THE ORGANS OF EXCRETION.
The earliest stages in the development of the excretory system have already
been described in a previous chapter[335] of this memoir, and up to the
present time no investigator, with the exception of Dr Alex. Schultz[336],
has gone over the same ground. Dr Schultz' descriptions are somewhat brief,
but differ from my own mainly in stating that the segmental duct arises
from an involution instead of as a solid knob. This discrepancy is, I
believe, due to Dr Schultz drawing his conclusions as to the development of
the segmental duct from its appearance at a comparatively late stage. He
appears to have been unacquainted with my earlier descriptions.
Footnote 335: Chapter VI. p. 345, _et seq._
Footnote 336: _Archiv f. Micr. Anat_. Bd. XI.
The adult anatomy and later stages in the development of the excretory
organs form the subject of the present chapter, and stand in marked
contrast to the earlier stages in that they have been dealt with in a
magnificent monograph[337] by Professor Semper, whose investigations have
converted this previously almost unknown field of vertebrate embryology
into one of the most fully explored parts of the whole subject. Reference
is frequently made to this monograph in the succeeding pages, but my
references, numerous as they are, give no adequate idea of the completeness
and thoroughness of Professor Semper's investigations. In Professor
Semper's monograph are embodied the results of a considerable number of
preliminary papers published by him in his _Arbeiten_ and in the
_Centralblatt_. The excretory organs of Elasmobranchii have also formed the
subject of some investigations by Dr Meyer[338] and by myself[339]. Their
older literature is fully given by Professor Semper. In addition to the
above-cited works, there is one other paper by Dr Spengel[340] on the
Urinogenital System of Amphibians, to which reference will frequently be
made in the sequel, and which, though only indirectly connected with the
subject of this chapter, deserves special mention both on account of the
accuracy of the investigations of which it forms the record, and of the
novel light which it throws on many of the problems of the constitution of
the urinogenital system of Vertebrates.
Footnote 337: "Urogenital System d. Plagiostomen," Semper,
_Arbeiten_, Vol. II.
Footnote 338: _Sitzungsberichte d. Naturfor. Ges. Leipzig_,
1875. No. 2.
Footnote 339: "Preliminary account of the development of
Elasmobranch Fishes," _Quarterly Journal of Microscopical
Science_, 1874. "Origin and History of the Urinogenital Organs
of Vertebrates," _Journal of Anat. and Physiol._ Vol. X.
Footnote 340: _Arbeiten_, Semper, Vol. III.
_Excretory organs and genital ducts in the adult._
The kidneys of Scyllium canicula are paired bodies in contact along the
median line. They are situated on the dorsal wall of the abdominal cavity,
and extend from close to the diaphragm to a point a short way behind the
anus. Externally, each appears as a single gland, but by the arrangement of
its ducts may be divided into two distinct parts, an anterior and a
posterior. The former will be spoken of as the Wolffian body, and the
latter as the kidney, from their respective homology with the glands so
named in higher Vertebrates. The grounds for these determinations have
already been fully dealt with both by Semper[341] and by myself.
Footnote 341: Though Professor Semper has come to the same
conclusion as myself with respect to these homologies, yet he
calls the Wolffian body Leydig's gland after its distinguished
discoverer, and its duct Leydig's duct.
Externally both the Wolffian body and the kidney are more or less clearly
divided into segments, and though the breadth of both glands as viewed from
the ventral surface is fairly uniform, yet the hinder part of the kidney is
very much thicker and bulkier than the anterior part and than the whole of
the Wolffian body. In both sexes the Wolffian body is rather longer than
the kidney proper. Thus in a male example, 33 centimetres long, the two
glands together measured 8-1/4 centimetres and the kidney proper only
3-1/2. In the male the Wolffian bodies extend somewhat further forwards
than in the female. Leaving the finer details of the glands for subsequent
treatment, I pass at once to their ducts. These differ slightly in the two
sexes, so that it will be more convenient to take the male and female
separately.
A partly diagrammatic representation of the kidney and Wolffian body of the
male is given on Pl. 20, fig. 1. The secretion of the Wolffian body is
carried off by a duct, _the Wolffian duct (w.d.)_, which lies on the
ventral surface of the gland, and receives a separate ductule from each
segment (Pl. 20, fig. 5). The main function of the Wolffian duct in the
male is, however, that of a vas deferens. The testicular products are
brought to it through the coils of the anterior segments of the Wolffian
body by a number of vasa efferentia, the arrangement of which is treated of
on pp. 487, 488. The section of the Wolffian duct which overlies the
Wolffian body is much contorted, and in adult individuals at the generative
period enormously so. The duct often presents one or two contortions beyond
the hind end of the Wolffian body, but in the normal condition takes a
straight course from this point to the unpaired urinogenital cloaca, into
which it falls independently of its fellow of the opposite side. It
receives no feeders from the kidney proper.
The excretion of the kidney proper is carried off not by a single duct, but
by a series of more or less independent ducts, which, in accordance with
Prof. Semper's nomenclature, will be spoken of as _ureters_. These are very
minute, and their investigation requires some care. I have reason, from my
examinations of this and other species of Elasmobranchii, to believe that
they are, moreover, subject to considerable variations, and the following
description applies to a definite individual. Nine or possibly ten distinct
ureters, whose arrangement is diagrammatically represented in fig. 1,
Pl. 20, were present on each side. It will be noticed that, whereas the
five hindermost are distinct till close to their openings into the
urinogenital cloaca, the four anterior ones appear to unite at once into a
single duct, but are probably only bound up in a common sheath. The ureters
fall into the common urinogenital cloaca, immediately behind the opening of
the Wolffian duct (so far as could be determined), by four apertures on
each side. In a section made through the part of the wall of the cloaca
containing the openings of the ureters of both sides, there were present on
the left side (where the section passed nearer to the surface than on the
right) four small openings posteriorly, viz. the openings of the ureters
and one larger one anteriorly, viz. the opening of the Wolffian duct. On
the other side of the section where the level was rather deeper, there were
five distinct ducts cut through, one of which was almost on the point of
dividing into two. This second section proves that, in this instance at
least, the two ureters did not unite till just before opening into the
urinogenital cloaca. The same section also appeared to shew that one of the
ureters fell not into the cloaca but into the Wolffian duct.
As stated above both the Wolffian duct and the ureters fall into an
unpaired urinogenital cloaca. This cloaca communicates at one end with the
general cloaca by a single aperture situated at the point of a somewhat
conspicuous papilla, just behind the anus (Pl. 20, fig. 1, _o_), and on the
other it opens freely into a pair of bladders, situated in close contact
with each other, on the ventral side of the kidney (Pl. 20, fig. 1, _sb_).
To these bladders Professor Semper has given the name _uterus masculinus_,
from having supposed them to correspond with the lower part of the oviducts
of the female. This homology he now admits to be erroneous, and it will
accordingly be better to drop the name uterus masculinus, for which may be
substituted _seminal bladder_--a name which suits their function, since
they are usually filled with semen at the generation season. The seminal
bladders communicate with the urinogenital cloaca by wide openings, and it
is on the borders of these openings that the mouths of the Wolffian duct
and ureters must be looked for. My embryological investigations, though
they have not been specially directed to this point, seem to shew that the
seminal bladders do not arise during embryonic life, and are still absent
in very young individuals. It seems probable that both the bladders and the
urinogenital cloaca are products of the lower extremities of the Wolffian
duct. The only other duct requiring any notice in the male is the
rudimentary oviduct. As was first shewn by Semper, rudiments of the upper
extremities of the oviducts, with their abdominal openings, are to be found
in the male in the same position as in the female, on the front surface of
the liver.
In the female the same ducts are present as in the male, viz. the Wolffian
duct and the ureters. The part of the Wolffian duct which receives the
secretion of the Wolffian body is not contorted, but is otherwise similar
to the homologous part of the Wolffian duct in the male. The Wolffian ducts
of the two sides fall independently into an unpaired urinal cloaca, but
their lower ends, instead of remaining simple as in the male, become
dilated into urinary bladders. Vide Pl. 20, fig. 2. There were nine ureters
in the example dissected, whose arrangement did not differ greatly from
that in the male--the hinder ones remaining distinct from each other, but a
certain amount of fusion, the extent of which could not be quite certainly
ascertained, taking place between the anterior ones. The arrangement of the
openings of these ducts is not quite the same as in the male. A somewhat
magnified representation of it is given in Pl. 20, fig. 3, _o.u._ The two
Wolffian ducts meet at so acute an angle that their hindermost extremities
are only separated by a septum. In the region of this septum on the inner
walls of the two Wolffian ducts were situated the openings of the ureters,
of which there were five on each side arranged linearly. In a second
example, also adult, I found four distinct openings on each side similarly
arranged to those in the specimen described. Professor Semper states that
all the ureters in the female unite into a _single duct_ before opening
into the Wolffian duct. It will certainly surprise me to find such great
variations in different individuals of this species as is implied by the
discrepancy between Professor Semper's description and my own.
The main difference between the ureters in the male and female consists in
their falling into the urinogenital cloaca in the former and into the
Wolffian duct in the latter. Since, however, the urinogenital cloaca is a
derivative of the Wolffian duct, this difference between the two sexes is
not a very important one. The urinary cloaca opens, in the female, into the
general cloaca by a median papilla of somewhat smaller dimensions than the
corresponding papilla in the male. Seminal bladders are absent in the
female, though possibly represented by the bladder-like dilatations of the
Wolffian duct. The oviducts, whose anatomy is too well known to need
description, open independently into the general cloaca.
Since the publication of Professor Semper's researches on the urinogenital
system of Elasmobranch fishes, it has been well known that, in most adult
Elasmobranchii, there are present a series of funnel-shaped openings,
leading from the perivisceral cavity, by the intermediation of a short
canal, into the glandular tubuli of the kidney. These openings are called
by Professor Semper, _Segmentaltrichter_, and by Dr Spengel, in his
valuable work on the urogenital system of Amphibia, _Nephrostomen_. In the
present work the openings will be spoken of as segmental openings, and the
tubes connected with them as segmental tubes. Of these openings there are a
considerable number in the adults of both sexes of Scy. canicula, situated
along the inner border of each kidney. The majority of them belong to the
Wolffian body, though absent in the extreme anterior part of this. In very
young examples a few certainly belong to the region of the kidney proper.
Where present, there is one for each segment[342]. It is not easy to make
certain of their exact number. In one male I counted thirteen. In the
female it is more difficult than in the male to make this out with
certainty, but in one young example, which had left the egg but a short
time, there appeared to be at least fourteen present. According to Semper
there are thirteen funnels in both sexes--a number which fairly well agrees
with my own results. In the male, rudiments of segmental tubes are present
in all the anterior segments of the Wolffian body behind the vasa
efferentia, but it is not till about the tenth segment that the first
complete one is present. In the female a somewhat smaller number of the
anterior segments, six or seven, are without segmental tubes, or only
possess them in a rudimentary condition.
Footnote 342: The term segment will be more accurately defined
below.
A typical segment of the Wolffian body or kidney, in the sense in which
this term has been used above, consists of a number of factors, each of
which will be considered in detail with reference to its variations. On
Pl. 20, fig. 5, is represented a portion of the Wolffian body with three
complete segments and part of a fourth. If one of these be selected, it
will be seen to commence with (1) a segmental opening, somewhat oval in
form (_st.o_) and leading directly into (2) a narrow tube, the segmental
tube, which takes a more or less oblique course backwards, and, passing
superficially to the Wolffian duct (_w.d_), opens into (3) a Malpighian
body (_p.mg_) at the anterior extremity of an isolated coil of glandular
tubuli. This coil forms the fourth section of each segment, and starts from
the Malpighian body. It consists of a considerable number of rather
definite convolutions, and after uniting with tubuli from one or two
(according to size of the segment) accessory Malpighian bodies (_a.mg_),
smaller than the one into which the segmental tube falls, eventually opens
by a (5) narrowish tube into the Wolffian duct at the posterior end of the
segment. Each segment is completely isolated (except for certain
rudimentary structures to be alluded to shortly) from the adjoining ones,
_and never has more than one segmental tube and one communication with the
Wolffian duct_.
