colpoda (of Ehrenberg). These very soon fix
themselves ; and a fine pedicle is developed at
the place of attachment. In other instances
the Vorticella-cyst was observed to send
forth long contractile processes from its sur-
face, and then assumed very much the form
and appearance of an Acineta or Actinophrys;
and in this case a new Vorticella was formed
in the interior in the manner of a bud. The
Vorticella, therefore, it would appear, is ca-
pable of reproduction in two modes, by the
development of embryoes from the divided
nucleus, which Stein on this account proposes
to call nucleus germinativus (the testis of
Ehrenberg) ; and by gemmation from an
intermediate Acineta form. The first form
Stein would regard as the equivalent of sex-
* See the Article VEGETABLE OVUM for an ac-
count of this process in the lower forms of plants,
t Zeitsch. fur Wissensch. Zool. Feb. 1852.
j Ann. dcs Scien. Nat. 1845 and 1848.
OVUM.
ual production ; the second as coming under
the category of alternate generation ; and the
Vorticella embryo of the Acineta-form either
repeats its gemmal multiplication, or becomes
encysted, and gives rise then by its nuclear
division to embryonal production. Other
new forms of Infusoria are described by Stein
under the names Spirochona gemmipara,
Dendrocometes paradoxus, and Lageno-
phrys vaginicola, ampulla, and nassa, in which
the mode of reproduction is somewhat similar.
These observations at once show the im-
portance of the views entertained by some
authors as to the share the nucleus may take
in new production, and strongly indicate that
much still remains to be known from ob-
servation of the processes of reproduction
among the Infusoria.
Should these observations be confirmed,
another analogy, in addition to those already
observed, will be shown to exist between the
organisation and functions of the Protozoa,
and those of the lowest plants.* The ten-
dency of various other recent researches, to
which it has been impossible to refer more
particularly in this place, seems to be to show
that, in addition to the more common and
obvious mode of multiplication by division
and gemmation, by which the Infusoria, when
vigorous and well nourished, are reproduced,
there are other means by which, in dif-
ferent circumstances, the more permanent re-
production of the species may be secured;
that minute cells are formed within them
for that purpose, which may at present be
called reproductive cell-germs rather than
ova, till a more complete knowledge shall
have been obtained of their nature and of
the circumstances attending their formation ;
and that it is very probable that in the
protozoa, as in the simplest plants, the com-
bination of the contents of two cells, to all
appearance similar, may, as in the process of
conjugation, be the necessary preliminary step
to the development of the reproductive germs.
It ought at the same time to be kept in
view that the Infusoria may, like many other
animals, be subject, some to metamorphosis,
and others to alternate generations. Already,
since the publication of the great work of
Ehrenberg, most important modifications of
his system of these animals have been found
necessary, and it seems almost certain that it
is destined to undergo still farther changes,
many of those forms which are now recog-
nised as belonging to distinct genera and
species being possibly no more than different
stages of development of the same animal.
2nd. Of the possibility of primary r , direct, or
non-parental production of animals, or of so-
called spontaneous and equivocal generation.
From what has before been stated as to the
very general, and almost universal, existence
of the sexual mode of generation among ani-
mals, and from the reasons that have been
given for the belief that in those few and
simple animals in which a sexual distinction
* See the recent work of Alex. Braun, entitled
Die Verjungung in der Xatur, Freiburg, 1849.
has not yet been ascertained, there may still
be propagation by means of minute germs,
the reader will already have drawn a con-
clusion as to the very' in sufficient nature of
the proof that can now be adduced in favour
of the view that certain animals may arise
independently of pre-existing individuals of
the same species. The hypothesis might, per-
haps, be at once dismissed with the remark
of a recent writer*, " that it is safer to trust
to generally prevailing laws, than to confide in
such of our observations as are contrary to
them." But as in the article GENERATION f,
the author was led by a careful examination
of the evidence then available on the subject,
to admit the probability of the non-parental
mode of production as an exceptional occur-
rence, at least among the lowest tribes of
animals and plants, and as that hypothesis has
since gradually lost more and more of its pro-
bability, from the accumulated opposing proofs
resulting from more recent researches, so as,
in his opinion, to be now no longer tenable, it
may be proper at this place to review briefly
the bearing of the present more advanced
knowledge "of the generative process upon
this long and keenly debated question.
