INDEX. Pp. 584-590.
APPENDIX.
EMBRYOLOGY.
INTRODUCTION.
Embryology forms a large and important department of Biology. Strictly
interpreted according to the meaning of the word, it ought to deal
with the growth and structure of organisms during their development
within the egg membranes, before they are capable of leading an
independent existence. Modern investigations have however shewn that
such a limitation of the science would have a purely artificial
character, and the term Embryology is now employed to cover the
anatomy and physiology of the organism during the whole period
included between its first coming into being and its attainment of the
adult state.
The subject-matter of the science of Embryology admits of a twofold
classification. It may be placed under a series of heads, each dealing
either with a special group of organisms, or with a special department
of the whole science. If classified in the first of these ways the
science will naturally be divided into an Embryology of Plants, and an
Embryology of Animals; each of which admits of further subdivision. In
the second way the subject falls under two primary heads; viz.
Physiological Embryology and Anatomical Embryology.
The present treatise deals only with the Embryology of Animals, and is
further confined to those animals known as Metazoa. The science is
moreover treated from the morphological or anatomical, rather than
from the physiological side.
The marvellous phenomenon of the evolution of a highly complicated
living being from a simple undifferentiated germ in which it needs the
aid of the most modern microscopical appliances to detect any visible
signs of life, has not unnaturally attracted the attention of
biologists from the very earliest periods. Before the establishment of
the cell theory the origin of the organism from the germ was not known
to be an occurrence of the same nature as the growth of the fully
formed individual, and Embryological investigations were mixed up with
irrelevant speculations on the origin of life[1].
[1] To this general statement Wolff forms a remarkable exception,
for though without any clear knowledge of what we call cells he
had very distinct notions on the relations of growth and
development.
The difficulties of understanding the formation of the individual from
the structureless germ led anatomists at one time to accept the view
"according to which the embryo preexisted, even though invisible, in
the ovum, and the changes which took place during incubation consisted
not in a formation of parts, but in a growth, _i.e._ in an
expansion with concomitant changes of the already existing germ."
Great as is the interest attaching to the simple and isolated life
histories of individual organisms, this interest has been increased
tenfold by the generalizations of Mr Charles Darwin.
It has long been recognized that the embryos and larvæ of the higher
forms of each group pass, in the course of their development, through
a series of stages in which they more or less completely resemble the
lower forms of the group[2]. This remarkable phenomenon receives its
explanation on Mr Darwin's theory of descent. There are, according to
this theory, two guiding, and in a certain sense antagonistic
principles which have rendered possible the present order of the
organic world. These are known as the laws of heredity and variation.
The first of these laws asserts that the characters of an organism at
all stages of its existence are reproduced in its descendants at
corresponding stages. The second of these laws asserts that offspring
never exactly resemble their parents. By the common action of these
two principles continuous variation from a parent type becomes a
possibility, since every acquired variation has a tendency to be
inherited.
[2] Von Baer who is often stated to have established the above
generalization really maintained a somewhat different view. He
held (_Ueber Entwickelungsgeschichte d. Thiere_, p. 224)
that the embryos of higher forms never resembled the adult stages
of lower forms but merely the embryos of such forms. Von Baer was
mistaken in thus absolutely limiting the generalization, but his
statement is much more nearly true than a definite statement of
the exact similarity of the embryos of higher forms to the adults
of lower ones.
The remarkable law of development enunciated above, which has been
extended, especially by the researches of Huxley[3] and Kowalevsky,
beyond the limits of the more or less artificial groups created by
naturalists, to the whole animal kingdom, is a special case of the law
of heredity. This law, interpreted in accordance with the theory of
descent, asserts that each organism in the course of its individual
ontogeny repeats the history of its ancestral development. It may be
stated in another way so as to bring out its intimate connection with
the laws of inheritance and variation. Each organism reproduces the
variations inherited from all its ancestors at successive stages in
its individual ontogeny which correspond with those at which the
variations appeared in its ancestors. This mode of stating the law
shews that it is a necessary consequence of the law of inheritance.
The above considerations clearly bring out the fact that Comparative
Embryology has important bearings on Phylogeny, or the history of the
race or group, which constitutes one of the most important branches of
Zoology.
[3] Huxley was the first to shew that the body of the
Coelenterata was formed of two layers, and to identify these
with the two primary germinal layers of the Vertebrata.
Were it indeed the case that each organism contained in its
development a full record of its origin, the problems of Phylogeny
would be in a fair way towards solution. As it is, however, the law
above enunciated is, like all physical laws, the statement of what
would occur without interfering conditions. Such a state of things is
not found in nature, but development as it actually occurs is the
resultant of a series of influences of which that of heredity is only
one. As a consequence of this, the embryological record, as it is
usually presented to us, is both imperfect and misleading. It may be
compared to an ancient manuscript with many of the sheets lost, others
displaced, and with spurious passages interpolated by a later hand.
The embryological record is almost always abbreviated in
accordance with the tendency of nature (to be explained on the
principle of survival of the fittest) to attain her ends by the
easiest means. The time and sequence of the development of parts is
often modified, and finally, secondary structural features make their
appearance to fit the embryo or larva for special conditions of
existence. When the life history of a form is fully known, the most
difficult part of his task is still before the scientific
embryologist. Like the scholar with his manuscript, the embryologist
has by a process of careful and critical examination to determine
where the gaps are present, to detect the later insertions, and to
place in order what has been misplaced.
The aims of Comparative Embryology as restricted in the present work
are two-fold: (1) to form a basis for Phylogeny, and (2) to form a
basis for Organogeny or the origin and evolution of organs. The
justification for employing the results of Comparative Embryology in
the solution of the problems in these two departments of science is to
be found in the law above enunciated, but the results have to be
employed with the qualifications already hinted at; and in both cases
a knowledge of Comparative Anatomy is a necessary prelude to their
application.
In accordance with the above objects Comparative Embryology may be
divided into two departments.
The scientific method employed in both of these departments is that of
comparison, and is in fact fundamentally the same as the method of
Comparative Anatomy. By this method it becomes possible with greater
or less certainty to distinguish the secondary from the primary or
ancestral embryonic characters, to determine the relative value to be
attached to the results of isolated observations, and generally to
construct a science out of the rough mass of collected facts. It
moreover enables each observer to know to what points it is important
to direct his attention, and so prevents that simple accumulation of
disconnected facts which is too apt to clog and hinder the advance of
the science it is intended to promote.
In the department of Phylogeny the following are the more important
points aimed at.
(1) To test how far Comparative Embryology brings to light ancestral
forms common to the whole of the Metazoa. Examples of such
forms have been identified by various embryologists in the ovum
itself, supposed to represent the unicellular ancestral form of the
Metazoa: in the ovum at the close of segmentation regarded as the
polycellular Protozoon parent form: in the two-layered gastrula, etc.,
regarded by Haeckel as the ancestral form of all the Metazoa[4].
[4] The value of these identifications as well as of those below
is discussed in its appropriate place in the body of the work.
Their citation here is not to be regarded as necessarily implying
my acceptance of them.
(2) How far some special embryonic larval form is constantly
reproduced in the ontogeny of the members of one or more groups of the
animal kingdom; and how far such larval forms may be interpreted as
the ancestral type for those groups.
As examples of such forms may be cited the six-limbed Nauplius
supposed by Fritz Müller to be the ancestral form of the Crustacea;
the trochosphere larva of Lankester, which he considers to be common
to the Mollusca, Vermes, and Echinodermata; the planula of the
Coelenterata, etc.
(3) How far such forms agree with living or fossil forms in the adult
state; such an agreement being held to imply that the living or fossil
form in question is closely related to the parent stock of the group
in which the larval form occurs. It is not easy to cite examples of a
very close agreement of this kind between the larval forms of one
group and the existing or fossil forms of another. The larvæ of some
of the Chætopoda with long provisional setæ resemble fossil Chætopods.
The Rotifers have many points of resemblance to the trochosphere,
especially to that form of trochosphere characteristic of the
Mollusca. The Turbellarians have some features in common with the
Coelenterate planula. Some of the Gephyrea in the presence of a
præoral lobe resemble certain trochosphere types. The larva of the
Tunicata has the characters of a simple type of the Chordata.
Within the limits of a single group agreements of this kind are fairly
numerous. In the Craniata the tadpole of the Anura has its living
representative in the Pisces and perhaps especially in the Myxinoids.
The larval forms of the Insecta approach Peripatus. The stalked larva
of Comatula is reproduced by the living Pentacrinus and Rhizocrinus
etc. Numerous examples of the same phenomenon are found
amongst the Crustacea.
(4) How far organs appear in the embryo or larva which either atrophy
or become functionless in the adult state, and which persist
permanently in members of some other group or in lower members of the
same group. Cases of this kind are of the most constant occurrence,
and it is only necessary to cite such examples as the gill slits and
Wolffian body in the embryos of higher Craniata to illustrate the kind
of instance alluded to. The same conclusions may be drawn from them as
from the cases under the previous heading.
(5) How far organs pass in the course of their development through a
condition permanent in some lower form. Phylogenetic conclusions may
be drawn from instances of this character, though they have a more
important bearing on Organology than on Phylogeny.
The considerations which were used to shew that the ancestral history
is reproduced in the ontogeny of the individual apply with equal force
to the evolution of organs. The special questions in Organology, on
which Comparative Embryology throws light, may be classified under the
following heads.
(1) The origin and homologies of what are known as the germinal
layers; or the layers into which the embryo becomes divided
immediately after the segmentation.
(2) The origin of primary tissues, epithelial, nervous, muscular,
connective, etc., and their relation to the germinal layers.
(3) The origin of organs. The origin of the primitive organs is
intimately connected with that of the germinal layers. The first
differentiation of the segmented ovum results in the cells of the
embryo becoming arranged as two layers, an outer one known as the
epiblast and an inner one as the hypoblast. The outer of these forms a
primitive sensory organ, and the inner a primitive digestive organ.
(4) The gradual evolution of the more complicated organs and systems
of organs.
This part of the subject, even more than that dealing with questions
of Phylogeny, is intimately bound up with Comparative Anatomy; without
which indeed it becomes quite meaningless.
REPRODUCTION.
A study of reproduction logically precedes that of Embryology.
Reproduction essentially consists in the separation of a portion of an
organism which has the capacity of developing into a form similar to
that which gave it origin. The simplest modes of reproduction are
those which occur amongst the Protozoa.
In this group, reproduction may take place in a great variety of ways.
These may be classified in three groups: (1) fission, (2) budding or
gemmation, (3) spore formation.
Reproduction in all these ways may take place either subsequently to
and apparently in consequence of a very important process known as
conjugation, which consists in the temporary or permanent fusion of
two or more individuals, or spontaneously, _i.e._ independently
of any such previous conjugation.
Reproduction by fission consists simply in the division of the
organism into two similar parts, the nucleus when present becoming
divided simultaneously with the cell body. This mode of reproduction
is the simplest conceivable, and is not followed by a development,
since the two organisms produced are exactly similar, except in size,
to the parent form. Besides single fission, a process of multiple
fission may take place, as amongst the Flagellata, where Drysdale and
Dallinger have shewn that an individual enclosed within a
structureless cyst may divide first into two, then into four, and so
on.
The process of budding differs mainly from that of simple fission in
the fact that the two organisms produced are dissimilar in size, and
also that the separation of the smaller organism from the larger is
preceded by a process of growth in the latter, so that in the
separation of the bud no essential part of the parent form is removed.
This mode of reproduction is found amongst the Infusoria, Acineta, &c.
An interesting variation in it is the internal gemmation of many of
the Acineta, where a portion of the internal protoplasm with part of
the nucleus is separated off to form a fresh individual. This mode of
gemmation is connected by a series of gradations with the normal
external gemmation. The organisms produced by gemmation are
not always similar at birth to the parent; _e.g._ Acineta.
Both fission and gemmation when incomplete lead to the formation of
colonies.
The third mode of reproduction, by spore formation, does not
essentially differ from that by multiple fission. It consists in the
breaking up of the organisms into a number (usually very considerable)
of portions; each of which eventually develops into an organism like
the parent form. All gradations between a simultaneous division of the
organism into such spores and simple multiple fission are to be found,
but this process of reproduction may be sometimes distinguished from
that by such fission by the fact that the two processes may coexist in
a single form, _e.g._ the biflagellate monad of Drysdale and
Dallinger. In the majority of cases the spores produced differ at
first from the parent organism not only in size but in other points,
such as the possession of a flagellum, etc. They may even be without a
nucleus when the parent organism is nucleated, as in the Gregarinidæ.
The encystment, which in many cases precedes reproduction by any of
the above processes, and more especially by spores, is not an
essential condition of their occurrence; and is probably in the first
instance a protective arrangement which has become secondarily adapted
to and connected with reproduction.
