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On the so-called interpleural opening in the opossum didelphys virginiana (Kerr).

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The obseisvations 1-epartedin tliis paper were niacle clwing
the study of t i n miclescyibeil stage of pouc~li young of thv
opossum, part of a prograin to complete infoi*matioii coimi‘iiing Didelpliys virginiana ( K e r r ) , nndcrtakcn by the Zoology
D e p y t m e n t of the University of Pennsylvania and The TVistar
Institute of Anatomy and Biology. The author is indebted to
The Wistar Institute for permission to study their slides,
and to Dr. Olin E. Nelsen for the loan of serial sections, and
for specimens of all ages for dissection.
Various authors, basing their opinioiis on reported morpliological peculiarities, have considered tliat the opossum differs
fro111 typical maiiimals, arid liave coiist ructed hypotheses t i s io
the origin of the Eutlicria, and us to the position of the
Narsupialia with respect to the Saul-opsida and the Eutheria.
One of the regions mentioned most frequently as being
atypical, is the thoracic region. I t has been reported (Jazuta,
’32 ; Hartman-Weinbcrg, ’24) that the pleural cavities in
Didelphys communicate with each other by means of the socalled ‘interpleural opening. ” I n this paper it will be demoiistrated that no such opening between the right and left pleural
cnvities exists in Didelpliys virginiana, and that the plenrtil
cavities of the opossum resemble markedly those of other
ri~arnnialsnot closely related to Ihc mai*supials.
As has been noted by various authors, the left lung of the
opossum has only one lobe. The right lung has in addition to
the usual three lobes, a fourth lobe, called the accessory or
infracardiac lobe. I t has been noted, too, that the right lung
overlaps the heart, and that the ventral part of the mediastinal
septum runs diagoiially towards the left of the median line
of the body; in fact, it joins tlic diaphragm to the left of tlic
pcricurditil cavity (fig. 2 A).
Several investigators have observed that the oesophagus is
long in Didelphys, especially in its ventral portion. I t s course
is diagonal, from dorsornedial in the cephalic portion of the
thorax t o more ventromcdial where it passes through the
diaphragm. As a result, there is dorsad to the oesophagus a
triangular space bounded rouglily by the oesophagus, aorta,
a i d diaphragm. Across this is stretched the thin mediastinal
septum, composed of R double layer of splanchnic pleura. It is
in this region that the interpleural opening is supposed to be.
I n dissections and scrial sections of embryos arid of 10- arid
15-day-old pouch young the author could find no interpleural
opening. I n all specimens the right and left pleural cavities
were completely separated (figs. 1, 2). Older stages were then
examined: pouch young of 50 days, young of 100, 109, 150,
and 185 days of age, and g number of young adults of unknown age. I n no case was there found to be any cornmunications between the right and left pleural cavities. I n all the
specimens studied, the thin mcdiastinal scpt um was intact
from the oesophagus to the aorta, and from the aortic arch
to the diaphragm.
One peculiarity of the thoracic region seems to have escaped
notice; that is, the infixcardiac lobe of the right lung often
lies in a separate cavity, a pocket iii the mediastinum. Thus,
especially in young opossums, tliere may he three pleural
spaces (figs. 1 and 2).
The pocket in which the accessory lobe lies is caudad and
ventrad to the heart. On the left siclo it is separated from the
left pleural cavity lip the ohliqnc septum nictliastinalis. A fold
of pleura scparates the main part of the right luiig from the
accessory lobe either coniplctely or partially. As this pleural
fold courses ventrad it covers the postcava and coritiiiueb
ventro-caudally to the diaphragm. This partition may be incomplete dorsally, between the postcava and the oesophagus,
or ventrally, where it may fail to join the body wall o r thc
pleural sac around the main part of the right lung. Caudoventrally the cavity lies between the cliaphra-gm and the pericardial sac. Thus, in a ventral clissectioii of the thoracic
Fig. 1 Diagrammatic drawing of tranvrersc scctioii through heart region of
15-day pouch young. A, mediastinum. B, plcural cavity of left lung. C, pocket
where the infraeardiae lobc lies. D, heart. Figure viewed looking caudad.
Fig. 2 Diagrammatic drawing of transversc section through region caudad
t o hcart of 15-day pouch young. A, ventral p a r t of mediastinal septum. B, infracardiac lobe of tlic right lung. C, fold of pleure forming p a r t of pocket i n which
the infracardiac lobe lies. E, postcava. I), p a r t of diaphragm. Figure viewed
looking caudad.
region, a coiiipartment niay sometiiiies be seen below the heart,
in which is visible the tip of the accessory lobe of the right lung.
The presence of an accessory lobc to the right lung lying
completely or partially in a pocket in tlie mcdiastinum is not
peculiar t o the opossum. 111 the rat the right luiig possesses
ail accessory lobe which is notched to perniit passage of the
postcava. This lobe is called the postcaval lobe. As in the
opossum, the left lung is not lobed and the mediastinal septum,
ventrally, is diverted to the left of the pericardial sac. The
greater part of the postcavd lobc lies caudad to the heart in
a membranous pocket of the niediastinum, closed completcly
tlic loft pleural cavity but open into tlic right pleural
cavity dorsad to the postcava.
