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             By JAMES S. BAXTER, M.Sc., M.B., F.R.C.S.I.
           The Department of Anatomy, Queen's University, Belfast

A CONSIDERABLE amount of evidence has now been brought forward to show
that developmentally the vagina is a compound organ, in at least several
mammalian forms. Further, this evidence indicates that the mode of develop-
ment of the vagina differs very considerably in different orders and even
among the members of one order of Mammalia, and it appears certain that the
descriptions which seek to establish a uniform developmental process, differing
only in details, for the vagina in the mammals above the marsupials can now no
longer be accepted.
    In a recent paper (1) I have brought forward evidence to show that the
variations in the developmental process are closely correlated in the rodents
with the anatomical differences of the lower part of the female genito-urinary
tract. A study of the development of the vagina in the rabbit showed that the
upper portion was derived from the Mullerian ducts, while the lower and lesser
portion arose from the fusion of two Wolffian bulbs with one another. In the
rabbit the vagina communicates with the exterior through a fairly long uro-
genital sinus. In the rat (Mijsberg (2)) the same structures, Mullerian ducts and
Wolffian ducts, are concerned in the formation of the vagina, and, in addition,
the urogenital sinus divides into two canals and the dorsal canal forms the
lowest part of the vagina. In the rat the vagina opens directly to the exterior,
the genital tract being thus more completely emancipated from the urinary
tract than in the rabbit; and this emancipation is to be associated with the
additional developmental process at the lower end.
    The developmental processes of the rabbit and the rat suggest the nature of
the compound formation of the vagina, but before a complete understanding of
the morphology of the vagina becomes possible, descriptions of the develop-
mental processes in other mammalian orders are required and their correlation
with the varying anatomical forms must be established. The Ungulata have
not yet been extensively studied. The descriptions of the development of the
vagina in this order are not very complete and in several points they are
contradictory. I have studied the developmental processes in the pig as part
of the wider investigation of the development and morphology of the vagina
in the Mammalia on which I am at present engaged. In this paper I present the
description of the development of one form as a necessary preliminary to the
consideration of the vagina in the Ungulata generally.
240                                  James S. Baxter
    Felix and Bfihler(3) give the same description for ungulates as they do for
other mammals, namely, that in all the vagina is formed solely from the fused
Mullerian ducts. Henneberg(4) has described the formation of the external
genitals in the female pig, and he notes in his account that the urogenital sinus
is split into two parts (dorsal and ventral) from above downwards by a frontal
division. The anterior channel becomes the urethra, the posterior one aids in
the building of the vagina. In a female foetal pig of 12 cm. c.R. length Wood-
Jones (5) has described the fusion of the lower ends of the Mullerian ducts with
the Wolffian ducts and the entry of the latter ducts into the urogenital sinus.
He has interpreted this as a stage in development where there exists a median
Wolffian chamber into which the Millerian ducts will open later in foetal life.
Tourneux (6) gives an account of the vaginal anlage in a foetal horse, and shows
that in its lower part the Mullerian and Wolffian ducts blend to form a common
canal. On the other hand, Retterer (7) found that the lower part of the vagina in
the horse and the calf was split off from the upper part of the urogenital sinus
by the formation of a frontal septum. Bergschicker (quoted by Mijsberg)
studied the Mullerian and Wolffian ducts in the cow and found that the
Mullerian ducts fused with the urogenital sinus for some distance at their lower
ends. The Wolffian ducts thus obtained a definitive opening into the lower part
of the vagina.
                                            THE PIG
    The Mullerian ducts in the pig fuse with one another in the genital cord
to form the utero-vaginal canal in the same manner as in other mammals.
According to Tourneux and Legay (8) the fusion occurs when the foetus measures
about 60 mm. from vertex to rump and commences towards the middle of the
genital cord. My material does not show the place of first fusion of the
Mullerian ducts, but it does indicate clearly that the lower ends of the Mullerian
ducts remain separate until quite late in foetal life (see text-fig. 2).
    Shortly after its formation (8-2 cm. stage) the vaginal anlage consists of an
epithelial tube, oval in transverse section, which bifurcates at its lower end;
and each limb of the tube comes into separate contact with the dorsal wall of
the urogenital sinus. The wall of this tube and of the limbs consists of a single
layer of columnar epithelial cells, but the extreme lower ends of the separated
extremities are solid, each end being filled up with a mass of polyhedral cells.
This solid portion of each duct corresponds with the part in contact with the
urogenital sinus'.
    The Wolffian ducts appear to open into the urogenital sinus cephalad to the
level of contact of the Mullerian ducts with the sinus. At first sight this rela-
tionship between the ducts seems distinctly unusual, but closer examination
    1 The presence of a distinct basement membrane on the dorsal wall of the sinus, between it and
the tips of the Mullerian ducts, would make it seem very unlikely that an upward proliferation of
sinus epithelium into the lower ends of the ducts (" conus vaginalis " of Spuler) had occurred. These
cells are almost certainly Mullerian in origin and have been formed in situ by proliferation from the
growing tip of the duct.
                     Development of the Vagina in the Pig                                      241
suggests, I think, the interpretation. Inspection of the dorsal wall of the uro-
genital sinus in a wax-plate reconstruction of this stage (text-fig. 1) indicates
that the terminal part of the Wolffian duct has the form of an elongated funnel.
(This is confirmed by the study of a graphically reconstructed paramedian
section through the ducts and sinus (text-fig. 5 A).) There is an inwardly
projecting ridge on each side on the inner wall of the sinus. When traced
upwards this ridge is found to be continuous with the upper margin of the
apparent opening of the Wolffian duct; followed downwards it runs upon the
lateral wall of the sinus to the level of the termination of the Miillerian duct, and

