ferred to.

The interesting discovery of an aperture in
the external membrane of the ovum of osseous
fishes is due to Dr. Ransom of Nottingham,
who observed it first in two species of Stickle-
back or Gasterosteus, and afterwards in other
fishes. This author made the farther interest-
ing observation in the first-mentioned fish,
that in impregnation the spermatozoa entered
the ovum only through this aperture or mi-
cropyle. As this is the first instance in
which the existence of this aperture and its
relation to the process of fecundation have
been ascertained by direct observation in a
vertebrate animal, I will describe it more fully
from Dr. Ransom's paper to the Royal So-
ciety of London*, and from farther inform-
ation which he has obligingly furnished to me
in private. I may also mention that I have
fully confirmed Ransom's observations as to

* Proceedings of Roy. Soc. 1854, Nov. 23rd.



OVUM.



Fig. 68*.



;^:g

XY-. ;.''; ':>:'
>:.'i;'r:' .-..'




Micropyle of the ovum in Osseous Fisfies.

A. Enlarged view of a quadrangular portion of the
surface of the mature ovarian egg of the Stickleback
containing the micropyle from above. In the outer
part of this figure the'general dotted appearance of
the membrane is seen, and here and there the pedicu-
lated flask-like processes attached to the membrane
in this fish in the vicinity of the micropyle ; the
radiated shading represents the appearance of the
funnel-shaped depression leading to the aperture of
the micropyle, which is seen in the centre of the
space it encloses.

B. Transverse section of the dotted membrane
and funnel of the micropyle of the same egg some-
what more enlarged, seen in profile ; the aperture
of the micropyle is seen towards the point of the
funnel. This view is semidiagrammatic, and the
fine canals passing through the membrane are re-
presented fewer and wider than they are in nature.

The diameter of the whole ovum was about " ;
the thickness of the external membrane ^^' ; the
width of the base of the funnel about T | 5 // ; the
depth of the funnel ^" ; the diameter of the micro-
pyle aperture at the apex 5^5".

c. Small portion of the membrane at the apex of
the funnel containing the aperture of the micropyle
pressed flat, magnified 500 diameters; from the
trout's egg.

D. A similar portion of the membrane magnified
1000 diameters. The lumen of the canals is seen, and
an indication of hexagonal division of the spaces
between them, represented somewhat too distinctly
in the figure.



the existenc6 pf ,tfre rnuTOpyle " itf tl ie*0va/ o *
several fishes ; and though I have not yet
been so fortunate as to perceive the sperma-
tozoa actually passing into the ovum through
this aperture, the accuracy of Ransom's obser-
vations on this as well as on other points leave
little doubt as to the fact stated by him.

The micropyle in the Gasterosteus, as de-
scribed by Ransom and observed by myself,
is a considerable funnel-shaped depression in
the outer membrane, which projects inwards
on the granular substance of the yolk, so as
to indent this layer to some depth, and pro-
bably to reach near to the germinal vesicle,
which lies imbedded within the germinal layer.
The inner narrow end of the funnel terminates
in a distinct rounded or elliptical mark, with a
fine but distinct line bounding it, which has
all the appearance of a foramen, and which is
either an open passage or one which is closed
only by an extremely delicate structure.
The funnel-shaped depression leading to the
micropyle may be easily seen on the surface
of the egg of the salmon or trout when slightly
dried of the adhering moisture, and is of such
a size that it may be perceived with the naked
eye or with a lens of low magnifying power.
In order to perceive the micropyle itself, how-
ever, or pore in the point of the funnel, it is
necessary to remove from the egg that portion
of the dotted shell membrane containing the
funnel; and having freed it from the adherent
granules of the yolk-substance by careful wash-
ing, for which Ransom has recommended a
solution of acetate of potash, this part of it
may be viewed under pressure with great ease
with a magnifying power of 200 or 300 diame-
ters. The porous structure of the membrane
is then seen to continue very nearly up to the
margin of the micropyle. This last has a
diameter of from 3^00" to ^oW* The ap-
pearance of a double outline surrounding the
micropyle proceeds from the circumstance
that, in looking through the funnel we see
at once two portions of the narrowing wall
of the passage of different widths.

