ 1853.

. Leypic, F.—‘ Lehrbuch der Histologie,’ 1857.
. Lzypic, F.—‘ Ueber Organe eines Sechsten Sinnes, &.’ 1868.

. Leypic, F.—“ Ueber die allgemeinen Bedeckungen der Amphibien,”

‘ Archiv f. Micros. Anat.,’ Bd. xii, 1876.

. Leypic, F.—“ Die Hautdecke und Hautsinnesorgane der Urodelen,”

‘Morphologisches Jahrbuch,’ Bd. ii, 1876.

. Matpranc, M.—“ Von der Seitenlinie und ihren Sinnegorganen bei

Amphibien,” ‘Zeit. wiss. Zool.’ Bd. xxvi, 1876.

. Scuuttzs, F. E.— Ueber die Nervenendigung in den sogen-

annten Schleimcaniilen der Fische, &c.,” ‘ Archiv. fiir Anat.
und Physiol.’ 1861.

Scuuttzs, F, E.—“ Ueber die beckerformigen Organe der Fische,”
‘Zeitschr. f. wiss. Zool.,’ Bd. xii, 1863.

Scuuttze, F, E.—“ Ueber die Sinnesorgane der Seitenlinie bei
Fischen und Amphibien,” ‘ Archiv fiir Micros. Anat.,’ Bd. vi,
1870.

Semper, C.—‘ Die Verwandschafts-beziehungen der gegliederten
Thiere.” ‘Arbeiten a, d. Zool.-zoot. Institut. zu Wiirzburg,’
Bd. iti, 1876-7.

Soteer, B.—“ Zur Kentniss der Seitenorgane der Knochenfische,”
‘Centralblatt f. d. med. Wiss.,’ 1877, No. 37.

Sotcer, B.—“ Zweite Mittheilung iiber Seitenorgane der Knochen-
fische,” ditto,

SontcER, B.—‘“‘ Ueber die Seitenorgane der Fische,” ‘ Kais. Leop.
Akad. der Naturforscher,’ Heft 14, 1878.

SoneER, B.—‘ Bemerkung iiber die Seitenorganen Ketten der
Fische,” ‘Zool. Anzeig.,’ No. 127, 1882.

Spencer, W. B.—“ Notes on the Early Development of Rana tem-
poraria,” ‘Quart. Journ, Micr. Sc.,’ Supplement, July, 1885,

218
37.

38.

39.

12.
13.

JOHN BEARD.

Van Wisue, J. W.—‘ Ueber die Mesodermsegmente und tiber die
Entwickelung der Nerven des Selachier Kopfes,’ Amsterdam,
1882.

WrepersHeimm, R.—‘ Lehrbuch der vergleichenden Anatomie der
Wirbelthiere,’ 1883. |

Wricut, P. R.—“ Contributions to the Anatomy of Amiurus,”
‘Proceed. Canadian Institute,’ 1884, Toronto.

OTHER WoRKS QUOTED IN THIS PAPER.

. AutBorn, F.—“Ueber die Segmentation des Wirbelthier-K6érpers,”

‘ Zeitschr. wiss. Zool.,’ Bd. xl.

. Bateson, W.—“ The Later Stages in the Development of Bala-

noglossus,” ‘Quart. Journ. Micr. Se.,’ Supplement, July, 1885.

. Dourn, A.—““Studien zur Urgeschichte des Wirbelthier-Korpers,”

‘Mittheil. a. d. Zool. Stat. zu Neapel.’
1. “Der Mund der Knochenfische,” Bd. iii.
2, 3. “Der Hypophysis bei Petromyzon planert,” Bad. iii.
7. “Enstehung, c&c., des Zungenbein und Kieferappara-
tes der Selachier,” Bd. vi.

. GrcEnBauR, C.—‘ Die Kopfnerven von Hexanchus.’
. MarsHaty, A. M.—‘ On the Development of the Cranial Nerves in

Birds,” ‘Quart. Journ. Micr. Sc.,’ 1878.

. MarsHatt, A. M.—“On the Morphology of the Vertebrate

Olfactory Organ,” ditto, 1879.

. MarsHaty, A. M.—“ On the Head Cavities and Associated Nerves

in Elasmobranchs,” ditto, 1880.

. Marsuatt, A. M., and Spencer, W. B.—‘‘ On the Cranial Nerves

of Scyllium,” ditto, 1881.

. Marsuatt, A. M.—“ The Segmental Value of the Cranial Nerves,”

‘Journ. Anat. and Physiol.,’ 1882 ; also separate.

. ScowaLBe.—‘ Das Ganglion Oculomotorii.’
. Stannius, H.—‘ Das Peripherische Nervensystem der Fische,’

Rostock, 1849.
Stannius, H.—‘ Handbuch der Anatomie der Wirbelthiere,’ 1854.
WirpERsHEIM, R.—‘ Anatomie d. Gymnophionen, 1879.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 219

DESCRIPTION OF PLATES VII, VIII, AND IX.

In most cases the objective and ocular used for each drawing are
denoted by letters, such as Z. D, oc. 2, which signify Zeiss’s objective
p, ocular No. 2. The figures are mostly camera drawings, and are all
reduced to one third of their apparent enlargement,

ALPHABETICAL LIST OF REFERENCES,

I, III, V, vil, &c., Olfactory, motor oculi, trigeminal, facial, &c., nerves.
al.c. Alimentary canal. aud. and au.o. Auditory organ. aw.gl.
Auditory ganglion. aun. Auditory nerve. 67. Brain. br.gl.
Branchial ganglion. 67.0. Branchial sense organ. c.b. Nerve con-
necting ciliary and Gasserian ganglia. c.gl. and cil.gl. Ciliary gan-
glion. cz, Ciliary. cl. Cleft. cl. vi Sixth cleft. ep. Epiblast. fdr.
Fore-brain. f.gl. Facial ganglion. fac. Facial. Gass. Gasserian.
G.gl. Gasserian ganglion. gl.gl. Glossopharyngeal ganglion. gloss.
Glossopharyngeal. A.br. Hind-brain. h.c. Head-cavity. h.c.. Second
head-cavity. hy.cl. Hyoid cleft. «ae. Indifferent epiblast. 1.7.
Lateral line. 7m. Lateral nerve. J.m. Lateral muscle plates. m.
Mouth. m.br. Mid-brain. me. Mesoblast. ms. Inter-muscular septa.
nm. Notochord. xz.c.gl. Nerve of ciliary ganglion. 2s. Nervous
system. olf.gl. Olfactory ganglion. o/fn. Olfactory nerve. olf.o.
Olfactory organ. oph.pro. Ophthalmicus profundus. op.s. Ophthal-
micus superficialis of fifth nerve. y.b7.0. Primitive branchial sense
organ. p.b.n. Pre-branchial nerve. p.f. Portio facialis of ophthal-
micus superficialis—one supra-branchial nerve of facial. p.n. Post-
branchial nerve. .7. Posterior root. 7.6. Ramus buccalis, the second
supra-branchial nerve of the facial. sb.n. Supra-branchial nerve. sp.c.
Spinal cord. sp.gl. Spinal ganglion. sm.b. Smell-buds. s.¢.v, Supra-
temporal branch of vagus 1. s.¢.g. Supra-temporal branch of glosso-
pharyngeal. v9.91. Vagus ganglion. v9.1. Vagus ganglion 1.

Puate VII.
Fig. 1. Olfactory nerve just fusing with olfactory thickening. Torpedo
ocellata. Z. D, oc. 2, camera. olf.n. Olfactory nerve. off.o.
Olfactory thickening.
Fig. 2. Olfactory ganglion (ol/f.g/.) and olfactory thickening (o/f0.)
fused together. Torpedo ocellata. Z.D, oc. 2, cam. luc.

220 JOHN BEARD.

Fig. 3. Transverse section of olfactory organ (o/f.0.) and nerve (olf.n.)
in Rana temporaria. Z. c, oc. 2, cam. lue.

Fig, 4. Transverse section of embryo of Rhodeus amarus, showing
olfactory nerve and thickening both fused with skin.
Letters as before. fbr. Brain. Z. F, oc. 2, cam. luc.

Fig. 5. Transverse section through fore-brain and olfactory organ of
an embryo of 7” ocellata. Combined from several sections.
Shows olfactory nerve and ganglion fused with thickening
and connected with brain. Letters as before. Z. a, oc. 2,
cam. luc.

Fig. 6. Somewhat horizontal section through mid-brain, showing
nerve of ciliary ganglion (n.¢.gl.) just fusing with skin,
T. ocellata. Z. F, oc. 2, cam. luc.

Fig. 7. Low-power view of same section. Z.c, oc. 2, cam. luc.

Figs. 8 & 9. High- and low-power drawings respectively of a some-
what horizontal section through fore- and hind-brain.
Shows ciliary ganglion rudiment (c.g/.) and its primitive
branchial sense organ (p.br.0.). The ganglion is in course
of formation from the epiblast. fdr. Fore-brain. h.br.
Hind-brain. Zorpedo ocellata. Z. D and A, oc. 2, cam. luc.

Fig. 10. Horizontal section through a young Torpedo embryo, showing
ciliary ganglion still fused with its sensory thickening.
Algo shows motor oculi nerve, (111). e.gl. Ciliary ganglion.
p.br.o. Primitive branchial sense-organ. 1. Motor oculi.
G.gl. Gasserian ganglion. hy.cl. Hyoid cleft. m.br. Mid-
brain. h.c. Head-cavity. f.gl. Facial ganglion. Z,. a, oc. 2,
cam. luc.

Fig, 11. Drawing under high power of ciliary ganglion and its primi-
tive sense organ of the preceding section. Late stage,
but still intimate fusion with skin. Also origin of supra-
branchial nerve of ciliary ganglion (ophth. profund.) from
skin. Supra-branchial nerve (s.b7.n.). Z. F, oc. 2,
cam. luc. ;

Figs. 12 & 13. Similar drawings to figs. 10 and 11 respectively. Let-
ters as before. gl.gl. Glossopharyngeal ganglion.

Fig. 14. Horizontal section through mid-brain and anterior portion of
hind-brain. Shows course of fifth nerve, which lies just
under skin, but is not yet fused with it. No ganglion yet
present. 7. ocellata. Z. 0, oc. 2, cam. luc.

Fig.

malts

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19.

ig. 20.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 221

Fifth nerve fused with its sensory thickening (p.b7.s.0.), and
proliferation of Gasserian ganglion from the skin. G‘gl.
Gasserian ganglion. 7’ ocellata. Z. 0, oc. 2, cam. luc.

Similar figure to preceding one. JZ". ocellata. Z. 0, oc. 2,
cam. luc.

Section through hind- and fore-brain. Shows Gasserian
ganglion just before its separation from the skin, Z.
ocellata. Z. A, oc. 2, cam. luc.

Small piece of a horizontal section of a Torpedo embryo.
Shows hyoid pree-branchial nerve (pb.n.) lying in epiblast
and not yet separated from it. Gigli. Gasserian ganglion.
f.gt. Facial ganglion.

Puate VIII.

Transverse section through hind-brain of a Torpedo embryo.
Facial nerve (vi1) just on point of fusion with its sensory
thickening. Gill-cleft (hyoid) just about to form. Z. o,
oc. 2, cam. luc.

A similar section. Facial nerve just fused with skin, and its
post-branchial (p.n.) passing on to muscles of cleft. Z. a,
oc. 2, cam. luc. A later stage of facial ganglion in fig. 42.

. Facial ganglion leaving skin, and still connected by two

supra-branchial nerves (s.0.2. 1, and s.6.n. 2). Z. D, oc. 2,
_ cam. luce.

. Horizontal section of a Torpedo embryo. Facial ganglion

fused with auditory, but line of demarcation is obvious.
Facial has just left the skin, and is leaving a supra-
branchial nerve (s.0.n.) behind it.

. Part of a transverse section through the auditory region of a

Torpedo embryo, Auditory nerve (vi) not yet fused
with auditory thickening (au.o.). Z. F, oc. 2, cam. luc.

& 25. Auditory just fused with auditory thickening, and
ganglion proliferating. Letters as before. JZ’. ocellata.
Z. F, oc. 2, cam. luc.

. Low-power drawing of a horizontal section, such as the two

preceding figures form part of.

Transverse section, rather oblique, through hind-brain of a
frog embryo. Shows auditory nerve and thickening on one
side, and vagus nerve and ganglion on the other. awn

222

Fig.

Fig.

Fig.

Fig.

Fig.

Figs,

Fig.

Fig.

Fig.

28.

29.

. 30.

moll

37.

38.

JOHN BEARD.

Auditory nerve. vg. Vagus nerve. vg.gl. Vagus ganglion.
Z. A, oc. 2, cam. luc.

High-power drawing of auditory portion of preceding. Shows
auditory nerve not yet separated from skin. Z. F, oc. 2,
cam. luc.

Transverse section through auditory region of an Elasmo-
branch embryo. Shows auditory ganglion and auditory
thickening intimately fused together. Auditory involu-
tion as yet only partial.

Similar section under low power. Auditory involution com-
plete.

Highly-magnified drawing of auditory portion of last sec-
tion. Shows intimate fusion of ganglion and thickening,
and proliferation of cells of thickening into ganglion,
Many nuclear figures near proliferating portion.

. Transverse section through hind-brain of a Torpedo embryo.

Shows glossopharyngeal nerve (1x) just fused with its
primitive branchial sense organ (p.br.o.). Z. ©, oc 2,
cam. luc.

. Similar section to preceding. Vagus nerve (x) just before

fusion. Z. c, oc. 2, cam.

. Similar section. Vagus nerve just fused with its thickening.

Post-branchial branch (p.n.) passing on to muscles of cleft.
Z. D, oc. 2, cam.

& 36. Portions of similar sections to preceding. Portions of
vagus ganglion (vg.gl.) above gill-cleft, and just separating
from skin; in separating, leaving a nerve behind. p.br.o.
Branchial sensory thickening. pir. Pharynx. cl. Cleft.
T. ocellata. Z. D., oc. 2, cam, luc.

Puate IX,

Horizontal section through head of a Torpedo embryo. Shows
hyoid pre-branchial nerve (p.br.n.) formirig in epiblast.
High-power view of small piece of preceding section, showing
hyoid pre-branchial nerve (p.br.n.) in epiblast. Z. F, oc. 2,

cam.

. Horizontal section through Torpedo embryo. Vagus ganglion

separating from the’skin. Lateral line (/./.) growing back-
wards and pushing indifferent epiblast (7.¢.) away. sp.gl.
Spinal ganglion. Z. a, oc. 2, cam, luc.

Fig.

Fig.

Fig.

Fig.

Fig.

Fig.

Fig.

BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 223

40, High-power view of part of preceding section, showing lateral
line forming (//.), indifferent epiblast (.¢.) being pushed
away, and. lateral nerve (/.n.) splitting off from thickening.
me. Mesoblast. 7’. ocellata. Camera luc.

41, Later stage of lateral line. Further back in trunk. High-
power, camera lucida. Shows same things as preceding
drawing. 7’. ocellata.

42. Drawing combined under camera from several horizontal
sections of an Elasmobranch embryo. Shows several cra-
nial ganglia fused with their branchial sense organs. p.br.o.
Primitive branchial sense organ. m.br. Mid-brain. e.gl.
Ciliary ganglion. G‘gl. Gasserian ganglion. jf.gl. Facial
ganglion. au.gl. Auditory ganglion. gl.gl. Glossopharyn-
geal ganglion. vg.gl. 1. First vagus ganglion. vg.gl.e.
Second, third, and fourth vagus ganglion. 0. Lateral
line. Jd.n. Lateral nerve. au. Ear. . Notochord. sp.c.
Spinal cord.

43, Diagrammatic horizontal section through the various bran-
chial sense organs and their ganglia. The reader should
conclude nothing from the cerebral vesicles figured here :
there is probably at least one between the trigeminal and
seventh nerves, and it is not figured here.

44, Part of a horizontal section of a six week’s old salmon.
Shows the position and segmental arrangement of the
branchial sense organs (07.0.) in the trunk. 7s. Intra-
muscular septa. ». Notochord. me. Mesoblast.

45. Diagram of lateral view of an Elasmobranch embryo. Shows
the central nervous system as plate not yet involuted, the
posterior roots of the cranial nerves (p.r.) the branchial
sense organs, the dorsal eye (oc.), mouth, and gill-clefts.
Letters as before.

46. Similar diagram, to show the branches of nerves to gill-clefts.
The aborted branches in dotted lines. Also shows for-
mation and direction of various supra-branchial nerves
(s.0.n.). Vagus represented as supplying in all five clefts.
This figure is a more diagrammatic view of fig. 51, which
represents nature more or less accurately.

47. Horizontal section of a Torpedo embryo, showing rudiment
(cl. v1) of a sixth true branchial cleft.

224 JOHN BEARD.

Fig. 48. Low-power drawing of transverse section through nose of an
adult Triton showing Blaue’s smell-buds (sm.0.).

Fig. 49. High-power drawing of two such smell-buds. Z. F, oc. 2,
cam. luce.

Fig. 50. Diagrammatic transverse section through the gill-bearing
region of an Elasmobranch or other Ichthyopsid. Nervous
system not yet closed in. On the left side the gill muscle
plate is shown, and on the right the gill-cieft. h.c. Head
cavity. m.s. Nervous system. p.r. Posterior root. x. Noto-
chord. y.b7.0. Branchial sense organ. Or.gl. Branchial
ganglion. Um. Lateral muscle plate. y.n. Post-branchial
nerve. al.c, Alimentary canal.

Fig. 51. Diagram taken partly from my own drawings and partly
from Prof. Marshall’s. Shows the ganglia and various
branches of the cranial nerves. Also mouth (m.) and gill-
clefts (cl.1, cl.z), &c. For lettering, see general list.

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A SIMPLIFIED VIEW OF THE HISTOLOGY OF THE
STRIPED MUSCLE FIBRE.

By B. Metuanp, B.Sc. Platt Physiological Scholar in the
Owens College.

[PLATE X.]

INTRODUCTION.

Everyone who has considered the subject must admit the essential
identity from a physiological point of view of all those tissues which
possess in a special degree contractility. The contraction of a white
blood corpuscle or amceba is essentially the same phenomenon as the
contraction of an involuntary fibre cell or a striped muscle fibre.

When we consider these three contractile tissues from a histological
point of view we are struck by an apparently essential difference in
character between the striped muscle fibre and the elements of the
other two contractile tissues, and indeed cells generally. The voluntary
muscle fibre is morphologically a cell like a muscle fibre cell and the
ameeboid corpuscle. Yet it differs from the latter and from all other
cells in showing a characteristic transverse striation.

According to Klein,! the protoplasm of the simpler contractile
tissues, (1) the amceboid cell, (2) the ciliated cell, and (3) the involun-
tary fibre cell, agrees, inasmuch as it consists of two parts—a matrix
and an arrangement of fine fibrils, the intracellular network. The
actual arrangement of the fibrils differs somewhat in the three cases.

1 ‘Klein, ‘ Atlas of Histology,’ diagrams 1 and 4, and fig 2, pl. xv.

226 B, MELLAND.

Tn the white blood corpuscle they are arranged in a network or
meshwork with polygonal meshes. In the ciliated cell they also form
a network which seems to be in peculiar relation with the cilia. In
the ciliated cell of the Mollusc, according to Engelmann,’ the fibrils
are arranged in a longitudinal manner as fine varicose filaments
running the whole length of the cell, and in connection with the
bases of the cilia. In the protoplasm of the involuntary fibre cell
the fibrils are arranged in a central or axial bundle, anastomosing at
the poles of the nucleus with the intra-nuclear network.

Observations on which I have been engaged for some time past, and
which have been partly worked out in the Physiological Laboratory of
Owens College, lead me to the belief that the striated muscular fibre
really agrees fundamentally as regards histological structure with
the other contractile tissue elements, in containing an intracellular
network, differing from them merely in the greater amount of differen-
tiation, and more regular arrangement, of the network.

I believe, further, that the various conflicting descriptions given by
different observers, and those points on which competent histologists
differ more materially, can be explained and brought into harmony
with one another by this view.

I have observed this network in the fibres of Dytiscus, the Bee,
Crayfish, Lobster, Frog, and Rat, prepared by a somewhat special
method of gold staining, the network being the only part of the fibre
stained by the gold.

It may be specially stained also by treating the fibre with acetic
acid, and subsequently staining with hematoxylin.

It may be demonstrated, though not so completely, in the living
fibre, and in acetic and osmic acid preparations. I have submitted
my drawings and preparations to the examination of Prof. A. Gamgee
and Prof. Milnes Marshall.

DEMONSTRATION OF AN INTRACELLULAR NETWORK IN THE STRIPED
Muscie Fire.
1. Tue Muscite FIBRE PREPARED WITH GOLD CHLORIDE.
(a) Dytiscus marginalis.
Method of gold staining.—Decapitate a Dytiscus, open the thorax,
remove a portion of a leg muscle, and place in 1 per cent acetic acid

1 Engelmann, ‘ Pfliiger’s Archiv,’ xxiii, 1880, and ‘Quain’s Anatomy,’ 9th edition, vol. ii,
fig. 240.

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE, 227

from five to fifteen seconds, then into gold chloride solution 1 per cent
for forty-five minutes, and leave in formic acid 25 per cent for
forty-eight hours in the dark, Tease and mount in glycerine.

If now examined with a magnifying power of about 700 diam. the
appearance shown in figs, 1, 2, 3, 12, 13, and 14 will be seen in certain
of the fibres. The method of preparation has a great tendency to
soften the fibre, so that it becomes much expanded on compression
by the cover-slip ; it also has a great tendency to split the fibre into
transverse discs.

Fig. 1 represents a fibre which has retained its natural size and form.
Narrow transverse bands of granular substance, deeply stained with
the reduced gold, are seen crossing the fibre, separated by wider bands
of lighter substance. These deeply-stained granular bands correspond
in position to Krause’s “membranes.” The usual separation into light
and dim discs of about equal thickness is lost by this method of
preparation. ‘Traversing the wider unstained discs, and giving the
fibre the appearance of longitudinal striation, are seen fine longitudinal
lines.

In fig. 2 is seen a portion of a fibre which has been more flattened
out by pressure. In it the deeply-stained, narrow granular band is
seen to consist of a transverse row of dots. The longitudinal lines are
seen to represent fine rod-like bodies traversing the position usually
occupied by the dim stripe, and being continued into the dots at
either end. Insome fibres a minute thickening of the rod is apparent
midway in the position of the so-called “Hensen’s disc” (in the middle
of the dim stripe).

This method, as was before stated, has a tendency to split the fibre
into transverse discs. These isolated discs are found in many parts of
the preparation; they present the appearances seen in figs. 4 and 5.
They are seen plainly in all cases to consist of two parts—(1) a net-
work of fine highly refracting lines, stained by the gold, and having
thickenings at the nodes ; and (2) an unstained substance lying in the
interstices of the network.

The appearance of this network differs somewhat with the degree of
compression of the discs. When much compressed the network appears
more open, and the nodal dots less marked. Towards the outside of
the fibre the meshes appear more oblong, the network extending
mostly in a radial direction. This network evidently corresponds
when it is in its transverse position in the fibre with the deeply-stained,

228 B. MELLAND.

beaded disc occupying the position of ‘‘Krause’s membrane.” This
is shown in certain fibres in which the discs are not seen perfectly
edgeways but in perspective (fig. 6). The beaded disc at each
membrane of Krause is here seen to consist of a transverse or
horizontal network, united to the discs above and below by fine
thread-like lines. This method of gold staining, then, brings out a
network arranged in a manner represented diagrammatically in dia-
grams 1, 2, 3, and 4.

This network differs chemically from the rest of the fibre, inasmuch
as it resists to a larger extent the action of acetic acid, and possesses
in a greater degree the power of reducing gold.

Tt will be shown later, by other methods of preparation, that this
network differs again from the matrix in its physical properties. The
network is isotropous and highly refractile. The refractive power is
somewhat altered by gold staining, but certain optical effects are still
produced by the refractive action of the network upon light. These
optical effects can be more definitely seen in isolated portions of the
network than in the whole fibre.

Optical Effects produced by the Network.

Fig. 12 represents a small piece of the network isolated from the
yest of the fibre, consisting of nine or ten rows of dots and the
connecting longitudinal bars. There is a single layer only of network
aud dots. This isolated piece seems to be a portion of sarcolemma
stripped off the fibre, along with the portion of network immediately
below the sarcolemma, and attached to it by each transverse network.

When exactly focussed (fig. 14, 1) each dot appears as a dark
granule surrounded by a bright halo. The blending of these haloes
causes a crenated bright transverse band. The effect of alternating
light and dim bands is thus obtained, the bright band being crossed
transversely by a row of dots, the dim band longitudinally by a series
of fine lines.

On altering the focus (raising ‘0025 millimetre, about), the refractive
effects are to a certain extent transposed (fig. 14, vu). The dots now
appear bright, surrounded by a dark border. By coalescence the
appearance of a narrow bright disc is produced, separated from the
dim disc at each side by a dark crenated line.

Similar refractive effects and transposition on focussing are seen in
the dises isolated by transverse splitting of the fibre.

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE, 929

Transposition of the Bands.

The effect known as “ transposition” of the bands has been noticed
by many observers. On raising the objective what was previously the
bright band appears now darker than the dim band.

This so-called transposition is seen in fibres prepared by the gold
method, better in fibres prepared with osmic acid; diag. 6, u represents
a fibre at the upper focus. The light band in the position of Krause’s
membrane appears very bright, and is bordered by a dark line at the
junction of the light and dim bands. On focussing about 0025 mm.
lower down (with Zeiss D obj.) the appearance seen in 1 is obtained.
The darkest part of the fibre is now in the centre of what was the
bright band, that is, in the position of Krause’s membrane. Bordering
on this dark band, and separating it from the dim band, is a bright
zone. The dim band remains much the same as before, though by
contrast with the now dark Krause’s membrane it may appear lighter.

The bright haloes round the nodal dots of the network may be
compared with the similar effects observed whenever any highly
refractile particle, such as a micrococcus or minute oil globule, is
observed in a medium of lower refracting index.

In the oil globule suspended in water similar and very definite
transposition effects are seen on altering the focus. If focussed low
it appears as a dark spot surrounded by a bright halo or border
(u, diag. 7). On raising the objective (about 0025 mm., Zeiss D)
the oil globule appears bright, surrounded by a dark border.

The effect produced when a row of oil globules are seen side by
side is, at the lower focus (x), a bright band (formed by the coalesced
haloes), with a series of dark dots traversing it; at the upper focus
(u) a narrower bright band, bordered by dark edges. The beads at
the nodes of the transverse network may be looked upon as refracting
and reflecting the light, in the same way as an oil globule in water,
and as causing the so-called “transposition” of the bands seen on
altering the focus.

Identity of Network with Schdfer’s Muscle Rods.
We cannot but be struck by the resemblance of the appearances
brought out by gold staining with those described by Schiifer! in the
living fibre as muscle rods. The two views differ, however, on two

1H. A. Schafer, “On the Minute Structure of the Leg Muscles of the Water-beetle.”
* Phil. Trans.,’ xii, 1873.

eee

230 B. MELLAND.

points : (1) Schafer describes in a transverse section of the fibre a
bright ground substance with a number of minute specks or dots; no
appearance of a network. (2) He considers that there is typically a
double transverse row of dots in the middle of each bright stripe.

Concerning the appearance on transverse section we must not forget
that Schifer’s conclusions were drawn from the living fibre in optical
transverse section. Probably he saw all that it is possible to see of
the transverse network in the living fibre, namely, the thickenings or
dots at the nodal points, the fine network, seen so plainly in a
transverse view when stained with gold, not being visible in the fresh
fibre examined in this way.

