development or metamorphosis into a perfect
Distoma, but lias gradually formed from germ-
cells within it a progeny, sometimes of one,
more frequently a number of bodies, which,
when they arrive at maturity, present each one
an external form and internal structure and
locomotive powers, entitling them to be con-
sidered as independent animals. Nor are these
directly converted into Distomata ; but again
there is formed within the body of each, and
in the same gradual manner from germ-cells,
a new progeny of animals nearly similar to
those producing them and equally differing
from the complete Distomata. Each of this
new progeny, as it increases in size, has formed
within it by development from germ-cells the
third progeny of the series, and the last of
the cycle ; but these are different from their
immediate parents, and in their internal or-
ganisation soon manifest the type of the
true Trematode. These animals are endowed
for a time with very active locomotive powers,
to which a long caudal appendage con-
tributes ; their two progenitors have been
confined in the parasitic condition, but these
Fig. 28.
Series of changes in the development and generations of Distoma. (From Steenstrup.~)
o, Ovum with embryo or larva developed in it. e, this embryo in a free moving state ; e>, another
embryo in its interior. (These are of Monostomum mutabile, from V. Siebold.)
E, this last embryo farther advanced. 1, first stage, soon after it becomes free ; 2 and 3, farther on,
with g, the second generation, within them in various stages.
G, 1, one of this second generation at an early period of its advancement ; 2 and 3, farther on, with
c, c, Cercariae or Distoma-larvae, within them ; g', one of the granular globules from which the Distoma
larvae and previous generations arise near the posterior part of the body.
c, one of the Cercariae or Distoma larva with its caudal appendage, p, the same, passed into its en-
cysted or pupa state, having previously lost its tail.
D, Distomata. 1, young Distoma immediately after it has quitted the cyst, and has penetrated a
short distance into the body of the snail ; 2, Distoma found deep in the viscera.
are in general freed from confinement, and
move about with great vivacity for a time
in the water surrounding the animals which
their progenitors have infested. In this state
they have long been known as Cercariae, and
as they have been supposed to be the young
of Distomata, have attracted peculiar notice
among Helminthologists.*
The free Cercariae are not, however, directly
converted into Distomata; but appear always
to undergo a previous metamorphosis in a
chrysalis state, or enclosed in a pupa cyst.
* Kitsch, Beitragzur Infusorienkunde, &c., Halle,
1817; Bojanus, in Isis, 1818; A remarkable and
interesting series of papers by V. Baer, in Nov. Act.
Nat. Curios. 1826, vol. xiii. ; Rud. Wagner, in Isis,
1834 ; V. Siebold, in Burdach's Physiol. vol. ii. of
German edit. p. 187., or vol. iii. of French transl.,
p. 32., &c.
Previous to the formation of this cyst the
Cercarise adhere to, and bore into, the sub-
stance of the animal infested by the Disto-
mata; the tail is cast off, an exudation from
their own bodies forms the cyst, which en-
closes them : within this they remain for
many weeks, and even months, moving all the
while, and undergoing changes of develop-
ment, by which they are at last converted
into the complete Distoma.
The greater number of the observations
from which this remarkable process of gene-
ration has been ascertained to occur are due
to V. Siebold and Steenstrup ; but the whole
succession of changes has not yet been ob-
served in any one species, and it is to the
latter observer especially that the scientific
world is indebted for the ingenious com-
bination and interpretation of the scattered
OVUM.
31
observations of previous inquirers, as well as
the addition of new facts, from which an
almost entire certainty is acquired that the
various phenomena do actually succeed each
other in the order above stated, and that the
occurrence of alternate or intermediate gene-
rations in these animals is established.
Von Siebold had in 1835 described in the
Monostomum mutabile the development of the
first embryo from the ovum in the Gregarina-
like or animalcular form, and had shown the
next change to consist in the formation within
the first embryo of a second body endowed
with locomotive power, and independent vita-
lity, and differing both from its immediate
parent and from the acKilt.* V. Siebold, as well
as others, had ascertained the Cercariae to be
themselves incomplete animals, and to proceed
from others by a process of internal production
of a non-sexual kind. Steenstrup therefore di-
rected his attention particularly to trace these
Cercariae on the one hand, in their development
into complete Distomata, and on the other,
backwards through their progenitors towards
the first origin from an ovum. His observa-
tions were made principally in three kinds of
Cercaria, which, along with their antecedent
and succeeding conditions, are found in great
numbers in the fresh water snails, Lymneus
stagnalis, Paludina vivipara, Planorbis, &c.,
and which had been previously named Cer-
caria echinata, C. armata, and C. ephemera.
