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THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

On, Stone, ly J.ErxLeljtsi.

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norv omnzrw '.' GOETHE .

ON

PARTHENOGENESIS,

OR

THE SUCCESSIVE PRODUCTION OF

PROCREATING INDIVIDUALS

FROM

A SINGLE OVUM.

A DISCOURSE

INTRODUCTORY TO THE H-UNTERIAN LECTURES ON

GENERATION AND DEVELOPMENT,

FOR THE YEAR 1849,

DELIVE-RED AT THE

ROYAL COLLEGE OF SURGEONS OF ENGLAND.

BY

RICHARD OWEN, F.R.S. &c.,

HUNTERIAN PROFESSOR AND CONSERVATOR OF THE MUSEUM.

LONDON:

JOHN VAN VOORST, PATERNOSTER ROW.

MDCCCXLIX.

By the same Author.

ON THE ARCHETYPE AND HOMOLOGIES OF THE VER-
TEBRATE SKELETON. Svo. 1848.

Two Folding Plates and Twenty-eight Woodcuts. Price 1 Os.

ON THE NATURE OF LIMBS. Svo. 1849.
Three Plates and Eleven Woodcuts. Price 6s.

ON

PARTHENOGENESIS,

THERE is a natural and irrepressible tendency in the hu-
man mind to penetrate the mystery of the beginning of
things, and above all that of the origin of living things,
involving our own origin.

But it is plainly denied to finite understandings to as-
cend to the very beginning, and to comprehend the nature
of the operation of the First Cause of anything.

And perhaps the best argument from reason for a future
state and the continued existence of our thinking part, is
afforded by the fact of our being able to conceive the pos-
sibility of the enjoyment of such knowledge, and the con-
sequent yearning to possess it, — that ^avrevfjua n of Plato,
or parturient vaticination of some higher knowledge which
cannot be fulfilled in the present state of our existence.

The ablest endeavours here to penetrate to the beginning
of things do but carry us, when most successful, a few
steps nearer that beginning, and then leave us on the verge
of a boundless ocean of the unknown truth, dividing the
secondary or subordinate phenomena in the chain of cau-
sation from the great First Cause.

The brief record of creation in the Sacred volume leaves
us to infer that certain plastic and spermatic qualities of

A 2

common matter were operative in the production of the first
organized Beings of this planet. " The earth brought forth
grass and herb yielding seed after his kind, and the tree
yielding fruit whose seed is in itself." " The waters brought
forth abundantly the moving creature that hath life;" and
" the earth brought forth the living creature after his kind,
cattle, and creeping thing, and beast of the earth." But of
our own species it is written, " God created man after his
own image, in the image of God created he him ; male and
female created he them." And " God said unto them, Be
fruitful, and multiply and replenish the earth." (Gen. i.
27 and 28.)

Since that first fiat went forth, the propagation of the
species of plants, animals and mankind has been left to the
operation of certain natural secondary causes, which we
sum up as the ' act of generation.'

Botanists and physiologists have observed and progres-
sively analysed the phenomena until they have reduced
them to a great degree of simplicity, the essential condi-
tions being the same, or closely similar, in both realms of
organic nature.

With regard to the animal kingdom, the generation of
which here concerns us, the essential conditions of the act
appear to be a nucleated cell, and the product of a nucle-
ated cell, with the combination of the two : the nucleated
cell is the c germinal vesicle/ and is the essential part of the
ovum ; the other nucleated cell is the ( sperm-cell/ and its
product is the spermatozoon.

It is essential to the development of the germ that the
ovum receive the matter of the spermatozoon : it is then said
to be impregnated.

The phenomena that thence ensue are essentially the
same up to a certain point in all animals, and consist in
the formation of a germ- cell (PI. I. fig. 4, c), and its propa-
gation of a numerous offspring (fig. 10) at the cost of the
germ-yelk (fig. 4, a), by a series of reiterated spontaneous

divisions. The right and clear comprehension of the pur-
pose of this process, or the object effected by it, is essential
to the elucidation of the nature and relations of the sub-
sequent modifications and varieties in the course of deve-
lopment. The progeny of the c primary impregnated
germ-cell * may be called e secondary ' or e derivative
impregnated germ-cells/ and the whole is the 'germ-
mass.3

The progeny of the impregnated germ-cell resemble
their parent, with a diminution of size, to a certain stage of
descent, when they may be ultimately reduced to their es-
sential parts or nuclei (fig. 11). "When they cease to exist as
germ-cells or nuclei of such, either by coalescing with others
or by liquefaction, they do not lose their vitality : as indivi-
duals, indeed, they may be said to die, but by their death
they minister to the life of a being higher than themselves
(e.g. figs. 12, 13). They combine to construct its tissues
or dissolve and impart properties to its fluids ; these me-
tamorphoses being mysteriously governed by a plastic
nature or mode of force operating unconsciously upon the
matter, but according to a law of order and harmony, and
to a fore- ordained and definite end, resulting in a distinct
and specific form of animal adapted by its organization for
a particular sphere of existence, and forming a more or less
valuable, but not, as once was thought, an essential link
in the great chain of organic life.

Not all the progeny of the primary impregnated germ-
cell are required for the formation of the body in all animals :
certain of the derivative germ-cells may remain unchanged
and become included in that body which has been composed
of their metamorphosed and diversely combined or confluent
brethren : so included, any derivative germ-cell or the nu-
cleus of such may commence and repeat the same processes
of growth by imbibition, and of propagation by sponta-
neous fission, as those to which itself owed its origin ; fol-
lowed by metamorphoses and combinations of the germ-

masses so produced, which concur to the development of
another individual ; and this may be, or may not be, like
that individual in which the secondary germ-cell or germ-
mass was included.

It has been found that in proportion as the subjects of
anatomical investigation descend in the scale of animal life,
the number of the derivative germ-cells and nuclei which
retain their individuality and spermatic power is greater,
and the number of those that are metamorphosed into
tissues and organs less.

Cells predominate in the tissues of the vegetable king-
dom, the lower members of which consist exclusively of
them, and have been thence called e plantae cellulares : ' the
lowest of all consist of a single nucleated cell.

The animal kingdom starts from the same elementary
beginning : a cell- wall forms the smooth elastic and con-
tractile integument of the Gregarina*: a fluid with granules,
and a firm nucleus which sometimes contains one or more
nucleoli, — the ordinary cell-contents, — are the sole repre-
sentatives of organs or viscera. Yet the power of the Gre-
garinte to live and grow independently by assimilating fo-
reign nutriment, the vital contractility of their tegumentary
tunic, their chemical composition and their definite forms,
with such well-marked specific characters, in a few instances,
as the Greg, brevirostris and Greg. Sieboldii present, render
their interpretation by Kolliker as a low and primitive form
of parasitic animal, the most accordant with actual physio-
logical and zoological knowledge f.

The Gregarina is a single- celled animal, which differs
from the single-celled plant by the vital contractility of its
tissue, and the solubility of its cell-wall in acetic acid.
Devoid' of mouth, stomach, or any other organ properly so

* A genus of microscopic parasites which infest gregariously the in-
ternal cavities and canals of insects and worms.

t Kolliker, ' Ueber die Gattung Gregarina,' Zeitschrift fur Wissen-
schaftliche Zoologie, Bd. i. p. 10, 1848,

called, it reduces our definition of an animal to the differ-
ence indicated in the preceding comparison.

The Polygastric Infusoria exhibit the next step in the
progress of individualizing a higher independent embodi-
ment of animal life. A firm central nucleus in which, as in
the Gregarina, resides the self-repellent property of spon-
taneous division, indicates, however, their essential charac-
ter as animated cells. But the granular contents have been
developed into secondary cells ; and some of these have
combined and coalesced to form special organs, such as
cavities for digestion, pulsatile cells for circulating a clear
plasmatic fluid, an irritable and contractile integument beset
with vibratile cilia : yet a large proportion of the contents
of this modified primary cell- wall consists of unchanged
secondary cells.

In the freshwater polype, Hydra, an external layer of cells
is partially condensed into an integument, and an internal
layer modified to form the gastric secerning villi : in the ten-
tacula a greater proportion of the derivative cells have been
metamorphosed into the muscular bands, the nodosities,
the prehensile darts and tactile cilia. But the chief point
that I have now in view is to draw attention to the large
proportion of retained and unaltered nucleated cells and
nuclei, which are identical in all recognizable characters
with the progeny of the primary impregnated germ-cell,
and which are ready, therefore, when favourable circum-
stances concur, to repeat the acts of assimilation and spon-
taneous fission, and, each individually, thus to lay the basis
of a new polype.

A large proportion of the derivative germ-cells is re-
tained unchanged in the compound hydriform polypes and
in the parenchymatous Entozoa : a smaller proportion in
the Acalephae and cavitary Entozoa. Derivative germ-
cells are aggregated in the last segment of the Nais,
and of the young of other Annelides. We find derivative
germ-cells, and masses of nuclei like those resulting from

8

the final subdivision of germ-cells, retained unchanged at
the filamentary extremities of the branched uterus forming
the ovaria of the larval Aphides.

In most of the lower classes of animals the course of de-
velopment is temporarily arrested at certain stages, though
growth may go on ; the embryo moving and feeding, and
perhaps propagating, as if it were a completed individual,
usually under a form very different from that which itself
or its progeny are destined ultimately to assume ; whence
these arrested forms have been termed e Larva?/ the true
lineaments of the fully developed form being hidden, as it
were, beneath a mask.

The earlier the individual in any of these larval stages
may have been arrested from the commencement of its
development from the germ-mass, the greater is the pro-
portion of the derivative impregnated germ-cells and nuclei
that continue unchanged in its constitution : and the result
of the retention of these, in the hydriform larvae of Aca-
lephes, e. g., is the exercise, as in the mature freshwater
Hydra, by one or more of such retained progeny of the
primary impregnated germ-cell, of the powers derived from
the legacy of the portion of the spermatic virtue which they
received from their parent nucleated cell.

In the Polygastria, e.g. when favourable influences of
warmth, light and abundant nutriment concur, a central
body, which represents the nucleus of the impregnated
germ-cell, sets on foot the special act of assimilation and
spontaneous fission ; and its divisions seem to repel each
other to positions equidistant from each other, and from
the pole or end of the body to which they are nearest.

The influence of these distinct centres of assimilation is
to divert the flow of the plasmatic fluid from a common
course through the body of the Polygastrian to two special
courses about those centres. So much of the primary de-
velopmental processe is renewed, as leads to the insulation
of the sphere of the influence of each assimilative centre

from that of the other by the progressive formation of a
double party-wall of integument, attended by progressive
separation of one party-wall from the other, and by con-
comitant constriction of the body of the Polygastrian, until
the vibratile action of the superficial cilia of each separating
moiety severs the narrowed neck of union, and they be-
come two distinct individuals. An eye-speck, a pulsatile
sac, a proboscis, or whatever organ may be required to
complete the specific characters of the particular Polygas-
trian, are likewise developed, and the individualization of
each moiety is thus completed.

This mode of propagation is termed e spontaneous fission/

In the freshwater polype the progeny of the primary
impregnated germ-cell retained unaltered in the body, may
set up, under favourable stimuli of light, heat and nutri-
ment, the same actions as those to which they owed their
own origin : certain of the nucleated cells do set up such
actions, those, e. g., in the Hydra fusca, which are aggre-
gated near the adhering pedicle or foot ; and the result of
their increase by assimilation and multiplication is to push
out the contiguous integument in the form of a bud, which
becomes the seat of the subsequent processes of growth
and development : a clear cavity or centre of assimilation
is first formed, which soon opens into the stomach of the
parent : but the communication is afterwards closed, and
the young Hydra is ultimately cast off from the surface of
the parent.

This mode of propagation is termed e Gemmation/ It
differs from the development of the Hydra ab ovo inasmuch
as the impregnated germ-cell, which set on foot the process,
is derivative and included in the body of the adult, instead
of being primary and included in a free ovum. But the
germ-cell is the essential part of the ovum, and the chorion
an accessory and non-essential part.

The very small size in relation to the entire body, and the
superficial position, of the derivative germ-cell which takes

10

on the processes of development in the Hydra, appear to
be the chief conditions influencing that modification of the
generative process by which a small portion only of the
Hydra is taken into the system of the new individual, in-
stead of one-half of the body, as in the case of the Monad.
So insignificant is the distinction between gemmation and
spontaneous fission ; the essential condition of both being,
as in the development of the ovum, the presence of the
pellucid nucleus of a derivative germ-cell, as the centre
from which all the processes in the formation of the new
individual radiate.

The Hydra propagates by ova as well as by buds. Cer-
tain of the retained fertile germ-cells multiply themselves,
and coalesce to form a larger central cell, surrounded by
others of the ordinary smaller size, the exterior of which
are metamorphosed into a chorion. Certain other germ-
cells are converted into sperm-cells, and develope sperma-
tozoa. The ova are extruded and fertilized by these: each
constitutes the first term of a new series of life and deve-
lopes a Hydra, which retains however a large proportion
of unchanged cells in its composition. Accordingly this
Hydra may propagate by buds, and the Hydra so de-
veloped may propagate again by ova, and these two kinds
of generation may alternate indefinitely.

In most of the marine hydriform polypes, the delicate
tissue of the body is supported in the waves and breakers,
and protected from the briny element by an external horny
integument. Were such a polype to propagate by gem-
mation, and the external crust to grow with the growth
of the bud, and expand to protect the soft digestive sac
of the new polype, an apparently compound animal would
result from the number of individuals so held together.
Such is the nature of a vast family of marine zoophytes
which our Lyncean Ellis has so accurately described and
beautifully figured as ' Corallines */ and which are grouped

* An Essay towards a Natural History of Corallines, &c., 4to. 1755.

11

together in modern systems of zoophytology under the
name of Hydrozoa*' or Anthozoa Hydroideaf.

These also must propagate by ova, in order that the spe-
cies may be dispersed. In some species the germ-cells are
metamorphosed into ova at particular parts, and the con-
comitant growth of the soft tissue and outer crust furnishes
those ova with a capsule (PL I. fig. 2,/) ; which modifica-
tion in the growth of the coralline Prof. E. Forbes compares
with that " metamorphosis in flowering plants in which the
floral bud is constituted through the contraction of the
axis and the whorling of the individuals borne on that
axis, and by their transformation into the several parts of
the flower."

The ova may escape from the ovi-capsule in the condition
of ciliated locomotive bodies, called ' planulae 9 by Sir John
Graham Dalyell ; or the planulae may be hatched in the
interior of a polype-individual developed from the summit
of the ovi-capsule, and which, after liberating them, may
wither and fall like the flower of a plant, as in some Cam-
panulariae%. Or a generative individual of a particular
form § may be developed and become detached, and, by its
own power of locomotion, carry the contained ova to a
distance from the composite and fixed group of nutritive
individuals.

The ova may be developed within the bell-shaped Aca-
lephoid prior to its detachment, as in the Coryne vulgaris,
observed by Wagner ||, or not until it has become de-

* Owen, Lectures on the Corap. Anat. and Physiology of the Inver-
tebrata, p. 82, 8vo, 1843.

f Johnston, History of British Zoophytes, p. 5, 8vo, 1846.

I Lister, Phil. Trans. 1834, p. 375, pi. 10. fig. 1, 5, 6. (See PI. I.
fig. 2, g, t.)

§ E. g. the Medusa octocilia and duodocilia of Dalyell, from Euden-
drium ramosum ; and the Tintinnabulum or Bell-medusa (PI. I. fig. 2, 0
observed by the same author to be developed from the Campanularia
dichotoma. — Edinb. New Philosophical Journal, vol. xxi. 1836, p. 91.

|| Isis, 1833, p. 256, t. xi.

12

tached and acquired the full characters of a bare- eyed
Medusa*

This remarkable phaenomenon is best shown in the cla-
viform Corallines, and has been especially described by
Lovenf in an excellent memoir on the Syncoryne ramosa,
and by Steenstrup in the Coryne Fritillaria%. This spe-
cies originally developes a many-armed digestive polype or
individual, which retains a large proportion of unchanged
germ-cells : these, by the stimulus of the excess of nutri-
ment, begin to repeat the process, and push out buds in an
analogous position to that in the Hydra fusca, viz. around
the base of the stomach of the first or parent animal : but
the buds, instead of repeating the form and condition of
that animal, take on a higher form, resembling that of a
bell-shaped Medusa ; they become detached and swim off
to a distance, forming and discharging the ova, which, as
Steenstrup conjectures, in their turn develope the fixed
polype-shaped Coryne.