In Ransom's experiments, very soon after
spermatic fluid was placed in the water round
the ovum of the Stickleback, several of the
spermatozoa were perceived to pass in at the
micropyle ; and immediately upon this water
was imbibed, and the space named the respira-
tory chamber was formed between the yolk
surface and the external membrane; a change
which in this fish did not take place in the
unfecundated ova, but which in some others
occurs without impregnation. It is from
this fact apparently that Ransom is inclined
to the opinion that the micropyle may be
closed by a very delicate membrane, which in
fecundation is removed or broken through by
the entrance of the spermatozoa; but with
regard to this point there is still some uncer-
tainty. The germinal vesicle previous to its
disappearance is imbedded below the super-
ficial layer of yolk-substance in a stratum of
granular matter ; and Ransom conceives that
at the time of the rupture of the vesicle, this
^ranular matter being mingled with the con-
[H 3]



[J02]



OVUM.



tears of ike vesicle,. the more immediately
germinal part of the egg is formed from the
mixture of the two. However this may be, it
seems not improbable, from the observations
now referred to, that the spermatozoa are
conveyed directly to the germinal part of
the egg by the funnel of the micropyle. I
shall afterwards have to state the more
numerous instances in which, following its
first discovery by J. Miiller in the Holo-
thuria, a micropyle has been detected in the
ova of Invertebrate animals ; and I may at-
tempt to show the great importance of this
aperture in connection with fecundation in ova
with thick external coverings to which the
spermatic substance does not gain access till
the later periods of their formation. The ac-
companying figures of the micropyle in the
Stickleback will give a sufficiently clear view
of this remarkable structure. At present
it may be permitted to remark that, if
we consider the size of this aperture, and
the ease with which it may be found in
the ova of fishes by an observer whose at-
tention has been called to its existence, to-
gether with the fact of its having been so long
overlooked previously, there is much ground
for caution as to negative statements as to
the existence of a similar aperture in the ova
of other animals. I have already made al-
lusion to this subject in the previous sections,
in which I have stated that Dr. Ransom has
expressed to me his firm conviction, founded on
observations, that the micropyle exists also in
the ova of Batrachia. At the same time it is
quite probable that such an aperture may only
exist or be required for the admission of the
spermatozoa when fecundation is of late oc-
currence, and when the covering membrane
of the ovum is so dense as to resist the pene-
tration of the spermatozoa through its solid
substance.

It is right also to mention that the exist-
ence of this aperture, or rather the funnel lead-
ing to it, did not entirely escape the observa-
tion of preceding physiologists. The accurate
Von Baer, in his work on the development of
Fishes*, has described in the Bream (Cy-
prinus blicca) a funnel-shaped depression of
the external membrane, which reached nearly
to the surface of the germ ; and he ob-
served that this funnel was effaced as soon
as the imbibition of water took place. He
considered this aperture as most probably
owing to the escape of the germinal vesicle
from the surface of the yolk and through the
coverings of the ovum, "in the same manner
as he had described in the frogf, and did not
therefore conceive it to serve any immediate
purpose in connection with the introduction
of the spermatozoa. Dr. Ransom has ob-
served that the effacement of the funnel which
he had seen in the Stickleback is not inva-
riably the consequence of fecundation in the
Fish's ovum; for in the salmon and trout

* Entwickelungsgeschichte der Fische, Leipzig 1 ,
1835, p. 9. figs. 1. and 2.
t De Ovi Mammal. &c., pi. xxv.



Fig. 69*.




Development of the ova of Gasterosteus.

A. B. c. D. Four ova of the Stickleback in the
earlier stages of their development within their
ovisacs.

In that figured at A, which is the earliest, ^"
in diam., the germinal vesicle placed near the cen-
tre has scarcely any perceptible membrane or wall,
hut resembles a gelatinous mass in which the small
number of maculae are developed : there is as yet no
yolk, but only a slightly turbid fluid substance
filling the space between the ovisac and the ger-
minal vesicle: delicate epithelial cells project from
the inner surface of the ovisac.

In B. ^y the maculse have increased in number,
the germinal vesicle, as well as all the other parts, has
increased in size, the fine granules of the yolk sub-
stance have begun to be deposited towards the
periphery, but there is as yet no vitelline mem-
brane. The wall of the ovisac is now more distinct,
and besides the internal cells, there are seen on the
exterior the nuclei of external flattened cells.

In c. -fig" the maculai have become more numerous
and distinct; the yolk granules are more opaque
and in greater quantity, and the mass of the j-olk
more circumscribed, a clear space now intervening
between it and the wall of the ovisac.