             Text-fig. 1.                                      Text-fig. 2.
Text-fig. 1. Drawing of a wax-plate reconstruction of the lower end of the genital cord of an
     8-2 (Im. female foetal pig. The reconstruction is viewed from in front and slightly from ahove.
     The left half of the anterior wall of the urogenital sinus has been removed: (1) fused Miillerian
     (lucets; (2) Wolifian duct; (3) opening of left Wolifian duct into sinus recess; (4) overhanging
     edge of this opening continuous with fold on lateral wall of sinus; (.5) point of contact of
     Mijllerian duct with sinus.
Text-fig. 2. Drawing of a wax-plate reconstruction of the lower end of the genital cord in a female
     foetal pig of 12 4 (in. caR. length. Only the epithelial structures are represented, and the
     reconstruction is viewed from in front and from the left side: (1) fused Miillerian ducts;
     (2') Wolifian duct; (3J) Miillerian vaginal cord; (4) upgrowth of Wolifian epithelium on anterior
     aspect of left vaginal cord; (5) median canal; (6) urogenital sinus.

there it turns fairly sharply medialwards and disappears. A transverse section of
this part of the sinus (Plate I, fig. 1) shows that the two ridges are the anterior
boundaries of dorso-lateral bays, each bay being perfectly continuous in an
upward direetioni with the lumen of the corresponding Wolifian duct. The
epithelium lining the sinus and the bays is stratified, the cells being polymlor-
phous. It is lower in the bays (about three cells thick) than elsewhere on the
sinus walls, where it averages five to six cells in thickness, but there are no
definite cytological differences to be detected between the epithelia of the dorsal
and ventral walls of the sinus.
242                            James S. Baxter
     The tubular vaginal anlage now grows and enlarges both in length and
breadth. Longitudinal ridges begin to be evident upon the inner surface of the
hollow portion, especially towards the lower end (12.4 cm. stage). The lining
epithelium of this part is still seen, in sections perpendicular to the free surface,
to consist of a single layer of columnar cells. Traced downwards the vaginal
anlage bifurcates, each separate part being formed by a solid cord of epithelial
cells and representing the terminal non-fused part of a Mullerian duct (Plate I,
fig. 2). The corresponding Wolffian duct lies anterior, and somewhat lateral, to
the Mullerian cord. As these structures are traced caudally Wolffian cells
are found on the anterior aspect of each Mullerian cord (Plate I, fig. 3). These
cells are in the form of a solid mass which, when traced caudally, becomes
patent and soon establishes an open connection with the Wolffian duct proper,
the duct itself being now definitely enlarged (see text-fig. 5 B for the relations
of Wolffian and Mullerian epithelium at this stage). At a lower level the Wolffian
ducts fuse with one another in the middle line, the lumina however remaining
separate fo; a short distance (Plate I, fig. 4). The Mfillerian epithelium has
almost disappeared at this level, being represented only by a few cells on the
dorsal aspect of the left duct. A short distance further caudally the Mfillerian
epithelium has completely disappeared and the Wolffian ducts form a single
median canal. The wall of this canal consists of a stratified epithelium, the
constituent cells of which are characterised by the clearness of the cytoplasm.
The canal continues for some distance further caudally and then opens into
the dorsal part of the urogenital sinus (Plate I, fig. 5). In the sections at this
level the cells of the Wolffian canal quite markedly differ from the cells of the
sinus in the better staining of the cytoplasm of the sinus cells with Orange G.
     In a foetus of 17'5 cm. C.R. length the solid Mfillerian epithelial cords have
commenced to fuse in the middle line and they have become relatively shorter.
