abortion or fusion) attached by a mesentery to the dorsal wall of a
more or less extended region of the abdominal cavity. This ridge is at
first identical in the two sexes, and arises at an early period of
embryonic life. It is essentially formed of a thickening of the
peritoneal epithelium, and in Osseous Fish, Ganoids (?) and Amphibia
the ovary remains during embryonic life nearly in this condition,
though a small prominence of the adjacent stroma also becomes formed.
In other Craniata the ridge, though at first in this condition, very
soon becomes much more prominent, and is formed of a central core of
stroma enclosed in the germinal epithelium (fig. 18).
[FIG. 18. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO OF
SCYLLIUM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (_po_) LYING
IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.]
The thickened germinal epithelium gives rise (in the case of the
female) to the ova and the follicular epithelium. Whether the genital
ridge is provided with a core of stroma or no, the germinal epithelium
is always in contact on one side with the stroma, from which it is at
first separated by a well-marked boundary line; but after a certain
time there appear numerous vascular ingrowths from the stroma, which
penetrate through all parts of the germinal epithelium, and break it
up into a sponge-like structure formed of trabeculæ of germinal
epithelium interpenetrated by vascular strands of stroma. The
trabeculæ of the germinal epithelium form the egg-tubes of Pflüger.
With reference to the distribution of the stroma in the germinal
epithelium, it may be said in a general way that there is a special
layer close to the surface of the ovary, which, after the formation of
fresh ova has nearly ceased, completely isolates a superficial layer
of the germinal epithelium from the deeper and major part of it. The
superficial layer is frequently (but erroneously) regarded as
constituting the whole of the germinal epithelium. The layer of stroma
below the superficial epithelium forms in the mammalian ovary the
tunica albuginea. As the follicles are formed. in the trabeculæ of
germinal epithelium the stroma grows in around them, and forms for
each one of them a special tunic.
The adult ovaries differ in a corresponding manner to the embryonic
genital ridges as to the presence of a core of stroma. The ovaries
which are without such a core in the embryo, are also without it in
the adult, and are formed of a double layer of tissue entirely derived
from the germinal epithelium with its ingrowths of stroma, and
composed, for the most part, of ova in all stages of development. In
the case of the other ovaries there is a hilus of stroma--the
zona vasculosa--internal to the egg-bearing region.
In Mammalia, proportionately to the ovary, the zona vasculosa is at a
maximum, and in Birds and Reptiles it is relatively far less
developed. In these forms the germinal epithelium covers the whole
surface of the ovary. In Elasmobranchii the structure of the ovary is
somewhat different, owing to the presence in the ovarian ridge of a
large quantity of a peculiar lymphatic tissue, which has no homologue
in the other ovaries; and still more to the fact that the true
germinal epithelium is in most forms entirely confined to the outer
surface of the ovary, on which it forms a layer of thickened
epithelium in the embryo (fig. 17), and of ovigerous tissue in the
adult.
In the ovary of Mammalia and Reptilia and possibly other forms there
are present in the zona vasculosa during embryonic life cords of
epithelial tissue derived from the Malpighian bodies; these cords have
no function in the female, but in the male assist in forming the
seminiferous tubules.
In considering the development of the ova it is again convenient to
distinguish between Amphioxus and the Craniata.
In Amphioxus the germinal cells destined to become ova are first
distinguished by the larger size of their germinal vesicles and by the
presence of certain refracting granules in their protoplasm. They
subsequently rapidly enlarge and form protuberances on the surface of
the ovary, which are enveloped for three-quarters of their
circumference by the flattened epithelioid cells of the peritoneal
membrane, which thus form a kind of follicle. As the ova become ripe
yolk granules are deposited in their protoplasm, first in the
superficial layer and subsequently throughout. The germinal vesicle
also passes from the centre to the surface. A vitelline membrane is
formed when the ova are mature.
In the Craniata the ova are developed from the cells of the germinal
epithelium. In the types with larger ova (Teleostei, Elasmobranchii,
Amphibia, Reptilia, Aves), at a very early period, sometimes
(Elasmobranchii) even before the formation of the genital ridge,
certain of the cells which are destined to form ova become
distinguished by their greater size, and by the possession of an
abundant clear protoplasm and a large spherical granular nucleus.
(Fig. 18, _po._) Such special cells form primitive germinal
cells, and are common to both sexes.
For a considerable period after their first formation these cells
remain stationary in their development; but their number increases,
partly, it appears, by an addition of fresh ones, and partly
by division. Owing to the latter process the germinal cells come to
form small masses or nests. The following description of the further
changes of these cells in the female refers in the first instance to
Elasmobranchii, but holds good in most respects for other types as
well.
It is convenient to distinguish two modes in which the primitive
germinal cells may become converted into permanent ova, though the
morphological difference between the two modes is of no great
importance.
[FIG. 19. SECTION THROUGH PART OF THE GERMINAL EPITHELIUM OF THE
OVARY OF SCYLLIUM AT THE TIME WHEN THE PRIMITIVE GERMINAL CELLS ARE
BECOMING CONVERTED INTO OVA.
_nn._ Nests formed of agglomerated germinal cells. The nuclei of
these cells are imbedded in undivided protoplasm. _do._ developing
ova. _o._ ovum with follicle. _po._ primitive germinal cell. _dv._
blood-vessels.]
In the first mode the protoplasm of all the cells forming a nest
unites into a single mass containing the nuclei of the previously
independent ova (fig. 19, _nn_). The nuclei in the nest increase
in number, probably by division, and at the same time the nest itself
increases in size. The nuclei while increasing in number also undergo
important changes. A segregation of their contents takes place, and
the granular part (nuclear substance) forms a mass close to one side
of the membrane of the nucleus, while the remainder of the nucleus is
filled with a clear fluid. The whole nucleus at the same time
increases somewhat in size. The granular mass gradually assumes a
stellate form, and finally becomes a beautiful reticulum, of
the character so well known in nuclei (fig. 19, _do_). Two or
three special nucleoli are present, and form the nodal points of the
reticulum, while its meshes are filled up with the clear fluid
constituents of the nucleus. Not all the nuclei undergo the above
changes; but some of them stop short in their development, undergo
atrophy, and appear finally to be absorbed as pabulum by the
protoplasm of the nest. Such nuclei in a state of degeneration are
shewn in fig. 19. Thus only a few nuclei out of a nest undergo a
complete development. At first the protoplasm of the nest is clear and
transparent, but as the nuclei undergo their changes the protoplasm
becomes more granular, and a specially large quantity of granular
protoplasm is generally present around the most developed nuclei, and
these with their protoplasm gradually become constricted off from the
nest, and constitute the permanent ova (fig. 19, _do_). The
relative number of ova which may develop from a single nest is subject
to great variation. The object of the whole occurrence of the fusion
of primitive ova and the subsequent atrophy of some of them is to
ensure the adequate nutrition of a certain number of them.
In the second and rarer mode of development of permanent ova from
primitive germinal cells, the nuclei and protoplasm undergo the same
changes as in the first mode, but the cells either remain isolated,
and never form part of a nest, or form part of a nest in which no
fusion of protoplasm takes place, and in which all the cells develop
into permanent ova.
The isolated ova and nests are situated, during the whole of the above
changes, amongst the general undifferentiated cells of the germinal
epithelium, but as soon as a permanent ovum becomes formed the cells
adjoining it arrange themselves around it as a special layer, and so
give rise to the epithelium of the follicle (fig. 19, _o_). The
growths of stroma into the germinal epithelium appear shortly after
the formation of the earlier follicles.
_Mammalia._ The development of the ovary in Mammalia differs
mainly from that just described in that the formation of primitive
germinal cells from the indifferent cells of the germinal epithelium
takes place at a relatively much later period.
The stroma grows into the germinal epithelium while it is still formed
of rounded indifferent cells, and divides it into trabeculæ as
described above. At a later period a number of the cells in the deeper
layer of the epithelium, as well as certain cells in the superficial
part, become primitive germinal cells, while the remainder of the
cells become smaller and are destined to form the follicle cells.
The most conspicuous primitive germinal cells are situated in the
superficial layer of epithelium; and the primitive germinal cells in
the deeper layers of the germinal epithelium are not nearly so marked
as in most Craniata, so that it is difficult in some cases to be sure
of their destination till their nucleus commences to undergo its
characteristic metamorphosis.
The change of the primitive ova into permanent ova takes place in the
same manner in Mammals as in Elasmobranchii, except that the fusion of
the primitive ova into polynuclear masses is much rarer. The formation
of the at first quite simple follicles takes place while the ova are
still aggregated in large masses; and the first follicles are formed
in the innermost part of the germinal epithelium. Soon after their
formation the follicles become isolated by connective-tissue growths.
_Post-embryonic development of the ova._
The ova of the Vertebrata differ greatly in size and structure. The
differences in size depend upon the quantity of the food-yolk. In the
Amphioxus and Mammalia, in which the ova are smallest, the
comparatively insignificant amount of food-yolk is distributed
uniformly through the ovum. A larger quantity of it is present in the
ova of Amphibia, Marsipobranchii and Teleostei, and it attains an
immense development in the ova of Elasmobranchii, Reptilia, and Aves.
The food-yolk originates from a differentiation of the protoplasm of
the egg. It arises as a number of small highly refracting particles in
a stratum slightly below the surface.
In the Mammalian ovum these particles spread through the protoplasm of
the egg, but do not attain any considerable development. In other
forms the case is different. In Elasmobranch Fishes the refracting
particles appear to develop into vesicles, in the interior of which
there arise solid oval or even rectangular highly refracting bodies,
in the substance of which a stratification may usually be observed,
which gives them an appearance not unlike that of striated muscle. In
Teleostei the yolk assumes very different characters in different
cases. It is often formed of larger or smaller vesicles containing in
their interior other bodies. Stratified plates like those of
Elasmobranchii are also not uncommon. In the ripe ovum of Teleostei
the food-yolk usually resolves itself into a large vitelline sphere,
which occupies the greater part of the ovum, and is formed of a highly
refracting fluid material which coagulates on the addition of water.
It contains in many instances one or more highly refracting bodies
known as oil globules, and is invested by a granular protoplasmic
layer continuous with the germinal disc, in which a number of normal
yolk-spherules are frequently present. In the ovum of the Herring[24]
no distinct investing protoplasmic layer or germinal disc is present
till after impregnation, but the ovum is formed of a superficial layer
with minute yolk-spherules, and of a central portion with larger
yolk-spheres.
[24] Kupffer, _Laichen u. Entwicklung des Ostsee-Härings_.
Berlin, 1878.
In Amphibia the yolk very often appears in the form of oval or
quadrilateral plates. In Reptilia the yolk-spherules are vesicles,
somewhat similar to the white yolk-spheres of Aves, but as a rule
without the highly refracting spheres in their interior. The peculiar
and complicated arrangement and structure of the white and yellow yolk
in Birds is fully described in the "Elements Of Embryology," and it
need only be said that the yolk develops in Birds in the same manner
as in other types, and that at first all the yolk-spherules appear in
the form of white yolk. The yellow yolk-spheres are a peculiar
modification of white yolk-spheres, formed comparatively late in the
development of the egg (fig. 20).
[FIG. 20. YOLK ELEMENTS FROM THE EGG OF THE FOWL.
_A._ Yellow yolk. _B._ White yolk.]
In the eggs of many Amphibia a dark granular mass known as the yolk
nucleus makes its appearance; and is supposed, without any very clear
evidence, to be related to the formation of the yolk.
A body in the form of a shell enclosing a dark nucleus, which is
perhaps of the same nature, has been described by Eimer in the
Reptilian egg: it eventually resolves itself into a number Of angular
fragments. In Elasmobranchii a similar body is perhaps present.
The food-yolk just described is imbedded in the active protoplasmic
portion of the body of the ovum. In the case of the mammalian
ovum the food-yolk is fairly uniformly distributed, but in the case of
all other craniate ova the protoplasm of the ovum is especially
concentrated at one pole, which is known as the upper or animal pole,
and the food-yolk is more especially concentrated at the opposite
pole. The Herring's ovum forms an apparent exception to this
statement, in that the concentration of the protoplasm to form the
germinal disc does not take place till after impregnation. In Amphibia
the animal pole is mainly marked by the smaller size of the
yolk-spherules, but in most other forms a small portion of the ovum in
the region of the germinal vesicle is nearly free from yolk-spherules,
and then forms a more or less specialized part known as the germinal
disc. In Aves, Reptilia, and Elasmobranchii the germinal disc shades
off insensibly into the yolk; but in Teleostei it is more sharply
marked off, and is continued more or less completely round the
periphery of the ovum. In ova with true germinal discs it is the
germinal disc alone which undergoes segmentation. The protoplasm of
vertebrate ova frequently exhibits a reticulate or sponge-like
structure (fig. 21) and the reticulum in many cases, _e.g._
Elasmobranchii and Reptilia, serves to hold the yolk-spheres together.
In the Tench it has been observed by Bambeke to penetrate into the
vitelline sphere.
In the ova of the Craniata the germinal vesicle is generally
polynucleolar. In Amphioxus and Petromyzon there is however but a
single nucleolus, and in Mammalia there is usually one special
nucleolus and two or three accessory ones. The opposite extreme is
reached in many osseous fish where the nucleoli are extremely
numerous. The protoplasmic reticulum of the embryonic germinal vesicle
may in some instances be retained till the ovum is nearly ripe, but
usually assumes a very granular form. It is at first connected with
the nucleoli which form nodal points in it, but this relation cannot
always be detected in the later stages. A membrane, which in the case
of the larger ova becomes very thick, is always present round the
germinal vesicle. It is said to be perforated in some Reptilian ova
(Eimer). As to the position of the germinal vesicle, it is at first
situated in the centre of the ovum, but always eventually travels to
the animal pole, and as the egg becomes ripe undergoes changes which
will be more especially detailed in the next chapter. In the ova with
a large amount of food-yolk it assumes an eccentric position very
early.