This stage of the cycle has not yet been the subject of
observation ; but, by the analogy of the larger Medusae, is
the more probable process than that direct metamorphosis
of the medusiform individual into the pedunculate poly-
poid individual, which V. Beneden has described by the
aid of a conjectural figure in the Tubularia §.

The medusiform ovigerous locomotive or distributive
individual of the Coryne and Campanularia dichotoma is
obviously homologous with the polypiform ovigerous in-
dividual, which seems to nurse, as it were, the ova into
6 planulae y in the Campanularia geniculata ; and the nutri-
tive gemmiparous polypiform individuals in all the com-

* See the beautiful and philosophically treated monograph on the
Gymnophthalmata, by Prof. Edw. Forbes ; published by the Ray Society.
4to, 1848.

f Wiegmann's Archiv fur Naturgeschichte, 1837, p. 322, t. vi.

I ' On the Alternation of Generations/ translation by Raj Society,
8yo, 1845, p. 26, pi. 1. figs. 41-45.

§ Recherches sur I'embryogenie des Tubulaires, 4to, 1844, pi. 2. fig. 5.

13

pound Radiaries would seem, rather than the oviparous
medusiform ones, to manifest the typical form of the spe-
cies ; as the leaf is a more typical form of the plant than
the parts of the flower; and as the free-swimming Cirri-
pedal larva, with its pedunculated eyes, is more typical of
its class than is the blind and fettered multivalve Barnacle
into which it is so marvellously transformed. Superadd,
however, distinct nutritive and circulating organs to the
free-moving ovigerous individual from the rooted polype,
and prolong its existence, and it would then cease to have
the ancillary character of a nurse to the ova of the fixed
individuals, and would assume that of the perfected form
of the species, and such in fact is the case with the larger
gelatinous Radiaries called Medusa.

The egg of the species called Cyancea aurita is devoid of
a chorion*, and consists, after impregnation, of the germ-
mass only, formed according to the before-defined process
for the dissemination of the spermatic principle f. By the
combination and metamorphosis of the peripheral series of
secondary germ-cells a ciliated epithelium is formed J : an
assimilative cavity § results from the liquefaction of certain
central germ-cells : the germ-mass grows, elongates, and is

* Siebold, Beitrage zur Naturgeschichte der Wirbellosen Thiere, 4to,
1839, p. 21.

f The external signs of these universal preliminary steps in develop-
ment ab ovo are figured in the above-cited work, in tab. 1. figs. 1-13.
The preliminary internal changes may be inferred from what was ob-
served by Siebold and Bagge in the ova of the Strongylus auricularis (De
Evolutione Strongyli, 4to, 1841), some of whose figures are copied in my
'Lectures on the Invertebrata/ 8vo, 1843, p. 77, figs. 33-44, where I
remark, as Dr. Martin Barry had, also, independently remarked (see
the note which he communicated to the same work) : — "This prelimi-
nary division of the clear central cell to the spontaneous fission of the
yelk is closely analogous to that division of the central cell in the poly-
gastrian animalcule, preparatory to the spontaneous division of its body
into two individuals, which Ehrenberg described." (See PI. I. figs.
4 — 13 of the present ' Discourse/)

J Siebold, loc. tit. fig. 19. § Ib. fig. 15.

14

thus metamorphosed into an animal, which, like the ( pla-
nula' of the Hydrozoa, typifies the ciliated infusorial Leu-
cophrys-, ciliated lobes, like those of a Rotifer, are next
developed from one extremity ; these grow into arms, and
the whole animal assumes the form of a polype : as such it
was originally described by Sir J. G. Daly ell, under the
name of Hydra Tuba, and in this state it propagates by
gemmation.

The condition of this * Parthenogenesis,' or power of
propagation by the virgin larval polype, appears to reside,
as in the Hydra fusca, in the following structure : " The
body and tentacula of the larva are composed of two
distinct layers, an internal and external. The internal
layer chiefly consists of nuclei and nucleated cells of va-
rious sizes, some of them containing a large number of
nuclei ; while the external is chiefly composed of a struc-
tureless substance with numerous minute nuclei dissemi-
nated through it *." Sir J. G.' Dalyell obtained a colony
of eighty-three Hydrae Tuba, by buds, from one parent.
But the procreative force is not hereby exhausted. After
one, two or more years of captivity the body of the polype
has been observed to become thickened and impressed by
circular grooves : these, deepening, divide the outer surface
into rings, and this annulated part of the body may be sup-
ported on a contracted and smooth base ; the whole polype
being lengthened by the successive development of the seg-
ments so indicated : the margins of these segments next
shoot out short tentacles in eight pairs, and then the whole of
the annular partf is resolved by a series of spontaneous fis-
sions into as many small medusiform discs, which by growth
and a minor amount of metamorphosis are developed into
true Medusae. In certain of these individuals sperm-sacs

* Dr. J. Reid's ' Observations on the Development of the Medusae.'—
Taylor's Annals of Natural History, 1848, p. 26.

•\ The base may remain, reproduce the arms of the Hydra, and again
propagate by gemmation and spontaneous fission.

15

and spermatozoa are developed ; in others, ovisacs and ova :
thus distinct males and females are produced, combining
the nutritive with the generative functions. The ova are
impregnated, are sheltered for a time in special marsupial
pouches, the parent also performing the part of a nurse, and
then the young brood issues forth under its ciliated or in-
fusorial form. From the polygastric type it passes into the
rotiferous one, from this into the polype type, and all the
individuals propagated by gemmation under the latter form
are ultimately resolved by spontaneous fission into the male
or female oviparous Medusae.

I will cite a few other instances in which a species is
represented by a series of individuals of different powers
and forms succeeding each other in a cycle. Certain fresh-
water snails are infested by Entozoa of the order Trema-
toda or fluke- worms ; as the Limncea stagnalis e. g. by the
Distoma tarda. The ova, or products of the ova, of this
species are found, in early summer, adhering in vast num-
bers to the inner surface of the respiratory cavity and to
the exterior of the lobes of the liver and generative organs
of the snail ; where they increase in size, and detach them-
selves as free animalcules, assuming a bright yellow colour,
whence they were called by Bojanus " konigsgelben Wiir-
mern," and manifesting a twisting vermicular motion. If
one of these be microscopically examined, none of the
lineaments of the organs of the future Distoma can be dis-
cerned ; they resemble in structure rather the Gregarince,
consisting in fact of little else than the germ-mass, a small
proportion of which may have been metamorphosed to form
the smooth outer skin.

As the growth of this Gregariniform parasite proceeds,
a progeny is seen to rise in its interior by the aggregation
round separate pellucid centres of the nuclear matter
into numerous independent germ-masses, and their de-
velopment into embryos : these are first manifested by
the appearance of an anterior and a median circular
sucker, and then they acquire cephalic spicula and a

16

vibratile caudal appendage: they escape from the parent
cyst, bore their way out of the snail, and disperse them-
selves as free swimming ciliated Cercariform animal-
cules in the water. After a brief enjoyment of this free
and active state of existence, they shrink in size, the vi-
bratile tail is cast off, and they attach themselves to the
skin of the snail. Here they become buried, form for
themselves a pupa-case out of the condensed mucus, and
are metamorphosed into true Distomata, which gain their
parasitic habitat by piercing the soft integument of the
water-snail. Thus we have a Trematode entozoon succes-
sively assuming the form of a Gregarina, a Cercaria, and
a Distoma ; one impregnated ovum developing many in-
dividuals of the second form, each of which finally passes
into the third form*.

But this does not give the entire metagenetic cycle of
the Entozoon. The gregariniform parent of the cercarial
larvae is most probably the product of the ovum of the
Distoma tarda, not the ovum itself, the development of
which, in this species, has not yet been seen. From the
analogy of those Trematoda in which that development has
been traced, e. y. Monostoma mutabile, Distoma tereticolle,
Dist. hians-f, Dist. hepaticum, Dist. cylindraceum, Dist.
globiporum%) Dist. duplicatum, Dist. longicolle and Amphi-
stoma subclavata, the first product of the ovum is a free-
swimming -ciliated animalcule, and so like a polygastric
infusory as to have been described as a Param&cium. Sie-
bold, to whom we owe the most precise and valuable in-
formation of this first link in the chain of self-active indi-
viduals, found that this animalcule was hatched in the
oviduct of the Monostoma mutabile, was excluded from the
egg by pushing off its opercular cap, and then escaped
from the vulva. The ciliated integument inclosed a fluid

* Steenstrup, op. cit. p. 52, pi. 3. Siebold, Archiv fiir Naturge-
schichte, 1843, p. 321.

f Mehlis, Oken's Isis, 1831, p. 190.

| Nordmann, M ikrographische Beitrage, Heft 2. p. 139.

17

with fine granules, and a subcentral colourless, transparent'
structureless body, which, if the whole animalcule were
compared to a ciliated single-celled animal, would stand in
the relation of a nucleus to such cell. But this nucleus
has a very definite form : it is elongated, pointed at one
end, obtuse at the other end, on each side of which stands
out a short thick process. The ciliated integument of the
animalcule, after a brief course, ruptures, decomposes, and
sets free the body we have compared to the nucleus. This
body then increases in size, manifests a smooth non-ciliated
skin, and begins to show an extremely fine granular struc-
ture, but no other organ. Siebold, however, who could
not but recognize it as an independent individual organism,
asks if it might be a parasite of the infusorial-like embryo
of the Moriostoma, but observes that the constancy of its
existence in such wras opposed to that idea, and he points
out its resemblance to the parents of certain Cercarice de-
scribed by Bojanus and V. Baer. The solitary progeny of
the ciliated embryo of the trematode Entozoon is closely
similar in form and structure to those gregariniform animal-
cules, since described and figured by Steenstrup*, in which
are developed the numerous cercariform individuals that he
saw ultimately metamorphosed into trematode Entozoa.

The testimony of different good and independent wit-
nesses at different periods to different stages of the succes-
sive generations, which, when compared with one another,
are seen to link together and complete the metagenetic
cycle, is perhaps the best foundation for scientific confi-
dence in the truly marvellous result.

Siebold t observes the development and birth of the cili-
ated monadiform embryo from the egg of the viviparous
trematode Entozoon, and the escape of the gregariniform
non-ciliated worm from the ciliated one.

* ' On the Alternation of Generations/ Ray Society's Translation,
8vo, 1845, tab. 2. figs. 2 a— d.

f Wiegmann's Archiv fur Naturgeschichte, 1835, Bd. i. p. 67.

B

18

Bojanus* and V. Baerf trace the development of the
cercariform individuals from the ( king's-yellow worm/
which in its form and simple structure corresponds with
the vermiform offspring of the first ciliated embryo.

Nitzsch J traces the cereariform progeny of that worm to
their pupa state.

Steenstrup §, confirming the origin of the Cercarise from
the multiparous ' king's-yellow worm/ completes the ob-
servation of their metamorphosis through the pupa-state
into the trematode Entozoon.

The conditions of structure to be considered in the at-
tempt to explain the successive parthenogeneses are the
following.

In the Distoma tereticolle \\ the first germ-cell, which is
the result of impregnation, multiplies itself in the usual
geometrical ratio, progressively absorbing and assimilating
the germ-yelk, until a germ-mass of countless minute cells
is formed ; but the subdivision and diffusion of the sper-
matic force is carried further by the reduction of the
progeny of the primary germ-cell to the size and apparent
state of nuclei or granules ; in which state, with a clammy
fluid vehicle, inclosed in (a vermiform sac, usually composed
of a ciliated integument and sometimes provided with one
or more eye-specks, the germ-mass quits the ovum. We
perceive, therefore, that it has undergone no other change
than that which has contributed to the formation of such
peripheral parts. There are no proper internal organs. In
the Monostoma mutabile it would seem that the true nu-
clear matter of the germ-cells had, after their ultimate sub-
division, aggregated itself together in the centre of the cili-
ated embryo or modified germ-mass. In that aggregated

* Isis von Oken, 1818, p. 729-

t Beitragezur Kenntniss der Niederen Thiere, Nova Acta Leop. Car.
torn. xiii. pt. ii. p. 629.

J Beitrag zur Infusorienkunde, Halle, 1817.
§ On tha Alternation of Generations, p. 52.
II Kolliker, Miiller's Archiv fiir Physiologic, 1843, p. 99-

19

state it is liberated from the locomotive germ-mass; but
with the exception of a smooth peripheral extensible in-
tegument which now invests it, the nuclear matter has
exhausted no force in the formation of organs. The nu-
clear particles begin first to operate, apparently by self-
repulsion, thus making themselves discernible ; and by the
attraction and assimilation of the foreign organic matter of
the animal which the ciliated parent may have penetrated,
a certain growth ensues ; the nuclear contents then resolve
themselves into numerous distinct germ-masses. These are
in a great degree metamorphosed into the organs of as many
Cercariae : the unused- up cells, with the remnant of the
spermatic force, are exhausted in converting each Cercaria
into an androgynous Entozoon. In this final form the
spermatic force is mysteriously renewed in the appropriate
male organ ; ova are simultaneously prepared for its re-
ception, after which its dissemination recommences by the
formation of the germ-mass in each impregnated ovum.

Here, amongst other topics suggested by the phaeno-
mena just described, we may ask, whether all the anima,
or manifestations of self-perceptivity and self-activity, of
the several e Paramaecioids/ ' king's-yellow worms/ e Cer-
cariae' and 'Trematoda/ pre-existed as independent prin-
ciples in the single impregnated ovum from which those
animals are to proceed. Ajid we might enter upon the inter-
esting and important inquiry how, and how far, these phae-
nomena of Parthenogenesis may determine the long-mooted
question of the relation of such vital principles or manifes-
tations to the organization of the individual. But this is
foreign to my present object, if even time permitted, and
I proceed therefore to the other instances of cyclical ge-
neration.

V. Baer, who led the way in experimenting and ob-
serving the artificially impregnated eggs of the Sea-urchin
(Echinus), found them to be first developed into a ciliated
monadiform animalcule, like that of the Medusa aurita.

20

In the course of four days this animalcule had become me-
tamorphosed into (or had given birth to ?) a minute trans-
parent acalephbid creature which he compares to a Beroe.
Here his observations ceased, the animals having perished
on the fifth day.

In the autumn of 1845 Prof. Miiller had discovered in
the sea at Heligoland a transparent acalephoid animalcule,,
referable by its mode of motion to the e ciliograde ' order,
but which, from its singular form, somewhat resembling
a painter's easel, he described under the name of Pluteus
paradoxus*.

Returning in the following year to the same field of ob-
servation, he pursued with admirable detail the change of
this species into an Ophiura (Brittle star-fish), and that of
another kind of Pluteus into an Echinus^.

The larva of the Echinus, like that of the Ophiura, has a
quadrilateral pyramidal body, of a colourless, transparent,
or hyaline substance, dome-shaped above, slightly excavated
at the base, the corners of which are prolonged into straight
slender legs, strengthened by filiform rods of calcareous
matter, reaching to the summit of the dome : the mouth is
prolonged from the middle of the concave base into a four-
sided proboscis, the angles of which are also produced into
slender processes shorter than the four outer legs. Were
the creature to stand upon these it would resemble a table-
clock case, and the proboscidiform mouth and appendages
might swing to and fro like the pendulum. The larva is,
however, a free-swimming animalcule, and propels itself,
with the base and processes forwards, chiefly by means of
strong cilia, grouped on two tubercles at the sides of the
dome, which are compared to epaulets. In the centre of the
dome-shaped body is a subspherical granular mass, which

* Archiv fur Anat. und Physiol. 1846, p. 101.

f ' Ueber die Larven und die Metamorphose der Ophiuren und See-
igel/ Vorgetragen in der Kbn. Akad. der Wissenschaften, 29 October
1846; 4to, 1848.

21

Prof. Miiller calls the stomach, and which doubtless sur-
rounds the cavity to which the mouth and pharynx conduct.