In these, especially in the first, the conversion
of an encysted Cercaria by metamorphosis
into a Distoma, and the descent of the Cercaria
(by metagenesis) through two progenitors,
not themselves Distomata, was ascertained,
but he did not succeed in tracing these bodies
back to their origin from ova. By a com-
parison, however, of the body formed within
the animalcular embryo of the ovum of the
Monostomum mutabile, as observed by V.
Siebold, with the first progenitor of the Cerca-
ria, to which it was found to present a remark-
able similarity, the chain of evidence seemed
to be complete, and Steenstrup found himself
in a position to announce the general views
of alternate generation, which have ever since
their first publication attracted the greatest
attention, and contributed in a powerful de-
gree to modify and direct the investigation of
the generative processes in the lower animals.
To the immediate progenitor of the Cercaria
Steenstrup gave the name of nurse (altrix,
Amme), in allusion to its nursing or nourishing
function, and to the immediate progenitor of
this one he gave the appellation of " parent or
grand-nurse." These terms may be objection-
able, but an unnecessary amount of criticism
seems to have been bestowed on them by
some writers. They are adopted hypotheti-
cally by Steenstrup ; they do not appear to
withdraw him from the matter-of-fact state-
ment of his observations; and they seem to
be, in many respects, short and convenient
terms in the description of the phenomena.
These bodies have in the Cercaria echinata all
the appearance of distinct animals, that is, a
* See Wiegmann's Archiv, 1835.
body with a head separated by a neck or col-
lar, a tail or caudal projection, and two pro-
cesses of the integument similar to limbs, a
mouth and alimentary cavity, and they move
with all the appearance of spontaneity ; but
it ought to be remarked that the form and
powers of these nursing or formative cases
differ considerably in various other species,
and in some present so little of the external
form or endowments of an independent ani-
mal, that the more general appellations of
germ-cases, or germ-sacs, or sporo-cysts, may
be more appropriate to them.*
It is chiefly among the aquatic Gastero-
pod Mollusca, and a few land ones, that these
observations have been made ; but V. Siebold
has extended them to some of the Trematoda
inhabiting the air-sacs and other parts of
water fowls, which no doubt come from the
same Mollusca, and obtain access to the seat
of their final parasitic habitation from the
water or along with food, into which they
have come as Cercaria?, after having previously
been parasitic in the Mollusca. It is easy to
understand how the ova of the Distomata
discharged from the bodies of the water
fowl may gain their place in the Mollusca.
V. Siebold has observed in a very interesting
manner also the passage of the Cercariae
into the bodies of water insects (larvae of
Ephemera and Perlida), which he placed
together with a quantity of Lymneus stag-
nalis, from the various parts of whose bodies
the Cercariae were discharged in numbers out
of their nursing capsules : the penetration of
the integument of the insect by the Cercaria
and the mode of casting its tail being precisely
the same as that observed by Steenstrup in the
Mollusca.f
Both these observers agree that the first
and second germ-cases (or nurses), and the
Cercariae, or Distoma-larvae, arise by a process
of gradual development from extremely minute
granular spherules, which are at first situated
in the posterior region of the body, or between
the alimentary cavity and the integument.
These are certainly not ova : but we are at a
loss to state to what class of reproductive
germs they may be referred with greatest
accuracy.:}:
It is known that the bodies which inhabit
the aqueous chamber of the eyes of many
fishes are imperfect Distomata. Steenstrup
has frequently observed these larvae in the
pupa state adhering to the inside, and some-
times to the outside, of the cornea, and he has
occasionally noticed a delicate streak through
the cornea, indicating the track through which
the animal has penetrated ; and he considers
it as extremely probable that all the Trema-
toda of the eyes of fishes, of which a vast
variety has been described by Nordmann$, are
* See Victor Carus, iiber den Generations -wechsel,
for a figure of these more simple forms of sporo-
cysts.
t See the Article PARASITES, in R. Wagner's
Handworterbuch der Physiologic.
t See Fig. 28. </.