The homologies of the primary egg membranes of Craniata are still
involved in some obscurity. There seem to be three membranes, which
may all coexist, and of which one or more are almost always present.
These membranes are--
(1) An outermost usually homogeneous non-perforated membrane, which is
by most authors regarded as a chorion, but is probably a vitelline
membrane--by which name I shall speak of it.
(2) A radiately striated membrane (internal to the former when the two
coexist) which can be broken up into a series of separate columns.
These give to the membrane its radiate striation, but it is probable
that between the columns there are pores sufficiently large to admit
of the passage of protoplasmic filaments. This membrane will be spoken
of as the zona radiata. It is a differentiation of the outermost layer
of the yolk.
(3) Within the zona radiata a third and delicate membrane is
occasionally found, especially when the ovum is approaching maturity.
[FIG. 21. 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.]
In Elasmobranchii the first membrane to be formed is the vitelline
membrane, which appears in some instances before the formation of the
follicle--a fact which appears to shew that it is really formed as a
differentiation of the protoplasm of the egg. In most Elasmobranchii
this membrane attains a very considerable development. A zona radiata
is generally (if not always) present in Elasmobranchii, but arises at
a later period than the vitelline membrane (fig. 21, Zn). The zona
radiata always disappears long before the ovum is ripe. The vitelline
membrane also gradually atrophies, though it lasts much longer than
the zona radiata. When the egg is taken up by the oviduct all trace of
both membranes has vanished. In Reptilia precisely the same
arrangements of the membranes are found as in Elasmobranchii, except
that as a rule the zona radiata is relatively more important. The
vitelline membrane is thin except in the Crocodilia. The third
innermost membrane is found according to Eimer in many Reptilia. In
birds both vitelline membrane and zona radiata are present, but the
latter atrophies early, leaving the former as the sole membrane when
the egg is ripe.
In osseous fish the vitelline membrane is usually either absent or may
perhaps in some instances, _e.g._ the Perch, be imperfectly
represented. In the ripe ovum of the Herring there is a distinctly
developed membrane external to the zona radiata which is probably the
vitelline membrane. The zona radiata attains a very great development,
and is generally provided with knobs of various shapes on its outer
surface. A delicate membrane internal to this--my third membrane--has
often been described, but there is still some doubt about its
existence. In some cases an external less granular layer of the ovum
itself has been described as a special membrane. In the Perch a
peculiar mucous capsule, penetrated by irregular branched
prolongations of the follicle cells, is present in addition to the
ordinary membranes. In Petromyzon a zona radiata appears to be
present, which in the adult is divided into two layers, both of them
radiately striated according to Calberla, but according to Kupffer and
Benecke the outer one is not perforated, and would appear therefore to
be a vitelline membrane as defined above. A delicate membrane is
formed at a comparatively late period around the ova of the Amphibia,
and is stated (Waldeyer, No. 6, and Kolessnikow) to have a delicate
radial striation. It probably corresponds with the zona radiata.
In Mammalia a radiately striated membrane--the zona radiata--is
generally described as being present, and internal to it, in the
nearly ripe egg, a delicate membrane has been shewn by E. van Beneden
to exist. Externally to the zona radiata there may be observed a
granular membrane irregular on its outer surface on which the cells of
the discus are supported. This membrane is more or less distinctly
separated from the zona radiata; and by tracing back its development
it appears very probable that it is the remnant of the first-formed
membrane in the very young ovum, and therefore the vitelline membrane.
A micropyle (first discovered by Ransom, No. 74) is present in a large
number of osseous fish and in Petromyzon (Calberla). Doubts
have been thrown on its existence in the latter form by Kupffer and
Benecke; and at any rate it would only seem to perforate the zona
radiata. In the osseous fish in which it has been detected, Salmonidæ,
Percidæ (Gasterosteus), Clupeidæe, etc., it forms a minute perforation
of the zona radiata at the animal pole, just large enough to admit a
single spermatozoon. Its characters differ slightly in different
cases, but there is usually a shallow depression, in the centre of
which it is situated.
The eggs of all Craniata (except Petromyzon (?)) appear to be enclosed
in a cellular envelope known as the follicle. The cells which form
this are, as has been already explained, derived from the germinal
epithelium[25], and frequently arrange themselves around the ovum
before the appearance of the growths of stroma into the epithelium.
All young follicles are nearly alike, but as they grow older they
exhibit various modifications in the different groups. They retain
their simplest condition as a flat epithelial layer in most osseous
fish and Amphibia. In most other forms the cells become at some period
columnar, and are generally arranged in two or more layers. There is
formed externally to the epithelium a delicate membrane--the membrana
propria folliculi--which is in its turn enclosed in a vascular
connective-tissue sheath.
[25] For the different views maintained by Foulis, Kölliker, etc.
the reader is referred to the writings of these authors. The
grounds for the view here adopted will be found in my paper (No. 64).
In Elasmobranchii and many Reptilia (_Lacertilia_, _Ophidia_) some of
the cells become much larger than the others, and assume a
funnel-shaped form with the narrow end in contact with the egg
membrane. These large cells, which have a regular arrangement in the
epithelium, are probably in some way connected with the nutrition.
They have only been noticed in large-yolked ova. Many observers have
described prolongations of the follicle cells through the pores of the
zona radiata in Aves, Reptilia and Teleostei.
The most remarkable modification of the follicle is that which is
found in Mammalia. At first the follicle is similar to that of other
Vertebrata, and is formed of flat cells derived from the germinal
cells adjoining the ovum. These cells next become columnar and then
one or two layers deep. Later they become thicker on one side
than on the other, and there appears in the thickened mass a cavity,
which gradually becomes more distended and is filled with an
albuminous fluid. As the cavity enlarges, the ovum with several layers
of cells around it forms a prominence projecting into it. The whole
structure with its tunic is known as the Graafian follicle. The
follicle cells are known as the membrana granulosa, and the
projection, in which the ovum lies, as the discus or cumulus
proligerus. The cells of the discus in immediate contiguity to the
ovum usually form a more or less specialized layer and are somewhat
more columnar than the adjoining cells.
THE SPERMATOZOON.
Although there is no doubt that the spermatozoon in most instances
plays as important a part as the ovum in influencing the characters of
the organism which is evolved from the coalesced product of the ovum
and spermatozoon, yet the actual form of the spermatozoon has not,
like the form of the ovum, a secondary influence on the early phases
of development. A comparative history of the spermatozoon is therefore
of less importance for my purpose than that of the ovum; and I shall
confine myself to a few remarks on its general structure, and mode of
growth. The primary origin of the male germinal cells, and their
relation to the sperm-forming cells, is dealt with in the second part
of the treatise.
Although the minute size of most spermatozoa places great difficulties
in the way of a satisfactory investigation of them, yet there can be
but little doubt that they always have the value of cells. In the vast
majority of instances the spermatic cell or spermatozoon is composed
of (1) a spherical or oval portion known as the head, formed of a
nucleus enveloped in an extremely delicate layer of protoplasm, and
(2) of a motile protoplasmic flagellum known as the tail; which
together with the investing layer of the head forms the body of the
cell.
As might be anticipated, the proportion, size, and relations of the
parts of the spermatozoon are subject to great variations. The head is
often extremely elongated; and it is in many cases rather on
theoretical grounds, than as a result of actual observation,
that a protoplasmic layer is stated to be continued round the nucleus
which forms the main constituent of the head. In some of the elongated
forms of spermatozoa, _e.g._ in Insecta, there is no marked
distinction, except in the character of the protoplasm, between the
head and the tail. A connecting element is frequently interposed
between the head and tail, which appears however to be constituted of
the same material as the tail, and sometimes forms a thickening on the
tail close below the head (Amphioxus). A very remarkable modification
of the tail is found in many Amphibia, Reptilia and Mammalia. In these
types there is attached to what appears to be a normal tail a delicate
membrane, the outer edge of which is thickened to form a kind of
secondary filament. In the living spermatozoon this filament is in a
state of constant movement. The membrane winds spirally round the
tail.
In the majority of forms the tail of the living spermatozoon exhibits
sinuous cilia-like movements. In two groups the movements are however
of an amoeboid character. These groups are the Nematoda and the
Crustacea; and the spermatozoa in both of them frequently present very
abnormal forms. In Nematoda they are pear-shaped, cylindrical,
spine-shaped, etc., and are mainly formed of protoplasm with a highly
refracting nucleus. In the Crustacea the variations of form are still
greater. In the Malacostraca they are sometimes simply spherical
(Squilla), while in Astacus and a large number of Decapoda they are
composed of a nucleated body with stellate rays. In Paludina amongst
the Mollusca there are two _forms_ of completely developed
spermatozoa existing side by side in the same individual.
The spermatozoa are formed by the breaking up of the male germinal
cells, or of cells secondarily derived from them by division. The
cells which directly give rise by division to the spermatozoa may be
called spermospores and are equivalent to the ova or oospores.
Amongst the Sponges (Halisarca, Schultze, No. 141) a germinal cell,
similar to that which in the female becomes an ovum, repeatedly
divides and eventually gives rise to a ball of cells (a spermosphere
or sperm-morula), each constituent cell of which becomes converted
into a spermatozoon, and may be designated by the special term
'spermoblast.'
In most Hydrozoa the subepithelial epiblastic cells become converted
into germinal cells (spermospores), and then break up to form
spermoblasts, each of which becomes a spermatozoon.
In most higher Metazoa the spermospores usually form the epithelium of
an ampulla or tube, though more rarely (many Chætopoda, Gephyrea,
etc.) they may be derived from cells lining the body cavity, as in the
case of ova. The spermatozoa are formed either by the direct division
of the spermospores into a number of cells, spermoblasts, each of
which grows into a spermatozoon; or by the nucleus of the spermospore
becoming subdivided within the cell body, the latter differentiating
itself into the tails of the spermatozoa while the segments of the
nucleus give rise to the main part of the heads.
In many instances interstitial cells which do not give rise to
spermatozoa, are intermingled with the spermospores.
In a good many cases, as first pointed out by Blomfield[26], the whole
of each spermospore does not become converted into spermatozoa, but
part, either with or without a segment of the original nucleus,
remains passive, and carrying as it does the off-budded spermoblasts
may be called the 'sperm-blastophor.' This passive portion of
protoplasm is not employed in the regeneration of the spermoblast.
This very singular phenomenon has been observed in Elasmobranchii, the
Frog, the Earthworm, Helix, etc.[27], and probably has a much wider
extension. In Elasmobranchii (Semper) the passive portions of
protoplasm are nucleated, and are placed on the outer side of the
columnar spermospores which line the testicular ampullæ; they are not
distinctly differentiated till the nuclei, segmented from the nucleus
of the primitive spermospore to form the heads of the spermatozoa,
have become fairly numerous. In the Frog the passive blastophor also
occurs as a nucleated mass of protoplasm on the outer side of the
spermospore. In the Earthworm the blastophor forms a central
non-nucleated portion of the spermospore; and the whole periphery of
each spermospore becomes converted into spermoblasts.
[26] _Quart. Journ. of Micro. Science_, Vol. XX. 1880.
[27] Blomfield, _loc. cit._, p. 83, states that he has observed
this fact in Lumbricus, Tubifer, Hirudo, Helix, Arion, Paludina,
Rana, Salamandra, and Mus.
It has been already stated in the introduction that the male and
female generative products are homodynamous, but the consideration of
the development of the products in the two sexes shews that a single
spermatozoon is not equivalent to an ovum, but rather _that the
whole of the spermatozoa derived from a spermospore are together
equivalent to one ovum_.
CHAPTER II.
THE MATURATION AND IMPREGNATION OF THE OVUM.
_Maturation of the ovum and formation of the polar bodies._
In the preceding chapter the changes in the ovum were described nearly
up to the period when it became ripe, and ready to be impregnated.
Preparatory to the act of impregnation there take place however a
series of remarkable changes, which more especially concern the
germinal vesicle.
The attention of a large number of investigators has recently been
directed to these changes as well as to the phenomena of impregnation.
The results of their investigations will be described in the present
chapter; but for an historical account of these investigations, as
well as for a determination of the delicate questions of priority, the
reader is referred to Fol's memoir (No. 87), and to a paper by the
author (No. 81).
[FIG. 22. RIPE OVUM OF ASTERIAS GLACIALIS ENVELOPED IN A
MUCILAGINOUS ENVELOPE, AND CONTAINING AN ECCENTRIC GERMINAL VESICLE
AND GERMINAL SPOT (copied from Fol).]
The nature of the changes which take place in the maturation of the
ovum may perhaps be most conveniently displayed by following the
history of a single ovum. For this purpose the eggs of Asterias
glacialis, which have recently formed the subject of a series of
beautiful researches by Fol (87), may be selected.
The ripe ovum (fig. 22), when detached from the ovary is formed of a
granular vitellus enveloped in a mucilaginous coat, the zona
radiata. It contains an eccentrically-situated germinal vesicle and a
germinal spot. In the former is present the usual protoplasmic
reticulum. As soon as the ovum reaches the sea-water the germinal
vesicle commences to undergo a peculiar metamorphosis. It exhibits
frequent changes of form, the reticulum vanishes, its membrane becomes
gradually absorbed, its outline indented and indistinct, and finally
its contents become to a certain extent confounded with the vitellus
(fig. 23).