The first evidence of the change to the Echinus is the
formation of a circular disc, like a clock-face, on one side
of the granular mass : five double lines, like pointers, radiate
from the centre of this disc, and their extremities expand
into circles with a double outline. These circles form the
bases of as many hollow tentacles, like those that traverse
the ambulacral pores of the adult Echinus, but which are
here single, not in pairs. Triradiate pedicellariae appear,
two on each side the disc, of the kind called ' gemmiform '
in the adult Echinus. The disc progressively expands, ex-
tending over the cellular mass to which it owes its origin,
anddevelopes tubercles, which push through the transparent
dome of the Pluteus, and are transformed into spines and
tentacles, both of which, together with the pedicellariae,
manifest their characteristic motions and combine these
seemingly voluntary actions with the automatic continued
vibrations of the ciliated epaulets of the larva ; so that it
can now both swim and creep. By the time the disc has
grown over half the granular sphere little of the larva re-
mains, except some of the slender calcareous rods : the per-
forated oviducal plates of the nascent Echinus are formed
round the pentagonal space, as yet imperforate, in the
centre of the original disc, determining the anal pole of the
developing spiny globe, close to which the cicatrix of the
ruptured pharynx of the Pluteus is completed by the so-
called ' madreporiform plate/ The mouth of the Echinus
is quite distinct and remote from that of the Pluteus, and
is first indicated in the centre of the naked or unspined
moiety of the granular sphere by the formation of the hard
summits of the five characteristic jaws and teeth of the
Echinus. The outer skeleton is completed by the progres-
sive extension of the ' ambulacral' and ( interambulacral/ or
spine-bearing plates, and by the less regular 'oral' plates.

Thus the Echinus is fully manifested, and becomes either

22

a male by the development of testes/or a female by that
of ovaria. The ova are impregnated, and develope the
monadiform embryo ; this gives origin to the beroiform
Pluteus, which produces by e metagenesis/ rather than by
metamorphosis, the young Echinus, and the cycle is com-
pleted by its growth into the oviparous adult.

Passing to another great group of Invertebrate animals,
the Mollusca, we learn from the observations of M. Milne-
Edwards*, Dalyellf and Sars J, that the embryo developed
from the ovum of the compound Ascidiae quits the recep-
tacle of the fixed parent in the form of a large Cercaria,
i.e. with a caudate appendage, as a freely swimming animal-
cule ; then finally settles, becomes attached to some foreign
body, developes the organization of the Ascidian, and pro-
pagates by gemmation the compound group of such Mol-
lusca, which in their turn develope ova producing freely-
swimming tadpole-like larvae.

Chamisso was long ago led by his observations during
Kotzebue's circumnavigatory Voyage to the conclusion,
that the solitary Salpa3 gave birth to embryo Salpae of a
different form, and connected together in chains ; from
each individual or link of which a solitary embryo is pro-
duced, which grows and assumes the characters of that
Salpa from which the chain of embryo Salpae again pro-
ceeds.

The Nais and the young of many marine Annelides pro-
pagate by gemmation from the terminal segment of the
body a succession of other individuals like themselves, and
the characteristics of the head of the parent, whether a
proboscis or eye-specks, antennae or branchiae, are recog-
nisably developed upon the anterior segment of the still
attached offspring. Before this is cast off, the gemmiparous
segment of the parent repeats its procreative office, and a

* 'Observations sur les Ascidies Composees,' 1841.
t Edinb. New Philosophical Journal, 1839.
I Cited by Steenstrup, op. cit. p. 48.

23

second young annelide is formed: sometimes a third, a
fourth, a fifth, and even a sixth may be generated, and thus
seven individuals may be seen organically connected in a
linear series*. They are ultimately separated by sponta-
neous fission. The gemmiparous parent has no generative
organs: these are developed in its offspring, which pro-
pagate by impregnated ova, producing in the Nereids a
ciliated monadiform embryo, metamorphosed into the seti-
gerous worm, which produces by gemmation and sponta-
neous fission the true males and females of its species f.

Closely analogous to this alternating mode of reproduc-
tion, and to that cyclical succession of differently formed
individuals which has been adduced from the Medusa and
the Distoma, are the generative phenomena presented by
certain members of the class of Insects, in which many
individuals are the product of a single impregnated ovum,
and are produced in successive generations from the indi-
vidual proceeding from that ovum.

It is now more than a century since Bonnet} first at-
tracted the attention of physiologists and naturalists to
this mode of generation in the Aphides or Plant-lice. And
because it was the first of a large class of phenomena, till
then utterly unknown and unsuspected, it was received
with considerable doubt, or met by total incredulity.

The facts are briefly these.

The impregnated ova of the Aphis are deposited, at the
close of summer, in the axils of the leaves either of the
plant infested by the species or of some neighbouring
plant, and the ova retaining their latent life through the
winter, are hatched by the returning warmth of spring:
a wingless hexapod larva is the result of the develop-
ment. This larva, if circumstances, such as warmth and
food, be favourable, will produce a brood, and indeed a

* Milne-Edwards, 'Annales des Sciences/ 1845, pi. 11. fig. 65.

f Ibid. p. 170.

% ' Traitc d'Insectologie, ou Observations sur les Pucerons/ 8vo, 1745.

24

succession of broods, of eight larvae, like itself, without any
connection with the male. In fact no winged males at this
season have appeared. If the virgin progeny be also kept
from any access to the male, each will again produce a
brood of the same number of aphides : and carefully pro-
secuted experiments have shown that this procreation from
a virgin mother will continue to the seventh, the ninth, or
the eleventh generation before the spermatic virtue of the
ancestral coitus has been exhausted.

When it is so exhausted, a greater proportion of the
nuclear germ-masses retained by the last procreant larvae
is used up : individual growth and development proceed
further than in the parent : some members of the last larval
brood are metamorphosed into winged males, others into
oviparous females. By these the ova are developed, impreg-
nated and oviposited, and thus provision is made for dis-
seminating the individuals and for continuing the existence
of the species over the severe famine-months of winter.

The metamorphosis in all insects is attended with the
casting off or creeping out of a certain proportion of the
precedent individual, which process is called the 'ecdysis*
or moult. With regard to the so-called metamorphoses
which issue in the succession of a fixed, blind, sessile
multivalve barnacle to a free-swimming crustacean wTith
pedunculated eyes*, or in the succession of a rooted
vermiform parasite to a natatory animal with articulated
setigerous limbs t — when these phaenomena are closely
traced, they are seen to depend in a greater degree
upon the action and coalescence of retained cells, than
upon a change of pre-existing tissues. If the development
of the ovum in the pedunculate ovarian sac of the low
organized crustaceous epizoon of a fish be closely traced,
the peripheral cells of the germ-mass are seen to combine
and coalesce to form the smooth transparent skin of the

* J. V. Thompson, Zoological Researches, 8vo, p. 69.
•f Norclmann, Mikrographische Beitrage, Heft ii. 1832.

25

embryo Lernaea, from which also tubular processes extend
in two (Achtheres) or three (Lernceocera) pairs, including
setae which project from their extremities.

In the Lernaocera the anterior pair is directed forward
like antennae, but they are unjointed; and the head is
further indicated by a coloured eye-speck. Another layer
of germ-cells has perished, as such, in order to form the
parietes of a straight and simple intestine, with a mouth
and anus. Thus the annelidous type is first manifested.

But a large proportion of the minute germ-cells remain
in the wide abdominal interspace, amassed around the
alimentary tube, and aggregated in groups at the base of
the tubular arid setigerous feet. With respect to the latter
we might say that the same provision is made for the re-
production of the limbs as is retained throughout life in
regard to those of the Lobster. In the larval Lernaea, how-
ever, those reserve-cells commence the formation of new
limbs irrespective of any injury to the old ones. The
whole peripheral stratum of the retained germ-mass in con-
tact with the primary integument is transformed into a new
integument. These germ-cells have increased and propa-
gated at the expense of the aliment assimilated by the
alimentary canal. The formation of the new integument
and of the new feet proceeds connectedly and contempo-
raneously : but the new parts are not moulded upon the
inner surface of the old ones. The plastic force has
changed its course of operation. A hinder segment of the
body is added to the front one which answers to the whole
of the body of the first larva. If antennae did not before
exist, a jointed pair is now developed. Instead of two
pairs of tubular setigerous limbs, three pairs of uncinated
prehensile limbs are developed from the anterior or cepha-
lothoracic segment, and as many pairs of articulated seti-
gerous limbs from the abdominal segment. New muscles,
new nerves, and new vessels are formed for the support
and exercise of these various instruments. The outer

26

case, and all that gave form and character to the precedent
individual^ perish and are cast off: they are not changed
into the corresponding parts of the new individual. These
are due to a new and distinct developmental process ; ren-
dered possible through the retention of a certain proportion
of the unchanged germ-cells and nuclei. The process is
essentially the same as that which developes the cercariform
larva of the Distoma within the gregariniform one, or the
external bud from the Hydra, or the internal bud from the
Aphis. It is a slightly modified parthenogenesis : and the
phases by which the locomotive annelidous larva of the
Lernaea passes through the entomostracous stage before
retrograding to the final condition of the oviparous, limb-
less, bloated and rooted parasite, are much more those of
a metagenesis than a metamorphosis.

The instances which I have adduced are a few out of
many that have been truly observed and faithfully re-
corded by zealous and acute men who have devoted them-
selves to the observant service of Nature, in hope of re-
ward by an insight into her secret operations for the multi-
plication of the species of the lower invertebrated classes
of animals. These observations have already demonstrated
the generation of many individuals from a single impreg-
nated ovum to be, in those classes, the rule and not the
exception i they have shown that the mode of that manifold
generation is similar in its essentials, though diverse in its
accessory circumstances, as in all other acts and things in
Nature. Bonnet, Chamisso, Von Baer, Ehrenberg, Dalyell,
Milne-Edwards, Sars, Loven, Siebold, Lister, Steenstrup,
Miiller, Kolliker and others have enriched their science
with valuable stores of such facts ; and they have mutually
corrected or confirmed the impressions derived from the
observation of the remarkable phenomena. These pheno-
mena remain, however, at present on record as such simply :
they have been generalized as being dependent on a law* :
* By Steenstrup, in the Essay above cited.

27

but this law, or the condition essential to their occurrence,
has not, so far as I can learn, been recognized, i arrived,
however, some years ago*, at what I felt to be a clear in-
sight into the circumstances which rendered the successive
generations from virgin Aphides possible and conceivable,
and I have the greater confidence in the truth of that
insight from finding it equally explanatory of the analogous
phaenomena of 'Lucina sine concubitu' in other animals.

These phaenomena, first observed, as I have said, by
Bonnet, in the genus Aphis, were the first to which the
thoughts of physiologists were bent to explain. But being
viewed in the light of a strange and anomalous exception,
and at a period when the phaenomena of embryonic deve-
lopment were not known, the earliest steps more especially,
success could not be expected.

Reaumur eluded the difficulty of the fact which Bonnet
had discovered, by affirming the Aphides to be androgy-
nous. The vagina in the perfect oviparous females has
appendages called spermatheca and colleterium : and
Reaumur might have even appealed to the microscope in
support of his idea, for he would have detected, by its aid,
spermatozoa in the spermatheca. But this would not have
proved the hermaphroditic structure ; for the spermatheca
receives the intromittent organ of the male, and retains the
semen in store for the successive impregnation of the ova
as they pass out : the ova at the same time being coated
by the adhesive and protective matter of the colleterium.
These appendages of the vagina are found in most ovipa-
rous insects : and the true male Aphis is as well known now
as that of any other species of Insect. Moreover, it is found
that the viviparous virgin larvae of the Aphides have not got
a trace of those appendages of the vagina, which Reaumur
supposed to be male organs. They were not required in
her mode of generation and are not developed : the germ-

* Lectures on the Comparative Anatomy and Physiology of the In-
vertebrate Animals ; 8vo, 1843, pp. 234, 366.

28

cell already exists in her with sufficient spermatic and
plastic force for its development : no semen, therefore,
needed to be retained, and there is no spermatheca : the
embryonic development is completed in utero, and no se-
cretion for the protective covering of ova was needed. The
structures therefore which Reaumur, under a misconception
of their nature, cited in order to solve the problem of
the alleged virgin procreation, are present only in that
perfect form of Aphis where no such phaenomena are mani-
fested*.

Leon Dufour, whose extent of research and comparison
of the generative organs of insects led him to a true appre-
ciation of the nature and function of the appendages to
the female organs of the oviparous Aphides, referred the
phaenomena of the generation of the larviparous Aphides to
* spontaneous or equivocal generation/ Now if we con-
sider what we actually learn from these words, — that the
larvae produced by the virgin Aphides are produced by
( spontaneous ' or equivocal generation, — it will seem to be
little more than another mode of stating the fact. The
condition or mode of the fact, i.e. the phaenomena rendering
it possible, are not explained by them : M. Leon Dufour,
however, meant to record his belief in a hypothetical mode
of generation, in which, as he expresses it, f the act of im-
pregnation was in no degree concerned/ Having detected
the male Aphis and well scrutinized the structure of its
organs, having witnessed the coitus with the winged female,
and carefully excluded the male in repeating the observa-
tions and experiments of Bonnet, M. Dufour satisfied
himself, and affirmed, that impregnation had no share what-
ever in the phaenomena of the development of the larval
aphis in the body of another virgin larval aphis.

With regard to the hypothesis of spontaneous generation,
the reasons which have led me to concur with most physio-
logists of the present day in rejecting it were fully given in
* Siebold, in Froriep's Notizcn, Bd. xii. p. 308.

29

a former Course of Lectures on the subject of Generation,
nine years ago, and every exact observation and experiment
subsequently recorded serve to render that hypothesis less
tenable and more gratuitous.

The learned and ingenious authors of the deservedly
popular e Introduction to Entomology5 admit it to be "an
incontestable fact that female Aphides have the faculty of
giving birth to young ones without having had any inter-
course with the other sex/' and they suppose "that one
conjunction of the sexes suffices for the impregnation of
all the females that in a succession of generations spring
from that union." They adduce, in order to show that
such a supposition is not contradictory to the general
course of nature in the production of animals, the case of
the hive-bee, " in which a single intercourse with the male
fertilizes all the eggs that are laid for the space of two
years ;" and the case of a common spider, showing (e that
the sperm preserves its vivifying powers unimpaired for a
long period, indeed a longer period than is requisite for the
impregnation of all the broods that a female Aphis can pro-
duce.^ But these instances do not touch the question how
one of such a brood, insulated from all connection, should
give birth to other broods. Admitting that this pheno-
menon may depend on the inheritance of the impregnating
principle transmitted from generation to generation, the
problem for the natural philosopher to explain is, how this
is brought about. The superaddition of the { spermatheca *'
to the vagina of the queen- bee, as of other oviparous in-
sects, plainly accounts for the fact in the ceconomy of that
insect which Messrs. Kirby and Spence quote, according
to the function of the part determined by the well-devised
experiments of Hunter on the silk-moth*. To say that one
conjunction of the sexes suffices to impregnate the females
of the successive generations of Aphides springing from that
union, is little more than a statement of the fact ; and it
* On Bees,, Philos. Trans. 1792, p. 175.

30

seems, indeed, to have been so felt by the able entomologists
cited, who conclude their remarks by confessing, — " It is
however one of the mysteries of the CREATOR that human
intellect cannot fully penetrate*."

Prof. Morren, a comparatively recent observer of the
anatomy and generative oeconomy of the Aphides, and
impressed by analogous feelings of the extreme difficulty
of the problem, having accepted the hypothesis of spon-
taneous generation as it has been applied to the Entozoa,
propounded, though not without reserve, a theory that the
larval aphides were developed in the body of the virgin
parent, like Entozoa, " by the individualization of a pre-
viously organized tissue f«" Here also is a phrase which,
when the meaning of it is analysed, does little more than
express the old facts in a new way. When a larval aphis
is developed, a new individual exists ; in other words, it
has been ' individualized5; and as nothing can come out
of nothing, it must have been by the individualization of a
previously existing something. The question to be solved
is, what is that something, and what has happened to that
something to make its individualization under the form of a
larval aphis possible and conceivable by us according to the
known analogies of other embryonic developments or indi-
vidualizations ? That would be the explanation of which
we are in quest, — one going as far as the explanation which
we are able to give, for example, of the development of an
ordinarily impregnated ovum ; and, by the proved analogy
of the essential condition of the development in the virgin
Aphis with that condition in the impregnated ovum, capable

* Introduction to Entomology, vol. iv. p. 16 1.

f " A dire vrai, je me refuse a emettre une opinion au milieu d'une
tel dedale, et je tiens pour plus philosophique d'avouer son ignorance
dans un phenomene ou la Nature nous refuse rneme Tapparence d'une
explication. S'il fallait une explication a toute force j'admettrais que la
generation se fait ici, comme chez quelques Entozoaires, par individuali-
sation d'un tissu precedemment organise"." — Annales des Sciences Natu-
relles, t. v. 1836, p. 90.

