follow it, but it may be proper to state here
in a general way the relation which subsists
between the earliest changes occurring after
fecundation, and the commencement of those
phenomena of a histogenetic nature which
precede the formation of the embryo itself.
The most obvious and constant of these
changes is that known as the cleavage of
segmentation of the yolk, a process which
has been observed in the ova of all animals,
and is not less interesting from its own na-
ture, than from the bearing of its phenomena
upon the explanation of the earliest organising
process of embryonic development, and upon
the whole subject of histogenesis.
The segmentation affects only that part of
the ovum of animals which is directly ger-
minal or formative ; and it results in the pro*
duction of that layer of organised cells, of
variable extent, in the centre of which, in a
determinate position and direction, the rudi-
ments of the embryo are first formed. The
process of segmentation is, therefore, the pre-
lude to the formation of the Blastoderma or
germinal membrane of Pander.
The extent, therefore, to which segmenta-
tion affects the yolk differs greatly according
to the amount of the yolk-substance which is
directly germinal ; that being in some animals
the whole, and in others only a fraction of
the yolk, in proportion to the part which is
only indirectly nutritive. In that group of
ova, then, to which those of Mammalia belong,
and which we have called the small-yolked,
the entire yolk, or, at all events, its superficial
layer, being directly formative, or being in-
volved from the first in the production of the
Blastoderm, the segmentation is complete, or
the process of cleavage affects the whole mass
of the finely granular yolk within the 2ona or
vitelline membrane. In those ova again, such
as we find in the bird among vertebrate, and
the cuttlefish among the invertebrate animals,
in which the formative yolk has the most
limited extent, and consists only of a finely
granular disc near the surface of the much
larger mass of the cellular nutritive yolk, the
segmentation is confined to that disc alone,
and is therefore, in some respects, widely
different from that which occurs in Mam-
malia. In the intermediate group of ova be-
longing to Batrachia and Osseous Fishes, there
are many gradations of transition from the
complete to the partial cleavage, so that in
some, as the common frog, it is nearly, but
not entirely, over the whole yolk ; while in
others, as in the salmon or osseous fishes, it
does not extend over more than a third of
the surface of the yolk.*
* The more important phenomena of the yolk-
germ cleavage or segmentation have been ascer-
tained principally by the following observations:
viz. 1st. of Prevost and Dumas in Batrachia, as
early as the year 1824, and subsequently of Rusconi
and Von Baer in the same animal ; 2nd. of Bischoff
and Barrj- in Mammalia, in 1838-39, their obser-
vations being confirmed by myself in 1810, and
greatly extended by Bischoff before the publication
of his work upon the development of the rabbit, in
1842 j 3rd. of Bagge in 1841, and of Kolliker in
OVUM. [139]
In the greater number of instances there is
recognised in the mass of the whole yolk, or
in its germinal part, immediately previous to
its undergoing segmentation, a clear simple
cell, generally nucleated, which was not be-
fore perceptible ; to this the name of embryo-
cell has been given, in order to distinguish it
from the germinal vesicle, from which it has
hitherto been believed it is in some measure
distinct. In other instances a clear spherule
or space only is observed in the place of the
embryo -cell, and in a few animals no clear
part of this nature has yet been detected.
The division of the embryo-cell accompanies,
or rather immediately precedes, that of the
germ-yolk, so that each mass formed by the
cleavage, grooving, or segmentation, as the
case may be, contains as its nucleus or centre
an embryo-cell, or clear spherule of its own,
descended from the first cell or spherule of
the same description.