[FIG. 23. TWO SUCCESSIVE STAGES IN THE GRADUAL METAMORPHOSIS OF THE
GERMINAL VESICLE AND SPOT OF THE OVUM OF ASTERIAS GLACIALIS
IMMEDIATELY AFTER IT IS LAID (copied from Fol).]
The germinal spot at the same time loses its clearness of outline and
gradually disappears from view.
At this stage, and between it and the stage represented in fig. 26,
the action of reagents brings to light certain appearances the nature
of which is not yet fully cleared up for Asterias, which have been
described somewhat differently by Fol for Ast. glacialis and Hertwig
for Asteracanthion.
Fol finds immediately after the stage just described that a star is
visible between the remains of the germinal vesicle and the surface of
the egg, which is connected with an imperfectly-formed nuclear spindle
extending towards the germinal vesicle[28]. At the end of the nuclear
spindle may be seen the broken up fragments of the germinal spot.
[28] By the term 'nuclear spindle' I refer to the peculiar form
of a double striated cone assumed by the nucleus just before
division, which is no doubt familiar to all my readers. I use the
term star for the peculiar stellate figure usually visible at the
poles of the nuclear spindle. For a further description of these
parts the reader is referred to Chapter IV.
[FIG. 24. OVUM OF ASTERIAS GLACIALIS SHEWING THE CLEAR SPACES IN THE
PLACE OF THE GERMINAL VESICLE. FRESH PREPARATION (copied from Fol).]
At a slightly later stage, in the place of the original germinal
vesicle there may be observed in the fresh ovum two clear spaces (fig.
24), one ovoid and nearer the surface, and the second more irregular
in form and situated rather deeper in the vitellus. In the upper space
parallel striæ may be observed. By treatment with reagents the first
clear space is found to be formed of a horizontally-placed spindle
with two terminal stars, near which irregular remains of the germinal
spot may be seen. Slightly later (fig. 25) there may be seen on the
lower side of the spindle a somewhat irregular body, which may
possibly be part of the remains of the germinal spot, though Fol holds
that it is probably part of the membrane of the germinal vesicle. The
lower clear space visible in the fresh ovum now contains a round body,
fig. 25. Fol concludes that the spindle is formed out of part of the
germinal vesicle and not from the germinal spot, while he sees in the
round body present in the lower of the two clear spaces the
metamorphosed germinal spot. He will not, however, assert that no
fragment of the germinal spot enters into the formation of the
spindle.
[FIG. 25. OVUM OF ASTERIAS GLACIALIS, AT THE SAME STAGE AS FIG. 24,
TREATED WITH PICRIC ACID (copied from Fol).]
The following is Hertwig's (No. 92) account of the changes in the
germinal vesicle in Asteracanthion. Shortly after the egg is laid the
protoplasm on the side of the germinal vesicle towards the surface of
the egg develops a prominence which presses inwards the wall of the
vesicle. At the same time the germinal spot develops a large vacuole,
in the interior of which is a body consisting of nuclear substance,
and formed of a firmer and more refractive material than the remainder
of the germinal spot. In the prominence first mentioned as projecting
inwards towards the germinal vesicle first one star, formed by radial
striæ of protoplasm, and then a second make their appearance; while
the germinal spot appears to have vanished, the outline of the
germinal vesicle to have become indistinct, and its contents to have
mingled with the surrounding protoplasm. Treatment with reagents
demonstrates that in the process of disappearance of the germinal spot
the nuclear mass in its vacuole forms a rod-like body, the free end of
which is situated between the two stars which occupy the prominence
indenting the germinal vesicle. At a later period granules may be seen
at the end of the rod and finally the rod itself vanishes. After these
changes by the aid of reagents there may be demonstrated a spindle
between the two stars, which Hertwig believes to grow in size as the
last remnants of the germinal spot gradually vanish, and he maintains
that the spindle is formed at the expense of the germinal spot. The
stage with this spindle corresponds with fig. 25.
Several of Hertwig's figures closely correspond with those of Fol, and
considering how conflicting is the evidence before us, it seems
necessary to leave open for Asterias the question as to what
parts of the germinal vesicle are concerned in forming the first
spindle.
A clearer view of the phenomena which take place at this stage has
been obtained by Fol in the case of Heteropods (Pterotrachæa). In the
ovum a few minutes after it has been laid the germinal vesicle becomes
very pale, and two stars make their appearance round a clear substance
near its poles. The nucleus itself is somewhat elongated, and
commences to exhibit at its poles longitudinal striæ, which gradually
extend towards the centre at the expense of the nuclear reticulum,
from a metamorphosis of which they are directly derived. When the
striæ of the two sides have nearly met, thickenings may be observed in
the recticulum between them, which give rise, where the striæ of the
two sides unite, to the central thickenings of the fibres (nuclear
plate). In this way a complete nuclear spindle is established[29].
[29] For the further details on the nuclear spindle _vide_
the next Chapter.
The important result of Fol's observations on Heteropods, which
tallies also with what is found in Asterias, is that a spindle with
two stars at its poles is formed from the metamorphosis of the
germinal vesicle and surrounding protoplasm (fig. 25).
[FIG. 26. PORTION OF THE OVUM OF ASTERIAS GLACIALIS AT THE MOMENT OF
THE DETACHMENT OF THE FIRST POLAR BODY AND THE WITHDRAWAL OF THE
REMAINING PART OF THE SPINDLE WITHIN THE OVUM. PICRIC ACID
PREPARATION (copied from Fol).]
Polar cells. The spindle has up to this time been situated with
its axis parallel to the surface of the egg, but in somewhat older
specimens a vertical spindle is found, with one end projecting into a
protoplasmic prominence which makes its appearance on the surface of
the egg (fig. 26). Hertwig believes that the spindle simply travels
towards the surface, and while doing so changes the direction of its
axis. Fol asserts, however, that this is not the case, but that
between the two phases of the spindle an intermediate one is found in
which a spindle can no longer be seen in the egg, but its place is
taken by a body with a dentated outline. He has not been able to
arrive at a conclusion as to what meaning is to be attached to
this occurrence, which does not appear to take place in Heteropods.
[FIG. 27. PORTION OF THE OVUM OF ASTERIAS GLACIALIS, WITH THE FIRST
POLAR CELL AS IT APPEARS WHEN LIVING (copied from Fol).]
In any case the spindle which projects into the prominence on the
surface of the egg divides into two parts, one in the prominence and
one in the egg (fig. 26). The prominence itself with the enclosed
portion of the spindle becomes constricted off from the egg to form a
body, well known to embryologists as the polar body or cell (fig. 27).
Since more than one polar cell is formed, that which is the earliest
to appear may be called the first polar cell.
[FIG. 28. PORTION OF THE OVUM OF ASTERIAS GLACIALIS IMMEDIATELY
AFTER THE FORMATION OF THE SECOND POLAR CELL. PICRIC ACID
PREPARATION (copied from Fol).]
The part of the spindle which remains in the egg becomes directly
converted into a second spindle by the elongation of its fibres,
without passing through a typical nuclear condition. A second polar
cell next becomes formed in the same manner as the first (fig. 28),
and the portion of the spindle remaining in the egg becomes converted
into two or three clear vesicles (fig. 29), which soon unite to form a
single nucleus (fig. 30). The new nucleus which is clearly derived
from part of the original germinal vesicle is called the female
pronucleus, for reasons which will appear in the sequel.
[FIG. 29. PORTION OF THE OVUM OF ASTERIAS GLACIALIS AFTER THE
FORMATION OF THE SECOND POLAR CELL, SHEWING THE PART OF THE SPINDLE
REMAINING IN THE OVUM BECOMING CONVERTED INTO TWO CLEAR VESICLES.
PICRIC ACID PREPARATION (copied from Fol).]
The two polar cells appear to be situated between two membranes, the
outer of which is very delicate, and only distinct where it covers the
polar cells, while the inner one is thicker and becomes, after
impregnation, more distinct, and then forms what Fol speaks of as the
vitelline membrane. It is clear, as Hertwig has pointed out, that the
polar bodies originate by a regular process of cell division
and have the value of cells.
A peculiar phenomenon makes its appearance in the eggs of Clepsine
shortly after the formation of the polar cells, which has been spoken
of by Whitman (No. 100) as the formation of the polar rings. The
following is his description of the occurrence.
"Fifteen minutes after the elimination of the polar globules
(_i.e._ cells) a ring-like depression or constriction appears in
the yolk around the oral pole, and in this depression a transparent
liquid substance (nuclear?) is collected forming the first polar
ring.... The same phenomena repeat themselves later at the aboral
pole.... The rings concentrate to form two discs.... Before the first
cleavage both discs plunge deep into the egg."
The nature of these rings is at present quite obscure.
[FIG. 30. OVUM OF ASTERIAS GLACIALIS WITH THE TWO POLAR CELLS AND
THE FEMALE PRONUCLEUS SURROUNDED BY RADIAL STRIÆ, AS SEEN IN THE
LIVING EGG (copied from Fol).]
Considering how few ova have been adequately investigated with
reference to the behaviour of the germinal vesicle, any general
conclusions which may at present be formed are to be regarded as
provisional.
There is however abundant evidence that at the time of maturation of
the egg the germinal vesicle undergoes peculiar changes, which are, in
part at least, of a retrogressive character. These changes may begin
considerably before the egg has reached the period of maturity, or may
not take place till after it has been laid. They consist in an
appearance of irregularity and obscurity in the outline of the
germinal vesicle, the absorption of its membrane, the partial
absorption of its contents in the yolk, the disappearance of the
reticulum, and the breaking up and disappearance of the germinal spot.
The exact fate of the single germinal spot, or the numerous spots
where they are present, is still obscure.
The retrogressive metamorphosis of the germinal vesicle is followed in
a large number of instances by the conversion of what remains into a
striated spindle similar in character to a nucleus previous to
division. This spindle travels to the surface of the ovum and
undergoes division to form the polar cell or cells in the
manner above described. The part which remains in the egg forms
eventually the female pronucleus.
The germinal vesicle has up to the present time only been observed to
undergo the above series of changes in a certain number of instances,
which, however, include examples from several divisions of the
Coelenterata, the Echinodermata, and the Mollusca, some of the
Vermes [Turbellarians (_Leptoplana_), Nematodes, Hirudinea,
Alciope, Sagitta], Ascidians, etc. It is very possible, not to say
probable, that such changes are universal in the animal kingdom, but
the present state of our knowledge does not justify us in saying so.
In the Craniata especially our knowledge of the formation of the polar
bodies is very unsatisfactory. In Petromyzon Kupffer and Benecke have
brought forward evidence to shew that one polar body is formed prior
to the impregnation, and a second in connection with a peculiar
prominence of protoplasm after impregnation. Part of the germinal
vesicle remains in the egg as the female pronucleus. In the Sturgeon
the germinal vesicle atrophies and breaks up before impregnation, and
afterwards part is found as a granular mass on the surface of the egg,
while part forms a female pronucleus.
In Amphibia the observations of Hertwig (90) and Bambeke (77) tend to
shew that after the germinal vesicle has assumed a superficial
situation at the pigmented pole of the ovum its contents become
intermingled with the yolk, and are in part extruded from the ovum as
a granular mass after impregnation. Part of them remains in the ovum
and forms a female pronucleus. Whether there is a proper division of
the germinal vesicle as in typical cases is not known.
Oellacher (95) by a series of careful observations upon the egg of the
trout, and subsequently of the bird, demonstrated that in the ovum
while still in the ovary, the germinal vesicle underwent a kind of
degeneration and eventually became ejected, in part at any rate. My
own observations on Elasmobranchs, which require enlargement and
confirmation, tend to shew that this part may be the membrane. Ed. van
Beneden (78) has contributed some important observations on the
rabbit. His account is as follows. As the ovum approaches maturity the
germinal vesicle assumes an eccentric position, and fuses with the
peripheral layer of the egg to constitute the _cicatricular
lens_. The germinal spot next travels to the surface of the
cicatricular lens and forms the _nuclear disc_: at the same time
the membrane of the germinal vesicle vanishes, though it probably
unites with the nuclear disc. The plasma of the nucleus then collects
into a definite mass and forms the nucleoplasmic body. Finally the
nuclear disc assumes an ellipsoidal form and becomes the nuclear body.
Nothing is now left of the original germinal vesicle but the nuclear
body and the nucleoplasmic body, both still situated within the ovum.
In the next stage no trace of the germinal vesicle can be
detected in the ovum, but outside it, close to the point where the
modified remnants of the vesicle were previously situated, there is
present a polar body which is composed of two parts, one of which
stains deeply and resembles the nuclear body, and the other does not
stain but is similar to the nucleoplasmic body. Van Beneden concludes
that the parts of the polar body are the two ejected products of the
germinal vesicle. We may be perhaps permitted to hold that further
observations on this difficult object will demonstrate that part of
the germinal vesicle remains in the ovum to form the female
pronucleus.
With reference to invertebrate forms attention may be called to the
observations of Bütschli (80). Although in Cucullanus a normal
formation of the polar bodies takes place, yet in the Nematodes
generally, Bütschli has been unable to find the spindle modification
of the germinal vesicle, but states that the germinal vesicle
undergoes degeneration, its outline becoming indistinct and the
germinal spot vanishing. The position of the germinal vesicle
continues to be marked by a clear space, which gradually approaches
the surface of the egg. When it is in contact with the surface a small
spherical body, the remnant of the germinal vesicle, comes into view,
and eventually becomes ejected. The clear space subsequently
disappears.