OVUM.



[103]



In D. T J 5 ", although the number of maculae has
greatly increased by endogenous multiplication, the
germinal vesicle has not now undergone an enlarge-
ment proportional to that of other parts of the egg
and ovisac : the granules of the yolk, especially to-
wards the surface, are much increased, and a narrow
clear marginal space on the surface now indicates
the commencement of the formation of a zona or
vitelline membrane. This appearance ia also
slightly perceptible in fig. c.

The "dimensions of the several parts in these dif-
ferent specimens were as follows :

A. B. C. D.

Ovisac - - -0025 -004 -0056 -007

Yolk -0042 -005

Germinal vesicle - -001 -0016 -0025 -0026

Maculae - - -00015 -00018 -00025 -0003

he found the funnel-shaped aperture to re-
main for some time after the completion of
fecundation, and in none of the fishes he has
observed does he conceive the aperture of
the micropyle to be closed.

The ova of osseous fishes appear to take
their origin within the rudimentary follicles
or ovisacs of the ovary mueh in the same
manner as those of the Batrachia. The ear-
liest part of the ovum that can be distinctly
seen within the follicle is a vesicle of about
half the diameter of the primitive follicle it-
self. A little later this vesicle is seen to be
surrounded with a dear, jelly-like substance,
in which some small dark granules are depo-
sited chiefly towards the surface of the vesi-
cle. There is as yet no enclosing membrane,
but the follicle is seen to be lined by a layer
of extremely delicate hyaline ceils, often dif-
ficultly perceptible. The earliest recognisable
part of the ovum, therefore, is the germinal
vesicle ; which, as in other animals, has soon
deposited round it the clear gelatinous base-
ment-substance of the yolk, in which the
opaque yolk granules soon make their appear-
ance. There is not at first any vitelline or
other membrane enclosing the primitive parts
of the egg, and indeed it is a considerable
time before any such membranes are formed.
The deposit of vitelline granules increases ra-
pidly, so as to give the yolk considerable opa-
city ; afterwards larger globules appear, and
seem to increase by endogenous multiplica-
tion. * The oil globules are at first small,
and equally diffused through the whole
yolk ; it is only in the later stages of for-
mation that they unite into fewer and larger
globules.f The granular or primitive yolk-
substance continues to surround more imme-
diately the germinal vesicle till the period
when this vesicle is ruptured, and is probably
spread over the germinal disc of the egg. Si-
milar granules also occupy, however, in a
layer the surface of this part of the egg pre-
vious to the rupture of the germinal vesicle ;
so that it is not probable that the germinal
disc owes its origin, as Coste states^:, entirely
to the effusion of the contents of the germinal
vesicle.

* Lereboullet, loc. cit.
t Retzius, loc. cit.

j Hist. ge'n. et part, du Develop?, des Corps
organ, torn. i.



The ovum receives its firm porous mem-
brane while within the ovarian capsule, but
only in the latter part of the time of its forma-
tion. This membrane lies very close to the
inside of the ovisac, is at first comparatively
thin and destitute of apparent structure, and
gradually increases in thickness towards the
time of its approach to maturity. At the
same time a remarkably thin pellicle may be
distinguished close to the surface of the
granular yolk-substance, scarcely meriting the
name of membrane. As already remarked, it
is difficult to determine what is the true homo-
logical signification of these membranes. The
inner one might by some be regarded as a re-
presentative of the zona pellucida, or a conso-
lidated pellicle on the surface of the yolk,
though it must be admitted that Ransom's ob-
servation, that it follows the segmentation, is
opposed to this view, and makes it more
probable that it is only a part of the yolk
itself. The origin of the external porous
membrane I am inclined to connect rather
with the interior of the ovarian follicle ; but
whether by exudation from it, or by amalga-
mation of the innermost layer of epithelial
cells of the follicle, I have not yet been able
to determine. I am inclined to regard the
latter as most probable, and that this is the
true vitelline membrane.

