the ovary. Externally the nucleated remains of
the ovicapsule are represented ; inside this the
thick albuminous layer marked with radiated lines,
and lined closely by the vitelline membrane ; both
these, as well as the ovicapsule, being perforated by
the micropyle formed at the place where the pedicle
formerly existed.
The micropyle "aperture has also been ob-
irved in other Echinodermata, viz. by J.
Mliller in Ophiothrix fragilis, in which he
states its diameter to be yaW* anc * by n * s
son Max Miiller in Sternaspis thalassemoides.*
This aperture has not yet been observed in
the ovum of Echinus. In the fecundated
ova of this genus, however, Derbes observed
spermatozoa to have passed through the thick
external albuminous covering, but not within
the more delicate vitelline membrane ; but in
this animal the external covering is more like
a layer of soft albumen than a dense mem-
brane as in Holothuria.
The ova of Echinodermata take their origin,
like those of other animals, by the formation
of the germinal vesicles. These have been
* The micropyle was represented in the ovum of
Holothuria tubulosa by R. Wagner in his Icones
Zootomicae, tab. xxxii., fig. 12., before its nature
was known. The first discovery of a micropyle in
the animal ovum is therefore due to J. Miiller. The
next observations of a similar nature are those of
Leuckart and Keber.
observed by Leuckart in the Holothuria tu-
bulosa, beginning to be formed in the ovarian
substance, which they cause to bulge or pro-
ject when they enlarge, so as to hang into the
ovarian cavity. The yolk-granules then come
to be deposited round the vesicles, rendering the
ova opaque, but during all this time the ovum
is attached and enveloped by the original
capsule derived from the ovary ; the albu-
minous layer is then deposited, and the ovum
being detached, the micropyle remains, as al-
ready stated, as the perforation in the pedicle
of attachment.*
Polypma. Although the greater number
of the Polypi are commonly multiplied by a
process of gemmation, as has already been
stated in a former part of this article, yet they
are all capable of attaining sexual complete-
ness, and are also reproduced by means of
fecundated ova. From the varieties, however,
presented by the form both of the gemmules
and true ova in different genera of Polypes,
considerable difficulty has been experienced
in determining the exact circumstances in
which the ova are produced, and the distinc-
tion between the germs from which true ova
and those from which gemmae are formed.
This is more especially the case among the
ciliobrachiate Polypes or Bryozoa, which in
their general organisation approach very nearly
the tunicate Mollusca, but which in their mode
of reproduction resemble closely some of the
Polypes.
The ova of the common Hydca, already re-
ferred to in a previous part of this article,
present the character common to the class, of
being enveloped by a firm covering or shell
membrane, which seems to be formed from
modified cells, and which is sometimes beset
with rough processes or projecting bristles or
barbed spines somewhat like those of the
Bryozoa.
In the Tubularidae and Sertularidae the
ova are formed in ovigerous capsules, which
may be regarded as modified individuals or
polype-heads of the compound animal formed
by gemmation. In some instances these are
detached from the parent stem, as in Tubu-
laria indivisaf ; in other genera they remain
attached, and their ova, or the ciliated em-
bryos developed from them, are discharged
from the cavities in which they are formed J ;
but as the phenomena of the production of
these ova have been fully described by Pro-
fessor Rymer Jones in the article POLYPIFER A,
it is unnecessary to enter into farther details
with regard to the process in this place.
* In addition to the memoirs previously quoted,
descriptions of the ova of Echinodermata will be
found in the following : viz., those of Comatula by
J. Muller, in Mem. of the Berlin Academy for
1841 ; of Asteracanthion, in Wagner's Prodromus,
and in the 5th Part of Carus and Otto's Tabulae
Anat. Compar. ; those of Echinus by Derbes, loc. cit. ;
and by Krohn in Beitr. zur Entwick. der Seeigel-
larven, Heidelberg, 1849, &c.
f Sir John Dalzell, Remarkable Animals of
Scotland, &c.
J Dumortier and Van Beneden's Researches, in
Mem. of the Acad. of Belgium, 1842, torn. xvi.
OVUM.
In Hydractinia rosea, Van Beneden ascer-
tained the existence of the germinal vesicle
and nucleus within the ova while still con-
tained in the capsule ; and it appears that in
all true ova of the Hydrozoa the vitellus,
which consists of finely granular substance,
undergoes a complete segmentation in the
same manner as in other animals in which it
presents a similar structure.