In addition to the types just quoted, which may very probably turn out
to be normal in the mode of formation of the polar bodies, there is a
large number of types, including the whole of the Rotifera and
Arthropoda with a few doubtful exceptions[30], in which the polar
cells cannot as yet be said to have been satisfactorily observed.
[30] The best instance of what appears like a polar cell in
Arthropoda is a body recently found by Grobben
("Entwicklungsgeschichte d. Moina rectirostris." Claus'
_Arbeiten_, Vol. II., Wien, 1879) near the surface
of the protoplasm at the animal pole of the summer and
parthenogenetic eggs of _Moina rectirostris_, one of the
Cladocera. The body stains deeply with carmine, but differs from
normal polar cells in not being separated from the ovum; and its
identification as a polar cell must remain doubtful till it has
been shewn to originate from the germinal vesicle.
The more important of the doubtful cases amongst the Rotifera and
Arthropoda are the following.
Flemming (83) finds that in the summer and probably parthenogenetic
eggs of _Lacinularia socialis_ the germinal vesicle approaches
the surface and becomes invisible, and that subsequently a slight
indentation in the outline of the egg marks the point of its
disappearance. In the hollow of the indentation Flemming believes a
polar cell to be situated, though he has not definitely seen one.
Hoek[31] believes that he has found a polar body in the ovum of
_Balanus balanoides_, but his observations are not perfectly
satisfactory.
[31] "Zur Entwicklung d. Entomostraken." _Niederlandischer
Archiv. f. Zoologie_, Vol. III. p. 62.
Bütschli, who has expressly searched for the polar bodies in the ova
of Rotifera, was unable to find any trace of them, though he found
that as the egg became ripe the germinal vesicle became half its
original size. In the parthenogenetic eggs of Aphis he also failed to
find a trace of polar bodies, though the germinal vesicle, after the
germinal spot had broken up into fragments, approached the surface and
disappeared.
Whatever may be the eventual result of more extended investigation, it
is clear that the formation of polar cells according to the type
described above is a very constant occurrence. Its importance is
increased by the discovery by Strasburger of the existence of an
analogous process amongst plants. Two questions about it obviously
present themselves for solution: (1) What are the conditions of its
occurrence with reference to impregnation? (2) What meaning has it in
the development of the ovum or the embryo?
The answer to the first of these questions is not difficult to find.
The formation of the polar bodies is independent of impregnation, and
is the final act of the normal growth of the ovum. In a few types the
polar cells are formed while the ovum is still in the ovary, as, for
instance, in some species of Echini, Hydra, etc., but, according to
our present knowledge, far more usually after the ovum has been laid.
In some instances the budding-off of the polar cells precedes, and in
other instances follows impregnation; but there is no evidence to shew
that in the latter cases the process is influenced by the contact with
the male element. In Asterias, as has been shewn by O. Hertwig and
Fol, the formation of the polar cells may indifferently either precede
or follow impregnation--a fact which affords a clear demonstration of
the independence of the two occurrences.
To the second of the two questions it does not unfortunately seem
possible at present to give an answer which can be regarded as
satisfactory.
The retrogressive changes in the membrane of the germinal vesicle
which usher in the formation of the polar bodies may very probably be
viewed as a prelude to a renewed activity of the contents of the
vesicle; and are perhaps rendered the more necessary from the
thickness of the membrane which results from a protracted period of
passive growth. This suggestion does not, however, help us to explain
the formation of polar bodies by a process identical with cell
division. The ejection of part of the germinal vesicle in the
formation of the polar cells may probably be paralleled by the
ejection of part or the whole of the original nucleus which, if we may
trust the beautiful researches of Bütschli, takes place during
conjugation in Infusoria as a preliminary to the formation of a fresh
nucleus. This comparison is due to Bütschli, and according to it the
formation of the polar bodies would have to be regarded as assisting,
in some as yet unknown way, the process of regeneration of the
germinal vesicle. Views analogous to this are held by Strasburger and
Hertwig, who regard the formation of the polar bodies in the light of
a process of excretion or removal of useless material. Such hypotheses
do not, unfortunately, carry us very far.
I would suggest that in the formation of the polar cells part of the
constituents of the germinal vesicle, which are requisite for its
functions as a complete and independent nucleus, is removed, to make
room for the supply of the necessary parts to it again by the
spermatic nucleus.
My view amounts to the following, viz. that after the formation of the
polar cells the remainder of the germinal vesicle within the ovum (the
female pronucleus) is incapable of further development without the
addition of the nuclear part of the male element (spermatozoon), and
that if polar cells were not formed parthenogenesis might normally
occur. A strong support for this hypothesis would be afforded were it
to be definitely established that a polar body is not formed in the
Arthropoda and Rotifera; since the normal occurrence of
parthenogenesis is confined to these two groups. It is certainly a
remarkable coincidence that they are the only two groups in which
polar bodies have not so far been satisfactorily observed.
It is perhaps possible that the part removed in the formation of the
polar cells is not absolutely essential; and this seems at first sight
to follow from the fact of parthenogenesis being possible in instances
where impregnation is the normal occurrence. The genuineness of the
observations on this head is too long a subject to enter into
here[32], but after admitting, as we probably must, that there
are genuine cases of such parthenogenesis, it cannot be taken for
granted without more extended observation that the occurrence of
development in these rare instances may not be due to the polar cells
not having been formed as usual, and that when the polar cells are
formed the development without impregnation is impossible.
[32] The instances quoted by Siebold, _Parthenogenesis d.
Arthropoden_, are not quite satisfactory. In Hensen's case, p.
234, impregnation would have been possible if we can suppose the
spermatozoa to be capable of passing into the body cavity through
the open end of the uninjured oviduct; and though Oellacher's
instances are more valuable, yet sufficient care seems hardly to
have been taken, especially when it is not certain for what
length of time spermatozoa may be able to live in the oviduct.
For Oellacher's precautions, vide _Zeit. für Wiss. Zool._,
Bd. xxii., p. 202. A better instance is that of a sow given by
Bischoff, _Ann. Sci. Nat._, series 3, Vol. II.,
1844. The unimpregnated eggs were found divided into segments,
but the segments did not contain the usual nucleus, and were
perhaps nothing else than the parts of an ovum in a state of
disruption.
Selenka found in the case of _Purpura lapillus_ that no polar
body was formed in the eggs which did not develop, but in the case of
Neritina, Bütschli has found that this does not hold good.
The remarkable observations of Greeff (No. 88) on the parthenogenetic
development of the eggs of _Asterias rubens_ tell, however, very
strongly against the above hypothesis. Greeff has found that under
normal circumstances the eggs of this species of starfish will develop
without impregnation in simple sea-water. The development is quite
regular and normal, though much slower than in the case of impregnated
eggs. It is not definitely stated that polar cells are formed, but
there can be no doubt that this is implied. Greeff's account is so
precise and circumstantial that it is not easy to believe that any
error can have crept in; but neither Hertwig nor Fol have been able to
repeat his experiments, and we may be permitted to wait for further
confirmation before absolutely accepting them.
To the suggestion already made with reference to the function of the
polar cells, I will venture to add the further one, _that the
function of forming polar cells has been acquired by the ovum for the
express purpose of preventing parthenogenesis_.
The explanation given by Mr Darwin of the evil effects of
self-fertilization, viz. the want of sufficient differentiation in the
sexual elements[33], would apply with far greater force to cases of
parthenogenesis.
[33] Darwin, _Cross- and Self-Fertilization of Plants_, p. 443.
In the production of fresh individuals, two circumstances are
obviously favourable to the species. (1) That the maximum number
possible of fresh individuals should be produced, (2) That the
individuals should be as vigorous as possible. Sexual differentiation
(even in hermaphrodites) is clearly very inimical to the production of
the maximum number of individuals. There can be little doubt that the
ovum is potentially capable of developing _by itself_ into a
fresh individual, and therefore, unless _the absence_ of sexual
differentiation was very injurious to the vigour of the progeny,
parthenogenesis would most certainly be a very constant occurrence;
and, on the analogy of the arrangements in plants to prevent
self-fertilization, we might expect to find some contrivance both in
animals and in plants to prevent the ovum developing by itself
without fertilization. If my view about the polar cells is correct,
the formation of these bodies functions as such a contrivance.
Reproduction by budding or fission has probably arisen as a means of
increasing the number of individuals produced, so that the coexistence
of asexual with sexual reproduction is to be looked on as a kind of
compromise for the loss of the power of rapid reproduction due to the
absence of parthenogenesis. In the Arthropoda and Rotifera the place
of budding has been taken by parthenogenesis, which may be a frequent,
though not always a necessary occurrence, as in various Branchiopoda
(_Apus_, _Limnadia_, etc.) and Lepidoptera (_Psyche helix_, etc.); or
a regular occurrence for the production of one sex, as in Bees, Wasps,
Nematus, etc.; or an occurrence confined to a certain stage in the
cycle of development in which all the individuals reproduce their kind
parthenogenetically, as in Aphis, Cecidomyia, Gall Insects
(_Neuroterus_, etc.), Daphnia[34].
[34] Mr J. A. Osborne has recently shewn (_Nature_, Sept. 4,
1879), that the eggs of a Beetle (Gastrophysa raphani) may
occasionally develop, up to a certain point at any rate, without
the male influence.
On my hypothesis the possibility of parthenogenesis, or at any rate
its frequency, in Arthropoda and Rotifera is possibly due to the
absence of polar cells. In the case of all animals, so far as is known
to me, fertilization of the ovum occasionally occurs[35], but there
are instances in the vegetable kingdom where so-called parthenogenesis
appears to be capable of recurring for an indefinite period. One of
the best instances appears to be that of Coelebogyne, an introduced
exotic Euphorbiaceous plant which regularly produces fertile seeds
although a male flower never appears. The recent researches of
Strasburger have however shewn that in Coelebogyne and other
parthenogenetic flowering plants, embryos are formed by the
_budding_ and subsequent development of cells belonging to the
ovule. This being the case, it is impossible to assert of these plants
that they are really parthenogenetic, for the embryos contained in the
seed of a flower which has certainly not been fertilized, may have
been formed, _not by the development of the ovum_, but by budding
from the surrounding tissue of the ovule.
[35] Dicyema, which is an apparent exception, has not yet been
certainly shewn to develop true ova. If its germs are true ova it
forms an exception to the above rule.
The above view with reference to the nature of the polar bodies is not
to be regarded as forming more than an hypothesis.
_Impregnation of the Ovum._
A far greater amount of certainty has been attained as to the effects
of impregnation than as to the changes of the germinal vesicle which
precede this, and there appears, moreover, to be a greater uniformity
in the series of resulting phenomena.
[FIG. 31. SMALL PORTIONS OF THE OVUM OF ASTERIAS GLACIALIS. THE
SPERMATOZOA ARE SHEWN ENVELOPED IN THE MUCILAGINOUS COAT. IN A. A
PROMINENCE IS RISING FROM THE SURFACE OF THE EGG TOWARDS THE NEAREST
SPERMATOZOON; AND IN B. THE SPERMATOZOON AND PROMINENCE HAVE MET.
(Copied from Fol.)]
[FIG. 32. PORTION OF THE OVUM OF ASTERIAS GLACIALIS AFTER THE
ENTRANCE OF A SPERMATOZOON INTO THE OVUM. IT SHEWS THE PROMINENCE OF
THE OVUM THROUGH WHICH THE SPERMATOZOON HAS ENTERED. A VITELLINE
MEMBRANE WITH A CRATER-LIKE OPENING HAS BECOME DISTINCTLY FORMED.
(Copied from Fol.)]
It will be convenient again to take Asterias glacialis as the type.
The part of the germinal vesicle which remains in the egg, after the
formation of the second polar cell, becomes converted into a number of
small vesicles (fig. 29), which aggregate themselves into a single
clear nucleus, which gradually travels toward the centre of the egg
and around which, as a centre, the protoplasm becomes radiately
striated (fig. 30). This nucleus is known as the female pronucleus. By
the action of reagents a nucleolus may be shewn in it. In Asterias
glacialis the most favourable period for fecundation is about an hour
after the formation of the female pronucleus. If at this time the
spermatozoa are allowed to come in contact with the egg, their heads
soon become enveloped in the investing mucilaginous coat. A
prominence, pointing towards the nearest spermatozoon, now arises from
the superficial layer of protoplasm of the egg, and grows till it
comes in contact with the spermatozoon (fig. 31, A and B). Under
normal circumstances the spermatozoon which meets the prominence is
the only one concerned in the fertilization, and it makes its
way into the egg by passing through the prominence. The tail of the
spermatozoon, no longer motile, remains visible for some time after
the head has bored its way in, but its place is soon taken by a pale
conical body, which is, however, probably in part a product of the
metamorphosis of the tail itself (fig. 32). It eventually becomes
absorbed into the body of the ovum.
At the moment of contact between the spermatozoon and the egg the
outermost layer of the protoplasm of the latter raises itself as a
distinct membrane, which separates from the egg and prevents the
entrance of other spermatozoa. At the point where the spermatozoon
entered a crater-like opening is left in the membrane, through which
the metamorphosed tail of the spermatozoon may at first be seen
projecting (fig. 32).
[FIG. 33. OVUM OF ASTERIAS GLACIALIS, WITH MALE AND FEMALE
PRONUCLEUS AND A RADIAL STRIATION OF THE PROTOPLASM AROUND THE
FORMER. (Copied from Fol.)]
