as different from a mere change of form by
growth or by metamorphosis in the same
individual.
At the same time it is deserving of no-
tice, that, among the compound Polypina,
the continuous multiplication of individuals
proceeds to such an extent, and with such
remarkable regularity, as to give rise to com-
posite masses, often of very large size, the
general arrangement of which bears a con-
siderable resemblance to that of plants : and
we are led still more to institute a comparison
between these animals and plants by the cir-
cumstance of the similarity of their continuous
growth by the addition of new sets of polype
individuals to the extension of the leaves and
branches of a tree or ramified plant, and by
the correspondence of the occasional forma-
tion of sexual individuals on the polype stock
OVUM.
39
with the development of flowers. It is not
without considerable interest, that the same
conditions as to temperature, season, supply
of nourishment, &c., seem to determine the
one or the other kind of production in both
the animal and the vegetable bodies. Many
botanists regard the plant as an assemblage
of individuals; and zoologists are for the most
part agreed as to the distinct individuality
of the parts united in a compound polype.
But the tendency to spontaneous separation
of these individuals, especially in their sexual
form, is very frequently exhibited in the
animal kingdom, while it is rarely, if ever,
met with among plants ; and among the
polype tribes, as well as in other examples
of alternate generation, the striking dif-
ference of form, structure, mode of life,
and functions, of some of the sexual indivi-
duals developed by non-sexual generation,
seem to warn us against extending the compa-
rison farther than the admission of the general
analogy above adverted to, or, at all events,
precludes us in the mean time from drawing
any arguments as to the nature of animal pro-
duction from that which is as yet only imper-
fectly understood in the vegetable kingdom.*
These considerations raise another ques-
tion on which recent writers are at issue in
regard to the theory of Alternate Generations,
viz. whether the various animal bodies formed
by the non-sexual process within one act of
sexual generation are to be regarded as so
many individuals composing the species, or
whether they are to be considered only as the
different states of one and the same indi-
vidual. I have abstained from entering
directly on the discussion of this question,
from the desire to avoid the confusion which
is apt to arise in it from the use of terms in
other than their usual significations. In re-
gard to connected animal forms, such as those
which coexist in a Compound Polype, less
difficulty might be felt than in those instances
in which a complete separation of the progeny
from its producer has taken place ; but it
seems to require a greater departure from the
ordinary signification of a common term than
is warranted by our present imperfect know-
ledge of the phenomena, arbitrarily to de-
termine to regard as merely one individual all
those bodies which may be formed by a non-
sexual process from the product of a single
ovum, notwithstanding the great variations in
their structure and mode of life, and the com-
plete separation and apparent independence
to which they may attain. It is unquestion-
ably important to acknowledge the integrity
and permanence of the species as maintained
in the midst of all these variations by gene-
ration from an ovum ; but there does not
seem to be any obvious impropriety in the
instances in question in regarding the species
as made up of individuals differently con-
stituted among themselves, and produced one
out of another by a non-sexual process. The
* For farther remarks on this subject the reader
is referred to an account of vegetable production,
under the Article VEGETABLE OVUM.
term Zooid suggested by Huxley, or Zoonite
previously employed by Milne Edwards and
some other French authors, or any such term
agreed upon as implying a relation of affinity
among the various bodies included in one act
of true sexual generation, may perhaps re-
move some of the ambiguity; but I confess I
do not think the present state of the inquiry
warrants the total abnegation of individuality
to the various animal bodies produced in the
non-sexual manner.*
In reviewing, then, the whole subject of
Alternate Generation, it seems to be equally
premature to refer the whole of the pheno-
mena included under this term, which may
hereafter be discovered to be very various in
their nature, to a simple process of gemmation
or individual development, or to attribute
them to the existence of certain powers, such
as a germ-force or spermatic power, remaining
in certain germ-cells, or to reject altogether
the hypothesis of Steenstrup of Alternate
Generations, which indeed is little more than
the expression of the course of the observed
phenomena ; until we shall know more exactly
the minute structure of the germ from which
a bud arises, and the difference between that
and the germ of an ovum, and until we shall
be more fully acquainted with the whole
structure and series of changes of the various
animal forms that have been the subject of
consideration in the preceding section.
