still visible, though obscured by the yolk granules ;
the ovum has now assumed an ovoid shape.
D. Softened state of the ovum at a slightly later
stage, when it has met with the spermatic cor-
OVUM.
[121]
puscles ; which are held by Nelson thus to pene-
trate or gain access to the vitelline substance.
K. Ovum more advanced ; the vitelline and
albuminous membranes formed; clear highly re-
fracting spaces resembling altered spermatic cor-
puscles are seen in the yolk substance.
F. Ovum after fecundation; uniform structure
of the yolk substance previous to the appearance of
the embryonic cell and commencement of segmenta-
tion. The chorion has now become tuberculated.
of the ova appears to consist, first, in the
production of minute cell-germs in the upper-
most part of the ovarian tube immediately
adjoining its coecal termination. It does not
appear to be fully ascertained whether these
germs are originally, as some have supposed,
the maculae or nuclei, or rather, as others hold,
the germinal cells or vesicles themselves : the
latter opinion appears to be the most probable.
Second, the granules of the yolk-substance
very soon collect round the exterior of the ger-
minal vesicles. These granules appear at first
to be suspended in fluid ; but a little later,
as they come to collect round the germinal
vesicles, they are united together in a mass by
a firmer but clear basement substance, and
when the minute ova have somewhat in-
creased in size, the outline of this clearer
basement substance of the yolk is distinguish-
able. There is not, however, at first any ex-
ternal or vitelline membrane ; of this Dr.
Nelson and I have convinced ourselves by re-
peated observations in Ascaris mystax.*
The ova, as they continue to descend in the
vitelligenous part of the tube in immense
numbers closely pressed together, assume the
form of subtriangular flattened bodies, and
come to be arranged in series of three, four, or
more, in a short spiral round the centre of that
part of the ovarian tube which constitutes
the yolk organ, as round a central axis, but
without being united together by any com-
mon stalk or other structure. A prodigious
number of ova are thus packed together in a
very small space.
In passing through the next part of tube,
which forms an oviduct, the ova are detached
from the spiral and closely-set position, and
being surrounded by fluid, which must here
be secreted within the tube, descend one by
one through its narrower part. At this place
they encounter the spermatic corpuscles when
they are present, and undergo the change of
fecundation ; but whether fecundated or not,
the ova now lose their germinal vesicles, alter
their form from that of flattened triangles to
oval, become for a time much more yielding
and soft, and somewhat later begin to acquire
an external covering which they have not
previously possessed.
The peculiar motionless and tailless sper-
matic corpuscles appear, therefore, to come
into contact with the ova when the yolk is
exposed directly to their action. According
to the interesting observations of Dr. Henry
* See Nelson's paper on the Reproduction of the
Ascaris Mystax in the Trans. Roy Soc. of Lond.
1852, p. 563., pi. 28, figs. 48. and 50.
Development of Spermatic Corpuscles in Ascaris
mystax.
This figure is introduced to show the several
stages of development of the peculiar acaudal and
motionless spermatic corpuscles of the Ascaris
mystax.
A. shows various stages of the primary sperm -
cells or rather sperm-germs; in the more advanced
of which towards the right, internal cells are seen
forming by endogenous production within the
primary germ-cells.
B. & c. show the second stage, in which the sepa-
rated germ-cells have each become covered by a
finely granular mass collected round them ; in B.
this process is beginning ; in c. it is completed, and
the sperm cells thus formed have assumed an ovoid
shape.
D. Two views of sperm-cells in the third stage, in
which a quadrifid division of the whole cell has
taken place preparatory to the escape or separation
of the spermatozoon-cells, usually four in number,
proceeding from each sperm-cell.
E. Various views of these spermatozoon cells in
which the radiated linear marking (seen in D.) has
disappeared, and is again resolved into granules ;
the nucleus is seen from above in the left-hand
figure ; in the three others being viewed in profile
the appearance of the bell-shaped spermatic cor-
puscle with the nucleolus is perceptible.
F. Exhibits from right to left the various pro-
gressive stages of the bell-shaped corpuscle into the
test tube form ; the remains of the nucleolus and
granular substance are seen towards the mouth of
the flask-shaped bodies.
o. Illustrates the effect of water in developing
" Sarcode " on the exterior of these corpuscles in
two different stages of their advancement.
[122]
OVUM.
Nelson,* a peculiar softening of the ova, which
may be caused by the rapid imbibition of fluid
at the time the changes above mentioned are
taking place, renders them peculiarly liable to
be impressed by the spermatic corpuscles at
Fig. 87*.