The number and general arrangement of the segmental tubes have already been
spoken of. Their openings into the body-cavity are, in Scyllium, very
small, much more so than in the majority of Elasmobranchii. The general
appearance of a segmental tube and its opening is somewhat that of a spoon,
in which the handle represents the segmental tube, and the bowl the
segmental opening. Usually amongst Elasmobranchii the openings and tubes
are ciliated, but I have not determined whether this is the case in Scy.
canicula, and Semper does not speak definitely on this point. From the
segmental openings proceed the segmental tubes, which in the front segments
have nearly a transverse direction, but in the posterior ones are directed
more and more obliquely backwards. This statement applies to both sexes,
but the obliquity is greater in the female than in the male.
As has been said, each segmental tube normally opens into a Malpighian
body, from which again there proceeds the tubulus, the convolutions of
which form the main mass of each segment. This feature can be easily seen
in the case of the Malpighian bodies of the anterior part of the Wolffian
gland in young examples, and sometimes fairly well in old ones, of either
sex[343]. There is generally in each segment a second Malpighian body,
which forms the commencement of a tubulus joining that from the primary
Malpighian body, and, where the segments are larger, there are three, and
possibly in the hinder segments of the Wolffian gland and segments of the
kidney proper, more than three Malpighian bodies.
Footnote 343: My observations on this subject completely
disprove, if it is necessary to do so after Professor Semper's
investigations, the statement of Dr Meyer, that segmental tubes
in Scyllium open into lymph organs.
The accessory Malpighian bodies, or at any rate one of them, appear to have
curious relations to the segmental tubes. The necks of some of the anterior
segmental tubes (Pl. 20, fig. 5) close to their openings into the primary
Malpighian bodies are provided with a small knob of cells which points
towards the preceding segment and is usually connected with it by a fibrous
band. This knob is most conspicuous in the male, and in very young animals
or almost ripe embryos. In several instances in a ripe male embryo it
appeared to me to have a lumen, and to be continued directly forwards into
the accessory Malpighian body of the preceding segment. One such case is
figured in the middle segment on Pl. 20, fig. 5. In this embryo segmental
tubes were present in the segments immediately succeeding those connected
with the vasa efferentia, and at the same time these segments contained
ordinary and accessory Malpighian bodies. The segmental tubes of these
segments were not, however, connected with the Malpighian body of their
proper segment, but instead, turned forwards and entered the segment in
front of that to which they properly belonged. I failed to trace them quite
definitely to the accessory Malpighian body of the preceding segment, but,
in one instance at least, there appeared to me to be present a fibrous
connection, which is shewn in the figure already referred to, Pl. 20,
fig. 5, _r.st_. In any case it can hardly be doubted that this peculiarity
of the foremost segmental tubes is related to what would seem to be the
normal arrangement in the next few succeeding segments, where each
segmental tube is connected with a Malpighian body in its own segment, and
more or less distinctly with an accessory Malpighian body in the preceding
segment.
In the male the anterior segmental tubes, which even in the embryo exhibit
signs of atrophy, become in the adult completely aborted (as has been
already shewn by Semper), and remain as irregular tubes closed at both
ends, which for the most part do not extend beyond the Wolffian duct
(Pl. 20, fig. 4, _r.st_). In the adult, the first two or three segments
with these aborted tubes contain only accessory Malpighian bodies; the
remaining segments, with aborted segmental tubes, both secondary and
primary Malpighian bodies. In neither case are the Malpighian bodies
connected with the aborted tubes.
The Malpighian bodies in Scyllium present no special peculiarities. The
outer layer of their capsule is for the most part formed of flattened
cells; but, between the opening of the segmental tube and the efferent
tubulus of the kidney, their cells become columnar. Vide Pl. 20, fig. 5.
The convoluted tubuli continuous with them are, I believe, ciliated in
their proximal section, but I have not made careful investigations with
reference to their finer structure. Each segment is connected with the
Wolffian duct by a single tube at the hinder end of the segment. In the
kidney proper, these tubes become greatly prolonged, and form the ureters.
It has already been stated that the semen is carried by vasa efferentia
from the testes to the anterior segments of the Wolffian body, and thence
through the coils of the Wolffian body to the Wolffian duct. The nature of
the vasa will be discussed in the embryological section of this chapter: I
shall here confine myself to a simple description of their anatomical
relations. The consideration of their connections naturally falls under
three heads: (1) the vasa efferentia passing from the testes to the
Wolffian body, (2) the mode in which these are connected with the Wolffian
body, and (3) with the testis.
In Pl. 20, fig. 4, drawn for me from nature by my friend Mr Haddon, are
shewn the vasa efferentia and their junctions both with the testes and the
kidney. This figure illustrates better than any description the anatomy of
the various parts. Behind there are two simple vasa efferentia (_v.e._) and
in front a complicated network of vasa, which might be regarded as formed
of either two or four main vessels. It will be shewn in the sequel that it
is really formed of four distinct vessels. Professor Semper states that
there is but a single vas efferens in Scyllium canicula, a statement which
appears to me unquestionably erroneous. All the vasa efferentia fall into a
_longitudinal duct (l.c)_, which is connected in succession with the
several segments of the Wolffian body (one for each vas efferens) which
appertain to the testis. The hind end of the longitudinal duct is simple,
and ends blindly close to its junction with the last vas efferens; but in
front, where the vasa efferentia are complicated, the longitudinal duct
also has a complicated constitution, and forms a network rather than a
simple tube. It typically sends off a duct to join the coils of the
Wolffian body between each pair of vasa efferentia, and is usually swollen
where this duct parts from it. A duct similar to this has been described by
Semper as _Nierenrandcanal_ in several Elasmobranchii, but its existence is
expressly denied in the case of Scyllium! It is usually found in Amphibia,
as we know from Bidder and Spengel's researches. Spengel calls it
_Längscanal des Hoden_; the vessels from it into the kidney he calls _vasa
efferentia_, and the vessels to it, which I speak of as vasa efferentia, he
calls _Quercanale_.
The exact mode of junction of the separate vasa efferentia with the testis
is difficult to make out on account of the opacity of the basal portion of
the testis. My figure shews that there is a network of tubes (formed of
four main tubes connected by transverse branches) which is a continuation
of the anterior vasa efferentia, and joined by the two posterior ones.
These tubes receive the tubuli coming from the testicular ampullæ. The
whole network may be called, with Semper, the _testicular network_. While
its general relations are represented in my figure, the opacity of the
testes was too great to allow of all the details being with certainty
filled in.
The kidneys of Scyllium stellare, as might be expected, closely resemble
those of Scy. canicula. The ducts of the kidney proper, have, in the former
species, a larger number of distinct openings into the urinogenital cloaca.
In two male examples I counted seven distinct ureters, though it is not
impossible that there may have been one or two more present. In one of my
examples the ureters had seven distinct openings into the cloaca, in the
other five openings. In a female I counted eleven ureters opening into the
Wolffian duct by seven distinct openings. In the remaining parts of the
excretory organs the two species of Scyllium resemble each other very
closely.
As may be gathered from Prof. Semper's monograph, the excretory organs of
Scyllium canicula are fairly typical for Elasmobranchii generally. The
division into kidney and Wolffian body is universal. The segmental openings
may be more numerous and larger, _e.g._ Acanthias and Squatina, or absent
in the adult, _e.g._ Mustelus and Raja. Bladder-like swellings of the
Wolffian duct in the female appear to be exceptional, and seminal bladders
are not always present. The variations in the ureters and their openings
are considerable, and in some cases all the ureters are stated to fall into
a single duct, which may be spoken of as the ureter _par excellence_[344],
with the same relations to the kidneys as the Wolffian duct bears to the
Wolffian body. In some cases Malpighian corpuscles are completely absent in
the Wolffian body, _e.g._ Raja.
Footnote 344: I feel considerable hesitation in accepting
Semper's descriptions of the ureters and their openings. It has
been shewn above that for Scyllium his statements are probably
inaccurate, and in other instances, _e.g._ Raja, I cannot bring
my dissections to harmonise with his descriptions.
The vasa efferentia of the testes in Scyllium are very typical, but there
are some forms in which they are more numerous as well as others in which
they are less so. Perhaps the vasa efferentia are seen in their most
typical form in Centrina as described and figured (Pl. XXI) by Professor
Semper, or in Squatina vulgaris, as I find it, and have represented it on
Pl. 20, fig. 8. From my figure, representing the anterior part of the
Wolffian body of a nearly ripe embryo, it will be seen that there are five
vasa efferentia (_v.e_) connected on the one hand with a longitudinal canal
at the base of the testes (_n.t_) and on the other with a longitudinal
canal in the Wolffian body. Connected with the second longitudinal canal
are four Malpighian bodies, three of them stalked and one sessile; from
which again proceed tubes forming the commencements of the coils of the
anterior segments of the Wolffian body. These Malpighian bodies are clearly
my primary Malpighian bodies, but there are in Squatina, even in the
generative segments, secondary Malpighian bodies. What Semper has described
for Centrina and one or two other genera, closely correspond with what is
present in Squatina.
_Development of the Segmental Tubes._
On p. 345, _et seq._ an account was given of the first formation of the
segmental tubes and the segmental duct, and the history of these bodies was
carried on till nearly the period at which it is taken up in the exhaustive
Memoir of Professor Semper. Though the succeeding narration traverses to a
great extent the same ground as Semper's Memoir, yet many points are
treated somewhat differently, and others are dealt with which do not find a
place in the latter. In the majority of instances, attention is called to
points on which my results either agree with, or are opposed to, those of
Professor Semper.
From previous statements it has been rendered clear that _at first_ the
excretory organs of Elasmobranchii exhibit no division into Wolffian body
or kidney proper. Since this distinction is merely a question of the ducts,
and does not concern the glandular tubuli, no allusion is made to its
appearance in the present section, which deals only with the glandular part
of the kidneys and not with their ducts.
Up to the close of stage K the urinogenital organs consist of a segmental
duct opening in front into the body-cavity, and terminating blindly behind
in close contact with the cloaca, and of a series of segmental tubes, each
opening into the body-cavity on the inner side of the segmental duct, but
ending blindly at their opposite extremities. It is with these latter that
we have at present to deal. They are from the first directed obliquely
backwards, and coil close round the inner and dorsal sides of the segmental
duct. Where they are in contact (close to their openings into the
body-cavity) with the segmental duct, the lumen of the latter diminishes
and so comes to exhibit regular alternations of size. This is shewn in
Pl. 12, fig. 18, _s.d_. At the points where the segmental duct has a larger
lumen, it eventually unites with the segmental tubes.
The segmental tubes rapidly undergo a series of changes, the character of
which may be investigated, either by piecing together transverse sections,
or more easily from longitudinal and vertical sections. They acquire a
Lambda-shaped form with an anterior limb opening into the body-cavity and
posterior limb, resting on a dilated portion of the segmental duct. The
next important change which they undergo consists in a junction being
effected between their posterior limbs and the segmental duct. In the
anterior part of the body these junctions appear before the commencement of
stage L. A segmental tube at this stage is shewn in longitudinal section on
Pl. 21, fig. 7_a_, and in transverse section on Pl. 18, fig. 2. In the
former the actual openings into the body-cavity are not visible. In the
transverse section only one limb of the Lambda is met with on either side
of the section; the limb opening into the body-cavity is seen on the left
side, and that opening into the segmental duct on the right side. This
becomes quite intelligible from a comparison with the longitudinal section,
which demonstrates that it is clearly not possible to see more than a
single limb of the Lambda in any transverse section.
After the formation of their junctions with the segmental duct, other
changes soon take place in the segmental tubes. By the close of stage L
four distinct divisions may be noticed in each tube. Firstly, there is the
opening into the body-cavity, with a somewhat narrow stalk, to which the
name segmental tube will be strictly confined in the future, while the
whole products of the original segmental tube will be spoken of as a
segment of the kidney. This narrow stalk opens into a vesicle (Pl. 18,
fig. 2, and 21, fig. 6), which forms the second division. From the vesicle
proceeds a narrower section forming the third division, which during stage
L remains very short, though in later stages it grows with great rapidity.