In the common fresh-water polype, in which
ovigerous capsules, or ova, and spermatic cap-
sules were found coexistent on the same in-
dividuals, I observed sometimes the spermatic
capsules brought into contact with the surface
of the ova by the bending round of the body of
the polype at the time when the spermatozoa
were being discharged. This took place pre-
vious to the formation of the firm external
covering ; but I could not determine whether
fecundation had thus taken place or whether
any spermatozoa had penetrated the ovum.
In some of the Hydrozoa, as in the com-
mon green polype, the ova are single, while in
others as in thdra fusca, figured by R. Wag-
ner*, there are several ova enclosed in the
same capsule.
It is remarkable that, while in some Hy-
drozoa the ova are developed from animals
which retain the polype form in their com-
plete sexual condition, or from modified po-
lype heads, in others, as in Coryne, Fritil-
laria and Campanularia dichotoma, it is only
from a medusoid progeny separated from the
polype stock that the true fecundated ova are
produced.
In Anthozoa, the most of which, as Actinia,
Alcyonium, Veretillum, Gorgonia, and the
Corallines are hermaphrodite, the ova consist
of finely granular yolk, germinal vesicle and
macula, and undergo complete segmentation.
The Bryozoa may be most appropriately
considered in this place, as they present con-
siderable analogy to the compound polypes
in the mode of their reproduction. They are
of separate sexes, and appear to be propagated
in three modes, viz. : 1st, by gemmation ;
2nd, by true fecundated ova ; and Srdly, by
bodies which have long been regarded as ova,
but which according to Professor Allman's
recent researches may rather be considered as
peculiar encysted gemmules, and may pro-
bably be analogous to the so-called winter
ova of Daphnia and Lacinularia to which
reference has previously been made.
The development of the true ova of Pedi-
cellina observed by Van Beneden has been
already described.f In this instance the ova
are arranged in clusters surrounded by a
transparent capsule. In each ovum the finely
granular yolk undergoes a complete segmen-
tation. The germinal vesicle possesses a sin-
gle macula.
According to Van Beneden and Dumor-
tierj, the ova of Alcyonella are developed in
ovarian sacs connected with the inner end of
* Icones Zootomicae.
t See p. 23. and Jig. 19. of this article.
t Mem. sur les Polypes d'Eau douce. Acad. de
Belgique, 1812.
the stomach. They are described as com-
mencing by the formation of germinal vesicles
with nuclei or maculae, and as having subse-
quently the granular yolk-substance deposited
round each vesicle ; and these authors de-
scribe the same ova as acquiring at a later
period the peculiar horny or cellular covering
which forms the two-valved shell membrane
long known as belonging to the winter ova
of this and several other genera of fresh-
water polypes. But with regard to the na-
ture of these bodies and the mode of their
formation some doubts may arise in conse-
quence of the researches of Professor Allman.
The bodies in question are at first nearly
spherical and of a light or milky colour ; they
become later of an oval form, and flattened or
discoid, and the cells of the shell -covering
are then developed, and acquire the deep
brown colour which very generally prevails
among these bodies when arrived at maturity,
and which makes it impossible to trace farther
the changes within the ovum. These cells
are developed to a greater extent round the
widest margin of the disc, so as to form there
a thick ring or border, which is afterwards
cleft in two when the valves of the shell open
to allow the escape of the embryo.
The same authors have described the pro-
pagation of the Paludicella to take place in
summer by means of buds, and in winter by
Fig. 92*.
Formation and Structure of the ova of Lophopus
Bakeri. (From Van Beneden.)
These represent, according to Professor Allman,
not the true ova, but the Winter ova or " Stato-
blasts." a. The ovum previous to the deposit of the
cellular covering and marginal plate, b. This co-
vering now in progress of formation, c. and d. pro-
file and front view of the ovum, when completed,
showing the structure of the cellular border which
is afterwards cleft in two at the edge, when the em
bryo is about to escape.
e. An ovum at an earlier stage showing the ovi-
capsule in part removed from one side of the ovum
and its cellular covering.