Development of ova in Mernris albt'cans, belonging to
the Gordiacei. (From Meissner.}
a. Germ-cells from the upper or coecal end of the
ovarian tube, their nuclei undergoing subdivision.
b. Various stages of farther multiplication of the
internal cells, which in the more advanced are seen
to approach the surface of the original cell, and to
cause the bulging of its membrane by the enlarge-
ment of the internal cells, which last constitute
the primitive ova.
* Loc. cit., p. 576.
c. d. Groups of primitive ova thus formed ; some
of them much more developed than others, present-
ing internally the nucleated germinal vesicles and
yolk'granules and attached in pediculated capsules,
which are formed by the extension of the membrane
of the primary germ cells.
e. A group of these ova more advanced; the
opaque granular yolk increased in quantity so as to
obscure in part the germinal vesicles ; the pedicles
much narrowed and somewhat elongated ; the ex-
ternal ova are nearly mature ; those in the centre
remain abortive.
f. Two similar ova now ripe, a part of one of
them is artificially burst, showing the escape of the
yolk granules and germinal vesicle with a double
macula. The remains of the pedicles when detached
from the central mass constitute, according to
Meissner, the micropyle aperture.
this period; and Nelson is of opinion that
these corpuscles even penetrate completely
into the yolk-substance, and ultimately com-
bine with it. Little doubt can be entertained
that a combination of the spermatic and vitel-
line elements in some manner takes place at
this time, whether by the direct interpene-
tration after the mode described by Nelson,
some may be inclined to doubt; but at all
events the spermatozoa act immediately on
the vitelline substance at this stage of the
progress of the ovum. *
As the ovum descends in the next part of
the tube or uterus, the external membrane
becomes more dense, additional layers are
deposited upon it, and at last it acquires more
* Professor Bischoff has, in his recently published
tract " Wiederlegung des von Dr. Keber bei den
Naiaden und Dr. Nelson bei den Ascariden behaupte-
ten Eindringens der Spermatozoiden in das Ei," &c.,
Giessen, 4to., 1854, called in question the accuracy
of Nelson's observations, and asserted that Nelson's
spermatozoa are only epithelial particles belonging
to the female passages. In a subsequently pub-
lished paper, entitled, " Bestatigung des von Dr.
Newport bei den Batrachiern und Dr. Barry bei
den Kaninchen behaupteten Eindringens der Sper-
matozoiden in das Ei, Giessen, 25th March, 1854,"
although Bischoff has seen reason to alter his pre-
vious views as to the phenomena of fecundation in
the Ascaris mystax, he still in that paper, and in a
special memoir on the subject, published in the
Zeitsch. fur Wissensch. Zool., 1854, vol. vi. p. 377.
adheres to the view that the bodies which I, along
with Nelson and Meissner, regard as spermatozoa
are no more than epithelial cells. I have elsewhere
shown that this view is altogether untenable, and
that no doubt can now prevail as to the corpuscles in
question being the product of development from
the spermatic cells of the male Ascaris, and as to
the possibility of their direct action on the ova
within the female previous to the formation of the
vitelline membrane. Meissner has also given
the most satisfactory evidence on the same point
in his memoir on the penetration of the sperma-
tozoa into the ova of animals, contained in the
same volume of the last quoted work, though this
author takes a different view from Nelson and my-
self as to the manner in which the spermatozoa are
admitted into the ovum in Ascaris mystax, believing
in the existence of a vitelline membrane and micro-
pyle, in the same manner as in Mermis and other
Gordiacei, which he has so well described. With
regard to this view as applied to the Ascaris mystax,
Bischoff's observations, Nelson's, and my own, give
me the greatest confidence in asserting that there is
at first no vitelline membrane in this animal at the
time when the ova first meet with the spermatic
corpuscles.
OVUM.
[123]
or less of a minutely tuberculated structure
on its external surface. The ovum becomes
of a regular short oval or nearly spherical
form. If fecundation shall have" occurred,
the embryonic vesicle or cell makes its ap-
pearance, and the phenomena of segmentation
follow in rapid succession.
Fig. 88*.