There is as yet no communication between the upper tubular (Mullerian) part
of the vagina and the median (Wolffian) canal below (Plate I, fig. 6). The Wolf-
fian ducts open into the Wolffian canal a short distance below the junction of
the solid Mullerian cords with it. Above its solid ends the Mullerian vaginal
anlage is lined with a single layer of columnar cells; and with the further-growth
the longitudinal folds of the epithelium in its upper part have become more
     At a slightly later stage (18.2 cm. C.R. length) the upper part of the vagina
is still lined with a single layer of columnar epithelial cells. The Wolffian ducts
lie anterior to this part of the vagina, one on each side of the middle line, and
are clothed with a cubical epithelium. As the sections are followed caudally
small isolated areas which show the first appearances of stratification of the
 epithelium are found in the vaginal mucosa. Further caudally these areas
become larger and more numerous, and they are finally continuous, so that for
 some distance above the terminal solid epithelial cord the vagina possesses a
 stratified epithelial lining, two or three cells thick. This part of the vagina
terminates in a very short epithelial cord, evidently the result of fusion of the
                   Development of the Vagina in the Pig                   243
paired cords of earlier stages. The lower end of this Mullerian cord shows a
sudden transition in cell type to the form which was previously recognised as
Wolffian in the mass of cells which connected the Mullerian epithelial cords
with the sinus at the 12-4 cm. stage. This Wolffian cord divides into two when
traced further caudally, each part becomes progressively larger, acquires a
lumen and finally re-unites with its fellow and with the corresponding Wolffian
duct. The united structure passes on as the median Wolffian canal and termi-
nates in the sinus.


                                                      ...   .....

              of urethra into
     (4) opening                urogen~~~~~~~~~~~~~~~~~~~~italsinus.

   A young sow (6 weeks old) measuring 35 cm. from vertex to rump afforded
me an opportunity to study the vagina with particular reference to the presence
or  absence of " hymeneal " structures. According to Weber (9) there is a
circular constriction where the vagina joins the urogenital canal in many
ungulates, and this constriction may be supplemented by transverse folds
of the mucous membrane. On laying open the vagina and urogenital sinus
of this specimen from the ventral aspect, the appearance figured in text-fig. 3
was found. There is clearly seen to be a circular fold of mucous membrane
constricting the vagina and separating off an upper part from the remainder.
This upper portion shows numerous delicate longitudinal ridges and furrows,
244                                     James S. Baxter
and quite definitely corresponds with the upper tubular part of the vaginal
anlage of early stages. A short distance below the circular fold there is seen
on each side the opening of Gartner's duct, and below this level is the opening
of the urethra into the urogenital sinus. A few longitudinal ridges of mucous
membrane run downwards from the openings of Girtner's ducts, but this
part of the vagina is relatively smooth walled. The "hymeneal" fold lies
therefore between the Mullerian vagina above and the median Wolffian canal
below, and in this position it does not guard the opening of the vagina into the
sinus. A vertical section through the "hymeneal" fold is shown in text-fig. 4.
It has the form of a transverse shelf in the vagina, more apparent from the
upper aspect. Both surfaces of this shelf are clothed with stratified squamous

                    2                                                        o

                                        *.       ...    ...            ...

