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==Chapter XXIV. 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.
Ccelenterata. The greatest confusion prevails as to the
germinal layer from which the male and female products are
derived in the Ccelenterata 1 .
1 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).
742 CCELENTERATA.
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 Hydromedusae, 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 : Gonothyraea, Campanularia, Hydractinia, Clava.
In view of the somewhat surprising results to which the
researches on the origin of the genital products amongst the
Ccelenterata 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 Ccelenterata, 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 Ccelenterata.
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
GENERATIVE ORGANS. 743
that the endodermal position of the generative organs in the Actinozoa and
acraspedote Medusre 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.
\Ve might perhaps, on this view, formulate the origin of the generative
products in the Ccelenterata 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.
Chaetopoda and Gephyrea. In the Chaetopoda 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 Ccelenterata 1 .
Chaetognatha. In Sagitta the history of the generative
cells, which was first worked out by Kowalevsky and Biitschli,
has been recently treated with great detail by O. Hertwig 2 .
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 ifl my diagram, takes place in such a way that two
1 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.
2 O. Hertwig, Die Chcetognathen. Jena, 1880
744
CH^ETOGNATHA.
cells are placed nearer the median line, and two externally. The
two inner cells form the eventual testes, and the outer the
FIG. 408. THREK STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after
Biitschli, 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 (in) has become continuous with the alimentary tract, and the blastopore is closed.
///. mouth ; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. perivisceral cavity ; sp, splanchnic mesoblast ; so. somatic mesoblast ; ge. generative
organs.
ovaries, one half of each primitive cell thus forming an ovary, and
the other a testis.
FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal
surface. B, from the side. (After Biitschli.)
m. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ;
<.'/. epiblast ; c.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchnopleure ; ge. generative organs.
GENERATIVE ORGANS.
745
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 Chsetognatha. 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 1 .
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 2 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 ;
1 Fide Vol. n. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189.
2 C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zool. Instil.
Wien. Vol. II. 1879.
746
CHORDATA.
sp.c
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.
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 4io,/0). 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.
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. 41 1),
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 ;
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. sub-notochordal 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 ;
i>.o. primitive generative cells.
GENERATIVE ORGANS.
747
though with reference to certain stages in the history further
researches are still required 1 .
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
OK 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 ampullae, mainly formed of
germinal cells, and each provided with a central lumen ; and
these ampullae 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 ampulla;; 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.
1 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.
MODE OF EXIT OF GENITAL PRODUCTS.
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 1 .
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. Lemons s. la generation des Vcrlebrcs. Paris, 1879.
(555) F. M. Balfour. "On the structure and development of the Vertebrate
ovary." Quart, J. of Micr. Science, Vol. xvm.
(556) E. van Beneden. "De la distinction originelle dutecticule et clel'ovaire,
etc." Bull. Ac. roy. belgique, Vol. xxxvil. 1874.
(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zcit.
f. -wiss. Zool., Vol. xxxv. 1881.
(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.zoot. Inslit. Wilrzburg, Vol. I. 1874.
(559) C. Semper. "Das Urogenilalsystem d. Plagiostomen, etc." Arbeit, a.
d. zooL-zoot. Ins tit. Witrzbiirg, Vol. II. 1875.
(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen bei
den Hydroiden." Zool. Anzeiger, No. 55, 1880.
Fitffcalso O. and R. Hertwig (No. 271), Kolliker (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 Ccelenterata there are no generative ducts.
In the Hydromedusae 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
1 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).
GENITAL DUCTS.
749
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 Chaetognatha (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
Chaetopoda, 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 Polychaeta,
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
750 DERIVATION FROM EXCRETORY ORGANS.
function ; while in other instances, e.g. Bonellia, such modification is very considerable.
The generative ducts of the Oligochaeta 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 Miillerian 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 Oligochasta 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 Polychaeta (vide Marion and Bobretzky), analogous to that just offered for the
Oligochaeta.
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 Mullerian 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 Mullerian 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
In vertebrata.
GENITAL DUCTS.
751
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 1
(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.
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
Pcecilopoda, 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
1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordnungen d.
Thierreichs, Vol. in. p. 404.
752 DERIVATION FROM EXCRETORY ORGANS.
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 (i) 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 tc the
epithelium of the vas deferens, is only formed at the hinder ond
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
GENITAL DUCTS. 753
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 Hydromedusae, 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 1 the remaining triploblastica.
Their ducts have been evolved in forms in which the
generative products continued to be liberated directly to the
exterior, as in the Hydromedusae ; 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 2 .
1 It would be interesting to have further information about Balanoglossus.
2 These views fit in very well with those already put forward in Chapter xm. on
the affinities of the Echinodermata.