The process of segmentation, whether it
involves the whole ovum, or is limited to a
larger or smaller disc of the yolk, proceeds
in most animals with a certain degree of geo-
metric regularity, so that the number of germ-
yolk segments are successively multiplied so
as to be in the numbers 2, 4, 8, 16, 32, 64,
&c., until by the ultimate division a vast
number of small globular masses are formed,
which occupy principally the surface of the
yolk over all its germinal portion.*
The last result of the segmentation is the
production of the blastoderma or germinal
membrane in which, by other changes, the
rudiments of the embryo subsequently make
1843, on Xematoid Worms ; 4th. of C. Vogt in the
Salmonidae and in the Alytes Obstetricans in 1842 ;
5th. of the same author, of Quatrefages, and many
others in various invertebrate animals ; 6th. in its
most limited form the phenomenon was first well
described by Kolliker in Cephalopoda in 1844 ; and
7th. in birds by Bergmann in 1846, by Coste in
1848, and by myself in the following years. The
observations on this subject are far too numerous
for quotation; those especially which have been
made in experiments by artificial fecundation are
most favourable to the investigation of the seg-
mentation and other phenomena which follow
immediately on fecundation. And in all these
instances, as well as in very numerous others, the
occurrence of segmentation and the regularity of
its phenomena are so constant that we may regard
it as one of the best established series of facts in
organic nature. The observations with regard to
segmentation in the ova of insects, which are still
imperfect, form the only exception to the foregoing
statement with which I am acquainted.
* Reference is here made chiefly to the best-
known and most common kind of segmentation, in
which this process consists in the massing of the
granular and fluid substance of the yolk round the
embryo-cells or clear spheres as centres ; but it is
right to state that there is another form of this
process, as yet only observed in some of the Cestoid
and Nematoid Entozoa, in which the yolk, either
clear or granular in its structure, does not appear
to follow the divisions of the embryo-cells, but the
gradually increasing progeny of the latter assi-
milate or combine more and more with the yolk,
so that at last, when the germinal part of the ovum
is entirely occupied with new cells, the original
yolk has quite disappeared. The nature of this
process, as compared with the more common form
of yolk segmentation, is not perhaps as yet fully
understood.
[HO]
OVUM.
their appearance. According to most ovo-
logists, the last globules formed by segmenta-
tion are the nucleated organised cells im-
mediately constituting the blastoderma. A
different view of the process, however, in
Mammalia, has been taken by Bischoff, very
decidedly set forth in his two most recent
works on the development of the guinea-pig
and the deer; according to which the last
resulting spherules formed by segmentation
are not true cells, and that previous to the
formation of the blastodermic cells, the yolk-
germ falls completely into an amorphous or
homogeneous finely granular substance, out of
which, secondarily, the blastodermic cells are
produced by a process of cytogenesis. It
seems probable that, in the different classes of
animals, there may be considerable variety in
the degree of perfection in organisation or ad-
vance in cell-structure to which the segments
of the yolk have attained at the period when
the development of the embryo begins to ma-
nifest itself. But in the higher animals at least
the weight of evidence appears to me in favour
of the view that the process of segmentation
results directly in the formation of blastodermic
cells. The fact now established by the obser-
vations of Reichert in Entozoa, in 1841, of
Ransom in osseous fishes, and more particu-
larly those of Remak in Batrachia, that a de-
licate membrane is formed over the surface of
each of the segments as they appear, and that
the last and smallest segments possess a deli-
cate membranous envelope, appear to show
that, in these animals, each segment has the
structure of an organised cell, and is very si-
milar to, if not identical with, those of the
blastodermic lamina.
The origin of the embryo-cell is still in-
volved in obscurity. Most ovologists are dis-
posed to connect it in some way or other
with the previously existing germinal vesicle, or
some part of its contents, and more especially
the nucleus. For the solution of this ques-
tion, as already remarked, a more accurate
knowledge of what happens to the germinal
vesicle at the time of that disappearance which
has been so commonly observed at the period
of the maturation of the ova of almost all ani-
mals, will be required. Does the macula re-
main, as has been alleged by some, to form the
nucleus or the whole of the embryo-cell?
Or, in other cases, if the multiplied maculae
are dispersed among the granules of the ger-
minal yolk, are they again collected together
into a mass or spherule to form the embryo-
cell? Or, again is the embryo-cell formed
out of other materials, and not necessarily
either partially connected with, or wholly de-
rived from, the germinal vesicle ? And finally,
might it not be, according to some recent ob-
servations, such as those of J. Miiller on En-
tochoncha and those of Remak on the frog,
that the disappearance of the germinal vesicle
is not attended with the dispersion of its con-
tents, but is a phenomenon caused only in a
certain number of animals by the solution of
the delicate external wall of the vesicle, and
by some change in the position and consist-
ence of its contents ? Further observations
will be required to determine this point ; but
if in the meantime we regard it as most pro-
bable that the embryo-cell is in some way or
other connected in its origin with the germinal
vesicle, we might further found upon this
the speculative view that the blastodermic
cells and the blastema from which unques-
tionably, by a histogenetic process of cell-di-
vision and multiplication, the various textures
and organs of the animal body are produced,
may be regarded as the descendants of the
original cell-germ from which the ovum was
developed combined with the sperm. We
should thus trace the organic cellular connec-
tion between the succession of parents and
offspring, which I have stated to be one of the
most general facts in organised nature.