We regard it, therefore, as more consistent
with the actual state of our knowledge of the
facts to describe the phenomena of Alternate
Generation as a peculiar mode of existence
belonging to some of the simplest kinds of
various classes of Invertebrated animals,
which seems to have especial reference to the
preparation of the sexual organs ; and of this
nature, that the animal immediately developed
from the fecundated ovum does not usually
arrive at sexual completeness, but has formed
from it, by a non-sexual process of production,
another individual of a different form, or a
succession of them, which finally attain to
sexual completeness, and produce the fecun-
dated ova that originate the generative cycle ;
and the effect of which is, to render two or
more successive generations of dissimilar ani-
mals necessary to the completion of the
species to which they belong.
* Some acute and interesting remarks by Mr.
Huxley on this subject will be found appended to
a sketch of J. Muller's discoveries on the Echino-
dermata, in the Annals of Natural History, 1851,
vol. vii. p. 1. I would also refer at this place to
the Lectures of M. Agassiz on Comparative Em-
bryology, Boston, 1849, as presenting a most en-
gaging view of the influence which the study of
the metamorphosis of animals, along with the
history of alternate generations, must exercise on
the systematic views in Zoology and Comparative
Anatomy, and I also take this opportunity of refer-
ring to some remarks by Mr. C. Spencer Bates in a
paper on the Development of the Cirripedia, in the
Ann. and Mag. of Xat. Hist, for 1851. vol. viii. p.
331. for a statement of the relations subsisting be-
tween the various forms of animals considered in
their sedentary and free states, either in individual
species or in different 'genera or families of the
various classes of animals.
D 4
OVUM.
Additional Remarks. Since the foregoing
observations on alternate generation were
printed in" August, 1852, the knowledge of
these peculiar phenomena and of the whole
forms of reproduction in various classes of
animals has been considerably augmented by
several important contributions. A short
reference to some of these researches seems
necessary in this place, in order to complete
our notice of the phenomena referred to.
The statements of Stein, referred to at
p. 7 of this article, as to the encysted condi-
tion of the Vorticellae previous to their under-
going multiplication, having been called in
question by Ehrenberg*, the subject has been
farther investigated by F. Cohn, with the re-
sult both of a full confirmation of Stein's state-
ments, and of its being ascertained that a
considerable number of other infusoria are
subject to a similar change ; among which he
mentions species belonging to the genera
Euglena, Prorodon, Chilodon, Notophyra,
Trachelius, Trachelocera, and Stentor.f Stein
holds it to be fully established that the nucleus
of the Vorticella is its true reproductive or-
gan. From this nucleus a progeny is formed
in two modes, in both of which the parent
animal becomes encysted. In one mode the
encysted Vorticella is converted into an Aci-
neta-form by the prolongation of narrow con-
tractile processes of its substance from the
external surface, and from the nucleus of this
Acineta-like animal successive single ones are
produced by gemmation. In the other form, the
nucleus of the encysted Vorticella undergoing
subdivision, becomes converted into the
germs or spores of numerous new Vorticellae.
This process is probably in these Protozoa
the equivalent of a sexual production of ova;
and appears to correspond nearly with that
which takes place in the production of the
Navicella-like progeny from Gregarinae, with
this difference, that in the latter case two
Gregarinae are united or fused together into a
sphere previous to the production of the new
progeny or spores.
The view of Leydig, therefore :, that the
parasitic infusoria named Gregarinas are only
an imperfect or metamorphosed condition of
a Filaria, does not appear to be confirmed.
In reference to the Entozoa, recent experi-
ments have* furnished ample confirmation of
the views now adopted by almost all physiolo-
gists, that these animals are the product of a
true sexual generation, and that their ova, or
embryoes, or larvae, are introduced from with-
out into the bodies of those animals which
they parasitically inhabit ; while at the same
time the knowledge of their migrations and
remarkable transformations becomes more
and more precise from additional and renewed
investigation of their different forms.
Herbst, who had previously failed in some
experiments to cause the transmission of the
* Bericht of Berlin Acad. for 1851.
f Zeitsch. fur Wissen. Zool., vol. iv. p. 253.
J On Psorospermia and Gregarinae, in Mtiller's
Archiv. for 1851, translated in Quart. Journ. of
Micro. Science.