The manner in which the micropyle takes
its origin has not yet been ascertained. It
will afterwards be shown, that in a consider-
able proportion of those invertebrate animals
in which this aperture in the egg coverings
is found, it has existed from a very early
period, and proceeds from the remains of the
pedicle by which the ovum is originally con-
nected with the ovarian substance. Such a
pediculated connection has certainly not yet
been observed by most of those who have in-
vestigated the ovarian ovum of fishes.*
Rathke, indeed, observed the appearance of
the remains of a pedicle in the detached ova
of the Blennius viviparus -j- ; according to
Ransom the micropyle in the Pike is not
a depression, but projects from the surface
like a trumpet-shaped process; and in the
earliest stage of development of the ovarian
ovum of Trigla hirundo, according to Ley-
dig J, the shape is somewhat pyriform or
pediculated, in the same manner as in some
of the invertebrate animals.

On the other hand, Ransom expressly
states that he has never been able to observe
the slightest connection in Gasterosteus be-
tween the pedicle of the ovum by which
it is attached to the ovary, and the mi-
cropyle. This aperture he says is always
situated at that side of the ovum towards
which the germinal vesicle and the germinal
disc are placed ; but these parts have no
regular connection with the pedicle. The pe-



* The pedicle here spoken of is not that of the
ovarian capsule containing the ovum, but of the
ovum itself within the capsule,

\ Abhandlung. zur Entwick. part. ii. p. 4.

t MUller's Archiv. for 1854, p. 376. fig. 6.



OVUM.



dicle, he affirms consists only of the ovarian
structure, and of no part of the membranes of
the ovum. From his observations on Gaster-
osteus, in which the projecting bodies from
the porous or outer membrane in the vicinity
of the micropyle enable this part to be easily
recognised, he feels confident that if any
pediculated connection had existed it could
hardly have escaped notice.

When the ovarian ovum has attained ma-
turity it falls into the cavity of the ovary, or
that which may be regarded as ovary and
oviduct united, by the rupture of the ovarian
capsule in which it is contained. The walls
of the ovi-capsules have by this time become
extremely thin ; but according to Von Baer a
small stigma or non-vascular mark may be dis-
tinguished where the rupture takes place.
After the ova have fallen into the common
cavity they are surrounded by a considerable
amount of secreted albuminous matter, by
which in some fishes the ova are covered
when excluded. In some this albuminous se-
cretion serves to unite the spawn in chains or
networks. In other fishes the ova are covered
externally with villous projections ; but the
manner in which these are formed has not
yet, so far as I am aware, been observed.

One of the most remarkable, but as yet
quite unexplained, varieties in the external
coverings of the ovum in one of the osseous
fishes, is that discovered and recently de-
scribed by Ernst Hackel, as occurring in the
family of Scomberesoces** This consists in
the formation, in the space between the sur-
face of the yolk and the vitelline membrane
(that is, the porous membrane), of a layer of
long and very distinct fibres, which are wound
somewhat spirally, but irregularly, over the
surface of the yolk. Hackel has traced the
gradual formation of these in fresh specimens
of Belone from points on the surface of the
yolk-substance; and in other genera he has
observed several varieties in the forms of the
fibres. They are on an average about -S-^Q-Q"
thick, and long enough to surround the egg
several times ; and they appear to resemble the
fibres of the elastic yellow tissue more than
any other animal substance, but do not entirely
agree with them. In the meantime we must
suspend our judgment as to this very extraordi-
nary addition to the surface of the ovum until
farther observations shall have been made as
to their distribution in various fishes or other
animals, and as to their relation to the deve-
lopment of the embryo, f

* Muller's Archiv. 1855, p. 23. See plates IV.
andV.

f Some time after the above was in the hands of
the printer, I received the first and second parts of
the seventh volume of the Zeitsch. fur Wissen.
Zool., containing a notice of the discovery of the
micropyle in the Salmo salar, and S. fario, by
Professor Bruch of Basle. The observations leading
to this discovery were made in the winter of 1854-5 ;
and it is right to state here, that Dr. Ransom's dis-
covery of the micropyle in the gasterosteus, which
was communicated to the Royal Society on the 23rd
of November, 1854, was made in the months of June
and July previous; and these observations had been