31

of having every advance of knowledge of the operation of
such essential condition applied to it.

When however M. Morren affirms " que la generation se
fait ici, comme chez quelques Entozoaires, par Findividuali-
sation d'un tissu precedemment organise/3 the objection
immediately arises, that no one has ever seen a portion of
mucous membrane, muscular fibre, or other organized tissue
detach and transform itself into an entozoon : such a pro-
cess is as gratuitously assumed, and as little in accordance
with observed phaenomena, as ( spontaneous generation * in
the abstract. In a former Course I objected that " The fis-
siparous nucleated cells of the ovum, once metamorphosed
into a tissue, can produce nothing higher, and nothing
else save by their decay, which products are excreted : but
the cells which retain their primitive state amidst the
various tissues which the coalescing cells have constituted
in building up the body of the new animal may, by virtue
of their assimilative and fissiparous forces, lay the founda-
tion for a new organism.5' I shall not however here pursue
the argument which is carried out in my published e Lec-
tures on the Invertebrate Animals/ but shall proceed in
the next Lecture to compare the phaenomena of generation
from virgin larvae, and the successive production of many
individuals from a solitary ovum, which other classes ex-
hibit, and to consider the explanations that have been
offered of such phaenomena, and how they have been gene-
ralized.

32

LECTURE II.
ON PARTHENOGENESIS (continued).

IN the former part of this Introductory Discourse I ad-
duced some of the instances in the animal kingdom in
which a species is propagated in different ways, and is re-
presented by procreative individuals of different forms ; the
group of such individuals representing nevertheless one
and the same species, and returning into itself. I cited
the hypotheses which naturalists and physiologists of de-
servedly high repute had propounded for one of the earlier
known instances of this cyclical generation, and showed
how they failed in their end ; and I proceed now to con-
sider the explanations that have been offered by those who
have recognized and pondered over a wider range of these
phaenomena, and have endeavoured to generalize them.

With a view to illustrate one of these generalizations,
permit me to revert to some of the more simple manifesta-
tions of the phaenomena which we were last considering.

Suppose the Monad to be developed from a germ-cell,
and to propagate by spontaneous fission, each individual
so formed developing and liberating germ-cells, and these
again producing fissiparous individuals*; just as the Hy-

* On the authority of Ehrenberg. No one however has yet wit-
nessed the development of the germ- cell into the Polygastrian ; but this
negative fact seems to me to have undue weight given to it when it is
made to preponderate against the analogies which have led the masterly
Micrographer to his conclusions. The germ -cells, which in well-fed
Polygastria occupy the interspaces of the digestive sacs, the rythmical
pulsatile sacs and the nucleus, are too regular in their cell -form, and for
the most part too much alike, to allow of our dismissing them as of no
functional significance under the name of ' sarcode.' M. Dujardin
would seem also not to have comprehended the essentials of an ovum
and of development from such, in the lowest forms of animals, when
he objects to Ehrenberg's interpretation of the cells in question, " pour
etre fixe definitivement sur leur nature il faudrait avoir vu au moins des
coques vides apres 1'eclosion." (Annales des Sciences Naturelles, t. x.

33

dra is developed from an impregnated ovum, and propa-
gates by gemmation an individual which in its turn deve-
lopes fertile ova. Not that the processes always do alter-
nate with this regularity, but they may do so.

It is obvious that by calling them e alternate generation/
we should not explain their nature any more than if we
were to say that the polype from the bud, or the monad
from the fission, was " spontaneously or equivocally gene-
rated independently of any impregnation/' or that they
resulted " from the individualization of a previously orga-
nized tissue." All these would be but so many different
modes of expressing the fact, with the addition of demon-
strable misconception of its nature by the advocates of the
equivocal generation hypothesis.

Let us further suppose the hydra from the bud to as-
sume a different form from the egg-born hydra out of which
it proceeded, and to be oviparous ; but the ova to produce
only the lower form of hydra, which is gemmiparous ; a
lower and higher form alternately appearing, the one from
an egg, the other from a bud. These phenomena might
likewise be defined to be ' an alternating generation' ; but

1838, p. 298; reproduced by Siebold, Lehrbuch der Vergl. Ana .
p. 23.) Such an objection would have force only were a shell or
chorion the essential part of an ovum. But this essentiality being the
impregnated germ-cell, which alone represents the ovum in the Poly-
gastria, it may be developed into the embryo, without the phenomenon
of exclusion, as in the case of the monadiform embryo of the Medusa,
and as Steenstrup describes to be the development of the gregariniform
larvae of the Distoma tarda. This is the course of development which
might be anticipated in the representative of an ovum in a micro-
scopic monad which itself represents an animated cell or ovum. The
relation however of those minutest forms of Polygastria, e. g. Chlamy-
domonas pulvisculus, to the larger and more complex forms has still to
be determined. If they should prove to be embryonic forms, the sig-
nificance of their close repetition of the first stages in the formation of
a germ-mass in the ova of higher animals by undergoing their spon-
taneous fission within a common covering would be intelligible. Com-
pare fig. 14, p. 24, and figs. 33-44, p. 77, of my " Lectures on the
Invertebrate Animals," 8vo, Longmans, 1843.

C

34

we should not, by that definition, have penetrated one step
into the mystery of the development by the gemmiparous
hydra of an offspring, without the preliminary act of im-
pregnation in its own person.

Again : the simple hydra, which by gemmation produces
the hypothetical higher form of oviparous hydra, may be
designated a e larva/ as containing potentially, and con-
cealing, as it were by a mask, such higher form of animal.
Or, by another figure of speech, the simple gemmiparous
hydra may be called a e wet-nurse/ in the character of
carrying or contributing sustenance to a young indivi-
dual of a higher grade than itself. But neither of these
metaphors would convey to us an idea of the organic con-
ditions essential to the development by the larval or nursing
hydra of a bud producing another individual : they would
prove, I opine, to every thinking mind, very unsatisfactory
substitutes for such knowledge.

The terms Generationswechsel, e alternate generation/
and Amme, e wet-nurse/ have been, however, proposed by
Dr. J. J. Sm. Steenstrup, a learned and ingenious Danish
Professor of botany, in explanation of the " natural phae-
nomena of an animal producing an offspring, which at no
time resembles its parent, but which itself brings forth a
progeny that returns in its form and nature to the parent •"
and these phaenomena, he writes, were "till now inexpli-
cable n (p. 1), meaning until the publication of his work
" Ueber den Generationswechsel," which has been trans-
lated by the * Ray Society ' with the title " On the Alter-
nation of Generations, or the Propagation and Develop-
ment of Animals through Alternate Generations, a peculiar
form of fostering the young in the lower classes of animals/^
8vo, 1845*.

I have carefully studied this work in the hope of ob-
taining some further insight into the mystery of Parthe-
nogenesis, or procreation by an animal without sexual
* This is the edition cited in the present discourse.

35

concourse, beyond that which ray observations on the
Aphides in 1841 and 1842 had furnished me with*. But,
although I laid down the volume with most grateful
feelings towards the ingenious author for his useful collec-
tion of scattered memoirs detailing the phenomena, for
his confirmation by personal researches of some of them,
and his generalization of the whole, I sought in vain for
any observation which really explained them, or anything
purporting to be explanatory of them, save the phraseology
and figurative expressions above cited : no organic condi-
tion is pointed out as being related or essential to the ( Lu-
cina sine concubitu.'

With regard also to figurative language in science, unless
the metaphor be exact, it misleads rather than explains.

Thus, as to the term Amme. The development of the
secondary germ-cell in the body of a larva is at the ex-
pense of the surrounding cells and fluids of that larva,
which therefore contributes food to the embryo ; but this
is rather as the mother of the viviparous vertebrate animal
contributes nutriment to its embryo, than as the wet-nurse
does to her foster-child : the relation of the offspring of the
parturient larva to its procreator is also very different from
that of the infant to the nurse that suckles and dandles
it ; and more closely resembles that of the foetus in utero.

To rightly and clearly understand the phenomena we
must adhere closely to them and to nature, and be careful
not to be led astray by ambiguous or remotely figurative
expressions.

No naturalist or physiologist has hesitated to call the
polype which propagates by gemmation a ' parent } in re-
lation to the young polype so propagated : it is true that
the parent has produced without preliminary sexual inter-
course in her own person, and she may have received ab ovo,
and not herself have developed the germ-cell w^hich was the
essential basis of the bud ; yet the term ' parent5 is never-

* 'Lectures on the Invertebrate Animals/ pp. 233-235.

c 2

36

theless more appropriate than that of ( nurse/ And were
the bud to be developed into a higher form of animal than
that which produced it, the term e parent } would still, as it
seems to me, be the most appropriate one to designate the
relation; since the bud, in regard to the individual pro-
ducing it, would still be ' its own flesh and blood/ as the
parental relation is expressed in homely parlance.

It is by no means easy, however, to find a name for the
relation in which the fissiparous monad stands to the two
monads between which it has been equally divided. A
parent retains its individuality distinct from its progeny ;
but the monad has become a part, and indeed the chief
part, of the two that have resulted from its spontaneous
fission. Both separated moieties are, in an equal degree,
the same individual as the whole from which they pro-
ceeded ; and in an infinitesimal though conceivable degree
the actual monad is the same individual as the first created
one from which it may have proceeded by an uninterrupted
succession of spontaneous fissions : and in that degree it
may be viewed as one of the oldest individuals in creation,
the individual having never wholly or bodily become de-
ceased.

These phenomena, as you will perceive, are suggestive
of reflections on personal identity and individuality which
might exercise and perplex as keen wits as those of ancient
Greece that debated the question of the identity of the old
ship of Theseus which had undergone so much repair that
no part of the original wood remained.

But the true and rational explanation of natural phaeno-
mena is usually the result of the exercise of common sense
and careful observation ; and, returning to those before us,
I may recall to your recollection that the Medusae actually
manifest the phaenomena that I have ascribed to the
Hydra for the sake of illustration : that is to say, they
resemble the polype at one stage of their development or
under one form of iridividualization, and the medusiform

37

individuals are developed from segments separated from
the larval polype. Analogous phsenomena, as we have
seen, are presented by some of the compound Hydriform
polypes, by Echinoderms, by certain Entozoa, by com-
pound Ascidians and Salpae amongst the Mollusks, and,
with various modifications, in other Invertebrata up to
species of Insecta, where, in the Aphis, the ( Luciua sine
concubitu' has ever excited the wonder if not the incre-
dulity of those who for the first time have been made
acquainted with the facts*.

The value of the ( alternate-generation 9 theory is well
tested by its alleged power of affording the explanation
of this very remarkable mode of Parthenogenesis. The
virgin-procreation of the Aphides is described by Professor
Steenstrup in his work (p. 108), agreeably with the account
which I have already given and which is generally received.
He calls the virgin larviparous Aphides, Ammen, or ' wet-
nurses t'; says the species is propagated by e alternate gene-

* " The multiplication of these little creatures is infinite and almost
incredible. Providence has endued them with privileges promoting fecun-
dity, which no other insects possess : at one time of the year they are vivi-
parous, at another oviparous ; and what is most remarkable and with-
out parallel, the sexual intercourse of one original pair serves for all the
generations which proceed from the female for a whole succeeding year.
Reaumur has proved that in five generations one Aphis may be the pro-
genitor of 5,904,900,000 descendants ; and it is supposed that in one
year there may be twenty generations/' — Kirby and Spence, Intr. to
Entomology, vol. i. p. 175.

•f* The adoption of the extreme metaphor of Amme was least to be
expected from the physiologist who objected to the comparatively close
and true application of the Linnsean figure larva to the answerable
stage in the metamorphic parthenogenesis of the Radiata. " It is quite
erroneous," says Steenstrup, "to term the Scyphistoma the larval con-
dition of Medusa aurita, since a Scyphistoma never becomes a Medusa,
but is the quasi mother of some scores of them." (p. 6.) Not more
erroneous, however, than to term the wingless procreating Aphis a
' larva/ which may be the mother of a winged Aphis, without herself
becoming one. Yet under certain conditions the ' larva ' does become
an ' imago,' as in A. Coryli and A. Quercus, e. g. It seems better there-
fore to extend, in like manner, the signification of the term 'larva' to

38

ration'; and affirms that " no true ovary has been discovered
in the larval and larviparous Aphides, but that the germs,
as soon as they are perceptible, are situated in organs which
must be regarded as oviducts and uteri." (p. 112.) Accord-
ing to my own observations the germs are perceptible in the
embryo Aphis, above the simple digestive sac, before any
organs have been formed for their reception. And, with
regard to the nature of the organs when formed, I may
remark that the continuity of the ovarian tubes with the
oviducts in all insects is such as to render the negation of
the term ( ovary' to those two bodies from which the slender
extremities of the eight oviducal and uterine tubes proceed
in the larval Aphis, to say the least, quite arbitrary. My
examinations agree with those of Siebold, in determining
scarcely any appreciable difference between the ovaria of
the oviparous and those of the viviparous females. The
contents of the ovarian tubes differ, inasmuch as they con-
tain oval masses of granules or nuclei, comparable to the
germ-mass in its state of minutest subdivision, in the virgin
aphides, and not ova with the germinal vesicle as in the
oviparous females. The most marked and essential di-
stinction, however, between the generative organs of these
two kinds of female is the multilocular form of the uterus
and simple vagina in the viviparous one, and the super-

those individuals in other classes that contain ' potentially ' the higher
form, and generate it by gemmation, than to restrict it to those that are
actually themselves metamorphosed into such form, which metamor-
phosis is, after all, but a minor degree of metagenesis. Throughout
Prof. Steenstrup's essay we meet indeed with instances where Nature,
as it were, compels him so to use the Linnsean term or an equivalent ; as
e. g. where speaking of the larval Medusse he writes : " But there is also
a most essential difference between the young fry and the full-grown
animal." " Let us now trace these young Medusae further" (p. 14),
meaning the individual or ' larva* in the mask of the ' planula' of Sir
J. G. Dalyell. "This polypiform animal, which we shall afterwards
consider as a pedunculated Medusa." (p. 15.) In what respect is it less
erroneous, on Prof. Steenstrup's own argument, to call this Scyphistoma
' a pedunculated Medusa' than a ' larval Medusa ' ?

39

addition to the vagina of the oviparous copulating female
of a sperm-sac and mucus-glands. In denying the pre-
sence of ovaria in the larviparous Aphis it would seem that
Prof. Steenstrup had been biased by a predilection for the
metaphorical name ofAmme, which he applies to them, and
by an a-priori conclusion, which he thus enunciates, viz.
" that as in the more perfect females an ovary especially is
formed, so in the e nursing y individuals a much-developed
uterus is presented, in consequence of which, they, as in-
dividualized uteri, have assigned to them, as the object of
their existence, the performance of the functions of a ute-
rus" (p. 113.) And, no doubt, the relations of the larval
Aphis to the embryo developing within it are much more
truly uterine than mammary — more maternal than nu-
tricial.