The observations respecting the very re-
markable movements of the yolk, before and
during the earlier stages of the segmenting
process which have now been recorded by
several physiologists, must excite the liveliest
interest, and suggest subject for much reflection
as to the evidence they may afford of the
causes of this change, or, if we may use the
expression, of the forces by which segmenta-
tion is brought about. There seems to be
little doubt that the embryo-cell (and its nu-
cleus first of all) is the earliest to become di-
vided, and that the process of cleavage then
proceeds from the surface of the segmenting
mass inwards towards the cell ; but in what
relation the nucleus, granular substance of
the yolk, and ovicell-membrane stand to each
other in this process, must be left to be de-
termined by future researches.
Of the other early changes in the ovum
which immediately follow fecundation and
precede embryonic development little need
here be said. They consist principally in the
greater degree of consolidation and compact-
ness acquired by the germinal part of the
yolk, and in the formation in most animals of
a clear space between the surface of the yolk-
substance and the enclosing vitelline mem-
brane. It is in this clear space, or, as it has
been called by Newport, respiratory chamber,
that the spermatozoa have been observed in
those instances in which they have been as-
certained to penetrate into the cavity of the
ovum. There is another phenomenon of the
same period, which has now been so frequently
observed, and which is of so peculiar a nature,
that it must not be passed over without no-
tice ; I allude to the appearance in the re-
spiratory space of one or more clear and
highly refracting spherules, nearly of the size
of the germinal vesicle, but quite independent
of it. These clear hyaline-like globules have
been observed in the ova of Gasteropodous Mol-
lusca after fecundation by almost all those who
have attended to the ovology of this class of
animals, among whom may be mentioned
Dumortier, Pouchet, Van Beneden, Nord-
mann, and Vogt; in the Annelida by Quatre-
fages ; in Mammalia by Bischoff' and Barry ;
and in Batrachia by Newport. From the
observations of Quatrefages in Hermella they
appear to be excluded or expressed, as it
were, from the clear basement-substance of
OVUM.
the yolk ; and Bischoff states that they gra-
dually disappear, or are dissolved without
obvious change. We are at a loss to deter-
mine what office these globules may have in
connection with the changes of the ovum at
the time they appear.
Lastly, I would notice the interesting re-
lation which appears to subsist between the
situation of the germinal vesicle and the cen-
tre of the germinal membrane afterwards
formed, or the germinal pole of the ovum,
and the conformity in the direction of the
line of the first cleavage of the yolk with that
of the principal axis of the embryo in verte-
brate animals. The first fact has been observed
in all animals, and the latter has been ascer-
tained by Mr. Newport's researches in Ba-
trachia, and by observations which I have
myself made in the bird's egg during its de-
scent through the oviduct. These facts, as
yet inscrutable in their nature, point to in-
teresting laws relating to the connection of
the first phenomena of development, which
may be worked out by the further prosecution
of these inquiries.
In the preceding part of this article we
have considered chiefly the anatomical struc-
ture of the ova of animals, and have made
little mention of their chemical composition.
The knowledge of the latter subject is as yet
very imperfect. In a recent Memoir* Messrs.
Valenciennes and Fremy have given an ac-
count of an extended series of experimental
researches in which they have been engaged,
with a view to determine the differences in
the chemical composition of the ova of dif-
ferent animals, and although this investigation
is still necessarily incomplete and fragmentary,
they appear already to have arrived at some
interesting results. The following are some
of the more important of these results.
The albumen or white is not exactly of
the same composition in the eggs of different
birds ; but it generally contains albumen with
salts and a compound of sulphur in solution.
In the yolk of birds' eggs they recognise the
principle first distinguished by Dumas and
Cahours as Vitellme y a substance precipitable
by mixture with a large quantity of water,
and apparently more nearly resembling fibrine
than albumen in its composition and some of
its properties. The phosphuretted fat of the
yolk is somewhat similar to the cerebral fatty
matter.