As has been already stated, all the above modes of reproduction take
place in some of the Protozoa without any anterior process which can
be regarded as of a sexual nature; but very often they are preceded by
the temporary or permanent fusion of two or more individuals, such
fusion being known as conjugation.
In most cases reproduction by spores is the consequence of
conjugation, but in the Infusoria etc. where the fusion at conjugation
is temporary (except Vorticella), there is probably merely a renewed
activity--a rejuvenescence--which most likely results in active
fission or budding. In the Gregarinidæ reproduction by spores usually
follows conjugation, but may also take place without it. In some
Flagellata reproduction by spores follows the conjugation of two
individuals in a different stage of development. Thus in the
springing Monad, described by Drysdale and Dallinger, a form produced
by the fission of a monad in an amoeboid condition fuses with an
ordinary monad to produce an individual, which then breaks up into
spores. Another instance of the fusion of dissimilar individuals is
afforded by Vorticella, where a free-swimming individual conjugates
and is permanently united with a fixed one (Engelmann, Bütschli).
Conjugation often consists in the fusion of more than two individuals.
In conjugation where the fusion is permanent, the nuclei of the
conjugating forms usually unite before the product breaks up into
spores and where temporary fusion occurs in the Infusoria a division
of the paranuclei and often of the nuclei takes place, followed by the
ejection of parts of them, and a reproduction of new paranuclei and
nuclei from the remainder of the original structures.
In order to understand the meaning of conjugation in connection with
reproduction, it is important to understand how the two became in the
first instance related. For the solution of this question the fact
that many Protozoa have the capacity of temporarily or permanently
fusing together without an _immediate_ act of reproduction is of
great importance. A good example of such fusion is supplied by
Actinophrys. We must suppose in fact that the simple coalescence of
two or more individuals gives a sufficient amount of extra vigour to
their product, to compensate the race for the loss in number of
individuals so caused. This extra vigour probably first exhibited
itself especially by increased activity in reproduction, till finally
the two processes, viz. that of conjugation and that of reproduction,
came to be inseparably connected together.
The reproduction of the forms above the Protozoa, which are known as
the Metazoa, takes place by two methods, viz. a sexual and an asexual
one. The sexual process, which occurs in every known Metazoon[5],
consists essentially, as is shewn in the second chapter of this work,
in the fusion of two cells budded off from the parent organism, viz.
the female cell or ovum, and the male cell or spermatozoon, and of the
subsequent division of the compound cell so produced into a number of
parts which build themselves up into an organism resembling
one of the parents. The sexual process has obviously at first sight a
very close resemblance to the process of conjugation. Since it is a
question of fundamental importance to determine how sexual
reproduction originated, it becomes necessary to examine how far this
apparent resemblance is a real one, and how far sexual reproduction
can be derived from reproduction following upon conjugation.
[5] Dicyema, if it is a true Metazoon, would seem to form an
exception to this rule.
In spite of the general similarity between the two processes there is
an obvious difficulty in comparing them, in that the result of
conjugation is usually the breaking up of the individual formed by the
fusion of two other individuals into a _number of new organisms_,
while the result of the fusion which takes place in sexual
reproduction is the formation of a _single new organism_. This
difference between the two processes, great as it is, is perhaps
apparent rather than real. It must be remembered that a single
individual Metazoon is equivalent to a number of Protozoa coalesced to
form a single organism in a higher state of aggregation. It results
from this that the segmentation of the ovum which follows the sexual
act may be compared to the breaking up of the product of conjugation
into spores, the difference between the two processes consisting in
the fact that in the one case the spores separate each to form an
independent organism, while in the other they remain united and give
rise to a single compound organism.
If the above considerations are well founded it seems permissible to
accept the general view according to which sexual reproduction is
derived from conjugation. It is necessary to suppose that, in a colony
of Protozoa in the course of becoming a Metazoon, the capacity of
reproduction by spores became localized in certain definite cells, and
although the formation of spores from these cells may have been
possible without previous conjugation, yet that conjugation gradually
became established as the rule. The differentiation of primitively
similar conjugating cells into male and female cells was probably a
very early occurrence, since indications of an analogous
differentiation, as has already been mentioned, are found in certain
existing Protozoa (Monads, Vorticella, etc.). I have attempted to shew
in the second chapter that the breaking up of the cell into spores
without previous conjugation is perhaps provided against in
the extrusion of the so-called 'directive body'.
With the differentiation of special germinal cells, to take the place
of the whole individual in the act of conjugation, the possibility of
each act of conjugation resulting in the production of only a single
organism became introduced. Germinal cells can be indefinitely
produced, and the reproductive capacity of a single individual is
therefore unlimited; while if two whole individuals conjugated and
only produced _one_ from the process, the result would be a
diminution instead of an increase in the race[6].
[6] In the vegetable kingdom there are numerous types of
Thallophytes, which throw a considerable amount of light on the
relation between sexual reproduction and conjugation. Subjoined
are a few of the more striking cases. In Pandorina at the time of
sexual reproduction the cells which constitute a colony divide
each into sixteen, and the products of their division are set
free. Pairs of them then conjugate and permanently fuse. After a
resting stage the protoplasm is set free from its envelope after
division into two or four parts. Each of these then divides into
sixteen coherent cells and constitutes a new Pandorina colony. In
OEdogonium the fertilization is effected by a spermatozoon
fusing with an oosphere (ovum). The fertilized oosphere (oospore)
then undergoes segmentation like the ovum of an animal; but the
segments, instead of uniting to form a single organism, separate
from each other, and each of them gives rise to a fresh
individual (swarm-spore) which grows into a perfect OEdogonium.
In Coleochæte the impregnation and segmentation take place nearly
as in OEdogonium, but the segments remain united together,
acquire definite cell walls, and form a single embryo. There is
in fact in Coleochæte a true sexual reproduction of the ordinary
type. (_Vide_ S. H. Vines "On alternation of generation in
the Thallophytes." _Journal of Botany_, Nov., 1879.)
It must be admitted that, in the present state of our knowledge, the
passage from reproduction by spores following conjugation, to true
sexual reproduction, can only be traced in a very speculative manner,
and that a further advance in our knowledge may prove that the steps
which I have attempted to sketch out are far from representing the
true origin of sexual differentiation. The peculiar conjugation and
fusion of two individuals to form _Diplozoon paradoxum_ may be
alluded to in this connection. This fusion merely results in the
attainment of sexual maturity by the two conjugating individuals. It
does not appear to me probable that this conjugation is in any way
connected with the conjugation of the Protozoa, but the reverse must
be borne in mind as a possibility.
It is not easy to decide whether the hermaphrodite or the
dioecious state is the primitive one, or in other words whether the
two conjugating cells, from which I have supposed the sexual products
to originate, were derived in the first instance from one or from two
colonies of Protozoa. On purely _à priori_ grounds it seems
probable that they were originally formed in one colony, and that
their derivation from two colonies or individuals was inaugurated when
the spermatozoon became motile. There can be no doubt that the
dioecious state is a very early one, and that the majority of
existing cases of hermaphroditism are secondary.
The above considerations with reference to the male and female cells
appear to indicate that they were primitively homodynamous; a
conclusion which is on the whole borne out by the history of their
development.
Although the modes of reproduction amongst the Metazoa have been
divided into the classes sexual and asexual, there is nevertheless one
mode of asexual reproduction which ought to be classified with the
sexual rather than with the asexual modes. I mean parthenogenesis,
which consists essentially in the development of the ovum into a fresh
individual without previous coalescence with the male element. This
mode of reproduction, which has a very limited range in the animal
kingdom, being confined to the Arthropoda and Rotifera, is undoubtedly
secondarily derived from sexual reproduction. The conditions of its
occurrence are discussed in the second chapter.
It is remarkable that in certain cases the absence of fertilization
causes the production of males (Bees, a Saw-fly, Nematus ventricosus,
etc.); more usually it results in the production of females only, and
there are very often in the Arthropoda a series of successive
generations of females all producing ova which develop parthenogenetically
into females; eventually however, usually in direct or indirect
connection with a change of food or temperature, or other conditions,
ova are formed which give rise without fertilization both to males and
females.
The true asexual modes of reproduction amongst the Metazoa consist of
fission and gemmation. Gemmation is by far the most widely
disseminated of the two. Various as are the methods in which it takes
place, it seems nevertheless that cells derived from all the germinal
layers, and very frequently from all the important organs of
the adult, assist in forming the bud. Into the details of the process,
which require in many points a fuller elucidation, it is not my
purpose to enter.
Gemmation is a far commoner occurrence amongst the simpler than
amongst the more highly organised forms. It appears to have been
superadded to the sexual mode of reproduction quite independently in a
number of different instances.
While there is no difficulty in understanding how gemmation may have
started in such simple types as the Coelenterata, the manner in
which it first originated in certain highly organised forms, as for
instance the Ascidians, is somewhat obscure, but it seems probable
that it began with the division of the developing germ into two or
more embryos, at a very early stage of growth.
Such a division of the germ is, as has been shewn by Kleinenberg,
normal in Lumbricus trapezoides[7] and Haeckel has shewn that an
artificial division of the germ in the Siphonophora leads to the
development of two individuals. It has been pointed out by various
naturalists that the production of double monsters is often a
phenomenon of the same nature. While it is next to impossible to
understand how production of a bud could commence for the first time
in the adult of a highly organised form, it is not difficult to form a
picture of the steps by which the fission of the germ might eventually
lead to the formation of buds in the adult state.
[7] The case of Pyrosoma, which might be cited in this
connection, is probably secondary.
The coexistence of sexual reproduction with normal asexual
multiplication, or with parthenogenesis, has led to a remarkable
phenomenon in the animal kingdom known as alternations of
generations[8].
[8] For an excellent account of this subject, _vide_ Allen
Thompson's article Ovum in Todd's _Cyclopædia_. The
metamorphosis of the Echinoderms included under this head in
Thompson's article is now known not to be a proper case of
alternations of generations.
For the details of the various types of alternations of generations,
and their origin, the reader is referred to the body of the work; but
a few general remarks on the nature and origin of the process, and on
its nomenclature, may conveniently be introduced in this place. The
simplest cases are those in which an individual which produces
by sexual means gives origin to asexual individuals differently
organised to itself, which produce by budding the original sexual
form, and so complete a cycle. Instances of this kind are supplied by
the Hydrozoa, Annelida and Tunicata. In the case of the Tunicata
(Doliolum) two different asexual generations may be interpolated
between the sexual generations. In all these cases the origin of the
phenomenon is easily understood. It appears, as is most clearly shewn
in the case of the Annelida, that the ancestors of the species which
now exhibit alternations of generations originally reproduced
themselves at the same time both sexually and by budding, though
probably the two modes of reproduction did not take place at the same
season. Gradually a differentiation became established, by which
sexual reproduction was confined to certain individuals, which in most
instances did not also reproduce asexually. After the two modes of
reproduction became confined to separate individuals, the
dissimilarity in habits of life necessitated by their diverse
functions caused a difference in their organization; and thus a
complete alternation of generations became established. The above is
no merely speculative history, since all gradations between complete
alternations of generations and simple budding combined with sexual
reproduction can be traced in actually existing forms.
The alternation of generations as it is found amongst the
Entoparasitic Trematodes and most Cestodes, is to be explained in a
slightly different way.
It appears that in these parasitic forms a complicated metamorphosis
first arose from the parasite having to accommodate itself to the
different hosts it was compelled to inhabit, owing to the liability of
its primitive and subsequent hosts to be devoured[9]. A capacity for
asexual multiplication--obviously of immense advantage to a
parasite--appears to have been acquired in some of the stages of this
metamorphosis, and an alternation of generations thus established.
[9] The appearance of Vertebrata on the globe as the forms which
most frequently preyed on Invertebrate forms, and were themselves
not so liable to be devoured, has no doubt had a great influence
on the metamorphosis of internal parasites, and has amongst other
things resulted in these parasites usually reaching their sexual
state in a vertebrate host.
A nearly parallel series to that exhibiting alternations of sexual
generations with generations which produce by budding is supplied by
the cases where sexual generations alternate with parthenogenetic
ones, or in some instances even with larvæ which reproduce sexually or
else parthenogenetically.
The best known examples of this form of alternations of generations
are found amongst the Insecta[10]. A simple case is that of the
Aphides. The ova deposited by impregnated females give rise to forms
differently organised to the parents but provided with an ovary[11].
The eggs from the ovary develop parthenogenetically within the
oviduct, and so long as there is plenty of food and warmth the
generations produced are always parthenogenetic forms. The failure of
warmth and nutriment causes the production of true males and females,
and so the cycle is completed. We must suppose that the capacity
possessed by so many female insects of producing eggs capable of
developing without the influence of the male element, has been, so to
speak, taken hold of by natural selection, and has led to the
production of viviparous parthenogenetic forms, by which, so long as
food is abundant, a clear economy in reproduction is effected. The
continuance of the species during winter is secured by the production
of males and females, the females laying eggs in autumn which are
hatched in the spring.
