GENERATIVE ORGANS AND GENITAL DUCTS.
_Generative organs._
The structure and growth of the ovum and spermatozoon were given in
the first chapter of this work, but their derivation from the germinal
layers was not touched on, and it is this subject with which we are
here concerned. If there are any structures whose identity throughout
the Metazoa is not open to doubt these structures are the ovum and
spermatozoon; and the constancy of their relations to the germinal
layers would seem to be a crucial test as to whether the latter have
the morphological importance usually attributed to them.
The very fragmentary state of our knowledge of the origin of the
generative cells has however prevented this test being so far very
generally applied.
Porifera. In the Porifera the researches of Schulze have clearly
demonstrated that both the ova and the spermatozoa take their origin
from indifferent cells of the general parenchyma, which may be called
mesoblastic. The primitive germinal cells of the two sexes are not
distinguishable; but a germinal cell by enlarging and becoming
spherical gives rise to an ovum; and by subdivision forms a
sperm-morula, from the constituent cells of which the spermatozoa are
directly developed.
Coelenterata. The greatest confusion prevails as to the germinal layer
from which the male and female products are derived in the
Coelenterata[268].
[268] E. van Beneden (No. 556) was the first to discover a
different origin for the generative products of the two sexes in
Hydractinia, and his observations have led to numerous subsequent
researches on the subject. For a summary of the observations on
the Hydroids _vide_ Weismann (No. 560).
The following apparent modes of origin of these products have been
observed.
(1) The generative products of both sexes originate in the ectoderm
(epiblast): Hydra, Cordylophora, Tubularia, all (?) free Gonophores of
Hydromedusæ, the Siphonophora, and probably the Ctenophora.
(2) The generative products of both sexes originate in the entoderm
(hypoblast): Plumularia and Sertularella, amongst the Hydroids, and
the whole of the Acraspeda and Actinozoa.
(3) The male cells are formed in the ectoderm, and the female in the
entoderm: Gonothyræa, Campanularia, Hydractinia, Clava.
In view of the somewhat surprising results to which the researches on
the origin of the genital products amongst the Coelenterata have led,
it would seem to be necessary either to hold that there is no definite
homology between the germinal layers in the different forms of
Coelenterata, or to offer some satisfactory explanation of the
behaviour of the genital products, which would not involve the
acceptance of the first alternative.
Though it can hardly be said that such an explanation has yet been
offered, some observations of Kleinenberg (No. 557) undoubtedly point
to such an explanation being possible.
Kleinenberg has shewn that in Eudendrium the ova migrate freely from
the ectoderm into the endoderm, and _vice versa_; but he has given
strong grounds for thinking that _they originate_ in the ectoderm. He
has further shewn that the migration in this type is by no means an
isolated phenomenon.
Since it is usually only possible to recognise generative elements
after they have advanced considerably in development, the mere
position of a generative cell, when first observed, can afford, after
what Kleinenberg has shewn, no absolute proof of its origin. Thus it
is quite possible that there is really only one type of origin for the
generative cells in the Coelenterata.
Kleinenberg has given reasons for thinking that the migration of the
ova into the entoderm may have a nutritive object. If this be so, and
there are numerous facts which shew that the position of generative
cells is often largely influenced by their nutritive requirements, it
seems not impossible that the endodermal position of the generative
organs in the Actinozoa and acraspedote Medusæ may have arisen by a
continuously earlier migration of the generative cells from the
ectoderm into the endoderm; and that the migration may now take place
at so early a period of the development, that we should be justified
in formally holding the generative products to be endodermal in
origin.
We might perhaps, on this view, formulate the origin of the generative
products in the Coelenterata in the following way:--
Both ova and spermatozoa primitively originated in the ectoderm, but
in order to secure a more complete nutrition the cells which give rise
to them exhibit in certain groups a tendency to migrate into the
endoderm. This migration, which may concern the generative cells of
one or of both the sexes, takes place in some cases after the
generative cells have become recognisable as such, and very probably
in other cases at so early a period that it is impossible to
distinguish the generative cells from indifferent embryonic cells.
Very little is known with reference to the origin of the generative
cells in the triploblastic Invertebrata.
Chætopoda and Gephyrea. In the Chætopoda and Gephyrea, the germinal
cells are always developed in the adult from the epithelial lining of
the body cavity; so that their origin from the mesoblast seems fairly
established.