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata
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This historic 1885 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.



Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol. III. A Treatise on Comparative Embryology 2 (1885)

Chapter XXIV. 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.

Ccelenterata. The greatest confusion prevails as to the germinal layer from which the male and female products are derived in the Ccelenterata 1 .

1 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).


742 CCELENTERATA.


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 Hydromedusae, 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 : Gonothyraea, Campanularia, Hydractinia, Clava.

In view of the somewhat surprising results to which the researches on the origin of the genital products amongst the Ccelenterata 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 Ccelenterata, 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 Ccelenterata.

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


GENERATIVE ORGANS. 743

that the endodermal position of the generative organs in the Actinozoa and acraspedote Medusre 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.

\Ve might perhaps, on this view, formulate the origin of the generative products in the Ccelenterata 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.

Chaetopoda and Gephyrea. In the Chaetopoda 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 Ccelenterata 1 .

Chaetognatha. In Sagitta the history of the generative cells, which was first worked out by Kowalevsky and Biitschli, has been recently treated with great detail by O. Hertwig 2 .

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 ifl my diagram, takes place in such a way that two

1 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.

2 O. Hertwig, Die Chcetognathen. Jena, 1880


744


CH^ETOGNATHA.


cells are placed nearer the median line, and two externally. The two inner cells form the eventual testes, and the outer the



FIG. 408. THREK STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after

Biitschli, 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 (in) has become continuous with the alimentary tract, and the blastopore is closed.

///. mouth ; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. perivisceral cavity ; sp, splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs.

ovaries, one half of each primitive cell thus forming an ovary, and the other a testis.



FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal

surface. B, from the side. (After Biitschli.)

m. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ; <.'/. epiblast ; c.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchnopleure ; ge. generative organs.


GENERATIVE ORGANS.


745


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 Chsetognatha. 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 1 .

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 2 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 ;

1 Fide Vol. n. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189.

2 C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zool. Instil. Wien. Vol. II. 1879.


746


CHORDATA.


sp.c


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.

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 4io,/0). 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.

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. 41 1), 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 ;



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. sub-notochordal 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 ; i>.o. primitive generative cells.


GENERATIVE ORGANS.


747


though with reference to certain stages in the history further researches are still required 1 .

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 OK 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 ampullae, mainly formed of germinal cells, and each provided with a central lumen ; and these ampullae 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 ampulla;; 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.

1 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.


MODE OF EXIT OF GENITAL PRODUCTS.


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 1 .

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. Lemons s. la generation des Vcrlebrcs. Paris, 1879.

(555) F. M. Balfour. "On the structure and development of the Vertebrate ovary." Quart, J. of Micr. Science, Vol. xvm.

(556) E. van Beneden. "De la distinction originelle dutecticule et clel'ovaire, etc." Bull. Ac. roy. belgique, Vol. xxxvil. 1874.

(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zcit. f. -wiss. Zool., Vol. xxxv. 1881.

(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.zoot. Inslit. Wilrzburg, Vol. I. 1874.

(559) C. Semper. "Das Urogenilalsystem d. Plagiostomen, etc." Arbeit, a. d. zooL-zoot. Ins tit. Witrzbiirg, Vol. II. 1875.

(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen bei den Hydroiden." Zool. Anzeiger, No. 55, 1880.

Fitffcalso O. and R. Hertwig (No. 271), Kolliker (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 Ccelenterata there are no generative ducts.

In the Hydromedusae 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

1 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).


GENITAL DUCTS.


749


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 Chaetognatha (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 Chaetopoda, 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 Polychaeta, 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


750 DERIVATION FROM EXCRETORY ORGANS.

function ; while in other instances, e.g. Bonellia, such modification is very considerable.

The generative ducts of the Oligochaeta 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 Miillerian 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 Oligochasta 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 Polychaeta (vide Marion and Bobretzky), analogous to that just offered for the Oligochaeta.

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 Mullerian 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 Mullerian 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 In vertebrata.


GENITAL DUCTS.


751


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 1 (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.

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 Pcecilopoda, 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

1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordnungen d. Thierreichs, Vol. in. p. 404.


752 DERIVATION FROM EXCRETORY ORGANS.

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 (i) 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 tc the epithelium of the vas deferens, is only formed at the hinder ond 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


GENITAL DUCTS. 753


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 Hydromedusae, 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 1 the remaining triploblastica.

Their ducts have been evolved in forms in which the generative products continued to be liberated directly to the exterior, as in the Hydromedusae ; 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 2 .

1 It would be interesting to have further information about Balanoglossus.

2 These views fit in very well with those already put forward in Chapter xm. on the affinities of the Echinodermata.