Trichinae of the muscles by implantation in
wounds of animals, at last succeeded in
1850-51 in causing the muscles of several
young dogs to be infected with these parasites
by giving them to eat the flesh of a badger which
had lived with him for some time in confine-
ment, and in which the Trichinae existed in great
quantity.* About the same time Kuchen-
meister found that by causing young dogs to
swallow along with their food a quantity of the
Cysticercus pisiformis (of the hare and rabbit),
the intestines of these animals were in a few
weeks invariably occupied by a Taenia (T. ser-
rata). Von Siebold, who, as has before been
remarked, was the first to advance the opinion
(in 1844), that the Cysticercus fasciolaris in-
habiting the liver of the rat and mouse is
only the early condition of the Taenia crassi-
colis of the intestine of the cat, has with the
assistance of Lewald performed a series of
experiments of a nature similar to those of
Kuchenmeister, which have established be-
yond doubt that various,, kinds of Cysticerci,
and also the Caenurus of the sheep's brain,
and the Echinococcus veterinorum, are always
converted into Taeniae or Cestoid Worms within
a short time after they have been transferred,
as by feeding, into the alimentary canal of
suitable animals. These experiments have
also afforded to V. Siebold the means of ob-
serving in a most interesting manner the pro-
cess of development and gradual conversion
of the cystic into the cestoid entozoon.f
Since the publication of V. Siebold's ex-
periments, farther researches on the same
subject have been laid before the Academy of
Sciences of Paris by Van Beneden and by
Kuchenmeister, in Memoirs presented by
them in competition for the grand prize of-
fered by the Academy in 1853 for the scien-
tific investigation of the Development and
Transmission of Intestinal Worms. J
The observations and experiments com-
municated by Van Beneden in this prize essay,
and those contained in his highly interesting
Memoir on the Cestoid Entozoa, published
in the Memoires de 1' Academic Royale de
Belgique for 1850, have also established in
the most satisfactory manner the relation be-
tween the encysted or scolex condition and
the cestoid form of the Tetrarhynchi ; facts
which have also been in part confirmed by R.
Wagner. Van Beneden has observed the
first development of the ova of the Tsenia
dispar of the Rana temporaria || into the small
embryo provided with its three pairs of boring
booklets, and has watched with care the ac-
tive motions of these instruments, by which
* Ann. des Sc. Nat., 1852, torn. xvii. p. 63., and
in Quart. Journ. of Micro. Science, No. 3. p. 209.
t Zeitsch. fur Wissen. Zool., vol. iv. pp. 400. 409.
See particularly Plate xvi. A. Also in Annal.
des Sciences Nat, 1852, and L'lnstitut, No. 974.
J See the Report of the Commissioners, by Qua-
trefages, in Comptes Rendus for Jan. 30th, 1854.
p. 166.
See extract of a letter in Ann. des Sc. Nat. for
1853, vol. xix. p. 179.
|l Comptes Rendus, 1853, p. 788., and in Annals
of Nat. Hist., vol. xiii. p. 157.
OVUM.
41
the minute embryoes, scarcely larger than the
blood-discs of the frog, penetrate into the
tissues or organs in which they afterwards be-
come encysted ; thus affording the most direct
proof from observation of the manner in which
the young of these parasites become established
in the internal parts of animals. The head,
with the circle of booklets and the four
suckers, is then formed at the anterior part
of the embryo, constituting now the scolex of
Van Beneden ; and this author proposes to
give the name of proscolex to the previous
embryonic stage. In connection with the for-
mation of the head of the encysted animal, it
may also be noticed that Stein has observed
the three pairs of embryo booklets remaining
for a time irregularly scattered over the en-
larged vesicular part of the body.* When
the. encysted animal (Cysticercus) has been
introduced into the stomach and intestine of
a suitable animal, generally a carnivorous one,
it resists the digestive action of these organs,
but speedily loses its caudal vesicle, and
gradually acquires the new joints which are
formed from the head. It is thus brought to
the condition of the strobila, if we choose to
liken the jointed state of the tape-worm to the
multiple or divided polype stock from which
medusae are thrown off; and lastly, as the
sexual organs become fully developed in each
of these joints, beginning in the posterior ones,
which are first formed, these are detached one
by one, and constitute in the separate condi-
tion the bodies named proglottis by Dujardin
and Van Beneden. The latter author regards
these as alone the perfect animals of the
Cestoid, the jointed strobila being only a pre-
paring stock; and he adheres to the view,
previously referred to in this article, that each
proglottis approaches in some degree to the
organisation of a Trematode animal. Ac-
cording to the views of Van Beneden, there-
fore, the different stages of a Cestoid worm
are the following: 1st, the ovum; 2nd, the
first embryo or proscolex; 3rd, the cysti-
cercus, or encysted vesicular body, or scolex ;
4th, the jointed stock, tape-worm, or strobila;
and 5th, the separate sexual individuals, or
proglottidea.