                                   ..        ..' ....    ..;.

Text-fig. 4. Vertical section through "hymeneal" fold in a 35 cm. pig, x 25. (1) Circular muscle of
     Mullerian vagina; (2) circular muscle surrounding Wolffian vagina and separated from the
     first layer by a connective tissue septum. Mallory triple connective tissue stain.

epithelium. The vaginal wall a short distance above the level of the fold is
lined with a single layer of columnar cells. Probably this part of the vagina does
not become stratified until the time of sexual maturity. Wilson (1o) describes
it to be three or four cells thick in the -adult pig at the resting stage of the
sexual cycle. The muscular coat of the upper part of the vagina consists mainly
of bundles of smooth muscle circularly arranged. External to this layer are a
few longitudinal bundles. At the level of the hymen a new layer of circularly
disposed smooth muscle is found which replaces the other. It lies outside the
circular stratum of the upper part of the vagina and is separated from it by a
definite connective tissue septum. Both layers are, however, internal to the
stratum vasculare. At the region of the opening of the urethra into the
                  Development of the Vagina in the Pig                         245
urogenital sinus, striated muscle fibres from the pelvic floor are found to blend
with this second layer of smooth muscle.
     It has been shown in the foregoing description that, developmentally con-
sidered, there are three regions in the vagina of the pig: (1) an upper portion
which is formed from the lower part of the tubular utero-vaginal canal;
(2) below this, there is a region, quite short and narrow, derived from the solid
tips of the Mullerian ducts; and (3) next the sinus is a median canal into which
open the Wolffian ducts (ducts of Gartner in the adult). I propose now to
consider each of these parts and I shall designate them, for convenience,
the upper, the middle, and the lower vaginal segments.
     Upper segment. This part is formed from the fused Mullerian ducts in the
lower portion of the genital cord. It has for long the form of a simple epithelial
tube lined by a single layer of columnar cells. During development numerous
vertical ridges are formed on its inner surface. The transformation of the
columnar epithelial lining into a stratified one commences very late in foetal
life and is first to be observed at the lower end. From this part it spreads
slowly and irregularly upwards, and probably does not reach the cervix until
the time of sexual maturity. It is noteworthy that this segment of the vagina
never becomes solid during development; and in this, as also in the slow trans-
formation undergone by its epithelium, it is in strong contrast with the upper
part of the vagina in the human female. The retention of the primitive tubular
condition in the upper region of the vagina seems to be the rule in lower mam-
mals (it is found at least in several rodents, in some bats, and in the mole);
but to what extent this segment of the vagina is represented in higher forms, if
at all, can be determined only by the study of vaginal development in the
    Middle segment. The lower ends of the Mullerian ducts are separate from
one another in the early stages of development, and union between them does
not occur until the foetus has attained a considerable size (17.5 cm.). When
first observed the separated parts are solid at their tips, being formed there of
polyhedral epithelial cells. According to Nigela1) this solid condition is to be
expected, since these structures represent the growing parts of the Mullerian
ducts. At first these cell masses are in contact with the dorsal sinus recesses or
bays which, as I have already mentioned, I believe to be the expanded funnel
openings of the Wolffian ducts; later, the lower end of each duct is fused with
a small diverticulum from the median (Wolffian) canal of the lower segment
(12-4 cm. stage). This diverticulum is later drawn out on each side into an
epithelial cord, the cells of which are different in appearance from those of
the middle'segment proper. (These diverticula are further referred to below.)
The lower separated rudiments of the middle segment remain short, become
flattened antero-posteriorly and may be termed Mullerian vaginal cords
(Plate I, fig. 6). Union of these cords in the middle line occurs about the 17-5 cm.
246                           James S. Baxter
stage, and subsequent canalisation gives rise to the constricted part of the
vagina which is the "hymen." My preparations do not show canalisation to
