FATHERLESS FROGS
One of the most interesting discoveries of recent date in
regard to the processes which go on in that all-important
material--protoplasm--which is the physical basis of life and the
essential constituent of "cells"--those minute corpuscles of which all
living bodies are built--was made in 1910 by a French naturalist, M.
Bataillon, and has been examined and confirmed by another French
biologist, M. Henneguy. To explain this discovery, a few words as to
well-known facts are necessary. It is well known that if we isolate a
female frog at the egg-laying season and let her swim in perfectly
pure filtered water, and proceed to deposit some of her eggs in that
water, the eggs will not germinate; they remain unchanged for a time
and then decompose--become, in fact, "rotten." It is a matter of
common knowledge that it is necessary for the eggs to be "fertilised"
in order that they may start on that series of changes and growth
which we call "development," and become tadpoles and eventually young
frogs. The "fertilisation" of the frog's eggs is effected in ordinary
conditions by the presence in the water of the pond, into which the
female sheds them, of microscopic sperm-filaments (often called
spermatozoa, or simply "sperms") which are shed into the water at the
same time by the male frog.
The egg (the blackish-brown spherical body, as big as a rape-seed,
which is imbedded in a thin jelly, and is familiar to those who are
drawn by curiosity to look into the waters of wayside ponds in spring)
is a single cell or corpuscle of protoplasm distended with
dark-coloured and other granules of nutrient substance. A single sperm
(though requiring the microscope to render it visible) is also a
single cell. It is a minute oval body, with a long serpentine tail of
actively undulating protoplasm. Hundreds of thousands of these are
shed into the water at the breeding season by the male frog. One is
enough to fertilise the egg. The sperm-cells swim in the water, and
are chemically attracted by the eggs. As there are so many sperms, one
of them is sure to reach each black egg-sphere. It drives its way into
the substance of the egg, making a minute hole in its surface; then
the protoplasm of the sperm fuses with the protoplasm of the egg, and
becomes intimately mixed with it. The egg-cell has a "nucleus," that
dense, peculiar, deep-lying, and well-marked "kernel" of its
protoplasm which all cells have. It is of essential importance in the
life and activity of the cell. The sperm-cell has also a "nucleus,"
and now (as has been carefully ascertained) the nucleus of the sperm
and the nucleus of the egg-cell unite and form one single nucleus. The
egg is thereupon said to be "fertilised"--that is to say, "rendered
fertile." It at once commences to move. Its surface ripples and
contracts and nips in deeply, so that the sphere is marked out into
two hemispheres. These are two "cells," or masses of protoplasm,
adhering to each other. Each is provided with its own distinct nucleus
or cell-kernel, for the first step in the division of the egg-sphere
is the division within it of its newly constituted nucleus into two,
each half consisting of nearly equal proportions of the mingled
substance of the sperm-nucleus and the egg-nucleus. The two first
cells or hemispheres again divide, and so the process goes on until
the little black egg has the appearance of a mulberry, each granule of
the berry being a cell provided with its own nucleus derived from the
original nucleus formed by the fusion of the nuclei of the paternal
and maternal cells. In the course of a day or two the division has
proceeded so far that the resulting "cells" are so small as to be
invisible with a hand-glass, and require one to use a high magnifying
power in order to distinguish them. And there are hundreds of them;
the whole mass of the "egg" within, as well as on the surface, has
divided into separate cells. They go on multiplying, take up water,
and nourish themselves on the granular nutritive matter present from
the first in the egg-cell. The little mass elongates, increases in
size, and gradually assumes the form of a young tadpole.