It leads into the fourth division, which constitutes the posterior limb of
the Lambda, and has the form of a dilated tube with a narrow opening into
the segmental duct.
The subsequent changes of each segment do not for the most part call for
much attention. They consist mainly in the elongation of the third
division, and its conversion into a coiled tubulus, which then constitutes
the main mass of each segment of the kidney. There are, however, two points
of some interest, viz. (1) the formation of the Malpighian bodies, and (2)
the establishment of the connection between each segmental tube and the
tubulus of the preceding segment which was alluded to in the description on
p. 486. The development of the Malpighian body is intimately linked with
that of the secondary connection between two segments. They are both
products of the metamorphosis of the vesicle which forms the termination of
the segmental tube proper.
At about stage O this vesicle grows out in two directions (Pl. 21,
fig. 10), viz. towards the segment in front (_p.x_) and posteriorly into
the segment of which it properly forms a part (_mg_). That portion which
grows backward remains continuous with the third division of its proper
segment, and becomes converted into a Malpighian body. It assumes (Pl. 21,
figs. 6 and 10) a hemispherical form, while near one edge of it is the
opening from a segmental tube, and near the other the opening leading into
a tubulus of the kidney. The two-walled hemisphere soon grows into a nearly
closed sphere, with a central cavity into which projects a vascular tuft.
For this tuft the thickened inner wall of cells forms a lining, and at the
same time the outer wall becomes thinner, and formed of flattened cells,
except in the interval between the openings of the segmental tube and
kidney tubulus, where its cells remain columnar.
The above account of the formation of the Malpighian bodies agrees very
well with the description which Pye[345] has given of the formation of
these bodies in the embryonic Mammalian kidney. My statements also agree
with those of Semper, in attributing the formation of the Malpighian body
to a metamorphosis of part of the vesicle at the end of the segmental tube.
Semper does not however enter into full details on this subject.
Footnote 345: _Journal of Anatomy and Physiology_, Vol. IX.
The elucidation of the history of the second outgrowth from the original
vesicle towards the preceding segment is fraught with considerable
difficulties, which might no doubt be overcome by a patient investigation
of ample material, but which I have not succeeded in fully accomplishing.
The points which I believe myself to have determined are illustrated by
fig. 10, Pl. 21, a longitudinal vertical section through a portion of the
kidney between stages O and P. In this figure parts of three segments of
the kidney are represented. In the hindermost of the three--the one to the
right--there is a complete segmental tube (_s.t_) which opens at its upper
extremity into an irregular vesicle, prolonged _behind_ into a body which
is obviously a developing Malpighian body, _m.g_, and in _front_ into a
wide tube cut obliquely in the section and ending apparently blindly
(_p.x_). In the preceding segment there is also a segmental tube (_s.t_)
whose opening into the body-cavity passes out of the plane of the section,
but which is again connected with a vesicle dilating behind into a
Malpighian body (_m.g_) and in front into the irregular tube (_p.x_), as in
the succeeding segment, _but this tube is now connected_ (and this could be
still more completely seen in the segment in front of this) _with a vesicle
which opens into the thick-walled collecting tube (fourth division) of the
preceding segment_ close to the opening of the latter into the Wolffian
duct. The fact that the anterior prolongation of the vesicle ends blindly
in the hinder-most segment is due of course to its terminal part passing
out of the plane of the section. _Thus we have established between stages O
and P a connection between each segmental tube and the collecting tube of
the segment in front of that to which it properly belongs; and it further
appears that in consequence of this each segment of the kidney contains two
distinct coils of tubuli which only unite close to their common opening
into the Wolffian duct!_
This remarkable connection is not without morphological interest, but I am
unfortunately only able to give in a fragmentary manner its further
history. During the greater part of embryonic life a large amount of
interstitial tissue is present in the embryonic kidneys, and renders them
too opaque to be advantageously studied as a whole; and I have also, so
far, failed to prepare longitudinal sections suitable for the study of this
connection. It thus results that the next stage I have satisfactorily
investigated is that of a nearly ripe embryo already spoken of in
connection with the adult, and represented on Pl. 20, fig. 5. This figure
shews that each segmental tube, while distinctly connected with the
Malpighian body of its own segment, also sends out a branch towards the
secondary Malpighian body of the preceding segment. This branch in most
cases appeared to be rudimentary, and in the adult is certainly not
represented by more than a fibrous band, but I fancy that I have been able
to trace it (though not with the distinctness I could desire) in surface
views of the embryonic kidney of stage Q. _The condition of the Wolffian
body represented on Pl. 20, fig. 5 renders it probable that the accessory
Malpighian body in each segment is developed in connection with the
anterior growth from the original vesicle at the end of the segmental tube
of the succeeding segment._ How the third or fourth accessory Malpighian
bodies, when present, take their origin I have not made out. It is,
however, fairly certain that they form the commencement of two additional
coils which unite, like the coil connected with the first accessory
Malpighian body, with the collecting tube of the primitive coil close to
its opening into the Wolffian duct or ureter.
The connection above described between two successive kidney segments
appears to have escaped Professor Semper's notice, though I fancy that the
peculiar vesicle he describes, _loc. cit._ p. 303, as connected with the
end of each segmental tube, is in some way related to it. It seems possible
that the secondary connection between the segmental tube and the preceding
segment may explain a peculiar observation of Dr Spengel[346] on the kidney
of the tailless Amphibians. He finds that, in this group, the segmental
tubes do not open into Malpighian bodies, but into the fourth division of
the kidney tube. Is it not just possible that in this case the primitive
attachment of the segmental tubes may have become lost, and a secondary
attachment, equivalent to that above described, though without the
development of a secondary Malpighian body, have been developed? In my
embryos the secondary coil of the segmental tubes opens, as in the Anura,
into the fourth section of a kidney tubulus.
Footnote 346: _Loc. cit._ pp. 85-89.
_Development of the Müllerian and Wolffian ducts._
The formation of the Müllerian and Wolffian ducts out of the original
segmental duct has been dealt with in a masterly manner by Professor
Semper, but though I give my entire assent to his general conclusions, yet
there are a few points on which I differ from him. These are for the most
part of a secondary importance; but they have a certain bearing on the
homology between the Müllerian duct of higher Vertebrates and that of
Elasmobranchii. The following account refers to Scy. canicula, but so far
as my observations go, the changes in Scy. stellare are nearly identical in
character.
I propose treating the development of these ducts in the two sexes
separately, and begin with the female.
Shortly before stage N a horizontal split arises in the segmental
duct[347], commencing some little distance from its anterior extremity, and
extending backwards. This split divides the duct into a dorsal section and
a ventral one. The dorsal section forms the Wolffian duct, and receives the
openings of the segmental tubes, and the ventral one forms the Müllerian
duct or oviduct, and is continuous with the unsplit anterior part of the
primitive segmental duct, which opens into the body-cavity. The nature of
the splitting may be gathered from the woodcut, fig. 6, p. 511, where _x_
represents the line along which the segmental duct is divided. The
splitting of the primitive duct extends slowly backwards, and thus there is
for a considerable period a single duct behind, which bifurcates in front.
A series of transverse sections through the point of bifurcation always
exhibits the following features. Anteriorly two separate ducts are present,
next two ducts in close juxtaposition, and immediately behind this a single
duct. A series of sections through the junction of two ducts is represented
on Plate 21, figs. 1A, 1B, 1C, 1D.
Footnote 347: For the development of the segmental duct, vide
p. 345, _et seq._
In my youngest example, in which the splitting had commenced, there were
two separate ducts for only 14 sections, and in a slightly older one for
about 18. In the second of these embryos the part of the segmental duct
anterior to the front end of the Wolffian duct, which is converted directly
into the oviduct, extended through 48 sections. In the space included in
these 48 sections at least five, and I believe six, segmental tubes with
openings into the body-cavity were present. These segmental tubes did not
however unite with the oviduct, or at best, but one or two rudimentary
junctions were visible, and the evidence of my earlier embryos appears to
shew that the segmental tubes in front of the Wolffian duct never become in
the female united with the segmental duct. The anterior end of the Wolffian
duct is very much smaller than the oviduct adjoining it, and as the reverse
holds good in the male, an easy method is afforded of distinguishing the
two sexes even at the earliest period of the formation of the Wolffian
duct.
Hitherto merely the general features of the development of the oviduct and
Wolffian duct have been alluded to, but a careful inspection of any good
series of sections, shewing the junction of these two ducts, brings to
light some features worth noticing in the formation of the oviduct. It
might have been anticipated that, where the two ducts unite behind as the
segmental duct, their lumens would have nearly the same diameter, but
normally this appears to be far from the case.
To illustrate the formation of the oviduct I have represented a series of
sections through a junction in an embryo in which the splitting into two
ducts had only just commenced (Pl. 21, fig. 1), but I have found that the
features of this series of sections are exactly reproduced in other series
in which the splitting has extended as far back as the end of the small
intestine. In the series represented (Pl. 21) 1A is the foremost section,
and 1D the hindermost. In 1A the oviduct (_od_) is as large or slightly
larger than the Wolffian duct (_w.d_), and in the section in front of this
(which I have not represented) was considerably the larger of the two
ducts. In 1B the oviduct has become markedly smaller, but there is no
indication of its lumen becoming united with that of the Wolffian duct--the
two ducts, though in contact, are distinctly separate. In 1C the walls of
the two ducts have fused, and the oviduct appears merely as a ridge on the
under surface of the Wolffian duct, and its lumen, though extremely minute,
_shews no sign of becoming one with that of the Wolffian duct_. Finally, in
1D the oviduct can merely be recognised as a thickening on the under side
of the segmental duct, as we must now call the single duct, but a slight
bulging downwards of the lumen of the segmental duct appears to indicate
that the lumens of the two ducts may perhaps have actually united. But of
this I could not be by any means certain, and it seems quite possible that
the lumen of the oviduct never does open into that of the segmental duct.
The above series of sections goes far to prove that the posterior part of
the oviduct is developed as a nearly solid ridge split off from the under
side of the segmental duct, into which at the utmost a very small portion
of the lumen of the latter is continued. One instance has however occurred
amongst my sections which probably indicates that the lumen of the
segmental duct may sometimes, in the course of the formation of the oviduct
and Wolffian duct, become divided into two parts, of which that for the
oviduct, though considerably smaller than that for the Wolffian duct, is
not so markedly so as in normal cases (Pl. 21, fig. 2).
Professor Semper states that the lumen of the part of the oviduct split off
from the hindermost end of the segmental duct becomes continuously smaller,
till at last close to the cloaca it is split off as a solid rod of cells
without a lumen, and thus it comes about that the oviduct, when formed,
ends blindly, and does not open into the cloaca till the period of sexual
maturity. My own sections do not include a series shewing the formation of
a terminal part of the oviduct, but Semper's statements accord precisely
with what might probably take place if my account of the earlier stages in
the development of the oviduct is correct. The presence of a hymen in young
female Elasmobranchii was first made known by Putmann and Garman[348], and
subsequently discovered independently by Semper[349].
Footnote 348: "On the Male and Female Organs of Sharks and
Skates, with special reference to the use of the claspers,"
_Proceed. American Association for Advancement of Science_,
1874.
Footnote 349: _Loc. cit._
The Wolffian duct appears to receive its first segmental tube at its
anterior extremity.
In the male the changes of the original segmental duct have a somewhat
different character to those in the female, although there is a fundamental
agreement between the two sexes. As in the female, a horizontal split makes
its appearance a short way behind the front end of the segmental duct, and
divides this into a dorsal Wolffian duct and a ventral Müllerian duct, the
latter continuous with the anterior section of the segmental duct, which
carries the abdominal opening. The differences in development between the
two sexes are, in spite of a general similarity, very obvious. In the first
place, the ventral portion split off from the segmental duct, instead of
being as in the female larger in front than the Wolffian duct, is very much
smaller; while behind it does not form a continuous duct, but in some parts
a lumen is present, and in others again absent (Pl. 21, fig. 6). It does
not even form an unbroken cord, but is divided in disconnected portions.
Those parts with a lumen do not appear to open into the Wolffian duct.