Admitting, in the meantime, that the ova,
or separate germs of Infusoria, have not yet
been discovered with certainty, there are not
wanting direct experiments which demonstrate
that in an infusion of organic matter which
would, when exposed to the air, naturally
furnish a rapid succession of these produc-
tions, the development of living organisms
is entirely suspended, if the arrangements
are made such as to render it impossible
for any germ or other part of a previously
existing infusorian animalcule or plant to be
communicated to the infusion. The experi-
ments of Schultze and of Schwann are
valuable, as appearing to have secured, in
a great measure, the above-mentioned con-
ditions, without otherwise interfering with the
validity of the result. The first of these ob-
Fig.6.
Apparatus employed by Schultze to prevent the access
of germs by the air to an infusion.
a, flask for infusion ; b, tube, with caustic potash ;
c, tube, with sulphuric acid.
* Eschricht, in Edinr. Xew Phil. Journ. vol. xxxi.
1841. p. 355.
t P. 42U.
10
OVUM.
servers* placed in a glass flask an infusion of
organic matter, a portion of which was known
from comparative trials, when left exposed to
the open air, soon to have animalcules deve-
loped in it in great quantity, and he connected
this vessel with a tubular apparatus, by two
apertures, in such a manner that the air, which
was made to pass frequently through the
vessel containing the infusion, should be drawn
through strong sulphuric acid, or potash solu-
tion, before reaching it ; and Schwann | ar-
ranged a similar experiment, having in view to
secure the like conditions, by causing the air,
which had access to the infusion, to be pre-
viously passed through an iron tube at a red
heat. Before the commencement of these
experiments, the infusion and the apparatus
were carefully subjected to the temperature
of boiling-water, by which it was presumed
the vitality of all ova, or germs, or other or-
ganic particles must have been destroyed :
and the result was the same in both the series
of experiments, viz., that, after a consider-
able lapse of time, no animalcules nor con-
fervoid plants were formed : but when the
atmospheric air was afterwards allowed to
pass freely over the same infusion, without
being subjected to the processes before men-
tioned, a rapid production of infusory ani-
malcules took place in the usual manner.
The results of these experiments appear to
be on the whole satisfactory, and nearly to
decide the question as far as relates to the
probability of the introduction of the germs
of Infusoria, &c., into infusions by the air.
But, indeed, the failure of many experiments
of this kind, when not performed with the
most scrupulous accuracy, need not excite
surprise, when the very indestructible nature
of some kinds of infusory animalcules is con-
sidered. It has long been known, and has
been ascertained by the careful experiments
of Spallanzani, Bauer, and Doyere, that some
of the Rotifera and Tardigrada are capable of
supporting a high temperature without loss of
life, and of being kept for years even in the
state of complete dryness, without loss of
vitality : and, although it must be admitted
that these animals differ greatly in their or-
ganisation from the Polygastric Infusoria, and
the latter appear to be very liable to destruc-
tion from slight causes, yet it is possible that
their germs may resist destruction in a greater
degree than their adult forms : and, should
only one of these animalcules, or its germ,
be left in any situation favourable to its
development, it is easily understood, from
what is known of the production of these
beings, with what rapidity a vast multitude
of them may be brought into existence by
their ordinary process of fissiparous increase.
Most physiologists are inclined to reject as
fanciful and inaccurate the alleged observa-
tions of the actual conversion of particles of
organised or organic matter into living in-
* Poggendorff's Annalen, 1837, and Edin. New
Phil. Journ. vol. xxiii. p. 165.
t In paper on Fermentation, &c. in Poggen-
dorff's Annalen, 1837, p. 184.
fusoria. At all events, statements of this
kind are to be received with the greatest
caution : such, for example, as the observa-
tions stated to have been made by Pineau*,
who affirms that he has seen the direct con-
version of particles of disintegrating muscular
fibre, isinglass, and wheat-flour, into various
forms of living infusoria.