Formation and fecundation of the ova of Nematoid
Worms. (According to Meissner.)
a. A portion of the ovarian axis and early ova
attached to it from the ovarian tube of Strongylus
armatus. The axis column occupies the centre of
the tube, and the ova are suspended to it by
pedicles, supposed by Meissner to form micropyle
apertures when they are detached.
b. View given by Meissner of a set of the nearly
ripe ova of Ascaris mystax, which he conceives are
thus connected by pedicles to a central axis.
c. Two mature" ova of the same surrounded and
in part penetrated by spermatic corpuscles. At the
narrow angles of these ova a spermatozoon is seen
passing into the interior by what Meissner has
regarded as a micropyle formed by the detached
pedicle. In the ovum to the right a spermatic cor-
puscle is seen in the vitelline substance. The
existence of such a micropyle aperture and pedicu-
lated attachment of the ova in the Ascarides 1 re-
gard as doubtful.
In others of the Nematoid Worms and more
especially in Strongylus and the Gordiacei, it
would appear from the researches of Meiss-
ner, that the first germs of ova which take
origin in the uppermost part of the ovarian
tube multiply by an endogenous production,
and that in this manner groups or bunches of
the primitive ova are produced which are con-
nected together by pedicles arising from the
Fig. 89*.
Formation of ova and fecundation in Gordius Sub-
bifurcus. (From Meissner.)
a. A small portion of the ovarian tube with
groups of the ova partly within and partly escaping
from it.
b. Three of the mature ova from the lower part
of the oviduct surrounded by the spermatozoa. The
ova are now isolated, and the pedicle of each is
open, and is regarded by Meissner as a micropyle,
by which spermatozoa, as represented in two of
them, enter the ova. The germinal vesicle is still
to be seen.
elongated membrane of the original germ-cell
which remains as a covering of the whole. A
certain number of these ova make progress in
development while others probably become
abortive. As the ova enlarge they are more
spread out in the tube and take something of
the spiral disposition which exists in the Asca-
rides, but with this difference, as already noted,
that the various ova remain connected to-
gether by the attachment of their pedicles to
a central axis or stem running down the
middle of the ovarian tube. On the subse-
quent detachment of the ova by the break-
ing of these pedicles, according to Meissner,
a micropyle aperture is formed in each
ovum for the admission of the spermatozoa.
The accompanying drawings from Meissner's
Memoir will give a sufficiently clear idea of
his views on this subject.
The ova of the nematoid worms constitute
a marked example of the simpler kind of ovum
in which the formative yolk is present, and
[124]
OVUM.
in most but not in all of which segmentation
is complete. This process was first made
known through the interesting researches of
Kdlliker*, in Muller's Archiv., 1843, p. 68,
and Bagge, in his Inaugural Dissertation.-j-
The memoir of Reichert in Muller's Archiv.,
1847, contains very correct views as to the
formation of the spermatic cells.
The accompanying figure from Meissnerjl,
gives a representation of a remarkable form
of the external capsule of the ova occurring in
some of the Gordiacei (Mermis nigrescensj.
Fig. 90*.
Mature ova of Mermis nigrescens. {From Meissner.)
This figure is introduced to show the very pe-
culiar capsule in which the ovum is enclosed.
a. Ovum taken from the uterus with embryo
enclosed ; the chorion and shell capsule with cha-
lazae or brush-like processes attached to the latter.
b, c. The shell capsule c burst across the equa-
torial groove, allows the ovum b to escape with the
chorion and embryo contained within it.
The ova of Trematoda are generally of a
long-oval form, and of middle size. They are
enveloped by a shell membrane of consider-
able firmness, and which is not unfrequently
of a dark brown colour. The yolk-sub-
stance contains fat corpuscles simple and
compound ; and there is a germinal vesicle
present, which, however, from the deep
colouration and other circumstances, is often
very difficult of detection.
In these animals an interesting peculiarity
in the arrangement of the reproductive organs
exists, in the separation of the germ-forming
and yolk-forming portions from each other ;
in the first of these organs germinal vesicles
or clear nucleated cells alone being produced,
in the other the opaque granular fatty matter
which furnishes the vitellus. This arrange-
ment was first described by Von Siebold in
1836.$ The germ organ is generally in the
form of a rounded sac, which is filled with
the nucleated germ-cells or vesicles in various
* See his admirable memoir on the first changes
in the fecundated ovum, principally referring to the
Entozoa.
f Dissert, inaug. de Evolutione Strongyli auri-
cularis et Ascaridis acuminatae, Erlangae, 1841.
t ZeiLsch. fur Wissen. Zool. vii. pi. ii.