Is there a single or a double row of dots in the middle of the bright
stripe? In the fresh fibre sometimes a single, sometimes a double,
row of dots is seen, the two appearances often alternating with a
higher or a lower focus. The same variation is seen in alcohol and
some other preparations.

In the gold preparations, when the fine granular disc or transverse
network is seen perfectly edgeways and in focus, it appears invariably
made up of a single transverse line of dots.

When the transverse network is not seen perfectly edgeways,
through not lying in a plane quite at right angles to the longitudinal
axis, but slightly obliquely or in perspective, it may appear as a
double row of dots or as a granular or dotted band crossing the disc
transversely,

In a perspective view of the fibre (figs. 3, 6, and 17), not only the
dots (nodal points of the network) at the near side of the fibre are
seen, but at the same time those deeper down or at the far side.
Hence the appearance of two or more rows of dots crossing the fibre.
When, by raising the focus, the nearer edge of one of these obliquely-
arranged discs is alone focussed it is seen to consist of a single row of
dots.

It was noticed a few moments ago, when speaking of transposition
of the bands, that at the upper focus (diag. 6, uv) the coalesced bright
dots form a bright band bordered at each side by a dark crenated
line. Each dark line is not unlike a row of dots. Schifer’ seems to
have figured muscle at this upper focus, and hence describes two lines
of dots traversing the light disc where it borders on the dim disc.

1 *Quain’s Anatomy,’ vol. ii, 9th edition, fig. 119.

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE. 231

(b) Bee.

Insect muscle may be very conveniently obtained from the thorax
or leg of a bumble bee.

Prepared with acetic acid and gold chloride, by the method already
described, it shows a network identical with that described in Dytiscus.

In order to obtain muscle in as uncontracted a condition as possible,
gold preparations were made from the leg muscles of a bee, rendered
insensible and immovable by chloroform vapour, in which presumably
there was complete relaxation of the muscle fibre. These prepara-
tions, however, could not be distinguished from those prepared without
chloroform,

As the fibres are rendered soft by the method of preparation their
size and the size of their elements varies with the pressure of the
cover-slip ; hence measurements are of little or no value.

(c) Frog.

The fibres from the gastrocnemius of the frog treated by the
same gold method as before yield an unmistakeable network. The
fibres when examined are seen to be more changed by the process
than is the case with insect muscle. They become very much softened,
and when pressed upon by the cover-slip expand to many times their
natural diameter, and thus often altogether lose their shape. Owing
to this disturbance of the fibre the network usually shows no distinct
differentiation into horizontal or transverse, and longitudinal portions.
Hence there is no transverse striation.

In many places in the preparation isolated portions of fibre show a
network with polygonal meshes as in fig. 7. This network is also seen
at the ends of certain fibres which, curling up, show a transverse
section. The meshes are often, when the fibre is much expanded by
compression, large enough to be seen with Zeiss A. obj., at other times
much smaller, approximating in size to the meshes of the horizontal
networks in insects’ muscle. The size of the meshes seems to depend
entirely on the degree of compression of the fibre. When the meshes
are small, distinct thickenings or dots are seen at the intersections of
the fibres composing the network. This network is particularly sharply
defined and is plainly seen to be a true network, that is, the lines
represent linear fibres only. It is not a honeycomb work. The lines
do not represent the edges of plates of interfibrillar material,

ee eet eee ee eee ee ae ee nee oer

cov

232 B. MELLAND.

(d) Crustacean.

_ An exactly similar network can be brought out in the muscle
of the lobster. My friend Mr. C. F. Marshall has made prepara-
tions of lobster muscle with acetic acid and gold which show this
network in a most beautiful manner. The muscle in this case was
left in 15 per cent acetic acid for fifteen minutes (a much longer time
than I use), in gold chloride thirty minutes, and in 25 per cent formic
acid in a warm chamber for three hours exposed to the light.

This network represents the transversely and longitudinally arranged
network described in insects’ muscle pulled out of shape. In some of
the fibres indeed it is still seen arranged in the rectangular manner.
Fig. 8 represents a portion of a fibre in which transverse are crossed
by longitudinal lines with dots at the intersections. In this case the
ordinary light and dim transverse striation is obtained by refraction
round the nodal dots.

At first sight the meshes of the irregular network described in the
frog and lobster look too large to correspond in size with the meshes of
the horizontal network in Dytiscus, that is, with the end view of sarcous
elements. But we must not forget the effect of pressure ; it expands
the fibre to about ten times its normal diameter, and a corresponding
increase in the-size of the meshes takes place. Fig. 11 represents a
transverse section of the fibre of the frog cut fresh with the freezing
microtome and stained by the gold method. It has not been much
enlarged by pressure, and hence the meshes of the network are small.

Fig. 10 represents a portion of a fibre of the lobster which has split
into fibrils ; an uncommon effect in gold preparations. When muscle
splits into fibrils the fibres of the transverse network rupture midway
between the nodal points; the longitudinal threads and dots remain
often attached to the fibril of sarcous substance, and cause it to appear
transversely striated.

The muscular fibres of the crayfish show exactly the same network,
the precise method of gold staining seems to make little difference,
Isolated portions of network are seen pulled out of shape, and thus
with polyhedral meshes as in fig. 7. At other points the network is
seen still arranged in its typical manner as in fig. 8.

(ce) Rat.
In the Rat most of the fibres show the typical arrangement into
transverse and longitudinal portions (fig. 9). The transverse network

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE. 233

is most marked. In certain isolated portions the dots at each nodal
point of the network are seen surrounded by bright haloes as already
described.

Such then is the effect of gold staining on the muscular fibre.
Can this network be demonstrated in any other way? Any method
which fixes the fibre in that condition in which it is when living gives
rise to appearances closely resembling those described. Acetic and
osmic acids seem to act in this way.

Ii, Acetic AciD PREPARATIONS.

Muscular fibres from the leg of the bee were placed in dilute acetic
1 per cent for from five to fifteen seconds, then into glycerine, and
mounted,

On examination they are seen to present a transverse row of dots
at each membrane of Krause and longitudinal connecting rods. The
network, like the sarcolemma, seems to resist the action of acetic acid
more than the matrix or sarcous substance. If the fibre be stained in
heematoxylin after the action of the acetic, the network becomes stained
to a greater extent than the matrix, which remains relatively unstained.

The fibre now presents the appearance seen in fig, 15. Thin
granular deeply-stained discs are seen crossing the fibre in the position
of each Krause’s membrane. They are attached to the sarcolemma
at the edges, and appear to divide the fibre into compartments. If
the near edge of one of these discs be focussed it appears as a
transverse row of dots crossing the fibre, and in many fibres fine
longitudinal lines may be seen joining the dots of two adjacent discs.

In some fibres the appearance of a double row of dots crossing the
fibre in the position of the transverse network is seen. ‘This is repre-
sented in fig. 16. It is noticed in the preparations made with acetic
acid, that the double rows of dots are met with, as a rule, in those
fibres which have undergone least pressure. In fibres expanded by
pressure a single row of transverse dots is alone observed.

Fig. 17 represents a fibre treated with acetic acid and afterwards
stained in watery solution of logwood. At the upper part of the fibre
the thin dotted transverse discs are not seen edgeways but partially
from below. Lower down in the fibre the discs are seen more nearly
edgeways, and appear in perspective view as narrow granular bands.
These granular bands appear crossed longitudinally, and more or less
broken up into short parallel longitudinal segments, by fine bright

eer

234 B. MELLAND.

lines. These bright lines are caused by refraction from the longitudinal
rods of the network.

III. Osmic Actip PREPARATIONS.

Preparations made by placing living muscles from the bee in osmic
acid 1 per cent for ten minutes, and mounting in balsam, give on
examination the appearances figured in fig. 18 and diag. 6. Thickenings
(Engelmann’s “fixed waves of contraction”) are seen on many of the fibres.

In diag. 6, u the fibre is seen crossed at intervals by a dark well-
marked line, Krause’s ‘‘membrane,” or the horizontal network. On
focussing upwards this line appears as a thin bright disc, and the
appearance U is obtained.

In certain fibres (fig. 18), by careful examination, it can be seen
that this dark line consists of a row of dots, and occasionally fine
longitudinal lines may be seen joining them.

A fined wave of contraction is shown in this figure. The contracted
part of the fibre is widened out transversely and the distance between
the transverse networks diminished. The series of haloes round the
rows of dots extends to the whole of the now diminished interval
between the successive rows. There is consequently a bright band in
the position usually occupied by the dim band. Traversing this bright
band longitudinally are seen fine lines joining the dots of adjacent
networks. Between this fully contracted and the relaxed part of the
fibre is the portion showing the “homogeneous stage” of Engelmann.
The transverse marking is here to a large extent lost, and this can be
easily understood, when we consider that at the onset of contraction
the transverse network would be probably more or less pulled out of
shape. The individual dots would no longer lie in the same transverse
plane, and hence the haloes would not blend into a continuous bright
transverse disc. This agrees with the fact mentioned by Schiifer,!
that mechanical shifting of the elements of a fibre causes a dis-
appearance of the transverse striations.

Another point often observed in osmic acid preparations is a caving
in of the sarcolemma between each transverse network, that is opposite
the dim stripe. In other preparations the sarcolemma usually bulges
at these points, and appears to be contracted at its attachment to the
transverse network or Krause’s membrane. This may be explained
if it be supposed that in osmic acid preparations there is a certain

1 Quain’s Anatomy,’ vol. ii, 9th edition, p. 129.

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE. 235

amount of contraction of the matrix or sarcous substance, by exosmosis
for instance. The sarcolemma will follow this decrease in bulk but
will be prevented from doing so at those points where it is held out-
wards by the more rigid transverse networks.

IV. Tue Livine FIsre.

The fibres from the leg of Dytiscus, or the bee, mounted without
the addition of any fluid, and examined whilst fresh or living, give
the appearances seen in figs. 19 and 20. Most of the fibres are seen
to present the appearance of alternate dim and bright batids, the dim
bands being the thicker. Each dim band is traversed by a series of
longitudinal lines of a highly refractile substance. Running across the
middle of the bright band transversely is seen a single row of dots. The
fine dark lines crossing the dim stripe are traceable at either end into
the dots of the bright stripe. In this case, just as in the acetic acid
preparations, there often appears to be a double row of dots in the
centre of the bright stripe. Fibres are seen side by side, one with a
single row, another with a double row of dots in this position. When
a double row is present, the corresponding dots of the two rows appear
to be always joined longitudinally by fine lines across the middle of
the bright stripe. This is mentioned by Haycraft' but not by Schafer.

Sometimes again the appearance shown in fig. 20 is observed. A
series of short parallel longitudinal lines is seen in the position of the
transverse network. These lines appear dotted on careful examination.
This appearance is similar to that described in the acetic acid prepara-
tion (fig. 17), and may be explained in the same way as a perspective
view of the network crossed by longitudinal bright lines, caused by
refraction from the longitudinal rods. “Transposition” of the bands
may be seen on altering the focus, similar to that already described.
The line of dark dots, with its series of bright haloes forming the
bright disc, becomes now a line of bright dots bordered by two crenated
dark lines. The above observations on the living fibre were made by
means of the gas chamber. The chitinous integument of the leg of
the bee was slit longitudinally, the muscle scooped out, and quickly
teased on a cover-glass and inverted over the moist gas chamber.
This method may be used for studying the phenomena of contraction,
by blowing air charged with alcohol vapour into the chamber, and
thus causing the fibre to contract by chemical stimulus.

2 *Quart. Journ. Micr, Sc.,’ April, 1881, p. 23.

236 B. MELLAND.

On contraction the fibre becomes shorter and thicker, the transverse
rows of dots approach one another and appear darker, probably by
contrast with the now bright “dim” disc. These appearances are
similar to those seen in the “fixed waves of contraction,” described
in the osmic acid preparations.

In a preparation of fresh muscle I have seen a fibre undergo slow
rigor mortis, commencing at one end and gradually extending towards
the other. It exactly resembled a very slow contraction wave passing
over the fibre, and the changes undergone by successive discs, as
the contraction affected them, were similar in appearance to those
described in fig. 18, and could be observed with more deliberation
than usual.

The Fibre under Polarised Light.—The effects observed in the
living fibre with crossed Nicols were exactly similar to those figured
and described by Briicke and Schafer (‘ Quain’s Anat.,’ 9th ed., vol. ii,
fig. 125). Briicke’s drawing is almost identical with diagram 3.

The fibre is chiefly made up of doubly refractile or anisotropous
material, but a band of singly refractile or isotropous material crosses
the fibre transversely in the position of each Krause’s membrane, and
this band is geen with a high power to consist of a row of rhomboidal
dots. Fine lines of isotropous material are described running longi-
tudinally across the anisotropous discs and joining the rhomboidal
dots. The appearance of the muscle fibre under polarised light leads
us to the belief that the network consists of isotropous, the matrix or
ground substance of anisotropous or doubly refracting, material.

VY. AtcoHot PREPARATIONS.

Alcohol preparations of muscle show, in most cases, a somewhat
different character to those prepared by the preceding methods.

Spirit has a tendency to split the fibre into fibrils and sarcous
elements. After the muscle has been in alcohol it may be stained
with some reagent ; Kleinenberg’s hematoxylin, for instance, gives
excellent results. Alum carmine may also be used. Mount in Canada
balsam.

Absolute alcohol has a somewhat different effect from ordinary spirit.
It sometimes seems to fix the fibre as it appears during life—that is,
there is no differentiation into sarcous elements, but transverse rows of
dots, and longitudinal lines are alone seen, as in the living fibre. Fixed
waves of contraction may also be found,

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE. 237

Fig. 21 represents a portion of a fibre of Dytiscus stained in heema-
toxylin after the action of spirit. It shows an alternation of bright
and dim discs, the dim discs stained a deep purple and made up of a
series of sarcous elements side by side. Across the middle of the
bright discs a granular or transverse line is seen. Fine longitudinal
lines, the longitudinal bars of the network, may occasionally be seen
crossing the bright discs.

This account agrees for the most part with that given by Klein! as
to the structure of muscle. He, however, figures a continuous line—
the homogeneous Krause’s membrane—in the middle of the bright
stripe, and no longitudinal fibrillation in the bright disc.

Let us consider the influence of the intracellular network in pro-
ducing the appearances known as sarcous elements, and Cohnheim’s
areas, in the muscle fibre.

The matrix, or substance which lies in the interstices of the network,
is of far greater bulk than the network. It is homogeneous through-
out ; nevertheless, it may be looked upon as being partially divided
into columns or fibrils by the longitudinal bars of the network, and
partially into dises—the contents of muscle compartments—by the
transverse networks. By the action of spirit the matrix becomes split
into fibrils. The reagent causes this ‘sarcous substance” to shrink
(possibly by abstraction of water), and the homogeneous mass now
separates into fibrils along the lines of greatest weakness—that is,
along the guide lines formed by the longitudinal bars of the network.
These fibrils may again divide transversely at the horizontal networks,
producing sarcous elements (diag. 8). Thus the appearance of sarcous
elements is seen, as described by Klein,? to be a post-mortem phenomenon.
In consequence of shrinking the sarcous substance no longer entirely
fills up the skeleton “muscle caskets,” and the division into sarcous
elements, which was foreshadowed only before by the bars of the
network, becomes evident by the development of intervening spaces
between adjacent elements. The appearance known as Cohnheim’s
areas is somewhat differently described by different observers. For
the present we may follow Klein’s® description. The prismatic sarcous
elements which lie side by side in the living fibre with no intermediate
substance, shrink through coagulation on dying, and become separated
from one another by a transparent interstitial fluid substance. In a

1¢ Atlas of Histology,’ p. 77.
2 Loc. cit., p. 76s
3 Loe. cit.

238 B. MELLAND.

transverse view there are thus seen small polygonal areas separated
by clear lines, each polygonal area corresponds to a sarcous element.

Cohnheim’s areas may be described as the appearances produced by
coagulation and splitting of the matrix along the guide lines formed
by the transverse network ; they represent an end view of sarcous
elements, and are post-mortem phenomena (diag. 9).

Previous VIEWS.

I think it unnecessary to give a historical account of the different
views which have been published with regard to the structure of the
striped muscle fibre.

An epitome of the historical results may be found in Schifer’s' paper
on the leg muscles of the water-beetle; or by the same author in
‘Quain’s Anatomy,’ 9th ed.

Reference has already been made to most of the appearances des-
cribed by different observers, and the way in which these appearances
may be explained as caused by the presence of a highly refracting
network.

The relation of this network to Krause’s? views may be noticed.
Krause’s “muskel-kastchen” are bounded above and below by Krause’s
membrane, and laterally by the boundaries of Cohnheim’s areas.
Briicke® regards the isotropous lines which traverse the anisotropous
disc as optical sections of the partitions between ‘‘ muskel-kistchen.”
These partitions correspond with the longitudinal bars of the network
and with Schiifer’s rods. The alternation of bright and dim transverse
bands has been looked upon by several observers as an optical effect,
and not due to any anatomical differentiation here present.

Heppner‘ and Stricker look upon the bright band as the expression
of total reflexion, which occurs at the line of demarcation between
Krause’s membrane and the chief substance of the fibre.

Bowman suggested that the transverse striping shown by the fibrille
was caused by the moniliform shape. Haycraft’ has recently developed
this view, and extended it to the whole tibre.

Striped muscular fibres are met with in the animal kingdom,
from the Coelenterata upwards; there is no reason to suppose that the
+ Loc. cit.

2 “ Ueber den Bau der quergestreiften Muskelfaser,” ‘Zeitschr. f. rat. Med.,’ xxiii.
3 © Quain’s Anatomy,’ p. 127; and “ Muskelf. im polarisirten Licht,” ‘Wiener Denkschr.,’ xv.

4 *Stricker’s Handbook’ (Syd. Socy.), p. 548, vol. iii.
5 *Quart. Journ, Micr. Sc.,’ April 1, 1881,

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE. 239

cause of the transverse striation is different here from that in the
insect.

I have received the greatest sympathy during this investigation
from my friend Mr. C. F. Marshall, with whom I have verified most
of my results. The drawings of the network in the fibres of the Rat
and Lobster are from gold preparations by him. Mr. Marshall is at
present working on the histology of the muscle fibre, from the lowest
types of the animal kingdom in which it occurs upwards, and has
already obtained interesting results. A study of the comparative
development or phylogeny of this network, and at the same time of
its embryology, may lead to its undoubted recognition as an ordinary
intracellular network.

My thanks are also due to Professor Milnes Marshall, who has kindly
examined my drawings and specimens, and suggested alterations in the
paper, and to Mr, J. Priestley.

BriEF SUMMARY OF RESULTS.

The chief results at which I have arrived may be summarised as
follows :

There is an intracellular network present in the striped muscle fibre
of Dytiscus, Bee, Lobster, Crayfish, Frog, and Rat, which may be
most clearly demonstrated by certain methods of gold staining. The
network alone is stained by the reduced gold, and, owing to this
differentiation, is plainly visible even with comparatively low powers.
This network may be demonstrated, though not so completely, in the
living fibre, and in acetic and osmic acid preparations.

Crossing the fibre transversely, united to the sarcolemma, and more
or less separating the muscle fibre into compartments, are network
partitions—the transverse network.

Running longitudinally down each compartment, and joining the
dots at the intersections of the fibres of the transverse network, are a
series of fine rods. The arrangement of this network will be made
evident by reference to diagrams 1, 2, 3, and 4.

This network consists of an isotropous material, and is more highly
refractile than the rest of the muscle substance, which is anisotropous.
This network serves to explain the transverse striation and other
complicated appearances presented by the muscle fibre, and brings

into harmony many of the conflicting statements of histologists on
this subject.

240 B. MELLAND.

DESCRIPTION OF PLATE X.

Diags. 1, 2, 3, and 4.—Diagrammatic views of the network in striated
muscle.

Diag. 1.—Perspective view of the fibre, showing the transverse
network, a, at each membrane of Krause, and the longitu-
dinal lines.

Diag. 2.—Perspective view of a portion of the network,
showing:—a. The transverse networks, with polygonal
meshes and dots at the nodes. 6. The longitudinal bars
of the network ending in the dots.

Diag. 3.—The fibre geen in longitudinal view. The transverse
network, a, appears as a row of dots crossing the fibre (in
the position of Krause’s membrane). c. Minute thicken-
ings on the longitudinal bars of the network, midway
between the transverse networks.

Diag. 4.—The fibre seen in transverse section.

Diag. 5.—Network as seen in a longitudinal view of the fibre, showing’
the production of alternating bright and dim bands by
refraction around the nodal dots.

Diag. 6.—So-called transposition of the bands, as seen in an osmic
acid preparation of muscle of Bee. v. Appearance at
upper focus. wt. Appearance at lower focus.

Diag. 7.—Oil globules in water, showing their refractive effect upon
light. vu. At the upper focus, each globule surrounded
by a dark border. 1. At the lower focus, each globule
surrounded by a bright halo.

Diag. 8.—Production of sarcous elements by contraction of the matrix
and splitting along the guide lines formed by the bars of
the network (seen in spirit preparations).

Diag. 9.—Formation of Cohnheim’s areas by contraction of the matrix
as above. In this transverse view of the fibre the pris-
matic sarcous elements are seen on end, and appear as
polygonal areas separated by bright lines.

Fig. 1.—Fibre of Dytiscus, prepared by the gold method. Zeiss,
D obj., No. 5 oc.

THE HISTOLOGY OF THE STRIPED MUSCLE FIBRE, 241

Fig. 2.—Dytiscus, gold method, portion of a fibre more compressed
than in Fig. 1.

Fig. 3.—Fibre of Bee, prepared by the gold method; transverse
networks in perspective.

Figs. 4 and 5.—-Dytiscus, gold method, showing isolated discs con
sisting of a network.

Fig. 6.—Fibre of Dytiscus, gold method, splitting into discs.

Fig. 7.—Lobster fibre, gold chloride ; isolated portion of a fibre, net-
work pulled out of shape. Exactly similar networks are
seen in the Frog and Crayfish.

Fig. 8.—Frog, gold method; network arranged typically, and showing
transverse striping. |

Fig. 9.—Rat, gold chloride; longitudinal view of a portion of a fibre.
(Preparation by C. F. Marshall.)

Fig, 10.—Lobster, gold chloride, splitting into fibrils.

Fig. 11.-—Frog. Transverse section of the frozen fibre, stained by
the gold method.

Fig. 12.—Dytiscus, gold method ; isolated portion of the network.

Fig. 13.—The same, more highly magnified. (Zeiss, F obj., No. 5
eyepiece.)

Fig. 14.—The same, showing refracting effect of the network. L.
Lower focus. v. Upper focus.

Figs. 15 and 16.—Fibres of Bee, treated with acetic acid, then
Kleinenberg’s heematoxylin.

Fig. 17.—Fibre of Bee, treated with acetic acid, then watery solution
of logwood. ‘The transverse networks seen more or less
obliquely.

Fig. 18.—Fibre of Bee, prepared with osmic acid, shows a fixed wave
of contraction.

Fig, 19.—Living fibre of Bee, showing longitudinal view of network,
(ts immersion obj).

Fig. 20.—Living fibre of Bee, transverse networks seen somewhat
obliquely.

Fig. 21.—Portion of a fibre of Dytiscus, stained in hematoxylin after
the action of spirit. Shows sarcous elements.

Where not otherwise stated, the drawings were made from Zeiss,

D. obj., No. 5 oc.

Diag. 5

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THE CAMBRIDGE SCIENTIFIC INSTRUMENT COMPANY.

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THE ANATOMY OF THE MADREPORARIA. I.

By G. Herpert Fow er, B.A., Keble Coll., Oxon; Berkeley Fellow of
the Owens College.

[Puates XI, XII, & XIII.)

By the kindness of Professor H. N. Moseley I have been enabled to
study the anatomy of certain Madreporaria obtained by him during
the voyage of H.M.S. “Chailenger.” As in lecture-courses and text-
books but little information is given relative to this ancient and
interesting group, and the few papers on the subject are scattered, a
short sketch of the more recent researches is prefixed to my own
results. Two forms only are described in this paper, P/labellum pata-
gonicum and Rhodopsammia parallela: others, it is hoped, will follow
shortly. The text will be reduced as far as possible throughout.

An acquaintance with the anatomy and development of an ordinary
Actinia is presupposed in the reader, this being the type by which
comparisons are made; but a list of the more technical names used to
describe the anatomical parts of the polyp is given, with the synonyms
for the use of those desirous of consulting the literature of the subject.

Mouth-dise = Mundscheibe, Peristome.

Body wall = Leibeswand.

Stomatodeum (cesophagus) = Schlund-, oder Magen-rohr.

Ceelenteron = Darmhdéhle, Leibeshohle, Estomac.

Mesenteries (sarcosepta) = Scheidewiinde, Parietes, Replis mesenteroides.
Mesenterial filament (craspedon) = Mesenterialfaden, Cordon pelotonné,
Septa (sclerosepta) = Sternleisten, Cloisons, Lames.

Theca = Mauerblatt, Muraille.

944 G. HERBERT FOWLER.

A “pair” of mesenteries is constituted by two mesenteries whose
longitudinal muscle fibres are ranged on their adjacent faces (except
in the case of the two “directive pairs,” each of which is placed at
one end of the longer axis of the mouth oval, and in which the arrange-
ment of the muscles is reversed). For the chambers (Radial-taschen,
Loges) into which the coelenteron is periaxially divided by the mesen-
teries, I am compelled to coin new names. ‘To those chambers which
lie between a “pair” of mesenteries the term entocele ig applied
(fig. 1, B); to those chambers of which one lies between every two pairs
of mesenteries the term exocele (fig. 1, a). The septa lying in these
two classes of chambers are similarly called exosepta and entosepta.

The classification adopted will be found at the end of the paper,
together with the bibliography.

RECENT RESEARCHES INTO THE MorPHOLOGY oF THE Group.

In 1873 Lacaze Duthiers (i), studying the development of Astrozdes
calycularts on the coast of Algiers, found that it agreed in every
important point with the development of Actinia, his observations
on which (2) were corroborated and corrected by the Brothers Hert-
wig (3). With regard to the developing skeleton, he recorded two facts
of importance—firstly, as appears in his pl. xiv, fig. 27, that it
was formed outside the polyp; and secondly, that the theca arose
independently of the septa. Owing to various practical difficulties his
investigation was incomplete.

The chief worker in this field has been Georg von Koch, who, in the
course of several investigations, has arrived at the conclusion that the
theca is a secondary structure, derived from fusion of the peripheral
ends of the septa. The evidence adduced in support of this theory
appears to me to be at present insufficient for complete proof, though
from our slight knowledge of the group it is injudicious to absolutely
deny its truth.

Von Koch first published this theory in 1879, founding it on the
following observations on Caryophyllia (4). There is no living tissue
on the greater part of the exterior of the corallum; but at the apex
the peripheral edge of the mouth-disc overlaps the lip of the calyx in
such a way that in the highest sections the septa appeared to stand
free in the ccelenteron, in «sections a little lower to have fused
peripherally into a theca. The cost are, according to him, and

THE ANATOMY OF THE MADREPORARIA, 2945

as will be seen by the figures, the outermost ends of the septa
(EEA, 8B):

Further, the mesenteries and chambers between them appeared to
be continued into this external part of the polyp. These appearances
he explained by supposing that, as the peripheral ends of the septa
approximated and fused, they surrounded the mesenteries, dividing
them ultimately into a central and a peripheral part. Asa further
proof he adduced the observation that in microscopic sections of the
corallum sutures were visible in the theca at the points where he
supposed the septa to have fused.