Some of the progeny of the primary germ-cell in the
human species are discernible in the ovaria of the human
embryo, and may have laid the basis of such ovaria, since
the like unchanged progeny of the impregnated germ-cells
have undoubtedly constituted the ovaria in the larviparous
Aphis : only in the human embryo the spermatic virtue has
been so far exhausted in the scanty remnant of the remote
progeny of the primary cell, which remnant goes to form
the ovarian germinal vesicles, that the stimulus of a new
coitus is requisite to set on foot the developmental processes.
In the Aphis, on the contrary, the presence of such common
external stimuli as warmth, light and nutriment suffice to
call in action the latent remnant of the spermatic virtue, and
to produce such a multiplication of the derivative germ-cells
and nuclei, as serves not only for the formation of the tissues
of the embryo, but to spare ; and such unmetamorphosed
germ-masses are stored up in its ovaria with their inherited
spermatic powder, which, notwithstanding its reiterated sub-
division, is still capable of responding to the action of the
same external stimuli to which, as the exciting cause of the
procreative act, the entire animal owed its origin. Had

40

the Danish physiologist rightly recognized the anatomical
structure and contents of the generative organs of the larval
Aphis, he might have discerned a much closer relationship
between the ovaria and the act of larval propagation than
between this and the uterus.

When, in regard to the analogous generation of the
Medusa, Prof. Steenstrup says that the polypiform Scy-
phistoma " is a precedent generation of the kind in ques-
tion," viz. e alternate generation/ and that the Strobila (e is
a precedent individual or preparative individual/5 it seems
to me that we have here only a different and more formal
mode of expressing Siebold's and Sars's facts : viz. that the
individual form of the Medusa, which Sir J. G. Dalyell
called the Hydra Tuba, precedes the more perfect form, and
that the next larval individual, which Sars called ' Strobila,9
is preparatory to the production of the young Medusa.

It does not explain why the Hydra precedes the Strobila,
or how the Strobila prepares the Medusa. The conditions
of the organization that render the gemmation of the Hydra
Tuba and the spontaneous multifission of the Strobila pos-
sible are not pointed out; these phenomena are left as
unexplained as they were before : at least the words that
" they take place agreeably with the law of alternate gene-
ration," and the metaphorical comparison of the Hydra
and Strobila with ( wet-nurses } (Ammeri), bring no in-
formation as to the organic conditions of the phaenomena :
the facts, as discovered by Sars and Siebold, remain just
as absolute and empirical as they were before*.

They have, however, been grouped together with corre-
sponding phaenomena in other species, and some of their
analogies have been pointed out for the first time by Prof.

* This the clear-headed Norwegian observer strongly felt when he
asked : — " Und wie will man denn die Stolonenbildung, die doch deut-
lich genugund schon von Siebold an den achtarraigen Medusen-ammen
beobachtet worden ist erklaren?" — and was compelled to answer:
" Davon sch weight Steenstrup." — Sars, Fauna Littoralis Norvegica,
fol. 1849, p- H.

41

Steenstrup. These characteristics of his Essay have procured
for it a wide and favourable reception ; and the terms in
which he has summed up the facts are now commonly
received both here * and on the Continent in the sense in
which he has proposed them, viz. as explanatory of the
phenomena so generalized. It is against this arrest of
inquiry and judgment that I now chiefly argue, from a con-
viction that a better explanation of the phenomena may be
given, or, I should rather say, a real explanation, so far as
it goes, as compared with the seeming explanation accord-
ing to the terms of the e alternate-generation5 theory.

I am not, however, the only English physiologist who
has submitted the true value of this theory to examination.
The able author of the article on Sir John Graham Dalyell's
beautiful work " On the Rare and Remarkable Animals of
Scotland" in the e Annals and Magazine of Natural Hi-
story5 (1848, p. 312), has pointed out some of the facts
which he deems irreconcileable with Steenstrup's theory :
and Dr. Carpenter, in a valuable paper on the same subject
in the Medico-chirurgical Review, 1848, has rejected the
theory in stronger terms of condemnation than I think
are applicable to it. The latter reviewer says that Steen-
strup " has built up a very plausible hypothesis upon the
foundation of the phaenomena of metamorphosis pre-
sented by the zoophytes and acalephae," and charac-
terizes it as " a very premature, erroneous and limited
expression of the real facts" (p. 192) ; affirming that "Pro-
fessor Steenstrup has laboured under a total misconception
of the most important part of the process" (ib.), his propo-
sitions being " totally inapplicable to the vegetable king-
dom." And Dr. Carpenter offers what he believes to be " a

* " I see no reason, therefore, to dissent from the theory of Steen-
strup : it is the simplest and most intelligible, as well as the most
original expression hitherto offered of the astonishing facts which he
was the first to generalise." Prof. Ed. Forbes, ' Monograph on the
Naked-eyed Medusae/ p. 88.

42

clearer and more philosophic view of the subject" (p. 193).
This view may be briefly stated to be based upon the belief
that ( gemmiparity ' or the process of gemmation is e alto-
gether distinct ' from oviparity or the process of develop-
ment in and from an ovum ; viviparity being rightly shown
by the reviewer to be a mere modification of the process of
development from an ovum (pp. 193 and 194).

Oviparity " is effected by the concurrent agency of two
sets of sexual organs, situated on the same individual or
appertaining to two different beings ;" and " where there is
no evidence of such concurrence, the presumption is" held
to be " strong that the reproduction is really carried on
upon the gemmiparous plan/' which is defined as " a mul-
tiplication of cells by a process of continuous growth"
(p. 193.)

Before entering into the inquiry how far what is known
of the process of gemmation is rightly defined as one of
' continuous growth/ as contra-distinguished from the
growth of the germ-mass and embryo in an ovum, I feel
desirous to offer a few remarks in mitigation of the censure
which Dr. Carpenter has passed on the ingenious Essay
on the Alternation of Generations.

When Prof. Steenstrup, for example, in reference to the
classes of Invertebrate animals whose generation he has
described from his own or others' observations, says that
the offspring never resembles its immediate parent, and that
the parent " does not meet with its resemblance in its own
brood, but in its descendants of the second, third, or fourth
degree/' he simply enunciates a matter of fact, and sets forth
no hypothesis plausible or otherwise. Unless it can be shown
that the ovum of a Medusa does not produce a Hydra Tuba,
and this a Strobila from which the Medusa ultimately comes;
and unless the cercariform individual which intervenes be-
tween the gregariniform one and the Distoma, developing
the ova of the Entozoa of the freshwater Snail, have no exist-
ence in nature, the language of the ' alternate-generation '

43

theory cannot be justly described as " a very premature
and erroneous expression of the real facts.55 It merely
disappoints the expectation raised by Prof. Steenstrup as
to its affording an explanation of the facts. And inasmuch
as the seed of a plant produces a leaf and its root, which
proceeds to develope other leaves before it finally pro-
duces the flower with the seed like that from which the
first cotyledonal leaf or pair of leaves originated ; and since
therefore the plant-individual, in the form of a leaf, inter-
venes between the seed and the male and female indivi-
duals, called ' stamens and pistils,' concerned in the repro-
duction of the seed, the generation of such plant, as Prof.
Ed. Forbes has well argued*, may be called ( alternate,5
in the sense in which Steenstrup applies the term to the
generation of the Medusa and the Aphis ; and such lan-
guage, it seems to me, would be very far from implying
" a total misconception of the most important part of the
process,55 though it might not necessarily involve a full and
clear recognition of its nature or essential condition.

The reviewer defines this part of the process to be
e growth in continuity with the parent,5 effecting gemma-
tion and also spontaneous fission, in contradistinction to
6 multiplication by means of germs detached from the pa-
rent.5 But the nuclear germ-mass in the virgin larviparous
Aphis is as really distinct and detached from the parent as
the germinal vesicle which is developed into an egg in the
wedded oviparous Aphis. In this, when impregnated, a
germ-mass must be formed, identical in character with the
foregoing, prior to embryonic formation. The first steps
of development are identical in both : they might equally
be defined as "a multiplication of cells by continuous
growth and independent vitality;55 but this definition
would be simply an expression of the well-known fact
that cells do multiply, and especially germ-cells, and so
produce what may be called e continuous growth,5 as in

* Loc. cit. p. 87.

44

the Conferva?, and that both the cells and the parts they
form have vitality. What we are in quest of is the con-
dition of such growth by multiplication of cells, which is
essential to their forming a germ-mass and laying the
foundation of a future individual ; and our conception of
this condition will be in the ratio of our comprehension
of the purport of the process by which the germ-mass is
formed in all animals.

When we find the development of the Aphis by multi-
plication of cells, and its continuous growth by the meta-
morphosis of the resulting germ-mass, taking place within
the body of the larval Aphis, without the concurrent agency
of male and female sexual organs, and without any previous
detachment of an ovum properly so called, or any pre-
existent ovum, like that in the larviparous Blue-bottle Fly,
we recognise a phenomenon essentially similar to the pro-
cess which by multiplication of cells in so-called ( continuous
growth5 developes the uncinated arid suctorious head and
neck from the common cyst of the Ccenurm*, and which
produces the bud in the adult Hydra viridis and in the larval
Hydra Tuba. But in the Aphis the gemmation, instead of
protruding through the side of the abdomen, extends into
an oviduct ; the progressive growth distends the tube into
a uterine sac, and the progeny escapes 'per vaginam.5

The presence of female organs determines the sex of this
viviparous larva, and by this analogy Prof. Steenstrup may
havebeen guided to the opinion that the gemmiparous larva?
of the Medusas and other species, which he calls f Arnme/
were females. Dr. Carpenter affirms that " he had no more
reason for so calling them than the botanist would have in
speaking of a budding plant as of the female sex up to the
time of the evolution of the flowering system." (p. 194.)
And this will seem to be true to the botanist who may
view such a plant as an individual whole, with its nutritive,

* H. D. Goodsir, 'On the Development, &c. of the Acephalocysts/
Trans. Royal Soc. of Edinburgh, 1844, vol. xv. p. 565, pi. 16.

45

respiratory and generative organs, comparable to those in
a complex animal. But the true test of any opinion as to
the sex of a gemmiparous larva can only be applied and
appreciated by the physiologist who has learnt to view the
relations of the plant to the animal in the spirit of John
Hunter*. If the budding polype be analogous to the leaf
and leaf-bud on the one hand and to the larviparous Aphis
on the other, — if these be severally examples of true gemmi-
parous individuals differing only in the scale of their com-
plexity,— then the analogy of the Aphis would justify the
opinion that the budding polype is essentially female. And
if it can be proved that the growth by cell-multiplication
producing a bud, instead of being ' altogether distinct from 9
the growth by cell-multiplication in an egg, is essentially
the same kind of growth or developmental process, that
same analogy will furnish the botanist with a reason that
he perhaps had not before, for viewing a budding plant as
essentially ' female/ and for regarding the so-called males
of the Palms and other Linnaean Dicecia, — plants which
terminate their series of successive gemmations by ela-
borating the true male individuals or ( stamens/ — as stem-
bound aggregates of female gemmiparous individuals in
the form of leaves and buds ; and these, by the light of
zoological analogy, may be viewed as ( larvae' in comparison
with the more advanced and more individualized stamens
or males associated with them.

The green petiolate discoid ' gemmae ' of the Liverwort
(Marchantia polymorphd), which are detached whilst in the
condition of a mere aggregate of cells in an envelope, the

* " In the vegetable this power or mode of propagation arises from
two principles ; one is that every part of a vegetable is a whole ; the
second arises from some having the power in every part (under certain
circumstances) of retaining life, bringing in nourishment at every part,
and of producing parts called roots. So that a vegetable under these
circumstances is capable always of being multiplied as far as it can be
divided into distinct plants." — ' Introduction to the Organs of Genera-
tion/ Physiological Catalogue of the Hunterian Collection, vol.iv. p. 4.

46

' spores * that are thrown off from the tissue of the fronds
of ferns, and the ' bulbels ' that drop from the axils of the
Lilium bulbiferum, are precisely analogous to that aggregate
of cells in an envelope that falls into the anterior chamber
of the locular uterus of the virgin Aphis : only in the plant
the aggregate of cells is detached from an external surface,
but in the animal from an internal surface.

When it is asserted that in the propagation of the Hydra
by buds or by artificial division there is not ' anything like
oviparous reproduction/ but that it is the result of the
simple action of continuous growth, we feel the want of a
clear definition of the meaning of that phrase. In the
growth of the Conferva, which the reviewer rightly admits
to be analogous to that of gemmation, each filament ori-
ginates from a germ, which is first developed into a single
cell ; this cell increases, and it either divides, or its germ-
granules give origin to two cells within it, which take the
place of the former ; and by the repetition of this increase
and multiplication a long and single series of tubular cells,
attached at one end, free and constantly growing at the
other, is at last produced, as linger and other mycologists
have clearly demonstrated.

Now, short of the mere formal and non-essential modi-
fication of the juxtaposition of the cells, end on end, the
process here described is the same as that with which the
development in ovo of the mammal, and doubtless also of
Man himself, begins : the germ-cell gives origin to two ;
these new cells themselves undergo the same increase by
subdivision, which is repeated through a long series of cell-
generations until a coherent mass or body of countless cells
is produced, which differ from those in the Conferva, inas-
much as they are in contact with each other at many and
indefinite points, instead of being arranged in a linear series.

The first step in the formation of the bud in the Hydra,
as in the Ccenurus, is the multiplication of cells, which also
are aggregated together as in the ovum, and not set end on

47

end as in the Conferva. The mutual contact of these delicate
cells suffices for mutual action and reaction on one another ;
excess of nutriment or other quality in one is balanced by
the endosmotic action of neighbouring cells, just as the
nutriment is transmitted from cell to cell to the last deve-
loping-cell in the chain forming the conferval filament.

The young Hydra from the bud is identical in organic
structure and character with that which comes from the
ovum : and when the effects of organic development are
the same, their efficient causes cannot be e altogether di-
stinct ' : only the non-essential accessories of the process
may be the subject of variation.

The retention of unused-up germ-cells in the tissues
of the Hydra is the condition which explains the fact,
that, when favourable external circumstances of light, heat,
and food concur, it may be propagated by buds. It is not,
however, absolutely correct to say that buds " may arise
from any portion of the body of the parent structure;" in
the Hydra fusca, at least, they are pretty constantly de-
veloped from a special part of the animal, as was long ago
shown by Roesel*, and has been recognized by all sub-
sequent observers : the part in question is at or near to
the bottom of the digestive sac ; between it and what might
be termed the foot or attaching pedicle of the polype ; — and
this is precisely the part where those nucleated cells are
assembled in greatest number, which most closely resemble
the secondary or derivative germ-cells in the ovum.

It might be specified as the ( ovarium' of the polype, but
that the true ova are matured from cells not situated exclu-
sively at this part ; nevertheless the germ-cells situated be-
tween the stomach and pedicle are the essential elements of
ova, and are only not converted into ova, and cast off with a
chorion, because their development proceeds directly and
at once to the business of the formation of an embryo : and

* See his beautiful 86th plate in the ' Historise Polyporum/ of the
Supplements to the ' Insekten-Beliistigiingen.'

48

the difference between this mode of generation and that of
the virgin larviparous Aphis is merely in the protrusion of
the embryo in the one case through the skin, and in the
other through the vagina : both secondary modifications or
non-essential accidents of the generative process. In suf-
fering to pass uncorrected the term e ovum/ applied figura-
tively to these germ-cells of the budding Hydra, in a former
Course of Lectures*, published too hastily perhaps, as they
were reported by Mr. Cooper, I freely confess that I was
justly amenable to the criticism, based on the fact that a
true ovum with a bristled chorion had not been shown to
exist at the base of the bud. What I believe myself to
have stated was, that the true ovum of the Hydra had
received the influence of the spermatozoon, and that the
primary germ-cell had divided that influence amongst its
progeny, which stood in the same relation to the bud as
the corresponding derivative germ-cells were afterwards
explained to stand to the offspring of the virgin Aphis f.

No buds are developed from the complex tentacles of the
Hydra. Those cells that have been metamorphosed into tis-
sues have lost the power of reproducing anything but those
tissues or the products of their decay. Had this truth
been apprehended by Morren, he would not have imagined
that an entozoon could result from the individualization of
a previously organized fibre. Many derivative germ-cells
and nuclei are retained in the body of the Hydra, each being,
in its degree, impregnated ; and this I continue to regard as
the condition of its power of gemmation, and also of the
fact, that when cut into numerous parts, many Hydrse may
be thus artificially propagated.

The reproduction of parts of higher animals has also
been found to depend on pre-existing cells retained as such.
Mr. H. D. S. Goodsir has shown that in the Lobster e.g.,
so noted for the power of reproducing its claws, the rege-

* 'Lectures on the Invertebrata,' 8vo, 1843, p. 85.
t Ibid, pp. 234, 366.