Wiegmann's Archiv., 1836, p. 217, Tafl. vi.,
and Muller's Archiv. 1836, p. 232, Tafl. x., fig. 1.
stages of development. The vitelline organ is
double, each one consisting of ccecal tubes, in
which the opaque granular yolk-substance is
secreted.* The ducts of these two organs
meet in a common cavity or uterus, and the
germs descending into this cavity are there
enveloped by a portion of the vitelline mass,
and shortly afterwards an enclosing vitelline
membrane is formed. These animals being
hermaphrodite, the vas deferens of the male
organ or testicle leads into the uterine cavity ;
and it would appear, therefore, that in many
cases, if not in all, impregnation takes place
by the access of the spermatic corpuscles to
the elements of the yolk and germinal vesicle,
just at the time when they are brought toge-
ther to form the ovum.
This separation of the germ-forming and
yolk-forming parts of the ovarian organ, which
is so apparent in the Trematoda, is not in
truth so great a departure from the more
familiar structure of other animals as might at
first be thought ; for, as Leuckart has well
observed, there are other examples of the
same disposition, or an approach to it. Thus
in Insects and in Nematoid Worms, as we have
seen, it is from distinct parts of the genital
tube that the germs and yolk are produced ;
and more or less of the same arrangement
exists in all instances in which the form of
the ovary is tubular.
The Cestoidea present a great similarity
to the Trematoda in the arrangement of the
organs by which the ovum is formed. Indeed,
notwithstanding the difference of their antece-
dent stages of development, the structure of
the mature sexual joint or proglottis of the
tapeworm, offers so great a resemblance to that
of some of the Trematoda, that they have been
regarded as identical by several recent obser-
vers. In each sexual joint of the tapeworm,
the testicle and the two parts of the ovarian
organ coexist, and, as stated in an earlier
part of this article, arrive at maturity simul-
taneously in the posterior or oldest segments
of the body. Van Beneden has, in his recent
work on the Cestoid Wormsf , described very
clearly the structure and relations of the ger-
migenous and vitelligenous parts of the repro-
ductive organs in the complete segments or
proglottides of a variety of Cestoid worms.
The ova originate in the first mentioned of
these organs as germinal vesicles, and, passing
into the vitelligenous part, meet with the
yolk-masses formed there. Near the same
place is situated the seminal vesicle, from
which, doubtless, the spermatic substance
easily reaches the ovum as it descends in the
course of its formation. The ova then ac-
quire an external envelope, and pass into the
cavity termed a uterus. As they corne to be
accumulated in gradually increasing quantity
in the latter cavity, they distend it to a great
degree, so as to cause it to pervade in various
forms, ramified and others, the whole body of
* See also Thaer on this subject, in Muller's
Archiv., 1850, p. 626.
f Mem. sur les Vers Cestoides. Acad. Roy. de
Belgique, torn. xxv. 1850, see plate B.
OVUM.
[125]
the proglottis ; and finally they are dis-
charged from this, usually after the separation
of the joint from the main tapeworm, by the
irregular rupture of the outer wall, or by a
genital aperture. Here, then, we have another
instance of the combi nation of the several com-
ponent elements of the ovum together with
the sperm, previous to the enclosure of the
whole by a membrane so as to give the form
of a complete ovum.
The ova of most of the Cestoidea, as in the
common tapeworms, are of proportionally
small size. The external envelope is firm,
thick, and nearly homogeneous ; sometimes,
however, presenting a slight appearance of
fine radiated strias passing through it, which
recalls the structure of the thick membrane
of the Fish's ovum. The vitelline substance
is very finely granular, or almost clear ; the
germinal vesicle is perceived with difficulty,
but is of large size.* In some Cestoids the
external envelope is of a brown colour, as in
the Trematoda, and in others presents pecu-
liar forms and prolongations from its surface.
A delicate vitelline membrane is described
within the outer covering by some authors.f
The segmentation of the yolk appears to be
complete ; but this process has been observed
only in a few instances.
Of the ova of the Cystic Entozoa nothing
need here be said, seeing that it has already
been shown that the several genera of this
order, viz., Cysticercus, Ccenurus, and Echi-
nococcus, are only larral and aberrant forms of
the Cestoid worms, and being immature
animals, never produce ova, excepting through
their more advanced stage of cestoid develop-
ment.
Echinodermata. The different orders and
families of this class are all of distinct sex, so
far as is yet known, with the single exception
of one of the Holothurida, viz., Synapta(S.
Duvernaea), described by Quatrefages^l as
presenting a combination of the testicles and
ovaries in one organ, resembling in some
measure that which exists in the Gasteropo-
dous Mollusca.