T venture to think with Moseley (5) that this explanation is erroneous;
that the appearances in the first section (Pl. XI, 4) are due merely
to the fact that in this, as in many corals, the secretion of calcium
carbonate is most active about the septa, which consequently rise
slightly above the level of the theca, as may be seen in any figure of
Caryophyllia ; and further that, in the second section (PI. XI, B), the
apparent continuation of the mesenteries and chambers between them
over the lip of the calyx is not due to their having been cut into two
portions by fusion of the septa, but to more or less abnormal contrac-
tion due to the use of alcohol ; in life the polyp, when fully expanded,
undoubtedly stretches over the lip, but in these forms, so far as I can
ascertain, in natural contraction it is completely within the calyx.
Further, as appears from his own researches and those of others on
different forms, the whole skeleton, instead of being, as he describes,
free in the ccelenteron, is shut off from it by a layer of endoderm and
mesoderm, and as much outside it as the rest of the corallum ; these
layers he himself figures as clothing another part of the septum,
though of this portion no histological details are given. Von Heider,
in a paper shortly to be referred to, states that Von Koch has over-
looked the fact that the whole of the corallum is covered externally by
ectoderm and mesoderm ; certainly this form requires more complete
investigation, Again, having ground many microscopic sections of
corals, I can afford no credence to “sutures ;” in the process cracks
fly through the coral in all directions. But if evidence of a directly
contrary character is needed, the case of Flabellum may be adduced,
in which (5), according to Moseley, sutures run not between the fused
ends of the septa—ie. through the theca—but down the centre of
each septum,

246 G. HERBERT FOWLER.

Though Von Koch gives no detailed description of the anatomy of
Caryophyllia, the following account may be inferred from his figures
and text (4) (6). The polyp is built on the Actinian type, consisting
of mouth-disc, stomatodzeum, mesenteries ; the muscles of the latter
being arranged as in Actinia. No external body wall, its place being
taken by the theca; inner body wall of mesoderm and endoderm,
lining the coelenteron, and clothing the interior of the calyx, both
theca and septa. Mouth-dise drawn down in abnormal (?) alcoholic
contraction over the lip of the calyx. Entosepta and exosepta both
present. No mention made of tentacles.

Of Madrepora variabilis he records, in 1880, the following facts (6):
Structure Actinian ; in the end-polyps of the colony six pairs of
mesenteries, six entosepta, and six exosepta; in the side-polyps also
Six pairs of mesenteries, but six entosepta only.

Von Koch has also studied Stylophora digitata in somewhat greater
detail (7). The form of the colony resembles that of Aleyonium
digitatum ; the polyps live in small calyces on the surface of the
colony, but the living tissues are not continued down into its centre,
as in Alecyonium ; the lower part of the cavity formerly inhabited by
the polyp being shut off by a kind of tabula as it grows upwards.
Over the surface of the colony lies the ccenosare, the fleshy rind of the
otherwise calcareous colony, which puts the polyps in communication
with one another, being permeated by canals which are continuous
with their ccelentera, and similarly lined by endoderm. The polyp
possesses six pairs of mesenteries, six larger tentacles, six smaller
tentacles, and six entosepta. There are two distinct types of nema-
tocyst. Longitudinal muscles occur on the mesenteries, but the small-
ness of the latter rendered it impossible to detect whether their arrange-
ment agreed with Actinia or not.

In 1881 Dr. von Heider, of Graz, published a description of Clado-
cora astrearia and Cl. cespitosa (8). These species are also built on
the Actinian type; and Heider describes for them the same con-
tInuation of the mesenteries and mesenterial spaces that v. Koch
mentions as occurring in Caryophyllia cyathus. I have examined
macroscopically and by sections Cl. cespitosa in a completely retracted
state, and can find no trace of such a condition, an observation which
confirms my belief that this appearance is due to partial contraction,
owing to the use of alcohol. There is no true coenosare such as occurs
in Stylophora, just as there is no coenenchyme, the calyces being free

THE ANATOMY OF THE MADREPORARIA. 247

outwardly from the rest of the colony. In luxuriant growth and
budding, however, according to Heider, both skeletons and soft tissues
of adjacent polyps may fuse ; an observation interesting as probably
indicating the history of the formation of the ccenosare and ccenen-
chyme which characterise many other forms, There is one correction
to be made in his work which, for the sake of future workers in this
field, ought to be mentioned here, namely, that in hig Pl. III he
frequently figures as endodermal cells small spherical bodies with a
well-staining nucleus, which are zooxanthellee or symbiotic unicellular
Algze, living free in the ccelenteron in such numbers as often to
completely obscure the true endoderm, with which they, of course,
have no connection. While accepting v. Koch’s theory as to the
origin of the theca from fusion of the septa, he differs from it in some
details, regarding the “sutures” as merely cracks artificially produced
in the corallum. Septa and tentacles both entoccelic and exoccelic ;
mesenteries and their muscles arranged as in Actinia. For further
details, which are -very thoroughly worked out, his paper should be
consulted. One point of importance deserves mention: between the
corallum and the structureless mesoderm-lamella which overlies it
immediately, and was generally understood to secrete it, v. Heider
detected certain cells, for the most part scattered, but in some places
forming a definite layer. To these he gave the name calycoblasts, and
assigned the function of coral-secretion, with great justice, as later
researches proved, though their origin was a matter of doubt till
cleared up by v. Koch.

The latter, in a paper on the development of Astrovdes calycularis,
brought into notice the following facts (9). When first fixed, and before
the secretion of the skeleton has commenced, the embryo is plano-
convex, and its ectoderm may be divided into two regions, correspond-
ing to its surfaces, the plane disc of attachment, or basal ectoderm,
and the convex portion, or lateral ectoderm, the centre of which is
invaginated as the stomatodeum. ‘The skeleton first appears as small
pellets of calcium carbonate lying between the basal ectoderm and the
foreign body to which the embryo is attached, and is therefore outside
the animal, and consequently the result of secretion by the ectoderm. As
the corallum is always described in text books as a product of the
mesoderm, this observation cannot be too strongly insisted upon.
These pellets become first a ring-shaped disc, then a complete disc
lying between the basal ectoderm and the foreign body to which the

248 G. HERBERT FOWLER.

embryo is attached. Where septa are to be formed the three body
layers, endoderm, mesoderm-lamella, and basal ectoderm, rise upwards
asa fold into the coelenteron; and as they rise, coral is deposited
beneath them which fuses with the original disc; the septa are thus
also deposited outside the basal ectoderm. Then they begin to
bifurcate at their distal ends. The originally basal ectoderm to
which the secretion of the skeleton is attributable persists in the adult
as the calycoblasts of v. Heider.

Von Koch further asserts that the theca results from the fusion of
the bifurcating ends of the septa; but, though not venturing to deny
this, I would point out that he neither describes the process nor gives
figures to illustrate it ; whereas, on the other hand, we have the direct
evidence of L, Duthiers to the effect that the theca and septa arise
independently of each other (‘‘les septa et la muraille ne sont pas
unis”), and a figure, which appears to bear out his statement. It must,
however, be borne in mind that Lacaze Duthiers may have described
as theca what v. Koch terms epitheca, a secretion of the lower portion
of the lateral endoderm of the embryo which fuses with the periphery
of the original basal disc, and ultimately combines also with what he
terms the true theca, formed as above mentioned, to become the outer
wall of the corallum. Were this the case, however, the coste could
not be, as he regards them, the peripheral ends of the septa. But the
question can only be finally settled by a study of the embryonic
development of widely different forms.

Professor Moseley has published a preliminary note on Seriato-
pora and Pocillopora (10). These forms were originally classed with the
Tabulata, but his account of their anatomy brings them into close
connection with the other Madreporaria at present described. The
polyps of Seriatopora are oval in outline, with twelve short tentacles,
which in complete retraction are covered over by the indrawn margins
of the disc, a condition common in Actiniaria, but very rare in
Madreporaria. There are twelve mesenteries, only two of which,
the same two in every polyp, are enormously long and bear
mesenterial filaments and generative organs. The elongation of
this pair of mesenteries deep into the colony suggests an inevitable
comparison with the Alcyonaria; and the similarity is strengthened

_ by the marked orientation of the polyp, for a division into “dorsal”

and “ventral” halves is clearly distinguishable in both soft
tissues and corallum. Two of the septa are very rudimentary, and

THE ANATOMY OF THE MADREPORARIA, 249

both this fact and the absence of mesenterial filaments on ten of the
mesenteries would seem to indicate a degeneration, of which I hope to
bring forward a second instance in a future paper. Between the
polyps runs a similar canal system to that already described by v. Koch
in Stylophora. The anatomy of Pocillopora, so far as mentioned,
appears to agree in all respects with that of Seriatopora, and the polyps
exhibit the same marked orientation.

Moseley has also described the microscopic anatomy of three
other Madreporarian polyps (II). His observations on Flabellum are
mostly incorporated with my own below, and need not, therefore, be
recapitulated here ; and of Stephanophyllia I hope to give a detailed
description in a future paper.

Of Bathyactis, which is planoconvex in shape, the plane being the
basal surface, he records that on decalcification a lamina of ectoderm
and mesoderm separates off from the base. This fact, together with
its shape, suggests that the original basal ectoderm of the embryo
persists in this species throughout life, in its primitive position, except
for such part as grows up with the skeleton (the calycoblasts).

To sum up the undoubted facts elucidated by these observers :—

1. The adult Madreporarian polyp is built distinctly on the Actincan
type, except for the absence of an external body-wall in some cases
(Caryophyllia, Cladocora), which is then replaced physiologically by
the imperforate theca.

2. The corallum is a product of the ectoderm, and deposited outside
the embryo.

This ectoderm persists in the adult as the layer of calycoblasts, to
which the continual growth of the corallum is attributable ; thus the
skeleton is morphologically external to the polyp throughout life.

4, Between this layer and the cavity of the ccelenteron, and clothing
every part of the skeleton, is a layer of mesoderm and endoderm,
forming the internal body-wall.

5. Septa, when present, always lie between a pair of mesenteries
(entosepta), sometimes also in the spaces intermediate between pairs of
mesenteries (exosepta). | |

6. Tentacles may be exoccelic as well as entoccelic, but exosepta may
be present without corresponding tentacles.

The present classification of the Madreporaria is admittedly
unscientific. I have therefore laid stress on what may perhaps seem
the trivial point of the relations of septa and tentacles to the mesen-

250 G. HERBERT FOWLER.

terial spaces, as it is probable that, since the morphological differences
of the whole group of Zoantharia hexacoralla are very slight, such
structural variations might be useful for a new classification, which, if
based upon the relations of polyp to skeleton, will be on a far sounder
foundation than the present one, which rests upon the skeleton alone.

FLABELLUM PAaTAGONICcUM (Moseley).

This is an imperforate Madreporarian, belonging to the family Turbi-
nolide. As Moseley (II) has given a full description of the specific
characters of the corallum in his ‘‘ Challenger” Report (to which
reference should be made for figures of the complete calyx), only a few
of them will be mentioned here.

i. The corallum is solitary and conical, the apex of the cone forming
a pedicle by which the polyp is attached when young; in the adult
the pedicle becomes obliterated and the coral free (vide figs. 2, 3, Pe.).
The outline of the mouth of the calyx is oval (fig. 1). There are four
orders of septa, all of which are entoccelic ; six of the first order, which
meet in an elementary form of columella; six of the second, which are
nearly as long as the primary septa; twelve of the third, and twenty-
four of the fourth order. In some specimens the full number is not
developed. The corallum is about 2 cm. high in a well-grown specimen ;
and the longer axis of the calyx mouth about 24 cm., the shorter axis
2 cm. in length.

Along the lines which correspond on the exterior surface of the
theca with the attachments of the septa on the interior, are shallow
but distinct grooves running from lip of calyx to tip of pedicle, each
corresponding exactly in position with a septum. ‘These do not agree
with v. Koch’s views as to the origin of the theca from fusion of the
septa, to accord with which costee should be developed in this position,
such as occur in many forms. .

The whole of the exterior surface of the theca shows well-marked
lines of growth (fig. 4), so arranged as to appear to indicate that the
chief centres of activity for the secretion of coral ‘lie in the septa.
Hence the lip of the calyx is slightly dentate (figs. 3, 4).

While the upper fourth of the external surface of the theca is, like
the whole of the interior of the calyx, glistening, white, and hard, the
lower three-fourths are soft in texture and brownish. This latter
portion was described by Moseley as a “light-browu epitheca.” But
on decalcification the brown substance falls off as soft flakes, which, by

THE ANATOMY OF THE MADREPORARIA. 951

means of sections are found to consist of dead tissues and algal (1)
parasites. There is really no epitheca present, recognisable as such,
in the adult.

The columella (fig. 3, col.) is incomplete, the septa not always
meeting regularly along their free edges.

In the retracted condition of the polyp there is no tissue external
to the corallum (figs. 1, 2), nothing corresponding to the condition
described by Heider in Cladocora and by v. Koch in Caryophyllia. When
expanded, however, the soft tissues almost certainly stretch outwards
and downwards over the upper fourth of the exterior of the theca,
which is thus kept white and hard, as mentioned above. Were the
polyp thus completely expanded to be plunged into a killing fluid, the
same appearances would ensue as the above-named observers have
described.

ii. Anatomy.—This agrees in all essential details with the Actinian
type, except in the absence of an external body-wall, the whole polyp
being enclosed in the corallum (figs. 1, 2). Moseley mentions that in
some specimens tissues external to the theca were observed round the
lip, and figures them (II), pl. xvi, fig. 10, as consisting of ectoderm
and mesoderm, but had not the means of studying them by sections,
None of my specimens had any trace of such, and from observations
on Desmophyllum, a closely-allied form, I imagine that these tissues
were simply due to the expansion of the polyp, and contained a con-
tinuation of the coelenteron such as was described by v. Heider in
Cladocora. On decalcification the polyp appears conical, and divided
into a series of wedges by the spaces where the septa had been. At
_ the base of the polyp—z.e. the apex of the cone—these wedges appear
to be connected together by little bridges of tissue. These latter are
of no morphological importance, being due apparently merely to the
incompleteness of the columella, and their arrangement varies in
different specimens. The polyp consists of a mouth-disc bearing
tentacles ; a stomatodzeum, which opens into the ceelenteron, the latter
being periaxially divided into exocceles and entocceles by the mesen-
teries.

The mouth-dise (fig. 2, mp) is peripherally fastened to the extreme
edge of the lip of the calyx, and is centrally invaginated into the
typical Anthozoan stomatodeeum.

On the disc are borne the tentacles, which are simple hollow
evaginations of the entocceles—z.e. one is placed over each septum.

SE a ee ee ee

252 G. HERBERT FOWLER.

They are covered by small prominences, each of which is a “ battery”
of nematocysts. I have not been able to determine whether they
possess an opening at the tip or not. They vary in size and position
according to the order to which they belong, the primary tentacles
being the largest and nearest to the mouth. (Vide Moseley (II), pl.
xvi, fig. 12.)

The mouth is oval in outline, and at each end of its long axis there
is in most cases a well-marked gonidial groove.

Through the periphery of the mouth-disc protrude the acontia. I
have by a fortunate section been able to satisfy myself that they are
ejected through definite openings, not by rupture of the disc ; these
are therefore directly comparable to the cinclides of Actiniz.

A mesentery of the first order is drawn in fig. 5 to show the general
trend of the muscles, though they are much more numerous than
there represented. They are best seen by mounting the mesentery
whole in glycerine.

In the arrangement of the longitudinal muscles on the inner
(entoceelic) faces of the mesentery, Flabellum agrees with Actinia ;
these are the retractors of the polyp. On the outer (exoccelic) faces are
ranged the protractors, oblique in direction ; these differ slightly in
the species, being confined in 1. alabastrum to the upper third of the
mesentery, while the longitudinal fibres extend for its whole length.
Both sets of fibres are continued into the tentacles, the oblique muscles
of the mesentery becoming their external longitudinal coat, the
longitudinal muscles of the mesentery passing into the internal and
approximately circular fibres of the tentacle. This apparent change
of direction will be understood by fig. 5.

The two pairs of “directive mesenteries” at the ends of the longer
axis of the mouth appear to possess the same general direction of the
muscle fibres, though bearing them on reverse faces; but the oblique
protractor muscles (in this case entoccelic) are, proportionately to the
retractors, somewhat more strongly developed, implying, perhaps, that
the expansion of the polyp is their especial function.

There are no perforations through the mesenteries, such as are
described in Actiniee, putting the chambers in communication.

Both the primary and secondary orders of mesenteries are attached
to the stomatodzeum for its whole length ; the tertiaries are attached to
the mouth-disc, but, as the latter passes imperceptibly into the stoma-
todzum, no importance is to be attached to this.

THE ANATOMY OF THE MADREPORARIA. 253

What Moseley has termed ‘the contorted mesenterial filaments,”
a mass of coils lying on the side of the mesenteries, appear to me, after
careful investigation to be, in part, at least, organs corresponding to
the acontia of Actiniz, namely, long lamellar offsets of the free edge of
the mesentery, with one edge thickened to correspond to the mesen-
terial filament, and charged with very large nematocysts. They pro-
trude in some instances, as above stated, through definite openings in
the mouth-dises. Their exact origin from and relation to the mesen-
teries I have not been able to detect, owing to the brittle condition of
the specimens, which did not allow of their being dissected out. |

The ova are developed on all three orders of mesenteries. As their
origin and position do not appear to differ from the type described
by the brothers Hertwig for Actinia, no figures are given. I have not
seen the testes, hence Flabellum may be regarded as dicecious. The
filament is present along the whole course of the free edge of the
mesentery, including that region in which ova are developed. The
latter is mostly below the part which is characterised by great con-
tortion of the free edge and by (?) the giving off of acontia.

iii. Histology.—The ectoderm of the mouwth-disc (fig. 6) is charac-
terised by deeply-staining, very numerous nuclei; and has distinctly
the appearance of a secreting layer. It probably produces a similar
secretion to the slime poured forth in quantities by an irritated
Actinia.

This figure (which is a section along the line a, fig. 2) is taken from
a well-grown polyp, and shows traces of the originally basal ectoderm
which secretes the corallum (the calycoblasts of v. Heider) (ch., fig. 6).
In a younger and actively-growing polyp these are much more
definitely marked (ch, fig. 7). The nuclei lie in a gelatinous-looking
matrix, which stains slightly with borax carmine, but in which no cell
outlines are distinguishable. In the calycoblast layer surrounding the
septum, at the same height and in the same polyp, the nuclei are
much rarer (ch., fig. 8).

The characters of the ectoderm alter considerably on the tentacles ;
as above mentioned, it is on them raised into a series of knobs, each
of which is a “ battery” of nematocysts. A transverse section through
the wall of a tentacle is shown in fig. 9, and exhibits the structure of
a battery ; the nematocysts are confined to the peripheral part, and
behind them lie a very large number of nuclei, probably instrumental,
as was first suggested by v. Heider, in the formation of the cells which

254 G. HERBERT FOWLER.

replace the ejected nematocysts. On the peripheral face of the
mesoderm-lamella lie longitudinal muscle fibres continuous with the
transverse fibres of the mesentery ; on the central face, oblique fibres.

The stomatodal ectoderm is not essentially different from that of
the mouth-disc ; and though there are well-marked gonidial grooves
(food grooves, Mundwinkelfurchen), they show no differentiation of
ectoderm comparable to that of Alcyonarians (the “siphonoglyphe” of
Hickson).

The whole of the celenteron is lined by endoderm of cubical ox
columnar cells ; generally it is only one cell deep, and in the living
animal presumably ciliated throughout, At the point where it passes
into the thickening known as the mesenterial jilament (if that be
indeed endodermal in origin) its characters change, and the number
of nuclei increases enormously, together with the length of the cells.
Its histological appearance entirely bears out what physiological
investigation has also shown for the similar filament in Actinie, that
it is secretory in character, producing a proteolytic fluid (tig. 10).

Nematocysts do not occur apparently in the true mesenterial filament,
but only on that portion of it which is continued on to the contorted
lamellee, which I regard, in part at least, as equivalent to the acontia
of Actinie. Those occurring on the tentacles are of a different size
and shape from those which characterise the acontial filament, though
in the latter both forms are found. The smaller, occurring on the
tentacles, is ‘06mm. x ‘01 mm.; the larger, which is only to be found
on the acontial filament, is‘l1mm. x ‘025mm. The thread of the latter
form is covered with minute barbs, which give it, when coiled up in
the capsule, a granular appearance.

RHODOPSAMMIA PARALLELA (Semper).

This form, belonging to the family Eupsammide, affords a very
good example of a perforate Madreporarian. Budding sparsely,
it forms no coenenchyme, so that the polyp can be studied easily and
without the complications incident to coenenchymatous species.

i. Of the Corallwm the systematic characters have been already
described by Semper (12), but certain corrections are to be made in
his account relative to the arrangements of the septa. Beautiful
figures of the colony will be found in his paper, which contains much
valuable and curious information about the group Madreporaria.

The corallum ofa polyp is about 30 cm. in height ; the calyx, which

THE ANATOMY OF THE MADREPORARIA. 255

is, as usual, oval in outline, measures about 18 mm. in the longer axis,
and 9-13 mm. in the shorter. Fresh polyps may be budded off from
the side, or, more rarely, from the calyx.

The theca has the porous appearance characteristic of the Perforata,
and is marked on the external surface by distinct spinous coste or
ridges ; each of which corresponds externally to the attachment of a
septum on the interior surface of the theca (fig. 14).

Both exosepta and entosepta occur in this form. Of true—ze.
entoccelic

septa there are only three orders, with occasional traces of
a fourth; from the sides of each primary and secondary entoseptum
erows out an exoseptum (fig. 14), and the relations of these two classes
to each other are rather complicated. Such a system as a-a in fig. 22
shows, in a transverse section taken high up in the polyp, the arrange-
ment diagrammatised in fig. 19, consisting of five true entosepta (each
of which lies between a pair of mesenteries), and four exosepta alter-
nating with them. In a lower section (fig. 20), the two exosepta
which grow out from the sides of adjacent primary and secondary
entosepta, fuse over and with the intermediate tertiary septum into
one. Lower yet (fig. 21) the two compound septa thus produced in
each system meet over and with the secondary septum, so that the
columella is due to the irregular fusion (fig. 15) of twelve primary
entosepta, distinct for their whole length, and twelve other septa thus
elaborately compounded.

ii. Anatomy.—In Rhodopsammia, which, like all the other forms as
yet described, bears a close resemblance to an Actinia, the mouth-dise,
unlike the case in Flabellum, passes into a distinct external body wall
of ectoderm, mesoderm, and endoderm (extending in some specimens
very much further down than is represented in the diagram, fig. 13).
Between this and the theca lies a narrow space in which run, parallel
to the long axis of the corallum, lamellee of tissue, connected on the
one hand with this external body-wall, on the other with the tissues
clothing the exterior surface of the theca (figs. 13,14, 17, M’). These
lamellze correspond externally to the attachments of the mesenteries
on the interior surface of the theca, and are apparently continuous
with them over the lip of the calyx (fig. 13). They thus divide the
space between body-wall and theca into a series of long chambers,
corresponding to the exocceles and entocceles, in each of which lies a
costa. Between these chambers and the exocceles and entocceles a
system of ramifying canals permeates the theca, placing the two sets

256 G. HERBERT FOWLER.

of cavities in communication with one another. The columella is
perforated by a similar system of canals, which unites the whole circle
of entocceles and exocceles; there is thus free communication through-
out the whole of the polyp, despite the comparative preponderance of
skeleton over soft tissue. The canals are composed of endoderm and
mesoderm, continuous with the same layers that clothe all the rest of
skeleton ; and in the meshes of the network lies the corallum, theca
or columella.

The polyp thus consists of an external body-wall, mouth-dise with
tentacles, stomatodzeum, and mesenteries ; with a ccelenteron divisible
into columellar canal system, exocceles, entocceles, thecal canal system,
and chambers exterior to the theca, corresponding to and continuous
over the lip with the mesenterial chambers.

The body-wall and mouth-disc are composed of simple ectoderm,
endoderm, and mesoderm, agreeing with those of other Hexactiniee.

The outline of the stomatodzeum is oval, as usual; but I have not
observed any trace of gonidial grooves at the ends of the longer axis.

The tentacles, which are simple evaginations, appear to be entoccelic
only ; they are so invaginated into pockets on each side of the septum
that it is impossible to make out their exact size and shape. This
condition is probably due merely to alcoholic contraction, and does
not imply that involution is the normal method of tentacular con-
traction. A similar invagination had taken place at the bases of the
tentacles of Flabellum. They are covered with nematocysts, which
are not so sharply defined into batteries as was the case in Flabellum.

At a varying depth below the lip of the calyx (but generally at a
lower point than is represented in fig. 13, which is considerably
shortened in the longer axis) the external body-wall perishes, owing
probably to the various parasites that infest the external surface of
most coral thecz and polyps; notably a sponge, which in some places
eats its way right into the theca. The cavity marked / in fig. 15 is
thus filled with sponge spicules. Below the point at which the body-
wall ends there is visible in some places a thin line of tissue indicated in
fig. 15, g, which may or may not be a part of the polyp. ‘The appear-
ance of the periphery of the theca in such a section suggests very
strongly that a secondary line of corallum has been deposited round
the circumference to protect the canals from communication with the
sea water and against the parasites. At the top of fig. 15 the semi-
circular outline of the canals seems to indicate such a formation.

THE ANATOMY OF THE MADREPORARIA, O57

The mesenteries vary in number, and are, like the entosepta,
generally of three orders. They are divisible into “ pairs,” as in the
other forms described, and possess the same arrangement of longitu-
dinal retractor muscles on their entoccelic faces, with the usual
difference in the two directive pairs. The trend of these muscles is
roughly indicated in fig. 13; but their minuteness renders it impos-
sible to recognise the arrangement of the protractor muscles, though
they are just visible in microscopic sections. There appears to be but
little contortion of the free edge of the mesentery, and the traces of
any organs resembling acontia are rare. This, however, may be due
to deficiency of material, which has much hampered my investigation
of this form. 3

Both primary and secondary mesenteries appear to be united to the
stomatodeeum for its whole length ; those of the third order become
disconnected from it very high up, and do not run deep down into
the colony, the cavities in which they lie disappearing among the
other perforations of the theca.

The number of pairs of mesenteries right and left of the “directives”
is not necessarily equal. Complete systems both of meseuteries and
septa (1, 3, 2, 3, 1, in notation) are generally found only at the ends
of the long axis of the calyx, z.e. in the neighbourhood of the direc-
tives. This has been noticed in many other corals.

That the almost exact correspondence of coste with septa, and of
the external lamellee (M’ in the figures) with the mesenteries, adds to
the probability of the correctness of vy, Koch’s view is undeniable. But
it is to be noted that no muscles are to be recognised on the mesoderm
plates of these lamellee, as would probably be the case had they once
been part of the mesenteries; nor in the highest section of the decal-
cified polyp are any cases of decaying tissue visible where the growing
theca is supposed to have cut them.

iil. Histology-—This is of such a simple character as to hardly
require comment. The ectoderm is composed of simple columnar
cells, the endoderm of similar but more cubical cells. Calycoblasts
are present, but in small numbers in comparison with Flabellum.
Nematocysts are of two forms and sizes, of which, as in Flabellum,
the smaller is the only one occurring. on the tentacles. Of the mesen-
terial filament, as unusual in outline, a sketch is given in fig. 16.

Tn conclusion, I have to acknowledge my obligations to Professor
Moseley for much kind assistance and most of my material; to Pro-

8

258 G. HERBERT FOWLER.

fessor Milnes Marshall for valuable advice; to Mr. John Murray, of
the ‘‘Challenger” office, for several specimens of Flabellum; and
lastly, to the anonymous donor of the Berkeley Fellowship, whose
generosity has enabled me to pursue the investigation,

CLASSIFICATION OF THE ZOANTHARIA (Hexacoratta.)