The glairy white of the eggs of cartilagi-
nous fishes is very different from that of birds'
eggs, being neither soluble in water nor coa-
gulable by heat nor acids to the same degree.
It seems to contain only traces of organic
matter. The angular and tabular particles
of the yolk of cartilaginous fishes are com-
posed of a principle which these authors re-
gard as peculiar, and have named IcJithine.
This substance is insoluble in water, alcohol,
and ether, and, on being dissolved by hydro-
chloric acid, gives no violet colour, as albu-
men does. It is dissolved by all the concen-
* See Journal de Pharmacie, &c., vol. xxv. pp.
321. and 415., and vol. xxvi. p. 5., 1854.
trated acids, and by dilute acetic and phos-
phoric acids. Its composition is stated to
be as follows: carb. 51 ; hyd. a 6*7; nit. 15;
ox. 25*4 ; phosph. 1*9.
In the ova of osseous fishes these authors
do not find the same organic principle, but
have detected two others in variable propor-
tions. One of these, which they have named
Ichtkidine, exists only in small "quantity, and
is absent in some fishes : it is quite soluble
in water. The other which is more generally
prevalent and in larger though variable quan-
tity is precipitated by water into a viscous
substance. This has been named Ichthuline.
Messrs. Valenciennes and Fremy have ascer-
tained the interesting fact that while these
principles, especially ichthuline, exist in large
quantity in the ova at an early stage of their
growth in the ovary, they gradually diminish
in quantity or are changed as the ova ap-
proach maturity, and give place chiefly to
albumen, which holds the phosphuretted fat
in suspension. In the salmon's egg there is
a large proportion of ichthuline, which is the
cause of their becoming opaque when water
enters the yolk. These authors propose in-
deed this character as a test of the maturity
of the ova, as they are not rendered opaque
by water when mature. It would be inte-
resting to know whether fecundation produces
any immediate chemical change on the prin-
ciples of the yolk. The composition of ich-
thuline is stated to be as follows : carb. 52'5 ;
hyd. 8 ; nit. 15'2; ox. 22'7 ; phosph. 0'6 ;
sulph. 1.
The ova of Batrachia seem to resemble
most nearly those of cartilaginous fishes, in
so far that the tabular particles of the yolk
are composed of ichthine. The external ge-
latinous covering is described as a tissue of
hyaloid membrane which absorbs water in a
determinate quantity.
The ova of Ophidia and Sauria resemble
nearly those of birds in the composition of
the white and yolk, containing the principle
vitelline in the latter. The yolk of the Viper
presents the singular peculiarity of becoming
gelatinous by immersion in water.
In the ova of several Chelonia they have
detected a different principle from vitelline to
which they give the name of vitelline. This
principle is soluble in potash, and has the
following composition : carb. 49'4< ; hyd. 7'4 ;
nit. 15'6 ; ox. and phosph. 26'7.
Among the invertebrate animals Messrs.
Valenciennes and Fremy have examined the
ova in several classes. In the Crustacea
they have given much attention to the in-
vestigation of the curious colouring principle
of the ova, which appears to be the same as
that existing in the shell, and which being
green in the moist state passes into red in a
variety of circumstances. They have isolated
this colouring matter by a very simple pro-
cess, and give an interesting account of its
properties, especially of the circumstances
causing it to change to red, such as the ac-
tion of alcohol, boiling, desiccation, placing
in a vacuum, friction, &c.
The ova of Arachnida am! Insects are
[142]
OVUM.
quite different from those of Crustacea in
their composition, containing albumen, fatty
matters, and a large quantity of a substance
precipitable by water.
The ova of Mollusca differ greatly in com-
position from those of other animals : more
particularly in the entire absence of fat from
them.
From these researches it appears that there
are considerable differences in the chemical
composition of the ova of animals of different
great groups, and even among those not far
removed from each other in the zoological
scale, and that there are also considerable
differences according to the state of advance-
ment of the ova of the same animal, more
especially it would appear that a marked
change of composition takes place at the
period of complete maturity. The researches
referred to appear to have brought to light
several new organic principles which are
modifications of albumen or belong to the
same class, and which may be considered as
Vitelline principles as belonging to the yolk
of different animals : such are Vitelline, Ich-
thine, Ichthidine, Ichthuline, and Emydine.