[128]
OVUM,
means of true ova, as well as by attached
buds, which last are then covered by a
strong corneous envelope, and have received
Fig. 93*.
winter ova, or the bodies provided with the
corneous envelope, are formed chiefly towards
the autumn and winter season ; and the
strength of their covering has generally been re-
garded as a provision for the protection of the
germ from the hurtful influences of the winter
season. During two seasons I have observed
the production of these bodies from the Plu-
matella repens ; and I have kept them through
the winter till the polypes were developed,
and issued from them in the ensuing summer.
From his careful observation of these bo-
Fig. 94*.
Formation of buds in Paludicella. (From Van
Beneden and Dumortier.)
a. One of the Polypes of Paludicella Ehrenbergii
contracted within its cell, showing at the upper
part towards the right the commencement of the
formation of the bud by the growth of cells be-
tween the outer and inner layers of the cell-wall.
b. The same bud a little more advanced and
more highly magnified, represented by itself. The
vesicular cells which separate the ectocyst and en-
docyst are seen more distinctly.
c. A more advanced stage of the same, internally ;
the part from which the embryo polyped arises is
seen bulging out from the rest.
This figure has been introduced to show the
difference between the process by which a true bud
arises and that by which ova are produced.
the name of propagula. In Fredericella
they describe a propagation by means of
buds and by ova provided with the strong
horny envelope. In Alcyonella and Lopho*
pus, besides the usual propagation by buds,
and by the common ova, these authors have
stated that there is also a viviparous produc-
tion of ciliated embryos from ova which re-
main within the parent animals ; but they
have not stated particularly the manner in
which these ova originate, nor their difference
from those which receive the corneous en-
velope. The difficulties presented by these
varieties seem to be in some measure re-
moved by the view offered by Professor All-
man of the nature of the bodies last men-
tioned, to which I will now advert.
It has Ions been known that the so-called
Winter ovum and embryo of Lophopus Crystallinus,
(From Van Beneden and Dvmortier.)
This is the same as that represented by Turpin
under the name of " Cristatella mucedo." In A. the
flat surface, and in B, the narrow edge of the ovum,
is represented. The two valves of the egg cover-
ing have opened superiorly, and the embryo, which
already possesses three crowns of tentacles, is seen
escaping.
dies in several genera, Professor All man has
arrived at the conclusion that they are not, as
was previously supposed, true ova, but rather
separated gemmules; and he conceives that
Van Beneden, who has described their form
and structure so well, must have confounded
them with some other bodies in their first or
earlier stages, or has failed to distinguish be-
tween them and the true ova. This distinc-
tion Allman has succeeded in making by as-
certaining that the true ova and these bodies
do not arise in the same situation, and that
these winter ova or gemmules do not in their
earliest stages present any germinal vesicle or
macula as the true ova do, and do not after-
wards undergo any segmentation. They are
formed, according to Allman, in the funiculus
which connects the bottom of the stomach
with the inside of the cell of the polypide, the
same body which was described by Van Be-
neden and Dumortier as an ovary, but which
Allman regards rather as analogous to the
gemmiferous stolon of the solitary Salpae.
These bodies Professor Allman proposes to
call stato- blasts. He farther discovered that
there is a true ovary with genuine ova which
may be distinctly observed in Alcyonella, and
which is situated in the walls of the endocyst
OVUM.
[129]
near the anterior extremity of the cell. A
number of ova were found in the ovary con-
taining the distinct germinal vesicle with
macula. He also observed the segmentation
of these ova in the usual manner, and the
conversion of the segmented mass into a
ciliated embryo, within which the new polype
is subsequently developed.-f-
Should these observations prove correct
and be applicable to the other instances of
similar winter ova among the Bryozoa, they
may tend to remove some of the difficulties
which exist in regard to the various repro-
ductive bodies occurring in these animals ;
but farther researches seem still necessary to
point out in these and in other polypine ani-
mals more fully and minutely the relation be-
tween the three kinds of reproductive bodies,
viz., true ova, separated gemmules, and at-
tached buds.
AcalephcB. It is remarkable that notwith-
standing the very close relation in which these
animals stand to the Anthozoid Polypes, the
form of their ova is not the same. The Dis-
cophora (Medusae) are of distinct sexes : the
Ctenophora (Beroes) are hermaphrodite ; the
Siphonophora (Diphyidae) are various, or bear,
in the manner of compound animal stocks, a
variety of zoids, sometimes of one sex alone,
at other times of different sexes on the same
stem.