If we are justified in holding the body cavity of these forms to be a
derivative of the primitive archenteron (_vide_ pp. 356 and 357) the
generative cells may fairly be held to originate from a layer which
corresponds to the endoderm of the Coelenterata[269].
[269] The Hertwigs (No. 271) state that in their opinion the
generative cells arise from the lining of the body cavity in all
the forms whose body cavity is a product of the archenteron. We
do not know anything of the embryonic development of the
generative organs in the Echinodermata, but the adult position of
the generative organs in this group is very unfavourable to the
Hertwigs' view.
Chætognatha. In Sagitta the history of the generative cells, which was
first worked out by Kowalevsky and Bütschli, has been recently treated
with great detail by O. Hertwig[270].
[270] O. Hertwig, _Die Chætognathen_. Jena, 1880.
The generative cells appear during the gastrula stage, as two large
cells with conspicuous nuclei, which are placed in the hypoblast
lining the archenteron, at the pole opposite the blastopore. These
cells soon divide, and at the same time pass out of the hypoblast, and
enter the archenteric cavity (fig. 408 A, _ge_). The division into
four cells, which is not satisfactorily represented in my diagram,
takes place in such a way that two cells are placed nearer the median
line, and two externally. The two inner cells form the eventual
testes, and the outer the ovaries, _one half of each primitive cell
thus forming an ovary, and the other a testis_.
[FIG. 408. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C
after Bütschli, and B after Kowalevsky.)
The three embryos are represented in the same positions.
A. Represents the gastrula stage.
B. Represents a succeeding stage, in which the primitive archenteron
is commencing to be divided into three.
C. Represents a later stage, in which the mouth involution (_m_) has
become continuous with the alimentary tract, and the blastopore is
closed.
_m._ mouth; _al._ alimentary canal; _ae._ archenteron; _bl.p._
blastopore; _pv._ perivisceral cavity; _sp._ splanchnic mesoblast;
_so._ somatic mesoblast; _ge._ generative organs.]
[FIG. 409. TWO VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal
surface. B, from the side. (After Bütschli.)
_m._ mouth; _al._ alimentary canal; _v.g._ ventral ganglion
(thickening of epiblast); _ep._ epiblast; _c.pv._ cephalic section
of body cavity; _so._ somatopleure; _sp._ splanchnopleure; _ge._
generative organs.]
When the archenteric cavity is divided into a median alimentary tract,
and two lateral sections forming the body cavity, the generative
organs are placed in the common vestibule into which both the body
cavity and alimentary cavity at first open (fig. 408).
The generative organs long retain their character as simple cells.
Eventually (fig. 409) the two ovaries travel forwards, and apply
themselves to the body walls, while the two testes also become
separated by a backward prolongation of the median alimentary tract.
On the formation of the transverse septum dividing the tail from the
body, the ovarian cells lie immediately in front of this septum, and
the testicular cells in the region behind it.
Polyzoa. In Pedicellina amongst the entoproctous Polyzoa Hatschek
finds that the generative organs originate from a pair of specially
large mesoblast cells, situated in the space between the stomach and
the floor of the vestibule. The two cells undergo changes, which have
an obvious resemblance to those of the generative cells of the
Chætognatha. They become surrounded by an investment of mesoblast
cells, and divide so as to form two masses. Each of these masses at a
later period separates into an anterior and a posterior part. The
former becomes the ovary, the latter the testis.
Nematoda. In the Nematoda the generative organs are derived from the
division of a single cell which would appear to be mesoblastic[271].
[271] _Vide_ Vol. II. p. 374; also Götte, _Zool. Anzeiger_, No.
80, p. 189.
Insecta. The generative cells have been observed at a very early
embryonic stage in several insect forms (Vol. II. p. 404), but the
observations so far recorded with reference to them do not enable us
to determine with certainty from which of the germinal layers they are
derived.
Crustacea. In Moina, one of the Cladocera, Grobben[272] has shewn that
the generative organs are derived from a single cell, which becomes
differentiated during the segmentation. This cell, which is in close
contiguity with the cells from which both the mesoblast and hypoblast
originate, subsequently divides; but at the gastrula stage, and after
the mesoblast has become formed, the cells it gives rise to _are
enclosed in the epiblast_, and do not migrate inwards till a later
stage. The products of the division of the generative cell
subsequently divide into two masses. It is not possible to assign the
generative cell of Moina to a definite germinal layer. Grobben,
however, thinks that it originates from the division of a cell, the
remainder of which gives rise to the hypoblast.