1, Actiniaria (Malacodermata).

Lia EVO RACINE Hi .(c,inisicnie ssisiat deteiawsiccleae tite oe Actinia.
ii, Edwardsiee.
iii, Zoanthee.
iv. Cerianthie.
2. Madreporaria (Sclerodermata).
A. Imperforata (Aporosa).
12s Durbimolideaiiey..idiaesecsacseeae reese Flabellum.
Caryophyllia.
tO Culini dea. SG ome heals Stylophora.
lis Pocilloporidee: sc... .ccssasteienceneen. Pocillopora.
3 Seriatopora.
UVES ERODES \eniuie tae slenacanclonceine antes Cladocora.
B. Fungida.
TBM OAC Ce HIarta select. Seve cie eeiers@ctslisies Sect Bathyactis.
C. Perforata.
PUM UP SANA ES ing, sloieiselecaelciccies lasts Stephanophyllia.
Rhodopsamnua.

LITERATURE OF THE GROUP.

|. LacazE Dutuiurs.—“ Développement des Coralliaires,” ‘Arch.
Zool. exp. et gén.,’ tome ii, 1873,

2. Lacaze Dututers.—“ Développement des Coralliaires,” ‘ Arch.
Zool. exp. et gén., tome i., 1872.

3, O. vu. R. Hertwic.—“ Die Actinien.” Jena, 1879.

4. von Kocu.—‘ Bemerk ii. d. Skelett d. Korallen,” ‘Morph. Jahrb.,’
Band v. 1879.

5. Mosetey.—‘ Remarks on some Corals,” ‘Proc. Zool. Soc.,’ 1880.

6. v. Kocu.—“ Notizen ti. Korallen,” ‘Morph. Jahrb.,’ Bd. vi, 1880.

7. v. Kocu.—Mitth. ti. Colenteraten,” ‘Jen. Zeitschr.,’ Bd. xi.

8. v. Herer.—-“ Die Gattung Cladocora,” ‘Sitz. d.k. Akad, Wiss.,’
1881,

THE ANATOMY OF THE MADREPORARIA. 259

9. v. Kocu.—“ Entwick. d. Kalkskeletes v. Astroides calycularis.”
‘Mitt. d. Zool. Sta, Neap.,’ Bd. iii.

10. Mosstey.—“Seriatopora, Pocillopora, &c.,” ‘Quart, Journ. Micr,
Se.,’ October, 1882.

Il. Mosrtzy.—‘ Rept. Voyage H.M.S. “Challenger,”’ Zoology, vol. ii.

12. Semper.—“Generationswechsel bei Steinkorallen,” ‘Zeitschr. f.
wiss. Zool.,’ Bd. xxii.

v. Kocu.—‘ Die Morphologische Bedeutung d. Korallenskelets,”
‘Biol. Centralblatt.,’ Bd. i1.

v. Kocu.—“ Mitt. ti. d. Kalkskelet d. Madreporaria,” ‘Morph.
Jahrb.,’ Bd, viii.

DESCRIPTION OF PLATES.

b.w. Cut edge of internal body-wall. ch. Calycoblast layer. Cel.
Celenteron. C. or Col. Columella. Cos. Costa. D. ‘“ Directive”
septum and mesenteries. Hct. Ectoderm. Hn. Endoderm. 7.5.
Entoseptum. Ex.S. Exoseptum. J. or Mes. Mesentery. MU’ “ Peri-
pheral” part of mesentery. J/.D. Mouth-disc. Me. Mesoderm. Me’.
Mesenterial muscles. J/./. Mesenterial filament. m.long. Longitu-
dinal muscles. .ob/. Oblique muscles. 2. Nematocyst. Pe. Pedicle.
S. Septum. Sé. Stomatodeum. Ze. Tentacle. Zh. Theca.

Fig, 1.—Section through two quarters of Flabellum, diagrammatic,
the right half showing the primary and secondary mesen-
teries attached to the stomatodzeum, taken along the line
b, Fig. 2, the left being lower down in the polyp, where the
meseuteries have all developed filaments, taken along the
line c, Fig 2. A. Exocele. &. Entocele. i, ii, iii,
Orders of septa and mesenteries, wrongly numbered in the
figure. The numbers should run, reckoning from the
central directive septum (JD), 1, 4, 3, 4, 2, 4, 3, 4, 1, &e.
Corallum coloured deep black throughout the figures.

Fig. 2. —Diagrammatic section along the line a, Fig. 1, @e. in an
exoccele, so that the external face of the mesentery is seen
flat, while the mouth-disc and internal body-wall are cut.
The contortions of the free edge of the mesentery are
omitted,

Fig.

G. HERBERT FOWLER,

. 3.—View of half of the corallum of Flabellum, showing the

relations of pedicle, theca, and septa, and the incomplete
union of the septa marked x, in Fig. 1, into a columella.

, 4.—Portion of the lip of the calyx of Flabellum, viewed from

the exterior by transmitted light, to show the grooves,
i, li, ill, corresponding to the septa of those orders, with
the lines of growth of the theca curving upwards at those
points.

. 5.—Primary mesentery and base of primary tentacles of Flabel-

lum, showing the direction of the muscles, contortions of
the free edge omitted.

. 6.—Section along the line a, Fig. 2, from a full-grown specimen,

with the layer of calycoblasts between mesoderm and theca.

. 7.—Similar section through the internal body-wall of a younger

polyp, in which the calycoblasts are much better marked.

. 8—Section through the tissues clothing the septum of a young

Flabellum.

9.—Section through the wall of a tentacle, including one com-

plete “battery.”

. 10,—Section through a mesenterial filament of Flabellum.
. 11.—Transverse section through part of a mesentery, to show

the mesodermal pleatings on which lie the muscles.

. 12.—Transverse section of an acontium of Flabellum.
ig. A. Transverse diagrammatic section of Caryophyllia (after v.

Koch). a, Septa. 6. Mesenteries.

, B, Similar section through Caryophyllia, in a lower plane than

A (after v. Koch). a. Septa. 06’. Central, 6”. peripheral
parts of the mesentery. c. Costa. th. Theca.

13.—Diagram of a longitudinal section of Rhodopsammia, con-
siderably shortened in the longer axis; the right half of
the figure taken along the lines ¢. c. in Figs. 14 and 15,
z.e.in an exocele; the left along the lines d. d. in the
same figures, and therefore cutting through a septum and
a tentacle. c. Cut edge of external body-wall. d. Cut
edge of tissues clothing theca and columella. 2. Tissue
clothing the entoseptum, which is seen projecting from
behind the mesentery. On the left side the inner face
of a mesentery (M) is seen similarly projecting from behind
the septum. The coste being in this form rows of spines,
appear as projections in both transverse (Fig. 14) and
longitudinal (Fig. 13) sections,

veal

THE ANATOMY OF THE MADREPORARIA. 261

Fig. 14.—Transverse section of half of the calyx of Rhodopsammia,

along the plane a, Fig. 13 (camera drawing), The
numerals 1, 2, 3 are placed in the entocceles formed by a
pair of mesenteries of those orders. Complete systems,
1, 3, 2, 3, 1, are only found in the region of the directives.
The dashed numerals, 1’, 2’, 3’, are placed in the external
chambers which correspond to the entocceles. ext.b.v.
External body-wall. Corallum deep black, soft tissues in
lighter black lines.

g. 15.—Similar section along the plane 0, Fig. 13. The septa have

fused into the columella, and are numbered 1, 2, 3,
according to their orders. f. Cavity filled with sponge
spicules. g. Line of tissue which may belong to the polyp.

. 16.—Mesenterial filament of Rhodopsammia in transverse section.

Fig. 17.—Part of Fig. 14, enlarged to show the relations of the

Fig

three body-layers. Mesoderm black, corallum grey. .
Thecal canal system in transverse section. ¢. External
chambers, corresponding. to exocceles and entocceles, in
each of which lies a costa.

. 18.—Part of the thecal canal system of Rhodopsammia, after

removal of the corallum by decalcification.

Figs. 19, 20, and 21.—Diagrams of the relations of a complete system

of septa of Rhodopsammia at different heights. The
numerals are placed at the bases of the entosepta.

Fig. 22.—Calyx of Rhodopsammia, viewed from above. From a

specimen in the British Museum.

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ON THE NERVOUS SYSTEM OF ANVTEDON ROSACEUS.

By A. Mines Marswatt, M.D., D.Sc, M.A., Fellow of St. John’s
College, Cambridge ; Beyer Professor of Zoology in Owens College.

[Puate XIV.]

During a recent visit to the Zoological Station at Naples, I devoted
some time to an investigation of the nervous system of Antedon,
with the object of testing by actual experiment the validity of the
rival doctrines which have been advanced concerning it of late years.

I propose in the present paper to give (1) a brief sketch of the
general organisation of Antedon, in order to define the terms employed,
and to make the following descriptions more readily intelligible; (2) a
short historical account of the controversy regarding the nervous
system of Antedon, including the present position of the question ;
(3) an account of my own experiments and observations; and (4) a
discussion of certain points of morphological interest affected by the
conclusions arrived at in the preceding section.

I. Generat Description oF ANTEDON.

Antedon! consists of a central disc from which radiate five pairs of
long arms, fringed with pinnules.

1 For fuller descriptions, vide Carpenter “ Researches on the Structure, Physiology, and
Development of Antedon rosaceus,” part i, ‘Phil. Trans.,’1866; and Ludwig, ‘ Morphologische
Studien an Echinodermen,’ Bd. i, Abth.i; and for a very excellent summary of recent re-
searches, vide P. H. Carpenter, ‘‘The Minute Anatomy of the Brachiate Echinoderms,”
Quart. Journ. Micr. Sc.,’ vol. xxi.

264 PROFESSOR MARSHALL.

The disc consists of a calcareous cup or calyx (vide fig. 1), and of
the visceral mass which is lodged within the cavity of the calyx, and
contains the whole of the alimentary canal and important parts of the
vascular, sensory, and other systems.

The surface of the visceral mass covered by the calyx is commonly
called the dorsal or aboral, the opposite one being the ventral or oral
surface. In, or near, the middle of the latter is the mouth (fig. 1, 7);
this leads into a convoluted alimentary canal (s) ending in an anus
placed at the top of a conical chimney-like projection, which arises
from the oral surface of the disc not far from its edge, and interradially,
a.¢. between two pairs of arms.

To the dorsal surface of the calyx are eatentel from twenty to
thirty jointed filaments or cirri (fig. 1, p), by which the animal attaches
itself to foreign bodies. The calyx itself consists of a number of
calcareous plates arranged as follows (cf. figs. 1 and 3):—In the centre
is a single pentagonal centro-dorsal plate (C.D.), to the dorsal surface
of which the cirri are attached, while the. ventral surface is hollowed
out in its centre to form a cup-shaped cavity closed above by a thin
calcareous plate—the Rosette (R.); more peripherally the centrodorsal

plate supports a ring of five plates called First Radials (R.1). To the ©

outer surfaces of these are connected five Second Radials (R.2) which
overlap and almost entirely conceal the First Radials from the dorsal
surface (fig. 1), and beyond the second comes a set of five Third
fadials (R.s).

Each Third Radial bears distally a pair of First Brachials (figs. 1
and 3, Br.), which are the first of a series of short calcareous joints
placed end to end and extending the whole length of the arms.

The spaces between the radials and between the basal joints of the
arms as far as the fourth brachials are filled up by uncalcified portions
of the perisome or body wall, which thus complete the calyx.

The several joints of the arms are moveable on one another. Move-
ment towards the oral or ventral surface, which will be called flexion,
is effected by muscles (figs. 1 and 2, w) running between the successive

segments ; extension or movement towards the dorsal surface is on the _
¢
other hand almost entirely due to the action of elastic ligaments

placed nearer the dorsal surfaces of the segments.

The dorsal and lateral surfaces of the arms are covered by an
extremely thin layer of integument, but along the ventral surface the
soft parts are much thicker and exhibit considerable complexity of

t

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 265

structure. Running along the ventral surface of each arm is a longi-
tudinal furrow, the ventral or ambulacral groove (fig. 2, 2), bordered on
either side by a fold of perisome, the edge of which is notched into a
series of concentric leaflets, at the base of each of which is a group of
three hollow tentacles (h).

The ambulacral groove is lined by a special ambulacral epithelium
which is columnar and ciliated and much thicker than the non-ciliated
epithelium covering the rest of the body. Beneath the columnar cells
is a fibrillar layer (fig. 2, ), spoken of as the subepithelial band. This
consists of very slender fibrils arranged for the most part longitudi-
nally, and so appearing as fine dots in transverse sections of the arm:
interspersed among the fibrils are very small nucleated cells. The
subepithelial band in Antedon rosaceus is continuous with the ambu-
lacral epithelium, of which it may be described as forming the deepest
layer; it is traversed vertically by strands which are continuous on
the one hand with certain cells of the columnar epithelium, and on
the other with a connective tissue stratum underlying the band. In
other species the subepithelial band appears from the descriptions of
Ludwig and others to be separated from the ambulacral epithelium by
a very thin connective tissue lamella.

At the bases of the arms the ambulacral grooves are continued on
to the disc ; those of each pair of arms unite together and so give rise
to five radial grooves which run over the surface of the disc to the
mouth, where they meet. Round the mouth the subepithelial bands
of the five radial grooves unite to form a pentagonal ring.

The tentacles, as described above, are hollow; their cavities com-
municate with a longitudinal canal (fig. 2, 7) which runs along the arm
just below the subepithelial band. These radial ambulacral canals are
continued into the disc and open into a circular canal round the mouth,
(fig. 1) from which short branching canals are given off ending in open
mouths communicating with the body cavity.

Besides the radial ambulacral canal, each arm contains also three
diverticula of the body cavity or celom. Of these the most ventrally

ituated (fig. 2, m) is called the subtentacular canal and is commonly
ae as in the figure, by a median vertical partition; the most
dorsally placed canal (fig. 2, 2) is called the celiac and communicates
at the end of the arm with the swbtentacular. The third or genital
canal is placed between the other two and lodges the cord-like genital
gland ; it is very small in the arm, but much larger in the pinnules,

966 PROFESSOR MARSHALL.

In the centre of the visceral mass is a plexiform structure (fig. 1, g),
the real nature of which has been much disputed, but which, according
to Ludwig and P. H. Carpenter, is part of the vascular system from which
branches are given to all parts of the body, and among others a radial
ventral vessel down each arm in the substance of the subepithelial
band. This central plexus passes down through the central canal
formed by the First Radials, passes through a hole in the. middle of
the rosette, and enters the cavity in the centrodorsal plate, where it
expands to forma sac divided by vertical septa into five radial com-
partments, and hence called the chambered organ (fig. 1, /).

The chambered organ is surrounded by a thick fibrillar investment
(d) known as the central capsule, and this is in connection with a
system of fibrillar bands which run down the arms in the substance of
the calcareous joints, and are hence called awial cords (figs. 1, 2, 3, a).

The connection between the central capsule and the axial cords
is rather complicated; but it is necessary to describe it in some
detail, as it is with these parts that we shail be specially concerned
later on.

The central capsule is lodged as we have seen in the hollow of the
centrodorsal plate and is covered on its ventral surface by the rosette ;
it forms a complete investment to the chambered organ (fig. 1) except-
ing where it is perforated by the central plexus in the middle of the
ventral surface. The dorsal and lateral walls are, as shown in the
figure, thicker than the ventral.

From the dorsal surface are given off processes to the cirri (fig. 1, e),
each of which is traversed down its centre by a vessel derived from
the central plexus.

From the margin of the central capsule arise five short interradial
processes (figs. 1 and 3), which, passing ventralwards and slightly
outwards, bifurcate into right and left branches between the centro-
dorsal plate and the First Radials. These branches, diverging from
one another, enter the substance of the First Radials and then unite
in pairs, the right branch of one interradial stem uniting with the left
branch of the one next to it, to form five stout radial nerves (fig. 3)
which run outwards in the substance of the First and Second Radials.
On reach'»g the boundary line between the Second and Third Radials
each of these radial cords divides into two branches right and left,
which, traversing the Third Radial, enter the right and left arms
respectively of the pair, along which they pass as the axial cords

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 267

(figs. 1, 2, 3 a) in the substance of the brachials or calcareous
segments of the arms.

Besides the connections described above there are certain others
which must be noticed. A pentagonal commissure (fig. 3) connects
all the branches together immediately after they have entered the
First Radial. There is also a further connection in each Third Radial
between the branches into which the radial cord divides to supply
the two arms of the pair; this connection, as shown in fig. 3, consists
of a transverse commissural band of fibres, and a chiasma formed by
two obliquely placed bands which cross one another and furnish
additional communications between the right and left axial cords.

In the arms the axial cords lie in tubular channels perforating the
calcareous joints (figs. 1 and 2), Each cord gives off alternately right
and left stout branches, which enter the pinnules (fig. 2, c), in which
their relations are the same as in the arms themselves. Besides these,
finer branches are given off, both from the axial cords themselves and
from the pinnule branches, which, passing through the calcareous
joints, can be traced into very intimate relation with the muscles
moving the arm-joints on one another, with the tentacles and the

crescentic leaflets bordering the ambulacral groove, and with the

tegumentary covering of the arms generally.

Histologically, the central capsule and the various cords in con-
nection with it consist principally of very delicate fibrils, arranged for
the most part longitudinally, and having interspersed among them
very small nucleated cells, both the cells and fibrils closely resembling
those of the subepithelial bands of the ambulacral grooves. Other
fibres, of more irregular size and distribution, which traverse both the
capsule and cords in various directions, appear to be of the nature of
connective tissue, and to correspond to the vertical strands in the
subepithelial bands. Externally both the capsule and the cords are
invested by a layer of cells, which are much larger than the small
ones found in the substance of the cords, stain deeply, and give off
branching processes which are in very close relation with the reticulum
forming the organic basis of the skeletal parts. This external layer
of cells appear to me to be a mere investment of the cords and to be
no part of their real substance.

The pinnules of each arm arise alternately from the right and left
sides, each of the brachials except the first bearing one pinnule. The
structure of the pinnules is, with certain exceptions, the same as that

268 PROFESSOR MARSHALW.

of the arms, each having an ambulacral groove, subepithelial band,
tentacles, ambulacral, subtentacular, genital and coeliac canals, a
branch of the axial cord, &c.; the genital rachis, however, which is
only a slender cord in the arms, dilates in the pinnules to form the
genital glands. The proximal or oral pinnules, 2.e. those borne by the
Second Brachials, differ markedly from the others; they are longer
than the rest, and habitually bend inwards, so as to arch over and
cover the disc; they have no tentacles’ and no ambulacral grooves,
the ciliated epithelium of the grooves and the subepithelial bands being
both absent ? they possess, however, like the other pinnules, branches
of the axial cords of the arms.

I]. HistoricaL SKETCH.

I propose, in this section, to notice briefly the principal views that
have been advanced concerning the nervous system of Antedon.

Miiller,® in 1841, gave the first account of the genital rachis in the
arms of Antedon, but mistook it for the nervous system, and described
it as such ; he also mentioned the axial cords, but described them as
vessels,

In 1865 Dr. Carpenter, in his ‘Memoir on the Structure, Physiology,
and Development of Antedon rosaceus, corrected Miiller’s mistake
concerning the genital rachis; and, with regard to the axial cords,
stated:* “It will be shown, in the second part of this memoir . . .
that a system of branching fibres proceeding from the solid cord that
traverses the axial canal of each calcareous segment of the rays and.
arms is traceable on the extremities of the muscular bundles, and
reasons will be given for regarding these fibres as probably having the
function of nerves, though not exhibiting their characteristic structure.”

In 1872 Baudelot? called attention to the anatomical and histological
resemblances between the axial cords of Antedon and the radial nerve-
cords of other Echinoderms. He mentioned the pentagonal commissure
in the calyx and the branches of the axial cords to the pinnules, and
described the cords as consisting of fibrils cemented together by a finely

2 Carpenter, ‘Phil. Trans.,’ 1886, p. 702.

2 P. H. Carpenter, ‘Remarks on the Anatomy of the Arms of the Crinoids,” part ii,
‘Journal of Anatomy and Physiology,’ vol. xi, p. 90.

3 J. Miiller, ‘Ueber den Bau des Pentacrinus Caput Meduse.” ‘Physikalische Abhandlun-
gen der Koniglichen Akademie des Wissenschaften zu Berlin.’

* Carpenter, ‘Phil. Trans.,’ vol. 156, 1866, p. 705.

5 Baudelot, ‘‘ Etudes Générales sur le Systéme Netveux,” ‘Archives de Zoclogie Expéri<
mentale,’ tome 1, 1872, p. 211,

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS, 269

granular substance and with very small cells imbedded in the fibrillar
tissue. He appears to have been unacquainted with Dr. Carpenter’s
work, and, in spite of the resemblances he points out so clearly, denies
absolutely the nervous nature of the axial cords without stating
definitely his reasons for so doing.

Semper, in 1874, published an independent refutation of Miiller’s
error as to the genital rachis, and suggests, concerning the nervous
system, in ignorance of Dr. Carpenter’s statement quoted above, “ It
might even be possible that the cord in the interior of the calcareous
skeleton (7.e. the axial cord) is a nervous cord; and if so, then the
so-called heart situated in the calyx would certainly have to be looked
upon asa ganglion.” Semper also suggests that a fibrous cord described
by Perrier? as lying above, ¢.e. on the ventral side of the tentacular
canal, may also belong to the nervous system. He confirms the
existence of this cord, and refers to it as # in a diagrammatic transverse
section of an arm.

In an addendum to the translation of Semper’s paper*® and in a second
communication to the Royal Society on the structure, physiology, and
development of Antedon rosaceus,* Dr. Carpenter further develops the
theory that the axial cords “are really nerve-trunks, and that the
five-chambered organ in the centrodorsal basin is their centre.” He
refers to the ‘quickness and consentaneousness” with which the
coiling and uncoiling of the arms are effected, and to the fact that
irritation of the oral pinnules causes the whole circlet of arms to close
together as strong evidence of the presence of a definite nervous
system, and suggests that the histological simplicity of the axial cords
may ‘‘be related to the fact that as the muscles are all flexors the
nerves have only one function to perform, and that there is conse-
quently no need of the insulation which they require where nerve-fibres
of very different functions are bound up in the same sheath.”

He further supports his theory by the following experiment made
at Oban in 1867, and which for convenience of reference I shall
describe as :—

Experiment A.—The entire visceral mass was removed from a living
Specimen so as to leave nothing but the calyx with the central capsule

1 Semper, ‘“Kurze Anatomische Bemerkungen ueber Comatula,” ‘ Arbeiten aus dem Zool-
Zoot. Institut in Wiirzburg,’ Band i, 1874, p. 259. Translated in the ‘Annals and Magazine
of Natural History,’ 1875, p. 202.

2 Perrier, ‘ Archives de Zoologie Expérimentale,’ tome ii, 1873, p. 55.

3 Carpenter, ‘Annals and Magazine of Natural History,’ 1875, p. 206.

* Carpenter, ‘ Proceedngs of the Royal Society,’ 1876, p. 226.

270 PROFESSOR MARSHALL.

and its prolongations and the arms. A needle was then passed down
the canal surrounded by the First Radials (cf. fig. 1) so as to irritate
the chambered organ, ‘All the ten arms then suddenly and con-
sentaneously closed up. On the withdrawal of the needle the arms
gradually straightened themselves again, and again coiled up as before
when the irritation of the central organ was renewed.”

In January, 1876, Greef* called attention to the thickened epithelium
forming the floor of the ambulacral grooves both of the arms and dise.
He pointed out the close correspondence both in position and_histo-
logical structure between this ambulacral epithelium of Antedon, and
the radial nerves and circumoral commissure of a Starfish, and suggested
that the former, like the latter, was nervous in function. At the same
time he denied the nervous character of the axial cords.

In the following month Ludwig,? without being acquainted with
Greef’s work, described for the first time a “ delicate fibrillar band”
immediately beneath the ambulacral epithelium, ae. what we have
named above the subepithelial band (fig. 2, 2), which he regarded on
histological and morphological grounds as the true nervous system of
Antedon, and as the representative of the radial nerves of other
Echinoderms.

In April of the same year P. H. Carpenter’ confirmed Ludwig’s
description of the subepithelial bands which he had himself inde-
pendently discovered, and agreed with him in regarding them as
nervous. He also showed that the cord w, described by Semper, was
not identical as he had supposed with Perrier’s fibrous cord, and that
neither of these structures corresponded to the subepithelial band,
Semper’s cord being merely a pigmented cellular thickening between
the ambulacral and subtentacular canals, while Perrier’s fibrous cord
is a muscular band in the ventral wall of the ambulacral canal.
P. H. Carpenter, however, differed from Ludwig in regarding not only
the subepithelial bands, but the axial cords also as nervous, and he
was the first to distinctly maintain the existence in Antedon of this
double nervous system, in spite of the morphological difficulties in-
volved in this view. He brought forward as additional evidence in
favour of the nervous character of the axial cords the fact that in the

1 Greef, ‘‘Ueber den Bau der Crinoideen,” ‘Sitzungsb. d. Gesellsch. z. Beford. der gesam.
Naturwiss. zu Marburg,’ No. 1, 1876, pp. 16—29.

2 Ludwig, ‘Nachrichten v. der Kénigl., Gesellschaft der Wissenschaften, und der Univer-
sitit zu Gottingen,’ No. 5, Feb. 28rd, 1876.

3 P. H. Carpenter, “Remarks onthe Anatomy of the Arms of the Crinoids,” ‘ Journal of
Anatomy and Physiology,’ April, 1876,

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. OTA

arms of Actinometra the cord enlarges in the centre of each ossicle
and gives off branches to both dorsal and ventral surfaces, some of
the latter reaching “the bases, or in some cases even the tips of the
respiratory leaves.” He even suspected a connection between some
of those branches of the axial cord and the subepithelial bands ; some,
he says, ‘appear to enter into the plexus of tissue forming the organic
base of the skeleton, others seem to become connected with epidermic
structures.”

In a supplemental note Dr. Carpenter also confirms the existence
of the subepithelial band, and considers that it is “by no means im-
probable, looking alike to its position and to its histological character,
that this band is a nerve.” On account mainly of its position he
suggests that it is ‘an afferent rather than a motor nerve.” He also
brings forward the following extremely important additional experi-
mental evidence in support of the nervous nature of the central capsule
and axial cords.

Experiment B.—The visceral mass was removed from a large and
vigorous Antedon, leaving the calyx with the central capsule and the
arms intact. On replacing the animal in the water 7 executed the usual
swimming movements as perfectly as the entire animal had previously done.

Experiment C.—From a second active specimen the entire centro-
dorsal basin with its contents and appendages were removed. On
replacing the animal in the water all the arms were rigidly straightened
out, apparently by the action of the elastic ligaments which the muscles
were powerless to antagonise.

Lupervment D.—In an active specimen the soft parts of one of the
arms were divided down to the calcareous segments. On replacing
the animal in water al/ the arms worked as usual without the slightest
disturbance of regularity.

Experiment E.—By means of nitric acid applied with a fine brush,
the dorsal half of one of the arms was dissolved away until the axial
cord was reached and destroyed. On replacing the animal in the
water the injured arm remained rigidly stretched out, while all the other
arms worked as usual.

From these experiments Dr. Carpenter concludes that the central
capsule is the co-ordinating centre of a nervous system whose peripheral
portion consists of the axial cords of the rays, arms and pinnules; also

1 Carpenter, ‘Proceedings of the Royal Society,’ vol. xxiv, 1876, p, 651.

212, PROFESSOR MARSHALL.

that the subepithelial band, if a nerve at all, has no immediate relation
to the swimming movements of the arms.