49

nerative faculty does not reside at any part of the claw
indifferently, but in a special locality at the basal end of
the first joint of each of the legs. This joint is almost
filled by a mass of nucleated cells surrounded by a fibrous
and vascular band; and other nucleated cells intervene
between this band and the outer crust. The vessels of
the band pass onwards for about half an inch and return
upon themselves, forming loops. When a claw is broken
or otherwise injured and disabled, the Lobster or Crab,
by a violent muscular effort, casts it off at the transverse
ciliated chink or groove which indents the reproductive
segment. The new claw is developed by the multiplication
of cells, which are soon divided into five groups answering
to the five joints of the future limb : these nascent joints
are folded upon each other in the Crab, but extended in
the Lobster : in both they are at first enveloped in a sac
formed by the distended cicatrix ; the budding limb ulti-
mately bursts this cicatrix and its growth is rapidly com-
pleted : a great proportion of the reproductive cells contained
in the basal extremity of the injured limb, is used up in the
production of the new limb ; but a mass of them is retained
unchanged at the basal joint, and ready to renew the re-
productive process when needed. In the lower Crustaceans
such groups of cells are found at more numerous joints.

Like Sars, the author of the article in the ( Medico-
chirurgical Review5 observes, with regard to Professor
Steenstrup's account of the propagation of the medusiform
from the strobiliform individuals : — " How these young
ones are developed — whether from ova or from gemmae —
he does not attempt to explain ; nor does he show, if they
are produced from ova, by what male influence they are
fertilized." (p. 204.) Steenstrup had not, in fact, any more
than his reviewer, recognized the nature of the process con-
sequent on fecundation, which carries the male influence
through successive generations of larvae. Yet he had per-
suaded himself that he was elucidating natural phenomena

D

50

which until his time had been e inexplicable5; quite un-
conscious, apparently, how little the term ' alternation of
generations/ and the figurative expression of ' wet-nurse/
applied to the fertile larval stage of an animal, serve to
acquaint us with the nature or essential conditions of the
phaenomena. And I may further observe, that if it were
proved that the ( multiplication of cells by a process of con-
tinuous growth, with independent vitality,' was a process
altogether distinct from that multiplication of cells in an
ovum which precedes the formation of the tissues of the
embryo ; yet the words e multiplication of cells, with or
without continuous growth/ would equally leave us devoid
of the idea of the essential force and condition of structure
that make the development of a young animal from a larval
polype or a wingless virgin Aphis possible. And we are
interested to learn, therefore, what that may be which the
reviewer calls " our philosophical interpretation of this won-
derful process " (p. 203). He truly compares the fertilized
ovum of the medusa-parent to the seed of the plant, the
polype that grows from it with its progeny by gemmation
to leaf-buds, and those discoid portions developed between
the base and summit of the Strobila to flower-buds. For
these discoid segments, in point of fact, when separated,
become either male or female Medusa3, and developing the
generative organs, justify the above comparison. The re-
viewer says, " It is evident to us that the pile of medusa-
discs is not formed by the constriction of the proper body
of the original polypoid animal;" yet he afterwards states
that the detachment of those discs " involves the separa-
tion of the tentacular ring from the body at the base/' —
that it separates one end of the polypoid animal from the
other, — which implies something more than constriction, —
constriction, viz. carried to the extent of absolute fission of
the original polypoid animal. But this fission is preceded,
as in the Nais and Nereis, by gemmation of the part to be
separated thereby.

51

There is doubtless a close and beautiful analogy between
the stages of the development of the Medusae and those of
the Tree ; between the larva of the one and the leaf of the
other, between the ovum of the animal and the seed of the
plant. Yet this comparison does not explain the essential
condition of gemmation in either : one thing unexplained
cannot be made to illustrate another unknown thing. One
seems to get some knowledge when it is stated that the
leaf is produced ' by continuous growth,5 and that the bud
of the Hydra is produced like the leaf. And had the re-
viewer recognized the essential concordance between the
'continuous growth5 of the conferval filament and that
which augments the germinal mass in the ovum, he might
have arrived at the desired explanation; but he affirms
them to be f altogether distinct/

Admitting the obvious analogies between the reproduc-
tive phaenomena of the Medusa and the Plant, Dr. Car-
penter says, " the whole of these phaenomena appear to us
to constitute but a single generation, instead of making
two, as represented by Steenstrup;" and, availing himself
of the popular idea that a tree is one individual living
thing, he turns those analogies with apparently great power
against the definitions of Professor Steenstrup.

It is quite true, as the able reviewer says, es that men
are not in the habit of speaking of leaf-buds and the flower-
buds of a plant as of two distinct generations." But before
the whole doctrine of the ( alternation of generations5 is
knocked to the ground (p. 205), those who desire to secure
for the ingenious Danish Professor the credit which he
really merits, may bring to his support that other and per-
haps truer interpretation of the phaenomena and analogies
arrayed against him, to which I have before referred in re-
gard to his opinion of the sex of the budding polype.

The fertilized ovum of the medusa-parent is like the fer-
tilized ovum of the Aphis, and the polype that grows from
it resembles the first larva into which the embryo Aphis

D2

52

expands. From this larva are produced other wingless
larvae, which are repetitions of the producer or parent;
these larvae in the Aphis remain disconnected from each
other, and do not combine to form a compound struc-
ture as in certain zoophytes ; they become detached like
the bulbels of the Marchantia or the lily, only they are
rather more advanced in individual development. But
under certain conditions a new and different series of
embryos forming male and female perfect insects are pro-
duced : these too become detached, and by their inherent
power of movement, through the possession of wings, they
carry the germs of a new generation to a distance from the
plants which their larval parents have devastated. The
whole of these phaenomena appear to me, as they did to
Bonnet, Reaumur, Dufour, and Morren, to constitute a
succession of generations, — seven, nine, or it may be eleven.
But as all the larvae are like each other, they collectively re-
present with the perfect insects two generations, in respect
of form, and the generation of larviparous and the genera-
tion of oviparous Aphides may thus be said to alternate
with each other, in accordance with Steenstrup's general
expression of the fact.

Now no physiologist has ever defined the series of virgin
larviparous Aphides, with the ultimately produced winged
males and oviparous females, as constituting a single gene-
ration, or the generation of the parts of one and the same
individual ; nor, if my comparison be true, have we better
grounds for so defining the succession of distinct animals
which have received the names of Hydra Tuba, Strobila, and
Medusa, or the succession of those which have been called
ciliated gemmules and planulae, digestive polypes, genera-
tive polypes, Tintinnabula or free medusiform polypes of
the Campanularia and Coryne ; or the successively deve-
loped cotyledons, leaves and flowers of a plant which are
combined by the stem into a seeming individual whole.
The analogy between the procreating larvae of the Aphis,

53

the Medusa, and the Coralline is so true and so close, that
if the larval Aphis be a distinct individual and not a part,
so must be the strobila, the planula, and the gemmiparous
leaf: if the succession of larval Aphides produced without
the access of the male be truly described, as it always has
been described, as a succession of generations, so must that
succession of planula, polype and strobila which leads to
the oviparous Medusa : and that succession of planulae and
nutritive polypes which precede the detachment of the free
procreative medusoid polypes in the Coryne, and the like
with the plant-generations preceding the flower.

The Botanist, in fact, arrived earlier than the Zoophy-
tologist at an intimate philosophical comprehension of the
nature of his composite subjects. After Linnaeus had
made known with his characteristic terseness and brevity
the phaenomena of the transformation of the leaf into the
bract, the sepal or the petal, and the retrograde change of
the stamen into the petal, of the petal into the sepal, and
of this again into the leaf, in his ' Philosophia Botanica*?
and with more detail in the e Prolepsis plantarum^'; and
when the great Goethe had gathered together these and
other analogous phaenomena in vegetable life, and had
combined them harmoniously, with true poetic insight
into their essential nature, in his famous doctrine of ' Ve-
getable Morphology/ a reconsideration of the nature of
the essentially individual plant could hardly fail to suggest
itself to the thinking mind.

* " Principium/onm etfoliorum idem est.
Perianthum sit ex connatis foliorum rudimentis.
Derivato nutrimento ad squamas amenti, destructis fiosculis, mutantur

in Folia.

Derivato nutrimento ad flosculos amenti, fiunt folia Calyces.
Luxurians vegetatio folia e floribus continuando producit.
Macro, vegetatio flores e foliis terminando producit." — Metamorphosis
Vegetabilis, Philos. Botan. p. 301.

\ " Quando flos nascitur abeunt folia gemmacea anni sequentis in
bracteas, tertii in calycem, quarti in petala, quinti in stamina, sexti in
pistilla, quod a situ judicatur." — Amcenitates Academicce, vol. vi. (1789)
p. 341. The hypothesis of the succession has long been abandoned.

54

The e Ur-pflanze ' or ideal type-vegetable with which
the e Morphologic ' of Goethe is illustrated (pi. iii. of the
French translation) is an aggregate of modifications of the
primitive type, not the fundamental pattern-plant itself.
My idea of such, from the analogy of the typical vertebra
or vertebrate skeleton*, is attempted to be expressed by
the more simple forms associated together in the Frontis-
piece, fig. 1, and the nature of the association of the
more or less modified individual plants is illustrated by
the accompanying figures from the animal kingdom. The
archetypal form of plant (phyton) nowhere perhaps exists in
actual nature, but is presented under various modifications
adapted to different conditions arid functions, and devia-
ting from the archetype as those functions become exalted.
The most familiar, if not the commonest form of the indi-
vidual plant, is the leaf, and it is sometimes cited as the
archetypal form. In the specimens of Bryophyllum from
the Physiological Series of the Museum (Nos. 2225 A & B),
you may perceive the manifestation of the power of re-
producing other individuals by the buds which have been
developed from the angles of the marginal crenations of the
leaf: in No. 2225 C. the buds have developed individuals
having the same form as the parent, viz. the leaf, and they
have sent down an organ for independent nutriment called
( root/ I long ago pointed out in our Physiological Cata-
logue (vol. iv. p. 8), with regard to those very specimens,
how characteristic of the leaf-individuals which enjoy un-
usual power of gemmiparous reproduction it was to retain
much of the primitive cellular structure.

The leaf, however, chiefly energises in the respiratory
function, with which also a nutrient power is combined,
since carbon is absorbed : with the leaf is connected that
slender chain of sap-vessels and air-tubes which is con-
tinued from its petiole down to the root, and forms part of
the same individual with the leaf: and the aggregate of
* On the Archetype and Homologies of the Vertebrate Skeleton,
pi. 2, 8vo, Van Voorst, 1847-

55

these vessels, with a common condensed integument or
* bark/ constitutes the branches and trunk of the com-
pound whole, which is called 'Tree/ A generation of
leaves, the breathing parts of the individual phytons, pe-
rishes each year ; but not before each individual has made
preparation for its successor by developing a bud at the
axil of the leaf, which remains when the leaf falls, and sup-
plies its place in the following year. Here the individuals
are successively propagated by gemmation, as in the Hydra
and Tubularia indivisa, but continue associated together
like the compound polypes. In order to effect a multipli-
cation of plants at a distance from the parent, the common
respiratory and gemmiparous form must be developed into
a higher condition of individual plant. This begins by the
suppression of the bud at the axil, and by the consequent
concentration of the formative force in the act of metamor-
phosis and individual development. The ( whorl/ or group
of such leaves in verticillate arrangement, constitutes what
is termed the f calyx': the calyx, then, consists of a group of
metamorphosed and often more or less connate or confluent
individuals, usually deprived of the power of gemmation :
but their tendency to resume the more typical form of the
individual — the leaf — must be familiar to all who have
looked at this part in the proliferous roses. The cowslip
shows the small amount of change which converts a leaf
into a sepal.

The corolla consists of a whorl of more modified e leaves/
called petals ; and incloses a third e whorl ' of still more
metamorphosed forms of the archetypal plant, called sta-
mens : in these we see that development has advanced to
the establishment of the true male individual, which, like
the male in the Lernaea according to Nordmann, the male
of the Argonaut according to Kolliker, and the male of the
Rotifer according to Brightwell* and Dairy mple, is re-
stricted almost to the function of elaborating the fertilizing
* ' Fauna Infusoria of Norfolk/ pi. 1. p. 2.

56

principle. Finally, we have the energies of the successive
generations of the compound plant exhausted in the per-
fection of the female individual, the pistillum, with its
ovarian base : the true nature of which is manifested by
its common return to the more archetypal form of petal or
leaf in those over-nourished plants that produce double
flowers, e.g. the Ranunculuses, Saxifrages and Wall-flowers.

The ovule is perfected in the part called ' ovary/ forming
the base of the female individual plant. By the passage of
the pollen-tube to the foramen of the ovule, as traced by
Brown in the Orchids and Asclepiads, impregnation results,
and various beautiful contrivances exist for conveying the
impregnated seed to a distance, where it may lay the foun-
dation of a new series of united gemmiparous individuals.

I am quite sensible that the composite plant or the com-
posite zoophyte, and the phenomena of their development,
may be explained agreeably with ordinary ideas and com-
mon belief in regard to trees ; viz. that the leaves are parts
of a whole, that they are the respiratory organs of the tree,
and the polypes the digestive organs of an individual com-
pound organism. And this explanation agrees with the
functions performed by the variously modified individuals
in relation to the association ; just as the several members
of a regiment or a corporation constitute one organized
whole. The question, however, is, whether the tree repre-
sents such a whole, or is equivalent to one of the or-
ganized individual members; whether the leaf and the
stamen may not answer rather to different individuals of
the 'body corporate/ than to the lung or the testis of
any single individual. Unquestionably our choice of ex-
planations ought to be governed by the results of the
most extended and accurate analogies. Our first (and
the common) notion of a tree and a coralline, as being
respectively individual organisms, derives seeming sup-
port from the fact that in each species of composite plants
and animals the aggregate of individuals assumes a de-

57

finite or specific form, whence the terms ' Oak/ 'Ash/
and ( Bell-coralline/ ' Fern-coralline/ ( Sea-oak-coralline/
&c. But, by parity of reasoning, the nests or ( combs' of
certain Hymenoptera and the nidamental capsules of most
testaceous mollusks might be regarded as individuals ; for
the separate ovigerous cells, of which they are respectively
the aggregates, are grouped together in each species, in so
constant and specific a form, as to be readily recognized
after having been once defined.

It would have been strangely at variance with that beau-
tiful principle of variety in non-essentials which pervades
nature, if the individuals of different species of plants and
animals, which cohere together after gemmation, had not
been grouped each after its own specific pattern ; but this
specific pattern of grouping no more constitutes a single
organic individual than it does in the case of the aggregate
of egg-capsules or larval cells just cited.

If, however, it should still be felt that the explanation
I have here offered in accordance with the opinions of
Hunter*, Steenstriipfj Prof. Ed. Forbes J, and other philo-
sophic naturalists, be somewhat forced or at least optional,
we may arrive perhaps at greater confidence in its exacti-
tude by extending the bounds of the analogies by which
it has been illustrated.

To facilitate the comparison I have devised the diagrams
engraved in the frontispiece, Plate I.

* Loc. cit. " Every part of a vegetable is a whole." The enunciation
of the idea is perhaps too general and extreme.

f Loc. cit. " It is certainly the great triumph of morphology that
it is able to show how the plant or tree (that colony of individuals ar-
ranged in accordance with a simple vegetative principle or fundamental
law) unfolds itself, through a frequently long succession of generations,
into individuals becoming more and more perfect." (p. 114.)

J " We are not in the habit of regarding a leaf as the individual —
popularly we look upon the whole plant as an individual. Yet every
botanist knows that it is a combination of individuals, and if so, each
series of buds must certainly be strictly regarded as generations." —
Monograph on British Naked- eyed Medusse, p. 87.

58

The pollen-tube or filament (a, fig. 1) discharged from
the pollen-cell (a1) in the plant represents the spermatozoon
(a, figs. 2 & 3) in the animal : its contents — whether by
endosmose or perforation is immaterial — are received by
the ovule (b, fig. 1), which is afterwards discharged and
becomes free. Under favourable circumstances the forma-
tion of the embryo takes place, with manifold modifications,
but essentially by the multiplication of cells, according to
a process which is as much entitled to be called continuous
growth as that process in the formation of the Conferva.
The embryo (c) proceeds to develope the radicle and the
plumula (d) by the metamorphosis and coalescence of certain
of the impregnated cells, retaining the major part, however,
as cells : and thus the first individual plant, or pair of in-
dividuals as in Dicotyledons, is established.