The structure of the ova in Medusas is
extremely simple. They are originally formed
from minute cytoblasts which soon acquire a
single nucleus or macula, and are enclosed in
a delicate external membrane. These consti-
Fii*. 95*.
Development of the ova of Acalepha.
These figures give magnified views of the diffe-
rent stages of formation of the ova taken from the
ovary of a large Rhizostoma. a. The primitive
germ. b. The germinal vesicle now present in the
primitive ovum. c. d. The same more advanced
and enlarged, the macula has appeared in the ger-
minal vesicle, and a few yolk granules are deposited
in the clear vitelline substance, e. The yolk gra-
nules greatly increased in quantity and becoming
opaque, a vitelline membrane is now formed, f.
The same somewhat more advanced, the yolk gra-
nules are now collecting together to form cor-
puscles. The macula is assuming the elongated
form.
t Proceedings of British Association for 1855.
See also Professor Allman's interesting Report on
the Polyzoa to the British Association. See
Trans, for 1850, p. 320.
Supp.
tute the germinal vesicles, round which the
granular yolk-substance is gradually deposited
in increasing quantity. The complete segmen-
tation of the yolk has been observed by Von
Siebold in Cyanea aurita.* The yolk-sub-
stance is often highly coloured, violet or
yellow. In the former part of this article I
have referred to the manner in which some
compound Hydroida are propagated through
their medusoid progeny. These medusoid
individuals, like the ordinary Medusae, are
of separate sex ; and they must therefore
be looked upon as the complete stage of
the polypine animals from which they have
proceeded, whether they have their young
developed while the parent remains at-
tached to the nursing polype stock, or have
assumed the separate and independent mode
of life in a more complete state of develop-
ment. There are many varieties in the de-
gree of perfection to which they attain even
while remaining attached to the polype ; but
the general principle of formation is the same
throughout the whole of the hydroid animals,
the remarkable and constant fact with regard
to the mode of their reproduction being this,
that the immediate product of development
from the ovum which has been formed by
sexual generation from a Medusa or medusoid
animal is invariably an attached Polype, and
that the medusa or medusoid is the product
of a non-sexual process of gemmation from
this polype stem.
Protozoa. With regard to the Protozoa, or
Infusoria and Rhizopoda, it is unnecessary
to add anything here to what has been stated
in the several articles on these subjects and in
a former part of this one, excepting the remark,
that continued researches appear to show that
as the sexual distinction has not been de-
tected, and may probably be absent in these
animals, the nucleus of the monocellular
forms of these beings may hold the place of
the germinal vesicle in them, and that the
processes of division and production of in-
ternal gemmules takes the place of true ovu-
lation. At the same time it must be admitted
that it is by no means improbable that the
sexual relations may yet be discovered in the
lowest monocellular animal bodies, as has re-
cently been the case in some of the simpler and
monocellular Alga?, and that as our knowledge
of the process of reproduction in these beings
is still very limited, it may be destined to un-
dergo even greater progressive changes than
those which it has suffered from the researches
of the last few years.f
Porifera. The bodies which have usually
been regarded as the ova of Sponges, and
to which a reference was made in the earlier
part of this article, are of two kinds, viz. gem-
mules or detached ciliated portions of the
* Beitr. zur Naturgesch. der Wirbellos. Thiere,
1839.
f See the papers of Focke, Cohn, and Stein re-
ferred to in the first part of this article, and the
more recent work of Stein, "Die Infusionsthiere
auf ihre Entwickelungsgeschichte untersucht." 4to.
Leipzig, 1854.
M
[130]
OVUM.
substance of the sponge, and certain spherical
bodies enclosed by dense capsules, which are
produced towards winter, and which appear
to contain a number of germs, each of which
is capable of being developed into a Protean
animalcule, from which probably a sponge
may proceed.* But it may be doubted whe-
ther these last-mentioned capsules are true ova
or may not rather be of the nature of the gem-
mules, winter ova, or statoblasts of Professor
Allman; and it is important to notice that
Mr. Huxley has recently discovered in Te-
thya a different set of bodies, which contain
all the essential parts of true ova, viz. vitel-
line membrane, yolk, germinal vesicle, and
macula, and that these bodies, which are si-
tuated between the cortical and central sub-
stance, are imbedded in a mass of cells together
with spermatozoa.f Although the individual
living particles of the sponge closely resemble
simple ciliated infusoria, and the mass may,
therefore, be viewed as an aggregate of these
minute beings, yet its analogies with and
transitions towards the fungiform polypes are
so great, that we may expect ere long that
the phenomena of its reproduction may be
placed in a new and clearer aspect by the
continuation of the researches now noticed,
and by others of a similar kind.