The head of the spermatozoon when in the egg forms a nucleus, for
which the name male pronucleus may be conveniently adopted. It grows
in size, probably by assimilating material from the ovum, and around
it is formed a clear space free from yolk-spherules. Shortly after its
formation the protoplasm in its neighbourhood assumes a radiate
arrangement (fig. 33). At whatever point of the egg the spermatozoon
may have entered, it gradually travels towards the female pronucleus.
The latter, around which the protoplasm no longer has a radiate
arrangement, remains motionless till the rays of the male pronucleus
come in contact with it, after which its condition of repose is
exchanged for one of activity, and it rapidly approaches the male
pronucleus, apparently by means of its inherent amoeboid
contractions, and eventually fuses with it (figs. 34-36).
As the male pronucleus approaches the female the latter, according to
Selenka, sends out protoplasmic processes which embrace the
former. The actual fusion does not take place till after the pronuclei
have been in contact for some time. While the two pronuclei are
approaching one another the protoplasm of the egg exhibits amoeboid
movements.
[FIGS. 34, 35, AND 36. THREE SUCCESSIVE STAGES IN THE COALESCENCE OF
THE MALE AND FEMALE PRONUCLEI IN ASTERIAS GLACIALIS. FROM THE LIVING
OVUM. (Copied from Fol.)]
The product of the fusion of the two pronuclei forms the first
segmentation nucleus (fig. 37), which soon, however, divides into the
two nuclei of the two first segmentation spheres.
[FIG. 37. OVUM OF ASTERIAS GLACIALIS, AFTER THE COALESCENCE OF THE
MALE AND FEMALE PRONUCLEI. (Copied from Fol.)]
The phenomenon which has just been described consists essentially in
the fusion of the male cell and the female cell. In this act the
protoplasm of the two cells as well as their nuclei coalesce, since
the whole spermatozoon which has been absorbed into the ovum is a cell
of which the head is the nucleus.
It is clear that the ovum after fertilization is an entirely different
body to the ovum prior to that act, and unless the use of the same
term for the two conditions of the ovum had become very familiar, a
special term, such as oosperm, for the ovum after its fusion with the
spermatozoon, would be very convenient.
Of the earlier observations on this subject there need perhaps only be
cited one of E. van Beneden, on the rabbit's ovum, shewing the
presence of two nuclei before the commencement of segmentation.
Bütschli was the earliest to state from observations on _Rhabditis
dolichura_ that the first segmentation nucleus arose from the
fusion of two nuclei, and this was subsequently shewn with greater
detail for _Ascaris nigrovenosa_, by Auerbach (76). Neither of
these authors gave at the first the correct interpretation of their
results. At a later period Bütschli (80) arrived at the conclusion
that in a large number of instances (_Lymnæus_, _Nephelis_,
_Cucullanus_, &c.), the nucleus in question was formed by the fusion
of two or more nuclei, and Strasburger at first made a similar
statement for _Phallusia_, though he has since withdrawn it. Though
Bütschli's statements depend, as it seems, upon a false interpretation
of appearances, he nevertheless arrived at a correct view with
reference to what occurs in impregnation. Van Beneden (78) described
in the rabbit the formation of the original segmentation nucleus from
two nuclei, one peripheral and the other central, and deduced from his
observations that the peripheral nucleus was derived from the
spermatic element. It was reserved for Oscar Hertwig (89) to describe
in _Echinus lividus_ the entrance of a spermatozoon into the egg and
the formation from it of the male pronucleus.
The general fact that impregnation consists in the fusion of the
spermatozoon and ovum has now been established for some forms in the
majority of invertebrate groups (Arthropoda and Rotifera excepted).
Amongst Vertebrata also it has been shewn by E. van Beneden that the
first segmentation nucleus is formed by the coalescence of the male
and female pronucleus. Calberla, and Kupffer and Benecke have
demonstrated that a single spermatozoon enters at first the ovum of
Petromyzon.
The contact of the spermatozoon with the egg membrane causes in
Petromyzon active movements of the protoplasm of the ovum, and a
retreat of the protoplasm from the membrane.
In Amphibia the appearance of a peculiar pigmented streak extending
inwards from the surface of the pigmented pole of the ovum, and
containing in a clear space at its inner extremity a nucleus, has been
demonstrated as the result of impregnation by Bambeke (77) and Hertwig
(90). There can be little doubt that this nucleus is the male
pronucleus, and that the pigmented streak indicates its path inwards.
Close to it Hertwig has shewn that another nucleus is to be found, the
female pronucleus, and that eventually the two join together. In
Amphibia the phenomena accompanying impregnation are clearly of the
same nature as in the Invertebrata. A precisely similar series of
phenomena to those in Amphibia has been shewn by Salensky to take
place in the Sturgeon.
Although there is a general agreement between the most recent
observers, Hertwig, Fol, Selenka, Strasburger, &c., as to the main
facts connected with the entrance of one spermatozoon into the egg,
the formation of the male pronucleus, and its fusion with the female
pronucleus, there still exist differences of detail in the different
descriptions, which partly, no doubt, depend upon the difficulties of
observation, but partly also upon the observations not having all been
made upon the same species. Hertwig does not enter into details with
reference to the actual entrance of the spermatozoon into the egg, but
in his latest paper points out that considerable differences may be
observed in the occurrences which succeed impregnation, according to
the relative period at which this takes place. When, in Asterias, the
impregnation is effected about an hour after the egg is laid, and
previously to the formation of the polar cells, the male pronucleus
appears at first to exert but little influence on the protoplasm, but
after the formation of the second polar cell, the radial striæ around
it become very marked, and the pronucleus rapidly grows in size. When
it finally unites with the female pronucleus it is equal in size to
the latter. In the case when the impregnation is deferred for four
hours the male pronucleus never becomes so large as the female
pronucleus. With reference to the effect of the time at which
impregnation takes place, Asterias would seem to serve as a type. Thus
in _Hirudinea_, _Mollusca_, and _Nematoidea_ impregnation normally
takes place before the formation of the polar bodies is completed, and
the male pronucleus is accordingly as large as the female. In
_Echinus_, on the other hand, where the polar bodies are formed in the
ovary, the male pronucleus is always small.
Selenka, who has investigated the formation of the male pronucleus in
_Toxopneustes variegatus_, differs in certain points from Fol. He
finds that usually, though not always, a single spermatozoon enters
the egg, and that though the entrance may be effected at any part of
the surface it generally occurs at the point marked by a small
prominence where the polar cells are formed. The spermatozoon first
makes its way through the mucous envelope of the egg, within which it
swims about, and then bores with its head into the polar prominence.
One important point has been so far only indirectly alluded to, viz.
the number of spermatozoa required to effect impregnation.
The concurrent testimony of almost all observers tends to shew that
one only is required for this purpose. But the number of cases tested
is too small to admit of satisfactory generalization.
Both Hertwig and Fol have made observations on the result of the
entrance into the egg of several spermatozoa. Fol finds that when the
impregnation has been too long delayed the vitelline membrane is
formed with comparative slowness, and several spermatozoa are thus
enabled to penetrate. Each spermatozoon forms a separate pronucleus
with a surrounding star; and several male pronuclei usually fuse with
the female pronucleus. Each male pronucleus appears to exercise a
repulsive influence on other male pronuclei, but to be attracted by
the female pronucleus. When there are several male pronuclei the
segmentation is irregular and the resulting larva a monstrosity. These
statements of Fol and Hertwig are up to a certain point in
contradiction with the more recent results of Selenka. In
_Toxopneustes variegatus_ Selenka finds that though impregnation is
usually effected by a single spermatozoon yet several may be concerned
in the act. The development continues, however, to be normal up to the
gastrula stage, at any rate, if three or even four spermatozoa enter
the egg almost simultaneously. Under such circumstances each
spermatozoon forms a separate pronucleus and star. Selenka is of
opinion (apparently rather on _a priori_ grounds than as a result of
direct observation) that normal development cannot occur when more
than one male pronucleus fuses with the female pronucleus; and holds
that, where he has observed such normal development after the entrance
of more than one spermatozoon, the majority of male pronuclei become
absorbed.
It may be noticed that, while the observations of Fol and Hertwig were
admittedly made upon eggs in which the impregnation was delayed till
they no longer displayed their pristine activity, Selenka's were made
upon quite fresh eggs; and it seems not impossible that the
pathological symptoms in the embryos reared by the two former authors
may have been due to the imperfection of the egg, and not to the
entrance of more than one spermatozoon. This, of course, is merely a
suggestion which requires to be tested by fresh observations.
Kupffer and Benecke have further shewn that although only one
spermatozoon enters the ovum directly in Petromyzon yet other
spermatozoa pass through the vitelline membrane, and are taken into a
peculiar protoplasmic protuberance of the ovum which appears after
impregnation.
The act of impregnation may be described as the fusion of the ovum and
spermatozoon, and the most important feature in this act appears to be
the fusion of a male and female nucleus; not only does this appear in
the actual fusion of the two pronuclei, but it is brought into still
greater prominence by the fact that the female pronucleus is a product
of the nucleus of a primitive ovum, and the male pronucleus is the
metamorphosed _head_ of the spermatozoon which, as stated above,
contains _part_ of the nucleus of the primitive spermatic cell. The
spermatic cells originate from cells indistinguishable from the
primitive ova, so that the fusion which takes place is the fusion of
morphologically similar parts in the two sexes.
These conclusions tally very satisfactorily with the view adopted in
the Introduction, that impregnation amongst the Metazoa was derived
from the process of conjugation amongst the Protozoa.
_Summary._
In what may probably be regarded as a normal case the following series
of events accompanies the maturation and impregnation of an ovum:--
(1) Transportation of the germinal vesicle to the surface of the egg.
(2) Absorption of the membrane of the germinal vesicle and
metamorphosis of the germinal spot and nuclear reticulum.
(3) Assumption of a spindle character by the remains of the germinal
vesicle, these remains being probably in part formed from the germinal
spot.
(4) Entrance of one end of the spindle into a protoplasmic prominence
at the surface of the egg.
(5) Division of the spindle into two halves, one remaining in the egg,
the other in the prominence; the prominence becoming at the same time
nearly constricted off from the egg as a polar cell.
(6) Formation of a second polar cell in the same manner as the first,
part of the spindle still remaining in the egg.
(7) Conversion of the part of the spindle remaining in the egg into a
nucleus--the female pronucleus.
(8) Transportation of the female pronucleus towards the centre of the
egg.
(9) Entrance of one spermatozoon into the egg.
(10) Conversion of the head of the spermatozoon into a nucleus--the
male pronucleus.
(11) Appearance of radial striæ round the male pronucleus, which
gradually travels towards the female pronucleus.
(12) Fusion of male and female pronuclei to form the first
segmentation nucleus.
(76) Auerbach. _Organologische Studien_, Heft 2. Breslau, 1874.
(77) Bambeke. "Recherches s. Embryologie des Batraciens." _Bull. de
l'Acad. royale de Belgique_, 2me Sér., T. LXI., 1876.
(78) E. van Beneden. "La Maturation de l'OEuf des Mammifères."
_Bull. de l'Acad. royale de Belgique_, 2me Sér., T. XL. No. 12, 1875.
(79) Idem. "Contributions à l'Histoire de la Vésicule Germinative,
&c." _Bull. de l'Acad. royale de Belgique_, 2me Sér., T. XLI. No. 1,
1876.
(80) O. Bütschli. _Eizelle, Zelltheilung, und Conjugation der
Infusorien._ Frankfurt, 1876.
(81) F. M. Balfour. "On the Phenomena accompanying the Maturation and
Impregnation of the Ovum." _Quart. J. of Micros. Science_, Vol.
XVIII., 1878.
(82) Calberla. "Befruchtungsvorgang beim Ei von Petromyzon Planeri."
_Zeit. f. wiss. Zool._, Vol. XXX.
(83) W. Flemming. "Studien in d. Entwickelungsgeschichte der Najaden."
_Sitz. d. k. Akad. Wien_, B. LXXI., 1875.
(84) H. Fol. "Die erste Entwickelung des Geryonideneies." _Jenaische
Zeitschrift_, Vol. VII., 1873.
(85) Idem. "Sur le Développement des Ptéropodes." _Archives de
Zoologie Expérimentale et Générale_, Vol. IV. and V., 1875-6.
(86) Idem. "Sur le Commencement de l'Hénogénie." _Archives des
Sciences Physiques et Naturelles._ Genève, 1877.
(87) Idem. _Recherches s. l. Fécondation et l. commen. d.
l'Hénogénie._ Genève, 1879.
(88) R. Greeff. "Ueb. d. Bau u. d. Entwickelung d. Echinodermen."
_Sitzun. der Gesellschaft z. Beförderung d. gesammten Naturwiss. z.
Marburg_, No. 5, 1876.
(89) Oscar Hertwig. "Beit. z. Kenntniss d. Bildung, &c., d. thier.
Eies." _Morphologisches Jahrbuch_, Vol. I., 1876.
(90) Idem. Ibid. _Morphologisches Jahrbuch_, Vol. III. Heft 1, 1877.
(91) Idem. "Weitere Beiträge, &c." _Morphologisches Jahrbuch_, Vol.
III., 1877, Heft 3.
(92) Idem. "Beit. z. Kenntniss, &c." _Morphologisches Jahrbuch_, Vol.
IV. Heft 1 and 2, 1878.
(93) N. Kleinenberg. _Hydra._ Leipzig, 1872.