We see, then that the process of fertilisation consists in two things,
the latter of which necessitates the former, viz. in the breaking or
penetration of the surface of the egg-cell by the active sperm
filament and second in the fusion of the substance of the sperm
filament with that of the egg in such a way that there is a distinct
and intimate fusion of the nucleus of the sperm filament with the
nucleus of the egg-cell. The recent discovery of M. Bataillon is this,
viz. that you can make the frog's egg develop in a perfectly regular
way and become a tadpole and then a young frog without the admission
to it of a sperm-filament or of any substance derived from the male
frog. All you have to do--and the operation, though it sounds easy and
simple, is an exceedingly delicate and difficult one--is to prick with
a fine needle the surface of the little black egg-sphere (not merely
of the jelly surrounding it) when it is shed by the female frog into
perfectly pure water free from sperms or anything of the sort. The
slight artificial puncture acts as does the natural puncture by the
swimming sperm-filament, and is sufficient! The egg proceeds to
develop quite regularly. There is no fusion of the nucleus of the
egg-cell with any matter from the outside; no paternal "material" is
introduced, but the nucleus of the egg-cell divides just as though
there had been! The whole progeny of cells, successively formed, are
the pure offspring of the maternal egg-cell and its nucleus. The
tadpoles and young frogs so produced are examples of what is called
"parthenogenesis"--that is to say, virginal reproduction--reproduction
without fertilisation by material derived from a male parent! The
needle, which gives off no material, but simply makes a tiny break in
the surface of the egg, does all that is necessary!
To those not acquainted with all that has been ascertained as to the
reproduction of lower animals such as insects, crustaceans, and worms,
this discovery will appear more astonishing than it really is. We
know of many lower animals in which the egg-cells produced by the
females do regularly and naturally develop without the intervention of
a male and without fertilisation. In an earlier volume[7] of this
"Easy Chair Series" I wrote of this curious subject, and described the
virgin reproduction or parthenogenesis of the hop-louse and
other plant lice, of some moths, of some fresh-water shrimps,
and of the queen bee (who produces only drones by eggs which are
not fertilised). But I had to point out then that no case was known of
"parthenogenesis"--that is to say, reproduction by unfertilised
eggs--among the whole series of vertebrate animals, the fishes,
amphibians, reptiles, birds, and mammals. The chief point of novelty
in M. Bataillon's discovery is that we have now an experimental
demonstration of parthenogenesis in a vertebrate animal, and in one so
highly organised as the frog. And equally interesting, indeed more
important from the point of view as to the real meaning and nature of
fertilisation, is the mode in which the parthenogenesis of the frog is
set going, namely, by a mere prick of the surface film of the ripe
egg!
There have, however, been important experiments on the subject of the
development of eggs without fertilisation in recent years, prior to
these discoveries as to the frog's egg. A favourite subject for such
inquiries is the sea urchin (Echinus of different kinds). The female
sea urchin, or sea egg, like its close allies the star fishes, lays a
great number of very transparent minute eggs (each about the 1/200th
of an inch in diameter) in sea-water, and they are there fertilised by
the mobile sperm filaments discharged by the males. The eggs are so
transparent and so easily kept alive in jars of sea-water that there
is no difficulty in watching under the microscope the penetration of
the egg by a sperm, and the fusion and other changes in the nuclei.
Delages of Paris, and Loeb of California, have made valuable studies
on these eggs. Loeb has shown that they may be artificially
started on the course of development and cell division without
fertilisation--simply by the action of minute quantities of simple
chemicals (fatty acids, etc.) introduced into the sea-water by the
experimenter. These chemicals appear to act on the delicate pellicle
which forms the surface of the egg-cell in much the same way as the
prick of a needle acts on a frog's egg. A limited and delicately
adjusted disturbance of the cohesion (or of the surface-tension) of
the egg-cell seems to be all that is necessary for starting the
egg-cell on its career of development. It becomes, in the light of
these experiments, not so much a wonder that egg-cells should develop
"on their own," but that they do not more frequently do so. It must be
remembered that the "germination" and development of unfertilised
eggs, even when the whole range of animals and plants is taken into
account (for plants also are reproduced by single cells identical in
character with the egg-cells and sperm-cells of animals), that is to
say, the existence of "parthenogenesis" as a natural, regularly
recurring process, is exceptional. We must distinguish cases in which
it regularly occurs as part of the life-history of an animal or plant
from cases in which it has been successfully brought about by
experimental "artificial" methods designed by man. The plant-lice
"naturally" reproduce through the summer by unfertilised eggs
producing only females, but in the first cold of autumn males are
hatched from some of the eggs, and the eggs of this generation are
fertilised and bide through the winter, hatching in the following
spring. Some few moths and flies also reproduce naturally during
summer by unfertilised eggs, and the brine-shrimps and some other
fresh-water shrimps produce "fatherless" broods from their eggs,
sometimes for years in succession, until "one fine day" some males are
hatched, owing to what causes we do not know. The queen bee naturally
and regularly lays a certain number of unfertilised eggs, and these
produce, not females as do the unfertilised eggs of plant-lice, etc.,
but male bees--the drones--and it is only from such eggs that the
drones of bees are born. These are the chief cases of regular and
natural parthenogenesis, but there are others which might be
enumerated.