have occurred at, or before, the 18-2 cm. stage. This does not agree with the
observations of MacCallum (12), who studied the Wolffian body and ducts by
the injection of colouring masses into the allantois. He noted that the Mullerian
ducts could be injected by this means in female foetuses of the pig measuring
120 mm. c.R. length or more. The pressure employed in this procedure might
force a passage between the two parts of the vagina before the histological
appearances of canalisation are to be appreciated. It seems certain to me, how-
ever, that nofunctional communication exists between the two parts up to the
18-2 cm. stage.
     The situation of the "hymen," marking out as it does the lower limit of the
Mullerian element in the vagina and delimiting this from the Wolffian contribu-
tion below, is precisely what I have previously described for the "hymen" in
the rabbit. Here, in the pig, the "hymen" is again seen to be placed at the
boundary zone between the actively growing Mullerian segment above and the
Wolffian duct segment below. The hymen in the human female (in which form
it has been most closely studied) would seem to be formed between the uro-
genital sinus below and whatever structures form the vagina above. What these
vaginal component parts are seems to me to be still undetermined in spite of the
large amount of work upon the subject. It is therefore not yet possible to say
if the hymeneal structures in these lower forms (pig and rabbit) have the same
developmental relationships as in the human subject. These would be truly
homologous only if the human vagina were a simple structure wholly derived
from the Miillerian ducts.
     Lower segment. This portion of the vagina develops rather later than the
other parts. The rudiment of it can be recognised at the 8&2 cm. stage, and it is
well developed in a foetus of 124 cm. C.R. length, where it forms a median canal
interposed between the Mullerian ducts and the urogenital sinus. The formation
of this canal appears to me to have been the result of the union from above
downwards of the two ridges found on the lateral wall of the sinus in the
8-2 cm. stage. The lower ununited ends of these folds are to be seen in Plate I,
fig. 5.
     Text-fig. 5 shows that there has been a "descent" of the Wolffian ducts
relative to the Mullerian ducts during the formation of the lower segment, but
the original contact area between the Mullerian ducts and the dorsal (Wolffian)
wall of the sinus remains the same. The Mullerian ducts consequently appear
to fuse with two projections from the dorsal wall of the median canal, and
further growth causes these projections to be drawn out into cords each con-
tinuous above with a Mullerian epithelial cord. When these cords fuse the
fusion process is continued and involves the Wolffian cords of the lower segment
as well.
    Two structures may enter into the formation of the lower vaginal segment,
namely, the Wolffian ducts and the urogenital sinus. My preparations would
                     Development of the Vagina in the Pig                                       247
indicate that the Wolffian ducts form this part of the genital canal. Harris (13)
has drawn attention to the fact that the dorsal wall of the urogenital sinus as
far down as Muller's tubercle is mesodermic in origin, being derived from the
Wolffian ducts in the series of changes which result in the formation of the base
of the bladder. If this is so for the pig (and my specimens indicate it), the
Mullerian ducts come in contact at their lower ends not with the urogenital
sinus but with the Wolffian contribution to this structure. When the median
canal is formed by union of the two sinus folds at least the dorsal wall of this
canal is Wolffian, and the Wolffian element is certainly increased by the changing
relationship of the Wolffian ducts to the Mullerian ducts. The cords of the lower


                A                                  B
Text-fig. 5. Paramedian sections through the vaginal anlagen and related structures in A an
     8-2 cm. foetus, and B a 12 4 cm. foetus. The drawings have been made from graphic recon-
     structions at the same magnification and represent comparable parts in the two stages. For
     convenience the Wolffian duct is represented as lying entirely in the plane of section in B;
     actually its upper part is further lateral. (1) Wolffian duct; (2) fused Mullerian ducts; (3) uro
     genital sinus; (4) Wolffian epithelium with which tips of Mullerian ducts lie in contact;
     (5) median canal formed by Wolffian ducts.