The ovum (#, fig. 2) of the zoophyte proceeds to develope
its free locomotive embryo (c) by an analogous multipli-
cation of cells, certain of which are metamorphosed into
an external skin with vibratile cilia: the embryo settles,
subsides, shoots out rays, analogous to the radicle of the
plant, but for attachment only, and grows afterwards, as a
stem, from which a polype (d) is speedily developed an-
swering to the first cotyledon al leaf, or leaves, in the plant
(d, fig. 1). Both plant and zoophyte proceed to develope
by gemmation, the one a succession of leaves (ee), the
other of polypes (e e), associated by the continuous growth
of the connecting parts : and finally the plant by a meta-
morphosis of part of the stem and certain leaves produces
the flower or fructification (/*, g, h, i), and the zoophyte by
a modification also of its stem and certain polypes produces
an ( ovarian vesicle ' (f) or a modified polype (g)9 or a me-
dusiform individual (/), which is set free : in both cases the
end to be attained is the diffusion of the species by means
of impregnated seeds or ova.

Now, if we compare with the preceding the third figure
(fig. 3), in which I have represented the corresponding

59

stages intervening between the ovum and the perfect male
and female individuals of the Aphis, the analogy between
these stages in the plant, the polype and the insect, will
be seen to be both true and close. The spermatozoon (a) of
the male Aphis (h) answers to the pollen-filament (0, fig. 1)
of the male leaf or ' stamen' (h). The ovum (b) of the
female Aphis (i) answers to the ovule (b) of the female leaf
or pistil (i): by their combination the impregnated ovum
results. The same processes of cell-formation ensue, and
the embryo Aphis (d) is formed by the combination and me-
tamorphoses of certain of these secondary germ-cells ; but
it retains the rest as a germ-mass in its interior, which may
be compared with the cells of the pith of the plant, and with
the cells or nuclear granules in the corresponding more
fluid part of the pith of the polype. Under favourable
circumstances of nutriment and warmth,, certain portions
of the retained germ-mass repeat the process of embryonic
formation, and a larval individual (e, fig. 3) like that from
the ovum is thus reproduced ; which is only not retained
in connexion with its parent, because the abdominal inte-
gument is not coextended with it.

The generation of a larval Aphis may be repeated from
seven to eleven times without any more accession to the
primary spermatic virtue of the retained germ-masses than
in the case of the zoophyte or plant : one might call the
generation an e internal gemmation,' but this phrase would
not explain the conditions essential to the process, unless
we previously knew those conditions in regard to ordinary
or external gemmation.

At length, however, the last apterous or larval Aphis, so
developed, proceeds to be f metamorphosed,' as it is termed,
into a winged individual, in which only the fertilizing fila-
ments are formed, as in the case of the stamens of the
plant (h) ; another larval Aphis (i) perfects the female ge-
nerative organs* and developes the ovules, as in the case

* This is not attended by the acquisition of wings ; or, if they be de-

60

of the pistil (i). We have, in fact, at length f male (h) and
female (i) individuals/ preceded by reproductive individuals
(e, e) of a lower or arrested grade of organization, analogous
to the gemmiparous polypes of the zoophyte (e, e, fig. 2)
and the leaves (e, e, fig. 1) of the plant.

I have described the process for its better intelligibility
in the Aphides as one of a simple succession of single in-
dividuals, but it is much more marvellous in nature. The
first-formed larva of early spring procreates not one but
eight larvae like itself in successive broods, and each of
these larvae repeats the process ; and it may be again re-
peated in the same geometrical ratio until a number which
figures only can indicate and language almost fails to ex-
press, is the result. The Aphides generated from virgin-
parents, by this process of internal gemmation, are as
countless as the leaves of a tree, to which they are so
closely analogous.

It generally happens that the metamorphosis which I
have described as occurring after the seventh or eleventh
generation takes place much earlier in the case of some of
the thousands of individuals so propagated (k, fig. 3) : just
as a leaf-bud near the root may develope a leaf- stem, a
flower and seed-capsule (k, fig. I) with much fewer antece-
dent generations of leaves from buds than have preceded
the formation of the flower at the summit of the plant ; or
just as one of the lower and earlier formed digestive polypes
may push out a bud to be transformed into an ovarian sac
(k) and a generative medusa (/). The analogy is beautifully
and closely maintained throughout.

The wingless larval Aphides are not very locomotive ;
they might have been attached to one another by continuity
of integument, and each have been fixed to suck the juices

veloped in the oviparous female they soon fall. I have, however, retained
them in the diagram for the better illustration of the analogy. Many
of the virgin viviparous Aphides acquire wings, but never perfect the
generative organs.

61

from the part of the plant where it was brought forth. The
stem of the rose might have been incrusted with a chain
of such connected larvae as we see the stem of a fucus in-
crusted with a chain of connected polypes, and only the
last developed winged males and oviparous females might
have been set free. The connecting medium might even
have permitted a common current of nutriment contributed
to by each individual to circulate through the whole com-
pound body. But how little of anything essential to the
animal would be affected by cutting through this hypo-
thetical connecting and vascular integument and setting
each individual free ! If we perform this operation on the
compound zoophyte, the detached polype may live and
continue its gemmiparous reproduction. This is more cer-
tainly and constantly the result in detaching one of the
monadiform individuals which assists in composing the
seeming individual whole called ( Volvox globator ' ; and so
likewise with the leaf-bud. And this liberation Nature has
actually performed for us in the case of the Aphis, and
she thereby plainly teaches us the true value or significa-
tion in morphology of the connecting links that remain to
attach together the different gemmiparous individuals of
the volvox, the zoophyte, and the plant.

The popular notion of a plant, as we have said, is that
it is one individual complex organism, of which the root
answers to the nutrient system, the trunk and branches to
the skeleton, the leaves to the lungs, and the flowers to
the generative organs of an animal : but we never see the
stomach converted into a lung, the lungs converted into a
testis or ovarium, or reciprocally, as we trace the leaf
changed into the stamen, or this by retrograde metamor-
phosis brought back to the state of a leaf. The stomach
of a man if detached would not live and reproduce its kind
like the polype of the Sertularia or the monad of the Vol-
vox ; nor would the lung manifest its individuality like the
leaf of the Bryophyllum under similar circumstances. No !

62

A flowerless plant is an associated colony of simple orga-
nized individuals (phytons) propagating by gemmation.

A flowering plant, with stamens and pistils in the same
flower, is not a hermaphrodite individual; but is an or-
ganically associated colony of phytons, most of which are
simple and gemmiparous, but some through a higher
energy have been developed into distinct male and female
individuals.

The so-called male of the Dioecious plants is an organi-
cally associated colony of gemmiparous phytons with true
male individuals ; and the so-called female plant is a similar
colony of gemmiparous phytons with true or perfect female
individuals.

From plants to Aphides inclusive the phenomenon of
Parthenogenesis is presented under manifold modifications,
alike in this essential that they are all examples of organized
beings from the impregnated ovum of which many indivi-
duals may be successively developed.

In the Vertebrated and in the higher Invertebrated ani-
mals only a single individual is propagated from each im-
pregnated ovum. Organized beings might be divided into
those in which the ovum is uniparous, and those in which
it is multiparous. This is the first and widest or most
general distinction which we have to consider in regard to
generation, and in proportion as we may recognize its
cause will be our insight into the true condition on which
Parthenogenesis depends.

To this end we must revert to what takes place in the
impregnated seed or ovum at the beginning of develop-
ment, and duly ponder over the nature of the first steps in
embryonic formation. These, observation has now esta-
blished to be essentially the same in all organisms. The
spermatozoon has been traced to the ovum in the lower
Invertebrata and in Insects (where a special organ, the
spermatheca, exists to ensure its application), in Mollusks,
in Fishes, in Batrachians and up to the mammal. After

63

impregnation the germinal vesicle disappears and the germ-
yelk (PI. I. fig. 4, a) contracts. This constant effect of im-
pregnation indicates the presence of an attractive force which
has produced a certain condensation of the germ-yelk. If
the yelk-membrane (b) were previously in contact with the
chorion (c), it has now receded from it, and an albuminous
fluid (d) occupies the interspace.

The primary germ-cell (e) soon appears at or near the
centre of the germ -yelk. Its nucleus divides (fig. 5), and
two germ-cells result ; these recede and establish two cen-
tres of attraction and assimilative force, around which, in
many species, the germ-yelk collects, and is thus divided
(fig. 6). Each secondary germ-cell becomes two (fig. 7)>
and the matter of the germ-yelk is collected around four
centres (fig. 8). The same process again takes place in
each of these four, producing an eight-fold division, then
a sixteen-fold one (fig. 9), and by its rapid repetition the
germ-yelk becomes subdivided and assimilated into count-
less germ-cells (fig. 10), and by their further subdivision
the nuclear or granular germ-mass (fig. 11) is established.
This is the most common mode in which the primary im-
pregnated germ-cell propagates itself at the expense of the
germ-yelk. But in some species the germ-yelk is not at-
tracted round the successively developed centres, but the
germ -cells as they are propagated push their way through
the germ-yelk and progressively assimilate it: the ulti-
mate result being the same — a germ-mass of countless
secondary germ-cells or nuclei. Much extraneous matter
has been assimilated and combined with the essential mat-
ter of the primary impregnated germ-cell in order to form
the material out of which the embryo is to be developed :
but something more is done besides increase of bulk by
assimilation and division.

The reception of the matter or principle of the sperma-
tozoon by the germinal vesicle is essential to the com-
mencement of this growth by multiplication of cells. When

64

the primary division of the impregnated germ-cell takes
place, it must divide its properties with its matter between
the two cells resulting from the spontaneous fission of its
nucleus : and this result must follow every subsequent di-
vision *. It is scarcely figurative therefore to say that the
primary or parent germ-cell has equally divided its sper-
matic virtue amongst its countless progeny.

What has here been propounded of the influence of the
spermatozoon is equally applicable to the pollen-filament,
and to those organisms in which the spermatic force has
not been concentrated in matter presenting the common
form of the spermatozoon or pollen-filament.

In the Polygastria, for example, the condition of sponta-
neous fission is due to the previous existence of a spermatic
nucleus in each moiety which is about to be individualized.

* Professor Kolliker (Muller's Archiv fur Physiologic, 1843, p. 137)
has summed up the subordinate modifications of this constant and im-
portant preliminary process, as follows : —

The germ-cells are developed free in the yelk.

The first germ-cells are small The first germ-cells are large

and assimilate the yelk slowly. and take up the yelk quickly.

Yelk granular. Yelk fluid.

Bothriocephalus. Ascaris dentata. Cucullanus elegans.
Taenia. Oxyurus ambiguus.
Distoma tereticolle.

The germ-cells clothe themselves with the yelk.

The germ-cells clothe them- The germ-cells clothe them-

selves with part of the yelk. selves with the whole yelk (as in

figs. 4-10, PI. I.).

(Partial division.) (Total division.)

Coregonus palcea. Rana. Triton.

Alytes obstetricans. Lepus cuniculus.

Sepia vulgaris. Canis familiar is.

Loligo sagittata. Ascarides permultse.

Strongyli nonnulli.

Nereis. Botryllus.

Pycnogonum. jfEolidia.

65

The preliminary division of that nucleus has been constantly
witnessed where the phaenomena have been clearly traced.
In many of Ehrenberg's beautiful plates the division of the
nucleus is shown before that of the entire body of the mo-
nad has commenced ; and according to the direction of the
division of the nucleus, that of the monad has been either
longitudinal* or transverse f. Two centres of assimilative

* See tab. 36. fig. 7. Chilodon cucullulus, 13, 14, 15.

f Ib. 16, 17, 18. Ehrenberg calls this nucleus of the Polygastria the
' testicle/ and views its division simply in the relation of the necessity of
each individual resulting from the general fission having such an organ :
meaning that each monad, developed by spontaneous fission, is perfected,
as regards its so-called testis, by the spontaneous division of the previous
testis, and not by the formation of a new one. But this is not the mode in
which the eye, or the circle of teeth, or the pulsating sac, is gained by
the second individual from the fission : the division usually takes place
so as to include the original organ in one or in the other moiety ; and that
in which it may be wanting gets the organ by a special and independent
development of it. The constancy of the preliminary fission of the
nucleus would therefore show that it related rather to the totality of the
act itself than to the partial completion of the individual in respect of
its being provided with a particular male organ of generation. How
then, we may inquire, does the division of the nucleus relate to the
performance of the general act of spontaneous fission ? Our hope of
any insight into this mysterious relationship would be from some light
to be derived by analogous phenomena. But with what phenomena
is the one in question analogous ? Obviously most closely with those
which have been observed in the successive fissions of the impregnated
germ-cell of those ova, such e.g. as the ova of the Strongy.lus,\)est adapted
to give a view of the fission analogous to those which the perseverance
of Ehrenberg enabled him to trace in the spontaneous fission of the
monad.

The same correspondence between the successive generations of the
Chlamydomonas and the formation of the germ-mass in the mammalian
ovum has been well discerned and illustrated by Dr. Martin Barry, who
has observed, " On examining the figures given by Ehrenberg of successive
generations of the Chlamydomonas, I see a resemblance to the two, four,
eight, &c. groups of cells in the mammiferous ovum too striking, not to
suggest that the process of formation must be the same in both ; the
essential part of this process consisting in the division of the pellucid
nucleus." See my 'Lectures on Invertebrata/ p. 24, 1843.

If this preliminary division of the germ-cell to the spontaneous fission

£

66

force have been set up in place of one, and the plastic power
has arranged the organic matter around each of these cen-
tres, as it was around the single nucleus of which they are

of the germ-yelk has not been seen in the ova of other animals, it is be-
cause hitherto only the coarser phaenomena of such division of the ger-
minal body in Medusae, Mollusca, Fishes, Frogs, &c. have been noticed.
In Dr. Barry's observations however on the development of the germ-
mass in the pellucid ova of the rabbit, phenomena closely analogous
to those described by Siebold and Bagge in the ovum of the entozoon
were detected.

In reflecting on the phsenomena in the monad and the ovum — that
a central something is first established, and the consequence of that
— I have been led to draw the same conclusion with respect to both,
and to regard the establishment of the special centre as the cause of the
confluence of the parts around it ; and I call it ' a centre of attractive
and assimilative force/ Since the pellucid centre of the germinal body
has not divided from the necessity of endowing the moiety to be sepa-
rated by the subsequent fission with a particular organ required for its
individual completeness, I infer that the same preliminary act in the
monad was not solely for the purpose of providing its separated moieties
with their respective testes, but that it had a higher significance.

As the pellucid centre in the ovum is the result of impregnation or of
the reception of the matter of the spermatozoon, so it may be concluded
that the nucleus of the monad is of a nature similar to, if not identical
with, that of the spermatozoon. It was doubtless a gross view of its
nature and analogies to regard it as the homologue of the whole pre-
paratory organ of the spermatic fluid, such as is required in the higher
animals ; because as the germ -cells exist in the body of the Polygastria
without the organ called ovarium, so we ought to expect that the essen-
tial matter of the sperm would likewise exist without a special testicular
envelope.

The objection to Ehrenberg's determination of the nucleus as the
' testis/ that it has never been observed to produce spermatozoa, is akin
to that which has been opposed to his determination of the ova, viz. that
the young have never been seen to quit them and leave the shell behind.
Neither of these objections will apply to the view of the nucleus as the
essential matter of the sperm, and to the germ -cells as the essential
elements of ova.

A filamentary spermatozoon is doubtless a very general form of the
essential matter of the sperm : but in tracing the modifications of the
spermatozoa from mammalia down the scale of animal life we find them
gradually reduced to the head or nuclear part, and discern in the
vibratile caudal appendage an accessory relating to the passage of the

67

the divisions. The same phsenomena occur in the develop-
ment of the cells during the growth of the conferval fila-
ment and of other cryptogamic plants.

fertilizing principle to the germ-cell, rather than to its essential ope-
rations when arrived there.