(94) C. Kupffer u. B. Benecke. _Der Vorgang d. Befruchtung am Eie d.
Neunaugen._ Königsberg, 1878.
(95) J. Oellacher. "Beiträge zur Geschichte des Keimbläschens im
Wirbelthiere." _Archiv f. mikr. Anat._, Bd. VIII., 1872.
(96) W. Salensky. "Befruchtung u. Furchung d. Sterlets-Eies."
_Zoologischer Anzeiger_, No. 11, 1878.
(97) E. Selenka. _Befruchtung des Eies von Toxopneustes variegatus._
Leipzig, 1878.
(98) Strasburger. _Ueber Zellbildung u. Zelltheilung._ Jena, 1876.
(99) Idem. _Ueber Befruchtung u. Zelltheilung._ Jena, 1878.
(100) C. O. Whitman. "The Embryology of Clepsine." _Quart. J. of Micr.
Science_, Vol. XVIII., 1878.
CHAPTER III.
THE SEGMENTATION OF THE OVUM.
The immediate result of the fusion of the male and female pronucleus
is the segmentation or division of the ovum usually into two, four,
eight, etc. successive parts. The segmentation may be dealt with from
two points of view, viz. (1) the nature of the vital phenomena which
take place in the ovum during its occurrence, which may be described
as the internal phenomena of segmentation. (2) The external characters
of the segmentation.
_Internal Phenomena of Segmentation._
Numerous descriptions have been given during the last few years of the
internal phenomena of segmentation. The most recent contribution on
this head is that of Fol (No. 87). He appears to have been more
successful than other observers in obtaining a complete history of the
changes which take place, and it will therefore be convenient to take
as type the ovum of _Toxopneustes (Echinus) lividus_, on which he
made his most complete series of observations. The changes which take
place may be divided into a series of stages. The ovum immediately
after the fusion of the male and female pronucleus contains a central
segmentation nucleus.
In the first stage a clear protoplasmic layer derived from the plasma
of the cell is formed round the nucleus, from which there start
outwards a number of radial striæ, which are rendered conspicuous by
the radial arrangement of the yolk granules between them. The
nucleus during this process remains perfectly passive.
In the second stage the nucleus becomes less distinct and somewhat
elongated, and around it the protoplasmic layer of the earlier stage
is arranged in the form of a disc-shaped ring, compared by Fol to
Saturn's ring. The protoplasmic rays still take their origin from the
perinuclear protoplasm. This stage has a considerable duration (20
minutes).
In the third stage the protoplasm around the nucleus becomes
transported to the two nuclear poles, at each of which it forms a
clear mass surrounded by a star-shaped figure formed by radial striæ.
The nucleus is hardly visible in the fresh condition, but when brought
into view by reagents is found to contain many highly refractive
particles, and to be still enveloped in a membrane.
In the fourth stage the nucleus when treated by reagents has assumed
the well-known spindle form. The striæ of which it is composed are
continuous from one end of the spindle to the other and are thickened
at the centre. The central thickenings constitute the so-called
nuclear plate. The clear protoplasmic masses and stars are present as
before at the apices of the nucleus, and the rays of the latter
converge as if they would meet at the centre of the clear masses, but
stop short at their periphery. There is no trace of a membrane round
either the nuclear spindle or the clear masses; and in the centre of
the latter is a collection of granules. The striæ of the polar stars
are very fine but distinct.
Between the stage with a completely formed spindle and the previous
one the intermediate steps have not been made out for Toxopneustes;
but for Heteropods Fol has been able to demonstrate that the striæ of
the spindle and their central thickenings are formed, as in the case
of the spindle derived from the germinal vesicle, _from the
metamorphosis of the nuclear reticulum_. They commence to be formed
at the two poles, and are then (in Heteropods) in immediate contiguity
with the striæ of the stars. The striæ gradually grow towards the
centre of the nucleus and there meet.
In the fifth stage the central thickenings of the spindle separate
into two sets, which travel symmetrically outwards towards the
clear masses, growing in size during the process. They remain however
united for a short time by delicate filaments--named by Fol connective
filaments--which very soon disappear. The clear masses also increase
in size. During this stage the protoplasm of the ovum exhibits active
amoeboid movements preparatory to division.
In the sixth stage, which commences when the central thickenings of
the spindle have reached the clear polar masses, the division of the
ovum into two parts is effected by an equatorial constriction at right
angles to the long axis of the nucleus. The inner vitelline membrane
follows the furrow for a certain distance, but does not divide with
the ovum. All connection between the two parts of the spindle becomes
lost during this stage, and the thickenings of the fibres of the
spindle give rise to a number of spherical vesicular bodies, which
pass into the clear masses and become intermingled with the granules
which are placed there. The radii of the stars now extend round the
whole circumference of each of the clear masses.
In the seventh stage the two clear masses become elongated and travel
towards the outer sides of their segments; while the radii connected
with them become somewhat bent, as if a certain amount of traction had
been exercised on them in the movement of the clear masses. Shortly
afterwards the spherical vesicles, each of which appears like a small
nucleus and contains a central nucleolus, begin to unite amongst
themselves, and to coalesce with the neighbouring granules. Those in
each segment finally unite to form a nucleus which absorbs the
substance of the clear mass. _The new nucleus is therefore partly
derived from the division of the old one and partly from the plasma of
the cell._ The two segments formed by division are at first
spherical, but soon become flattened against each other. In each
subsequent division of these cells the whole of the above changes are
repeated.
The phenomena which have just been described would appear to occur in
the segmentation of ova with remarkable constancy and without any very
considerable variations.
The division of the ovum constitutes a special case of cell division,
and it is important to determine to what extent the phenomena of
ordinary cell division are related to those which take place in the
division of the ovum. Without attempting a full discussion of
the subject I will confine myself to a few remarks suggested by the
observations of Flemming, Peremeschko and Klein. The observations of
these authors shew that in the course of the division of nuclei in the
salamander, newt, etc. the nuclear reticulum undergoes a series of
peculiar changes of form, and after the membrane of the nucleus has
vanished divides into two masses. The masses form the basis for the
new nuclei, and become reconverted into an ordinary nuclear reticulum
after repeating, in the reverse order, the changes of form undergone
by the reticulum previous to its division.
It is clear without further explanation that the conversion of the
nuclear reticulum of the segmentation nucleus into the striæ of the
spindle is a special case of the same phenomenon as that first
described by Flemming in the salamander. There are however some
considerable differences. In the first place the fibres in the
salamander do not, according to Flemming, unite in the middle line,
though they appear to do so in the newt. This clearly cannot be
regarded as a fact of great importance; nor can the existence of the
central thickenings of the striæ (nuclear plate), constant as it is
for the division of the nucleus of the ovum, be considered as
constituting a fundamental difference between the two cases. More
important is the fact that the striæ in the case of the ovum do not
appear, at any rate have not been shewn, to form themselves again into
a nuclear network.
With reference to the last point it is however to be borne in mind (1)
that the gradual travelling outwards of the two halves of the nuclear
plate is up to a certain point a repetition, in the reverse order, of
the mode of formation of the striæ of the spindle, since the striæ
first appeared at the poles and gradually grew towards the middle of
the spindle; (2) that there is still considerable doubt as to how the
vesicular bodies formed out of the nuclear plate reconstitute
themselves into a nucleus.
The layer of clear protoplasm around the nucleus during its division
has its homologue in the case of the division of the nuclei of the
salamander, and the rays starting from this are also found. Klein has
suggested that the extra-nuclear rays of the stars around the poles of
the nucleus are derived from a metamorphosis of the extra-nuclear
reticulum, which he believes to be continuous with the intra-nuclear
reticulum.
The delicate connective filaments usually visible between the two
halves of the nuclear plate would seem from Strasburger's latest
observations (No. 104) to be derived from the nuclear substance
between the striæ of the spindle, and to become eventually reabsorbed
into the newly-formed nuclei.
We are it appears to me still in complete ignorance as to the physical
causes of segmentation. The view that the nucleus is a single centre
of attraction, and that by its division the centre of attraction
becomes double and thereby causes division, appears to be quite
untenable. The description already given of the phenomena of
segmentation is in itself sufficient to refute this view. Nor
is it in the least proved by the fact (shewn by Hallez) that the plane
of division of the cell always bears a definite relation to the
direction of the axis of the nucleus.
The arguments by which Kleinenberg (93) attempted to demonstrate that
cell division was a phenomenon caused by alterations in the molecular
cohesion of the protoplasm of the ovum still in my opinion hold good,
but recent discoveries as to the changes which take place in the
nucleus during division probably indicate that the molecular changes
which take place in the cohesion of the protoplasm are closely related
to, and possibly caused by, those in the nucleus. These alterations of
cohesion are produced by a series of molecular changes, the external
indications of which are to be found in the visible alterations in the
constitution of the body of the cell and of the nucleus prior to
division.
BIBLIOGRAPHY.
In addition to the papers cited in the last Chapter, _vide_
(101) W. Flemming. "Beiträge z. Kenntniss d. Zelle u. ihrer
Lebenserscheinungen." _Archiv f. mikr. Anat._, Vol. XVI., 1878.
(102) E. Klein. "Observations on the glandular epithelium and division
of nuclei in the skin of the Newt." _Quart. J. of Micr. Science_, Vol.
XIX., 1879.
(103) Peremeschko. "Ueber d. Theilung d. thierischen Zellen." _Archiv
f. mikr. Anat._, Vol. XVI., 1878.
(104) E. Strasburger. "Ueber ein z. Demonstration geeignetes
Zelltheilungs-Object." _Sitz. d. Jenaischen Gesell. f. Med. u.
Naturwiss._, July 18, 1879.
_External Features of Segmentation._
[FIG. 38. VARIOUS STAGES IN PROCESS OF SEGMENTATION. (After
Gegenbaur.)]
In the simplest known type of segmentation the ovum first of all
divides into two, then four, eight, sixteen, thirty-two, sixty-four,
etc. cells (fig. 38). These cells so long as they are fairly large are
usually known as segments or spheres. At the close of such a
simple segmentation the ovum becomes converted into a sphere composed
of segments of a uniform size. These segments usually form a wall
(fig. 39, E), one row of cells thick, round a central cavity, which is
known as the segmentation cavity or cavity of Von Baer. Such a sphere
is known as a blastosphere. The central cavity usually appears very
early in the segmentation, in many cases when only four segments are
present (fig. 39, B).
[FIG. 39. THE SEGMENTATION OF AMPHIOXUX. (Copied from Kowalevsky.)
_sg._ segmentation cavity. A. Stage with two equal segments. B.
Stage with four equal segments. C. Stage after the four segments
have become divided by an equatorial furrow into eight equal
segments. D. Stage in which a single layer of cells encloses a
central segmentation cavity. E. Somewhat older stage in optical
section.]
In other instances, which however are rarer than those in which a
segmentation cavity is present, there is no trace of a central cavity,
and the sphere at the close of segmentation is quite solid. In such
instances the solid sphere is known as a morula. It is found in some
Sponges, many Coelenterata, some Nemertines, etc., and in Mammals;
in which group the segmentation is not however quite regular. All
intermediate conditions between a large segmentation cavity, and a
very minute central cavity which may be surrounded by more than a
single row of cells have been described.
The segmentation cavity has occasionally, as in Sycandra, the
Ctenophora and Amphioxus, the form of an axial perforation of the ovum
open at both extremities.
When the process of regular segmentation is examined somewhat more in
detail it is found to follow as a rule a rather definite rhythm. The
ovum is first divided in a plane which may be called vertical, into
two equal parts (fig. 39, A). This division is followed by a second,
also in a vertical plane, but at right angles to the first plane, and
by it each of the previous segments is halved (fig. 39, B.) In the
third segmentation the plane of division is horizontal or equatorial
and divides each of the four segments into two halves, making eight
segments in all (fig. 39, C). In the fourth period the segmentation
takes place in two vertical planes each at an angle of 45° with one of
the previous vertical planes. All the segments are thus again divided
into two equal parts. In the fifth period there are two equatorial
planes one on each side of the original equatorial plane, and
thirty-two spheres are present at the close of this period. Sixty-four
segments are formed at the sixth period, but beyond the fourth and
fifth periods the original regularity is not usually preserved.
In many instances the type of segmentation just described cannot be
distinctly recognized. All that can be noticed is that at each fresh
segmentation every segment becomes divided into two equal parts. It is
not absolutely certain that there is not always some slight inequality
in the segments formed, by which, what are known as the animal and
vegetative poles of the ovum, can very early be distinguished. A
regular segmentation is found in species in most groups of the animal
kingdom. It is very common in Sponges and Coelenterates. Though less
common so far as is known amongst the Vermes, it is yet found in many
of the lower types, viz. Nematoidea, Gordiacea, Trematoda, Nemertea
(apparently as a rule), _Sagitta_, _Chætonotus_, some Gephyrea
(_Phoronis_); though not usual it occurs amongst Chætopoda, E.G.
SERPULA. It is the usual type of segmentation amongst the
Echinodermata. Amongst the Crustacea it appears (for the earlier
phases of segmentation at any rate) not infrequently amongst the lower
forms, and even occurs amongst the Amphipoda (_Phronima_). It is
however very rare amongst the Tracheata, _Podura_ affording the one
example of it known to me. It is almost as rare amongst Mollusca as
amongst the Tracheata, but occurs in _Chiton_ and is nearly approached
in some Nudibranchiata. In Vertebrata it is most nearly approached in
_Amphioxus_[36].