RECAPITULATION AND CONCLUSION.
Having now stated in detail the principal
facts that have come under our knowledge
with regard to the form, structure, and mode
of origin of the ova of different animals, it may
be proper, in bringing this article to a close, to
endeavour shortly to deduce from these facts
the most general results to which they appear
to lead. These results, together with some re-
flections on our subject, may be stated under
the following heads, viz. 1. Definition of the
ovum, as related to its own structure, and its
history in connection with the reproduction of
the species. 2. Recapitulation of the most
general facts ascertained by the comparison of
the ova of different animals. 3. Morphology
of the ovum ; homology of its parts; and rela-
tion of the ovum to other organic structures.
4. Phenomena attendant on the maturation
of the ovum. 5. Relation of the ovum to
fecundation by the male sperm. 6. Immediate
effects of fecundation on the ovum ; and re-
lation of the ovum after fecundation to the
first commencement of the process of em-
bryonic development.
1. Definition of the ovum, as related to its
own structure, and its history in connection
with the reproduction of the species.
In the commencement of this article the
ovum was shortly defined as " the product of
parental sexual generation from which the
young of animals are developed (produced)."
This definition appears correct and sufficiently
comprehensive ; but should it appear desirable
to substitute for it a more precise description
of the characteristics of the animal ovum, the
* See Carter in Annals of Nat. Hist. vol. iv. p. 89.
f See Mr. Huxley's paper in Annals of Nat.
Hist., 2nd series, vol." vii. p. 370.
following may be proposed as applicable to
the ovum throughout the whole animal king-
dom, without involving any merely theore-
tical view of its structure and constitution,
viz. " the ovum may be shortly described
as a detached spheroidal mass of organised
substance, of variable size, enclosed by a
vesicular membrane, and containing in the
earlier periods of its existence an internal
cell or nucleus ; these parts, formed by the
female individual or organ of animals, are
capable, when fecundated by the male sperm
of the same species, of giving rise, by the series
of histogenetic and organogenetic changes
known under the general term of develop-
ment, to an embryo, from which either directly
or mediately the individuals of the animal
species to which the parents belong are re-
produced."
We thus separate from the category of
true ova all those bodies of an apparently
reproductive kind which are not the direct
product of an act of sexual generation. To
such bodies, the nature of which is as yet
doubtful, and probably somewhat various, the
indefinite appellations of buds, bud-germs,
gemmae, spores, winter ova, ephippial ova,
statoblasts, &c., have been given according
to the circumstances in which they are se-
verally produced.
In all animals, then, with the exception of
the Polygastric Infusoria and Rhizopoda, the
occurrence of sexual generation and the for-
mation of true ova are proved to be the
regular and constant means for the permanent
reproduction or maintenance of the species.
In the exceptional instances now mentioned,
and even in some others possessed of the
sexual distinction, the best known and most
common multiplication of individuals takes
place by a subdivision of the parent body,
either by fissiparous cleaving or by gemma-
tion; but in them also it can scarcely be
doubted that there are other means by which
the permanence of the species is maintained.
All the most accurate recent investigations
lead to the conclusion that the production of
the young of all organised beings, even the
simplest of the Protozoa, does only occur by
direct connection through some organised
medium with other beings of a similar kind
or species. We are forced, therefore, to con-
clude that in the propagation or production
of these simple beings, in circumstances where
their more ordinary fissiparous or gemmi-
parous mode of multiplication cannot be ad-
mitted to have taken place, there must have
passed from the bodies of the progenitors
minute particles of organised substance (ca-
pable, as we know, of being suspended in the
atmosphere, and of resisting during a long
period many of those influences which gene-
rally prove inimical to animal development),
which particles, when brought into circum-
stances favourable to the progress of the vital
processes, undergo the cycle of changes ne-
cessary for the reproduction of beings similar
to those from which they sprang. If there is