The process of splitting extends gradually backwards, so that there is a
much longer rudimentary Müllerian duct by stage O than by stage N. By stage
P the posterior portions of the Müllerian ducts have vanished. The anterior
parts remain, as has been already stated, till adult life. A second
difference between the male and female depends on the fact that, in the
male, the splitting of the segmental duct into Müllerian duct and Wolffian
duct never extends beyond the hinder extremity of the small intestine. A
third and rather important point of difference consists in the splitting
commencing far nearer the front end of the segmental duct in the male than
in the female. In the female it was shewn that about 48 sections intervened
between the front end of the segmental duct and the point where this became
split, and that this region included five or six segmental tubes. In the
male the homologous space only occupies _about 7 to 12 sections, and does
not contain the rudiment of more than a single segmental tube_. Although my
sections have not an absolutely uniform thickness, yet the above figures
suffice to shew in a conclusive manner that the splitting of the segmental
duct commences far further forwards in the male than in the female. This
difference accounts for two facts which were mentioned in connection with
the excretory organs of the adult, viz. (1) the greater length of the
Wolffian body in the male than in the female, and (2) the fact that
although a nearly similar number of segmental tubes persist in the adults
of both sexes, yet that in the male there are five or six more segments in
front of the first fully developed segmental opening than in the female.
The above description of the formation of the Müllerian duct in the male
agrees very closely with that of Professor Semper for Acanthias. For
Scyllium however he denies, as it appears to me erroneously, the existence
of the posterior rudimentary parts of the Müllerian duct. He further
asserts that the portions of the Müllerian duct with a lumen open into the
Wolffian duct. The most important difference, however, between Professor
Semper's and my own description consists in his having failed to note that
the splitting of the segmental duct commences much further forwards in the
male than in the female.
I have attempted to shew that the oviduct in the female, with the exception
of the front extremity, is formed as a nearly solid cord split off from the
ventral surface of the segmental duct, and not by a simple splitting of the
segmental duct into two equal parts. If I am right on this point, it
appears to me far easier to understand the relationship between the oviduct
or Müllerian duct of Elasmobranchii and the Müllerian duct of Birds, than
if Professor Semper's account of the development of the oviduct is the
correct one. Both Professor Semper and myself have stated our belief in the
homology of the ducts in the two cases, but we have treated their
relationship in a very different way. Professor Semper[350] finds himself
compelled to reject, on theoretical grounds, the testimony of recent
observers on the development of the Müllerian duct in Birds, and to assert
that it is formed out of the Wolffian duct, or, according to my
nomenclature, 'the segmental duct.' In my account[351], the ordinary
statements with reference to the development of the Müllerian duct in Birds
are accepted; but it is suggested that the independent development of the
Müllerian duct may be explained by the function of this duct in the adult
having, as it were, more and more impressed itself upon the embryonic
development, till finally all connection, even during embryonic life,
between the oviduct and the segmental duct (Wolffian duct) became lost.
Footnote 350: _Loc. cit._ pp. 412, 413.
Footnote 351: "The Urinogenital Organs of Vertebrates,"
_Journal of Anatomy and Physiology_, Vol. X. p. 47. [This
edition, p. 164.]
Since finding what a small portion of the segmental duct became converted
into the Müllerian duct in Elasmobranchii, I have reexamined the
development of the Müllerian duct in the Fowl, in the hope of finding that
its posterior part might develop nearly in the same manner as in
Elasmobranchii, at the expense of a thickening of cells on the outer
surface of the Wolffian duct. I have satisfied myself, in conjunction with
Mr Sedgwick, that this is not the case, and that the general account is in
the main true; but at the same time we have obtained evidence which tends
to shew that the cells which form the Müllerian duct are in part derived
from the walls of the Wolffian duct. We propose giving a full account of
our observations on this point, so that I refrain from mentioning further
details here. It may however be well to point out that, apart from
observations on the actual development of the Müllerian duct in the Bird,
the fact of its abdominal opening being situated some way behind the front
end of the Wolffian duct, is of itself a sufficient proof that it cannot be
the metamorphosed front extremity of the Wolffian (= segmental) duct, in
the same way that the abdominal opening of the Müllerian duct is the front
extremity of the segmental duct in Elasmobranchii.
Although the evidence I can produce in the case of the Fowl of a direct
participation of the Wolffian duct in the formation of the Müllerian is not
of an absolutely conclusive kind, yet I am inclined to think that the
complete independence of the two ducts, if eventually established as a
fact, would not of itself be sufficient (as Semper is inclined to think) to
disprove the identity of the Müllerian duct in Birds and Elasmobranchii.
We have, no doubt, almost no knowledge of the magnitude of the changes
which can take place in the mode of development of the same organ in
different types, yet this would have to be placed at a very low figure
indeed in order to exclude the possibility of a change from the mode of
development of the Müllerian duct in Elasmobranchii to that in Birds. We
have, it appears to me, in the smallness of the portion of the segmental
duct which goes to form the Müllerian duct in Elasmobranchii, evidence that
a change has already appeared in this group in the direction of a
development of the Müllerian duct independent of the segmental duct, and
therefore of the Wolffian duct; and it has been in view of this
consideration, that I have devoted so much attention to the apparently
unimportant point of how much of the segmental duct was concerned in the
formation of the Müllerian duct. An analogous change, in a somewhat
different direction, would seem to be taking place in the development of
the rudimentary Müllerian duct in the male Elasmobranchii.
It is, perhaps, just worth pointing out, that the blindness of the oviduct
of female Elasmobranchii, and its mode of development from an imperfect
splitting of the segmental duct, may probably be brought into connection
with the blindness of the extremity of the Müllerian duct or oviduct which
so often occurs in both sexes of Sturgeons (Accipenser).
I may, perhaps, at this point, be permitted to say a few words about my
original account of the development of the Wolffian duct This account was
incorrect, and based upon a false interpretation of an imperfect series of
sections, and I took the opportunity, in a general account of the
urinogenital system of Vertebrates, to point out my mistake[352]. Professor
Semper has, however, subsequently done me the honour to discuss, at
considerable length, my original errors, and to attempt to explain them.
Since it appears to me improbable that the continuation of such a
discussion can be of much general interest, it will suffice to say now,
that both Professor Semper's and my own original statements on the
development of the Wolffian duct were erroneous; but that both of us have
now recognised our mistakes; and that the first morphologically correct
account of the development was given by him.
Footnote 352: _Journal of Anatomy and Physiology_, Vol X.
1875. [This edition, No. VII.]
* * * * *
With reference to the formation of the urinal cloaca there is not much to
say. The originally widely separated openings of the two Wolffian ducts
gradually approximate in both sexes. By stage O (Pl. 19, fig. 1_b_) they
are in close contact, and the lower ends of the two ducts actually coalesce
at a somewhat later period, and open by a single aperture into the common
cloaca. The papilla on which this is situated begins to make its appearance
considerably before the actual fusion of the lower extremities of the two
ducts.
_Formation of Wolffian Body and Kidney proper._
Between stages L and M the hindermost ten or eleven segments of the
primitive undivided excretory organ commence to undergo changes which
result in their separation from the anterior segments as a distinct gland,
which was spoken of in the description of the adult as the kidney proper,
while the unaltered preceding segments of the kidney were spoken of as the
Wolffian body.
It will be remembered that each segment of the embryonic kidney consists of
four divisions, the last or fourth of which opens into the Wolffian duct.
The changes which take place in the hindermost ten or eleven segments, and
cause them to become distinguished as the kidney proper, concern alone the
fourth division of each segment, which becomes prolonged backwards, and its
opening into the Wolffian duct proportionately shifted. These changes
affect the foremost segments of the kidney much more than the hindermost,
so that the fourth division in the foremost segments becomes very much
longer than in the hindermost, and at last all the prolongations of the
kidney segments come to open nearly on the same level, close to the cloacal
termination of the Wolffian duct (Pl. 21, fig. 8). The prolongations of the
fourth division of the kidney-segments have already (p. 481) been spoken of
in the description of the adult as ureters, and this name will be employed
for them in the present section.
The exact manner in which the changes, that have been briefly related, take
place is rather curious, and very difficult to unravel without the aid of
longitudinal sections. First of all, the junction between each segment of
the kidney and the Wolffian duct becomes so elongated as to occupy the
whole interval between the junctions of the two neighbouring segments. The
original opening of each tube into the Wolffian duct is situated at the
anterior end of this elongated attachment, the remaining part of the
attachment being formed solely of a ridge of cells on the dorsal side of
the Wolffian duct. The general character of this growth will be understood
by comparing figs. 7_a_ and 7_b_, Pl. 21--two longitudinal vertical
sections through part of the kidneys. Fig. 7 _a_ shews the normal junction
of a segmental tube with the Wolffian duct in the Wolffian body, while in
figure 7_b_ (_r.u_) is shewn the modified junction in the region of the
kidney proper in the same embryo. The latter of these figures (fig. 7_b_)
appears to me to prove that the elongation of the attachments between the
segmental tubes and Wolffian duct takes place _entirely at the expense of
the former_. Owing to the length of this attachment, every transverse
section through the kidney proper at this stage either presents a solid
ridge of cells closely adhering to the dorsal side of the Wolffian duct, or
else passes through one of the openings into the Wolffian duct.
During stage M the original openings of the segmental tubes into the
Wolffian duct appear to me to become obliterated, and at the same time the
lumen of each ureter is prolonged into the ridge of cells on the dorsal
wall of the duct.
Both of these changes are illustrated in my figures. The fact of the
obliteration of the original opening into the Wolffian duct is shewn in
longitudinal section in Pl. 21, fig. 9, _u_, but more conclusively in the
series of transverse sections represented on Pl. 21, figs. 3A, 3B, 3C. In
the hindermost of these (3C) is seen the solid terminal point of a ureter,
while the same ureter possesses a lumen in the two previous sections, but
exhibits no signs of opening into the Wolffian duct. Sections may however
be met with which appear to shew that in some instances the ureters still
continue to open into the Wolffian duct, but these I find to be rare and
inconclusive, and am inclined to regard them as abnormalities. The
prolongation of the lumen of the ureters takes place in a somewhat peculiar
fashion. The lumen is not, as might be expected, _completely_ circumscribed
by the wall of the ureter, but only _dorsally and to the sides_. Ventrally
it is closed in by the dorsal wall of the Wolffian duct. In other words,
each ureter is at first an incomplete tube. This peculiarity is clearly
shewn in the middle figure of the series on Pl. 21, fig. 3B.
During stages M and N the ureters elongate considerably, and, since the
foremost ones grow the most rapidly, they soon come to overlap those
behind. As each ureter grows in length it remains an incomplete tube, and
its lumen, though proportionately prolonged, continues to present the same
general relations as at first. It is circumscribed by its proper walls only
dorsally and laterally; its floor being formed in the case of the front
ureter by the Wolffian duct, and in the case of each succeeding ureter by
the dorsal wall of the ureter in front. This is most easily seen in
longitudinal sections, and is represented on Pl. 21, fig. 9, or on a larger
scale in fig. 9A. In the latter figure it is especially clear that while
the wall on the dorsal side of the lumen of each ureter is continuous with
the dorsal wall of the tubulus of its own segment, the wall on the ventral
side is continuous with the dorsal wall of the ureter of the preceding
segment. This feature in the ureters explains the appearance of transverse
sections in which the ureters are not separate from each other, but form
together a kind of ridge on the dorsal side of the Wolffian duct, in which
there are a series of perforations representing the separate lumens of the
ureters (Pl. 21, fig. 4). The peculiarities in the appearance of the dorsal
wall of the Wolffian duct in fig. 9A, and the difference between the cells
composing it and those of the ventral wall, become intelligible on
comparing this figure with the representation of transverse section in
figs. 3B and 3C, and especially in fig. 4. Most of the ureters continue to
end blindly at the close of stage N, and appear to have solid posterior
terminations like that of the Müllerian duct in Birds.