[10] For details _vide_ Chapter on Insecta.
[11] The distinction drawn by Huxley between ova and pseudova
does not appear to me a convenient one in practice.
In Chermes there is less modification of the primitive condition in
that the parthenogenetic generations lay their eggs like the
impregnated females. In the gall-flies (Cynipidæ), there is frequently
an alternation of generations of the same kind as in Chermes; there
being no viviparous forms. The individuals of the different
generations differ from each other to some extent in all these cases.
A second type of alternations of parthenogenetic and sexual
generations is exemplified by the cases of Chironomus and Cecidomyia,
where the _larvæ_ which develop from the eggs of the fertilized
female produce parthenogenetically, by means of true ova, forms which
eventually after several generations (Cecidomyia) of larval
reproduction give rise to sexual forms. The explanation is
here practically the same as in the case of Aphis, and is paralleled
in the gemmiparous series by the production of _buds_ in the
_larval_ forms of Trematodes, etc. A very similar occurrence
takes place in Ascaris nigrovenosa (_vide_ chapter on Nematoidea),
except that larval forms, which carry on reproduction and then perish
without developing farther, do so by a true sexual process. Thus there
is an alternation of generations of adult and larval sexual forms. The
Axolotl is an intermittent example of the same phenomenon.
As might be anticipated from the mode in which alternations of
generations have become established, incomplete approximations to it
are not uncommon. Such approximations are especially found in the
Arthropoda, where alternations of sexual and parthenogenetic
generations frequently take place, in which the individuals of
different generations are similarly organised (Psychidæ, Apus, &c.).
Another approximation is afforded by the parthenogenetic winter eggs
of Leptodora amongst the Phyllopods, which give rise to Nauplius
larvæ, while the young hatched from the summer eggs do not pass
through a metamorphosis. Numerous transitional cases are also found
amongst the forms in which there is an alternation of sexual and
gemmiparous generations.
The whole of the cases to which allusion has been made in this section
may be conveniently classed under the term alternations of
generations, but the cases of alternation of two sexual generations,
and of sexual and parthenogenetic generations, are classified by
Leuckart, Claus, etc. as cases of heterogeny, which they oppose to the
other form of alternation of generations. If special terms are to be
adopted for the two kinds of alternation of generations, it would be
perhaps convenient to classify the cases of alternations of sexual and
gemmiparous generations under the term metagenesis, and to
employ the term heterogamy for the cases of alternation of
sexual and parthenogenetic generations.
The term Nurse (_German_ Amme), employed for the asexual
generations in metagenesis, may advantageously be dropped altogether.
CHAPTER I.
THE OVUM AND SPERMATOZOON.
THE OVUM.
The complete developmental history of any being constitutes a cycle.
It is therefore permissible in treating of this history to begin at
any point. As a matter of convenience the ovum appears to be the most
suitable point of departure. The question as to the germinal layer
from which it is ultimately derived is dealt with in a subsequent part
of the work; the present chapter deals with its origin and growth.
_General History of the Ovum._
Every young ovum (fig. 1) has the character of a simple cell. It is
formed of a mass of naked protoplasm (_a_), containing in its
interior a nucleus (_b_), within which there is a nucleolus
(_c_). The nucleus and nucleolus are usually known as the
germinal vesicle and germinal spot.
[FIG. 1. DIAGRAM OF THE OVUM. (From Gegenbaur.)
_a._ Granular protoplasm. _b._ Nucleus (germinal vesicle).
_c._ Nucleolus (germinal spot).]
The ovum so constituted is developed either (1) from one cell out of
an aggregation or layer of cells all of which have the capacity of
becoming ova; or (2) from one of a number of cells segmented off from
a polynuclear mass of protoplasm, not divided into separate cells. In
both cases the cells which have the capacity of becoming ova may be
spoken of as germinal cells, and in the case where the ova are
ultimately developed from a polynuclear mass of protoplasm the
latter structure may be called a germogen.
In some cases the whole of the germinal cells eventually become ova,
but as a rule only a small proportion of them have this fate, the
remainder undergoing various changes to be spoken of in the sequel.
Extended investigations have shewn that the distinction between
germinal cells which are independent cells from the first, or derived
from a germogen in which the nucleated protoplasm is not divided into
cells, is an unimportant one; and closely allied forms may differ in
this respect. It is moreover probable that a germogen of nucleated
protoplasm is less common than is often supposed: it being a matter of
great difficulty to determine the structure of the organs usually so
described. A germogen is stated to be found in most Platyelminthes,
Nematoidea, Discophora, Insecta, and Crustacea.
A more important distinction in the origin of the germinal cells is
that afforded by their position. In this respect three groups may be
distinguished. (1) The germinal cells may form the lining of a sack or
tube, having the form of a syncytium or of an epithelium of separate
cells (Platyelminthes, Mollusca, Rotifera, Echinodermata, Nematoidea,
Arthropoda). (2) Or they may form a specialized part of the epithelium
lining the general body cavity (Chætopoda, Gephyrea, Vertebrata). (3)
Or they may form a mass placed between the two elsewhere contiguous
primitive germinal layers (Coelenterata[12]).
[12] In all the Metazoa the generative organs are placed between
the primitive germinal layers; and the peculiarity of their
position in the Coelenterata depends on the absence of a body
cavity and of a distinct mesoblast.
Types of transition between the first and second group are not
uncommon. Such types, properly belonging to the second group,
originate by a special membranous sack continuous with the oviduct
being formed round the primitively free patch of germinal cells.
Examples of this are afforded by the Discophora, the Teleostei, etc.
It is very probable that all the cases which fall under the first
heading may have been derived from types which belonged to the second
group.
The mode of conversion of the germinal cells into ova is somewhat
diverse. Before the change takes place the germinal cells
frequently multiply by division. The change itself usually involves a
considerable enlargement of the germinal cell, and generally a change
in the character of the germinal vesicle, which in most young ova
(fig. 2) is very large as compared to the body of the ovum. The most
complicated history of this kind is that of the ovum of the Craniata.
(_Vide_ pp. 56, 57.)
[FIG. 2. OVUM OF CARMARINA (GERYONIA) HASTATA. (Copied from Haeckel.)
_gd._ Body of ovum. _gv._ Germinal vesicle. _gm._ Germinal spot.]
The ovum in its young condition is obviously nothing but a simple
cell; and such it remains till the period when it attains maturity.
Nevertheless the changes which it undergoes in the course of its
growth are of a very peculiar kind, and, consisting as they do in many
instances of the absorption of other cells, have led various
biologists to hold that the ovum is a compound structure. It becomes
therefore necessary to consider the processes by which the growth and
nutrition of the ovum is effected before dealing with the structure of
the ovum at all periods of its history.
[FIG. 3. FEMALE GONOPHORE OF TUBULARIA MESEMBRYANTHEMUM. CONTAINING
ONE LARGE OVUM (_ov_) AND A NUMBER OF GERMINAL CELLS (_g.c._).
_ep._ Epiblast (Ectoderm). _hy._ Hypoblast (Entoderm).
_ov._ Ovum. _g.c._ Germinal cells.]
The ovum is of course nourished like every other cell by the nutritive
fluids in which it is surrounded, and special provisions are made for
this, in that the ovary is very frequently placed in contiguity with
vascular channels. But in addition to such nutrition a further
nutrition, the details of which are given in the special part of this
chapter, is provided for in the germinal cells which do not become
ova.
In the simplest case, as in many Hydrozoa (fig. 3), the germinal cells
which do become ova are assimilated by the ovum much in the manner of
an Amoeba.
In other cases the ovum becomes invested by a special layer of cells,
which then constitutes what is known as a follicle. The cells which
form the follicle are often germinal cells, _e.g._ Holothuria,
Insecta (fig. 17), Vertebrata (fig. 19). In other cases they
seem rather to be adjoining connective-tissue or epithelioid cells,
though it is sometimes difficult to draw the line between such cells
and germinal cells. Examples of follicles formed of ordinary
connective-tissue cells, are supplied by Asterias, Bonellia (fig. 16),
Cephalopoda (fig. 14), etc.
A membrane enclosing the ovum without a lining of cells, as in many
Arachnida, _vide_ p. 51, has no true analogy with a follicle and
does not deserve the same name.
The function of the follicle cells appears to be, to elaborate
nutriment for the growth of the ovum. The follicle cells are not as a
rule directly absorbed into the body of the ovum, though in some
instances, as in Sepia (_vide_ p. 40), they are eventually
assimilated in this way.
In many cases some of the germinal cells form a follicle, while other
germinal cells form a mass within the follicle destined eventually to
be used as pabulum. Insects supply the best known examples of this,
but Piscicola, Bonellia (?) may also be cited as examples of the same
character. In the Craniata (pp. 56-58) some of the germinal cells
which advance a certain distance on the road towards becoming ova, are
eventually used as pabulum, before the formation of the follicle;
while other germinal cells form at a later period the follicular
epithelium. A peculiar case is that of the Platyelminthes (fig. 9),
where a kind of follicle is constituted by the cells of a specially
differentiated part of the ovary, known as the yolk-gland. The cells
of this follicle may either remain distinct, and continue to surround
the ovum after its development has commenced, and so be used as food
by the embryo; or they may secrete yolk particles, which enter
directly into the protoplasm of the ovum.
For further variations in the mode of nutrition the reader is referred
to the special part of this chapter. Suffice it to say that none of
the known modes of nutrition indicate that the ovum becomes a compound
body any more than the fact of an Amoeba feeding on another Amoeba
would imply that the first Amoeba ceased thereby to be a unicellular
organism.
The constitution of the ovum may be considered under three heads:--
(1) The body of the ovum.
(2) The nucleus or germinal vesicle.
(3) The investing membranes.
The body of the ovum. The essential constituent of the body of
the ovum is an active living protoplasm. As a rule there are present
certain extraneous matters in addition, which have not the vital
properties of protoplasm. The most important of these is known as
food-yolk, which appears to be generally composed of an albuminoid
matter.
The body of the ovum is at first very small compared with the germinal
vesicle, but continually increases as the ovum approaches towards
maturity. It is at first comparatively free from food-yolk; but,
except in the rare instances where it is almost absent, food-yolk
becomes deposited in the form of granules, or highly refracting
spheres, by the inherent activity of the protoplasm during the later
stages in the ripening of the ovum. In many instances the protoplasm
of the ovum assumes a sponge-like or reticulate arrangement, a fluid
yolk substance being placed in the meshes of the reticulum. The
character of the food-yolk varies greatly. Many of its chief
modifications are described below. There is not unfrequently present
in the vitellus a peculiar body known as the yolk nucleus,
which is very possibly connected with the formation of the food-yolk.
It is found in many Arachnida, Myriapoda, Amphibia, etc.[13]
[13] For details on the yolk nucleus _vide_ Balbiani, _Leçons s.
l. Génération d. Vertébrés_. Paris, 1879. In this work the author
maintains very peculiar views on the nature and function of the
yolk nucleus, which do not appear to me well founded.
[FIG. 4. _A._ OVUM OF HYDRA IN THE AMOEBOID STATE, WITH
YOLK-SPHERULES (PSEUDOCELLS) AND CHLOROPHYLL GRANULES. (After
Kleinenberg.)
_gv._ Germinal vesicle.
_B._ SINGLE PSEUDOCELL OF HYDRA.]
More important for the subsequent development than the variation in
the character of the food-yolk is its amount and distribution. In a
large number of forms it is distributed unsymmetrically, the yolk
being especially concentrated at one pole of the ovum, the germinal
vesicle, surrounded by a special layer of protoplasm comparatively
free from food-yolk, being placed at the opposite pole. In the
Arthropoda it has in most instances a symmetrical distribution.
Further details on this subject are given in connection with the
segmentation; the character of which is greatly influenced by the
distribution of food-yolk.
The body of the ovum is usually spherical, but during a period in its
development it not unfrequently exhibits a very irregular amoeboid
form, _e.g._ Hydra (fig. 4), Halisarca.
[FIG. 5. UNRIPE OVUM OF TOXOPNEUSTES LIVIDUS. (Copied from Hertwig.)]
The germinal vesicle. The germinal vesicle exhibits all the
essential characters of a nucleus. It has a more or less spherical
shape, and is enveloped by a distinct membrane which seems, however,
in the living state to be very often of a viscous semi-fluid nature
and only to be hardened into a membrane by the action of reagents
(Fol). The contents of the germinal vesicle are for the most part
fluid, but may be more or less granular. Their most characteristic
components are, however, a protoplasmic network and the germinal
spots[14]. The protoplasmic network stretches from the germinal spots
to the investing membrane, but is especially concentrated round the
former. (Fig. 5.) The germinal spot forms a nearly homogeneous
body, with frequently one or more vacuoles. It often occupies an
eccentric position within the germinal vesicle, and is usually
rendered very conspicuous by its high refrangibility. In many
instances it has been shewn to be capable of amoeboid movements
(Hertwig, Eimer), and is moreover more solid and more strongly tinged
by colouring reagents than the remaining constituents of the germinal
vesicle.