segment result from the drawing out of the Wolffian epithelium at the original
contact point, and at the same time more and more of the lower end of the
Wolffian ducts must be unrolled into the roof and anterior wall of the median
canal. The histological difference between the epithelium of the median canal
and the epithelium of the sinus in foetal life would also suggest that they are of
different derivation. The apparently anomalous relationship of the Wolffian
ducts to the lower ends of the Mullerian ducts requires further investigation,
and I hope to make this the subject of a subsequent communication. At
present it may be noted that all the growth processes involving the lower ends
of the Wolffian ducts in the pig seem to be delayed. Examination and recon-
struction of earlier stages in my possession show that the base of the bladder
248                           Jame8 S. Baxter
is late in formation, the ureters in a 35 mm. embryo still opening into the
Wolffian ducts.
    The conditions I have found at the 12-4 cm. stage confirm Wood-Jones'
observations, namely, that at this time a canal is interposed between the lower
ends of the Mullerian ducts and the urogenital sinus, and my specimens support
his hypothesis that this canal is probably derived from the Wolffian ducts.
    It may be thought that the formation of the lower vaginal segment by a
frontal septum in the sinus is the process observed by Henneberg. It does not
seem from Henneberg's description, however, that he actually observed any-
thing more than a descent of the point of union between the urethra and the
vagina from the level of the upper border of the symphysis pubis to below the
level of the lower border of that structure. Such a descent undoubtedly takes
place, but the conditions I have found do not warrant the assumption that it is
entirely due to septum formation in the urogenital sinus. On the contrary, the
septum formed in the sinus is relatively small, and the real process that
Henneberg observed is, I would suggest, a descent of the bladder and urethra
towards the pelvis.
    This investigation has not lent any support to the view of Felix and Biihler
that the vagina in ungulates is derived wholly from the Mullerian ducts. The
"hymen" limits the Mullerian contribution, and this structure is found some
distance from the opening of the vagina into the urogenital sinus.

                                CONCLUS IONS
    1. The development of the vagina has been investigated in the pig (Sus
scrofa). Three distinct regions have been recognised which differ in their
development. For convenience these have been called the upper, the middle,
and the lower vaginal segments.
    2. By the fusion of the Mullerian ducts in the genital cord there is formed a
simple epithelial tube which is the utero-vaginal canal. The lower part of this
tube is the anlage of the upper segment of the vagina. The wall of this tube
becomes folded during development so that a number of vertical ridges are
formed in this part. The epithelial lining commences to become stratified at the
lower part of this segment late in foetal life, and this change spreads very
gradually upwards. Even in a young sow, 35 cm. in length, the major portion
of the upper vaginal segment is lined with a single layer of columnar cells.
    3. The lower ends of the Mullerian ducts do not fuse with each other until
the foetus is about 17*5 cm. in length. Each duct tip is in contact with the
dorsal wall of the sinus, and each is solid through the proliferation of the con-
stituent epithelium. In this way two short Mullerian vaginal cords are formed.
These fuse with each other in the middle line and the resulting structure be-
comes canalised shortly before or at birth. This part of the vagina remains
narrow and forms a hymen in the vaginal canal. It has been termed the
middle vaginal segment.
                 Development of the Vagina in the Pig                                249
    4. The lower end of each Wolffian duct has a funnel-shaped opening into
the urogenital sinus. The posterior wall of this funnel (with that of the opposite
side) forms the dorsal wall of the urogenital sinus down to, and including, the
area where the Mullerian ducts are in contact with the sinus. The funnel
openings of the two Wolffian ducts are narrowed by the union of two side-
wall sinus folds. The result is the formation of a median Wolffian canal inter-
posed between the Mullerian ducts and the sinus. Disproportionate growth
between the Mfillerian ducts on the one hand and the Wolffian ducts and uro-
genital sinus on the other hand causes the tips of the Mullerian ducts to draw
out two Wolffian cords from the dorsal aspect of the median canal. These cords
fuse with one another in the middle line just later than the Mfillerian vaginal
cords, and the resulting structure is canalised before or at birth. The median
canal and the two cords derived from it form the lower vaginal segment into
which open the persisting Wolffian ducts.
    5. It is pointed out that the upper vaginal segment of the pig and other
lower mammals may not correspond at all with the upper part of the vagina in
higher forms. The late persistence of a columnar epithelial lining to this part is
also indicated.
    6. The hymen in this form is situated in the vagina. It marks off the
Mullerian element in the vagina from the Wolffian element lying below, and
thus has a similar situation to the hymen in the rabbit. Whether these hy-
meneal structures in lower mammals have the same developmental relation-
ships as the hymen in the human female is uncertain in the light of recent work
on the development of the human vagina. The hymen in the human would
seem to differ in that it is developed between the sinus below and whatever
structure or structures form the vagina above.