In 1877 Ludwig" published a more detailed account of the subepi-
thelial band in Antedon, in which he describes the band and the
columnar epithelium covering it as being sometimes directly continuous
with one another and sometimes separated by a delicate horizontal
lamella. This lamella he finds to be a more constant and evident
structure in Antedon Eschrichtii than in A. rosaceus. He considers
that the subepithelial band is alone to be regarded as the nerve, and
points out that the close histological similarity between this band in
Crinoids and the radial nerve of an Asterid, which latter, from the
position of the eyes, must certainly be nervous, is a strong argument
in support of his view. He also discusses the claim of the axial cords
to rank as parts of the nervous system ; but, while admitting the great
importance of Dr. Carpenter’s experiments, considers that the case is
not yet satisfactorily proved, and that the morphological difficulties
involved in the possession by Crinoids of a nervous system altogether
unknown in other Echinoderms, are too great to permit the acceptance
of Dr. Carpenter’s views. According to Ludwig, the axial cords are
parts of the connective tissue basis of the skeleton, which persist in
an uncalcified condition, and are probably nutritive in function.

P. H. Carpenter, in a further paper on the arms of Crinoids,? and
in a monograph on the genus Actinometra,’? brings forward strong
additional evidence in support of the nervous nature of the axial cords.
He shows that in Antedon rosaceus the oral pinnules differ from the
other pinnules, not only in being destitute of tentacles (as pointed out
by Dr. Carpenter in 1865), but also in having no ambulacral groove,
no thickened ambulacral epithelium, and no trace of the subepithelial
band, 2.e. that they are totally devoid of what Ludwig considers to be
the sole nervous system of Antedon ; and yet these oral pinnules are
peculiarly irritable, a slight touch being sufficient to cause all ten arms
to be suddenly coiled up over the disc.

He further finds that in Antedon LEschrichtii this absence of
ambulacral groove and epithelium, and of the subepithelial band, occurs
not only in the oral pinnules, but at the distal extremities of the

1 Ludwig, ‘Morphologische Studien an Echinodermen,’ Heft i, Abh. i; ‘Separat. Abdruck
aus der Zeitschrift f. wissenschaftliche Zoologie,’ Bd. 28.

2 P. H. Carpenter, ‘‘ Remarks on the Anatomy of the Arms of the Crinoids,” part ii,
‘ Journal of Anatomy and Physiology,’ vol. xi, October 1876.

3 Pp. H. Carpenter, “On the Genus Actinometra,” ‘Transactions of the Linnean Society,’
2nd series Zoology, vol. ii, part i, 1879,

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 273

arms and other pinnules. The allied genus Actinometra is still more
remarkable, for here entire arms may be completely devoid of ambu-
lacral groove and epithelium, and of the subepithelial band, and yet
such arms, though on Ludwig’s theory possessing no nerves at all, are
described, on Semper’s authority, as exhibiting as regular and active
movements while swimming as the other arms. On the other hand,
the axial cords or their branches extend along all the arms and
pinnules, whether possessing ambulacral grooves or not.

In all cases the absence of ambulacral grooves is associated with the
absence of tentacles. Non-tentaculiferous arms are met with in a
large number of species of Actinometra, no less than twenty-three out
of the forty-eight species collected by the ‘ Challenger”? having more
or fewer of such arms, the number of which varies greatly in different
individuals.

In a short paper published in 1883 Perrier® adopts very definitely
the views of the Carpenters concerning the nervous system. He traces
branches of the axial cords into connection, through the intermediation
of stellate cells, with the muscle fibres. Other branches were traced
by him into the tentacles. He gives no figures, however, and his
descriptions leave some doubt as to whether the stellate cells do not
rather belong to the connective tissue investment of the nerve or
muscle than to the nerves themselves.

P. H. Carpenter® has recently described tripolar cells intercalated in
the course of the axial cords and their branches in Antedon. He has
also traced in three species of Antedon a fibrillar plexus, derived from
the axial cords, into the connective tissue of the perisome forming the
ventral surface of the disc, and is ‘strongly inclined to believe that
extensions of this plexus are in direct connection with the fibrils of
the subepithelial bands.”

Finally, Dr. Carpenter has very recently* given a summary of the
investigations concerning the nervous system of the Crinoids which
have been published since his former paper in 1876. He points out
that the evidence accumulated in this interval is most strongly in

1 Pp. H. Carpenter, *‘ Preliminary Report upon the Comatule of the ‘Challenger’ Ex-
pedition,” ‘ Proceedings of the Royal Society,’ No. 194, 1879, p. 395.

2 Perrier, ‘‘ Note sur organisation des Crinoides,” ‘Comptes rendus,’ tome, xcvii, 1883,
pp. 187—189.

3 Pp. H. Carpenter, ‘‘ Notes on Echinoderm Morphology,” No. 6, ‘Quarterly Journal of
Microscopical Science,’ 1883.

“Carpenter, “On the Nervous System of the Crinoidea,” ‘Proceedings of the Royal
Society,’ 1884,

T

974 PROFESSOR MARSHALL.

favour of his view, which, on the other hand, is opposed merely “by a
theoretical homology, a preconceived notion of what Crinoids ought to
be.” He concludes with some important observations on the morpho-
logical aspects of the question, which will be noticed in a later section
of this paper.

The present position of the question may be briefly described thus.
The Carpenters and Perrier, on the one hand, maintain that the central
capsule and axial cords, with their branches, constitute the essential
and principal part of the nervous system, both motor and sensory,
while the subepithelial bands, if nervous at all, are of very subordinate
functional importance. On the other hand, Ludwig and the German
morphologists generally maintain that the subepithelial bands consti-
tute the sole nervous system. The former school cite in support of
their views a large mass of anatomical and histological observations
and certain direct experiments ; while the latter school rely entirely
on theoretical morphological objections to the views of their opponents.

III. ExpErRIMentTAL INVESTIGATION OF THE NeRvous SYSTEM OF
ANTEDON ROSACEUS.

This section of the paper, containing the account of my own
investigations made at Naples last April, I propose to subdivide under
the following heads :—A. The movements of uninjured specimens.
B. The effects of removal of the visceral mass. ©. The power of
regeneration. D. The functions of the central capsule. E. The
functious of the axial cords. F. The functions of the subepithelial
bands.

A. The Movements of Uniniured Specimens.

The normal position of Antedon rosaceus, the species on which all
my experiments were made, is a fixed one, the animal being attached
by the dorsal cirri to some foreign body, and the arms spread out
horizontally with their tips slightly flexed. The oral pinnules are
bent over the disc, crossing one another above it; the other pinnules
are spread out nearly at right angles to the arms.

In an aquarium containing a large number of specimens the great
majority will be found attached either to the bottom or sides of the
tank, z.e. with the oral surface directed either upwards or more or less
obliquely ; some specimens, however, are almost certain to be found,
if there be foreign bodies in suitable positions for attachment, inverted,
with the oral surface downwards.

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 275

An Antedon when once attached exhibits very little tendency to
alter its position, and may remain fixed in the same place for weeks,
If detached, either spontaneously or by force, it can, and usually does,
swim actively until it reaches a suitable place of rest, to which it
anchors itself by its cirri. The normal swimming movements, which
are peculiarly graceful, consist in strong flexion of the proximal half
of the arm, which is raised vertically over the disc, and then extension
of the whole arm, the distal half of which is thrown out something
like a whiplash or the line of a flyrod. During flexion the pinnules
are folded alongside the arm; during extension spread out so as to
expose as great a surface as possible. Usually two or three arms are
raised simultaneously, sometimes as many as five, and the only rule I
have noticed is that the two arms of each pair are always flexed
alternately and not simultaneously.

When attached by its cirri, the arms of Antedon exhibit but very
slight movements; they are usually spread out widely, apparently to
expose as large a surface as possible for the entanglement of food
particles, which, if they once come in contact with the ambulacral
epithelium, get carried by the action of its cilia to the mouth.

Irritation of the ambulacral groove at any part causes the adjacent
pinnules to be at once turned forwards, z.e. with their tips towards
the free end of the arm, and folded alongside the irritated part,
apparently to protect it from further injury. Slight irritation of a
pinnule or of an arm causes correspondingly slight and local move-
ments ; stronger irritation causes movements of the whole arm, which
may spread to other arms, or lead to the animal detaching itself and
swimming freely. Irritation of the oral pinnules, however slight,
causes them to be firmly closed over the disc, and stronger or prolonged
irritation causes the arms to be flexed strongly, so as to cover the disc,
or else the whole animal to detach itself and swim away.

If an Antedon be detached and placed with its oral surface down-
wards, it will right itself almost at once. If the surface on which it
is placed be a rough one, the righting movement is effected in a few
seconds or almost instantaneously. In a glass vessel it takes longer
to perform, but with an active specimen I have never seen more than
two minutes spent over the operation. In righting itself an Antedon
first flexes all the arms slightly, so as to raise the disc a little above
the ground ; then follows a moment of apparent uncertainty as to
which arm to use. One arm is then flexed more strongly than the

276 PROFESSOR MARSHALL.

others, so as to slightly lift the disc on that side, the pinnules of the
flexed arm being extended and apparently used to push against the
ground. Then after another pause, a rather sudden and violent flexion
of the arms immediately adjacent to the already flexed one causes the
animal to turn on its side, when a few energetic swimming movements
place it right way up. An active animal has apparently the strongest
objection to being placed mouth downwards, and will right itself again
and again if so inverted. When attached by the cirri, however, they
may, as noticed above, remain in the inverted position for days or weeks.

If an arm be cut off from an active Antedon, the detached arm will
retain its vitality for many hours. It will at first exhibit strong
movements of flexion, lasting from a few minutes to as long as a
couple of hours, the arm being alternately coiled up spirally, and then
extended with great force and rapidity.

Antedon, if kept in captivity, requires the water to be frequently
changed, or else very efficiently aérated. Specimens left over-night
in a small basin of sea-water were found dead the next morning. In
dead specimens, owing to the unopposed action of the elastic ligaments,
the arms are very strongly extended.

B. On the Effects of Removal of the Visceral Mass.

In a living specimen the visceral mass can be removed from the
calyx with great ease, as was pointed out long ago by Dr. Carpenter.
If the visceral mass be grasped with forceps an exceedingly slight pull
suffices to remove it. In such eviscerated specimens the central capsule
with its prolongations and the axial cords remain in the calyx intact,
excepting, of course, the branches of the cords described by P. H.
Carpenter as distributed to the oral perisome: the ambulacral grooves
and other soft parts, on the other hand, are torn across at the
bases of the arms, and the subepithelial bands consequently isolated
from one another.

Experiment 1.1—A large and vigorous specimen- was eviscerated.
without removal from the water. On being released it remained
quiescent for about a minute, and then swam about the tank actively
and in a perfectly normal manner. After a short time it came to rest
on the bottom in a perfectly normal position. Half an hour later,

1 For convenience of reference I propose to number the various experiments consecutively.
It will be understood that they were not made in the order given here, and that only those
which seem of distinct importance are recorded. No experiment is described from a single
observation only, and in most cases the experiments were repeated several times,

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. QT

without the slightest disturbance or irritation of any kind, it began
spontaneously to swim again actively and normally. Coming in
contact with a piece of stick, it attached itself to it by the dorsal cirri,
and remained there for more than a week.

The above experiment is the same as Dr. Carpenter’s Experiment B
described above. It is extremely important as proving that the
co-ordinating mechanism which regulates the complex swimming
movements of the arms is entirely without the visceral mass. As the
direct connection between the subepithelial bands of the several arms
is also destroyed, the experiment renders it extremely doubtful whether
these bands have any part in regulating the swimming movements of
the arms.

Experiment 2.—An active specimen was eviscerated, and allowed to
come to rest. The ventral surface of one of the arms was then
irritated gently with a needle; active movements both of the irritated
arm and of the others resulted. The same effect followed irritation of
one of the ordinary pinnules; while irritation of the oral pinnules
caused immediate and strong flexion of all the arms,

This shows that the effect of irritation of the arms or pinnules is
practically unmodified by the removal of the visceral mass ; the only
difference I have noted being that the response is slightly quicker and
more extensive in an eviscerated than in an uninjured specimen. The
nervous connection between the sensory epithelium of any one of the
arms or pinnules and the muscular system, not only of that arm, but
of all the others as well, must, therefore, be without the visceral mass.

As a source of irritation in this and other experiments I employed
at first scratching with a sharp needle. I found afterwards that nipping
with forceps was preferable, as the needle is apt to shake the whole
animal, and go cause disturbance of parts other than those it is desired
to irritate. The nip should be a sharp sudden one, and the irritated
part released at once. In all the experiments here recorded, except
when otherwise specified, both needle and forceps irritation were tried.
Tn some instances the application of acid by a fine brush was made use
of as an irritant; but this can only be done satisfactorily on specimens
removed from the water.

Experiment 3.—An active specimen was eviscerated and allowed to
come to rest in the normal position. It was then inverted and placed
mouth downwards on the bottom of the tank, After a short rest it
righted itself in the normal manner, but rather more slowly than usual,

9278 PROFESSOR MARSHALL.

the interval between inversion and completion of the righting manceuvre
being about two and a half minutes. This experiment was repeated
many times with different specimens. Some righted themselves
instantaneously, others took a longer or shorter time, but the general
average of the times taken by eviscerated specimens to right themselves
was about half a minute longer than that of uninjured ones.

This affords strong additional evidence that the co-ordinating centre
of the complex muscular movements of which an Antedon is capable
is situated not in the visceral mass, but in the calyx.

C. On the Power of Regeneration of Eviscerated Specimens.

It has been stated above that an eviscerated Antedon not only
attaches itself by its cirri in a perfectly normal manner, but that it
may remain so attached for a week or more. On experimenting one
day with a specimen that had been eviscerated about a fortnight
previously, I noticed that it righted itself when inverted rather more
readily than is usual in eviscerated specimens ; and on examination I
found that very considerable regeneration of the visceral mass had
occurred. The soft tissues lining the calyx were of some thickness ;
a mouth was already present in the centre of the oral surface, and
ambulacral grooves had formed converging from the arms to the
mouth. I at once took steps to secure a complete series of specimens,
showing all stages of this regeneration, and I hope to be able shortly
to describe the process in detail.

That Antedon possesses this very extensive power of regeneration,
greatly exceeding even that of Holothurians, was an entirely new fact
tome. Dr. Carpenter tells me that he was led to suspect this long
ago, and he has very kindly shown me specimens that have been in
his possession for many years, which seem to me to be clearly cases in
which regeneration has been partially effected. Dr. P. H. Carpenter
also tells me he has known this fact for some time, though I believe
no notice of it has yet been published. It is only fair to add that
while at Naples the possibility that an eviscerated Antedon might
regenerate its visceral mass was suggested to me in conversation by
Dr. Orley, of Buda-Pesth. I made very light of the suggestion at the
time, and was much astonished when a few days later I found the
specimen described above.

The influence of the nervous system on the regeneration of lost
parts isa point concerning which we know very little; but the apparent

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS, 279

ease with which this extensive regeneration is effected in Antedon
would certainly be still more surprising were the main centre of the
nervous system to be lodged in the part lost, and so far may be
regarded as an argument against such a location,

D. On the Functions of the Central Capsule.

Haperiment 4.—A specimen was eviscerated and allowed to come to
rest ; a needle was then passed from the oral surface down the canal
surrounded by the First Radials (fig. 1) so as to irritate the central
capsule ; the result was immediate flexion of the arms, and in many
cases active swimming movements of the whole animal.

Luperiment 5.—A specimen was eviscerated and then cut into two
parts, one having two pairs of arms and the other three. The central
capsule, which was divided and freely exposed by the operation, was
then irritated by a needle. The slightest irritation caused very active
and violent flexion of the arms.

Laperiment 6.—An active uninjured specimen was held under water,
and the dorsal half of the centrodorsal plate removed by a single snip
with a large pair of scissors so as to expose and partly remove the
central capsule (cf. fig. 1). On being released the animal fell to the
bottom with the arms very strongly extended, but in about twenty
minutes gradually righted itself and assumed the normal position.
The exposed central capsule was then irritated, first with a needle and
then with strong nitric acid applied by a small brush ; the effect of
irritation was to cause very strong and spasmodic flexion of the arms,
which in the first case ceased on removal of the stimulus, but in the
case of the acid persisted for several hours.

The three preceding experiments show that irritation of the central
capsule, whether mechanical or chemical, causes strong flexion of all
the arms, which persists as’ long as the stimulation is continued.
Experiment 4 is the same as Dr. Carpenter’s Experiment A, though
the results are not quite identical ; for while Dr. Carpenter describes
sudden and consentaneous flexion of the arms as following irritation
of the central capsule from the oral surface, I have found that
swimming movements quite as often result. The difference is a
slight one, and may, I believe, be accounted for by the oral pinnules
being accidently irritated in some of the experiments. If these were
clipped off I found that swimming movements of the arms almost
invariably followed irritation of the central capsule from above.

280 PROFESSOR MARSHALL.

The experiments prove in the most positive manner that the central
capsule is in direct physiological connection with the muscles of the
arms ; and the further fact that the experiments yield identical results,
whether performed on eviscerated or unmutilated specimens, proves
that the subepithelial bands form at any rate no part of the central
mechanism.

Experiment 7.—The centrodorsal plate of an active specimen was
removed with scissors and the central capsule carefully scooped out
with a small scalpel. The animal on being released fell to the bottom
of the water, where it lay on its side with the arms very strongly
extended ; it remained in this position for several hours without any
attempt to move. If taken from the water and thrown in again the
arms moved fairly actively, but there was no attempt at swimming,
each arm apparently acting quite independently of the rest. Finally,
if placed on its oral surface it remained there for an indefinite time
without making the slightest attempt to right itself.

Experiment 8.—The preceding experiment was repeated on an
eviscerated specimen, the results being in all respects the same.

These two experiments are of very great importance. They show
that removal of the central capsule completely destroys the co-
ordinating mechanism between the arms, as tested (a) by the power of
executing the normal swimming movements, (b) by the power of
righting itself when inverted; both these powers being permanently
destroyed by the operation. To obtain definite results it is necessary to
completely remove the central capsule, and this I have found cannot be
effected by simply cutting away the centrodorsal plate ; besides this the
capsule must be either scraped out with a fine scalpel or else destroyed
by free painting with strong acid. Specimens in which the centrodorsal
plate has been simply snipped off, though they lose temporarily the
power both of swimming and of righting themselves, yet regain these
more or less completely after an interval of half an hour to an hour,
If, however, sufficient care has been taken to entirely destroy the
central capsule the loss of power is absolute and permanent.

Experiment 9.—The centrodorsal plate of an active specimen was
removed, and the central capsule entirely destroyed ; the cavity was
also very freely painted with nitric acid so as to expose and destroy
the pentagonal commissure connecting the axial cords together at their
roots (cf. fig. 3). After being left at rest for an hour the arms were
irritated one by one; each arm responded readily and extensively to

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 281

the stimulation, but the movement was limited to the arm directly
irritated, none of the other arms sharing in it, except sometimes the
other arm of the pair to which the irritated arm belonged.

This experiment shows that the physiological connection between
the arms can be destroyed by removal of the central capsule and of
its branches, including the pentagonal commissure. After this operation
the several arms, with the exception of the two of each pair, are
physiologically isolated from one another, The experiment yields
identical results whether the visceral mass be present or not.

I have found it very necessary after severe operations to allow
sufficient time for recovery from shock before experimenting further,
and through failure to observe this precaution I obtained at first several
very contradictory and perplexing results. From half an hour to an
hour I usually found to be sufficient.

E. On the Functions of the Axial Cords.

Experiments on the functions of the axial cords and their branches
fall naturally under two heads, ¢.e. those concerned with the relations
of these structures to sensation and to motion respectively.

I propose to commence with the former of these, though, as it
sometimes happens that the same experiment is concerned with both
sensation and motion, it will not be advisable to draw too sharp a line
between the two divisions.

Luperiment 10.—Various parts of the surface, both of the disc
and the arms, of active uninjured specimens were irritated, both
mechanically and chemically, in order to determine the normal dis-
tribution of sensation. Ali parts of the surface were found to be
sensitive, but in very unequal degrees. Irritation of the dorsal surface
of the calyx caused only slight movements of the arms, unless the
irritation were severe or prolonged. Irritation of the dorsal or lateral
surfaces of the arms, where the layer of integument is very thin,
caused flexion of the arms, with extension of the pinnules close to the
irritated spot. ‘The response was usually ready, but the movement
only slight. Prolonged or more violent irritation caused exaggeration
of the movement, together with approximation of the adjacent arms
towards the irritated arm, as though to remove the source of irritation,
and in some cases active movement of the whole animal in a direction
away from the irritated arm. Irritation of a pinnule causes, according
to the degree and duration of the stimulation, movement of the pinnule,

282 PROFESSOR MARSHALL,

movement of the whole arm, approximation of the adjacent arms to
the affected one, or active movement of the whole animal away from
the source of irritation. Irritation of the oral pinnules causes, as
already noticed, immediate and very active flexion of all the arms, so
as to close in over the disc.

The epithelium of the ambulacral grooves is extremely sensitive,
and the results of stimulation are very definite. The slightest irritation
causes instantaneous movement of the four or five pairs of pinnules
immediately adjacent to the irritated spot, the pinnules being folded
alongside the ambulacral groove so as to close it in and grasp the
needle or other source of irritation. If the stimulation be continued
the arm is actively flexed and the adjacent arms applied to it, and
rubbed along the affected part, as though to remove the source of
irritation. Finally, irritation of the ventral surface of the disc between
the ambulacral grooves causes movements of the arms, but not nearly
s0 active as when the oral pinnules are touched.

Experiment 11.—An active specimen was eviscerated, and left for
half an hour. The calyx, arms, and pinnules were then successively
stimulated, as in the preceding experiment. The results were exactly
the same, showing that the communication between the sensitive
surface of any part of the calyx, arms, or pinnules, and the motor
mechanism of all the arms, is placed elsewhere than in the visceral
mass.

Experiment 12.—An active specimen was taken, and all the soft
parts scraped away with a knife from the ventral surface of one of the
arms, the scraped portion being about a quarter of an inch in length
and one inch from the disc. The pinnules were immediately folded
closely alongside the wound, and the animal on being released swam
actively in a direction away from the injured arm. It soon came to
rest in the normal position, and about six minutes after the operation
the distal end of the injured arm was nipped with the forceps. The
distal part of the arm, beyond the injury, was at once flexed actively,
the proximal part less actively, and the other arms did not move.
After a twenty minutes’ interval the distal end of the injured arm was
again nipped, when active movement of all the arms at once resulted,
the animal moving rapidly away from the source of irritation.

The above experiment shows that the communication between the
sensitive surface of an arm or pinnule and the motor mechanism of
all the arms is not effected by the subepithelial band. The practically

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 283

negative result obtained when the stimulation was applied very shortly
after the operation is, I think, most certainly to be ascribed to the
shock of the operation, which, as already noticed, must always be
kept in mind as a disturbing element.

If the communication is not effected by the subepithelial band, nor
by any of the soft parts of the ventral surface of the arm—all of which
were scraped away in the operation—it must take place either through
the integument of the dorsal and lateral surfaces of the arm, or
through the calcareous segments, or through the axial cords, for these
are the only parts left uninjured. To determine which of these is the
real path of communication the following crucial experiment was made.

Experiment 13.—A large and vigorous specimen was taken, and a
quarter of an inch of one of the arms, about an inch from the base,
thoroughly scraped witha scalpel all round so as to remove the soft
parts as completely as possible. The pinnules of the affected part and
for a quarter of an inch on either side of the wound were cut away to
prevent any possibility of contact communication between the parts
on either side of the injury. The injured part was then painted all
round very freely with strong nitric acid, the operation being repeated
until fully half the thickness of the calcareous segments had been
dissolved away. ‘The wound was then washed freely with sea water
and the animal returned to the tank. It fell at once to the bottom
on its side with the injured arm and the other one of the pair
stretched straight out horizontally, and the other arms rather strongly
extended. After a few minutes it began to move slowly, and in six
minutes had completely resumed the normal position. After half an
hour’s interval the distal end of the injured arm was sharply nipped
with forceps, when strong active movements of all the arms at once
resulted, the animal moving rapidly away from the source of irritation.

The above experiment, which was repeated several times, both on
entire and eviscerated specimens, proves conclusively that the com-
munication between the distal end of the irritated arm and the motor
mechanism of the arms is effected by the axial cord; in other words,
that the axial cord plays the part of an afferent or sensory nerve,
conveying impulses centripetally. Furthermore, that it is the normal
path of communication of such impulses is, I think, evident from the
response to stimulation being as ready when it alone remains as in the
uninjured animal. It remains, however, to show whether it is the
only path of communication, To test this I attempted several times

284 PROFESSOR MARSHALL.

to divide the axial cord between two of the segments by a fine scalpel,
but I failed, as Dr. Carpenter had done previously, owing to the fact
that as soon as the knife reached the axial cord the arm was at once
thrown off, usually at a point two or three segments nearer the disc
than the injury. I then tried the plan adopted by Dr. Carpenter,?
z.e. burning away the dorsal half of the arm with nitric acid so as to
expose and divide the axial cord, and with the following results.

Experiment 14.—An active specimen was removed from the water,
the dorsal surface of one of the arms carefully dried, and strong nitric
acid applied with a fine brush to the dorsal surface of the sixth and
seventh radials, which were dissolved away until the axial cord was
exposed and destroyed. If the arm were held during the operation it
was usually thrown off, but if the dise only were held and the arm
allowed merely to rest on the fingers, the operation was always
successful. The animal was then returned to the water, where it
assumed almost at once the normal position. After half an hour’s
rest, the distal end of the injured arm was nipped sharply with
forceps; active movements of the irritated arm beyond the injury
ensued, but no movement whatever of either the proximal part of the
injured arm or of any of the other arms.

This experiment also was repeated several times on both entire and
eviscerated specimens, the results being without exception as recorded
‘above. It is difficult to limit the action of the acid to the dorsal
surface of the arm, but by sufficient care it can be done, and on
several occasions the ambulacral epithelium, including of course the
subepithelial band, was left absolutely uninjured, responding to stimu-
lation in a perfectly normal manner. The experiment must, I think,
be considered, when taken in conjunction with Experiment 13, as
proving that the axial cord is the sole afferent communication between
the arm and the central motor mechanism, for the former experiment
shows that the communication is still perfect when it alone remains,
while the latter shows that division of the cord, other parts remaining
intact, destroys the communication absolutely.

Experiment 15.—One further and very obvious experiment is worth
recording. One of the arms of an active specimen was cut across about
its middle, and the animal held in the tank go that the stump of the
amputated arm was just above the surface of the water; the cut end
of the axial cord could then be very readily seen with the naked eye.

1 Carpenter, ‘Proc, Royal Soc.,’ 1876, p, 654.

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 285

hed

The stump was carefully dried and the axial cord touched with a
fine needle or with a finely-poiuted brush charged with nitric acid,
very violent movements of all the arms at once resulting. Similar
stimulation of the ambulacral epithelium or of other parts of the
section produced but very slight and local movements,

This concludes my experiments as to the afferent functions of the
axial cord, excepting certain points relating to the commissural
connections between these cords, which will be dealt with later on.
I propose now to enquire into the motor function of the axial cords.

Experiment 16.—As in Experiment 12, the soft parts were scraped
away from the ventral surface of about a quarter of an inch of one of
the arms, an inch from its base. On being returned to the water the
animal swam actively, all the arms moving vigorously and normally,
including the injured one, which, however, was rather less active than
the others, and a little stiff at the scraped part, probably from direct
injury to the muscles.