[14] In the germinal vesicles of very young ova the reticulum is
often absent.
In many instances there is only one germinal spot, or else one main
spot and two or three accessory smaller spots. In other cases,
_e.g._ Osseous Fishes, Echinaster fallax, Eucope polystyla, there
are a large number of nearly equal germinal spots which appear to
result from the division or endogenous proliferation of the original
spot. Sometimes the germinal spots are placed immediately within the
membrane of the germinal vesicle (Elasmobranchii and Sagitta). In many
Lamellibranchiata, in the earthworm, and in many Chætopoda the
components of the germinal spot become separated into two nearly
spherical masses (fig. 12), which remain in contiguity along a small
part of their circumference, and are firmly united together. The
smaller of the two parts is more highly refractive than the larger.
Hertwig has shewn that the germinal spot is often composed of two
constituents as in the above cases, but that the more highly
refractive material is generally completely enclosed by the less dense
substance. By Fol the germinal spot is stated to be absent in a
species of Sagitta, but this must be regarded as doubtful. In young
ova the relative size of the germinal vesicle is very considerable. It
occupies in the first instance a central position in the ovum, but at
maturity is almost always found in close proximity to the surface. Its
change of position in a large number of instances is accomplished
during the growth of the ovum in the ovary, but in other cases does
not take place till the ovum has been laid.
As the ovum attains maturity, important changes take place in the
constitution of the germinal vesicle, which are described in the next
chapter.
The egg membranes. A certain number of ova when ready to be
fertilized are naked cells devoid of any form of protecting covering,
but as a rule the ovum is invested by some form of membrane. Such
coverings present great variety in their character and origin,
and may be conveniently (Ludwig, No. 4) divided into two great groups,
viz. (1) those derived from the protoplasm of the ovum itself or from
its follicle, which may be called primary egg membranes; and
(2) those formed by the wall of the oviduct or otherwise, such as the
egg-shell of a bird, which may be called secondary egg membranes.
[FIG. 6. OVUM OF TOXOPNEUSTES VARIEGATUS WITH THE PSEUDOPODIA-LIKE
PROCESSES OF THE PROTOPLASM PENETRATING THE ZONA RADIATA (_zr_).
(After Selenka.)]
The primary egg membranes may again be divided into two groups (Ed.
van Beneden, No. 1), viz., (1) those formed by the protoplasm of the
ovum, to which the name vitelline membranes will be applied;
and (2) those formed by the cells of the follicle, to which the name
chorion will be applied.
The secondary egg membranes will be dealt with in connection with the
systematic account of the development of the various groups. They
coexist as a rule with primary membranes, though in some types
(Cephalophorous Mollusca, many Platyelminthes, etc.), they constitute
the only protecting coverings of the ovum.
The vitelline membranes are either simple structureless membranes or
present numerous radial pores. Membranes with the latter structure are
very widely distributed, Echinodermata, Gephyrea, Vertebrata, etc.
(_Vide_ figs. 5 and 7.) The function of the pores appears to be a
nutritive one. They either serve for the emission of pseudopodia-like
processes of the protoplasm of the ovum, as has been very beautifully
shown in the case of Toxopneustes by Selenka (fig. 6), or they admit
(?) processes of the follicular epithelial cells (Vertebrata). Their
presence is in fact probably caused by the existence of such
processes, which prevent the continuous deposition of the membrane.
The term zona radiata will be applied to perforated membranes
of this kind. Two vitelline membranes, one perforated and the other
homogeneous, may coexist at the same time, _e.g._ Sipunculida,
Vertebrata. (Fig. 7.)
[FIG. 7. SECTION THROUGH A SMALL PART OF THE SURFACE OF AN OVUM OF
AN IMMATURE FEMALE OF SCYLLIUM CANICULA.
_fe._ Follicular epithelium. _vt._ Vitelline membrane. _Zn._ Zona
radiata. _yk._ Yolk with protoplasmic network.]
The chorion is often ornamented with various processes, etc.
It is in many cases doubtful whether a particular membrane is a
chorion or a vitelline membrane.
All the membranes which surround the ovum may be provided with a
special aperture known as the micropyle. A micropyle is by no means
found in the majority of types, and there is no homology between the
various apertures so named. Micropyles have two functions, either (1)
to assist in the nutrition of the ovum during its development, or (2)
to permit the entrance of the spermatozoa. The two functions may in
some cases coexist. Micropyles of the first class are developed at the
point of attachment of the ovum to the wall of the ovary or to its
follicle. Good examples of this kind of micropyle are afforded by the
Lamellibranchiata (fig. 12), Holothuria, and many Annelida (Polynoe,
etc.). The micropyle of the Lamellibranchiata (p. 37) probably serves
also to admit the spermatozoa. The second type of micropyle is found
in many Insecta, Teleostei, etc.
GENERAL BIBLIOGRAPHY OF THE OVUM.
(1) Ed. van Beneden. "Recherches sur la composition et la
signification de l'oeuf," etc. _Mém. cour. d. l'Acad. roy. des
Sciences de Belgique_, Vol. XXXIV. 1870.
(2) R. Leuckart. Artikel "Zeugung," R. Wagner's _Handwörterbuch d.
Physiologie_, Vol. IV. 1853.
(3) Fr. Leydig. "Die Dotterfurchung nach ihrem Vorkommen in d.
Thierwelt u. n. ihrer Bedeutung." _Oken, Isis_, 1848.
(4) Ludwig. "Ueber d. Eibildung im Thierreiche." _Arbeiten a. d.
zool.-zoot. Institut Würzburg_, Vol. I. 1874[15].
(5) Allen Thomson. Article "Ovum" in Todd's _Cyclopædia of Anatomy and
Physiology_, Vol. V. 1859.
(6) W. Waldeyer. _Eierstock u. Ei._ Leipzig, 1870.
[15] A very complete and critical account of the literature is
contained in this paper.
_Special History of the Ovum in different types._
COELENTERATA.
(7) Ed. van Beneden. "De la distinction originelle d. testicule et de
l'ovaire." _Bull. Acad. roy. Belgique_, 3e série, Vol. XXXVII. 1874.
(8) R. and O. Hertwig. _Der Organismus d. Medusen._ Jena, 1878.
(9) N. Kleinenberg. _Hydra._ Leipzig, 1872.
Amongst the Coelenterata the ova are developed in imperfectly
specialized organs, which are situated in various parts of the body,
for the most part in the space between the epiblast and the hypoblast.
In Hydra the locality where the ova are developed only becomes
specialized at the time when an ovum is about to be formed. At one or
more points the interstitial cells of the epiblast increase in number
and form a protuberance of germinal cells, which may be called the
ovary. In this ovary a single ovum is formed by the special growth of
one cell. (Kleinenberg, No. 9.) In the free and attached gonophores of
Hydrozoa, the ova appear either around the walls of the stomach, or
the radial canals, or around other parts of the gastro-vascular canals.
[FIG. 8. RIPE OVUM OF EPIBULIA AURANTIACA. THE GERMINAL VESICLE HAS
BECOME INVISIBLE WITHOUT REAGENTS.
Copied from Metschnikoff, "Entwicklung der Siphonophoren."
_Zeitschrift f. wiss. Zool._, Vol. XXIV. 1874.
_p.d._ Peripheral layer of denser protoplasm. _p.m._ Central area
consisting of a protoplasmic meshwork.]
Their close relations to the gastrovascular canals are probably
determined by the greater nutritive facilities thereby afforded.
(Hertwig, No. 8.)
In the permanent Medusa forms the ova have similar relations to the
gastro-vascular system. Amongst the Actinozoa the ova are usually
developed between the epiblast and the hypoblast in the walls of the
gastric mesenteries. Amongst the Ctenophora the ova are situated in
close relation with the peripheral canals of the gastro-vascular
system, which run along the bases of the ciliated bands. There are
many examples amongst the Coelenterata of ova which retain
in their mature state the very simple constitution which has been
described as characteristic of all young ova; and which are, when
laid, absolutely without any trace of a vitelline membrane or chorion.
In many other cases both amongst the Medusæ, the Siphonophora, and the
Ctenophora, the ripe egg exhibits a distinction into two parts. The
outer part is composed of a dense protoplasm, while the interior is
composed of a network or more properly a spongework of protoplasm
enclosing in its meshes a more fluid substance. (Fig. 8.)
In some cases the ovum while still retaining the constitution last
described becomes invested by a very delicate membrane. Such is the
constitution of the ripe ovum of Hippopodius gleba amongst the
Siphonophora[16] and of the eggs of Geryonia amongst the permanent
Medusæ[17]. The ripe eggs of the Ctenophora usually present a similar
structure[18]. After being laid they are found to be invested by a
delicate membrane separated by a space filled with fluid from the body
of the ovum. The latter is composed of two layers, an outer one of
finely granular protoplasm and an inner layer consisting of a
protoplasmic spongework containing in its meshes irregular spheres.
These latter are stated by Agassiz to be of a fatty nature, and it is
probable that in most cases where a protoplasmic network is present,
this alone constitutes the active protoplasm and that the substance
which fills up its meshes is to be looked on as a form of food-yolk or
deutoplasm, though it appears sometimes to have the power of
assimilating the firmer yolk particles.
[16] Metschnikoff. _Zeitschrift f. wiss. Zoologie_,
Vol. XXIV. 1874.
[17] Herman Fol. _Jenaische Zeitschrift_, Vol. VII.
[18] Kowalevsky. "Entwicklungsgeschichte d. Rippenquallen."
_Mémoire de l'Acad. Pétersbourg_, 1866. And Alex. Agassiz.
"Embryology of the Ctenophoræ." _Amer. Acad. of Science and
Arts_, Vol. X. No. 111.
The membrane which invests the ovum of many of the Coelenterata is
probably a vitelline membrane.
The ova of the Hydrozoa take their origin, in most groups at any
rate[19], from the deeper layer of the epiblast (interstitial layer of
Kleinenberg). The interstitial cells in the ovarian region form
primary germinal cells, and by an excess of nutrition certain of them
outstrip their fellows and become young ova. Such ova differ from the
full-grown ova already described, mainly in the fact that they
have a proportionately smaller amount of protoplasm round the germinal
vesicle. They grow to a considerable extent at the expense of germinal
cells which do not become converted into ova.
[19] The view of van Beneden, according to which the ova have an
endodermal (hypoblastic) origin, has been shewn to be at any rate
confined to certain groups. The whole question of the origin of
the generative products from the germinal layers in the
Coelenterata is still involved in great obscurity.
The ova of many Coelenterata undergo changes of a more complicated
kind before attaining their full development. Of these ova that of
Hydra may be taken as the type. The ovary of Hydra (Kleinenberg, No.
9) is constituted of angular flattish germinal cells of which no
single one can be at first distinguished from the remainder. As growth
proceeds one of the cells occupying a central position becomes
distinguished from the remaining cells by its greater size, and
wedge-like shape. It constitutes the single ovum of the ovary. After
it has become prominent it grows rapidly in size, and throws out
irregular processes. The germinal vesicle, which for a considerable
time remains unaltered, also at length begins to grow; and the sharply
defined germinal spot which it contains after reaching a certain size
completely vanishes. After the atrophy of the germinal spot, there
appears in the middle of the ovum a number of roundish yolk granules.
The shape of the ovum becomes more irregular, and chlorophyll
granules, in addition to the yolk granules, make their appearance in
it. A fresh germinal spot of circular form also arises in the germinal
vesicle. Protoplasmic processes are next thrown out in all directions,
giving to the ovum a marvellous amoeboid character. (Fig. 4.) The
amoeboid form of the ovum serves no doubt to give it a larger
surface for nutrition. Coincidently with the assumption of an
amoeboid form there appear in the ovum a great number of peculiar
bodies. They are vesicles with a thick wall bearing a conical
projection into the interior which is filled with fluid. (Fig. 4B.)
These bodies are formed directly from the protoplasm of the ovum, and
are to be compared both morphologically and physiologically with the
yolk-spherules of such an ovum as that of the Bird. They are called
pseudocells by Kleinenberg, and are found with slightly varying
characters in many ova of the Hydrozoa.
They first appear as small highly refracting granules; in these a
cavity is formed which is at first central but is eventually pushed to
one side by the formation of a conical projection from the wall of the
vesicle.
After the growth of the ovum is completed the amoeboid processes
gradually withdraw themselves, and the ovum assumes a spherical form;
still however continuing to be invested by the remaining cells of the
ovary. It is important to notice that the egg of Hydra retains
throughout its whole development the characters of a single cell, and
that the pseudocells and other structures which make their appearance
in it are not derived from without, and supply not the slightest
ground for regarding the ovum as a structure compounded of more than
one cell.