By stage O all the ureters have become prolonged up to the cloacal end of
the Wolffian duct, so that the anterior one has a length equal to that of
the whole kidney proper. For the most part they acquire independent
openings into the end section of the Wolffian duct, though some of them
unite together before reaching this. The general appearance of the
hindermost of them between stages N and O is shewn in longitudinal and
vertical section in Pl. 21, fig. 8, _u_.
They next commence to develop into complete and independent tubes by their
side walls growing inwards and meeting below so as to completely enclose
their lumen. This is seen already to have occurred in most of the posterior
ureters in Pl. 21, fig. 8.
Before stage P the ureters cease to be united into a continuous ridge, and
each becomes separated from its neighbours by a layer of indifferent
tissue: by this stage, in fact, the ureters have practically attained very
nearly their adult condition. The general features of a typical section
through them are shewn on Pl. 21, fig. 5. The figure represents the section
of a female embryo, not far from the cloaca. Below is the oviduct (_od_).
Above this again is the Wolffian duct (_w.d_), and still dorsal to this are
four ureters (_u_). In female embryos more than four ureters are not
usually to be seen in a single section. This is probably owing to the
persistence, in some instances, of the intimate connection between the
ureters found at an earlier stage of development, and results in a single
ureter coming to serve as the collecting duct for several segments. A
section through a male embryo of stage P would mainly differ from that
through a female in the absence of the oviduct, and in the presence of
probably six[353], instead of four, ureters.
Footnote 353: This at least holds good for one of my embryos
at this stage, which is labelled Scy. canicula, but which may
possibly be Scy. stellare.
The exact amount of fusion which takes place between the ureters, and the
exact number of the ureters, cannot easily be determined from sections, but
the study of sections is chiefly of value in shewing the general nature of
the changes which take place in the process of attaining the adult
condition.
It may be noticed, as a consequence of the above account, that the
formation of the ureters takes place by a growth of the original segmental
tubes, and not by a splitting off of parts of the wall of the Wolffian
duct.
The formation of ureters in Scyllium, which has been only very cursorily
alluded to by Professor Semper, appears to differ very considerably from
that in Acanthias as narrated by him.
_The Vasa Efferentia._
A comparison of the results of Professor Semper on Elasmobranchii, and Dr
Spengel on Amphibians, suggests several interesting questions with
reference to the development of the vasa efferentia, and the longitudinal
canal of the Wolffian body.
Professor Semper was the first to describe the adult anatomy and
development of vasa efferentia in Elasmobranchii, and the following
extracts will fully illustrate his views with reference to them.
"In[354] dem frühesten Stadium finden sich wie früher angegeben ungefahr 34
Trichter in der Leibeshöhle, von diesen gehen die 27 hintersten in die
persistirenden Segmentaltrichter über, von denen 4 beim erwachsenen Thiere
auf dem Mesorchium stehen. Die übrigen 7 schliessen sich vollständig ab zu
den erwähnten länglichen und später mannigfach auswachsenden varicösen
Trichterblasen; von diesen sind es wiederum 3-4 welche untereinander in der
Längsrichtung verwachsen und dadurch den in der Basis der Hodenfalte
verlaufenden Centralcanal des Hodens bilden. Ehe aber diese Verwachsung zu
einem mehr oder minder geschlängelten Centralcanal vollständig wird, hat
sich einmal das Lumen der Trichterblasen fast vollständig geschlossen und
ausserdem von ihnen aus durch Verwachsung und Knospung die erste Anlage des
rete vasculosum Halleri gebildet (Taf. XX. Figs. 1, 2_c_). Es erstreckt
sich nämlich mehr oder minder weit in die Genitalfalte hinein ein
unregelmässiges von kleinen Zellen begränztes Canalnetz welches zweifellos
mit dem noch nicht ganz vollständigen Centralcanale des Hodens (Taf. XX.
Fig. 2_c_) in Verbindung steht. Von diesem letzteren aus gehen in
regelmässigen Abständen die Segmentalgänge (Taf. XX. Fig. 2 _sg._) gegen
die Niere hin; da sie meist stark geneigt oder selbst geschlängelt (bei
6{ctm} langen Embryonen) gegen die Niere zu verlaufen, wo sie sich an die
primären _Malpighi_'schen Körperchen und deren Bildungsblasen ansetzen, so
kann ein verticaler Querschnitt auch nie einen solchen nun zum vas efferens
gewordenen Segmentalgang seiner ganzen Länge nach treffen. Gegen die
Trichterfurche zu aber steht namentlich am hinteren Theile der Genitalfalte
der Centralcanal häufig noch durch einen kurzen Zellstrang mit dem
Keimepithel der Trichterfurche in Verbindung; mitunter findet sich hier
sogar noch eine kleine Höhlung, Rest des ursprünglich hier vorhandenen
weiten Trichters" (Taf. XX. Fig. 3_c_).
Footnote 354: _Loc. cit._ p. 364.
And again: "Dieser[355] Gegensatz in der Umbildung der Segmentalgänge an
der Hodenbasis scheint nun mit einem anderen Hand in Hand zu gehen. Es
bildet sich nämlich am Innenrande der Niere durch Sprossung und Verwachsung
der Segmentalgänge vor ihrer Insertion an das primäre _Malpighi_'sche
Körperchen ein Canal beim Männchen aus, den ich als _Nierenrandcanal_ oben
bezeichnet habe. Ich habe denselben bei Acanthias Centrina (Taf. XXI.
Fig. 13) und Mustelus (Taf. XV. Fig. 8) gefunden. Bei Centrina ist er
ziemlich lang und vereinigt mindestens 7 Segmentalgänge, aber von diesen
letzteren stehen nur 5 mit dem Hodennetz in Verbindung. Dort nun wo diese
letzteren sich an den Nierenrandcanal ansetzen (Taf. XXI. Fig. 13
sg.1-sg.5) findet sich jedesmal ein typisch ausgebildetes _Malpighi_'sches
Körperchen, mit dem aber nun nicht mehr wie ursprünglich nur 2 Canäle
verbunden sind (Taf. XXI. Fig. 14) sondern 3. Einer dieser letzteren ist
derjenige Ast des Nierenrandcanals welcher die Verbindung mit dem nächst
folgenden Segmentalgang zu besorgen hat. An den Stellen aber wo sich an den
Nierenrandcanal die hinteren blind gegen den Hoden hin endenden
Segmentalgänge ansetzen fehlen diese _Malpighi_'schen Körperchen (Taf. XXI.
Fig. 13 _sg_7) vollständig. Auch bei Mustelus (Taf. XV. Figs. 8, 10) findet
genau dasselbe Verhältniss statt; da aber hier nur 2 (oder 3)
Segmentalgänge zu vasa efferentia umgewandelt werden, so stehen hier am
kurzen Randcanal der Niere auch nur 2 oder 3 _Malpighi_'sche Körperchen.
Diese aber sind typisch ausgebildet" (Taf. XV. Fig. 10).
Footnote 355: _Loc. cit._ p. 395.
From these two extracts it is clear that Semper regards both the vasa
efferentia, and central canal of the testis network, as well as the
longitudinal canal of the Wolffian body, as products of the anterior
segmental tubes.
The appearance of these various parts in the fully grown embryos or adults
of such genera as Acanthias and Squatina strongly favours this view, but
Semper appears to have worked out the development of these structures
somewhat partially and by means of sections, a method not, in Scyllium at
least, very suitable for this particular investigation. I myself at first
unhesitatingly accepted Semper's views, and it was not till after the study
of the paper of Dr Spengel on the Amphibian kidney that I came to have my
doubts as to their accuracy. The arrangement of the parts in most
Amphibians is strikingly similar to that in Elasmobranchii. From the testis
come transverse canals corresponding with my vasa efferentia; these fall
into a longitudinal canal of the kidneys, from which again, as in Squatina
(Pl. 20, fig. 8), Mustelus and Centrina, canals (the vasa efferentia of
Spengel) pass off to Malpighian bodies. So far there is no difficulty, but
Dr Spengel has made the extremely important discovery, that in young
Amphibians each Malpighian body in the region of the generative ducts, in
addition to receiving the vasa efferentia, is connected with a fully
developed segmental tube opening into the body-cavity. In Amphibians,
therefore, it is improbable that the vasa efferentia are products of the
open extremities of the segmental tubes, considering that these latter are
found in their unaltered condition at the same time as the vasa efferentia.
When it is borne in mind how strikingly similar in most respects is the
arrangement of the testicular ducts in Amphibia and Elasmobranchii, it will
not easily be credited that they develop in entirely different methods.
Since then we find in Amphibians fully developed segmental tubes in the
same segments as the vasa efferentia, it is difficult to believe that in
Elasmobranchii the same vasa efferentia have been developed out of the
segmental tubes by the obliteration of their openings.
I set myself to the solution of the origin of the vasa efferentia by means
of surface views, after the parts had been made transparent in creosote,
but I have met with great difficulties, and so far my researches have only
been partially successful. From what I have been able to see of Squatina
and Acanthias, I am inclined to think that the embryos of either of these
genera would form far more suitable objects for this research than
Scyllium. I have had a few embryos of Squatina which were unfortunately too
old for my purpose.
Very early the vasa efferentia are fully formed, and their arrangement in
an embryo eight centimetres long is shewn in Pl. 20, fig. 6, _v.e_. It is
there seen that there are six if not seven vasa efferentia connected with a
longitudinal canal along the base of the testes (Semper's central canal of
the testis), and passing down like the segmental tubes to spaces between
the successive segments of the Wolffian body. They were probably connected
by a longitudinal canal in the Wolffian body, but this could not be clearly
seen. In the segment immediately behind the last vas efferens was a fully
developed segmental tube. This embryo clearly throws no light on the
question at issue except that on the whole it supports Semper's views. I
further failed to make out anything from an examination of still younger
embryos.
In a somewhat older embryo there was connected with the anterior vas
efferens a peculiar structure represented on Pl. 20, fig. 7, _r.st_? which
strangely resembled the opening of an ordinary segmental tube, but as I
could not find it in the younger embryo, this suggestion as to its nature,
is, at the best, extremely hazardous. If, however, this body really is the
remnant of a segmental opening, it would be reasonable to conclude that the
vasa efferentia are buds from the segmental tubes as opposed to their
openings; a mode of origin which is not incompatible with the discoveries
of Dr Spengel. I have noticed a remnant, somewhat similar to that in the
Scyllium embryo, close to the hindermost vas efferens in an embryo Squatina
(Pl. 20, fig. 8, _r.st_?).
With reference to the development of the longitudinal canal of the Wolffian
body, I am without observations, but it appears to me to be probably a
further development of the outgrowths of the vesicles of each segmental
tube, which were described in connection with the development of the
segmental tubes, p. 492. Were an anterior outgrowth of one vesicle to meet
and coalesce with the posterior outgrowth of the preceding vesicle, a
longitudinal canal such as actually exists would be the result. The central
canal of the base of the testes and the network connected with it in the
adult (Pl. 20, fig. 4), appear to be derivatives of the vasa efferentia.
I am thus compelled to leave open the question of the real nature of the
vasa efferentia, but am inclined to regard them as outgrowths from the
anterior segmental tubes, though not from their open terminations.
* * * * *
My views upon the homologies of the various parts of the urinogenital
system, the development of which has been described in the present chapter,
have already been expressed in a paper on Urinogenital organs of
Vertebrates[356]. Although Kölliker's[357] discovery of the segmental tubes
in Aves, and the researches of Spengel[358], Gasser[359], Ewart[360] and
others, have rendered necessary a few corrections in my facts, I still
adhere in their entirety to the views expressed in that paper, and feel it
unnecessary to repeat them in this place. I conclude the chapter with a
résumé of the development of the urinogenital organs in Elasmobranchii from
their first appearance to their permanent condition.
Footnote 356: _Journal of Anatomy and Physiology_, Vol. X.
[This edition, No. VII.]
Footnote 357: _Entwicklungsgeschichte des Menschen u. der
höheren Thiere._
Footnote 358: _Loc. cit._
Footnote 359: _Beiträge zur Entwicklungsg. d. Allantois d.
Müller'schen Gänge u. d. Afters._
Footnote 360: "Abdominal Pores and Urogenital Sinus of
Lamprey," _Journal of Anatomy and Physiology_, Vol. X. p. 488.