Among the Trematode animals the pro-
duction of a succession of new progenies
by a process of internal non- sexual ge
neration has received full confirmation. Van
Beneden has added the important fact, how-
ever, that some of the Trematoda are not
subject to any process of alternate genera-
tion or metagenesis. He has traced the
whole process of direct development of the
animal from the ovum of Udonella caligorum,
a viviparous Trematode with large ova. The
alternating generation belongs, according to
him, to the Oviparous Trematoda, in which
the ova are of small size.
In reference to the Compound Medusoid
Animals, Siphonophora and Physsophorida,
* In observations made on encysted non-sexual
round worms, and the encysted condition of a ces-
toid inhabiting the Tenebrio molitor, and its larva,
in Zeitsch. fiir Wissen. Zool., vol. iv. p. 206.
it appears from the researches of Leuckart,
Kolliker, Gegenbaur and .others *, that the
several joints of the connected chains of
these animals, as previously conjectured by
Milne, Edwards, and others, may fairly be
regarded as distinct, though imperfect in-
dividuals, some of which are destined for the
sexual production of ova, while the free or
floating polypine stock remains destitute of
sexual organs. The free polypine stock , is
first developed from the fecundated ovum,
and acquires its one or two swimming vesicles :
from it there is afterwards formed the long
chain of smaller bodies connected together by
an extension of the digestive cavity and ex-
ternal substance of the animal. Some of
these form swimming bells or vesicles with
prehensile and stinging filaments ; others are
digesting cavities or stomachs, and others,
most frequently those last formed in the
chain, contain one or other of the sexual
products, either male and female among the
individuals of the same stock, or distinct sexes
on separate stocks. But there is sometimes
a combination of the motor and nutritive
with the reproductive organs in the same in-
dividuals of the chain, which warrants fully
their being regarded as something more than
the mere repetition of organs belonging to
one animal. They are, in fact, the same as
the sexual individuals of a compound po-
lype ; and in some rare instances indeed
they have been observed to become detached
and to swim about in the separate condition.
In connection with the alternating gene-
ration of the Salpae, in the discovery of
which the whole series of facts now under
review may be said to have originated, some
doubts were in the previous part of the
article stated to prevail as to the rela-
tion of the sexes in the animals of the
aggregated chain. These doubts have now
been in a great measure removed, and the
knowledge of the whole phenomena of the
reproductive processes in these remarkable
animals greatly extended by the researches
of C. Vogt f and of H. Miiller.J From these
researches it appears that while, as was pre-
viously known, the Solitary Salpa3 are en-
tirely non-sexual, the animals united together
in the aggregated chain, formed by successive
gemmations from one spot of each solitary
individual, are hermaphrodite, or possess both
male and female sexual products. The ova,
however, according to Vogt are developed
at a much earlier period than the spermatic
substance : indeed, they are advanced to the
condition in which they are ready for fecun-
dation, while their producers are still attached
as a chain to their solitary gemmiparous parent.
The spermigerous organs only advance to
perfection after separation ; and the process
of fecundation must therefore be effected in
the animals of the attached chain by the
* Zeitsch. fur Wissen. Zool., vol. iv. p. 304., and
vol. v. p. 285. &c. ; and Kolliker's Memoir on the
Siphonophora of Messina. Leipzig, 1853.
t Bilder aus dem Thierieben, 1852, p. 59.
I Zeitsch. fur Wissen. Zool., 1853, p. 329.