This experiment, which was repeated on eviscerated specimens with
identical results, shows that the path by which motor impulses are
conveyed to the muscles of the arms is neither the subepithelial band
nor any part of the soft structures on the ventral surface of the arm.

Experiment 17.—The operation was the same as in Experiment 14,
the dorsal half of one of the arms, about an inch from the disc, being
dissolved away by nitric acid until the axial cord was exposed and
divided. The animal was then returned to the water, where it remained
quiescent for a few seconds, and then commenced to swim actively and
spontaneously, all the arms moving perfectly normally, except the
injured one, the proximal end of which moved slightly, while the distal
part beyond the injury was perfectly motionless and flexed spirally
into a coil, After a short time the animal came to rest in a perfectly
normal position, but for the spiral coiling of the distal part of the
injured arm, which persisted. After a quarter of an hour’s rest one of
the uninjured arms was irritated, causing at once active movements
of the uninjured arms and of the proximal part of the injured arm,
but none whatever of its distal part.

Experiment 18.—In a fresh Antedon two injuries, similar to that in
Experiment 17, were made in one of the arms at spots about an inch
and a half apart. Stimulation of the arm itself, or of the pinnules,
between the two wounds caused movements of the middle portion of
the arm, but none whatever of the proximal or distal portions.

ee SS ee ee —

286 PROFESSOR MARSHALL.

The two preceding experiments show that division of the axial cord
destroys the motor communication between the parts on either side of
the section as completely as we have already found it to destroy the
afferent or sensory communication. When combined with Experiment
16, which shows that the motor communication is not effected by any
other of the soft parts, the inference is irresistible that the sole motor
communication is that afforded by the axial cords. One additional
experiment may be mentioned in support of this conclusion.

Leperiment 19.—One of the urms of a vigorous specimen was
amputated by a snip of the scissors. The detached arm exhibited
extremely active movements for about a quarter of an hour, coiling
and uncoiling with great force and rapidity. After a time it became
quiescent. It was then held in the tank with the proximal end just
out of water. The end was carefully dried and the exposed section of
the axial cord touched with a needle and a fine brush charged with
nitric acid. The slightest irritation, whether mechanical or chemical,
caused violent and repeated flexion of the arm. Stimulation applied
to other parts of the cut end produced but very little effect.

It still remains to inquire into the functions of the commissural
bands which connect the axial cords together, for if the axial cords
are really nerves these connecting bands, which are identical with
them in histological structure, must be nerves also, and experiment
ought to throw light on their purpose. These commissures are of two
kinds (cf. fig. 3): there is, firstly, the great pentagonal commissure in
the First Radials which connect together the roots of the radial cords;
and, secondly, we have in each Third Radial a rather complicated
connection, by means of a transverse commissure and a chiasma,
between the two axial cords into which each radial cord divides. I
propose to deal with these two sets of fibres separately, taking the
ereat pentagonal commissure first.

Luperiment 20.—A specimen was eviscerated, and a needle passed
down from the oral surface into the chambered organ, and worked
about so as to destroy as completely as possible the central capsule
and chambered organ (ef. fig. 1). ‘The animal was then returned to
the water, and left at rest for half an hour. One of the arms was
then suddenly nipped with forceps, when all the arms exhibited active
movement, though the animal did not attempt to swim.

This experiment shows that the central capsule does not form the
sole physiological connection between the axial cords (nerves) of the

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 287

several arms. Figs. 1 and 3 show that the pentagonal commissure,
which is lodged in the First Radials, would not be touched by the
operation, and, as it furnishes an anatomical connection between
the axial cords, it was naturally suspected to be the physiological
connection as well. ‘To test this the following experiment was made :

Luperiment 21.—The same specimen employed in the preceding
experiment was taken, and the inside of the canal surrounded by the
First Radials freely painted with nitric acid, until the pentagonal
commissure was exposed and destroyed. The animal was then returned
to the water and left for half an hour on its oral face, where it
remained without auy attempt to right itself or to swim. The arms
were then strongly nipped with forceps one by one; each arm when
irritated responded by active movements, but none of the other arms
stirred except the other arm of the pair to which the irritated arm
belonged, which moved sometimes slightly, sometimes actively.

This last observation shows that there is a physiological connection
between the two arms of each pair still remaining after the several
pairs are isolated from one another by destruction of the pentagonal
commissure. There is, as we have seen, an anatomical connection in
the Third Radial (fig. 3), and the following experiments were made
to test whether this furnishes also the physiological connection in
question.

Lexperiment 22.—A pair of arms was cut off a specimen, the section

_ passing between the First and Second Radials. After half an hour's

interval one of the arms was stimulated, when both arms moved
actively.

Experiment 23.—Another specimen was eviscerated and a pair of
arms removed, the section passing between the Second and Third
Radials (cf. fig. 3). All the soft parts were scraped from the basal
portions of the arms, the basal pinnules were cut off, and the Third
Radial and basal joints of the arms freely scraped and painted with
nitric acid, so that the sole connection between the two arms was
through the substance of the Third Radial. After half an hour one
of the arms was sharply nipped ; the irritated arm moved freely, and
the other arm slightly but distinctly. The experiment was repeated
with a second specimen, and an interval of three hours allowed
between the operation and stimulation of the arm. In this case active

and extensive movements of both arms followed on irritation of either
one,

288 PROFESSOR MARSHALL,

As the radial cord (fig. 3) divides into the two axial cords before
entering the Third Radial, the sole anatomical connection between the
axial cords of the two arms in the above experiment is afforded by the
transverse commissure and the chiasma, one or other of which, or
both, must therefore furnish the physiological connection which the
experiment proves to exist. From the anatomical relations of the
parts, and from the fact that the proximal ends of the chiasma must
almost certainly have been injured in the operation, I think it probable
that the transverse commissure is the real connecting link in this
instance. As to the chiasma, the disposition of the fibres suggests
that it may be connected with the alternating movements of the two
arms of each pair which we have seen to occur in the act of swimming.

F. On the Functions of the Subepithelial Bands.

The subepithelial bands are supposed by Ludwig, as we have seen
above, to constitute the sole or main nervous system of Antedon.
The experiments detailed above demonstrate the incorrectness of this
view. They show that the central connection of the subepithelial
bands on the oral disc is in no way essential to, in fact, has nothing
whatever to do with the complicated and co-ordinated movements of
swimming, and of righting when inverted; they show, further, that
division or destruction of the subepithelial band at any place does
not destroy or even disturb either the sensory or motor communications
between the parts on either side of the injury. In fact, they not only
prove conclusively that these structures are not the sole nervous
system, but even raise doubts as to whether they belong to the system
at all.

I think, however, that the close histological resemblance between
the subepithelial bands and the axial cords, coupled with the close
correspondence as regards their relations to the ambulacral epithelium
which exists between Crinoids and other Echinodermata in which, as
in Asterids, they are most certainly nervous, must compel us to con-
sider these bands in Antedon as nervous in nature, though what their
exact function is has yet to be determined. The ambulacral epithelium
is extremely and exceptionally sensitive, and irritation of it is responded
to in a definite and peculiar manner, z.e. by the sudden folding of the
pinnules alongside the irritated spot. ‘The ambulacral grooves are
structures of great importance to the animal, for it is by them that
food particles are captured and swept along by the ciliary currents

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 289

to the mouth. Furthermore, the subepithelial band is in very intimate
relation with that most characteristic Echinoderm system, the ambu-
lacral vessels and their prolongations into the tentacles.

Seeing, then, that there are along the ventral surface of the arms
structures of great importance in very close anatomical relation with
these subepithelial bands, which agree histologically with what are
undoubtedly nerves, it seems probable that these bands form a special
part of the nervous system connected with one or other, or perhaps
all of these special structures.

That the connection between the subepithelial bands and the
ambulacral epithelium and tentacles is a very intimate one is shown
by P. H. Carpenter’s observation, alluded to above, that in both
Antedon and Actinometra all three structures disappear together,
both in the oral pinnules and in those arms or portions of arms
which are devoid of ambulacral grooves.

G. Summary of Results.

1. The central capsule, and its prolongations the axial cords and
their branches, constitute the main nervous system of Antedon.

2. The central capsule is specially connected with the complex
co-ordinated movements of swimming and of righting when inverted.

3. The axial cords act as both afferent and efferent nerves.

4, The subepithelial bands are probably also nerves, but their exact
function, probably a special and subordinate one in connection with
the ambulacral tentacles and epithelium, is not yet ascertained.

5. Evisceration apparently causes but little inconvenience to the
animal, and the visceral mass is regenerated completely in a few
weeks’ time.

These results are in complete accordance with the views so stead-
fastly advocated for many years past by the Carpenters, and recently
adopted by Perrier; while, on the other hand, they are in direct
opposition to the tenets of the German school.?

IV. Morphological Considerations.
Certain points of very considerable morphological interest arise in

2 Since this paper was written Dr. Jickeli, of Jena, has published an account of experiments
made on the nervous system of Antedon, which lead him to strongly uphold the correctness
of Dr Carpénter’s views. Many of Jickeli’s experiments are identical with ones described
above, and his paper, (‘Zoologischer Anzeiger,’ 23rd June, 1884) although at present incom:
plete, contains much valuable information,

U

290 PROFESSOR MARSHALL.

connection with the results detailed above, and I propose in this
concluding section to notice briefly a few of the more important of these.

In the first place the morphological difficulty arising from the
possession by Antedon of an antambulacral in addition to the typical
ambulacral nervous system of Echinoderms must be considered. This
objection has been strongly urged by Ludwig, and constitutes indeed
the real ground of his dissent from Dr. Carpenter’s views ; and it must
be admitted that the presence of a complicated nervous system in
Crinoids, which is apparently altogether unrepresented in other Echi-
noderms, is a feature which a morphologist might well shrink from
accepting until the fullest proof was forthcoming. This proof I have
attempted to supply in the preceding section; the morphological
puzzle, however, still remains to be considered.

Tiedemann! was the first to describe the ambulacral nervous system
of Echinoderms, and since his time the five radial bands with their
connecting circumoral commissure have been universally accepted as
constituting the typical Echinoderm nervous system. This nervous
system, as was pointed out by Tiedemann, is differently situated in
the different groups: in Asterids it is quite superficial, while in
Ophiurids, Echinids, and Holothurids it is much more deeply placed,
being separated from the surface by a thick layer of cutis which in the
two former groups is firmly calcified. Agassiz? urged this difference
of position as an objection to the homology of the radial bands in
Asterids and Echinids, but the objection was not sustained. More
recent researches, while confirming the presence and the nervous
nature of these radial bands and oral commissure, and adding
much to our knowledge of their minute structure and relations,’ have,
however, tended to show that they only form a part of what is really
a very widely-spread and diffuse nervous system.

Thus in Asterids it is very easy to demonstrate that the nerve-layer,
which is perfectly continuous with the epidermis, of which indeed
it forms the deepest stratum, is not confined to the floor of the
ambulacral groove, but extends, though as a thinner layer, over the

1 Tiedemann, ‘ Beobachtungen ueber das Nervensystem und die sensiblen Erscheinungen
der Seesterne ;? Meckel’s ‘Archiv. fiir Physiologie,’ Bd. i, 1815; and ‘ Anatomie des Rohren-
holothuries, des Seesterns und Steineigels,’ Landshut, 1816.

2 Agassiz et Desor, ‘Catalogue raisonné des familles des Echinodermes,” ‘Annales des
Sciences Naturelles,’ 1846.

3 Ludwig’s researches on the nervous system of Echinoderms, embodied in his ‘Morpho-~
logische Studien an Echinodermen,’ are of especial value and importance ; and a recent
paper by Hamann, referred to below, contains many new points of great interest.

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 291

tube feet.* The nervous layer can also be recognised with little
difficulty in the epidermis of the dorsal or antambulacral surface, and
Hamann* has shown that it really forms a continuous sheath over the
whole dorsal surface of the animal, which, though exceedingly thin
over the greater part of the back, thickens considerably at certain
places, notably at the bases of the respiratory processes. Hamann
describes the epidermis of Echinoderms as consisting of elements of
four kinds—(1) supporting cells, columnar cells whose deeper ends
are produced into fibres which pass down into the underlying dermis;
(2) sensory cells, columnar and sometimes ciliated cells whose deeper
ends are continuous with (3) the nerve-fibrils, delicate bands whose
direction is mainly parallel to the surface of the epithelium and which
are in places aggregated into bundles; and (4) ganglion cells, small
nucleated cells connected with the nerve-fibrils. Of these structures
the two latter form the nervous elements, which in Asterids are directly
continuous with the more superficially placed columnar cells,

In Ophiurids the radial nerves, though having the same histological
structure as in Asterids, are quite distinct from the epidermis,
and separated from it by a thick layer of calcified dermis. Here also,
however, branches from the radial nerves can be traced into the tube
feet, where they form a layer immediately beneath the epidermis.
Whether an epidermic nerve-sheath or plexus is present on the
ambulacral surface has not, I believe, yet been demonstrated. I have
been led to suspect the existence of such a sheath on physiological
grounds, but have not yet seen it.

According to Baudelot,? in Ophioderma longicauda each of the nerves
to the tube feet gives off a branch, which “se portait en haut et en
arriére, et m’a paru se perdre dans la région dorsale du bras.”

In Echinids Krohn‘ was the first to show that the radial nerves,
which, like those of Ophiurids, are separated from the external epider-
mis by a thick layer of calcified dermis, give off branches, which
accompany the tube feet through the pores in the ambulacral plates,
and run in the substance of the tube feet as far as their free ends.

1 Tbelieve Greef was the first to show this in 1871.

2 Hamann, ‘‘Beitrige zur Histologie der Echinodermen,” ‘Zeitschrift fiir wissenschaftliche
Zoologie,’ Bd. xxxix, 1883.

? Baudelot, ‘‘ Etudes Générales sur le Systeme Nerveux,” Archives de Zoologie Expéri-
mentale,’ tome i, 1872, p. 208.

* Krohn, “Ueber die Anatomie der Nervensystem der Echiniden und Holothurien,”
* Archiv fiir Anatomie,’ 1841, translated in ‘ Annales des Sciences Naturelles,’ tome xvi, 1841.

292 PROFESSOR MARSHALL.

Lovén' describes the branches which accompany the tube feet as
spreading out on the external surface of the test to form a network of
fibres with numerous ganglion-cells. He figures a part of this external
nerve plexus in Brissopsis lyrifera, and says concerning it: ‘On
concoit que tous les rameaux du trone nerveux se divisant de cette
maniére, il y aura, répandu a la surface du corps, un systeme nerveux
pérviphérique extrémement développé, fournissant des nerfs aux radioles,
aux pédicellaires, aux clavicles des fascioles, et en général 4 toutes les
parties externes.”

Fredericq? was led, from a series of experiments on Echinus and
Toxopneustes, to suspect the presence “d’un plexus nerveux situé
dans l’épaisseur de la peau qui recouvre le test 4 lextérieur,” but did
not succeed in demonstrating its existence anatomically.

More recently Romanes and Ewart® have described experiments on
living Echini, which lead them to believe in the existence not only
of an external nerve plexus outside the test, but also of an internal
plexus on its inner surface; they further believe that the two systems
are connected by nerve-fibres running through the plates of the test. The
external plexus they figure* and describe “‘as lying almost immediately
under the surface epithelium, and extending from the shell to the
spines and pedicellariz ;” and in a postscript they state that they
“have been successful in obtaining full histological demonstration of

)

the internal nervous plexus of Echinus,” and promise full descriptions
of “its character, distribution, and mode of communication with the
external plexus.”®

Concerning Holothurids, both Krohn and Baudelot describe, in the
memoirs cited above, branches from the radial nerves to the tube feet.
More recently Hamann,° in the paper already quoted, has added
valuable details concerning the distribution of these branches. He
shows that the branches to the tube feet, which are at first situated,
like the radial nerves from which they arise, beneath the dermis, soon

1 Lovén, ‘Etudes sur les Echinoidées,” ‘Kongl. Svenska Vetenskaps Academiens Hands
lingar,’ Bandet ii, No. 7, Stockholm, 1874, p. 8, and pl. ii, figs. 30 and 31.

2 Fredericq, ‘‘ Contributions a 1’étude des Echinides,” ‘Archives de Zoologie Expérimeii-
tale,’ tome v, 1876, p. 438.

3 Romanes and Ewart, ‘‘On the Locomotor System of Echinodermata,” ‘ Phil. Trans.,’
1881, part iii.

4 Romanes and Ewart, loc. cit., p. 836, pl. 80, figs. 16—18. These figures are very different
to Lovén’s, which, however, were drawn from another genus.

5 Loc. cit., p. 882.

® Hamann, loc. cit., p. 168, and pl. ii, figs. 61, 52, 53,

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 293

pass through this, and expand to form nerve sheaths around the tube
feet and immediately beneath the external epidermis.

From the above descriptions it follows that the ordinary text-book
accounts of the Echinoderm nervous system, which mention the radial
nerves and the circumoral commissure, but nothing more, require very
considerable modification.

We have in addition to the Crinoids four well-marked groups of
recent Echinoderms, the Asterids, Ophiurids, Echinids, and Holothurids.
Of these four there is, I think, no doubt that the Asterids must be
regarded as the most primitive group, while the apodous Holothurids
are perhaps the most modified. This primitive character of the Asterids
is well illustrated by their nervous system, which as we have seen
above, is in the form of a continuous nerve-sheath, enclosing the whole
body, and directly continuous with the external epidermis of which it
forms the deepest layer. This nerve-sheath is thickened at certain
places, notably along the ambulacral grooves, where it forms the five
radial or ambulacral nerves. Such a condition of the nervous system
there is very strong reason for regarding as a very primitive one. It
occurs in a slightly modified form in many Ceelenterates ; it occurs in
that primitive group of Nemertines which Hubrecht proposes to call
Paleonemertini; it occurs also in the young of Sagitta and in several
other cases. Even in Vertebrates the central nervous system really
remains throughout life continuous with the epidermis, for the epithe-
lium lining the central canal of the cord and the ventricle of the brain,
was originally part of the surface epidermis.*

The fact that the Asterid nerve system remains in this primitive
condition is of considerable importance from two points of view; in
the first place it shows us the parent form from which the more
modified nervous systems of other Echinoderms must have sprung,
and thereby throws great light on the mutual relations of these several
forms; in the second place it is of special interest in connection with
the subject of the present paper, as showing that the Asterids are, in
at any rate one extremely important respect, far more primitive than
the Crinoids. I propose to say a few words on each of these points.

Starting with the Asterid nervous system it is easy to derive from
it, theoretically, the nervous systems of other groups. The sinking
down of the radial nerves in Ophiurids and Echinids may possibly be

1 Attention has recently been directed to this point by Sedgwick in the ‘ Proceedings of
the Cambridge Philosophical Society,’ vol. iv, pl. vi.

994 PROFESSOR MARSHALL.

connected with the development of the protective calcareous plates on
the ambulacral surface, while the similar position they hold in Holo-
thurids is probably due to the descent of this group from mailed
ancestors provided with calcareous ambulacral plates, a line of descent
for which there is a considerable amount of evidence forthcoming.
That the radial nerves of Ophiurids, Echinids, and Holothurids are
really the same things as the radial thickenings of the nerve-sheath in
Asterids, in spite of their difference of position, is practically proved
by the identical relations of the branches of these nerves or thickenings
to the tube feet, which branches in all cases alike form sheaths im-
mediately beneath the epidermis. The external plexus of Echinids
may clearly be viewed as a somewhat modified nerve-sheath ; and the
internal plexus of Romanes and Ewart, which is said to be connected
directly with the external plexus through the substance of the test,
may be explained as due to this nerve-sheath having commenced to
shift inwards, just as the radial nerves have done, but at present
remaining entangled in the substance of the calcified dermis.

As regards the origin of the Crinoid nervous system, I think that
the Asterid again gives us an important clue, though much yet remains
to be explained. It is commonly assumed that the subepithelial bands
of the Crinoid are homologous with the radial nerve-bands of an
Asterid, and I think the homology must be accepted when we consider
how absolutely identical the relations of these two structures are to
what is perhaps the most characteristic feature in an Echinoderm, ie.
the ambulacral system. The histological identity is an additional
argument, though of less weight, on the same side.

Accepting this homology as proved, the fact that Crinoids possess
part of a nerve-sheath in a primitive and unmodified condition is, to
my mind, strong reason for viewing them as descended from forms
which agreed with the recent Asterids in possessing a complete nerve-
sheath (though possibly very unlike Asterids in other respects) ; and
I am, therefore, disposed to regard the antambulacral nervous system
of a Crinoid, ze. the central capsule and axial cords with their
branches, as being derived from the antambulacral part of the primi-
tive nerve-sheath, and not as an entirely new set of structures
possessed by no other Echinoderms. A certain amount of evidence
can be adduced in support of this view. Dr. Carpenter has shown?
that in an early stage of development of Antedon the radials do not

1 Carpenter, ‘Phil, Trans.,’ 1868,

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS. 295

enclose the radial cords, but merely form calcareous plates between the
cords and the integument, which Jater on thicken, grow round, and
enclose the cords completely.t In this early stage the relations of the
radial cords are very similar to those of the ambulacral nerves of an
adult Ophiurid or Echinid,? and as the latter have certainly acquired
their adult condition by becoming detached from the epidermis and
shifting inwards, so also may the same process be supposed to have
occurred in the Crinoid. The subepithelial bands of the Crinoid retain
their primitive positions, but the delicate connective-tissue lamella
that sometimes separates them from the overlying epithelium in
Antedon rosaceus, and is a far more evident structure in Antedon
Eschrichtti and in Actinometra, probably represents the earliest stage
in the process by which the nerve becomes detached from the epidermis
and shifted inwards, Again, the external and internal plexuses of
Echinus, with their connecting fibres in the substance of the calcareous
test offer us a condition of things in some respects approaching that
of the Crinoid.

Concerning the morphology of the central capsule, I feel in much
more doubt. Dr. Carpenter’s observations lead to the belief that, at
any rate in its present form, it is connected with the change from the
pedunculate to the free-swimming condition ; and it is worthy of notice
that the two actions with which it has been found to be specially
concerned physiologically, ae. the movements of swimming and of
righting, are ones that the pedunculate form, from the very nature of
things, can never exercise.

While, however, this theory of the derivation of the system of the
central capsule and axial cords of a Crinoid, by concentration from the
antambulacral portion of a continuous nerve-sheath, renders a compari-
son between the Crinoids and the Echinoderms possible, it still leaves
the gap between the two groups a very wide one. Crinoids are some-
times compared with Asterids or Ophiurids, but they differ from both
these groups in a great number of points of fundamental importance.

In the absence of any representatives of ambulacral ossicles, the
convoluted character of the alimentary canal, the position of the anus,
the permanent communication between the ambulacral system and the
coelom, the replacement (functionally if not morphologically) of the

1 That this condition is a primitive one is shown by its occurrence in some of the Palzo-
crinoids in which the axial cords often lie in grooves, and not in canals in the calcareous

plates (Carpenter).
2 Of course they do not correspond to these.

EEG

296 PROFESSOR MARSHALL.

madreporic plate by a number of ciliated openings, we have, quite
apart from the entirely exceptional features of the nervous system, a
list of characters, which could be very easily added to, which mark off
the Crinoids as a group widely separate from the other Echinoderms.

When we bear in mind that in a number of these points the Crinoid
condition is not only not a primitive one, but a very highly specialised
one, the gap becomes wider still. |

I do not propose at present to pursue further this point, which has
recently been noticed by both the Carpenters, and will close my paper
by venturing to call attention to the great importance of supplementing
morphological and histological inquiries by direct experimental investi-
gations. In this age of specialisation there is a very real danger of
men confining their attention too exclusively to one side of the problems
they attack, to the entire neglect of others, which are not only of equal
importance, but which would in many cases yield them far more ready
clues.

Comparative physiology is a phrase which has become well-nigh
extinct ; but it is the name of a very real and very necessary science,
which only requires better opportunities for development, such as we
hope shortly to see forthcoming in this country, in order to yield results
of first-rate importance to morphologists and physiologists alike.

rei ay .
ovat:

Plate XIV.

A.M.Marshall, del. ¥. Huth, Lith? Edin*

THE NERVOUS SYSTEM OF ANTEDON ROSACEUS, 297

DESCRIPTION OF PLATE XIV.

The figures are diagrammatic, and are intended merely to show the
position and relations of the nervous system ; the other systems being
omitted, either wholly or in part. The nervous system is coloured
black in all three figures. In figures 1 and 3, I have borrowed ideas
from figures given by Ludwig (‘Morphologische Studien’): all the
figures are, however, constructed from camera drawings of my own
preparations, and the two mentioned will be found to differ in some
important points from the corresponding ones of Ludwig.

Alphabetical List of References.

a. Axial cord. 6. Branches of axial cord. c¢. Branches of axial
cord to pinnule. d. Central capsule. e. Branches from central capsule
to cirri. jf Chambered organ. g. Central plexus. &. Subepithelial
band (ambulacral nerve). 7. Ambulacral groove. & Tentacle. 7. Am-
bulacral canal. m, Subtentacular canal. x. Ceeliac canal. o. Pin-
nule. . Cirri. 1. Mouth. ss. Intestine. ¢ Ciliated openings
in body-wall. «. Muscle. C.D. Centrodorsal plate. &. Rosette.
R,. First Radial. A, Second Radial. &,. Third Radial. Br,. First
Brachial. Br, Second Brachial.

Fig. 1. Diagrammatic vertical section through the disc and base of ©
one of the arms of Antedon rosaceus, showing the relations
of the central capsule, axial cords, and subepithelial bands.
The section is interradial, 7.e. passes between two pairs of
arms, on the left side, radial on the right.

Fig. 2. Transverse section of an arm of Antedon rosaceus, passing on
the right side through the base of one of the pinnules.
The figure is diagrammatic as regards the branches of the
axial cord, which are filled in from a considerable number
of sections.

Fig. 3. Diagrammatic plan of the central capsule and its branches in
Antedon rosaceus.

ON THE REGENERATION OF THE VISCERAL MASS
IN ANTEDON ROSACEUS.

By Artuur Denpy, B.Sc., Associate of the Owens College.

It has been known for a long time that the visceral mass of certain
species of Comatule is separable from the calyx with great ease.
Thus Fabius Columna in 1592, speaking of the visceral mass of
Antedon, says: ‘‘ipsum vero et facillime disjungitur a stella.”! This
passage is quoted by Dr. W. B. Carpenter in his memoir on Antedon
rosaceus.? The separation of the visceral mass is easily brought about
artificially, as mentioned by Professor Milnes Marshall in his paper on
the nervous system of Antedon rosaceus ;* and in some species it has
been observed, on the Challenger expedition,* by Dr. Carpenter,° and
by myself, that the animals are not infrequently dredged up without
any visceral masses at all, these latter, in like manner, being some-
times dredged up by themselves.

Not only do these Comatule thus easily part with their visceral
masses, but it has been shown that Comatule which have lost their
visceral masses have the remarkable power of developing new ones
(this has been proved for Antedon rosaceus alone). This fact, suspected
by Dr. Carpenter many years ago,® was observed by Professor Marshall

1 Phytobasanus, sive Plantarum aliquot Historia, by Fabius Columna, Neapoli, 1592.
2 Phil. Trans. Royal Soc., 1866.

8 “*On the Nervous System of Antedon rosaceus,” Quart. Journ. Micr. Se., July 1884.
* Challenger Report, Narrative of Cruise, vol. I., Pt. I., pp. 310—811.

5 Proc. Royal Soc., Lond., vol. 24, p. 215.
® Phil. Trans. Royal Soc., vol. 24, p. 215.

300 ARTHUR DENDY.

at Naples in 1884, and is referred to by him in the paper already
cited.