The development of the ova of the Tubularidæ, which has been supposed
by many investigators to present very special peculiarities, takes
place on essentially the same type as that of Hydra, but the germinal
vesicle remains permanently very small and difficult to observe. The
mode of nutrition of the ovum may be very instructively studied in
this type. The process is one of actual feeding, much as an Amoeba
might feed on other organisms. Adjoining one of the large ova of the
ovary there may be seen a number of small germinal cells. (Fig. 3.)
The boundary between these cells and the ovum is indistinct. Just
beyond the edge of the ovum the small cells have begun to undergo
retrogressive changes; while at a little distance from the ovum they
are quite normal (_g.c._)[20].
[20] The above description of the ova of the Tubularidæ is
founded on sections of the gonophores of Tubularia
mesembryanthemum. Dr Kleinenberg informs me however that the
absence of a distinct boundary between the germinal cells and the
ovum is not usual.
PLATYELMINTHES.
(10) P. Hallez. _Contributions à l'Histoire naturelle des
Turbellariés._ Lille, 1879.
(11) S. Max Schultze. _Beiträge z. Naturgeschichte d. Turbellarien._
Greifswald, 1851.
(12) C. Th. von Siebold. "Helminthologische Beiträge." Müller's
_Archiv_, 1836.
(13) C. Th. von Siebold. _Lehrbuch d. vergleich. Anat. d. wirbellosen
Thiere._ Berlin, 1848.
(14) E. Zeller. "Weitere Beiträge z. Kenntniss d. Polystomen." _Zeit.
f. wiss. Zool._, Bd. XXVII. 1876.
[_Vide_ also Ed. van Beneden] (No. 1).
This group, under which I include the Trematodes, Cestodes,
Turbellarians and Nemertines, has played an important part in all
controversies relating to the nature and composition of the ovum. The
peculiarity in the development of the ovum in most members of this
group consists in the fact that two organs assist in forming what is
usually spoken of as the ovum. One of these is known as the ovary
proper, and the other as the vitellarium or yolk-gland. In the sequel
the term ovum will be restricted to the product of the first of these
organs. In Trematodes the ovary forms an unpaired organ directly
continuous with an oviduct into which there open the ducts from paired
yolk-glands.
The ovary has a sack-like form and contains in some instances a
central lumen (Polystomum integerrimum). At the blind end of the organ
is placed the germinal tissue. This part is, according to the accounts
of the majority of investigators, formed of a polynuclear mass of
protoplasm not divided into distinct cells. Whether it is really
formed of undivided protoplasm or not, it is quite certain that a
little lower down in the organ distinct cells are found, which have
been segmented off from the above mass, and are formed of a large
nucleus and nucleolus, surrounded by a delicate layer of protoplasm.
These cells are the young ova. They usually assume a more or less
angular form from mutual pressure, and, in the cases where the ovary
has a lumen, constitute a kind of epithelial lining for the ovarian
tube. They become successively larger in passing down the ovary, and,
though in most cases naked, are in some instances (Polystomum
integerrimum) invested by a delicate vitelline membrane. Eventually
the ova pass into the oviduct and become free and at the same time
assume a spherical form.
In the oviduct the ovum receives somewhat remarkable investing
structures, derived from the organ before spoken of as the yolk-gland.
The yolk-gland consists of a number of small vesicles, each provided
with a special duct, connected with the main duct of the gland. Each
vesicle is lined by an epithelium of cells provided with doubly
contoured membranes, and containing nuclei.
As the yolk cells grow older refracting spherules become deposited in
their protoplasm, which either completely hide the nucleus, or
render it very difficult to see. In the majority of cases the entire
cells forming the lining of the vesicles constitute the secretion of
the yolk-gland. They invest the ovum, and around them is formed a
shell or membrane. In some cases (_e.g._ Polystomum integerrimum)
the yolk cells retain their cellular character and vitality till the
embryo is far developed. In other cases they lose their membrane and
nucleus shortly after the formation of the egg-shell, and break up
into a fluid, holding in suspension a number of yolk granules. A
partial disorganisation of the yolk cells can also take place before
they surround the ovum; while in some species of Distomum they
completely break up before leaving the yolk-gland.
[FIG. 9. GENERATIVE SYSTEM OF VORTEX VIRIDIS. (From Gegenbaur, after
Max Schultze.)
_t._ Testis. _v.d._ Vasa differentia. _v.s._ Seminal vesicle. _p._
Penis. _u._ Uterus. _o._ Ovary. _v._ Vagina. _g.v._ Yolk-glands.
_r.s._ Receptaculum seminis.]
There is thus a complete series of gradations between the investment
of the ovum by a number of distinct cells, and its investment by a
layer of fluid containing yolk-spherules in suspension. In neither the
one case nor the other do the investing structures take any share in
the direct formation of the embryo from the ovum. Physiologically
speaking they play the same part as the white in the fowl's egg.
The egg-shell, which is usually formed by a secretion of a special
shell-gland opening into the oviduct, exhibits one or two
peculiarities in the different species of Trematodes. In Amphistomum
subclavatum it presents at one extremity a thickened area, which is
pierced by a narrow micropyle. In other cases one extremity of the
egg-shell is produced into a long process, and sometimes even both
extremities are armed in this way. Opercula and other types of
armature are also found in different forms.
The mode of development of the ovum in Cestodes is very nearly the
same as in Trematodes.
The ovum becomes enveloped in the usual secretion of the yolk-gland;
and an egg-shell is always formed by the secretion of a special
shell-gland.
Amongst the Turbellarians and Nemertines, there are greater variations
in the arrangement of the female generative glands, than in
the preceding types. In most of the Rhabdocoela and fresh-water
Dendrocoela these organs resemble in their fundamental characters
those of the Trematodes and Cestodes. There are present a paired or
single ovary and a paired yolk-gland. The general arrangement of the
organs is shewn in fig. 9.
The blind end of the ovaries is usually (Ed. van Beneden, etc.) stated
to be formed of a polynuclear protoplasmic basis, but Hallez (No. 10)
has recently insisted that, even at the extreme end of the ovary, the
germinal cells are quite distinct, and not confounded together.
With one or two exceptions the yolk cells secreted by the vitellarium
retain their vitality till they are swallowed by the embryo, after the
development of its mouth. The few not so swallowed become
disintegrated. They are granular nucleated cells, and, as was first
shewn by von Siebold, are remarkable for exhibiting spontaneous
amoeboid movements.
Very important light on the nature of the vitellarium is afforded by
the structure of the generative organs in Prorhyncus and Macrostomum.
In Prorhyncus there is no separate vitellarium, but the lower part of
the ovarian tube functionally and morphologically replaces it. The
ovum becomes surrounded by yolk cells, which according to Hallez (No.
10) retain their vitality for a long time. According to Ed. van
Beneden yolk-spherules are formed in the protoplasm of the ovum
itself, in addition to and independently of the surrounding yolk
cells. In Convoluta paradoxa a special vitellarium is stated to be
absent; though a deposit of yolk is formed round the ovum (Claparède).
In Macrostomum again the yolk-glands are at most represented by a
lower specialized part of the ovarian tube. The ova in passing down
become filled with yolk-spherules. According to Ed. van Beneden these
spherules are formed in the protoplasm of the ovum itself; but this is
explicitly denied by Hallez, who finds that they are formed from the
lining cells of the ovarian tube, which, instead of retaining their
vitality as in Prorhyncus, break up and form a granular mass which is
absorbed by the protoplasm of the ovum.
In Prostomum caledonicum (Ed. van Beneden) the generative organs are
formed on the same plan as in other Rhabdocoela, but the
cells which form the yolk-gland give rise to yolk particles which
enter the ovum, instead of to a layer of yolk cells surrounding the
ovum.
Amongst the marine dendrocoelous Turbellarians the ova are formed in
separate sacks widely distributed in the parenchyma of the body
between the alimentary diverticula. In these the ova undergo their
complete development, without the intervention of yolk-glands.
The ovaries of the Nemertines more nearly resemble those of the marine
Dendrocoela than those of the Rhabdocoela. They consist of a
series of sacks situated on the two sides of the body between the
prolongations of the digestive canal. The eggs are developed in these
sacks in a perfectly normal manner, and in many cases become filled
with yolk-spherules which arise as differentiations of the protoplasm
of the ovum. The protecting membranes of the ova have not been
accurately studied. In some cases[21] two membranes are present, an
internal and an external. The former, immediately investing the
vitellus, is very delicate: the external one is thicker and hyaline.
[21] Amphiporus lactiflorius and Nemertes gracilis. McIntosh.
_Monograph on British Nemertines._ Ray Society.
The constitution of the female generative organs of the Trematodes was
first clearly ascertained by von Siebold (No. 12). He originally,
though not very confidently, propounded the view that the germinal
vesicles alone were formed in the ovary and that the protoplasm of the
ovum was supplied by the yolk-gland. This view has long been
abandoned, and von Siebold (No. 13) himself was the first to recognize
that true ova with a protoplasmic body containing a germinal vesicle
and germinal spot were formed in the ovary. The Trematodes have
however not ceased to play an important part in forming the current
views upon the development of ova, and have quite recently served Ed.
van Beneden as his type in exposing his general view upon this subject.
His view consists fundamentally in regarding the secretion of the
yolk-glands, which in most cases merely invests the ovum, as
homologous with the yolk-spherules which fill the protoplasm of many
eggs; and he considers the part of the ovary where in most forms the
ova receive their supply of yolk particles, as equivalent to the
vitellarium of the Platyelminthes. He further appears to regard the
primitive state as that exemplified in Trematodes, Cestodes, etc., and
holds that the ovarian types characteristic of other forms are
secondarily derived from this, by the coalescence of the primitively
distinct vitellarium with the ovary proper.
This appears to me a case of putting the cart before the horse. To my
mind the vitellarium is to be regarded, as has already been suggested
by Gegenbaur, Hallez, etc. as a special differentiation of the
primitively simple ovarian tube, and the instances Of Macrostomum and
Prorhyncus just cited appear to me to indicate some of the steps in
this differentiation. In Macrostomum the cells of the lower part of
the oviduct simply supply a kind of nutriment to the ovum in the form
of granular yolk particles, while in Prorhyncus the yolk cells of the
lower part of the ovarian tube form a complete investment of
independent cells for the ovum. If this lower part of the ovarian tube
were to grow out as a special diverticulum we should have produced a
normal vitellarium. But even with the above modification the theory of
van Beneden appears to me not completely satisfactory. The view that
the yolk-spherules are of the same nature as the yolk cells is mainly
supported by the case of Prostomum caledonicum, where the vitellarium
produces the yolk particles which fill the ovum. The cases Of
Prorhyncus and Macrostomum give a different complexion to that of
Prostomum caledonicum. From the first of these especially it appears
that, even when normal yolk cells surround the ovum, yolk particles
can be deposited independently in the protoplasm of the ovum.
The most probable view of the nature of the vitellarium is that of
Gegenbaur, Hallez, etc., according to which it is to be regarded as a
specially modified part of the ovarian tube. On this view the nature
and function of the yolk cells admit of a fairly simple explanation.
They are to be regarded as primary germinal cells like those in the
ovaries of Hydra, Tubularia, etc., which do not become converted into
ova. Like these cells they may in some instances, Macrostomum,
Prostomum, etc., serve directly in the nutrition of the ovum. In other
cases they retain their independence and serve for the late nutrition
of the embryo. In both instances they retain the faculty, normally
possessed by ova, of forming yolk particles in their protoplasm.
ECHINODERMATA.
(15) C. K. Hoffmann. "Zur Anatomie d. Echiniden u. Spatangen."
_Niederländisch. Archiv f. Zoologie_, Vol. I. 1871.
(16) C. K. Hoffmann. "Zur Anatomie d. Asteriden." _Niederländisch.
Archiv f. Zoologie_, Vol. II. 1873.
(17) H. Ludwig. "Beiträge zur Anat. d. Crinoiden." _Zeit. f. wiss.
Zool._, Vol. XXVIII. 1877.
(18) Joh. Müller. "Ueber d. Canal in d. Eiern d. Holothurien."
Müller's _Archiv_, 1854.
(19) C. Semper. _Holothurien._ Leipzig, 1868.
(20) E. Selenka. _Befruchtung d. Eies v. Toxopneustes variegatus_,
1878.
[_Vide_ also Ludwig (No. 4), etc.]
The eggs of the Echinodermata present in their development certain
points of interest.
The ovaries themselves are usually surrounded by a special vascular
dilatation. In the Asteroidea, the Echinoidea, and the Holothuroidea
the organs have the form of sacks; specially surrounded in the two
former groups, and probably the latter, by a vascular sinus formed as
a dilatation of one of the generative vessels. In the Crinoids they
have the form of a hollow rachis completely surrounded by a
blood-vessel. (Fig. 11, _b._) The proximity of the ovaries
(generative organs) to the vascular system in these forms has clearly
the same physiological significance as the proximity of the ovaries
(generative organs) to the radial vessels in the Coelenterata.
