J
wish to call attention to the numerical composition, if 1 may thi.1s
express myself, of the chromatic elements of the egg at successive moments in the history of its maturation.
At all stages of development 1ve find the chromatic mass of the egg
nucleus divided into two portions, each composed of four elemei1ts. At
times the latter are aggregated two by two.
In the germinative corpuscle [primary oocyte} there exist two
nuclear plates each composed of four chromatic globules ...
In the first polar body we find two chromatic bodies, each composed
of two [ chromatidsJ; through Meta phase II and perhaps of four [i.e.,
at Ana.phase II] parts more 9r less clearly separated.
In the deuthyalosome {seconda?y oocyte] we find two chromatic
:residues [chromosomes) which are separated into two. groups of elements
fchromatid pairs].
.·
In the second polar body there exist two chromatic bodies, ~ach
composed of two parts [arms] and each of these again consisting of two
agglutinated elements.
In the female pronucleus there are· two chromatic aggregations
[chromosomes). Each of these consists of two small cl1romatic masses each
composed of two deeply stained rods ...
As for the phenomena of maturation in the male pronucleus, they
are identical with those I lrnve described with regard to the female pr'onucleus: when they have reached their complete development .the two
elements are similarl}' constituted .... Then the two pronuclei approath
one another.toward the center of the egg. In order toj~>in its mate, the
female pronucleus, which always arises. in the neighborltood of the
superior pole of the egg, travels a much greater distance than the rnale
pronucleus; the latter is displaced xelatively little.
. ...
The segmentation of the egg titkes pface by indirect means [by
miwsisJ; the tw'o pronuclei, Without. fusing, both participate ill the
formation of a single dicentric figure. Both undergo simultmYeously, and
while they are still perfectly distinct, the same changes which occur in a
nucleus of an ordinary cell in the course of. karyokinetic division. Ea.ch
pronucleus furnishes tivo loops to the chromatic star of the equatorial
disc; the star is comprised of rwo male loops and two female loops. Each
of these divides longitudinally into two halves one of which goes to one
daughter nucleus and the other to. the second daughter nucletis. There
does not occur at. the outset any fusion between the male and female
chromatic elements. If such a fusion ever does occur, it cannot take place
except in the nuclei of the first two embryonic .cells. The chromatic star
of the equatorial plate supplied in part by the female pronucleus and in
part by the m:ale pronucleus, is identical with those :which form in the
blastomeres when they divide. On both sides the star consists of four
Edouard van Beneden (1883) Researches on the maturation of the egg and fertilization. From
Archives de Biologie 4: 265-640.
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chromatic loops. If the chromatic stars which are seen during the division
of the first two blastomeres represent a nucleus in the course of division,
it also follows that the first star, which develops out of the pronuclei, is
the equivalent of a single cellular nucleus.
I~ut since this manner of expression is also quite applicable to earlier
stages, we are thus led to consider the two pronuclei, though they are
distant one from another and entirely separated, as together representing
a single nucleus of a cell: each pronucleus is the equivalent of half a
nucleus. But this conclusion should not cause us to. forg·et that neither
before nor during karyokinesis, does there occur any fusion whatever, .at
least as far as the chromatic. elements of the pronuclei are concerned.
Neither does the achromatic substance of the two pronuclei mingle
into a single mass; in short, though the two pronuclei behave. as though
together they constitute a single nucleus, at none of the first stages of the
division do they cease to be distinct The conjugation of the two pro:
nuclei into a morphologically Single embryonic nucleus does not take
place. Before emb~irking upon a description of the phenomena :which
lead to the division of the egg into the first two blastomeres, we must
ask ourselves at what moment the egg should be considered as fertilized
and in what fortilization essentially consists.
It emerges clearly from the facts which I h~ve just summarized that.
the egg, provided with its two pronuclei, behaves like a single cell and
that the sum of the two nuclear elements which it contains are equivalent.
to an ordinary nucleus. The first cell of the embryo is fully constituted
from the moment whe.n the two pronuclei are formed; fertilization .therefore coincides with the genesis of these pronuclei.
I have shown abov.e that, as long as the second polar globule has' not
been eliminated, the elements of the sperfuatozoon. which are destined to
give birth to the male pronucleus do not undergo any kind of ciwnge.