Mikographische Beitrage, &c., 1832.
32
OVUM.
at one time encysted, and that of the two
principal forms distinguished by that observer,
the one is the more advanced and the other
is the larva.
From what has already been observed it
seems probable that other productions pre-
viously described as Entozoa of various kinds,
may, in reality, be nurses or larvae of different
Distomata; and that many of these maybe
brought under their several specific distinc-
tions, when the new paths of investigation
pointed out by the suggestions of Steenstrup
and V. Siebold have been diligently pursued.
Von Siebold has recently related* a very
interesting observation on the remarkable
double Trematode animal, discovered and
described by Nordmann+, under the name of
Diplozoon paradoxum, from which it seems
to be ascertained that this double animal is
produced by the actual union of two nearly
similar simple ones, by a process of partial
fusion, which, though much less complete,
seems to partake in some degree of the na-
ture of conjugation, such as occurs in the
Closterium and some of the lower vegetable
bodies. The single animal, first described by
Dujardin, under the name of DiporpaJ, was
observed infesting the minnow (Leuciscus
phoxinus)in the same streams with the gudgeon
(Gobio fluviatilis). This animal corresponds
nearly in form and structure with the half of the
Diplozoon, with the exception of its smaller
size, and the absence of generative organs.
On the side of the Diporpa a projecting sucker
exists, and the union between two of these
animals which gives rise to the Diplozoon,
begins by a mere adhesion of this sucker,
which becomes more and more complete, so
as at last to lead to that entire fusion and
combination of the adjacent parts of the in-
testinal canal and some other organs, which
has excited so much surprise in the Diplozoon.
The development of the genital organs in
both of the two united animals succeeds to
this union.
Annelida. Some phenomena in the repro-
duction of the Annelida are to be referred to
the indirect mode of generation now under
consideration. The most of these animals are
hermaphrodite ; they are all more or less
jointed, or formed in the adult of repetitions
of segments of similar structure, the ante-
rior and posterior alone differing from the
rest. The jointed structure does not exist
in the embryo when it first leaves the egg ;
but is gradually produced by a process of
gemmation, which ma} 1 be styled intervening
rather than fissiparous. In the multiplication
of these segments the new ones are always
formed in the interval between the caudal
segment and that which is next to it ; the seat
of new production differs therefore in the
Annelida and Cestoid worms ; for in the latter
* Zeitschrift fUr Wissensch. Zool. 1851.
t Op. cit.
j Hist. Nat. des Helminthes, 1845. Dujardin had
noticed the resemblance between this body and the
two parts of the Diplozoon, and had conjectured that
it might be in some way the young of the Diplozoon.
of these animals the new joints are developed
at the cephalic extremity, and there is also
some difference in its nature, as the multipli-
cation of joints in the Tasnia is in some degree
truly fissiparous.
A few of the jointed Annelida have long
been known to be subject to another kind of
development, by which one or more complete
segmented individuals are formed close to their
caudal extremity, and spontaneously separate
to enjoy for a time an independent life. This
remarkable fact was first described by Otto
Fred. Miiller, in the small Nais proboscidea*;
and Gruithuisen described accurately the same
phenomenon in a Nereis or Chaetogaster.f
This process was looked upon by these ob-
servers as an instance of accidental fissiparous
generation ; but it has received a different
signification and a greater interest from the
more recent researches of Quatrefages and
Milne-Edwards.
The first of these naturalists observed in a
number of individuals belonging to the genus
SyllisJ, at a certain period of their life, a new
individual to be formed at the caudal extremity
of each. The part was first marked off by
a notch or transverse groove, the form of the
parent individual gradually appeared in it, with
the head, eyes, the same joints, limbs, &c.,
and it was ultimately separated by spontane-
ous fission. But the resemblance between
the original individual and its offspring was
chiefly external ; for it was found that while
the parent animal continued to exercise as
before the functions of nutrition it was not
possessed of generative organs ; and, on the
other hand, the new individuals seemed de-
stined alone to perform the reproductive func-
tions, and contained the fully formed sexual
organs, while their alimentary canal appeared
to become atrophied, and was not employed
in the digestion of any newly assumed food.
These individuals lived long enough after
separation to complete the reproductive pro-
cess by the formation of fecundated ova.