The spermatic filaments also, which, so
long as they were looked upon as independ-
ent animals, were referred to as examples of
an undoubted spontaneous generation, furnish
no evidence in favour of that hypothesis in
the view in which they are now regarded by
physiologists : for they are to be considered
rather as a peculiar product of organic growth
within the spermatic cells, somewhat ana-
logous to the fine moving processes of the
ciliated texture, than as distinct organisms, -f-
In so far, therefore, as the theory of spon-
taneous generation may have been supposed
to derive support from the formation of the
lower forms of plants and animals in infusions
of organic matter, that hypothesis must be
considered as having lost the greater share of
its probability, if, indeed, it has not been
entirely disproved : but it must at the eame
time be admitted that a more precise ac-
quaintance with the nature of the germs from
which these organisms take their origin is
still required to render the arguments derived
from this source entirely conclusive.^
The external and internal parasites which
infest the bodies of almost all animals have
in former times been held to afford a still
stronger presumption in favour of sponta-
neous generation than the production of in-
fusoria ; but it will be found that in this
instance, to a much greater extent than in the
other, the probability of the view has gradu-
ally passed away before the increasing know-
ledge which modern research has afforded of
the various modes of propagation of these
animals.
The ready communication of various Epi-
zoa, or external parasites, from one animal to
another is now well known, and accurate ob-
servations have demonstrated that in almost
all instances this communication may be
traced to the implantation of ova, or pregnant
individuals into their parasitic abode, as in
the researches on the Sarcoptes scabiei, &c.
The parasitic fungi, also, of various cuta-
neous diseases, as tinea, porrigo, plica po-
lonica, foul ulcers, &c. ; the yeast-plant, the
vinegar-plant, and other minute fungi con-
nected with fermentation ; the contagious
algas of the batrachia and fishes ; the muscar-
dine of the silkworm, are all well proved to
be communicable by the deposit of their
spores, or some part of their substance, upon
the external surfaces of the bodies of the
animals on which they grow, or by their intro-
duction into cavities opening on the exterior.
Alltheinternal parasites, orEntozoa strictly
* Ann. d. Sc. Nat. March, 1845, p. 182.
t See Article SEMEN.
J Consult, especially, on the whole of this subject,
Dujardin's Hist. Nat. cles Infusoires, 1842.
OVUM.
11
so called, are now known to be capable of
true sexual generation, by means of ova, in
their perfect or complete condition, and the
whole class is remarkable for the great de-
velopment of the sexual organs, and the pro-
digious numbers of ova which they bring forth.
But it has been ascertained that their ova are
rarely developed into new being.s in the place
of the abode of the adult entozoa : they are
commonly subject, therefore, to migration
from one organ to another in the same indi-
vidual, or from one animal to another, or
from the parasitic to the free-living condition ;
and they have recently been discovered to
present very remarkable changes of external
form and internal organisation in their va-
rious habitations ; so great, indeed, that many
of them, previously believed to belong to
species, and even to genera and families
widely different, are now recognised as dif-
ferent conditions of the same animal or
species, and that many forms, whose mode
of generation was unknown, are found to be
derived by indirect production from ova, in
a manner which will be more particularly de-
scribed under the next section.
Thus it appears that the only entozoa
which are destitute of sexual organs, viz.
those belonging to the division cystica, are
very probably only imperfect forms of Taenia
or other cestoids, which, so long as they are in
the encysted or confined condition, do not
reach their full development : but many of
which, during their incomplete condition, are
capable of being multiplied by a process analo-
gous to gemmation.
The greater number of the entozoa breed
only when in the alimentary canal of animals,
and the ova are excreted along with the
foeces : it is obvious, therefore, that very
many ova must be destroyed, and that a few
only are liable to gain those peculiar situa-
tions which are fitted to maintain them in
their earlier conditions, or in their later stages,
to bring them, as parasites, to their full state
of development.