But, from .the moment when the egg has i·id. itself 0£ its second polar
globule and of its second perivitelline layer, .the spermatozopn engenders
the pronucleus, and. at this satne moment the female pronucleus is cqnstitutecl. It seems therefore that the egg only exerts its influence upori the
spermatozoon, that it only determines it tO play its role which consists. in
the formation of half a cellular nucleus (male pronucleus), after it has
cleared itself of these procluets. Only afterwards does its sexual character
manifest itself; of the primitive egg cell there rerriains only a reduced
vitellus, provided with haH a cellular nucleus.
The male pronucleus proceeds to complete this reduced cell which I
call the female gonocyte and to make of it a new complete cell; fertiliza- ~
' tion apparently consists essentially in this reconstitutiori of the firit
embryonic cell, revivified and provided with all the energy necessary to
transform itself, while passing tlu·ough a series of more and more complex
Edouard van Beneden (1883) Researches on the maturation of the egg and fertilization. From
Archives de Biologie 4: 265-640.
Fig. 2. Fertilization in Ascaris, showing the positionof the pronuclei at \':trious stages; lhc
two chrornosomes uf each pronudcus; the polar bodics.,VMI nl'.NEDEN, 1883.
stages, into an indiv.idual resembling the parent. This singular phenomenon of the origin of the cell ont of two different elements appears to .be
intimately linked with the rejection by the egg cell on the one hand, and
by the spcrmatocyte on the other, of certain ,products: of the polar
globules and the perivitellinc layers in the case of the egg, and of the
cytophoral portions in the spemiatocytc .... In this regard the egg cell
and the spermatocyte arc distinguished from all other cells. Just as the
Edouard van Beneden (1883) Researches on the maturation of the egg and fertilization. From
Archives de Biologie 4: 265-640.
egg supplied with its female pronucleus is no more than a portion of a
cell which I call a female gonocyte, similarly a spermatozoon is a reduced
cell, a spermatocyte less a cylophoral portion; I call it a male gonocyte.
It seems to me difficult not to coni1ect together these phenomena of reduction or elimination and of reconstitution or fertilization, not to see
in the latter phenomenon a replacement or a substitution ....
The achromatic contours of the two pronuclei become less and less
distinct; these bodies, pressed one against the other, now fo1m together a
clear mass which ... encloses four chromatic loops; they come to .be
oriented in one and the same plane, the equatorial plane of the cell in the
course of division .... The number of loops is constant: one regularly
finds four of them; they are approximately of the same length. It emerges
from the latest evidence of the study of the previous stages of development that two of these loops are derived from the male pronucleus, the
other two from the female pronuclcl.1s.
I must now call attention to another phenomenon encountered in a
great number of eggs which have arrived at the stage under consideration
and which sometimes has already appeared in the preceding stage; I
refer to the doubling of the chromatic loops. The fact of the longitudinal
division of the chromatic cords was discovered by Flemming in dividing
cells of the tissues of the salamander and confirmed by Retzius and by
Pfitzner. This is, in my opiriion, one of the most important facts of
karyokinesis ....
If: we recall that of the four loops constituting the equatorial disc
two were supplied by the male pronucleus and the other two derived from
the female pronucleus, we arrive at the conclusion that each daughter
nucleus receives half of its chromatic substance from the spermatozoon,
the other from the egg. If: the pronuclei have a sexual character, if one is
male, the other female, it is clear that the nuclei of the first few blastomeres are hermaphrodites.
The reason for the doubling of the chromatic cords at the time of
the division of the nuclei was suspected by Flemming: he wondered
whether each primary loop does not furnish a secondary loop to each of
the daughter nuclei. Pi:obable. as this hypothesis appeared to him, as it
made understandable the tVh'y of the doubling, he was not able to support
it by any factual observatiori; the l~rge number of loops which one sees
in the nuclei of the salamander do not allow one to follow each loop to
see what it beconies. Such difficulty is not encountered in the typ.ical
figures of the egg or the blastomeres of the Asc{lris of the horse. It was
this relative simplicity which allowed me to settle positively this point,
so important for the description of the indirect division of cells, and to
establish the descent of t11e chi·omatic substance of cellular nuclei from
the male and female gonocytes, the spermatozoon on: the one hand and
the mature egg on the other. . . .
.>
p
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Edouard van Beneden (1883) Researches on the maturation of the egg and fertilization. From
Archives de Biologie 4: 265-640.
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