Milne-Edwards observed in the Myrianida
fasciata$, a similar, but more numerous gem-
miparous production of sexual individuals. In
this animal, as many as six new individuals
were observed to be formed in gradual succes-
sion, one before the other, and between the
caudal and terminal segments of the original
body. Each one of these new individuals, as it
* Naturgesch. einiger Wurmarten des Stissen und
Saltzigen Wassers, Copenhagen, 1800 ; and in a
Nereis, in Zoologia Danica.
f Nov. Act. Nat. Cur. vol. xi. See also J. Mul-
ler's Physiol., by Baly, vol. ii. p. 1424; Owen's
Lect. on the Gener. and Develop, of the Inverte-
brated Animals, in 1849, in Med. Times, vol. xx.
p. 88, where he refers also to observations of Oersted
in an Eulosoma, and of Schmidt in a tubicolar An-
nelide, called Filograna.
I Annal. des Scien. Nat. 1844, torn. i. p. 22. Otto
F. Miiller had also noticed the phenomenon in the
same animal, and described it under the name of
Nereis prolifera, in his Zoolog. Danica, vol. ii. p. 15.
Annal. des Scien. Nat. 1845, torn. iii. p. 170.
See also Longet's Traite de Physiologie, torn. ii.
part 3rd, p. 47.
OVUM.
33
arrived at maturity, and acquired the external
configuration and structure (though of smaller
size)^of the parent, was found to be possessed
of reproductive organs, while the original
animal had not acquit ed any. The first
Fig. 29.
Hijrianida fasciata. (From Milne Edwards.)
Twice the natural size. At the posterior part of
the body are seen six young, produced between the
caudal and the next joint in succession, from 1 to 6.
formed was situated farthest back, and re-
mained for the time attached to the original
caudal joint, and the others followed in suc-
cession before it, the last produced being
attached to the terminal joint of the parent
body ; and each newer individual presented a
less developed structure than the preceding
one. In the animal observed by M. Ed-
wards, the anterior or youngest individual
had only ten rings, the second had fourteen,
the third sixteen, the fourth eighteen, the
fifth twenty-three, and the last, or caudal one,
thirty rings. It would appear, therefore,
that the new individuals take their origin
from the last joint of the parent Annelide.
The observations of M. Edwards farther
make it appear that the process of develop-
ment and multiplication of the segments in
each of the new individuals is the same as in
the young Annelide first formed from the
ovum ; that is, the embryo is at first without
segments or rings, the head and caudal part
existing alone, and the joints being gradually
formed between them, and in succession, from
the posterior of the segments previously pro-
duced.
These phenomena cannot fail to recal to
our recollection the production of sexual
individuals by a non-sexual process analogous
to gemmation from imperfect parents or nur-
Supp.
sing stocks; and as Mr. Owen has remarked*,
" Since the individuals so propagated alone
acquire the generative organs, an alternation
of generations may here be affirmed of such
species; the oviparous individuals producing
eggs from which the gemmiparous individuals
come, and these, in their turn, reproducing
the oviparous individuals."-}*
But it is to be observed, that in many
others of the Annelida, the generation is of
the ordinary kind, or consists in the produc-
tion of sexual individuals, by their direct or
metamorphic development from the ovum.
Insecta. Aphides. A remarkable example
of a similar modification of the reproductive
Fig 30.
Production of Aphides.
A. (After Owen.) Diagrammatic representation
of the succession of generations of Aphides, from the
fecundated ovum o, the first embryo e, the suc-
cessive non-sexual progenies, g to g (of each of
which only one individual is represented), to the
male and female insects, m and/.
B. (After Leydig.) Enlarged view of the cham-
bers of one of the ovarian oviducts of a viviparous
Aphis. In the uppermost chambers are seen the
fine nucleated cells, of which one in 1' and 2', larger
than the rest, descends ; and in 3, 4, and 5, are seen
the changes of this and other granular and cellular
blastema, from which the new. individual is formed.