[272] C. Grobben. "Die Entwick. d. Moina rectirostris." _Arbeit.
a. d. zool. Instit. Wien._ Vol. II. 1879.
Chordata. In the Vertebrata, the primitive generative cells (often
known as primitive ova) are early distinguishable, being imbedded
amongst the cells of two linear streaks of peritoneal epithelium,
placed on the dorsal side of the body cavity, one on each side of the
mesentery (figs. 405 C and 410, _po_). They appear to be derived from
the epithelial cells amongst which they lie; and are characterized by
containing a large granular nucleus, surrounded by a considerable body
of protoplasm. The peritoneal epithelium in which they are placed is
known as the germinal epithelium.
[FIG. 410. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY
YOUNGER THAN 28 F.
_sp.c._ spinal cord; _W._ white matter of spinal cord; _pr._
posterior nerve-roots; _ch._ notochord; _x._ subnotochordal rod;
_ao._ aorta; _mp._ muscle-plate; _mp´._ inner layer of muscle-plate
already converted into muscles; _Vr._ rudiment of vertebral body;
_st._ segmental tube; _sd._ segmental duct; _sp.v._ spiral valve;
_v._ subintestinal vein; _p.o._ primitive generative cells.]
It is at first impossible to distinguish the germinal cells which will
become ova from those which will become spermatozoa.
The former however remain within the peritoneal epithelium (fig. 411),
and become converted into ova in a manner more particularly described
in Vol. II. pp. 54-59.
The history of the primitive germinal cells in the male has not been
so adequately worked out as in the female.
The fullest history of them is that given by Semper (No. 559) for the
Elasmobranchii, the general accuracy of which I can fully support;
though with reference to certain stages in the history further
researches are still required[273].
[273] Balbiani (No. 554) has also recently dealt with this
subject, but I cannot bring my own observations into accord with
his as to the structure of the Elasmobranch testis.
In Elasmobranchii the male germinal cells, instead of remaining in the
germinal epithelium, migrate into the adjacent stroma, accompanied I
believe by some of the indifferent epithelial cells. Here they
increase in number, and give rise to masses of variable form, composed
partly of true germinal cells, and partly of smaller cells with deeply
staining nuclei, which are, I believe, derived from the germinal
epithelium.
[FIG. 411. 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.]
These masses next break up into ampullæ, mainly formed of germinal
cells, and each provided with a central lumen; and these ampullæ
attach themselves to tubes derived from the smaller cells, which are
in their turn continuous with the testicular network. The spermatozoa
are developed from the cells forming the walls of the primitive
ampullæ; but the process of their formation does not concern us in
this chapter.
In the Reptilia Braun has traced the passage of the primitive germinal
cells into the testicular tubes, and I am able to confirm his
observations on this point: he has not however traced their further
history.
In Mammalia the evidence of the origin of the spermatospores from the
germinal epithelium is not quite complete, but there can be but little
doubt of its occurrence[274].
[274] An entirely different view of the origin of the sperm cells
has been adopted by Balbiani, for which the reader is referred to
his Memoir (No. 554).
In Amphioxus Langerhans has shewn that the ova and spermatozoa are
derived from similar germinal cells, which may be compared to the
germinal epithelium of the Vertebrata. These cells are however
segmentally arranged as separate masses (_vide_ Vol. II. p. 54).
BIBLIOGRAPHY.
(554) G. Balbiani. _Leçons s. la génération des Vertébrés._ Paris,
1879.
(555) F. M. Balfour. "On the structure and development of the
Vertebrate ovary." _Quart. J. of Micr. Science_, Vol. XVIII.
(556) E. van Beneden. "De la distinction originelle du tecticule et de
l'ovaire, etc." _Bull. Ac. roy. belgique_, Vol. XXXVII. 1874.
(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium."
_Zeit. f. wiss. Zool._, Vol. XXXV. 1881.
(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." _Arbeit. a. d.
zool.-zoot. Instit. Würzburg_, Vol. I. 1874.
(559) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." _Arbeit.
a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875.
(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen
bei den Hydroiden." _Zool. Anzeiger_, No. 55, 1880.
_Vide_ also O. and R. Hertwig (No. 271), Kölliker (No. 298), etc.
_Genital ducts._
The development and evolution of the generative ducts is as yet very
incompletely worked out, but even in the light of our present
knowledge a comparative review of this subject brings to light
features of considerable interest, and displays a fruitful field for
future research.
In the Coelenterata there are no generative ducts.