In tlic cat t1iet.c is an accessor? niedian lobc of the right lung,
called the mediastinal lobe. It is divided into two alriiost equal
parts, the postcava passing bctween then). Thc left half of
the mediastinal lobe lies in a pocket in the niediastinum dorsnd
aiid caudad to the heart. This pocket is closed completely
from the left pleural cavity. As in the opossum, the membrane separating tlic pocket from the right pleural cavity is
open between the postcava arid thc ocsophagus.
hInch of the theoretical discussion as to the origiii aiicl evolution of the mamnials is based on reported anatoinical peculiaritics in the marsupials. In rccent literature (IlcCrady, '38)
one finds the following:
Jazuta ('32) observed tliat tlie right a d left pleural cavities
of the opossum coniiriunicate with cach otlicr in the dorsomedial linc. This interpleural opening has bceii found in
I)ii(clpliys, Pcrameles, and various birds. !"is
Jaxuta regards as sti*oiig cvitlencc that the rnai*supials
represent a connecting link bctwccn the Euthcria and tlic
. .
In tlie article cited by hlcCraciy, Jazuta writes:
Hinter Clem Herzbeutel weisen dic Rrustfclle gaiiz bezonderc
Verhaltnisse auf, und zwar : beginnend von 4 Brustwirbel bis
zuni 13 liaben beide mediastinal-pleurae eine gcmeinsamc,liingsgcstrecktc 18 mrn. lang 6ffnung, die durch folgende Gcbilclc
begrenzt ist : Vorne durch das Rrustfell, das die Spciscrolirc
bekleidet, hiiitcii durch das dic Aorte bedeckelide Bt*ustfell,
oben durdi den Aortwbogen, d e r sicli von €Icrxeii riickwiirts
zur Miirbelsaiilc biegt, uiitcii dui.cli ciiic sicltelformige serosc
Falte, die von der tlbsteigcriden Aorte zuiii Zmcrclifell geht.
Die Entstehung dieser Offnung kanii man sic11in solchen MTeise
vorstelten, dasz die beiden Ei-ustfclle zuerst zwischen der
Aorta und der Speiserolire verwachsen maren, dann entstand
aber 15ings ihrer groszten Strecke eine Koniiiiunikation
zwischen beiden Pleurahohlen.
Jazuta refers t o an earlier paper by Hartnian-T~reiiiberg,
describing an opening between the pleural cavities in several
species of iiiarsupials. He gives tlie riamc ' ' intcrpleural opeiiing" to this, stating
Bei Didelpliys marsupialis uiid Peramclcs Garragussi
komiiiunizieren die Pleurahohlen vermittelst einer besondern
Oeffnung in der Pleura inediastinalis,. die als Interpleural
Offnung bczcichnet werden lrann. na bcigenannten Tieren dic
rechte und linke mediastirialpleura unter Bildung einer
Septum mediastinale mitcinandcr rcrwacliscii, flicssen dict
hiedcn Plcuraloffnungen elmifalls zu ciilem eiiizigeii niil
ciiiaridcr zusammen.
On the basis of these observalioiis it lias been acceptcd that
the pleural cavities of the opossum ciiK'w froiii the typical
closed pleural cavity of Jlammalia.
None of the speciiiieiis exaiiiined during the studies rcportccl
in this paper showed a lengthwise split in the septum me&astirialis connecting tlie two pleural cavities.
It is evident from these studies that such ail orxiling clocs
not exist. The pleural cavities of Didelpliys \-ii*giniaiiaN re iiot
atypical, but conforin t o tlic mammaliaii type of closctl 1)lculal
Iforeover, there is a striking similarity amongst tlii*cv i q m sentatives of widely separated groups of mamiiials in tlic
possession of an accessory lobe to the right lung. The opossum,
the rat, and the cat have such an accessory lobe lying in tlic
median thoracic region dorsad and caudad to tlic pericardinl
sac. TI] each of these animals this lobc is honscd complc!tc~lyo r
1)artially in a special pocket in the niediastiiiuiii, septrutecl
froiii the left pleural cavity by the ventral part of the scptuiii
mediastinalis, communicating with the right pleural cavity by
an opening in the pleural fold dorsad to the postcava.
Careful comparative studies of other reported anatomical
peculiarities of the opossum need to be niade.
1. No so-called “interplcural” openings were found in
pouch young nor in older stages of Didelpliys virginiana.
2. The infracardiac lobe of the riglit lung lies in a special
cavity in the mediastinum.
3. There is a striking siniilarity between the opossum, the
rat, and the cat in the possession of an accessory lobe to the
right lung, and in the presence of a special cavity in thc
mediastinurn in which it lies.
I I A R T ~ I A N - W ~ I N A.
~ ~ : ~1924
~ ~ ; , ~ I l t c ~ ~ l J ~ ~ ~ i ~ ~ und
l ~ f f nLuftsackc
bpi Marsupialiern. Cotnpt. Belid. de l ’Acadcinic des Srirnees de Hussie 1924,
pp. 26-29.
KONSTANTIN1932 13aubesonderlieiten der Plcur:ihOl~lei~bciui Opossuni.
Anat. A m . , vol. 73, pp. 375-380.
MCCLURE,C. F. W. 1902 The anatomy and developincnt of the posterior wnx
cava in Didelphis virginiana. B i d . Bull., vol. 3, pp. 333-335.
JR. 1938 Embryology of the Opossuiu. American hnatonlical
Memoirs, No. 16.
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virginia, didelphys, interpleural, called, opening, opossum, kerr
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