The entozoa are usually found, therefore,
in their most advanced stage, in the alimentary
canal. There seems, on the whole, little dif-
ficulty in accounting for the entrance of en-
tozoa from without into the alimentary canal,
or the pulmonary air-cells and other open
cavities : and every new fact that has been
observed relative to the occurrence of entozoa
in man and animals, leads to the conclusion
that the ova, or perhaps more frequently the
earlier larval or undeveloped forms of the
entozoa, gain access to these situations by
introduction from without, and most fre-
quently along with food and drink ; in those
instances at least in which the entozoa migrate
from one animal to another, or from an
animal to the free state before returning
to the parasitic condition. But the entozoa,
which are, in general, in an incomplete state
when situated in the close cavities or solid
textures of the organs of animals, sometimes
injiku their way from these situations into the
alimentary canal, there to undergo their final
development. Such appears to be the case
with the Strongylus armatus, living in an
incomplete state in aneurismal sacs of the
blood-vessels of the horse, and in a fully
developed state in the intestine ; the Stron-
gylus vagans, in cysts of the porpoise, and
afterwards free in the lungs ; the Ligula or
Bothriocephalus solidus, in cysts of the ab-
dominal cavity of fishes, and afterwards in
their perfect state in the alimentary canal of
sea-birds. The Trichina and Echinorrhynchi,
imbedded in the muscular flesh in great quan-
tities, are no doubt imperfect forms of other
worms, which must migrate from these situa-
tions to attain to their complete state.
With regard to the manner in which the en-
tozoa inhabiting the close cavities of the body,
or imbedded in the solid substance of organs,
either in the free or encysted condition, gain
access to these situations, which has to many
appeared inexplicable, excepting on the hypo-
thesis of their arising actually in the places
which they inhabit, observations are no less
decided in proving them to be of external in-
troduction.
In the first place it may be stated that, al-
though the ova of a considerable number of
the entozoa are of so considerable a size as to
render it improbable that they have passed as
such through the capillary vessels, yet few, if
any, of these larger kinds are observed en-
cysted, and in others the ova are extremely
minute, and might, without difficulty, be car-
ried through most of the capillary vessels.
In the next place it may be mentioned that
the embryoes, or earlier forms of various
parasites, and the ova of others, have been
observed in considerable numbers in the cir-
culating blood of various animals *, as showing
that by this means the entozoa may be carried
in their small and early condition into any
part of the body of an animal which is fitted
to afford the conditions favourable to their
farther development.
But in what manner have these bodies
gained an entrance into the blood-vessels, or,
in other instances, how may entozoa have
penetrated into cavities or the parenchyma of
organs, without being conveyed through the
blood-vessels ? To this question, also, recent
observations seem to furnish a satisfactory
answer : for it has been ascertained that, in
a number of instances, smaller or larger en-
tozoa, but especially the former, pierce the
tissues of animals with great apparent facility,
being frequently provided in the young state
with an apparatus of sharp hooks for that
special purpose. Some of them have been
observed in the act of passing through the
* I may here refer to the original observations of
Schmitz, (Berlin, 1826), and the more recent ones of
Valentin Gruby, Gluge, Vogt, and others. See
Valentin, Repertorium for 1842 and 1843. The
Annual Report in Miiller's Archiv. for the same
years, and in Wiegmann's Archiv. for Xaturgeseh.
Valentin's account of the Ova of Distoma in the fluid
covering the medulla oblongata of a foetal sheep
(Miiller's Archiv. 1840, p. 317), and V. Siebold's
Article 'Parasites' in R. Wagner's Handworterbuch,
der Physiologic.
12
OVUM.
solid substance of organs or through mem-
branes ; and from the various stages of ad-
vancement of others already referred to,
seen in different parts of the same animal,
little doubt can prevail that they must have
done the same : but the aperture through
which they make their way, besides being in
most instances very minute, seems to close
very rapidly and completely after them. So
that the occurrence of entozoa in entirely
isolated cavities such as the aqueous cham-
ber of the eye, or in the parenchyma of solid
organs, does not now present to our minds
any valid objection to the view that in all in-
stances they are introduced from without ;
and it will be apparent, from the same con-
siderations, that even the occurrence of en-
tozoa in the foetus, of which there are un-
doubted instances, and to which great import-
ance has been attached as an argument in
favour of their spontaneous origin, may be
explained on the supposition of their ova, or
young, passing from the maternal parent,
through the blood-vessels of the umbilical
cord, as is known to happen with various
poisons.