The best microscopical examinations of the spermatozoa show "that
the spermatozoa are everywhere void of a special organization, and
consist of an uniform homogeneous substance which exhibits a yellow
amber-like glitter." — Art. SEMEN, Cyclop, of Anatomy, vol. iv. p. 502.
The nucleus of the Polygastrian offers the closest resemblance to this
character of tissue. And perhaps it may not be out of place here to
notice the close analogy of the modification of form which the nucleus
of some of the larger Polygastria, Stentor Roeselii e. g., presents to the
spirally disposed elongated head of the spermatozoon in the Torpedo,
Pelobates, and the Passerine birds.

The reception of the matter of the spermatozoon by the germ- cell is
the essential preliminary to the primary processes of its spontaneous
fissions : and when we see these fissions governed in the germ-cell of
the Ascaris by the act of impregnation, followed by the appearance of a
pellucid nucleus in the centre of the opake and altered germ-cell, and
when we further see its successive fissions governed by the preliminary
division of the pellucid centre, are we not naturally led to infer that that
centre is the seat or the chief seat of the spermatic principle which the
germ-cell has received ? The mind must either be a mere passive recipient
of these phenomena, or we must reason upon them and ask ourselves
their meaning. The most probable signification of the appearance of
the pellucid centre and of the initiative which it takes in the subsequent
changes may be that which I have just explained : but this is certain,
that the analogy between these phenomena in the multiplication of the
parts of the germ-mass and those of the nucleus in the multiplication
of the monads is so close, that one cannot reasonably suppose that the
nature and properties of the nucleus of the impregnated germ- cell and
that of the monad can be different.

Therefore I infer that the nucleus of the Polygastric Animalcules is
the seat of the spermatic force: it can only be called 'testis' figura-
tively : it is the essence of the testis. It is the force which governs the
act of propagation by spontaneous fission : and if Ehrenberg be correct
in viewing the interstitial cell- corpuscles (kornchen) as germ-cells,
these essential parts of ova may receive the essential matter of the sperm
from the nucleus which is discharged along with them in the breaking
up of the monad, which Ehrenberg regards as equivalent to an act of
oviposition ; and impregnated germ-cells may thus be prepared to
diffuse through 'space and carry the species of Polygastric Animalcules

68

Wherefore it may be concluded that the presence of the
spermatic force is essential to the process of growth by the
multiplication of cells, and that the phenomena of the for-
mation of the germ-mass are the provision for the presence
of that force. It is only therefore a question of the degree
of spermatic force which may suffice for any of the descend-
ants of the primary impregnated germ- vesicle, in order to
recommence and repeat the process to which the cell itself
owed its origin. The result of such process would be
always the same : the formation or accumulation of a
germ-mass ; that is to say, a mass of cells prepared as the
materials upon which the plastic force might operate in the
formation and adjustment of the different tissues and organs
of a new individual.

What then might be expected to be the conditions of
structure essential to the renewal of the process of forming
a germ-mass in an individual organism, independently of
the primal act of impregnation ? Obviously the retention
of some of the progeny of the primary germ-vesicle with
their inherited spermatic virtue.

Now we have seen that the power of propagating by
gemmation and spontaneous fission is in the ratio of the
retention of germ-cells, as such, in the constitution of the
individual first developed from the primary germ-mass.
Plants are more cellular than animals; monads than rotifers;
polypes than echinoderms ; the tape worms than the round
worms ; the radiata than the articulata and mollusca ; the
invertebrata than the vertebrata.

to a distance from the scene of life of the parent. A very peculiar
odour has always been recognized as one of the characteristics of the
semen, which leads me to quote the following remark by Ehrenberg : —
"Vast numbers of the Euglena viridis, dying contracted into a ball,
form a delicate green pellicle on the water, which first exhales, as du-
ring life, a spermatic and then a mouldy odour : sinks during cold, rises
during warmth, is decomposed into minute corpuscles and evolves gas,
and the mass finally is resolved into a greyish dust which contains
very minute ovules without chorion."

69

In proportion to the number of generations of germ- cells,
with the concomitant dilution of the spermatic force, and
in the ratio of the degree and extent of the conversion of
these cells into the tissues and organs of the animal is the
perfection of the individual, and the diminution of its power
of propagating without the reception of fresh spermatic
force.

In the vertebrate animal the whole of this force originally
diffused amongst the cells or nuclei of the germ-mass is
exhausted in the development of the tissues and organs of
the individual, in the mysterious renovation of the spermatic
power in the male by a special organ, and in the develop-
ment of ova or cells prepared fit for its reception in the
female. It now and then happens, even in the highest of
Vertebrata — the human species — that an ovarian germ-cell
sets up the process of embryonic development, but without
sufficient of the spermatic and plastic power to complete
even a larval form : some crude materials of the embryo
are the sole result : teeth, it may be, or hair, with irregular
amorphous ossifications, such as are met with occasionally
in ovarian cysts.

The completion of an embryonic or larval form by the
development of an ovarian germ-cell, or germ- mass, as in
the Aphis, without the immediate reception of fresh sper-
matic force, has never been known to occur in any verte-
brate animal.

The condition which renders this seemingly strange and
mysterious generation of an embryo without precedent
coitus possible, is the retention of a portion of the germ-
mass unchanged. One sees such portion of the germ-mass
taken into the semitransparent body of the embryo Aphis,
like the remnant of the yelk in the chick. I at first thought
that it was about to be inclosed within the alimentary canal,
but it is not so. As the embryo grows it assumes the
position of the ovarium, and becomes divided into oval
masses and inclosed by the filamentary extremities of the

70

eight oviducts, Individual development is checked and
arrested at the apterous larval condition. It is plain, there-
fore, that the essential condition of the development of
another embryo in this larva is the retention of part of
the progeny of the primary impregnated germ-cell.

What is really surprising in the phaenomena of the
Aphides is the potency of the mysterious virtue of the
quintessential excretion, which sustaining so great a de-
gree of subdivision, and of dilution with the material incor-
porated in the successive generation of cells, is nevertheless
equal to the renewal and repetition of embryonic develop-
ment through so many generations.

The explanation of the condition essential to this pro-
cess is still more clearly afforded by the development of the
Distoma tarda as described by Steenstrup. The entire
impregnated ovum consists, as in most of the lower Inver-
tebrata, of the germ-cells, which have assimilated the whole
of the germ-yelk; the ovum, therefore, grows with the
growth of the germ-mass which is formed by the usual pro-
cesses of imbibition and spontaneous fission producing the
multiplication of cells and nuclei : but certain of the outer-
most of these nuclei liquefy or coalesce, and form an addi-
tional layer to the chorion of a contractile or a ciliated
structure. The ovum is thus transformed into a sub-
elongate, subdepressed locomotive animal ; which contains,
however, nothing in its interior except the multiplied off-
spring of the primitive germ-cell. Here, therefore, we have
the condition which I regard as essential to the development
of an animal under circumstances which Steenstrup would
define as ( alternate generation by the vital powers and by
means of the bodies of the producing individuals/ and
wiiich the reviewer who has criticised him would prefer
to call a development ' by a multiplication of cells by a
process of continuous growth/ Neither of these definitions
explain the condition to which I refer. That condition is
the existence in each of the cell-progeny of the primary

71

germinal vesicle, of its share of that seminal virtue which
was incorporated with the parent-cell in the act of impreg-
nation and distributed by its successive divisions.

The course of development by which the ovum of the
Distoma has been converted into the Gregariniform larva
is as simple and as short as could be conceived, in order to
support the idea of such a conversion. A free moving
animalcular entozoon is, however, the result : a parasite
closely resembling those of the genus Gregarina as defined
by Kolliker and Siebold. This early larval form of the
Distoma is explained to be a wet-nurse (Amme), or rather
a grand-nurse (Grosse-amme), in the figurative language of
Steenstrup. What it truly is, and what its powers are, and
on what they depend, can only be understood by a recog-
nition of the essential nature and signification of the first
steps in its development, viz. the diffusion of the spermatic
force in the formation of the germ-mass.

Now we might expect, from the analogy of higher ani-
mals, that certain of the constituent corpuscles of this
germ-mass would perish as such in order to combine and
form the tissues and organs of the Distoma. But the fer-
tility of Nature's combinations and variations is inexhaust-
ible ! Instead of that ordinary process, several of the germ-
corpuscles of the Gregariniform larva set up independently
the centralizing actions of assimilation and spontaneous
fission, and so constitute as many secondary germ-masses,
in each of which a greater proportion of the constituent
corpuscles coalesce and combine to form the tissues of an
animal, like a Cercaria, than happened in the development
of the Gregariniform parent. Under the form of Cercariae
these secondary larvae, as they may be termed, burst the
integument of the primary one and are set free ; yielding
one of the most striking examples of the multiparous cha-
racter of the Trematode ovum ; the multiplication of the
larval forms which precede the ultimate oviparous indivi-
dual being simultaneous, not consecutive as in the Aphis.

F 2

72

One witnesses something like this in the rapid develop-
ment of the transverse segments of the Strobila, and their
almost simultaneous liberation by spontaneous fission.

It would be needless to multiply the illustrations of the
essential condition of these phenomena. That condition
is, the retention of certain of the progeny of the primary
impregnated germ-cell, or in other words, of the germ-mass
unchanged in the body of the first individual developed
from that germ-mass, with so much of the spermatic force
inherited by the retained germ-cells from the parent-cell or
germ-vesicle as suffices to set on foot and maintain the
same series of formative actions as those which constituted
the individual containing them.

How the retained spermatic force operates in the for-
mation of a new germ-mass from a secondary, tertiary, or
quaternary derivative germ-cell or nucleus, I do not profess
to explain ; neither is it known how it operates in develop-
ing the primary germ-mass from the impregnated germ- ve-
sicle of the ovum. In both we witness centres of repulsion
and of attraction antagonising to produce a definite result.

The physiologist congratulates himself with justice when
he has been able to pass from cause to cause, until he arrives
at the union of the spermatozoon with the germinal vesicle
as the essential condition of development — a cause ready to
operate when favourable circumstances concur, and without
which cause those circumstances would have no effect.

What I have endeavoured to do has been to point out
the conditions which bring about the presence of the same
essential cause in the cases of the development of an em-
bryo from a parent that has not itself been impregnated.
The cause is the same in kind though not in degree, and
every successive generation, or series of spontaneous fissions,
of the primary impregnated germ-cell must weaken the
spermatic force transmitted to such successive generations
of cells.

The force is exhausted in proportion to the complexity

73

and living powers of the organism developed from the pri-
mary germ-cell and germ-mass. It is consequently longest
retained and furthest transmitted in the vegetable king-
dom; the zoophytes manifest it in the next degree of
force ; and the power of retained germ-cells to develope a
germ-mass and embryo by the remnant of spermatic virtue
which they inherited, is finally lost, according to our actual
knowledge, in the class of Insects, and in the lower Mol-
lusca. It is interesting, however, to observe this power
energising, as it fades, in the reproduction of parts, e. g. the
limbs of the lobster, when it is no longer capable of deve-
loping a whole.

The secondary circumstances attending such develop-
ment of a germ-cell retained unaltered in the body, differ
from those which attend the development of an ovum, and
vary from one another as much as the accessory circum-
stances in the development of an ovum vary. An ovum is
essentially a nucleated cell called the ' germinal vesicle/
with a granular fluid called the e germ-yelk ' in an envelope :
it may contain a supplementary nutrient vitelline mass,
properly called yelk, or a yelk with chalazas and a calcareous
shell, or a nidamental case and appendages of various
kinds, &c., but the essential part is the germinal vesicle,
and this cell must be impregnated. The ovaria and the
conduits for the conveyance of the ovum outwards are
superadded and accessory parts in the business of gene-
ration. An impregnated germ-cell imparts its spermatic
power to its cell-offspring ; but when these perish, or when
the power is exhausted by a long descent, it must be re-
newed by fresh impregnation. But Nature is economical ;
and so long as sufficient power is retained by the progeny
of the primary impregnated vesicle, individuals are de-
veloped from that progeny without the recurrence of the
impregnating act.

The germinal vesicle is the essential part of an ovum.

An impregnated germ-cell is essential to the development

74

of the germ-mass and embryo in ovo : it is the primary im-
pregnated germ-cell.

A derivative germ-cell, the progeny of the primary one,
retaining a due amount of spermatic force, is essential to
the development of a secondary germ-mass and embryo
within the body of the primary one.

The gregariniform larva, or locomotive ovum, of the Di-
stoma tarda exemplifies the above propositions and plainly
illustrates the essential condition of its Parthenogenesis, or
power of propagating ( sine concubitu/

Were any one to assert that the development of the cer-
cariform larva from one of these secondary germ-masses was
altogether distinct from the development ab ovo, by being ( a
multiplication of cells by a process of continuous growth*,'
wre could regard such assertion in no other light than as a
purely arbitrary one, and as betraying a want of appre-
ciation of what must be effected when the primary impreg-
nated germ-cell subdivides itself into the secondary ones
in order to form the germ-mass.

Reverting again to the gregariniform larvae, let us sup-
pose that besides the cells of the germ-mass of the ovum of
the Distoma which have perished as such to form the larval
integument, others had coalesced to constitute special en-
velopes of parts of the unchanged germ-mass, and to form
canals leading from those ovarian envelopes outwards : if
any of such included germ-masses set on foot the processes
of forming an embryo, such embryo would be excluded
( per vaginam' as in the Aphis, instead of bursting through
the integument as in the gregariniform entozoal larva.

* The reverse of this idea is so obvious, that the veritable correspond-
ence between ovulation and gemmation could not escape the notice of
the equal observer of nature. Dujardin alludes to the common recog-
nition of it in his 'Memoir on the Infusoria': — ' Les gemmes, les
bourgeons qu'on voit se detacher du corps des zoophytes peuvent encore
etre compares jusqu'a un certain point avec les germes detaches de
1'ovaire des animaux plus parfaites." — Annales dps Sciences Nat. 1838,
p. 290.

75

Or let us suppose that certain other germ-cells in that
larva were to combine and coalesce to form a stomach, and
to push out prehensile arms around its mouth, pressing the
retained germ-cells into the substance of the thickened in-
tegument, or aggregating them near the base of the stomach :
if any of such germ-cells were to set on foot the process of
forming a germ-mass, it might protrude outwards, as in
the Hydra, and the embryo might include the stretched
portion of integument of the parent in the progress of its
development.

But this development by gemmation would differ from
that ab ovo only in accessory and non-essential particulars ;
the nature of the essential spermatic force would be the
same.

Such is the explanation which I have to offer of the
phenomena which meet us at the outset of our inquiries
into the generation of animals, and which I have grouped
together under the term c Parthenogenesis/ These phae-
nomena have hitherto remained the most obscure and seem-
ingly anomalous in the chapter of Comparative Physiology
relating to Generation; but, with this understanding of
their nature, they appear to me to enter into the ordinary
laws of the function, and to be disguised only by accessory
and non-essential modifications. Had I indeed, in the place
of such explanation, deduced from observation of the first
steps in development, salved these phenomena by the pro-
position " that they took place agreeably with the law of
alternate generation ;" " by the vital powers and by means
of the bodies" of the producing individuals; and had I
applied to the cercarial larva of the Distomata, to the po-
lypoid larva of the Medusae, and to the apterous larva of
the Aphides, the metaphorical phrase of f wet-nurses '
and ' nursing-generations/ I should have succeeded in
concealing my own ignorance of the organic conditions
essential to these remarkable reproductive processes, in the

76

same degree in which such propositions and phrases might
have seemed or sounded to you like explanations of the
facts : but I should have done no more : you would have
had apparent instead of real knowledge.

No one, as it appears to me, who carefully analyses the
language of the Generations-wechsel theory of Professor
Steenstrup, will be able to conceal from himself that it con-
veys no idea leading, even by a single step, to a closer
knowledge of the essential conditions of the development
of an Aphis in the body of a virgin parent. But all will
readily concede to him the merit of the assemblage of the
phaenomena from scattered memoirs into one convenient
work, and gratefully award to the ingenious author the
praise due for this useful labour, as well as for the confir-
mation of some of the facts and the addition of others, which
yet must have remained as unexplained and empirical phae-
nomena, until the organic conditions had been pointed out
which are essential to the * Lucina sine concubitu.'

END.

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