In the females of Echinus, Asterias, and
Holothuria, the ova have been studied with
care by different observers. In all of them
the ova present, when mature, more or less of
a deep yellow, orange, or red colour, which
belongs to the yolk-substance. This sub-
stance is finely granular, and is enclosed, at
least in some, as Echinus, by a delicate
vitelline membrane ; but in others, as Holo-
thuria, there is a considerable deposit of an
albuminous layer of a peculiar structure,
which, from its adhering closely to the vitel-
* See Kolliker in Mullet's Archiv. for 1843,
p. 92 ; Tafl. vii., fig. 44.
f Details as to the structure of these ova will be
found in the work of Von Siebold in Burdach's
Physiologie, vol. ii. ; in Dujardin's Hist. Nat. des
Helminthes, see pi. ix. and xii. ; in Blanchard's
memoirs in the Annal. des Scien. Nat. for 1848,
p. 321 ; in Van Beneden's work ; and in Kuchen-
meister's more recent Handbuch der Parasiteu des
Menschen, &c., Leipzig, 1855.
J Annal. des Scien. Nat., 1842, xvii.
line membrane, obscures the latter envelope,
and thus has made its existence doubtful to
some observers. This albuminous deposit
also exists in Echinus, but is in that animal
distinguishable from the vitelline membrane.*
The colour and opacity of the yolk-sub-
stance in the mature state of the ovum
usually prevent our perceiving the germinal
vesicle ; but in the earlier stages of formation,
when the ovum is of lighter colour or even
quite clear and transparent, a germinal vesicle
with a single distinct macula is easily per-
ceived. This vesicle has disappeared in the
ova which are deposited. The segmentation
of the yolk is complete in the Echinodermata :
the process has been fully traced by Sars in
Asterias f, and by various observers in some
other genera.
It was in the ovum of Holothuria tubulosa
that Professor Johannes Muller first made the
novel and interesting discovery of an aperture
leading through the thick external membrane
towards the yolk ; an observation which has
been confirmed by various other physiologists J,
and has been productive of important con-
sequences in its extension to a number of
other animals in which such an aperture was
not previously suspected to exist. MUller
brought this observation before the Berlin
Academy, and it was noticed in the printed
report of the proceedings in 1851. A more
detailed account of his observations on this
subject is given by Muller in his Archiv. for
1854 (p. 60.). The very thick covering of
the ovum of Holothuria presents an appear-
ance of radiated lines running through it per-
pendicularly to the surface, which resembles in
some degree the marking in the membrane
of the Fish's ovum, but is not so distinct, and
does not appear, as in it, to be produced by
visible tubes or pores passing through the
membrane. The canal of the micropyle pierces
the whole thickness of the radiated membrane ;
but Muller conceived that it did not perforate
the delicate vitelline membrane placed on its
inner surface. Leydig, however, and Leuck-
art are of opinion that the canal passes com-
pletely into the interior of all the egg-coverings,
and reaches the surface of the yolk, so that it
may convey the spermatozoa to that body.
The entrance of the spermatozoa has not,
however, as yet been actually observed.
According to Leydig, the thick membrane
may consist of several layers united together,
such as, internally the vitelline membrane, the
thick albuminous part in the middle, and ex-
ternally the nucleated layer formed by the
remains of the ovarian capsule. Leuckart and
Leydig have also pointed out the fact that the
formation of the canal of the micropyle in the
egg of Holothuria proceeds from or is con-
nected with the original attached and pedicu-
* Derbes, in Annal. des Scien. Nat. 1847, 3 e Se'r.
vol. viii., p. 80, and Leydig in Muller's Archiv. for
1854, p. 312.
f Wiegmann's Archiv. 1844, and Annal. des
Scien. Nat, 3 e se'r., vol. ii. p. 190.
J Leuckart in Bischoff's Wiederlegung, &c., 1854,
and Leydig, loc. cit.
[126]
OVUM.
lated condition of the ovum in the ovary, that
it is in fact the remains of the divided pedicle
after the ovum is separated from the place of
its original formation.
serve
Ovum and Micropyle in Holoihuria tubulosa.
(From Ley dig.}
a, b. A small portion of the ovary from the
inner surface, containing ova in various earlier
stages of their development ; three of them project
from the inner surface, of which a is the most de-
veloped. In this one the pediculated attachment
and enclosure of the ovum by the nucleated ovarian
membrane is seen, the yolk granules and the ger-
minal vesicle with its macula.
c. A more advanced ovum now separated from