Professor Marshall at once took steps to secure a complete series of
specimens, showing all stages of this regeneration, and on his return
placed his specimens in my hands for description. The greater part
of my work in connection with the subject has been done in the labora-
tory of the Owens College, under the direction of Professor Marshall,
to whom my best thanks are due. Unfortunately the series of speci-
mens. proved incomplete in many important respects, and in the
summer of 1885, in the hopes of completing it, I went up to continue
my investigations at the Scottish Marine Station for Scientific Re-
search, at Millport on the Clyde, being enabled to do so through the
kindness of Dr. Murray, Director of the Challenger Commission, to
whom I am much indebted for the valuable material which I was able
to collect there.

The following is a short account of the main features in the pro-
cesses of evisceration and regeneration as observed in Antedon rosaceus
(Comatula mediterranea), a species found very abundantly in the
Firth of Clyde.

Before proceeding, it may be well to say a preliminary word or two
with regard to the relations of the visceral mass to the arms and calyx
in an uninjured Antedon, although I shall have to deal with this ques-
tion more fully later on. The entire body of Antedon may be divided
roughly into three main parts, the calyx, the arms, and the visceral
mass. Of these parts the calyx appears to be the most important, for
when an arm is cast off it can easily be regenerated, and the same is
now known to be the case with the visceral mass. But no cases are
known of an arm or a visceral mass having been able to grow again
into a perfect animal after separation from the calyx. This fact, as
suggested by Professor Marshall,’ is to be connected with the fact that
the most important part of the nervous system is located in the calyx.
The concave surface of the calyx is lined by a thin layer of connective
tissue, on which the visceral mass rests and to which it is very closely
attached. The visceral mass when removed from the calyx, is a
roundish ball, from a quarter to half an inch in diameter and some-
what flattened on the ventral surface. This forms such a complete

1 Perrier, ‘‘Sur J’Anatomie et la Régénération des bras de la Comatula,” Archives de
Zoologie expérimentale, Tome II., p. 68.
2 Loc. cit., p. 22 (reprint),

THE VISCERAL MASS IN ANTEDON ROSACEUS. 301

thing in itself and looks so little like a mere part of an organism that
a specimen of the isolated visceral mass of one species of Comatula
was once described as a distinct animal. The dorsal surface of the
visceral mass, that is the one resting on the calyx, is covered by a
compact layer of connective tissue, which has a silky appearance on
the outside. In the centre of the dorsal surface is a minute hole
which serves for the passage of the central plexus of blood vessels
from the chambered organ into the visceral mass. The ventral sur-
face is covered by epidermis overlying connective tissue, and has the
mouth in the centre and the anal cone at one side. This layer of
epidermis and connective tissue, covering the ventral surface of the
visceral mass, has been called by Carpenter the “ oral perisome,” and
is continuous with a similar layer of tissue clothing the ventral sur-
face of each arm.

Tue Process oF EvisceraTion.—In some specimens of Antedon
rosaceus the visceral mass is very easily removed from the calyx by
pushing it aside with a blunt needle ; but it is a noteworthy fact that
this evisceration cannot be effected with equal ease in all cases, for,
while in some specimens the visceral mass comes away almost with
a touch, in others it adheres firmly to the calyx and cannot be brought
away at all except by tearing it to pieces.

While at Millport, I observed that specimens were dredged up by
the steam launch ‘ Medusa,” which had already lost their visceral
masses. At first I was inclined to think that the animals cast them
out of their own accord on being irritated, as some Holothurians cast
out their intestines, but this must be regarded as very doubtful, for,
owing to the slight manner in which they are generally attached to
the calyx the visceral masses might have been torn out by rough
treatment in the dredge. This view is perhaps supported by the fact
that isolated visceral masses have themselves occasionally been
dredged up.’

But although this evisceration is probably in most cases effected by
the dredge, yet I have very strong reason for believing that eviscera-
tion and subsequent regeneration of the lost visceral mass occasionally
occur quite independently of dredging operations. I have in my pos-
session a specimen of Antedon rosaceus, from the Firth of Clyde, in
which the visceral mass shows clear traces of having been regenerated ;

™ Challenger Report, Nav. of Cruise, vol. I., Pt. I., p. 311. f

302 ARTHUR DENDY.

this specimen was almost certainly dredged in its present condition :
it was certainly not intentionally eviscerated, and it is hardly possible
that it could have regenerated to such an extent in the aquarium
before I came to examine it, for owing to the great heat of the weather
and other causes I found it impossible to keep the Comatule alive in
the aquaria for more than a very few days. The specimen referred to
has arrived at the stage represented by one of the intentionally-evisce-
rated specimens from Naples which had been regenerating for about
nineteen days. In fact, although this Comatula in many respects
differs from a normal one in appearance, yet regeneration is fairly
complete. The differences may be enumerated as follows: (1) the disc
is of a very light colour, while in normal specimens it is usually con-
siderably pigmented ; (2) the disc is flat, while in normal specimens it
is usually strongly convex; (3) the lappets bordering the ambulacral
grooves around the mouth are very slightly developed ; (4) the anal
cone is very small; (5) the line of tear, caused by the removal of the
old visceral mass is still marked by the cessation of the pigment spots
bordering the ambulacral grooves. In all these particulars this speci-
men agrees with a regenerated specimen of about nineteen days and
differs from a normal one.

It has been already stated that in some specimens the visceral mass
may be very readily removed from the calyx, while in others it adheres
to it very tightly. I happened to be at Millport at what seemed to
be the height of the breeding season for Antedon rosaceus (July), the
pinnules being commonly much distended with ova which were fre-
quently discharged into the surrounding water in great numbers.

On examining my specimens after my return I noticed that out of
all which I had preserved of the specimens dredged without their
visceral masses, hardly one had the pinnules distended to any consider-
able extent with genital products. It appears to me also that usually
those specimens which most readily part with their visceral masses
are those in which the pinnules are least distended-with genital pro-
ducts, while, if the pinnules are much distended, the visceral mass is
more firmly attached to the calyx. It is perhaps possible to trace a
connection between these facts, for, when the energies of the animal
are being devoted to reproduction it would obviously be much less
capable of repairing the loss of such a large amount of tissue as is
represented by the visceral mass, and if evisceration were to occur at
this period it. would be likely to result in the death of the animal from

THE VISCERAL MASS IN ANTEDON ROSACEUS. 303

starvation, the whole alimentary canal being, of course, removed. In
order to avoid this danger we may imagine that the visceral mass at
this period becomes more firmly attached to the calyx. If this view
be correct, we must suppose that the loss of the visceral mags is by no
means an uncommon occurrence amongst Comatule, and I am strongly
inclined to believe that such is the case. What is the meaning of
this process, and whether it plays any special part in the economy of
the animal must for the present remain undecided ; I have, however,
thrown out a suggestion on this point at the end of the paper.

Errects oF EviscEraTion.—Removal of the visceral mass appears
to affect the animals but little. At first they fold their arms over the
calyx, but this condition does not generally last for long, and in an
hour or two, when they have once recovered from the shock, their
vigour seems to be unimpaired.

It is evident that evisceration involves tearing of the epidermis and
dermis, which together form the oral perisome, in a more or less
regular circle all round where the arms join on to the disc, and that
the ambulacral grooves and nerves, together with the ambulacral
vessels, must also be torn across. The central plexus of blood vessels
must also be broken across at some part of its course. The visceral
mass, as it appears after its removal from the calyx, has been already
described. Isolated specimens of the visceral mass of one species of
Comatula, dredged on the Challenger expedition, were observed by Sir
Wyville Thomson to perform slow creeping movements ;! but I have
noted nothing of the sort in Antedon rosaceus—on the contrary, the
visceral masses appeared to remain quite still, and slowly decay.

The central capsule, together with the nerves radiating from it to
the arms, is uninjured by the operation. It will be convenient for
purposes of reference to speak of the shallow, empty cup, left by the
removal of the visceral mass, as the “ visceral basin.”

The earlier stages in the process of regeneration will perhaps be
rendered clearer if I first of all give a somewhat more particular
account of the appearance presented by a freshly eviscerated specimen.
The following description is taken from a specimen which was dredged
in the eviscerated condition, but will apply equally well to a specimen
upon which the operation has been intentionally and successfully
performed. The line of tear, along which the oral perisome has been

> Challenger Report, Narr. of Cruise, vol. I., Pt I., p. 311.

_—_?

————

==

=

304 ARTHUR DENDY.

separated from the integument of the arms, is very distinctly marked.
Each ambulacral groove is torn sharply and suddenly across; gene-
rally at about the level of the base of the first (oral) pinnule. Between
any two adjacent arms, or pairs of arms, is a tightly stretched, trans-
lucent membrane, bounded by a free, incurved edge which marks the
line of tear interradially and radially. These membranes form an
important part of the floor of the visceral basin, and assist in sup-
porting the visceral mass. It will be seen that the line of tear forms
the edge of the visceral basin, This edge does not maintain an even
curvature all the way round, but is deeply notched. There are ten
of these notches, or bays, one between the two arms of every pair
(radial), and one between each two adjacent pairs of arms (interradial),
the latter being a good deal deeper than the former.

The floor of the visceral basin is lined by an exceedingly thin, trans-
parent membrane, of connective tissue, through which a series of
muscles is very distinctly visible. These are the muscles between the
calcareous plates of the calyx, and they are arranged in two concentric
circles. The inner of these two circles is made up of five pairs of
muscles, connecting the first and second radial plates in pairs. The
outer circle is composed of ten pairs of muscles, which connect in
similar manner the third radials with the first brachials. The degree
of distinctness with which these muscles are visible, beneath the
overlying regenerating tissues, forms, during the earliest stages, a
good indication of the amount of regeneration which has taken place.

On examining sections of a freshly eviscerated Antedon, and com-
paring these with sections of an uninjured specimen and of the isolated
visceral mass, it is fairly easy to determine the exact region in which
the separation of the visceral mass occurs. In sections of a freshly-
eviscerated Antedon the surface of the visceral basin, including the
muscles above described, is seen to be covered by a thin, smooth layer
of connective tissue, excepting in the centre, within the inner circle of
muscles, where this layer is incomplete. Here we have a deep pit,
the sides of which are formed by the first radial plates and the floor
by the perforated rosette. This pit is in the narrowest part a little
over half a millimetre broad and, in the one specimen I have mea-
sured, four fifths of a millimetre deep. The central plexus sticks up
into it through a hole in the middle of the rosette and is surrounded
by strands of connective tissue, which, attaching it to the walls of the
pit, support it. In the specimen measured the central plexus projects

THE VISCERAL MASS IN ANTEDON ROSACEUS. 305

above the surface of the rosette plate to a height of three-fifths of a
millimetre. The strands of connective tissue which support it appear
to be continuous with the organic basis of the first radials,

In sections of the isolated visceral mass we find that the aboral sur-
face is covered by a thin layer of connective tissue, which has on the
outside a very distinct outline, caused by an extremely thin, deeply
staining layer which is possibly epithelial. In sections of an uninjured
Antedon this layer is seen to rest on the layer of tissue already de-
scribed as forming the floor of the visceral basin, and is connected
with it by very delicate bands of connective tissue which run across
from the one to the other.

Thus, in the process of evisceration the separation of the visceral
mass takes place between two layers of connective tissue—the one
lining the surface of the visceral basin and the other covering the
aboral surface of the visceral mass.

Tue REGENERATION OF THE Lost ViscERAL Mass.—Unfortunately
the series of specimens at my disposal is still very incomplete and it
is as yet impossible to give a complete account of this process. For
the sake of convenience I*shall divide the description into several
parts, according to the length of time for which the specimens treated
of have been regenerating ; but it must not be thought that all speci-
mens which have been regenerating for the same length of time have
arrived at the same stage in the process; this is by no means the case,
and I find that different specimens of the same date present consider-
able variations. The regenerated specimens of forty-three hours are
from the Firth of Clyde, those of later dates from Naples.

Forty-three hours.—The connective tissue lining the floor of the
visceral basin has begun to thicken. Asa result of this the two circlets
of muscles already described are much less distinct than in a freshly-
eviscerated specimen, although still plainly visible. The thickening
is more marked around the edges of the injured area than in the
centre, so that the inner circle of muscles is much more distinctly
visible than the outer one; in one specimen of this date the outer
circle of muscles is nearly hidden by the ingrowing mass of connective
tissue. The line of tear, along which the visceral mass has been sepa-
rated from the calyx, no longer appears sharp and distinct.

Two days.—The new growth of connective tissue, which had com-
- menced at forty-three hours, has proceeded further. The circlets of
Vv

306 ARTHUR DENDY.

muscles on the floor of the visceral basin are both completely hidden
by the newly formed tissues. The line of tear is marked by the
sudden stopping of the lappets and pigment spots along the borders
of the ambulacral grooves.

In sections we see that the surface of the regenerating cushion of
tissue is covered by a thin, deeply staining layer, which is apparently
formed by the arching over, and ingrowth of the edges of the injured
area. This layer is formed of two parts, an upper layer, which is
probably epidermal, and a lower dermal layer. The deeper, connec-
tive tissue layer is continuous with strands of the same substance
which have grown out from the floor of the visceral basin. In the
centre, above the chambered organ, the superficial layer of this regene-
rating visceral mass is still incomplete; so that on looking at it from
above two or three little holes are visible in the centre, leading down
to the chambered organ. The regenerated cushion of tissue is still
very thin, averaging in thickness only about half a millimetre.

Regeneration, then, appears to commence in two chief ways; (1)
by a series of outgrowths from the thin layer of connective tissue
which forms the floor of the visceral basin; (2) by an ingrowth of
connective tissue and epidermis from the edges of the injured area,
forming a roof to the visceral basin.

Three days.—Of this date I have two specimens, of both of which
I have sections, but the one is so much further advanced in regene-
ration than the cther that I shall treat them as two separate stages,
taking the less advanced one first.

(a) The new visceral mass is seen to have grown and has thickened
considerably. The openings leading down to the chambered organ
are now closed over. In the centre of the regenerating visceral
cushion is a small opaque papilla, probably due to thickening of the
epidermis preparatory to the formation of a mouth, but unfortunately
the sections are not sufficiently well preserved to decide this point.
Sections show that regeneration has proceeded as far as the formation
of a cushion of loose connective tissue, much thicker than at two days
and covered by the deeply staining epidermic layer before mentioned.
The growth of connective tissue is especially strong underneath the
point where the mouth is subsequently to be formed ; so that we have
here a pillar of denser connective tissue, reaching from the top of the
rosette plate to the roof of the visceral basin and thickest just beneath
the epidermic layer. The central plexus is very well shown, but in

THE VISCERAL MASS IN ANTEDON ROSACEUS. 307

these sections, and also in those of later date, I have no means of
telling how much of it, if any, has regenerated and how much was left
after evisceration.

The ambulacral system has begun to regenerate, as is shown by the
presence of new ambulacral pores. On examining one of these more
closely, it is seen to be vesicular, lined by columnar epithelium and
opening to the outside. No internal opening is yet visible.

(6) This specimen’ is much more advanced than the preceding one,
in that mouth and alimentary canal are already present; in other
respects it agrees with it, so that it will only be necessary here to
speak of the alimentary canal. In the centre of the visceral mass is a
depression ; the epidermis covering the depressed area is thickened
and in the centre is a small mouth leading obliquely downwards into
a feebly developed alimentary canal. The alimentary canal has
already the characteristic form found in normal specimens; that is to
say it takes about one turn in an almost horizontal plane and ends in
an anus placed interradially. The most remarkable feature about it
is the rudimentary condition of its walls, In histological characters
the walls of the alimentary canal are almost indistinguishable from the
roof of the visceral basin, and to all appearance have been formed by
an invagination of this roof. The roof of the visceral basin is com-
posed of two layers ; externally a thin layer of minute, rounded, deeply
staining cells which are probably epidermic, and beneath this a thicker
layer of connective tissue which stains less deeply. In the floor of
the central depression already mentioned, the epidermic cells show a
slight tendency to become columnar. The walls of the alimentary
canal are composed of exactly the same two layers, a layer of more
deeply staining cells which corresponds to the epidermic layer of the
roof, but is now, of course, on the inside, and outside this a layer of
connective tissue. The cells of the inner layer show a tendency to
become columnar, and this layer is more distinct than the epidermic
layer of the roof. The lumen of the alimentary canal is very small
and its walls are very thin and but slightly folded. There is no trace
of an anal cone; the anus itself is represented by a minute perforation
in the roof of the visceral basin. The walls of the last part of the

1 The great amount of regeneration which has taken place inthis specimen compared with
the other of the same date suggests the possibility that some mistake may have been made,
either by myself or at Naples, with regard to the time of regeneration. There seems, how-
ever, to be no doubt that considerable variation in this respect does occur.

308 ARTHUR DENDY.

intestine, just before reaching the anus, are quite indistinguishable
from this roof.

I have thus no decisive evidence to bring forward as to the manner
in which the alimentary canal is formed ; but there appears to me to
be a strong probability in favour of the view that it is formed by in-
vagination, the inner, glandular layer of its walls being formed from
invaginated epidermis and the outer connective tissue layer from the
underlying dermis.

Five days.——This specimen agrees closely with the last. The ali-
mentary canal appears to be in nearly the same condition, It is very
small and almost solid, the inner layer of its walls being composed of
minute, deeply staining, nucleated cells, more or less rounded in shape
and showing a slight tendency to become columnar in places, and
being histologically indistinguishable from the tissue covering the
general surface of the disc. Here again the alimentary canal appears
to have been formed by invagination, accompanied by rapid prolifera-
tion of the cells of the epidermis. In the specimen described this
proliferation has formed a thick mass of minute cells projecting on the
surface at one side of the mouth. No anal cone is as yet visible.
The terminal portion of the intestine touches the roof of the visceral
basin, but I found no definite opening through; but it must be remem-
bered that the anus, on its first appearance, is so minute a structure
that it might easily escape observation, especially in imperfectly pre-
served sections.

No ambulacral nerves or canals are as yet visible round the mouth.
A number of new ambulacral pores have been formed, apparently by
invagination. The epithelium lining them is at first not distinctly
columnar.

Nine days.—Externally the boundaries of the ambulacral grooves
are seen to have met and formed a pentagon around the mouth. They
enclose a slightly depressed area with the mouth in the centre, and
appear as thickish white ridges, in some places notched, showing where
lappets are beginning to regenerate.

It appears to me that the ambulacral grooves are, from the first
commencement of regeneration, left as areas along which the thicken-
ing of the regenerating tissue is not so great as elsewhere. This
thickening takes place centripetally, in five distinct areas, one in each
interradius; as these grow inwards the ambulacral grooves and the
central depression around the mouth are left as less thickened portions.

THE VISCERAL MASS IN ANTEDON ROSACEUS. 309

The mouth is oval, runs obliquely downwards, and is now for the first
time very distinctly bounded. The beginning of the anal cone is
visible as a small conical papilla placed interradially.

In sections the alimentary canal is seen to be fairly well developed;
it contains food refuse, indicating that it has begun to be used again.
The epidermis covering the depressed area round the mouth is now
composed of columnar, nucleated cells, and passes gradually into the
inner lining of the cesophagus, the cells of which are still more colum-
nar. The walls of the stomach are also composed of the usual two
layers, an inner layer of columnar, nucleated cells, and an outer layer
of connective tissue ; this outer layer is connected with the loose con-
nective tissue filling the body cavity. In the specimen now described
the cells forming the inner wall of the alimentary canal have become
columnar right up to the anus. The anus is very minute and situated
on the top of a small papilla. The columnar cells lining the cavity
of the anus are very small and pass gradually into the epidermic cells
covering the rudimentary anal cone ; the epidermic layer at this point
is thickened. In the region of the stomach the walls of the alimen-
tary canal are beginning to become folded, especially on the axial side,
and on this side also the glandular layer is thicker.

In another specimen of this date, which appears not to have ad-
vanced quite so far in regeneration, the alimentary canal, as it ap-
proaches the anus, narrows very much; its inner wall, which in the
first part of its course is composed of the usual columnar cells, here
consists of a layer of very minute cells, such as line the whole of the
canal at an earlier date, and identical with the cells covering the
general surface of the disc. In thickness this inner layer is very
irregular and the cells composing it appear to be rapidly proliferating.
At its extreme end they block up the lumen of the canal, so that the
latter appears solid, and the end of the canal fuses indistinguishably
with the layer of similar cells covering the general surface of the disc.
At the point of fusion there is a thick masy of these minute round
cells, forming a little elevation on the surface of the disc. This indi-
cates the position of the future anal cone. In this specimen I have
been able to find no distinct anal opening. These facts appear to me
to support the view that the alimentary canal is formed by invagi-
nation from its oral end.

Twelve days—No great advance has been made on the condition
presented by a nine days specimen. In one specimen the pentagon

310 ARTHUR DENDY.

round the mouth, formed by the lips of the ambulacral grooves, has
become notched into lappets all the way round and in sections the
alimentary canal is seen to have become very considerably complicated
and folded upon itself. In sections of another specimen of this date
the ambulacral epithelium and canals can be traced across the disc
very nearly to the lip of the mouth. They present the same features
as in normal specimens ; thus the canals exhibit the transverse muscle
fibres found in ordinary specimens. The anal cone is still very
small.

Nineteen days.—Little advance is to be noted except in the further
erowth of the anal cone, which is now fairly well developed. Sections
show a large body cavity in the anal cone, surrounding the terminal
portion of the intestine, which latter is attached to the body wall by
strands of connective tissue.

Twenty-one days.—In sections of a specimen of this date a blind
diverticulum is visible, given off from the alimentary canal at the junc-
tion of the stomach with the cesophagus, as described by Ludwig in the
normal animal.t There is little to distinguish a regenerated specimen
of this date from a normal Antedon, excepting the smaller size of the
visceral mass and the want of pigment upon it.

COMPARISON WITH OTHER EcHINODERMS.—So little has as yet been
written on the regeneration of lost parts in the Echinodermata that it
is difficult to make any comparison between Antedon and other forms
in this respect. The most important paper as yet published on the
subject appears to me to be that by Professor Ernst Haeckel on Comet
Forms.”

It is a noteworthy fact that comet forms are, so far as I know, only
described as occurring in Asteroids (e.g. Linckia, Brisinga, Asteracan-
thion), none being described in Ophiuroids. Schizogony, on the other
hand, is known to occur in both groups (eg. Asteracanthion and
Ophiactis). .

Tn the formation of a comet form a single arm of a star-fish, after
separating itself from the disc, produces, by budding at the proximal
end, from the wounded surface, new arms; then a new disc appears
and thus the perfect condition is again attained. The mouth is at

1 Ludwig, Morphol. Stud. an Echinoderm.
2 “Die Kometenform der Seesterne und der Generationswechsel der Echinodermen,”
Zeit. fiir wiss, Zool., Bd. 30, Suppl. 1878,

THE VISCERAL MASS IN ANTEDON ROSACEUS, Stl

first formed simply from the open end of the gut diverticulum which
extends into the arm.

The non-occurrence of this phenomenon in Ophiuroids appears to
me to be due to the absence of any gut diverticula in the arms of the
latter, so that a separated arm is entirely without any portion of the
alimentary canal, by the growth of which the remainder can be regene-
rated. Schizogony, however, occurs in both Asteroids and Ophiuroids,
probably because, in this case, the disc itself divides into two parts
and hence each of the resulting halves will contain a portion of the
alimentary canal, whether or not the latter be produced into the arms.

It thus appears that regeneration, so far as it is known, in Asteroids
and Ophiuroids is very different from that which takes place in An-
tedon, for, in the two former groups regeneration never appears to
occur when the alimentary canal has been entirely removed.

Unfortunately Professor Haeckel has only been able to examine the
external characters of the comet forms described by him; further
research will probably throw much light on the subject, and, if comet
forms should ever be found to occur in Ophiuroids, it may be possible
to institute a closer comparison in respect of regeneration between
Antedon and Ophiuroids than between Antedon and Asteroids,

On the regeneration of lost individual arms in Asteroids and Ophiu-
roids it is needless to dwell; this phenomenon finds a parallel in the
regeneration of the arms of Antedon.

As yet little is known concerning the ejection and regeneration of
the viscera, including the alimentary canal, in Holothurians, beyond
the mere fact that it does occur, and this was shown to be the case by
Dalyell.’ It is possible that we have here a more exact parallel to
what takes place in Antedon than in Asteroids or Ophiuroids.

I have stated above that the meaning of the process in Antedon,
supposing it to be a normal occurrence, must for the present remain
undecided; suffice it here to offer, as a suggestion, the only explanation
which seems at all probable. Crinoids, unlike most Echinoderms, have
no selective power over their food supply; all sorts of food, good or
bad, wholesome or poisonous, are carried into the alimentary canal by
the action of the cilia in the ambulacral grooves. If any irritating or
poisonous particles, or even any dangerous parasite, were conveyed by

1 The Powers of the Creator, displayed in the Creation, London, 1851, vol.i., p. 49 et seq.
It was only after considerable difficulty that I obtained this reference, for which I am
indebted to Professor F. J. Bell, who also pointed out to me Semper’s remarks on the
subject; Reisen in Archipel der Philippinen, vol. i., Holothurien, p. 200 et seq.

312 ARTHUR DENDY.

this means into the alimentary canal, as might easily be the case, the
only way in which the animal could rid itself of the obnoxious matter
would be to cast out the alimentary canal, and this, owing to the
structure and relations of the parts concerned, which have probably
been specially adapted for the purpose, is most simply and readily
effected by the rejection of the entire visceral mass. This appears
to me to be a not improbable hypothesis, and the only one that will
explain the facts.

SOME INVESTIGATIONS ON THE PHYSIOLOGY OF THE
NERVOUS SYSTEM OF THE LOBSTER.

By ©. F. Marsuatt, B.Se., Platt Physiological Scholar in the
Owens College.

The following investigations were commenced in the Physiological
Laboratory of the Owens College during the past winter, under the
direction of Professor Gamgee and Professor Milnes Marshall, with
a view to determine the following points in the Physiology of the ner-
vous system of the Lobster :—

1. Are there distinct motor and sensory roots to the nerves arising
from the central nervous system, similar to those by which the spinal
nerves arise in vertebrates ?

2. Is there any marked decussation of the nerve fibres in the central
nervous system ?

The determination of these points in any invertebrate animal is a

‘matter of considerable morphological importance.

No definite results were obtained from the Lobsters experimented
upon at the Owens College, for the reason that they were not in a fit
state for experiment by the time they reached the laboratory.
However, through the kindness of Mr. J. T. Cunningham, I was
enabled during last August to continue my researches at the Scottish
Marine Station at Granton, a spot which is eminently suited for
carrying on experiments of this kind. There I was able to obtain
perfectly fresh lobsters which were kept in tanks through which sea
water was constantly circulating. I must here express my thanks to

314 Cc. F. MARSHALL.

Mr. Cunningham for much advice and help in conducting the experi-
ments, nearly all the more important of which were witnessed by him.

As this paper does not profess to be a complete account of the
physiology of the nervous system of the lobster, I shall not attempt
to give an exhaustive account of the literature of the subject, but only
refer to such papers as bear directly on the questions stated above.
The most important paper dealing with these questions is one by
Emile Yung, entitled “ Physiologie de la chaine ganglionaire chez les
crustacés.”+ This paper I was unfortunately unable to obtain access
to till after my experiments had been completed. Although many of
my results were anticipated by Yung, yet, since my methods of in-
vestigation differ for the most part from those employed by him, I
think it worth while publishing them. Confirmatory results arrived
at independently are always of value in experimental physiology.