In the Hydromedusæ and Siphonophora the generative products are
liberated by being dehisced directly into the surrounding medium;
while in the Acraspeda, the Actinozoa and the Ctenophora, they are
dehisced into parts of the gastrovascular system, and carried to the
exterior through the mouth.
The arrangement in the latter forms indicates the origin of the
methods of transportation of the genital products to the exterior in
many of the higher types.
It has been already pointed out that the body cavity in a very large
number of forms is probably derived from parts of a gastrovascular
system like that of the Actinozoa.
When the part of the gastrovascular system into which the generative
products were dehisced became, on giving rise to the body cavity, shut
off from the exterior, it would be essential that some mode of
transportation outwards of the generative products should be
constituted.
In some instances simple pores (probably already existing at the time
of the establishment of a closed body cavity) become the generative
ducts. Such seems probably to have been the case in the Chætognatha
(Sagitta) and in the primitive Chordata.
In the latter forms the generative products are sometimes dehisced
into the peritoneal cavity, and thence transported by the abdominal
pores to the exterior (Cyclostomata and some Teleostei, _vide_ p.
626). In Amphioxus they pass by dehiscence into the atrial cavity, and
thence through the gill slits and by the mouth, or by the abdominal
pore (?) to the exterior. The arrangement in Amphioxus and the
Teleostei is probably secondary, as possibly also is that in the
Cyclostomata; so that the primitive mode of exit of the generative
products in the Chordata is still uncertain. It is highly improbable
that the generative ducts of the Tunicata are primitive structures.
A better established and more frequent mode of exit of the generative
products when dehisced into the body cavity is by means of the
excretory organs. The generative products pass from the body cavity
into the open peritoneal funnels of such organs, and thence through
their ducts to the exterior. This mode of exit of the generative
products is characteristic of the Chætopoda, the Gephyrea, the
Brachiopoda and the Vertebrata, and probably also of the Mollusca. It
is moreover quite possible that it occurs in the Polyzoa, some of the
Arthropoda, the Platyelminthes and some other types.
The simple segmental excretory organs of the Polychæta, the Gephyrea
and the Brachiopoda serve as generative canals, and in many instances
they exhibit no modification, or but a very slight one, in connection
with their secondary generative function; while in other instances,
_e.g._ Bonellia, such modification is very considerable.
The generative ducts of the Oligochæta are probably derived from
excretory organs. In the Terricola ordinary excretory organs are
present in the generative segments in addition to the generative
ducts, while in the Limicola generative ducts alone are present in the
adult, but before their development excretory organs of the usual type
are found, which undergo atrophy on the appearance of the generative
ducts (Vedjovsky).
From the analogy of the splitting of the segmental duct of the
Vertebrata into the Müllerian and Wolffian ducts, as a result of a
combined generative and excretory function (_vide_ p. 728), it seems
probable that in the generative segments of the Oligochæta the
excretory organs had at first both an excretory and a generative
function, and that, as a secondary result of this double function,
each of them has become split into two parts, a generative and an
excretory. The generative part has undergone in all forms great
modifications. The excretory parts remain unmodified in the Earthworms
(Terricola), but completely abort on the development of the generative
ducts in the Limicola. An explanation may probably be given of the
peculiar arrangements of the generative ducts in Saccocirrus amongst
the Polychæta (_vide_ Marion and Bobretzky), analogous to that just
offered for the Oligochæta.
The very interesting modifications produced in the excretory organs of
the Vertebrata by their serving as generative ducts were fully
described in the last chapter; and with reference to this part of our
subject it is only necessary to call attention to the case of
Lepidosteus and the Teleostei.
In Lepidosteus the Müllerian duct appears to have become attached to
the generative organs, so that the generative products, instead of
falling directly into the body cavity and thence entering the open end
of a peritoneal funnel of the excretory organs, pass directly into the
Müllerian duct without entering the body cavity. In most Teleostei the
modification is more complete, in that the generative ducts in the
adult have no obvious connection with the excretory organs.
The transportation of the male products to the exterior in all the
higher Vertebrata, without passing into the body cavity, is in
principle similar to the arrangement in Lepidosteus.
The above instances of the peritoneal funnels of an excretory organ
becoming continuous with the generative glands, render it highly
probable that there may be similar instances amongst the Invertebrata.