In the Asteroidea, the Echinoidea and the Holothuroidea the ovaries
have the form of sacks lined by an epithelium of germinal cells, and
the ova are formed by the enlargement of these cells, which, when they
have reached a certain size, become detached from the walls, and fall
into the cavity of the ovarian sack. In Toxopneustes (Selenka) and
very probably in other forms only a few of the epithelial cells
undergo conversion into ova: the remainder undergo repeated division,
and, as in so many other cases, are eventually employed in the
nutrition of the true ova. In the nearly ripe ova of Asterias Fol has
described a flattened follicular epithelium the origin of which is
unknown.
[FIG. 10. OVUM OF TOXOPNEUSTES VARIEGATUS WITH THE PSEUDOPODIA-LIKE
PROJECTIONS OF THE PROTOPLASM PENETRATING THE ZONA RADIATA (_zr_).
(After Selenka.)]
In Holothuria (Semper) a further differentiation of the germinal
cells, not destined to become ova, takes place. They surround the
enlarged cell which forms the true ovum, for which they constitute a
kind of follicular capsule. This capsule is attached by a stalk to the
walls of the ovary, and the ovum lies freely in it except for an area
nearly opposite its (the capsule's) point of attachment, where the
ovum adheres to the wall of the capsule. Subsequently the follicle
cells which form the capsule fuse together, and form a definite
membrane in which only the nuclei remain distinct. Within the
membranous capsule there is formed for the ovum an albuminous zona
radiata. At the point where the ovum is attached to its capsule this
membrane cannot be developed, and therefore remains incomplete. The
perforation so formed, becomes the micropyle of the Holothurian egg,
which was first discovered by Joh. Müller. The albuminous membrane
just described for Holothurians is also found in Asteroids (fig. 5)
and Echinoids. In these groups there is no proper micropyle, though in
Ophiothrix a nutritive passage perforates the membrane at the
attachment of the ovum before the period when the ovum becomes free
(Ludwig). The formation of the zona radiata has been studied by
Selenka. It is secreted by the protoplasm of the ovum, and has a
gelatinous consistency, and after it is formed the peripheral layer of
the protoplasm of the ovum sends out through it pseudopodia-like
processes to absorb nutriment from without. These processes are at
first large and irregular, but soon become finer and finer (fig. 10),
and acquire a regular radiating arrangement. They are withdrawn when
the ovum is ripe, but they nevertheless give rise to the finely
radiated appearance of the membrane, the radii being in reality
delicate pores.
[FIG. 11. TRANSVERSE SECTION THROUGH THE PINNA OF A SEXUALLY MATURE
COMATULA. (From Gegenbaur, after Ludwig.)
_p._ Tentacle. _g._ Lumen of genital rachis. _w._ Water-vascular
vessel. _n._ Nerve cord. _b._ Blood-vessel on nerve cord and round
genital rachis. _cg._ Genital canal. _cd._ Dorsal section of the
body cavity. _cv._ Ventral section of body cavity.]
In the Crinoids the generative rachis consists of a tube, the
epithelium of which is formed of the primary germinal cells. (Fig.
11.) While some of these cells enlarge and become ova, the remainder
supply the elements for a follicular epithelium, which is established
round the ova, exactly as in Holothurians.
MOLLUSCA.
_Lamellibranchiata._
(21) H. Lacaze-Duthiers. "Organes génitaux des Acéphales
Lamellibranches." _Ann. Sci. Nat._, 4me série, Vol. II. 1854.
(22) W. Flemming. "Ueb. d. er. Entwick. am Ei d. Teichmuschel."
_Archiv f. mikr. Anat._, Vol. X. 1874.
(23) W. Flemming. "Studien üb. d. Entwick. d. Najaden." _Sitz. d. k.
Akad. Wiss. Wien_, Vol. LXXI. 1875.
(24) Th. von Hessling. "Einige Bemerkungen, etc." _Zeit. f. wiss.
Zool._, Bd. V. 1854.
(25) H. von Jhering. "Zur Kenntniss d. Eibildung bei d. Muscheln."
_Zeit. f. wiss. Zool._, Vol. XXIX. 1877.
(26) Keber. _De Introitu Spermatozoorum in ovula_, etc. Königsberg,
1853.
(27) Fr. Leydig. "Kleinere Mittheilung etc." _Müller's Archiv_, 1854.
_Gasteropoda._
(28) C. SEMPER. "Beiträge z. Anat. u. Physiol. d. Pulmonaten." _Zeit.
f. wiss. Zool._, Vol. VIII. 1857.
(29) H. Eisig. "Beiträge z. Anat. u. Entwick. d. Pulmonaten." _Zeit.
f. wiss. Zool._, Vol. XIX. 1869.
(30) Fr. Leydig. "Ueb. Paludina vivipara." _Zeit. f. wiss. Zool._,
Vol. II. 1850.
_Cephalopoda._
(31) AL. KÖLLIKER. _Entwicklungsgeschichte d. Cephalopoden._ Zurich,
1844.
(32) E. R. Lankester. "On the developmental History of the Mollusca."
_Phil. Trans._, 1875.
_Lamellibranchiata._
The ova of the Lamellibranchiata present several points of interest.
They are developed in pouches of the ovary which are lined by a
flattened germinal epithelium, or sometimes (?) a syncytium. Some of
the cells of this epithelium enlarge and become ova, but remain
attached to the walls of their pouches by protoplasmic stalks. Round
the ovum there appears in some forms (Anodon, Unio) a delicate
vitelline membrane, which is incomplete at the protoplasmic stalk, and
is therefore perforated by an aperture which forms the micropyle.
(Fig. 12.) As the ovum becomes ripe a large space filled with
albuminous fluid becomes established between the ovum and its
membrane, but the ovum remains attached to the membrane at the
micropyle. In Scrobicularia (von Jhering, No. 25) the membrane round
the ovum appears from the first as an albuminous layer, the outermost
stratum of which becomes subsequently hardened as the vitelline
membrane. In this form also the protoplasmic stalk becomes, in pouches
largely filled with ova, extremely long. The ova become eventually
detached by the stalk rupturing, and the portion of it which remains
attached to the vitelline membrane falling off. The function of the
stalk and of the micropyle during the development of the ovum is
undoubtedly a nutritive one.
In Anodon and Unio yolk granules similar to those deposited in the
protoplasm of the ovum are also found in the epithelial cells of the
ovarian pouches (Flemming, 22), and there can be but little doubt that
they are directly transported from these cells into the ovum. These
cells would seem therefore to play much the same part as the
yolk-glands of some Turbellarians (Prostomum caledonicum). In
Scrobicularia yolk granules are not found in the epithelium of the
pouches, but are contained in the dilated disc by which the ovum is
attached to the wall of its pouch, as well as in the ovum itself.
[FIG. 12. MEDIUM-SIZED OVUM OF ANODONTA COMPLANATA. (After
Flemming.)
_mp._ micropyle. _gs._ germinal spot.]
On the ovum becoming detached the micropyle still remains as an
aperture, which probably has the function of admitting the spermatozoa.
The shape and form of the micropyle vary greatly. In Anodon and Unio
it is a projecting trumpet-shaped structure, which after fertilization
becomes shortened and reduced to a mere aperture which is finally
stopped up. (Fig. 12.)
In other forms it is simply a perforation in the vitelline membrane
which is sometimes very large. In a species of Arca, which I had an
opportunity of observing at Valparaizo, it was equal to nearly the
circumference of the ovum.
The eggs of the Lamellibranchiata are not only remarkable in the
possession of a micropyle, but in certain peculiarities of the yolk
and of the germinal vesicle.
In the fresh-water mussels there is usually found in young and
medium-sized ova a peculiar lens-shaped body--Keber's corpuscle--which
is placed immediately internal to the micropyle. It is probably in
some way connected with the nutrition of the ovum, though the fact
that it is not always present shews that it cannot be of great
importance.
A dark body found by von Jhering in the neighbourhood of the germinal
vesicle in the ripe ovum of Scrobicularia is probably of a similar
nature to Keber's corpuscle. Both bodies may be placed in the same
category as the so-called yolk nucleus of the spider's and frog's ova.
In all except the youngest ova of Anodon and Unio the germinal spot is
composed of two nearly complete spheres united together for a small
part of their circumference. (Fig. 12, _gs._) The smaller of
these has a higher refractive index than the larger, and often
contains a vacuole: the two parts together appear to be the separated
components (though not by simple division) of the primitive nucleolus.
A nucleolus of this character is not universal amongst Lamellibranchiata,
but a similar separation of the constituents of the germinal spot has
been found by Flemming in Tichogonia, in which however the more highly
refracting body envelopes part of the less highly refracting body in a
cap-like fashion.
_Gasteropoda._
The ova of the Gasteropoda are developed, like those of the
Lamellibranchiata, from the epithelial cells of the ovarian acini or
pouches. In the hermaphrodite forms both ova and spermatozoa are
produced in the same pouches (fig. 13), some of the epithelial cells
becoming ova and others spermatozoa. The ova are usually formed in the
wall of the pouch, and the spermatozoa internally (Pulmonata) (fig. 13
_A_), or a further differentiation of parts may take place (fig.
13 _B_). The ova of Gasteropods are exceptional in the fact that
a vitelline membrane is rarely or never developed around them.
The ovum in its passage to the exterior becomes enclosed in a
secretion of the albuminous gland, which hardens externally to form a
special membrane.
[FIG. 13. FOLLICLES OF THE HERMAPHRODITE GLANDS OF GASTEROPODA.
(From Gegenbaur.)
_A._ Of Helix hortensis. The ova (_aa_) are developed on the wall of
the follicle, and the seminal masses (_b_) internally.
_B._ Of Aeolidia. The seminal portion of a follicle is beset
peripherally by ovarian saccules (_a_). _c._ Common afferent duct.]
_Cephalopoda._
Lankester (No. 32) has brought out some very interesting points with
reference to the nutrition of the eggs of Sepia during their growth.
The eggs develop in connective-tissue pouches which early give rise to
a double pedunculated capsule of connective tissue. The cells of the
inner layer of this capsule soon assume an epithelial character, and
become a definite follicular epithelium, while between the two layers
there penetrates a network of vascular channels. The follicular
epithelium becomes after the establishment of these vascular channels
folded in a most remarkable manner. The folds, which are shewn in
section in fig. 14, _ic_, project into and nearly completely fill
up the body of the ovum. An enormous increase is thus effected in the
nutritive surface exposed by the epithelium. Each fold is thoroughly
supplied with blood-vessels. The plications of the follicular
epithelium give rise to a basket-work tracery on the surface of the
ovum. During the stage when the follicular epithelium has the above
structure, its cells have a character similar to that of the
goblet-cells of a mucous membrane, and pour out their metamorphosed
protoplasm into the body of the ovum.
[FIG. 14. TRANSVERSE SECTION THROUGH AN OVARIAN EGG OF SEPIA.
(Copied from Lankester.)
_o.c._ outer capsular membrane. _i.c._ inner capsular membrane with
follicular epithelium. _b.v._ blood-vessels in section between the
outer and inner capsular membranes. _c._ vitellus.
The section shews the folds of the inner capsule with their
epithelium, which penetrate into the substance of the ovum for the
purpose of supplying it with nourishment.]
After the above mode of nutrition has gone on for a certain time a
change takes place, and the ridges gradually disappear. This is caused
by the epithelial cells passing off from the ridges into the
protoplasm of the ovum; and becoming assimilated, after retaining
their individuality for a longer or shorter period. When the
absorption of the ridges is completed the surface of the ovum assumes
a perfectly regular outline. The capsule of the ovum then bursts at
the opposite pole to the peduncle, and the ovum falls into the
oviduct.
The ova of the Cephalopoda, like those of the Gasteropoda, are quite
naked, being without a vitelline membrane or chorion. The egg-capsule
which is formed for them in their passage down the oviduct is
perforated in Sepia by a micropylar aperture.
CHÆTOPODA.
(33) Ed. Claparède. "Les Annelides Chætopodes d. Golfe de Naples."
_Mém. d. l. Sociét. phys. et d'hist. nat. de Genève_ 1868-9 and 1870.
(34) E. Ehlers. _Die Borstenwürmer nach system. und anat.
Untersuchungen._ Leipzig, 1864-68.
(35) E. Selenka. "Das Gefäss-System d. Aphrodite aculeata."
_Niederländisches Archiv f. Zool._, Vol. II. 1873.