* Lectures, 1849, Med. Times, vol. xx. p. 83.
t At the same time it ought to be mentioned in
connection with the above, that, according to some
D
34
OVUM.
process among animals higher in the scale, but
in them of an exceptional character, has long
been known to occur in the various species of
the common plant-louse, or Aphis, first dis-
covered by Reaumur * and Bonnet f, and
confirmed and more fully illustrated by a
variety of accurate entomologists in more
recent times. In this animal, successive gener-
ations, amounting each to a considerable num-
ber, and in the Aphis lanigera If. averaging
about a hundred, are produced for seven, nine,
or eleven times, according to the species,
from parents of no sex, or rather which seem
to possess the structure of females imper-
fectly developed. The course of the gene-
rative process is the following : perfect male
and female winged insects are observed only
towards the autumn season : these fly about
in great quantities, the impregnated females
deposit their eggs covered with a protecting
case of mucus in the axils and other recesses
of plants, where they remain during the winter.
In spring there are developed from these ova a
brood of larvae, or imperfect female Aphides,
which soon produce, by an act of viviparous
generation, and without any concurrence of the
male sex, a progeny of a similar kind, and this
is repeated, in successive generations, for nine
or ten times in the common species, or for
ten or twelve weeks during the summer, at
the end of which time the last brood brings
forth male and perfect female individuals,
both of which die after having provided by
the production of fecundated ova for the con-
tinued generation during the next season.
Upon the discovery of this very remarkable
mode of reproduction, various theoretical con-
jectures were made in regard to its nature ;
but no satisfactory explanation presented
itself, till the knowledge of the general
nature of the process of non- sexual larvation
came to be brought under a general principle
or law. It is now obvious^ that the production
of the successive generations of Aphis-larvae
may be regarded as an instance of the multi-
plication of individuals from the product of
a single ovum, previous to the development of
the true sexual organs and the exercise of
the sexual functions. But this example of non-
sexual larvation deserves attention, not only
from its occurrence among animals placed so
high in the 2oological scale of organisation as
insects, but also from the degree of perfection
of the larva? themselves, and from the circum-
stance that the new broods are formed, not
recent observations, the parent or stock individuals
of both Syllis prolifera and Nais proboscidea arrive
ultimately at sexual perfection 'after having given
off a number of sexual individuals by the caudal
gemmation. See Leuckart on Metamorphosis, Non-
sexual Reproduction and Alternate Generations, in
Zeitsch. fur Wissensch. Zool. 1851, in which he
refers to Frey and Leuckart, in Beitrage zur Kennt-
niss Wirbellos. Thiere, p. 96 ; and to Schultze, in
Archiv. fur Naturgesch, 1849, p. 287, for observa-
tions proving this fact.
* Histoire des Insectes, Tom. iii. Paris, 1738.
f Traits' d'Insectologie ou Observations sur les
Pucerons, 8vo, 1745.
t See Owen's Lect. on the Invertebrate Animals,
1813, p. 235.
as in the other examples of this process to
which the attention of the reader has already
been directed, by a division of the whole
body, or by gemmation from its external or
internal substance, but from germs arising
within a determinate organ, corresponding in
its general form and anatomical relation,
though not entirely similar, to the generative
organ of the perfect female.*
the genital oreans of the viviparous or larval
Aphides differ Trom those of the perfect or
oviparous females, principally in the want of
the receptaculum seminis, and the organ which
secretes the mucous investment for the ova,
and there is also some difference in the form
of the ovarium, or germiparous part of the
organs.-j-
The gradual development of the larva brood
within the oviduct of the viviparous parent
has been traced carefully by several observers.
J. Victor Carus has attempted to show that
this process is to be distinguished from the
usual process of development from an ovum by
the absence of cell multiplication, and by the
formation of the embryo of the larva from a
merely granular germ : but more recently
Ley dig t has shown that though there maybe
differences in the structure and mode of de-
velopment of the ovum and of the viviparous
germ, the latter arises as truly as the former
from cellular elements. The uppermost com-
partment of the oviduct contains, according to
Leydig, from eight to twelve distinct nu-
cleated cells, together with a quantity of
finely granular substance. One of these cells
appears to descend into the second compart-
ment, in which an outer layer of cells exists,
with finely granular substance internally : in the
third compartment the cells of the outer layer
have become still smaller and more numerous,
and have formed, in fact, a covering to the germ,
similar to that which proceeds from cleavage
in the ovum. The rudiments of the internal
organs now begin to be distinguishable, and
the various external organs are successively
developed out of the cellular mass. The cel-
lular structure of the ovum of insects is at all