As has been already pointed out by Gegenbaur there are many features
in the structure of the genital ducts in the more primitive Mollusca,
which point to their having been derived from the excretory organs. In
several Lamellibranchiata[275] (Spondylus, Lima, Pecten) the
generative ducts open into the excretory organs (organ of Bojanus), so
that the generative products have to pass through the excretory organ
on their way to the exterior. In other Lamellibranchiata the genital
and excretory organs open on a common papilla, and in the remaining
types they are placed close together.
[275] For a summary of the facts on this subject _vide_ Bronn,
_Klassen u. Ordnungen d. Thierreichs_, Vol. III. p. 404.
In the Cephalopoda again the peculiar relations of the generative
organs to their ducts point to the latter having primitively had a
different, probably an excretory, function. The glands are not
continuous with the ducts, but are placed in special capsules from
which the ducts proceed. The genital products are dehisced into these
capsules and thence pass into the ducts.
In the Gasteropoda the genital gland is directly continuous with its
duct, and the latter, especially in the Pulmonata and Opisthobranchiata,
assumes such a complicated form that its origin from the excretory
organ would hardly have been suspected. The fact however that its
opening is placed near that of the excretory organ points to its being
homologous with the generative ducts of the more primitive types.
In the Discophora, where the generative ducts are continuous with the
glands, the structure both of the generative glands and ducts points
to the latter having originated from excretory organs.
It seems, as already mentioned, very possible that there are other
types in which the generative ducts are derived from the excretory
organs. In the Arthropoda for instance the generative ducts, where
provided with anteriorly placed openings, as in the Crustacea,
Arachnida and the Chilognathous Myriapoda, the Poecilopoda, etc., may
possibly be of this nature, but the data for deciding this point are
so scanty that it is not at present possible to do more than frame
conjectures.
The ontogeny of the generative ducts of the Nematoda and the Insecta
appears to point to their having originated independently of the
excretory organs.
In the Nematoda the generative organs of both sexes originate from a
single cell (Schneider, Vol. I. No. 390).
This cell elongates and its nuclei multiply. After assuming a somewhat
columnar form, it divides into (1) a superficial investing layer, and
(2) an axial portion.
In the female the superficial layer is only developed distinctly in
the median part of the column. In the course of the further
development the two ends of the column become the blind ends of the
ovary, and the axial tissue they contain forms the germinal tissue of
nucleated protoplasm. The superficial layer gives rise to the
epithelium of the uterus and oviduct. The germinal tissue, which is
originally continuous, is interrupted in the middle part (where the
superficial layer gives rise to the uterus and oviduct), and is
confined to the two blind extremities of the tube.
In the male the superficial layer, which gives rise to the epithelium
of the vas deferens, is only formed at the hinder end of the original
column. In other respects the development takes place as in the
female.
In the Insecta again the evidence, though somewhat conflicting,
indicates that the generative ducts arise very much as in Nematodes,
from the same primitive mass as the generative organs. In both of
these types it would seem probable that the generative organs were
primitively placed in the body cavity, and attached to the epidermis,
through a pore in which their products passed out; and that, acquiring
a tubular form, the peripheral part of the gland gave rise to a duct,
the remainder constituting the true generative gland. It is quite
possible that the generative ducts of such forms as the Platyelminthes
may have had a similar origin to those in Insecta and Nematoda, but
from the analogy of the Mollusca there is nearly as much to be said
for regarding them as modified excretory organs.
In the Echinodermata nothing is unfortunately known as to the ontogeny
of the generative organs and ducts. The structure of these organs in
the adult would however seem to indicate that the most primitive type
of echinoderm generative organ consists of a blind sack, projecting
into the body cavity, and opening by a pore to the exterior. The sack
is lined by an epithelium, continuous with the epidermis, the cells of
which give rise to the ova or spermatozoa. The duct of these organs is
obviously hardly differentiated from the gland; and the whole
structure might easily be derived from the type of generative organ
characteristic of the Hydromedusæ, where the generative cells are
developed from special areas of the ectoderm, and, when ripe, pass
directly into the surrounding medium.
If this suggestion is correct we may suppose that the generative ducts
of the Echinodermata have a different origin to those of the majority
of[276] the remaining triploblastica.
[276] It would be interesting to have further information about
Balanoglossus.
Their ducts have been evolved in forms in which the generative
products continued to be liberated directly to the exterior, as in the
Hydromedusæ; while those of other types have been evolved in forms in
which the generative products were first transported, as in the
Actinozoa, into the gastrovascular canals[277].
[277] These views fit in very well with those already put forward
in Chapter XIII. on the affinities of the Echinodermata.