* * * * *
_Résumé._--The first trace of the urinary system makes its appearance as a
knob springing from the intermediate cell-mass opposite the fifth
protovertebra (woodcut, fig. 5A, _p.d_). This knob is the rudiment of the
abdominal opening of the segmental duct, and from it there grows backwards
to the level of the anus a solid column of cells, which constitutes the
rudiment of the segmental duct itself (woodcut, fig. 5B, _p.d_). The knob
projects towards the epiblast, and the column connected with it lies
between the mesoblast and epiblast. The knob and column do not long remain
solid, but the former acquires an opening into the body-cavity continuous
with a lumen, which makes its appearance in the latter.
[Illustration: FIG. 5.
TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL CLEFTS.
The sections illustrate the development of the segmental duct (_pd_) or
primitive duct of the kidneys. In _A_ (the anterior of the two sections)
this appears as a solid knob (_pd_) projecting towards the epiblast. In _B_
is seen a section of the column which has grown backwards from the knob in
_A_.
_spn._ rudiment of a spinal nerve; _mc._ medullary canal; _ch._ notochord;
_X._ string of cells below the notochord; _mp._ muscle-plate; _mp´._
specially developed portion of muscle-plate; _ao._ dorsal aorta; _pd._
segmental duct; _so._ somatopleure; _sp._ splanchnopleure; _pp._
pleuro-peritoneal or body-cavity; _ep._ epiblast; _al._ alimentary canal.]
While the lumen is gradually pushing its way backwards along the solid
rudiment of the segmental duct, the first traces of the segmental tubes, or
proper excretory organs, make their appearance in the form of solid
outgrowths of the intermediate cell-mass, which soon become hollow and open
into the body-cavity. Their blind ends curl obliquely backwards round the
inner and dorsal side of the segmental duct. One segmental tube makes its
appearance for each protovertebra, commencing with that immediately behind
the abdominal opening of the segmental duct, the last tube being situated a
short way behind the anus. Soon after their formation the blind ends of the
segmental tubes open into the segmental duct, and each of them becomes
divided into four parts. These are (woodcut 7) (1) a section carrying the
abdominal opening or segmental tube proper, (2) a dilated vesicle into
which this opens, (3) a coiled tubulus proceeding from (2) and terminating
in (4), a wider portion opening into the segmental duct. At the same time,
or shortly before this, each segmental duct unites with and opens into one
of the horns of the cloaca, and also retires from its primitive position
between the epiblast and mesoblast, and assumes a position close to the
epithelium lining the body-cavity. The general features of the excretory
organs at this period are diagrammatically represented on the woodcut,
fig. 6. In this fig. _p.d_ is the segmental duct and _o_ its abdominal
opening. _s.t_ points to the segmental tubes, the finer details of whose
structure are not represented in the diagram. The kidneys thus form at this
period an unbroken gland composed of a series of isolated coiled tubes, one
extremity of each of which opens into the body-cavity, and the other into
the segmental duct, which forms the only duct of the kidney, and
communicates at one end with the body-cavity, and at the other with the
cloaca.
[Illustration: FIG. 6.
DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN ELASMOBRANCH EMBRYO.
_pd._ segmental duct. It opens at _o_ into the body-cavity and at its other
extremity into the cloaca; _x._ line along which the division appears which
separates the segmental duct into the Wolffian duct above and the Müllerian
duct below; _st._ segmental tubes. They open at one end into the
body-cavity, and at the other into the segmental duct.]
The next important change concerns the segmental duct, which becomes
longitudinally split into two complete ducts in the female, and one
complete duct and parts of a second in the male. The manner in which this
takes place is diagrammatically represented in woodcut 6 by the clear line
_x_, and in transverse section in woodcut 7. The resulting ducts are the
(1) Wolffian duct dorsally, which remains continuous with the excretory
tubules of the kidney, and ventrally (2) the oviduct or Müllerian duct in
the female, and the rudiments of this duct in the male. In the female the
formation of these ducts takes place by a nearly solid rod of cells, being
gradually split off from the ventral side of all but the foremost part of
the original segmental duct, with the short undivided anterior part of
which duct it is continuous in front. Into it a very small portion of the
lumen of the original segmental duct is perhaps continued (Pl. 21, fig. 1A,
etc.). The remainder of the segmental duct (after the loss of its anterior
section and the part split off from its ventral side) forms the Wolffian
duct. The process of formation of the ducts in the male chiefly differs
from that in the female in the fact of the anterior undivided part of the
segmental duct, which forms the front end of the Müllerian duct, being
shorter, and in the column of cells with which it is continuous being from
the first incomplete.
[Illustration: FIG. 7.
DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A SCYLLIUM EMBRYO
ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND MÜLLERIAN DUCTS BY THE
LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.
_mc._ medullary canal; _mp._ muscle-plate; _ch._ notochord; _ao._ aorta;
_cav._ cardinal vein; _st._ segmental tube. On the one side the section
passes through the opening of a segmental tube into the body-cavity. On the
other this opening is represented by dotted lines, and the opening of the
segmental tube into the Wolffian duct has been cut through; _w.d._ Wolffian
duct; _m.d._ Müllerian duct. The section is taken through the point where
the segmental duct and Wolffian duct have just become separate; _gr._ The
germinal ridge with the thickened germinal epithelium; _l._ liver; _i._
intestine with spiral valve.]
The tubuli of the primitive excretory organ undergo further important
changes. The vesicle at the termination of each segmental tube grows
forwards towards the preceding tubulus, and joins the fourth section of it
close to the opening into the Wolffian duct (Pl. 21, fig. 10). The
remainder of the vesicle becomes converted into a Malpighian body. By the
first of these changes a connection is established between the successive
segments of the kidney, and though this connection is certainly lost (or
only represented by fibrous bands) in the anterior part of the excretory
organs in the adult, and very probably in the hinder part, yet it seems
most probable that traces of it are to be found in the presence of the
secondary Malpighian bodies of the majority of segments, which are most
likely developed from it.
Up to this time there has been no distinction between the anterior and
posterior tubuli of the primitive excretory organ which alike open into the
Wolffian duct. The terminal division of the tubuli of a considerable number
of the hindermost of these (ten or eleven in Scyllium canicula), either in
some species elongate, overlap, and eventually open by apertures (not
usually so numerous as the separate tubes), on nearly the same level, into
the hindermost section of the Wolffian duct in the female, or into the
urinogenital cloaca, formed by the coalesced terminal parts of the Wolffian
ducts, in the male; or in other species become modified in such a manner as
to pour their secretion into a single duct on each side, which opens in a
position corresponding with the numerous ducts of the other type (woodcut,
fig. 8). It seems that both in Amphibians and Elasmobranchii the type with
a single duct, or approximations to it, are more often found in the females
than in the males. The subject requires however to be more worked out in
Elasmobranchii[361]. In both groups the modified posterior kidney-segments
are probably equivalent to the permanent kidney of the amniotic
Vertebrates, and for this reason the numerous ducts of the first group or
single duct of the second were spoken of as ureters. The anterior tubuli of
the primitive excretory organ retain their early relation to the Wolffian
duct, and form the Wolffian body.
Footnote 361: The reverse of the above rule is the case with
Raja, in the male of which a closer approximation to the
single-duct type is found than in the female.
The originally separate terminal extremities of the Wolffian ducts always
coalesce, and form a urinal cloaca, opening by a single aperture situated
at the extremity of a median papilla behind the anus. Some of the abdominal
openings of the segmental tubes in Scyllium, or in other cases all the
openings, become obliterated.
In the male the anterior segmental tubes undergo remarkable modifications.
There appear to grow from the first three or four or more of them (though
the point is still somewhat obscure) branches, which pass to the base of
the testis and there unite into a longitudinal canal, form a network, and
receive the secretion of the testicular ampullæ (woodcut 9, _nt_). These
ducts, the vasa efferentia, carry the semen to the Wolffian body, but
before opening into the tubuli of this they unite into the _longitudinal
canal of the Wolffian body (l.c)_, from which pass off ducts equal in
number to the vasa efferentia, each of which normally ends in a Malpighian
body. From the Malpighian body so connected start the convoluted tubuli of
what may be called the generative segments of the Wolffian body along which
the semen is conveyed to the Wolffian duct (_v.d_). The Wolffian duct
itself becomes much contorted and acts as vas deferens.
[Illustration: FIG. 8.
DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT FEMALE
ELASMOBRANCH.
_m.d._ Müllerian duct; _w.d._ Wolffian duct; _s.t._ glandular tubuli; five
of them are represented with openings into the body-cavity; _d._ duct of
the posterior segmental tubes; _ov._ ovary.]
In the woodcuts, figs. 8 and 9, are diagrammatically represented the chief
constituents of the adult urinogenital organs in the two sexes. In the
adult female, fig. 8, there are present the following parts:
(1) The oviduct or Müllerian duct (_m.d_) split off from the segmental duct
of the kidneys. Each oviduct opens at its anterior extremity into the
body-cavity, and behind the two oviducts have independent communications
with the general cloaca.
(2) The Wolffian ducts (_w.d_), the other product of the segmental ducts of
the kidneys. They end in front by becoming continuous with the tubulus of
the anterior segment of the Wolffian body on each side, and unite behind to
open by a common papilla into the cloaca. The Wolffian duct receives the
secretion of the anterior part of the primitive kidney which forms the
Wolffian body.
(3) The ureter (_d_) which carries off the secretion of the kidney proper.
It is represented in my diagram in its most rare and differentiated
condition as a single duct.
(4) The glandular tubuli (_s.t_), some of which retain their original
openings into the body-cavity, and others are without them. They are
divided into two groups, an anterior forming the Wolffian body, which pour
their secretion into the Wolffian duct, and a posterior group forming the
kidney proper, which are connected with the ureter.
[Illustration: FIG. 9.
DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN AN ADULT MALE
ELASMOBRANCH.
_m.d._ rudiment of Müllerian duct; _w.d._ Wolffian duct, marked _vd_ in
front and serving as vas deferens; _st._ glandular tubuli; two of them are
represented with openings into the body-cavity; _d._ ureter; _t._ testis;
_nt._ central canal at the base of the testis; _VE._ vasa efferentia; _lc._
longitudinal canal of the Wolffian body.]
In the male the following parts are present (woodcut 9):
(1) The Müllerian duct (_md_), consisting of a small rudiment attached to
the liver representing the foremost end of the oviduct of the female.
(2) The Wolffian duct (_w.d_) which precisely corresponds to the Wolffian
duct of the female, but, in addition to functioning as the duct of the
Wolffian body, also acts as a vas deferens (_vd_). In the adult male its
foremost part has a very tortuous course.
(3) The ureter (_d_), which has the same fundamental constitution as in the
female.
(4) The segmental tubes (_st_). The posterior of these have the same
arrangement in both sexes, but in the male modifications take place in
connection with the anterior ones to fit them to act as transporters of the
testicular products.
Connected with the anterior ones there are present (1) the vasa efferentia
(VE), united on the one hand with (2) the central canal in the base of the
testis (_nt_), and on the other with the longitudinal canal of the Wolffian
body (_l.c_). From the latter are seen passing off the successive tubuli of
the anterior segments of the Wolffian body in connection with which
Malpighian bodies are typically present, though not represented in my
diagram.
_Postscript._
It was my original intention to have given an account of the development of
the generative organs. In the course, however, of my work a number of novel
and unexpected points turned up, which have considerably protracted my
investigations, and it has appeared to me better no longer to delay the
appearance of this monograph, but to publish elsewhere my results on the
generative organs. In chapter VI. p. 349 _et seq._ the early stages of the
generative organs are described, but in contemplation of the completion of
the account no allusion was made to their literature, and more especially
to Professor Semper's important contributions. I may perhaps say that I
have been able to confirm the most important result to which he and other
anatomists have nearly simultaneously arrived with respect to Vertebrates,
viz. _that the primitive ova give rise to both the male and female
generative products_.
EXPLANATION OF PLATES 20 AND 21.
COMPLETE LIST OF REFERENCE LETTERS.