OVUM.
spermatic matter derived from detached
chains which are frequently floating near
them. To this result, no doubt, the currents
of water in the respiratory cavity will materi-
ally contribute.
Since the publication of the first part of
this article, the general aspect also of the
questions involved in the facts referred to has
undergone revision by several authors. The
whole doctrine of alternate generation has, it
appears, been called in question by Reichert, in
a programme entitled Unisexual Reproduc-
tion*, which I have not had an opportunity
of seeing. In his recent very able and in-
structive treatise on Reproduction in the
Handworterbuch der Physiologic, Rud.
Leuckartf, though he retains the name of alter-
nate generation as designating the phenomena
referred to, gives the doctrine little place, and
seems still inclined to regard this mode of pro-
duction rather as a peculiar modification of
growth than as a true generation. He is per-
haps right in his remark^:, that physiologists
have been too much disposed to separate the
phenomena of alternate generation from other
forms of production of a non-sexual kind, of
which he considers them as only a variety ; the
alternation of different forms being, according
to him, only a subordinate and not an essential
phenomenon. In his recent very interesting
work on Animal Morphology, Victor CarusJ
has allowed more importance to the views of
Steenstrup, adopting at the same time Owen's
term Metagenesis, as most suitable for the
designation of this kind of production. This
author appears to me to have made the
nearest approach to a correct appreciation of
the nature of this process in its relation to the
whole phenomena of animal reproduction as at
present known.
Reviewing therefore finally the whole of the
facts and opinions on this subject, I am in-
clined to adhere to the views expressed in a
previous part of this article, that the pheno-
mena of alternate generation or metagenesis
ought to be grouped together, and to retain
their place as constituting one of the general
forms of reproduction among animals ; con-
sisting, as they constantly do in a certain
number of animals, in the combination, alterna-
tion, or succession of the sexual and non-sexual
production of individuals all proceeding origin-
ally from the development of one ovum. At
the same time it is to be noted, that this mode
of production does not exist in all the species
or genera of those tribes of the lower animals
among which it has been observed to occur,
and that in some it passes by gradual transi-
tions into other forms of the generative pro-
* Die Monogene Fortpflanzung. Dorpat, 1852.
t Article Zeugung, in the 5th and 6th parts of
vol. iv. of Wagner's Handworterbuch, 1853.
J Loc. cit. p. 979.
System der Thierischen Morphologic. Leipzig,
1853.
cess. It is not on that account the same as
them. Metamorphosis and Metagenesis, also,
as V. Carus remarks, may be combined, but
they are different. " Larvae, the subjects of
metamorphosis, arrive at the state of perfec-
tion by throwing off provisional structures
which belong to their larval condition, but
nurses, the subjects of metagenesis, are them-
selves entirely provisional structures."
Generation, therefore, or the production of
new individuals belonging to a species, may be
either of the sexual or of the non-sexual kind.
It is only in the Protozoa that the distinction of
sex has not yet been discovered. In all other
animals the production of new individuals of
the species is the result of the development of
ova fecundated by spermatic matter. In all
the Vertebrate animals and in the majority of
the Invertebrate, the development of the
ovum gives rise to a single new individual ; in
most of them by a continuous process of
formative growth, in some by successive
stages, or by metamorphosis. In a few Inver-
tebrate animals the development of the ovum
gives rise directly to more than one individual
by what may be named primary division of the
ovum. In a considerable number of Inverte-
brate animals the production of new indivi-
duals sexually perfect is not immediate from
the fecundated ovum, but secondary or in-
termediate by non-sexual formation from a
preparing stock which is the product of de-
velopment from the ovum. In such animals
an alternation of sexual and non-sexual for-
mation of individuals is necessary for the
completion of the act of generation or the
reproduction of the species. In a few of
these animals only one new individual is
formed ; but in by far the greater number the
product is multiple, thus leading to a great
increase in the number of individuals, either
in the distinct or in the aggregate form, which
have all derived their origin from a single ovum.
It is to be observed however, that in some in-
stances the alternating generation may co-exist
either with the non-sexual mode of multipli-
cation or with direct sexual reproduction.
In conformity with these views of the rela-
tion of the product of generation to the ovum
and its progenitors, there might thus be esta-
blished the following modes of reproduction
among animals viz.