   It is with pleasure that I acknowledge the kindness of Prof. Walmsley in
providing facilities for, and supervising, this investigation. I have also to
thank Miss M. E. Rea for text-figs. 1, 2 and 5.

        (1) BAXTER, J. S. (1933). J. Anat. vol. LXVII, p. 555.
        (2) MIJSBERG, W. A. (1925). Z. fur Anat. Bd. LXXVII, S. 650.
        (3) FELIX, W. and BtHLER, A. (1906). "Urogenitalsystem." Hertwig's
                Handbuch der Entwicklungslehre der Wirbeltiere, Bd. III, Teil 1.
        (4) HENNEBERG, B. (1922). Z. fuir Anat. Bd. LXIII, S. 431.
        (5) WOOD-JONES, F. (1914). J. Anat. vol. XLVIII, p. 268.
        (6) TOURNEUX, F. (1888). Competes Rendus SOc. Biol. Paris, T. v.
        (7) RETTERER, ED. (1891). Comptes Rendus Soc. Biol. Paris, T. IlI, p. 313.
        (8) TOURNEUX, F. and LEGAY, CH. (1884). J. de l'Anat. T. xx, p. 330.
        (9) WEBER, M. (1927). Die Siugetiere, 2. Aufi., 1. Band, S. 352.
       (10) WILSON, K. M. (1926). Amer. J. Anat. vol. XXXVII, p. 417.
       (11) NXGEL, W. (1891). Arch. mikr. Anat. Bd. XXXVII.
       (12) MACCALLUM, J. B. (1902). Amer. J. Anat. vol. I, p. 255.
       (13) HARRIS, H. A. (1926). J. Anat. vol. LX, p. 329.
250                                   Jame8 S. Baxter
                              EXPLANATION OF PLATE I
Fig. 1. Transverse section of the lower part of the genital cord in an 8.2 cm. female pig foetus, x 86.
     (1) Dorso-lateral sinus bay; (2) fused Mullerian ducts.
Fig. 2. Transverse section of the lower part of the genital cord in a female foetus of 12-4 cm. C.R.
     length, x 86. (1) Wolffian duct; (2) Wolffian epithelium on anterior aspect of the right
     Mullerian vaginal cord; (3) Milllerian vaginal cord.
Fig. 3. A section 75 microns caudal to the preceding one at the same magnification. Lettering as
     it fig. 2.
Fig. 4. A section 75 microns further caudal in the same foetus showing the union of the Wolffian
     ducts to form the upper part of the median canal. A few Mflllerian cells are to be seen on the
     dorsal aspect of the fused ducts.
Fig. 5. Transverse section of the genital cord in the same foetus showing the opening of the fused
     Wolffian ducts into the urogenital sinus. (1) Urogenital sinus; (2) fused Wolffian ducts.
Fig. 6. Coronal section of the lower part of the genital cord in a female foetal pig of 17-5 cm. c.R.
     length, x 25. (1) Mullerian vagina; (2) Mullerian vaginal cord; (3) median canal formed by
     fusion of Wolffian ducts; (4) Wolffian duct. Owing to spiral twisting of the vagina around the
     urethra this section does not appear as if truly coronal.
Journal of Anatomy, Vol. LXVIII, Part 2       Plate I