_amg._ Accessory Malpighian body. _cav._ Cardinal vein. _ge._ Germinal
epithelium. _k._ True kidney. _l.c._ Longitudinal canal of the Wolffian
body connected with vasa efferentia. _mg._ Malpighian body. _nt._ Network
and central canal at the base of the testis. _o._ External aperture of
urinal cloaca. _od._ Oviduct or Müllerian duct of the female. _od´._
Müllerian duct of the male. _ou._ Openings of ureters in Wolffian duct in
the female (fig. 3). _pmg._ Primary Malpighian body. _px._ Growth from
vesicle at the end of a segmental tube to join the collecting tube of the
preceding segment. _rst._ Rudimentary segmental tube. _ru._ Ureter
commencing to be formed. _sb._ Seminal bladder. _sd._ Segmental duct. _st._
Segmental tube. _sto._ Opening of segmental tube into body-cavity. _sur._
Suprarenal body. _t._ Testis. _u._ Ureters. _ve._ Vas efferens. _wb._
Wolffian body. _wd._ Wolffian duct.
PLATE 20.
Fig. 1. Diagrammatic representation of excretory organs on one side of a
male Scyllium canicula, natural size.
Fig. 2. Diagrammatic representation of the kidney proper on one side of a
female Scyllium canicula, natural size, shewing the ducts of the kidney and
the dilated portion of the Wolffian duct.
Fig. 3. Opening of the ureters into the Wolffian duct of a female Scyllium
canicula. The figure represents the Wolffian ducts (_wd_) with ventral
portion removed so as to expose their inner surface, and shews the junction
of the two W. ducts to form the common urinal cloaca, the single external
opening of this (_o_), and openings of ureters into one Wolffian duct
(_ou_).
Fig. 4. Anterior extremity of Wolffian body of a young male Scyllium
canicula shewing the vasa efferentia and their connection with the kidneys
and the testis. The vasa efferentia and longitudinal canal are coloured to
render them distinct. They are intended to be continuous with the
uncoloured coils of the Wolffian body, though this connection has not been
very successfully rendered by the artist.
Fig. 5. Part of the Wolffian body of a nearly ripe male embryo of Scyllium
canicula as a transparent object. Zeiss a a, ocul. 3. The figure shews two
segmental tubes opening into the body-cavity and connected with a primary
Malpighian body, and also, by a fibrous connection, with a secondary
Malpighian body of the preceding segment. It also shews one segmental tube
(_rst_) imperfectly connected with the accessory Malpighian body of the
preceding segment of the kidney. The coils of the kidney are represented
somewhat diagrammatically.
Fig. 6. Vasa efferentia of a male embryo of Scyllium canicula eight
centimetres in length. Zeiss a a, ocul. 2.
There are seen to be at the least six and possibly seven distinct vasa
going to as many segments of the Wolffian body and connected with a
longitudinal canal in the base of the testis. They were probably also
connected with a longitudinal canal in the Wolffian body, but this could
not be clearly made out.
Fig. 7. The anterior four vasa efferentia of a nearly ripe embryo.
Connected with the foremost one is seen a body which looks like the remnant
of a segmental tube and its opening (_rst?_).
Fig. 8. Testis and anterior part of Wolffian body of an embryo of Squatina
vulgaris.
The figure is intended to illustrate the arrangement of the vasa
efferentia. There are five of these connected with a longitudinal canal in
the base of the testis, and with another longitudinal canal in the Wolffian
body. From the second longitudinal canal there pass off four ducts to as
many Malpighian bodies. Through the Malpighian bodies these ducts are
continuous with the several coils of the Wolffian body, and so eventually
with the Wolffian duct. Close to the hindermost vas efferens is seen a body
which resembles a rudimentary segmental tube (_rst?_).
PLATE 21.
Figs. 1A, 1B, 1C, 1D. Four sections from a female Scyllium canicula of a
stage between M and N through the part where the segmental duct becomes
split into Wolffian duct and oviduct. Zeiss B, ocul. 2. 1A is the foremost
section.
The sections shew that the oviduct arises as a thickening on the under
surface of the segmental duct into which at the utmost a very narrow
prolongation of the lumen of the segmental duct is carried. The small size
of the lumen of the Wolffian duct in the foremost section is due to the
section passing through nearly its anterior blind extremity.
Fig. 2. Section close to the junction of the Wolffian duct and oviduct in a
female embryo of Scyllium canicula belonging to stage N. Zeiss B, ocul. 2.
The section represented shews that in some instances the formation of the
oviduct and Wolffian duct is accompanied by a division of the lumen of the
segmental duct into two not very unequal parts.
Figs. 3A, 3B, 3C. Three sections illustrating the formation of a ureter in
a female embryo belonging to stage N. Zeiss B, ocul. 2.
3A is the foremost section.
The figures shew that the lumen of the developing ureter is enclosed in
front by an independent wall (fig. 3A), but that further back the lumen is
partly shut in by the subjacent Wolffian duct, while behind no lumen is
present, but the ureter ends as a solid knob of cells without an opening
into the Wolffian duct.
Fig. 4. Section through the ureters of the same embryo as fig. 3, but
nearer the cloaca. Zeiss B, ocul. 2.
The figure shews the appearance of a transverse section through the wall of
cells above the Wolffian duct formed by the overlapping ureters, the lumens
of which appear as perforations in it. It should be compared with fig. 9A,
which represents a longitudinal section through a similar wall of cells.
Fig. 5. Section through the ureters, the Wolffian duct and the oviduct of a
female embryo of Scy. canicula belonging to stage P. Zeiss B, ocul. 2.
Fig. 6. Section of part of the Wolffian body of a male embryo of Scyllium
canicula belonging to stage O. Zeiss B, ocul. 2.
The section illustrates (1) the formation of a Malpighian body (_mg_) from
the dilatation at the end of a segmental tube, (2) the appearance of the
rudiment of the Müllerian duct in the male (_od´_).
Figs. 7_a_, 7_b_. Two longitudinal and vertical sections through part of
the kidney of an embryo between stages L and M. Zeiss B, ocul. 2.
7_a_ illustrates the parts of a single segment of the Wolffian body at this
stage, vide p. 491. The segmental tube and opening are not in the plane of
the section, but the dilated vesicle is shewn into which the segmental tube
opens.
7_b_ is taken from the region of the kidney proper. To the right is seen
the opening of a segmental tube into the body-cavity, and in the segment to
the left the commencing formation of a ureter, vide p. 502.
Fig. 8. Longitudinal and vertical section through the posterior part of the
kidney proper of an embryo of Scyllium canicula at a stage between N and O.
Zeiss A, ocul. 2.
The section shews the nearly completed ureters, developing Malpighian
bodies, &c.
Fig. 9. Longitudinal and vertical section through the anterior part of the
kidney proper of the same embryo as fig. 8. Zeiss A, ocul. 2.
The figure illustrates the mode of growth of the developing ureters.
9A. More highly magnified portion of the same section as fig. 9.
Compare with transverse section fig. 4.
Fig. 10. Longitudinal and vertical section through part of the Wolffian
body of an embryo of Scyllium canicula at a stage between O and P.
The section contains two examples of the budding out of the vesicle of a
segmental tube to form a Malpighian body in its own segment and to unite
with the tubulus of the preceding segment close to its opening into the
Wolffian duct.
XI. ON THE PHENOMENA ACCOMPANYING THE MATURATION
AND IMPREGNATION OF THE OVUM[362].
Footnote 362: From the _Quarterly Journal of Microscopical
Science_, April, 1878.
The brilliant discoveries of Strasburger and Auerbach have caused the
attention of a large number of biologists to be turned to the phenomena
accompanying the division of nuclei and the maturation and impregnation of
the ovum. The results of the recent investigations on the first of these
points formed the subject of an article by Mr Priestley in the sixteenth
volume of this Journal, and the object of the present article is to give
some account of what has so far been made out with reference to the second
of them. The matters to be treated of naturally fall under two heads: (1)
the changes attending the ripening of the ovum, _which are independent of
impregnation_; (2) the changes which are directly due to impregnation.
[Illustration: FIG. 1.--Unripe ovum of Toxopneustes lividus (copied from
Hertwig).]
Every ovum as it approaches maturity is found to be composed (Fig. 1) of
(1) a protoplasmic body or vitellus usually containing yolk-spherules in
suspension; (2) of a germinal vesicle or nucleus, containing (3) one or
more germinal spots or nucleoli. It is with the germinal vesicle and its
contents that we are especially concerned. This body at its full
development has a more or less spherical shape, and is enveloped by a
distinct membrane. Its contents are for the most part fluid, but may be
more or less granular. Their most characteristic component is, however, a
protoplasmic network which stretches from the germinal spot to the
investing membrane, but is especially concentrated round the former
(Fig. 1). The germinal spot forms a nearly homogeneous body, with
frequently one or more vacuoles. It occupies an often excentric position
within the germinal vesicle, and is usually rendered very conspicuous by
its high refrangibility. In many instances it has been shewn to be capable
of amoeboid movements (Auerbach, and Os. Hertwig), and is moreover more
solid and more strongly tinged by colouring reagents than the remaining
constituents of the germinal vesicle. These peculiarities have caused the
matter of which it is composed to be distinguished by Auerbach and Hertwig
as nuclear substance.
In many instances there is only one germinal spot, or one main spot, and
two or three accessory smaller spots. In other cases, _e.g._ Osseous Fish,
there are a large number of nearly equal germinal spots. The eggs which
have been most investigated with reference to the changes of germinal
vesicle are those with a single germinal spot, and it is with these that I
shall have more especially to deal in the sequel.
The germinal vesicle occupies in the first instance a central position in
the ovum, but at maturity is almost always found in close proximity to the
surface. Its change of position in a large number of instances is
accomplished during the growth of the ovum in the ovary, but in other cases
does not take place till the ovum has been laid.
The questions which many investigators have recently set themselves to
answer are the two following:--(1) What becomes of the germinal vesicle
when the ovum is ready to be impregnated? (2) Is any part of it present in
the ovum at the commencement of segmentation? According to their answers to
these questions the older embryologists roughly fall into two groups: (1)
By one set the germinal vesicle is stated to completely disappear and not
to be genetically connected with the subsequent nuclei of the embryo. (2)
According to the other set it remains in the ovum and by successive
divisions forms the parent nucleus of all the nuclei in the body of the
embryo. Though the second of these views has been supported by several very
distinguished names the first view was without doubt the one most generally
entertained, and Haeckel (though from his own observations he was
originally a supporter of the second view) has even enunciated the theory
that there exists an anuclear stage, after the disappearance of the
germinal vesicle, which he regards as an embryonic repetition of the monad
condition of the Protozoa.
While the supporters of the first view agree as to the disappearance of the
germinal vesicle they differ considerably as to the manner of this
occurrence. Some are of opinion that the vesicle simply vanishes, its
contents being absorbed in the ovum; others that it is ejected from the
ovum and appears as the _polar cell_ or _body_, or _Richtungskörper_ of the
Germans--a small body which is often found situated in the space between
the ovum and its membrane, and derives its name from retaining a constant
position in relation to the ovum, and thus serving as a guide in
determining the similar parts of the embryo through the different stages.
The researches of Oellacher (15)[363] in this direction deserve special
mention, as having in a sense formed the foundation of the modern views
upon this subject. By a series of careful observations upon the egg of the
trout and subsequently of the bird, he demonstrated that the germinal
vesicle of the ovum, while still in the ovary, underwent partial
degeneration and eventually became ejected. His observations were made to a
great extent by means of sections, and the general accuracy of his results
is fairly certain, but the nature of the eggs he worked on, as well as
other causes, prevented his obtaining so deep an insight into the phenomena
accompanying the ejection of the germinal vesicle as has since been
possible. Lovén, Flemming (6), and others have been led by their
investigations to adopt views similar in the main to Oellacher's. As a
rule, however, it is held by believers in the disappearance of the germinal
vesicle that it becomes simply absorbed, and many very accurate accounts,
so far as they go, have been given of the gradual atrophy of the germinal
vesicle. The description of Kleinenberg (14) for Hydra, and Götte for
Bombinator, may perhaps be selected as especially complete in this respect;
in both instances the germinal vesicle commences to atrophy at a relatively
early period.