[36] In the Rabbit and probably other Monodelphous Mammalia the
segmentation is nearly though not quite regular.
Most of the eggs which have a perfectly regular segmentation are of a
very insignificant size and rarely contain much food-yolk: in the vast
majority of eggs there is present however a considerable bulk of food
material usually in the form of highly refracting yolk-spherules.
These yolk-spherules lie embedded in the protoplasm of the ovum, but
are in most instances not distributed uniformly, being less closely
packed and smaller at one pole of the ovum than elsewhere. Where the
yolk-spherules are fewest the active protoplasm is necessarily most
concentrated, and we can lay down as a general law[37] that the
velocity of segmentation in any part of the ovum is roughly speaking
proportional to the concentration of the protoplasm there; and that
the size of the segments is inversely proportional to the
concentration of the protoplasm. Thus the segments produced from that
part of an egg where the yolk-spherules are most bulky, and where
therefore the protoplasm is least concentrated, are larger than the
remaining segments, and their formation proceeds more slowly.
[37] _Vide_ F. M. Balfour, "Comparison of the early stages
of development in Vertebrates." _Quart. Jour. of Micr.
Science_, July, 1875.
Though where much food-yolk is present it is generally distributed
unequally, yet there are many cases in which it is not possible to
notice this very distinctly. In most of these cases the segmentation
is all the same unequal, and it is probable that they form apparent
rather than real exceptions to the law laid down above. Although
before segmentation the protoplasm may be uniformly distributed, yet
in many instances, _e.g._ Mollusca, Vermes, etc., during or at
the commencement of segmentation the protoplasm becomes aggregated at
one pole, and one of the segments formed consists of clear protoplasm,
all the food-yolk being contained in the other and larger segment.
Unequal Segmentation. The type of segmentation I now proceed to
describe has been called by Haeckel (No. 105) 'unequal segmentation',
a term which may conveniently be adopted. I commence by describing it
as it occurs in the well-known and typical instance of the Frog[38].
[38] _Vide_ Remak, _Entwicklung d. Wirbelthiere_; and Götte,
_Entwicklung d. Unke_.
The ripe ovum of the common Frog and of most other tailless Amphibians
presents the following structure. One half appears black and the other
white. The former I shall call the upper pole, the latter the
lower. The ovum is composed of protoplasm containing in suspension
numerous yolk-spherules. The largest of these are situated at the
lower pole, the smaller ones at the upper pole, and the smallest of
all in the peripheral layer of the upper pole, in which also pigment
is scattered and causes the black colour visible from the surface.
[FIG. 40. SEGMENTATION OF COMMON FROG. RANA TEMPORARIA. (Copied from
Ecker.)
The numbers above the figures refer to the number of segments at the
stage figured.]
The first formed furrow is a vertical furrow. It commences in the
upper half of the ovum, through which it extends rapidly, and then
more slowly through the lower. As soon as the first furrow has
extended through the egg, and the two halves have become separated
from each other, a second vertical furrow appears at right angles to
the first and behaves in the same way (fig. 40, 4).
The next furrow is equatorial or horizontal (fig. 40, 8). It does not
arise _at the true equator of the egg_, but much nearer to its upper
pole. It extends rapidly round the egg and divides each of the four
previous segments into two parts, one larger and one smaller. Thus at
the end of this stage there are present four small and four large
segments. At the meeting point of these a small cavity appears, which
is the segmentation cavity, already described for uniformly segmenting
eggs. It increases in size in subsequent stages, its roof being formed
of the smaller cells and its floor of the larger. The appearance of
the equatorial furrow is followed by a period of repose, after which
two rapidly succeeding vertical furrows are formed in the upper pole,
dividing each of the four segments of which this is composed into two.
After a short period these furrows extend to the lower pole, and when
completed 16 segments are present--eight larger and eight smaller
(fig. 40, 16). A pause now ensues, after which the eight upper
segments become divided by an equatorial furrow, and somewhat later a
similar furrow divides the eight lower segments. At the end of this
stage there are therefore present 16 smaller and 16 larger segments
(fig. 40, 32). After 64 segments have been formed by vertical furrows
which arise symmetrically in the two poles (fig. 40, 64), two
equatorial furrows appear in the upper pole before a fresh furrow
arises in the lower; so that there are 128 segments in the upper half,
and only 32 in the lower. The regularity is quite lost in subsequent
stages, but the upper pole continues to undergo a more rapid
segmentation than the lower. While the segments have been increasing
in number the segmentation cavity has been rapidly growing in size;
and at the close of segmentation the egg forms a sphere, containing an
excentric cavity, and composed of two unequal parts (fig. 41). The
upper part, which forms the roof of the segmentation cavity, is formed
of smaller cells: the lower of larger yolk-containing cells.
[FIG. 41. SECTION THROUGH FROG'S OVUM AT THE CLOSE OF SEGMENTATION.
_sg._ segmentation cavity. _ll._ large yolk-containing cells. _ep._
small cells at formative pole (epiblast).]
The mode of segmentation of the Frog's ovum is typical for unequally
segmenting ova, and it deserves to be noticed that as regards the
first three or more furrows the segmentation occurs with the same
rhythm in the unequally segmenting ova as in those which have an
uniform segmentation. There appear two vertical furrows followed by an
equatorial furrow. The general laws which were stated with reference
to the _velocity_ of segmentation and the size of the resulting
segments are well exemplified in the case of the Frog's ovum.
The majority of the smaller segments in the segmented Frog's ovum are
destined to form into the epiblast, and the larger segments become
hypoblast and mesoblast.
With a few exceptions (the Rabbit, Lymnæus, etc.) the majority of the
smaller segments always become epiblast and of the larger segments
hypoblast.
The Frog's ovum serves as a good medium type for unequally segmenting
ova. There are many cases however in which a regular segmentation is
far more closely approached, and others in which it is less so.
One familiar instance in which a regular segmentation is nearly
approached is afforded by the Rabbit's ovum, which has indeed usually
been regarded as offering an example of a regular segmentation.
The ovum of the Rabbit[39] becomes first divided into two sub-equal
spheres. The larger and more transparent of the two may, from its
eventual fate, be called the epiblastic sphere, and the other the
hypoblastic. The two spheres are divided into four, and then by an
equatorial furrow into eight--four epiblastic and four hypoblastic.
One of the latter assumes a central position. The four epiblastic
spheres now divide before the four hypoblastic. There is thus
introduced a stage with twelve spheres. It is followed by one with
sixteen, and that by one with twenty-four. During the stages with
sixteen spheres and onwards the epiblastic spheres gradually envelop
the hypoblastic, which remain exposed on the surface at one point
only. There is no segmentation cavity.
[39] Van Beneden, "Développement embryonnaire des Mammifères."
_Bull. de l'Acad. Belgique_, 1874.
In Pedicellina, one of the entoproctous Polyzoa, there is a
sub-regular segmentation, where however the two primary spheres can be
distinguished much in the same way as in the case of the Rabbit.
A very characteristic type of unequal segmentation is that presented
by the majority of Gasteropods and Pteropods and probably also of some
Lamellibranchiata. It is also found in some Turbellarians, in
Bonellia, some Annelids, etc. In many instances it offers a good
example of the type where in the course of segmentation the protoplasm
becomes aggregated at one pole of the ovum, or of its segments, to
become separated off as a clear sphere.
The first four segments formed by two vertical furrows at
right angles are equal, but from these there are budded off four
smaller segments, which in subsequent stages divide rapidly, receiving
however, a continual accession of segments budded off from the larger
spheres. The four larger spheres remain conspicuous till near the
close of the segmentation. The process of budding, by which the
smaller spheres become separated from the larger, consists in a larger
sphere throwing out a prominence, which then becomes constricted off
from it.
In the extreme forms of this unequal segmentation we find at the end
of the second cleavage two larger spheres filled with yolk material
and two smaller clear spheres; and in the later stages, though the
large spheres continue to bud off small spheres, only the two smaller
ones undergo a regular segmentation, and eventually completely envelop
the former. Such a case as this has been described in Aplysia by
Lankester[40].
[40] _Phil. Trans._ 1875.
The types I have described serve to exemplify unequal segmentation.
The Rabbit's ovum stands at one end of the series, that of Aplysia at
the other; and the Frog's ovum between the two.
Great variations are presented by the ova with unequal segmentation as
to the presence of a segmentation cavity. In some instances,
_e.g._ the Frog, such a cavity is well developed. In other cases
it is small, _e.g._ most Mollusca, while not unfrequently it is
altogether absent.
Before leaving this important type of segmentation, it will be well to
enter with slightly greater detail into some of the more typical as
well as some of the special forms which it presents.
As an example of the typical Molluscan type the normal Heteropod
segmentation, accurately described by Fol[41], may be selected.
[41] Fol, _Archives de Zoologie Expérimentale_, Vol. IV.
1875.
The ovum divides into two and then four equal segments in the usual
vertical planes. Each segment has a protoplasmic and a vitelline pole.
The protoplasmic pole is turned towards the polar bodies. In the third
segmentation, which takes place along an equatorial plane, four small
protoplasmic cells or segments are segmented or rather budded off from
the four large segments, so that there are four small segments in one
plane and four large below these. In the fourth segmentation the four
large segments alone are active and give rise to four small and four
large cells; so that there are formed in all eight small and four
large cells. The four small cells of the third generation next
divide, forming in all twelve small cells and four large. The small
cells of the fourth generation then divide, and subsequently the four
large cells give rise to four new small ones, so that there are twenty
small cells and four large. The small cells form a cap embracing the
upper pole of the large segments. It may be noted that from the third
stage onwards the cells increase in arithmetical progression--a
characteristic feature of the typical gasteropod segmentation.
In the later stages of segmentation the large cells cease to give rise
to smaller ones in the same manner as before. One of them divides
first into two unequal parts, of which the smaller becomes pushed in
towards the centre of the egg. The larger cell then divides again into
two, and the three cells so formed occupy the centre of a shallow
depression. The remaining larger cells divide in the same way, and
give rise to smaller cells which line a pit which becomes formed on
one side of the ovum. The original smaller cells continue in the
meantime to divide and so form a layer enclosing the larger, leaving
exposed however the opening of the pit lined by the latest products of
the larger cells.
The eggs of Anodon and Unio serve as excellent examples of the type in
which the ovum has a uniform structure before the commencement of
segmentation, but in which a separation into a protoplasmic and a
nutritive portion becomes obvious during segmentation.
In Anodon[42] the egg is at first uniformly granular, but after
impregnation it throws out on one side a protuberance nearly free from
granules (fig. 42, 1)
[42] Flemming, "Entwick. der Najaden," _Sitz. d. Akad. Wiss.
Wien_, Bd. 4, 1875.
[FIG. 42. SEGMENTATION OF ANODON PISCINALIS. (Copied from Flemming.)
_r._ polar cells. _v._ vitelline sphere. 1. Commencing division into
two segments; one mainly formed of protoplasm, the other of yolk. 2.
Stage with four segments. 3. Formation of blastosphere, and
segmentation cavity. 4. Definite segmentation of the yolk sphere.]
In the case of this clear protuberance and of the similar
protuberances which follow it, the protoplasm is not at first quite
free from food-yolk, but only becomes so on being separated from the
yolk-containing part of the ovum. We must therefore suppose that the
production of the clear segments is in part at least due to the
yolk-spherules becoming used up to form protoplasm. Such a formation
of protoplasm from yolk-spherules has been clearly shewn to occur in
other types by Bobretzky and Fol.
The protuberance soon becomes separated off from the larger part of
the egg as a small segment composed of clear protoplasm. From the
larger segment filled with food-yolk, a second small clear segment is
next budded off, and simultaneously (fig. 42, 2) the original small
segment divides into two. Thus there are formed four segments, one
large and three small; the large segment as before being filled with
food-yolk. The continuation of a similar process of budding off and
segmentation eventually results in the formation of a considerable
number of small and of one large segment (fig. 42, 3). Between this
large and the small segments is a segmentation cavity.
Eventually the large yolk segment, which has hitherto merely budded
off a series of small segments free from yolk, itself divides into two
similar parts. This process is then repeated (fig. 42, 4) and there is
at last formed a number of yolk segments filled with yolk spheres,
which occupy the place of the original large yolk segment. Between
these yolk segments and the small segments is placed the segmentation
cavity.
The segmentation of the ovum of Euaxes[43] resembles that of Unio in
the budding off of clear segments from those filled with yolk, but
presents many interesting individualities.
[43] Kowalevsky, _Mem. Akad. Petersburg_, Series VII, 1871.
A very peculiar modification of the ordinary Gasteropod segmentation
is that described by Bobretzky for Nassa mutabilis[44].
[44] _Archiv. f. mikr. Anat._ Vol. XIII. 1877.
The ovum contains a large amount of food-yolk, and the protoplasm is
aggregated at the formative pole, adjoining which are placed the polar
bodies. An equatorial and a vertical furrow (fig. 43 A), the former
near the upper pole, appear simultaneously, and divide the ovum into
three segments, two small, each with a protoplasmic pole, and one
large entirely formed of yolk material. One of the two small segments
next completely fuses with the large segment (fig. 43 B), and after
the fusion is complete, a triple segmentation of the large segment
takes place as at the first division, and at the same time the single
small segment divides into two. In this way four partially
protoplasmic segments and one yolk segment are formed (fig. 43 C). One
of the small segments again fuses with the large segment, so that the
number of segments becomes again reduced to four, three small and one
large. The protoplasmic ends of these segments are turned towards each
other, and where they meet four very small cells become budded off,
one from each segment (fig. 43 D). Four small cells are again budded
off twice in succession, while the original small cells remain
passive, so that there come to be twelve small and four large cells.