The ova of the Chætopoda are in most cases developed from the special
tracts of the epithelial cells lining parts of the body
cavity, which constitute a germinal epithelium (fig. 15). Very
frequently (Aphrodite, Arenicola), as is so common in other types,
these tracts of germinal cells surround the blood-vessels. In some
cases the germinal epithelium thickens to form a compact organ, for
which the outermost cells may form a more or less definite membranous
covering (Oligochæta, etc.). The ova are formed by the enlargement,
accompanied by other changes, of these germinal cells. During their
early development the ova are frequently surrounded by a special
capsule, which is often stalked, and provided at its attachment with a
large micropylar aperture. In Aphrodite and Polynoe this arrangement,
which is clearly connected with the nutrition of the ovum, is very
easily seen. The ovum is dehisced into the body cavity by the bursting
of its capsule or the rupture of the stalk. The capsule is always
eventually thrown off; but a vitelline membrane is frequently
developed after the detachment of the ovum into the body cavity. The
vitelline membrane of Spio and other Polychæta is provided with an
equatorial ring of ampulliform vesicles.
[FIG. 15. A PARAPODIUM OF TOMOPTERIS. (From Gegenbaur.)
_o._ Collection of germinal epithelial cells lining the body cavity.]
DISCOPHORA.
(36) H. Dorner. "Ueber d. Gattung Branchiobdella." _Zeit. f. wiss.
Zool._, Vol. XV. 1865.
(37) R. Leuckart. _Die menschlichen Parasiten._
(38) Fr. Leydig. "Zur Anatomie v. Piscicola geometrica, etc." _Zeit.
f. wiss. Zool._, Vol. I. 1849.
(39) C. O. Whitman. "Embryology of Clepsine." _Quart. J. of Micr.
Sci._, Vol. XVIII. 1878.
The ovary of the Discophora is formed of a mass of cells enveloped in
a membranous sack. In Branchiobdella there is placed in the
central axis of these cells a column of nucleated protoplasm from
which the cells themselves are budded off. The development of the ovum
takes place by the enlargement, etc. of one of the peripheral cells,
which eventually bursts the wall of the sack and is freely dehisced
into the body cavity.
In most other Leeches (except Piscicola and its allies) there is found
a more specialized arrangement of the same nature as in
Branchiobdella. There are one or more coiled egg-strings which lie
freely in a delicate sack continuous with the oviduct. Each egg-string
is formed of a central rachis and of a peripheral layer of cells[22].
The ova are formed by the enlargement of the peripheral cells
accompanied by a deposition of food-yolk. Food-yolk appears to be
formed in the rachis even more energetically than in the protoplasm of
the ova. When ripe the ova fall into the ovarian sack.
[22] The rachis is stated by Whitman (No. 39), and other
observers to be formed of nucleated protoplasm, but further
investigations on this point are still required.
In Piscicola the development of the ovum is somewhat peculiar but
resembles in certain respects that of Bonellia (p. 45). The ova are
developed from the primitive germinal cells which fill up the ovarian
sack. The nuclei in these cells increase in number, and a nucleated
peripheral layer of each cell becomes separated from the central part,
which also contains nuclei. This latter part next divides into
numerous cells, of which one eventually forms the ovum, and the
remainder constitute a mass of cells adjoining it as in Bonellia (fig.
16). This mass of cells eventually disappears, and is probably
employed in the nutrition of the ovum.
The ovaries of the Leech appear to belong to the tubular type in that
the ova are not formed from part of the epithelium lining the body
cavity; but if, as seems probable, the true affinities of the Leeches
are with the Chætopoda, the investment of the ovaries must be of a
secondary nature. It should be noted that the ova are not, as in the
ordinary tubular ovary, developed from the epithelium lining the
ovarian tube.
GEPHYREA.
(40) Keferstein u. Ehlers. _Zoologische Beiträge._ Leipzig, 1861.
(41) C. Semper. _Holothurien_, 1868, p. 145.
(42) J. W. Spengel. "Beiträge z. Kenntniss d. Gephyreen." _Beiträge a.
d. zool. Station z. Neapel_, Vol. I. 1879.
(43) J. W. Spengel. "Anatomische Mittheilungen üb. Gephyreen."
_Tagebl. d. Naturf. Vers._ München, 1877.
In the Gephyrea, as in the Chætopoda, the ova are developed from the
lining cells of the peritoneum and frequently from the cells
surrounding parts of the vascular system (Bonellia, Thalassema). In
many cases (Sipunculus, Phascolosoma, Echiurus) the main growth of the
ovum takes place after it has been dehisced into the body cavity.
In Sipunculus the ova in the body cavity are surrounded by a follicle
which is thrown off before they become ripe.
Brandt denies the existence of this follicle or rather its cellular
nature. Spengel's (43) observations are conclusive in favour of the
correctness of the original interpretation Of Keferstein and Ehlers.
The follicles would seem to be formed after the ova have become free.
In Phascolosoma there is no follicle (Semper, Spengel).
In both Phascolosoma and Sipunculus a vitelline membrane with radial
pores--_zona radiata_--is formed, and in Phascolosoma the
external part of this is separated off as a structureless vitelline
membrane. The formation of both these membranes from the protoplasm of
the ovum is rendered certain in the latter case by the absence of a
follicular epithelium.
Some interesting observations on the growth and origin of the ovum in
Bonellia have been made by Spengel.
[FIG. 16. FOLLICLE OF BONELLIA AT A MEDIUM STAGE OF DEVELOPMENT.
(After Spengel.)
_ov._ ovum. _fe._ flattened follicular epithelium.]
The ova originate from certain cells (germinal cells) in the
peritoneal investment of the ventral vessel, overlying the nervous
cord. These cells, which are well marked off from the surrounding
flattened peritoneal elements, increase in number by division, and
form small masses surrounded by a follicle of peritoneal cells, and
attached by a stalk to the peritoneum. The central cell of each mass
grows larger than the rest, which arrange themselves in a columnar
fashion round it; it is not, however, destined to become the ovum. On
the contrary certain of the other cells adjoining the stalk grow
larger, and finally one of these becomes distinguished as the ovum by
its greater size and the character of its nucleus. The remainder of
the larger cells become of the same size as their neighbours. The ovum
now becomes more or less separate from the mass of germinal cells,
rapidly grows in size, and soon forms the most considerable
constituent of the follicle (fig. 16, _ov_). The remaining germinal
cells are quite passive, and though, with the exception of the central
cell, they do not appear to atrophy, they soon constitute a relatively
small prominence on the surface of the ovum. By the rupture of the
stalk the whole follicle becomes eventually detached, and the further
development of the ovum takes place in the body cavity. A vitelline
membrane is formed, and eventually the ovum is taken into the oviduct
(segmental organ). At this time or slightly before, the follicle cells
together with the germinal mass, which throughout exhibits no signs of
atrophy, become thrown off, and the ovum is left invested in its
vitelline membrane.
NEMATODA.
(44) Ed. Claparède. _De la formation et de la fécondation des oeufs
chez les Vers Nématodes._ Genève, 1859.
(45) R. Leuckart. _Die menschlichen Parasiten._
(46) H. Munk. "Ueb. Ei- u. Samenbildung u. Befruchtung b. d.
Nematoden." _Zeit. f. wiss. Zool._, Vol. IX. 1858.
(47) H. Nelson. "On the reproduction of Ascaris mystax, etc." _Phil.
Trans._ 1852.
(48) A. Schneider. _Monographie d. Nematoden._ Berlin, 1866.
The female organs consist as a rule of two cæcal tubes which unite
before opening to the exterior. Each of these is divided into a
vagina, uterus, oviduct, and ovary. The ovary constitutes the blind
end of the tube, and is formed of a common protoplasmic column,
holding a number of nuclei in suspension. The protoplasm becomes cleft
around the nuclei in the uppermost part of the tube; the
circumscription of the ova proceeds, however, very gradually, and
since it commences at the periphery of the column the ova remain
attached by stalks to a central axis with one end free. In this way
there is formed a rod-like structure known as the _rachis_, which
consists of a central axis with a series of half circumscribed ova
radiately arranged round it. In the lowest part of the ovary the ova
become completely isolated and form separate cells.
The protoplasm of the ova, which is clear in the terminal division of
the ovary, becomes in most forms filled lower down with yolk-spherules
secreted in the body of the ova. These commence to appear at the
uppermost extremity of the rachis.
In some instances, _e.g._ Cucullanus elegans, yolk-spherules are
not formed. In the Oxyuridæ the ova are directly segmented off from
the terminal syncytium of protoplasm without the intervention of a
rachis; and are therefore formed in the same way as amongst
Trematodes, etc.
The origin of the membrane around the ova of the Nematoda has been
much disputed.
At the time when the ovum is detached from the rachis no membrane is
present, but it nevertheless appears from Schneider's observations
that the region at which it is detached is softer than other parts, so
that a kind of micropyle is here formed which disappears after
impregnation. A delicate vitelline membrane then appears, around which
there is subsequently established an egg-shell, which is usually
stated to be formed as a secretion of the walls of the uterus; but
Schneider and Leuckart have given strong grounds for believing that it
is really a further differentiation of the vitelline membrane due to
the activity of the protoplasm of the ovum. The originally single
membrane becomes as it thickens split into two layers. The outer of
these forms the true egg-shell, and the fertilization of the ovum
appears to be a necessary prelude to its production. Round the
egg-shell the walls of the uterus often secrete a special albuminous
covering.
The egg-shell exhibits in many cases peculiar sculpturings as well as
terminal prolongations.
INSECTA.
(49) A. Brandt. _Ueber das Ei u. seine Bildungsstätte._ Leipzig, 1878.
(50) T. H. Huxley. "On the agamic reproduction and morphology of
Aphis." _Linnean Trans._, Vol. XXII. 1858. _Vide_ also _Manual of
Invertebrated Animals_, 1877.
(51) R. Leuckart. "Ueber die Micropyle u. den feinern Bau d.
Schalenhaut bei den Insecteneiern." Müller's _Archiv_, 1855.
(52) Fr. Leydig. _Der Eierstock u. die Samentasche d. Insecten._
Dresden, 1866.
(53) Lubbock. "The ova and pseudova of Insects." _Phil. Trans._ 1859.
(54) Stein. _Die weiblichen Geschlechtsorgane d. Käfer._ Berlin, 1847.
[Conf. also Claus, Landois, Weismann, Ludwig (No. 4).]
The ovum of Insects has formed the subject of numerous investigations,
and has played an important part in the controversies on the nature of
the ovum.
The ovaries are paired organs, rarely directly connected, each
consisting of more or fewer ovarian tubes which open into a common
oviduct. The oviducts unite into a vagina, usually provided with a
spermatheca and accessory glands, which need not be further alluded
to. Each ovary is invested by a peritoneal covering, which assumes
various characters, and either forms a loose network covering the
whole or a special tunic round each egg-tube. It is continuous with
the general peritoneal investment. Each ovarian tube (fig. 17)
consists of three sections: (1) a terminal thread, (2) the terminal
chamber or germogen, (3) the egg-tube proper.
[FIG. 17. _A._ OVARIAN TUBE OF THE FLEA, PULEX IRRITANS. (From
Gegenbaur, after Lubbock.)
_o._ ovum. _g._ germinal vesicle.
_B._ OVARIAN TUBE OF A BEETLE, CARABUS VIOLACEUS. (After Lubbock.)
_o._ ovarian segment, formed of an ovum _a_, and a mass of yolk
cells, _b_.]
The whole egg-tube is invested in a structureless tunica propria.
The terminal threads are fine prolongations of the ends of the
egg-tubes usually continued close up to the heart. At their
extremities they frequently anastomose, or even unite into a common
thread. In some cases they are absent. They form either direct
continuations of the germogen and have the same histological
structure, or in other cases are simply prolongations of the tunica
propria, and serve as ligaments.
The germogen usually consists of two parts: an upper, filled with
nuclei imbedded in protoplasm, and a lower, in which distinct cells
have become differentiated.
The lower part of the egg-tubes is filled with ova which advance in
development towards the oviduct, and lie in chambers more or less
distinctly constricted from each other. In these chambers there are in
most forms in addition to the true ova a certain number of nutritive
cells. The true egg-tubes are moreover lined by an epithelial
layer which passes in and forms more or less complete septa between
the successive chambers. The points which have been especially
controverted are (1) the relation of the ovum to the germogen, and (2)
the relation of the nutritive or yolk cells to the ovum. To the
controversies on these points it will only be possible to give a
passing allusion.
As has been already hinted there are two distinct types of ovaries,
viz. those without the so-called nutritive or yolk cells and those
with them[23].
[23] For a list of the genera with and without nutritive cells,
_vide_ Brandt, pp. 47 and 48.
The formation of the ovum is most simple in the type without yolk
cells, which will for that reason be first considered (fig. 17 _A_).