Two other papers, one by Richet, “Contribution & la physiologie
des centres nerveux et des muscles de I’écrévisse,”? and another by
Fredericq et Vandevelde, ‘‘ Physiologie des muscles et des nerfs du
homard,” * deal only with the phenomena of muscle and nerve and the
various conditions of muscular excitability, and do not bear on the
subject of this paper. Mr. J. Ward has written a short paper “Some
Notes on the Physiology of the Nervous System of Astacus Fluvia-
tilis.”* His results were obtained by cutting the nerves and replacing
the animals in running water and observing the subsequent movements.
Mr. Ward came to the conclusion that there was no evidence of decus-
sation of nerves in the central nervous system of the Crayfish. The
most recent contribution is a short note by Reichenbach, ‘ Beobach-
tungen tiber die Physiologie des Nervensystems vom Flusskrebs.”®
This I was unfortunately unable to obtain.

I shall now give a detailed account of my investigations dealing
first with the anatomy and then with the physiology, and compare my
results with those obtained by Yung.

A, ANATOMY.
1. Thoracic Nerves.— Arising from each of the thoracic ganglia, with
the exception of the supra- and sub-cesophageal ganglia, there are two

1 Archives de Zoologie, t. VII, 1878.

2 Archives de Physiologie normale et pathologique, t. VI, 1879.
8 Bulletin de VAcadémie royale de Belgique, 2° série, t. 47, 1879,
* Journal of Physiology, vol. I1., 1879-80.

5 In Humboldt, Bd. I., p. 26-27, 1883.

THE NERVOUS SYSTEM OF THE LOBSTER. 315

nerves on each side of considerable size which pass to the limb of the
corresponding side. Besides these there are other smaller nerves dis-
tributed to the wall of the thorax, but these do not concern the pre-
sent question. Of these two nerves the one is anterior and the other
posterior with regard to the normal position of the animal: but both
nerves arise in the same horizontal plane, z.e. one is not dorsal to the
other. The anterior nerve is much the smaller in the great chelae and
first and second ambulatory legs, where the posterior nerve is of great
thickness. In the last two ambulatory legs the two nerves are nearly
the same size. Throughout this paper the anterior will be spoken of
as the “small nerve” and the posterior as the “large nerve.”

On tracing these two nerves to their distribution in the large chelae
in specimens hardened in alcohol, the small nerve seemed to consist of
two parts, one of which could be traced to the divaricator muscle (ad-
ductor of Huxley +) which opens the claw. The other part of the nerve
appeared to go to the skin. But no such division into two parts was
found in the fresh specimen. The small nerve also supplies the
extensor muscles of the various joints of the limb.

The large nerve gives off branches to the flexor muscles of the limb,
but is chiefly distributed to the large occlusor muscle (adductor of
Huxley’) which closes the claw. Also there are numerous branches
to the skin of the claw.

In the case of the first or sub-cesophageal ganglion the nerves are so
crowded together that it is difficult to determine whether the nerves
to the masticatory appendages which arise from the ganglion are simi-
larly arranged.

2. Abdominal Nerves.—From each abdominal ganglion two nerves
arise on each side in a similar manner to the nerves of the thorax.
The anterior of these is distributed to the abdominal appendage of the
Same side, and in specimens hardened in alcohol appeared to consist
of two parts, but no such division was found in fresh specimens.

The posterior nerve, after passing between the abdominal muscles,
terminates in the skin along the side of the abdomen.

The abdominal muscles themselves are supplied by nerves which
arise chiefly from the abdominal nerve cord between the ganglia:
but according to Yung the superior (posterior) nerve ramifies in the
muscle of the abdomen.”

1 Huxley: The Crayfish, p. 93.
2 Loc. cit., p. 487,

316 Cc. F. MARSHALL.

In the case of the last abdominal ganglion there are two nerves on
each side passing to the telson, corresponding to the two divisions of
the nerves to the other abdominal appendages. There is another
nerve on each side passing to the skin corresponding to those from
the other abdominal ganglia, but differing from them in being placed
anterior to the nerves to the appendage. There are also several nerves
arising from the posterior end of the ganglion.

B. PHysioLoey.

All experiments were performed on the lobster immediately after
removal from the tanks. A “holder” consisting of a vertical block of
wood fixed in the centre of a board was employed, on which the animal
was secured by means of pieces of tape attached to screws placed in
convenient positions. ‘The chief difficulty experienced was the rapidity
with which the nerves when exposed lose their power of transmitting
the nervous impulse, the protoplasm of the nerve appearing to disin-
tegrate very rapidly. Another difficulty was the presence of the large
quantity of blood pouring out of the wounds which rapidly coagulated
and so hindered the operations.

The first difficulty was overcome after some practice by opening the
limbs and exposing the nerves as rapidly as possible. Also by opening
the thorax and letting out some of the blood, the amount pouring out
of the limbs was much diminished. For purposes of stimulation, a
Du Bois Reymond Induction coil with Magnetic Interruptor and a
single Daniell Cell were used. The electrodes used were of Platinum
wire. In most cases silk ligatures were used for the purpose of lift-
ing the nerves on to the electrodes.

I found very little trouble from the animal’s claws, because from
their great weight they cannot be moved quickly when out of water.

1. EXPERIMENTS ON THE NERVES IN THE LARGE CHELAE.

The chief experiments on the investigation of motor and sensory
roots were performed on the nerves of the large chelae, since from
their large size they are most suitable for experiment. The nerves
are most easily exposed on the ventral surface of the third joint of the
limb, where they lie close to the anterior border. The animal in these
experiments was in the supine position.

Experiment A.—The small nerve being intact, the large nerve was
ligatured and cut. On stimulating the distal end of the latter nerve

THE NERVOUS SYSTEM OF THE LOBSTER, 317

the limb was sharply raised and the claw closed. On stimulating its
central end there ensued sharp reflex movements of the thoracic appen-
dages, flapping of the abdomen and slight movements of the antennae
and masticatory appendages. Also in the most lively animals there
was a distinct opening of the claw.

This experiment shows: (1) that the large nerve contains the motor
fibres to the muscles which raise the limb and close the claw; (2) that
this nerve contains many afferent fibres and is hence mixed ; (3) that
it contains special afferent fibres which cause opening of the claw by
reflex action through the small nerve which alone supplies the divari-
cator muscle of the claw.

Haperiment B.—The large nerve being intact the small nerve was
ligatured and cut. On stimulating its distal end the claw was sharply
opened and the limb extended. On stimulating the central end the
claw was elosed and the limb raised. In some cases slight movements
of the thoracic and abdominal appendages were observed, but in most
cases there were no appreciable general movements.

This experiment shows (1) that the small nerve contains motor
fibres which supply the extensor muscles of the limb and especially
the divaricator muscle of the claw ; (2) that it contains afferent fibres
which cause reflex contraction of the claw through the large nerve
which supplies the occlusor muscle. This nerve is therefore mixed
also. Thus these two experiments show that each of the two nerves
is mixed, and hence separate motor and sensory roots do not exist
in the lobster.

Experiment C.—The following experiment shows that there is a
distinct sense of touch in the claws. In an active lobster the eyes
and antennae were removed and the animal again placed in the tank.
The animal then moved round and round in circles and appeared
unable to walk in a straight line unless guided by the side of the tank.
When a stick was placed in front of it, it did not grip the stick as it
did before the operation, but directly the stick was placed within the
claws the latter closed upon it; thus showing that there is a distinct
sense of touch in the claw, in spite of the great thickness and hardness
of the calcareous cuticle. This fact one would presuppose from the
large number of nerves which pass to the skin of the claw and which
are chiefly derived from branches of the large nerve.

Experiment D.—Mr. Cunningham found that on placing his finger
in the claw after the small nerve was cut he could feel a distinct

318 C. F. MARSHALL.

closing of the claw. ‘This was repeated several times. This experi-
ment again shows that the large nerve contains both afferent and
efferent fibres: for we have seen above that the skin is sensitive to
external stimuli, and these passing up the large nerve must have
caused closing of the claw by reflex action through efferent fibres in
the same nerve, because the small nerve was cut.

Lxperiment E.—In an active lobster the circum-cesophageal com-
missures were cut on both sides. This operation was performed by
making an aperture in the thorax and cutting with scissors where the
commissures were supposed to lie. That the commissures were really
cut was confirmed subsequently by post mortem examination, a pre-
caution the necessity of which is evident. The connection between
the cerebrum and the rest of the nervous system was thus severed.

The experiments A and B were then repeated. On cutting open the
limbs there ensued general movements much more marked than before,
the opposite limb being even brought over to the place of operation
manifestly for the purpose of removing the operating instruments.

On stimulating the distal end of the small nerve the claw was
sharply opened, and on stimulating the central end of the same nerve
the claw was sharply closed by reflex action through the large nerve.

In the opposite limb on stimulating the central end of the large
nerve the claw was sharply opened and the limb extended by reflex
action through the small nerve: and on stimulating the distal end of
the large nerve the claw was closed and the limb raised.

All these actions were much more strongly marked and more regular
than in the case of the animals in which the connection with the cere-
brum was intact. This experiment shows that all the effects described
above are due to reflex action, and are more regular in action and
more strongly marked when the cerebrum is cut off from the rest of
the nervous system; doubtless because inhibitory impulses which
might pass down from the cerebrum are stopped when the cesophageal
commissures are cut. This no doubt explains the fact that in some
of the animals experimented upon the effects described in A and B did
not all take place.

Experiment F.—In another lobster the nerve chain was cut both in
front of and behind the second thoracic ganglion: the latter ganglion
which supplies the chelae was thus isolated (confirmed by post mortem
examination). The experiments A and B were performed on this animal
with the same results (excepting, of course, the general movements of

THE NERVOUS SYSTEM OF THE LOBSTER. 319

the body and appendages). But the effects were much less marked
than in the case of E. The reflex divaricator action on stimulating
the central end of the large nerve was slight. The reflex closing of
the claw on stimulating the central end of the small nerve, however,
was distinct, and the effects on stimulating the distal end of each
nerve the same as usual. The animal in this case was soon exhausted.

This experiment shows that the second thoracic ganglion acts as a
special reflex centre for the great chelae; since the reflex closing and
opening of the claw on stimulating the central end of the small and
large nerve respectively take place when it is isolated. This is pre-
sumably the case with the other thoracic ganglia,

Yung came to a similar conclusion with regard to these ganglia ; he
says :-—“‘Chaque ganglion est un centre de sensibilité et de mouve-
ment pour le segment du corps auquel il appartient ; mais la sensi-
bilité est inconsciente et les movements réflexes lorsque le ganglion
est séparé de ceux qui le précedent” (pp. 492, 493). “Les gan-
glions thoraciques se comportent comme les ganglions abdominaux
pour les membres de leur segment respectif. Leur destruction en-
traine l’abolition des mouvements volontaires dans les appendices
situés en arriere” (p. 525). These results were obtained chiefly by
cutting the chain at various points and observing the subsequent
effects.

2. Tue NERVES IN THE AMBULATORY Lips.

Experiments on the nerves in the other ambulatory limbs were not
carried on in the same detail as in the case of the chelae, for the reason
that the nerves are much more difficult to expose.

On stimulating either nerve movements took place in the limb and
also general reflex movements of the body and appendages ; the latter
movements being strongly marked in the case of the large nerve, but
slight when the small nerve was stimulated. When the large nerve
was stimulated the limb as a whole was flexed, and extended when
the small nerve was stimulated.

Thus each nerve is mixed as in the case of the chelae.

3. THE ABDOMINAL NERVES.
The nerves arising from one of the abdominal ganglia were exposed
and cut. Sharp flapping of the abdomen took place when the central
end of either nerve was stimulated, but the effects were stronger in

320 Cc. F. MARSHALL,

the case of the posterior nerve which supplies the skin. On
stimulating the distal end of the latter nerve no effect was observed.
On stimulating the distal end of the anterior nerve which supplies
the abdominal appendage, the latter contracted. Hence the anterior
nerve would seem to be mixed, but the posterior nerve purely sensory.

These results differ somewhat from those obtained by Yung. He
states that mechanical excitation of the two nerves produce the same
effect, each nerve being mixed (p. 487).

With regard to the investigation of motor and sensory roots, Yung
does not appear to have experimented upon the nerves arising from
the thoracic ganglia. He says:—‘“ Les racines des nerfs irradiant de
la chaine ventrale sont 4 la fois motrices et sensitives” (p. 525) ; but
this result was obtained only on the nerves arising from the abdominal
ganglia (p. 486). This result, as stated above, differs slightly from
the result I obtained from the abdominal nerves, one of which I found
to be mixed and the other purely sensory. Most of Yung’s experi-
ments on the thoracic ganglia seem. to have been performed with
a view to investigate the statement previously made by some
investigators that the superior and inferior surfaces of the ganglia
held a different function with regard to motion and _ sensation.
On page 525 we find :—“L’opinion classique, que la face inférieure
de la chaine est sensitive, tandis que la face supérieure serait motvrice,
est infirmée par nos expériences.”

4, EXPERIMENTS ON DECcUSSATION.

(A) The cesophageal commissure of one side was cut and the an
terior end stimulated ; the antennae and antennules of the same side
moved sharply, those of the opposite side slightly. The same effect
took place when the other commissure was cut. Also the effects were
similar when the stimulation was performed on the other side.

On stimulating the posterior end of each commissure, the thoracic
appendages moved mainly on the side stimulated, but slightly also on
the opposite side. The same effects were observed when the commis-
sures joining the separate ganglia were stimulated.

These experiments show that there is no marked decussation of the
fibres as in vertebrates, for if it were so the effect would be more
marked on the opposite side of the body to that stimulated.

Yung came to the following conclusions with regard to decussation
(pp. 525, 526): “Chaque moitié droite et gauche du cerveau agit

THE NERVOUS SYSTEM OF THE LOBSTER. 321

sur la partie correspondante du corps.” ‘“Chaque portion de la chaine
agit également d’une maniére directe sur le c6té du corps qui lui
correspond. Il n’y a pas d’entrecroisement dans le parcours des fibres
nerveuses.” These results were obtained by stimulation with a needle
and by chemical stimulus. Electric stimulation was not employed.

Although the normal passage of the nervous impulse is along the
same side of the nerve chain as that stimulated, the following experi-
ments show that when the normal path is interrupted by section, the
impulse can pass across the chain.

(B) The nerve chain was cut completely across in front of the fourth
thoracic ganglion, and the interganglionic commissure on the right side
was cut just behind the same ganglion. On stimulating the stump of
the right commissure in front of and connected with the ganglion, the
fourth and fifth ambulatory limbs on the /eft side moved sharply ; this
shows that the impulse not being able to influence the limbs of the

‘same side, owing to the commissure being cut on that side, travelled
across the ganglion and acted on the limbs of the opposite side.

(C) One of the thoracic ganglia was isolated by section of the cords
in front and behind. The large nerve to the limb of one side was cut
and its central end stimulated ; the limb of the opposite side moved
sharply, the impulse having passed directly across the ganglion.

5, GENERAL EXPERIMENTS ON THE NeERvous SyYsTEM.

(A) Stimulation of the cerebrum or supra-csophageal ganglion
caused sharp movements of the antennae, thoracic appendages, and
abdomen. The animal crushed one claw within the other. The same
effects were observed when the cesophageal commissures were stimu-
lated, but the movements were even more violent.

Yung obtained the same result by stimulating the cerebrum with a
needle and he points out that “le cerveau ou ganglion sur-cesophagien
est sensible sur toutes ses faces comme les autres ganglions de la
chaine nerveuse, et contrairement 4 ce qui a lieu chez les insectes et
les vertébrés” (p. 525).

(B) Stimulation of the sub-cesophageal ganglion caused movements
of the masticatory appendages and maxillipedes ; also flapping of the
abdomen and movements of the thoracic appendages.

(C) Stimulation of each thoracic ganglion caused movements, mainly
of the appendages which it supplied, but also movements of the appen-
dages behind, and flapping of the abdomen.

Ww

322 C. F. MARSHALL.

Tn all these cases the movements of the appendages in front of the
ganglion stimulated were slight: hence the normal passage of the
nervous impulse is down the cord: but if the cord was cut behind the
ganglion stimulated, the appendages in front of the ganglion moved
more strongly when the latter was stimulated than before.

SUMMARY OF RESULTS.

1. Motor and sensory roots analogous to those by which the spinal

nerves of vertebrates arise do not exist in the lobster.

2. There is no marked decussation of the nerve fibres in the central
nervous system: but nervous impulses readily travel
across the ganglia from one side to the other.

. Each ganglion is a reflex centre for the appendages which it sup-
plies.

4, There is a distinct sense of touch which can be exercised through

the thick cuticle of all parts, especially in the large claws.

5. The cerebrum or supra-cesophageal ganglion is the seat of origin of

(St)

inhibitory impulses: reflex actions are much more marked
when the connection between this ganglion and the rest
of the nervous system is severed.

6, All the ganglia, including the cerebrum, are sensitive, ¢.e. respond
to stimulation.

7. The normal passage of the nervous impulse is down the cord, but
when this path is interrupted, it will pass up the cord.

It will be seen that the two most important results of these investi-
gations are negative ones. There is no marked decussation and there
are no separate motor and sensory roots. Nevertheless it is important
to have determined these points in such a highly organised inverte-
brate animal as the lobster. It would be interesting to investigate
these points in other highly organised invertebrates, such as the Jn-
secta, Arachnida, and Cephalopoda.

THE NERVOUS SYSTEM OF THE LOBSTER. 323

Motor and sensory roots are now recognised throughout the verte-
brate kingdom (with the exception of the Ascidians) ; they are now
even supposed to be present in Amphroxus, and it is a curious point
if this condition is confined to them alone and does not exist in any
invertebrate.

Concerning decussation, I believe complete crossing of the nerves
has been found to exist in every vertebrate in which this point has
been investigated. Now from the experiments described above,—
especially experiment B,—it appears that although there is no marked
decussation, yet partial decussation does exist in the lobster, at any
rate functionally, contrary to the opinion of Yung; but whether this
is a decussation of nerve fibres only, or whether nerve cells are con-
cerned in it, is not proved. This is of interest as affording a possible
clue to the condition found in vertebrates in which the crossing is
complete and in which nerve cells are concerned.

My thanks are due to Dr. Michael Foster and to Professor A. Milnes
Marshall, who have kindly examined the results of my experiments
and also suggested valuable alterations in the paper.

324 PROFESSOR MARSHALL.

THE MORPHOLOGY OF THE SEXUAL ORGANS OF HYDRA.

By A. Mitnns Marsuauz, M.D., D.Se., Beyer Professor of Zoology
in Owens College.

Hydra stands alone, or almost so, among Hydrozoa, inasmuch as its
reproductive organs, whether ovaries or testes, develop and ripen in
the body-wall of the animal instead of in special buds or gonophores.
Concerning the relationship in this respect between Hydra and other
Hydrozoa two diametrically opposite views have been held, one being
that Hydra exhibits the simplest and most primitive condition of the
reproductive organs, prior to the evolution of special sexual buds ; the
other that the condition in Hydra is one of extreme degeneration, the
sexual buds that were previously present having become completely
aborted.

A short time ago, Professor Weismann of Freiburg published some
extremely interesting and valuable researches on the development of
the sexual products in Hydrozoa,’ and it is the object of the present
paper to enquire into the bearing of these results on the problem
stated above concerning Hydra.

In one of the typical hydroid colonies such as Bodecone or Bou-
gainvillea the sexual products, whether ova or spermatozoa, are con-
tained in medusoid buds, and do not ripen until these medusze have
attained full development, and detached themselves from the colony
so as to lead a free-swimming existence. In many cases, however, the
sexual products ripen before the medusoid bud has completed its de-
velopment, in which case the bud remains attached to the colony in

1 Weismann, Die Entstehung der Sexualzellen bei den Hydromedusen. Jena, 1883,

THE MORPHOLOGY OF THE SEXUAL ORGANS OF HYDRA. 325

a more or less immature condition. In some instances the gonophore
is a fully-formed medusa, which, however, never detaches itself from
the colony, such a gonophore being called an attached medusa; in
other cases development stops at a still earlier stage, giving rise to a
disguised medusa, in which all the essential parts of the medusa are
present, but in an unexpanded condition; and, finally, development
may go no further than the production of a hollow diverticulum of
the body-wall of the parent known as a sporosac or sporophore, within
the walls of which the ova or spermatozoa are matured.

It is worthy of notice that the free medusa in the course of its de-
velopment passes through in succession the stages of sporosac, dis-
euised medusa, and attached medusa; so that these latter may be
regarded as due to arrested development of the medusa at an earlier
or later stage. That this view is correct rather than one which would
regard the sporosac, disguised medusa, and attached medusa as repre-
senting stages in the gradual progressive evolution of the free medusa,
is evident from the consideration that the disguised medusa and
attached medusa, which have all the parts of the free medusa fitting
it for independent existence, but never have an opportunity of em-
ploying them, could never have arisen by a process of natural selection
from the sporosac, for the possession of a swimming bell that is never
opened could clearly be of no advantage.

Hence the forms with free-swimming medusze must be regarded as
the most primitive, and those with attached or disguised medusz, or
with sporosacs, must be viewed as derived from these by abortion,
more or less complete, of the various parts of the free medusa, such
abortion being intimately associated with the early or premature
ripening of the sexual products.

Weismann, in the work alluded to above, has shown that the genital
cells of Hydrozoa may arise in parts other than those in which they
are ultimately lodged, and indeed before the appearance of these latter,
into which they migrate later on. In some cases this may be carried
so far that the genital cells arise in the body-wall of the primary zooid
not only before the commencement of the development of the gono-
phore, or sexual bud, but even before the first trace of the appearance
of the branch on which the gonophore will subsequently be borne. A
good example of this is afforded by the fresh-water genus Cordylo-
phora, in which the ova arise in what Weismann calls the germinal
zone of the primary zooid, then migrate into the lateral branch of the

326 PROFESSOR MARSHALL.

zooid when this is formed, and later on shift again into the gonophore
which arises as an offset from this lateral branch.

The explanation of this curious migration is probably to be found,
as Weismann suggests, in the advantage derived from commencing the
development of the sexual products ag early as possible. The for-
mation of the ovum, especially, is a long and complicated process,
which in most animals is commenced at a very early date; in the
highest mammals, for instance, the ovary contains either at or very
shortly after the time of birth all the ova that will ever be developed
init. The development of spermatozoa is a more rapid and lesg
elaborate process than that of ova, and we find accordingly that the date
of their appearance is not thrown back so far as that of the ova. For
instance, in Eudendrium the ova arise in the primary zooid before the
appearance of the lateral branches; the male cells, however, are not
formed till later, and appear first in the lateral branches, from which,
like the ova, they migrate into the gonophores.

The suggestion I would make with regard to Hydra is that it repre-
sents one step further in the process of migration beyond the stage
reached. by Cordylophora or Eudendrium; 7e. that in Hydra the
genital products not only make their first appearance in the wall of
the primary zooid, but remain and undergo their whole development
in the same position, no lateral bud or gonophore being formed.

Weismann’ himself takes the directly opposite view that Hydra
represents a primitive, and not, as I believe it to be, an extremely modi-
fied condition. He considers that in Hydra there has been no shifting
of the place of origin of the sexual cells, but that Hydra represents
in this respect the primitive and original condition.

In support of the contention that Hydra is, as regards its genera-
tive organs, a modified and not a primitive form I would submit the
following arguments :—

1. Hydra is hermaphrodite, being in this respect almost unique
among Hydrozoa. A hermaphrodite condition is altogether excep-
tional among Hydrozoa, and there is not the slightest evidence for
regarding it as primitive in them; while there is very strong reason for
viewing it as secondary and acquired, wherever it occurs in other
groups of animals.

2. Hydra is fresh water, differing in this respect also from almost all
other Hydrozoa. It is very generally accepted that fresh water forms

1 Weismann, op. cit., p. 254,

THE MORPHOLOGY OF THE SEXUAL ORGANS OF HYDRA. Olt

are in the great majority of cases derived from marine forms, and also
that they are very liable to undergo modification in consequence of
their change of habitat.

3. The other fresh water Hydroid, Cordylophora affords very inte-
resting evidence. In the first place, there is very strong reason for
regarding it having only recently migrated from the sea and adopted
a fresh water habitat.* Then as regards its sexual organs Cordy-
lophora is in an extremely modified condition, The genital cells,
whether ova-or spermatozoa, are lodged when fully developed in gono-
phores which never reach even the disguised medusa condition, but
are arrested at a stage very little in advance of the sporosac.?

This, however, is not all, for Weismann has shown’ that the genital
cells of Cordylophora do not arise in the gonophores, but in the body
walls of the primary zooids, where they may be recognised long before
the gonophores have commenced to develop, though on the appearance
of these latter they migrate into them.

These facts seem to me to speak very strongly in support of the
view advanced above concerning Hydra. Cordylophora is a genus
which has only ‘recently become a fresh water one, and in which the
condition of the reproductive organs is such that were the genital cells
to remain and ripen in the position in which they first appear, i.e. in the
body wall of the zooid, they would agree exactly in all essential points
with those of Hydra. It is surely more reasonable to suppose that in
Hydra, whose fresh water habits have been longer established, a pecu-
liarity which we find already very highly developed, in Cordylophora
should be carried just one step further, than that the two forms should
represent the opposite extremes of the series, for on Weismann’s
view Hydra is the most primitive, Cordylophora the most modified of
Hydroids.

4, Additional evidence of considerable value is afforded by the struc-
ture of the ovary itself in Hydra. In its early stages of development*
this consists of a plate-like mass of interstitial cells which at first are
all of about equal size, but of which one only, situated in the centre of

1 Concerning this migration, Semper states :—“It is, so far as I know, the only example
of an animal that can be proved to have originally lived in the sea or in brackish water, and
which, within our own time, has gradually accustomed itself to live in pure fresh water.”
Animal Life, pp. 151-153.

* For good figures of these gonophores, vide F. E. Schulze, Ueber den Baw und die Ent=
wicklung von Cordylophora lacustris. Pl. III & IV.

5 Weismann, op. cit., 29-33.

* For an account of the development of the ovum in Hydra, vide Kleinenberg, Hydia, 1872,

328 PROFESSOR MARSHALL.

the plate, develops into an ovum, the remaining cells supplying it
with nutriment. This must be regarded as a very highly specialised
condition, for all the cells of the ovary are at first alike and must be
supposed to be of equal value. It very usually happens among ani-
mals that of the cells composing the ovary, certain ones alone develop
into ova, the others serving to feed them, but it is altogether excep-
tional that only a single ovum should attain maturity : other instances
are indeed known, such as Moina and perhaps some other Entomos-
traca, and also some Ascidians, such as Salpa, but such cases are rare.

I am disposed to lay stress on this point, for if it be granted, as I
think it must, that the ovary of Hydra is in an exceptionally special-
ised condition, it becomes very difficult to believe, as Weismann would
have us do, that it is also from another point of view in a far more
primitive condition than that of other Hydroids,

5. Kleinenberg and others have denied that any direct comparison
is possible between the reproductive organs of Hydra and the gono-
phores of an ordinary Hydroid on the ground that the former consist
of ectoderm alone, while the latter even in their most degenerate con-
dition involve endoderm as well as ectoderm ; and it must be admitted
at once that the objection is a valid one.

However, it does not in the least affect the position I have attempted
to establish, which is that the reproductive organs of Hydra correspond
not to the gonophore of Cordylophora but to the zone of germination
round the necks of the zooids in which the genital cells arise in Cordy-
lophora and in which they both arise and ripen in Hydra.

T would in conclusion point out that the above argument only con-
cerns the sexual organs of Hydra.

As regards its general morphology I fully agree with Weismann’
that Hydra has strong claims to be regarded as having departed very
little from the condition of the ancestral form from which all Hydro-
medusee may have sprung.

1 Weismann, op. cit., p. 254.

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