Invertebrate Animals. The ova of Inverte
brata may be considered under two princi-
pal divisions, according as they present more
of the large-celled or of the finely granular
yolk-substance. The ova of the first kind
are usually of a larger size ; they possess a
larger germinal vesicle, and often a divided
or multiple macula ; and the process of seg-
mentation in them is either partial, that is,
limited to one part of the surface of the yolk,
or it occurs in a different manner on the upper
and lower sides of the ovum. In these there
is, in fact, nutritive as well as formative yolk.
In the other division of animals the yolk is
finally molecular, or is mainly composed of
smaller granules, and is chiefly of the formative
kind ; segmentation usually involves the whole
yolk, or if not so, is very nearly complete :
the germinal vesicle is generally clear, and
the macula most frequently single, and well
marked. It is true that the form and struc-
ture of the ova of Invertebrata presents many
and considerable varieties, as might indeed be
expected among animals of such diversity of
organisation as belongs to the great divisions
of the Radiata, Articulata, and Mollusca ;
but still it is to be observed that as a greater
degree of simplicity exists in the form and
structure of the primordial elements than in
the more developed textures and organs of ani-
mals, so also we find that much closer analogies
may be traced among these elements in the
lowest classes of the animal kingdom. We
meet, therefore, with little difficulty, even in the
most diverse tribes of the Invertebrate animals
in tracing the correspondence of the essential
parts of the ovum; and we are enabled also to
trace a more close analogy between these and
the corresponding parts in the Vertebrata
than might have been expected. We are there-
fore warranted in applying to them similar
designations ; and we have daily increasing
reason to trust to observations made on the
ovology of the lower animals as the means of
extending the knowledge of the reproductive
functions in Vertebrata and in Man. Thus
the recent discovery of the micropyle aperture
in some animals, and the certain and clear ob-
servation of the penetration of the sperma-

communicated to Professor Sharpey and myself in
August and September. In the beginning of
January, 1855, Dr. Ransom informed me by letter
of his having found the micropyle also in the Trout,
and a few days later in the Salmon. I then saw the
micropyle in the ova of both of these fishes ; and I
have since examined it minutely in the Stickleback,
and have confirmed in every particular Dr. Ransom's
statements. The existence of the micropyle in these
Vertebrate animals has thus been established by
several independent observations ; and I believe that
no one who uses the proper means can fail to detect
it in these and other fishes. Professor Bruch's ob-
servations were chiefly made on the ova after im-
pregnation, which may explain the reason of his
having failed to perceive the connection pointed out
between this aperture and the depression in the
centre of the germ disc. Bruch was like myself
unsuccessful in perceiving the entrance of sperma-
tozoa by the micropyle. His measurement of the
micropyle in the Salmon and Trout does not agree
with mine, making it much smaller.



OVUM.



[105]



tozoa into the ovum in others, suggest novel
and more general and extended views of the
process of fecundation, and while they add
certainty to the more limited observations of
the same kind made upon animals higher in
the scale, tend to prevent the adoption of
partial views in regard to these functions of
the animal economy.

It is principally among the more highly or-
ganised Invertebrata that we meet with that
form of ovum in which the nutritive is com-
bined in considerable quantity with the forma-
tive yolk, and in which segmentation is partial,
such as the Cephalopoda, Insecta, Arachnida,
Myriapoda, Crustacea, and some of the Arti-
culate Worms. In by far the greater number
of the Mollusca, such as Gasteropoda and
Acephala, the ova belong to the smaller kind
with more or less complete segmentation, as
also in most of the Annelida, as Hirudinea
and Lumbricina, the Nematoid, Cestoid and
Trematode worms, with the Planariae, the
Rotifera, Echinodermata, Bryozoa, Acalephae
and Polypina.

I now proceed to give a short statement of
the principal facts that have been ascertained
as to the structure of the ovum in these ani-
mals, and to state some details with regard to
some of those which are either best known
or which present phenomena of the greatest
interest.

1 st. Large-yolked Ova with partial Cleavage.
Cephalopoda. The ova of this class of ani-
mals have already been referred to in connec-
tion with those of birds, scaly reptiles, and
cartilaginous fishes, to which they present in
some respects a greater analogy than to those
of almost any of the Invertebrata. The con-
siderable size of the germinal vesicle with its
multiple maculae, the large mass of the coloured
yolk (nutritive), composed of conglomerated
masses of yolk corpuscles, and the very limited
extent of the process of segmentation, which
affects only a round disc of the germinal part
of the egg, are all characters in which the ova
of the Cephalopoda, at least the Sepia and
Loligo, which have been fully examined, are
ascertained to be similar to those of the large-
yolked group. We owe the most of our
knowledge of the ova of this class and their
development to Kolliker's interesting treatise,