The germogen is constituted of a number of nuclei imbedded in a scanty
cementing protoplasm. In the lower part of the germogen the nuclei are
larger, and become separated off from the nucleated protoplasm above,
as distinct cells with a thin layer of protoplasm round the germinal
vesicle. These cells are the ova. As they pass down the egg-tube their
protoplasm increases in bulk, and they become isolated by ingrowths of
the epithelial cells the origin of which is still uncertain, which
form round each ovum a special follicle, so that the egg-tube is
filled by a single row of ova each in an epithelial follicle (fig. 17
_A_). The larger the ova the more columnar is the epithelium of the
follicle. As the oviductal extremity of the egg-tube is approached the
ova increase in size, and their protoplasm is more and more filled
with yolk particles.
In the lower part of the egg-tube the epithelium gives rise to a
chorion.
The epithelium around each ovum has been spoken of as forming a
follicle, and it is implied that the epithelium round each ovum
travels down the egg-tube with the ovum. It is however by no means
clear from the observations of the majority of writers that this is
the case, and in fact the epithelium is generally spoken of as if it
were simply the epithelium of the egg-tube. In favour of the view here
adopted the following considerations may be urged.
Firstly, there is considerable evidence that the superficial layer of
the germogen gives rise to the epithelial cells, simultaneously with
the formation of the ova from the deeper layers.
Secondly, the fact that the epithelium grows in between the separate
ova appears to render it almost certain that this part of the
epithelium must travel down the egg-tubes with the ova.
Thirdly, the epithelium no doubt gives rise to the chorion, and
considering the peculiar structure of the chorion, this seems possible
only on the view that the epithelium travels down the egg-tube with
the ova.
Fourthly, when, or even before, the egg is laid the epithelium
undergoes atrophy, and the remains of it have been compared to the
corpora lutea.
If the view about the epithelium here adopted is correct, the
epithelium without doubt corresponds to the follicular epithelium of
other ova, and has the same origin as the ova themselves.
The ovaries with yolk cells differ in appearance from those without,
mainly in each ovarian chamber of an egg-tube containing two elements,
usually more or less distinctly separated. These two elements are (1)
at the lower end of the chamber, the ovum, and (2) at the upper, large
cells which gradually disappear as the ovum grows larger (fig. 17 _B_).
The uppermost part of the egg-tube is formed, as in the previous type,
by a mass of nucleated protoplasm, but the germinal cells formed from
it do not all become ova. The germinal cells leave the germogen in
batches, and in each batch one of the cells may usually be
distinguished from the very first as the ovum; the remainder forming
the nutritive cells. In the uppermost part of the egg-tube the whole
mass of each batch is very small, and the successive batches are very
imperfectly constricted from each other. Gradually however both the
nutritive cells and the ovum grow in size, and then as a rule, the
Diptera forming a marked exception, the chamber containing a batch
becomes constricted into an upper section with the nutritive cells and
a lower one with the ovum. The ovum in passing down the tube becomes
gradually invested by a layer of epithelial cells, which in many cases
pass in and partially separate the ovum from the nutritive cells. The
epithelium appears not unfrequently to be continued as a flat layer
between the nutritive cells and the wall of the egg-tube.
As was first shewn by Huxley and Lubbock, the protoplasm of the ovum
is often continued up as a solid cord, which terminates freely between
the nutritive cells, and serves to bring to the ovum the material
elaborated by them. It is present in its most primitive form in the
somewhat aberrant ovary of Coccus. In this ovary the terminal
chamber is filled with cells which are united to a central rachis, as
in Nematodes, and the prolongation from the ovum is continuous with
this rachis. This cord is known as the yolk-duct (Dottergang) by
German writers. Although it is not generally present in a distinct
form, there is always a passage connecting the ovum and yolk cells,
even when the follicular epithelium grows in and nearly separates them.
The number of nutritive cells varies from two (one ?) to several
dozen. After they have reached a maximum they gradually atrophy, and
are finally absorbed without apparently fusing directly with the ovum.
The two types of insect ovaries appear fundamentally to differ in
this. In the one type all the germinal cells develop into ova; in the
other the quantity is, so to speak, sacrificed to the quality, and the
majority of germinal cells are modified so as to subserve the
nutrition of the few. It is still undecided whether the yolk cells
absolutely elaborate yolk particles, or are merely conveyers of
nutriment to the ovum.
The egg membranes of Insects present many points of interest, which
are however for the most part beyond the scope of this work. There is
always a chorion formed as a cuticular deposit of the follicle cells,
which is frequently sculptured, finely perforated, etc., and is in
many instances provided with a micropyle, developed, according to
Leydig, at the upper end of the ovum.
Its development at this point appears to be due to the fact that the
follicle is here incomplete; so that the cuticular membrane deposited
by it is also incomplete.
A true vitelline membrane can in many instances be demonstrated
(Donacia, etc.).
ARANEINA.
(55) Victor Carus. "Ueb. d. Entwick. d. Spinneneies." _Zeit. f. wiss.
Zool._, Vol. II. 1850.
(56) v. Wittich. "Die Entstehung d. Arachnideneies im Eierstock, etc."
Müller's _Archiv._ 1849.
[Conf. Leydig, Balbiani, Ludwig (No. 4), etc.]
The ova of many Araneina are remarkable for the presence in the ovum
of the so-called yolk-nucleus. The ova develop from the epithelial
cells lining the ovarian sack. Certain of these cells grow large and
project outwards, invested by the structureless membrane of the
ovarian wall. The stalks of projections so formed are turned towards
the lumen of the ovary, and are plugged with the epithelial cells
which line the ovarian sack. When ripe, the ova pass from their sacks
into the cavity of the ovary. The yolk-nucleus, which appears very
early, is a solid body present in the protoplasm of the ovum. It is
not found in all genera of Araneina. At its full development it
exhibits in the fresh condition a granular structure, but very soon
shews an irregularly concentric stratification which becomes more
marked on the addition of reagents. According to Balbiani this
stratification is confined to the superficial layers, while internally
there is a body with all the characters of a cell. The yolk-nucleus is
still found in the nearly ripe ovum, though it always disappears
before development commences. It is probably connected with the
nutrition of the ovum, though nothing is certainly known about its
function.
CRUSTACEA.
(57) Aug. Weismann. "Ueb. d. Bildung von Wintereiern bei Leptodora
hyalina." _Zeit. f. wiss. Zool._, Vol. XXVII. 1876.
[For general literature _vide_ Ludwig No. 4 and Ed. van Beneden,
No. 1.]
Amongst the many interesting observations on the Crustacean ova I will
only allude to those of Weismann on the ova of Leptodora, a well-known
Cladoceran form.
The phenomena of the development of the ova in this form present a
close analogy with those in Insects.
The ovary is formed of (1) a germogen containing at its upper end
nucleated protoplasm and lower down germinal cells in groups of four;
(2) of a portion formed of successive chambers in each of which there
is a row of four germinal cells. Of the four cells only the third
develops into an ovum; the remainder are used as pabulum. This is the
mode of development in the summer. In the winter the sacrifice of a
larger number of germinal cells is required for the development of the
ova; and an ovum is produced only in the alternate chambers. In the
chambers where an ovum will not be formed an epithelial investment
becomes first established round the four germinal cells. The four
cells then coalesce, and form a spherical ball of protoplasm from
which portions are budded off and absorbed by the investing
epithelial cells, which at the same time lose their nuclei. When the
whole of the central ball is thus absorbed by the epithelial cells,
the latter become used by the winter ovum as food. The winter ovum at
its full development is formed of a central mass of food-yolk and
superficial layer of protoplasm.
CHORDATA.
_Urochorda._ (Tunicata.)
(58) A. Kowalevsky. "Weitere Studien ü. d. Entwicklung d. Ascidien."
_Archiv f. mikr. Anat._, Vol. VII. 1871.
(59) A. Kowalevsky. "Ueber Entwicklungsgeschichte d. Pyrosoma." _Arch.
f. mikr. Anat._, Vol. XI. 1875.
(60) Kupffer. "Stammverwandtschaft zwischen Ascidien u.
Wirbelthieren." _Arch. f. mikr. Anat._, Vol. VI. 1870.
(61) Giard. "Études critiques des travaux, etc." _Archives Zool.
expériment._, Vol. I. 1872.
(62) C. Semper. "Ueber die Entstehung, etc." _Arbeiten a. d.
zool.-zoot. Institut Würzburg_, Bd. II. 1875.
_Cephalochorda._
(63) P. Langerhans. "Z. Anatomie d. Amphioxus lanceolatus," pp. 330-3.
_Archiv f. mikr. Anat._, Vol. XII. 1876.
_Craniata._
(64) F. M. Balfour. "On the structure and development of the
Vertebrate Ovary." _Quart. J. of Micr. Science_, Vol. XVIII. 1878.
(65) Th. Eimer. "Untersuchungen ü. d. Eier d. Reptilien." _Archiv f.
mikr. Anat._, Vol. VIII. 1872.
(66) Pflüger. _Die Eierstöcke d. Säugethiere u. d. Menschen._ Leipzig,
1863.
(67) J. Foulis. "On the development of the ova and structure of the
ovary in Man and other Mammalia." _Quart. J. of Micr. Science_, Vol.
XVI. 1876.
(68) J. Foulis. "The development of the ova, etc." _Journal of Anat.
and Phys._, Vol. XIII. 1878-9.
(69) C. Gegenbaur. "Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier
mit partieller Dottertheilung." Müller's _Archiv_, 1861.
(70) Alex. Götte. _Entwicklungsgeschichte d. Unke._ Leipzig, 1875.
(71) W. His. _Untersuchungen üb. d. Ei u. d. Eientwicklung bei
Knochenfischen._ Leipzig, 1873.
(72) A. Kölliker. _Entwicklungsgeschichte d. Menschen u. höherer
Thiere._ Leipzig, 1878.
(73) J. Müller. "Ueber d. zahlreichen Porenkanäle in d. Eikapsel d.
Fische." Müller's _Archiv_, 1854.
(74) W. H. Ransom. "On the impregnation of the ovum in the
Stickleback." _Pro. R. Society_, Vol. VII. 1854.
(75) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." _Arbeiten
a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875.
[Cf. Ludwig, No. 4, Ed. van Beneden, No. 1, Waldeyer, No. 6, &c.]
There are some very obscure points connected with the growth of the
ovum of the Tunicata. When quite young the ovum is a naked cell with a
central nucleus containing a single large nucleolus. Around it is a
flat follicular epithelium enclosed in a membrana propria folliculi.
The follicle cells soon become larger and give rise to an envelope
round the egg of the nature of a chorion. At the same time they
frequently become cubical or even columnar, and filled with numerous
vacuoles.
During or after the completion of the above changes a number of bodies
usually spoken of as test-cells make their appearance in the
superficial protoplasm of the egg, which by the time the egg is ripe
arrange themselves in many species as a definite layer round the
periphery of the ovum. These bodies have received their name from the
opinion, now known to be erroneous (Hertwig and Semper), that they
eventually migrated into the test or mantle of the embryo which
becomes developed round the ovum. By Kowalevsky (No. 58) these bodies
are regarded as true cells, and are believed to be formed by some of
the cells of the original follicular epithelium making their way into
the vitellus of the ovum and multiplying there. By Kupffer (No. 60),
and Giard (No. 61), and Fol, they are also regarded as true cells but
are believed to originate spontaneously in the vitellus. Finally by
Semper they are believed not to be cells, but to be amoeboid
protoplasmic bodies which are pressed out from the vitellus under the
stimulus of the sea-water or otherwise.
They do not according to this author naturally appear till the ovum is
quite ripe, though they can be artificially produced at an earlier
period by the action of reagents or sea-water. When produced in the
natural course of things the vitellus undergoes a contraction. They
are without any apparent function, and play no part in the embryonic
development. Semper's results are very peculiar, but owing to the
careful study which his paper displays they no doubt deserve
attention. Further investigations are however very desirable.
Kowalevsky from his researches on Pyrosoma (No. 59) adheres to his
first opinion, though he abandons the view that these cells are
connected with the formation of the test.
In the passage of the egg through the oviduct the vacuolated follicle
cells grow out into very peculiar long processes or villi. In Ascidia
canina these processes become as long as the whole diameter of the
vitellus (Kupffer, No. 60).
In Amphioxus and the Craniata the ova are developed as in the
Chætopoda, Gephyrea, etc., from specialized germinal cells of the
peritoneal epithelium.
In Amphioxus the germinal epithelium which constitutes the essential
part of the ovary is divided into a number of distinct segments: in
the Craniata no such division is observable.
In young examples of Amphioxus the generative organs are in an
indifferent condition, and the two sexes cannot be distinguished. They
form isolated horse-shoe shaped masses of cells, which occupy a
position at the base of the myotomes, in the intervals between the
successive segments; and extend from the hinder end of the respiratory
sack to the abdominal pore. They are situated in the proper body
cavity, and are surrounded by the peritoneal membrane. Each generative
mass is at first solid, and is formed of an outer layer of more
flattened cells and an inner mass of large rounded or polygonal cells.
In its interior there appears at a somewhat later period a central
cavity. After the cavity has appeared the sexes can be distinguished
by the different behaviour of the cells.