On the other hand, examples of artificially induced development of
eggs, not fertilised, are very few. The first known came accidentally
to notice. Female silkworm moths reared in confinement sometimes lay
eggs when kept apart from the male, and these have been found to
hatch, and give rise to caterpillars, which were not reared to
maturity. Other moths bred by collectors behaved in the same way, but
the grubs were reared to maturity, and three successive generations of
"fatherless" moths were obtained. In these cases the hatching of
unfertilised eggs is not known to occur in a state of nature, although
it probably occurs occasionally. It has also been observed--an
important fact when considered with the history of the frog's egg and
the needle--that "brushing" the unfertilised eggs of the silkworm and
other moths, that is to say, gently polishing the little egg-shells
with a soft camel's-hair brush, has the effect of starting
development. Taking two lots of unfertilised eggs adhering to slips of
paper, as laid by the mother moth, it is found that those gently
brushed will hatch, whilst those not brushed will either not hatch at
all, or in very small number. The brushing seems to disturb the
equilibrium of the protoplasmic egg-cell within the egg-shell just
sufficiently to set it going--going on its course of division and
development. The only other case of "artificially-induced
parthenogenesis" at present recorded is that of the common frog, due
to M. Bataillon. There are questions of great interest still to be
made out as the result of his discovery. Can the fatherless brood be
reared to maturity and again made to yield a fatherless generation?
What is the precise structure of the nuclei of the cells which
originate from the nucleus of the egg-cell only, and not from a
nucleus formed by the fusion of that with a sperm-cell nucleus? These
and similar questions are the motive of further careful study now in
progress.
The important conclusion is forced upon us by these experiments with a
needle, that even in so typical and highly organised a creature as one
of the higher or five-fingered, air-breathing vertebrates, the
egg-cell does not require any material admixture from the sperm-cell
in order that it may successfully germinate and develop, but only a
disturbance of equilibrium, which can be administered as well by a
needle's point as by a sperm-filament! Yet the whole process of sexual
reproduction undoubtedly has, as its origin and explanation, the
fusion in the first cell of the new generation from which all the rest
will arise, of the material of two distinct individuals. Thus the
qualities of the young are not a repetition of the qualities of one
parent, nor are they a mere mixture of the qualities of both parents
(for contradictory qualities cannot mix). They are a new grouping of
qualities comprising some of the one parent and some of the other and
hence a great opportunity for variation, for departure from either
parent's exact "make-up," is afforded, and for the selection and
survival of the new combination. It is, it would seem, only in
exceptional cases and for limited periods that uni-sexual or
fatherless reproduction can be advantageous to a species of plant or
animal. Such cases are those in which abundant food, present for a
limited season, renders the most rapid multiplication of individuals
an advantage to the species. But after this exceptional abundance has
come to an end, the more usual process of reproduction by fertilised
eggs (also necessary and advantageous for the preservation of the race
by "natural selection in the struggle for existence" of the new
varieties so produced) is resumed until again the abundant food is
present, as in the annual history of plant lice and the plants on
which they feed.
FOOTNOTES:
[Footnote 7: "Science from an Easy Chair," Methuen & Co., 1910.]