Footnote 363: The numbers appended to authors' names refer to
the list of publications at the end of the paper.
Coming to the more modern period the researches of five workers, viz.
Bütschli, E. van Beneden, Fol, Hertwig, and Strasburger have especially
thrown light upon this difficult subject. It is now hardly open to doubt
that while part of the germinal vesicle is concerned in the formation of
the polar cell or cells, when such are present, and is therefore ejected
from the ovum, part also remains in the ovum and forms a nuclear body which
will be spoken of as the _female pronucleus_, the fate of which is recorded
in the second part of this paper. The researches of Bütschli and van
Beneden have been especially instrumental in demonstrating the relation
between the polar bodies and the germinal vesicle, and those of Hertwig and
Fol, in shewing that part of the germinal vesicle remained in the ovum. It
must not, however, be supposed that the results of these authors are fully
substantiated, or that all the questions connected with these phenomena are
settled. The statements we have are in many points opposed and
contradictory, and there is much that is still very obscure.
In the sequel an account is first given of the researches of the
above-named authors, followed by a statement of those results which appear
to me the most probable.
The researches of van Beneden (3 and 4) were made on the ovum of the rabbit
and of Asterias, and from his observations on both these widely separated
forms he has been led to conclude that the germinal vesicle is either
ejected or absorbed, but that it has in no case a genetic connection with
the first segmentation sphere. He gives the following description of the
changes in the rabbit's ovum. The germinal vesicle is enclosed by a
membrane, and contains one main germinal spot, and a few accessory ones,
together with a granular material which he calls _nucleoplasma_, which
affects, as is usual in nuclei, a reticular arrangement. The remaining
space in the vesicle is filled by a clear fluid. As the ovum approaches
maturity the germinal vesicle assumes an excentric 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
nucleoplasma 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 polar bodies are the two
ejected products of the germinal vesicle. In the case of Asterias, van
Beneden has not observed the mode of formation of the polar bodies, and
mainly gives an account of the atrophy of the germinal vesicle, but adds
very little to what was already known to us from Kleinenberg's (14) earlier
observations. He describes with precision the breaking up of the germinal
spot into fragments and its eventual disappearance.
Though there are reasons for doubting the accuracy of all the above details
on the ovum of the rabbit, nevertheless, the observations of van Beneden
taken as a whole afford strong grounds for concluding that the formation of
the polar cells is connected with the disappearance, partial or otherwise,
of the germinal vesicle. A very similar account of the apparent
disappearance of the germinal vesicle is given by Greeff (19) who states
that the apparent disappearance of the germinal spot precedes that of the
vesicle.
The observations of Bütschli are of still greater importance in this
direction. He has studied with a view to elucidating the fate of the
germinal vesicle, the eggs of Nephelis, Lymnæus, Cucullanus, and other
Nematodes; and Rotifers. In all of these, with the exception of Rotifers,
he finds polar bodies, and in this respect his observations are of value as
tending to shew the widespread existence of these structures. Negative
results with reference to the presence of the polar bodies have, it may be
remarked, only a very secondary value. Bütschli has made the very important
discovery that in perfectly ripe eggs of Nephelis, Lymnæus and Cucullanus
and allied genera a _spindle_, similar to that of ordinary nuclei in the
act of division, appears close to the surface of the egg. This spindle he
regards as the metamorphosed germinal vesicle, and has demonstrated that it
takes part in the formation of the polar cells. He states that the whole
spindle is ejected from the egg, and that after swelling up and forming a
somewhat spherical mass it divides into three parts.
In the Nematodes generally, Bütschli has been unable to find the spindle
modification of the germinal vesicle, but he 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. This description of
Bütschli resembles in some respects that given by van Beneden of the
changes in the rabbit's ovum, and not impossibly refers to a nearly
identical series of phenomena. The discovery by Bütschli of the spindle and
its relation to the polar body has been of very great value.
The publications of van Beneden, and more especially those of Bütschli,
taken by themselves lead to the conclusion that the whole germinal vesicle
is either ejected or absorbed. Nearly simultaneously with their
publications there appeared, however, a paper by Oscar Hertwig (11) on the
eggs of one of the common sea urchins (_Toxopneustes lividus_), in which he
attempted to shew that part of the germinal vesicle, at any rate, was
concerned in the formation of the first segmentation nucleus. He believed
(though he has himself now recognised that he was in error on the point)
that no polar cell was formed in Toxopneustes, and that the whole germinal
vesicle was absorbed, with the exception of the germinal spot which
remained in the egg as the female pronucleus.
The following is the summary which he gives of his results, pp. 357-8.
"At the time when the egg is mature the germinal vesicle undergoes a
retrogressive metamorphosis and becomes carried towards the surface of
the egg by the contraction of the protoplasm. Its membrane becomes
dissolved and its contents disintegrated and finally absorbed by the
yolk. The germinal spot appears, however, to remain unaltered and to
continue in the yolk and to become the permanent nucleus of the ripe
ovum capable of impregnation."
After the publication of Bütschli's monograph, O. Hertwig (12) continued
his researches on the ova of Leeches (_Hæmopis_ and _Nephelis_), and not
only added very largely to our knowledge of the history of the germinal
vesicle, but was able to make a very important rectification in Bütschli's
conclusions. The following is a summary of his results:--The germinal
vesicle, as in other cases, undergoes a form of degeneration, though
retaining its central position; and the germinal spot breaks up into
fragments. The stages in which this occurs are followed by one when, on a
superficial examination, the ovum appears to be absolutely without a
nucleus; but there can be demonstrated by means of reagents in the position
previously occupied by the germinal vesicle a spindle nucleus with the
usual suns at its poles, which Hertwig believes to be a product of the
fragments of the germinal spot. This spindle travels towards the periphery
of the ovum and then forms the spindle observed by Bütschli. At the point
where one of the apices of the spindle lies close to the surface a small
protuberance arises which is destined to form the first polar cell. As the
protuberance becomes more prominent one half of the spindle passes into it.
The spindle then divides in the normal manner for nuclei, one half
remaining in the protuberance, the other in the ovum, and finally the
protuberance becomes a rounded body united to the egg by a narrow stalk. It
is clear that if, as there is every reason to think, the above description
is correct, the polar cell is formed by a simple process of cell-division
and not, as Bütschli believed, by the forcible ejection of the spindle.
The portion of the spindle in the polar cell becomes a mass of granules,
and that in the ovum becomes converted without the occurrence of the usual
nuclear stage into a fresh spindle. A second polar cell is formed in the
same manner as the first one, and the first one subsequently divides into
two. The portion of the spindle which remains in the egg after the
formation of the second polar cell reconstitutes itself into a nucleus--the
female pronucleus--and travelling towards the centre of the egg undergoes a
fate which will be spoken of in the second part of this paper.
The most obscure part of Hertwig's work is that which concerns the
formation of the spindle on the atrophy of the germinal vesicle, and his
latest paper, though it gives further details on this head, does not appear
to me to clear up the mystery. Though Hertwig demonstrates clearly enough
that this spindle is a product of the metamorphoses of the germinal
vesicle, he does not appear to prove the thesis which he maintains, that it
is the metamorphosed germinal spot.
Fol, to whom we are indebted in his paper on the development of Geryonia
(7) for the best of the earlier descriptions of the phenomena which attend
the maturation of the egg, and later for valuable contributions somewhat
similar to those of Bütschli with reference to the development of the
Pteropod egg (8), has recently given us a very interesting account of what
takes place in the ripe egg of _Asterias glacialis_ (9). In reference to
the formation of the polar cells, his results accord closely with those of
Hertwig, but he differs considerably from this author with reference to the
preceding changes in the germinal vesicle. He believes that the germinal
spot atrophies more or less completely, but that in any case its
constituents remain behind in the egg, though he will not definitely assert
that it takes no share in the formation of the spindle at the expense of
which both the polar cells and the female pronucleus are formed. The
spindle with its terminal suns arises, according to him, from the contents
of the germinal vesicle, loses its spindle character, travels to the
surface, and reacquiring a spindle character is concerned in the formation
of the polar cells in the way described by Hertwig.
Giard (10) gives a somewhat different account of the behaviour of the
germinal vesicle in _Psammechinus miliaris_. At maturity the contents of
the germinal vesicle and spot mix together and form an amoeboid mass,
which, assuming a spindle form, divides into two parts, one of which
travels towards the centre of the egg and forms the female pronucleus, the
other remains at the surface and gives origin to two polar cells, both of
which are formed after the egg is laid. What Giard regards as the female
pronucleus is perhaps the lower of the two bodies which take the place of
the original germinal vesicle as described by Fol. Vide the account of
Fol's observations on p. 531.
Strasburger, from observations on _Phallusia_, accepts in the main
Hertwig's conclusion with reference to the formation of the polar bodies,
but does not share Hertwig's view that either the polar bodies or female
pronucleus are formed at the expense of the germinal spot alone. He has
further shewn that the so-called canal-cell of conifers is formed in the
same manner as the polar cells, and states his belief that an equivalent of
the polar cells is widely distributed in the vegetable subkingdom.
This sketch of the results of recent researches will, it is hoped, suffice
to bring into prominence the more important steps by which the problems of
this department of embryology have been solved. The present aspects of the
question 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 (9), may conveniently be selected.
The ripe ovum (Fig. 2), when detached from the ovary, is formed of a
granular vitellus without a vitelline membrane, but enveloped in a
mucilaginous coat. It contains an excentrically situated germinal vesicle
and 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, its membrane becomes gradually absorbed and its outline indented
and indistinct, and finally its contents become to a certain extent
confounded with the vitellus (Fig. 3).
The germinal spot at the same time loses its clearness of outline and
gradually disappears from view.
[Illustration: FIG. 2.--Ripe ovum of Asterias glacialis enveloped in a
mucilaginous envelope, and containing an excentric germinal vesicle and
germinal spot (copied from Fol).]
[Illustration: FIG. 3.--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).]
[Illustration: FIG. 4.--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. 4), one
ovoid and nearer the surface, and the second more irregular in form and
situated rather deeper in the vitellus. By treatment with reagents the
first clear space is found to be formed of a spindle with two terminal suns
on the lower side of which is a somewhat irregular body (Fig. 5). The
second clear space by the same treatment is shewn to contain a round body.
Fol concludes that the spindle is formed out of part of the germinal
vesicle and not of 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. It may be observed that Fol is
here obliged to fill up (so far at least as his present preliminary account
enables me to determine) a lacuna in his observations in a hypothetical
manner, and O. Hertwig's (13) most recent observations on the ovum of the
same or an allied species of Asterias tend to throw some doubt upon Fol's
interpretations.
[Illustration: FIG. 5.--Ovum of Asterias glacialis, at the same stage as
Fig. 4, treated with picric acid (copied from Fol).]
The following is Hertwig's account of the changes in the germinal vesicle.
A quarter of an hour 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 above-mentioned
prominence towards the germinal vesicle, first one sun is formed by radial
striæ of protoplasm, and then a second makes its appearance, while in the
living ovum 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 the vacuole forms a rod-like body, the free end of which is
situated between the two suns which occupy the prominence of the germinal
vesicle. At a slightly later period granules may be seen at the end of the
rod and finally the rod itself vanishes. After these changes there may be
demonstrated by the aid of reagents a spindle between the two suns, which
Hertwig believes to grow in size as the last remnants of the germinal spot
gradually vanish, and he maintains, as before mentioned, that the spindle
is formed at the expense of the germinal spot. Without following Hertwig so
far as this[364] it may be permitted to suggest that his observations tend
to shew that the body noticed by Fol in the median line, on the inner side
of his spindle, is in reality a remnant of the germinal spot and not, as
Fol supposes, part of the germinal vesicle. Considering how conflicting is
the evidence before us it seems necessary to leave open for the present the
question as to what parts of the germinal vesicle are concerned in forming
the first spindle.
Footnote 364: Hertwig's full account of his observations, with
figures, in the 4th vol. of the _Morphologische Jahrbuch_, has
appeared since the above was written. The figures given
strongly support Hertwig's views.