The whole history, then, of this remarkable
class of animals, as it is now known, tends to
support the general conclusion that they are
all capable in their complete state of sexual
reproduction, and that they gain the various
sites of their parasitic habitations by intro-
duction of their ova, or embryoes, or of more
advanced stages of their growth from without,
either directly into the open cavities, or more
indirectly, by piercing the coats of vessels,
membranes, *&c., into the close cavities and
the parenchyma of solid organs.*
A candid review of the whole evidence
on this question leads to the inevitable
conclusion, that, though all the difficulties
or doubts which surround it are by no
means completely removed, the hypothesis of
primary or spontaneous generation receives
little or no direct support from the accurate
observation of the mode of origin of those
animals which alone were supposed to afford
proofs of such a kind of production; and that
this view must, therefore, on the strongest
grounds of analogy, be in the meantime aban-
doned, for that which attributes the origin
and reproduction of all organised beings to an
undeviating connection through ova or germs,
seeds or spores, between new individuals and
others of identical species which have pre-
viously existed. And if the present some-
* As to the bearing of a knowledge of the habits
&c. of the Entozoa upon the question of their spon-
taneous origin, consult the able essay by Eschricht ;
" Inquiries concerning the Origin of Intestinal Worms
&c." in Edin. New Phil. Journ. vol. xxxi. p. 314.
1841, the article on Parasites by V. Siebold, in R.
Wagner's Handwork der Physiol. ; E. Blanchard's
Researches on the Structure &,c. f of Intestinal
Worms, in Ann. d. Sc. Nat. 1848 and 1849, parti-
cularly vol. vii. p. 121. Dujardin's systematic work,
Hist. Nat. des Helminthes, 1845. And in connec-
tion with this and the whole subject of spontaneous
generation, the Systematic Treatises on Physiology
of Uurdach, J. Miiller, Valentin, and Longet.
what imperfect state of knowledge does not
permit us to affirm this absolutely, as the
result of direct observation, the exceptions
are so few and unimportant, that they may be
disregarded in the overwhelming evidence of
a positive character in favour of the opinion,
derived from analogy, that every organic
being, if not produced in actual union with
another, derives its origin from a germ or
some such connecting part that has proceeded
from a being of the same kind.
If this be the present state of the argument
in respect to the hypothesis of the first origin
of organic beings, it need scarcely be added
that the opinion which has attributed the pro-
duction of various animals to conversion or
gradual transmutation out of other species or
genera, has still less of real to be adduced
in its support. In the long series of ages
in which authentic observations have been
made on animals, no such examples have
been ascertained, and there are no established
facts which give any substantial grounds for
believing that in the natural or wild state of
animals there is any departure from that un-
deviating succession of specific resemblance
between parent and offspring, which seems to
form one of the most constant of the laws
of organic nature with which we are ac-
quainted.
3rd. Production of dissimilar individuals
among sexual animals by a non-sexual process :
so-called Alternate Generations.
From the foregoing general views it ap-
pears that in all Vertebrated Animals, and
in by far the greater number of Invertebrated
animals, the process of permanent reproduc-
tion consists in the development of the new
being from the blastodermic mass formed by
a peculiar process of cytogenesis in the
fecundated ovum. But, as has already been
shortly stated, there are some varieties among
them in regard to the degree of directness
with which the product of development from
the ovum arrives at that state of maturity, or
sexual completeness, in which it is capable of
renewing the act of sexual generation. These
varieties may be classed as follows: 1st. The
product of the ovum, being single, attains by
a gradual process of development, when it
leaves the ovum at birth, to nearly the same
form and structure as its parents : this is
generally called Embryological Development.
2nd. The product of the ovum, being single,
is born or leaves the egg at an early period,
and while comparatively imperfect, or, as it is
called, in a larva state, and by one or more
successive changes of development of a marked
kind, afterwards reaches the specific or ty-
pical form : these changes are usually called
Metamorphoses. 3rd. The product of deve-
lopment from the ovum does not itself become
a complete animal, but gives rise, by a peculiar
mode of generation of a non-sexual character,
and therefore different from that by which
fecundated ova are formed, to a new body, or