The full citations of different works and
memoirs on the subjects of this article render
it unnecessary to give any detailed bibliogra-
phical list at its termination. I may, how-
ever, call the attention of the reader to the
following works already cited, as forming the
principal basis of modern knowledge of ovo-
logy and development, viz.: The Inaugural
Dissertation of Pander on the Development
of the Chick, published in 1817. The His-
tory of the Egg before Impregnation, by
Purkinje, in 1825. The Epistola of Von
Baer, in 1827. The contributions of Von
Baer and Rathke to Burdock's Physiology, in
1827 and 1828. The various Memoirs by
Rathke at different times, and Von fitter's
Lectures on Development, completed in
1837. The Systematic Manual of Develop-
ment by G. Valentin, in 1835. The Prodro-
mus and contributions by R. Wagner, in 1836.
and the Manual of Physiology by the same
author. J. Mullens Physiology, and especially
the English translation of the more recent
edition. The researches of Martin Barry, in
1838 and 1839. The various contributions
of JBischof, beginning in 1838: His Systematic
Treatise on Development in 1842, and his
Monographs on the Development of the
Rabbit in 1842, of the Dog in 1845, of the
Guinea Pig in 1852, and of the Deer in 1855.
The researches of Coste beginning in 1833;
his work on Comparative Embryology in
1837, and his large and beautifully illustrated
work, as yet unfinished, beginning in 1850.
The works of C. Vogt on the Alytes Obste-
tricans (Batrachia) and on the Embryology
of the Salmonidae, in 1842. Lastly, the re-
cent and valuable researches of Remak on the
Development of Vertebrata in 1853-1855;
and the republication of R. Wagner's Icones
Physiologicae by Ecker. The works relating
to the invertebrate animals are much too
numerous for quotation. I will only mention
the researches of Kolliker on the Cepha-
lopoda, of Qiialrefages, Vogt y and others on
the Mollusca, Annelida, &c., and those of
J. Miiller on the Echinodermata.
I would also refer the reader to the excel-
lent report on the progress of discovery in
regard to the Ovum by Thomas W. Jones in
the Brit, and For. Medical Review for Oct.
1843, to Bischoff's article on the History of
Discovery in Development, and its application
to the explanation of Malformations in Wag-
ner's Dictionary of Physiology, to Leuck-
art's Article on Generation in the same work,
and to Vrolik's Memoir on the Explanation
of Monstrosity from the History of Deve-
lopment, and to his article Teratology in this
Cyclopaedia. A large amount of information
on the whole of our subject will also be
found in C. Vogt's interesting Letters on
Living and Fossil Animals in 1851 ; in Victor
Carus's System of Animal Morphology in
1853 ,- in Van der Hceven's Manual of Zoo-
logy, with additions by Leuckart in 1850-
1856 ; and in the English works of Carpenter,
Oiven, and Rymer Jones on Comparative
Anatomy and Physiology.
In now bringing this article to a close, the
author owes an apology to the conductors
and the readers of this Cyclopaedia for the
extreme tardiness with which it has appeared.
The delay has arisen, in part, from personal
circumstances which need not be mentioned
here, and in part from the nature of the sub-
ject of which the article treats. In the ori-
ginal plan of the article, it was intended that
it should comprehend, along with the history
of the ovum, an account of the development
of the embryo. But as time advanced, and
every successive year added new and im-
portant matter to our knowledge of the science,
and greatly modified the statement of facts
previously regarded as established, it became
more and more difficult, especially in the
hands of one interested in the experimental
investigation of many of the individual facts,
to present a systematic and at the same time
clear and brief description of the researches
of physiologists on the subject of develop-
ment. The author regrets deeply that he
should thus be prevented from furnishing the
readers of the Cyclopaedia with this part of
the article as originally intended. But at the
same time he believes that when the recent
rapid progress of many departments of the
subject is considered, and the vast number of
details which would be required to embrace
even the shortest account of the origin and
formation of all the textures and organs of
the animal body, the knowledge of which
forms a science that is coextensive with the