In later stages the four first-formed small cells give rise to still
smaller cells and then the next-formed do the same. The large cells
continue also to give rise to small ones, and finally, by a continuous
process of division, and fresh budding of small cells from large
cells, a cap of small cells becomes formed covering the four large
cells which have in the meantime pressed themselves together (fig. 43
E). A segmentation cavity of not inconsiderable dimensions becomes
established between this cap of small cells and the large cells.
[FIG. 43. SEGMENTATION OF NASSA MUTABILIS (from Bobretzky). A. Upper
half divided into two segments. B. One of these has fused with the
large lower segment. C. Four small and one large segment, one of the
former fusing with the large segment. D. Each of the four segments
has given rise to a small segment. E. Small segments have increased
to thirty-six.]
Many eggs, such as those of the Myriapods[45], present an irregular
segmentation; but the segmentation is hardly unequal in the sense in
which I have been using the term. Such cases should perhaps be placed
in the first rather than in the present category.
[45] Metschnikoff, _Zeitschrift f. wiss. Zoologie_, 1874.
The type of unequal segmentation is on the whole the most widely
distributed in the animal kingdom. There is hardly a group without
examples of it.
It occurs in Porifera, Hydrozoa, Actinozoa and Ctenophora. Amongst the
Ctenophora this segmentation is of the most typical kind. Four equal
segments are first formed in the two first periods. In the third
period a circumferential furrow separates four smaller from four
larger segments.
This type is also widely distributed amongst the unsegmented
(Gephyrea, Turbellaria), as well as the segmented Vermes, and is
typical for the Rotifera. It appears to be very rare in Echinoderms
(_Echinaster Sarsii_). It is not uncommon in early stages of the
segmentation of the lower Crustacea.
For Mollusca (except Cephalopoda) it is typical. Amongst the Ascidia
it occurs in several forms (_Salpa_, _Molgula_) and amongst
the Craniata it is typical in the Cyclostomata, Amphibia, and some
Ganoids, _e.g. Accipenser_.
Partial segmentation. The next type of segmentation we have to
deal with has long been recognized as partial segmentation. It is a
type in which only part of the ovum, called the germinal disc,
undergoes segmentation, the remainder usually forming an appendage of
the embryo known as the yolk-sack. Ova belonging to the two groups
already dealt with are frequently classed together as holoblastic ova,
in opposition to ova of the present group in which the segmentation is
only partial, and which are therefore called meroblastic. For
embryological purposes this is in many ways a very convenient
classification, but ova belonging to the present group are in reality
separated by no sharp line from those belonging to the group just
described.
[FIG. 44. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION IN A
FOWL'S EGG. (After Coste.)
_a._ edge of germinal disc. _b._ vertical furrow. _c._ small central
segment. _d._ larger peripheral segment.]
The origin and nature of meroblastic ova will best be understood by
taking an ovum with an unequal segmentation, such as that of the frog,
and considering what would take place in accordance with the laws
already laid down, supposing the amount of food-yolk at the vitelline
pole to be enormously increased. What would happen may be conveniently
illustrated by fig. 44, representing the segmentation of a fowl's egg.
There would first obviously appear a vertical furrow at the formative
or protoplasmic pole. (Fig. 44 A, _b_.) This would gradually
advance round the ovum and commence to divide it into two halves.
Before the furrow had however proceeded very far it would come
to the vitelline part of the ovum; here, according to the law
previously enunciated, it would travel very slowly, and if the amount
of the food-yolk was practically infinite as compared with the
protoplasm, it would absolutely cease to advance. A second vertical
furrow would soon be formed, crossing the first at right angles, and
like it not advancing beyond the edge of the germinal disc. (Fig. 44
B.)
The next furrow should be an equatorial one (as a matter of fact in
the fowl's ovum an equatorial furrow is not formed till after two more
vertical furrows have appeared). The equatorial furrow would however,
in accordance with the analogy of the frog, _not be formed at the
equator, but very close to the formative pole_. It would therefore
separate off as a distinct segment (fig. 44 C, _c_), a small
central, _i.e._ polar, portion of each of the imperfect segments
formed by the previous vertical furrows. By a continuation of the
process of segmentation, with the same alternation of vertical and
equatorial furrows as in the frog, a cap or disc of small segments
would obviously be formed at the protoplasmic pole of the ovum,
outside which would be a number of deep radiating grooves (fig. 45),
formed by the vertical furrows, the advance of which round the ovum
has come to an end owing to the too great proportion of yolk spheres
at the vitelline pole.
[FIG. 45. SURFACE VIEW OF THE GERMINAL DISC OF FOWL'S EGG DURING A
LATE STAGE OF THE SEGMENTATION.
_c._ small central segmentation spheres; _b._ larger segments
outside these; _a._ large, imperfectly circumscribed, marginal
segments; _e._ margin of germinal disc.]
It is clear from the above that an immense accumulation of food-yolk
at the vitelline pole necessarily causes a partial segmentation. It is
equally clear that the part of meroblastic ova which does not undergo
segmentation is not a new addition absent in other cases. It
is on the contrary to be regarded merely as a part of the ovum in
which the yolk-spherules have attained to a very great bulk as
compared with the protoplasm; sometimes even to the complete exclusion
of the protoplasm.
An ordinary meroblastic ovum consists then of a small disc at the
formative pole, known as the germinal disc, composed mainly of
protoplasm in which comparatively little food-yolk is present. This
graduates into the remainder of the ovum, being separated from it by a
more or less sharp line. This remainder of the ovum, which almost
always forms the major part, usually consists of numerous
yolk-spherules, embedded in a very scanty protoplasmic matrix.
In some cases, _e.g._ the eggs of Elasmobranchii[46], the
protoplasm is present in the form of a delicate network; in other and
perhaps the majority of cases, too little protoplasm is present to be
detected, or the protoplasm may even be completely absent. In some
Osseous Fishes, _e.g._ Lota, the yolk forms a homogeneous
transparent albuminoid substance containing a large globule at the
pole furthest removed from the germinal disc. In this case the
germinal disc is sharply separated from the yolk. In other Osseous
Fishes the separation between the two parts is not so sharp[47]. In
these cases we find adjoining the germinal disc a finely granular
material containing a large proportion of protoplasm; this graduates
into a material with very little protoplasm and numerous
yolk-spherules, which is in its turn continuous with an homogeneous
albuminoid yolk substance. In Elasmobranchii we find that immediately
beneath the germinal disc there is present a finely granular matter,
rich in protoplasm, which is continuous with the normal yolk.
[46] _Vide_ Schultze, _Archiv. f. mikr. Anat._ Vol. XI.;
and F. M. Balfour, _Monograph on the Development of
Elasmobranch Fishes_.
[47] _Vide_ Klein, _Quart. Journal of Micr. Science_,
April, 1876. Bambeke, _Mem. Cour. Acad. Belgique_, 1875.
His, _Zeit. für Anat. u. Entwicklung_. Vol. I.
The Elasmobranch ovum may conveniently serve as type for the
Vertebrata. The ovum is formed of a spherical vitellus without any
investing membrane. The germinal disc is recognizable on this as a
small yellow spot about 1-1/2 millimetres in diameter. In the germinal
disc a furrow appears bisecting the disc, followed by a second furrow
at right angles to the first. Thus after the formation of the second
furrow the disc is divided into four equal areas. Fresh furrows
continue to rise, and eventually a circular furrow, equivalent to the
equatorial furrow of the frog's ovum, makes its appearance, and
separates off a number of smaller central segments from peripheral
larger segments. In the later stages the smaller segments at first
divide more rapidly than the larger, but eventually the latter also
divide rapidly, and the germinal disc becomes finally formed of a
series of segments of a fairly uniform size. So much may be
observed in surface views of the segmenting ovum, and it may be noted
that there is not much difference to be observed between the
segmentation of the germinal disc of the Fowl's ovum and that of the
Elasmobranchii. Indeed the figure of the former (fig. 44) would serve
fairly well for the latter. When however we examine the segmenting
germinal discs by means of sections, there are some differences
between the two types, and several interesting features which deserve
to be noticed in the segmentation of the Elasmobranchii. In the first
stages the furrows visible on the surface are merely furrows, which do
not meet so as to isolate distinct segments; they merely, in fact,
form a surface pattern. It is not till after the appearance of the
equatorial furrow that the segments begin to be distinctly isolated.
In the subsequent stages not only do the segments already existing in
the germinal disc increase by division, but fresh segments are
continually being formed from the adjacent yolk, and added to those
already present in the germinal disc. (Fig. 46.) This fact is one out
of many which prove that the germinal disc is merely part of the ovum
characterized by the presence of more protoplasm than the remainder
which forms the so-called food-yolk. During the latest stages of
segmentation there appear in the yolk around the blastoderm a number
of nuclei. (Fig. 46, _nx´._) These are connected with a special
protoplasmic network (already described) which penetrates through the
yolk. Towards the end of segmentation, and during the early periods of
development which succeed the segmentation, these nuclei become very
numerous. (Fig. 47 A, _n´_.) Around many of them a protoplasmic
investment is established, and cells are thus formed which eventually
enter the blastoderm.
[FIG. 46. SECTION THROUGH GERMINAL DISC OF A PRISTIURUS EMBRYO
DURING THE SEGMENTATION.
_n._ nucleus; _nx._ nucleus modified prior to division; _nx´._
modified nucleus of the yolk; _f._ furrow appearing in the yolk
adjacent to the germinal disc.]
The result of segmentation is the formation of a lens-shaped mass of
cells lying in a depression on the yolk. In this a cavity appears, the
homologue of the segmentation cavity already spoken of. It lies at
first in the midst of the cells of the blastoderm, but very
soon its floor of cells vanishes, and it lies between the yolk and the
blastoderm. (Fig. 47 A.) Its subsequent history is given in a future
Chapter.
Segmentation proceeds in Osseous Fishes in nearly the same manner as
in Elasmobranchii. In some cases the germinal disc is small as
compared with the yolk, in other cases it is almost as large as the
yolk. The only points which deserve special notice are the following:
(1) Nuclei, precisely similar to those in the Elasmobranch yolk,
appear in the protoplasmic matter around the germinal disc; (2) After
the deposition of the ova there is present in some forms a network of
protoplasm extending from the germinal disc through the yolk[48]. At
impregnation this withdraws itself from the yolk. It is to be compared
to the protoplasmic network of the Elasmobranch ovum.
[48] _Vide_ Bambeke, _loc. cit._
[FIG. 47. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A
PRISTIURUS EMBRYO AT STAGES PRIOR TO THE FORMATION OF THE MEDULLARY
GROOVE.
_ep._ epiblast; _ll._ lower layer cells; _m._ mesoblast; _hy._
hypoblast; _sc._ segmentation cavity; _es._ embryo swelling; _n´_.
nuclei of yolk; _er._ embryonic rim.]
There are two types of meroblastic ova. In one of these (Aves,
Elasmobranchii) the germinal disc is formed in the ovarian ovum. In
the second type the germinal disc is formed after impregnation by a
concentration of the protoplasm at one pole. This concentration is
analogous to what has already been described for Anodon and other
Molluscan ova (p. 100).
The ova of some Teleostei are intermediate between the two types.
The ovum of the wood-louse, Oniscus murarius[49], may be taken as an
example of the second type of meroblastic ovum. In this egg
development commences by the appearance of a small clear mass with
numerous transparent vesicles. This mass is the protoplasm which has
become separated from the yolk. It undergoes segmentation in a
perfectly normal fashion. Examples of other cases of this kind have
been described by Van Beneden and Bessels[50] in Anchorella, and in
Hessia by Van Beneden[51]. It appears from their researches that the
protoplasm collects itself together, first of all in the interior of
the egg, and then travels to the surface. It arrives at the surface
after having already divided into two or more segments, which then
rapidly divide in the usual manner to form the blastoderm.
[49] _Vide_ Bobretzky, _Zeitschrift für wiss. Zoologie_,
Vol. XXIV., 1874.
[50] _Loc. cit._
[51] _Bulletins de l'Acad. Belgique_, Tom. XXIX., 1870.
There are some grounds for thinking that the cases of partial
segmentation in the Arthropoda are not really quite comparable with
those in other groups, but more probably fall under the next type of
segmentation to be described. The grounds for this view are mentioned
in connection with the next type.
In most if not all meroblastic ova there appear during and after
segmentation a number of nuclei in the yolk adjoining the blastoderm,
around which cells become differentiated. (Figs. 46 and 47.) These
cells join the part of the blastoderm formed by the normal
segmentation of the germinal disc. Such nuclei are formed in all
craniate meroblastic ova[52]. In Cephalopods they have been found by
Lankester, and in Oniscus by Bobretzky. They have been by some authors
supposed to originate from the nuclei of the blastoderm, and by others
spontaneously in the yolk.
[52] Though less obvious in the ovum of the fowl than in that of
some other types, they may nevertheless be demonstrated there
without very much difficulty.
Some of the earliest observations on these nuclei were made by
Lankester[53] in the Cephalopods. He found that they appeared first in
a ring-like series round the edge of the blastoderm, and subsequently
all over the yolk in a layer a little below the surface. He observed
their development in the living ovum and found that they "commenced as
minute points, gradually increasing in size like other free-formed
nuclei." A cell area subsequently forms around them.
[53] _Quart. Journ. of Micr. Science_, Vol. XV. pp. 39, 40.