1122 
GEGENBAUR ‘) writes of the M. pyramidalis — also of that of 
the Monotremata and of the Marsupials: — “allein seine Lage nicht 
nur, sondern vielmehr sein Anschlusz an den Rectus so wie der 
Einschlusz in eine mit dem Rectus gemeinsame Scheide macht seine 
Entstehung aus dem Rectus wahrscheinlich und verweist auf die 
Thatsache, dasz bereits bei Amphibien mehrfache Rectus-bildungen 
vorkommen, von welchen die oberflächliche der Metamerie entbehrt, 
gleich dem Pyramidalis der Säugethiere, welcher auch nicht mit 
Unrecht als vorderer Rectus unterscheiden ward”. Also in Errmorr 
Cours’s inquiry, quoted above, we find a differentiation between a 
M. rectus externus and internus. This view is not plausible as 
regards the M. marsupialis. As appears from the diagram of the 
abdominal wall of Petrogale (fig. 1a) a leaf of the aponeurosis of 
the M. transversus passes between M. rectus and M. marsupialis. 
This renders it highly improbable that the M. marsupialis should 
arise from the M. rectus. The diagrams of Phascologale and Belideus 
(fig. 1c) also show that the M. marsupialis is not exactly invested 
by the rectus-sheath — which according to GEGENBAUR would speak 
for its arising from the M. rectus — but that it rather constitutes 
a part of the sheath itself. The M. obliquus internus does not play 
a part in the formation of the frontal leaf of the vagina muse. 
recti, owing to its insertion into the lateral border of the Os marsu- 
piale; now it is just the M. marsupialis which completes that same 
part of the rectus-sheath. Again, the fibres of the inferior part of 
the M.. marsupialis run at a right angle to the M. rectus which, 
though this is not a cogent proof of the reverse, renders it by no 
means plausible they should arise from the M. rectus. 
The fibre-course of the M. marsupialis corresponds much more 
to that of the M. obliquus internus. The fact that the M. marsu- 
pialis completes, as it were, the M. obliquus internus, and the location 
of this muscle between the M. obliquus externus and M. transversus 
— which appears above all in Petrogale (fig. 1a) — renders it more 
probable that the M. marsupialis is a part of the M. obliquus internus. 
This conception helps us to realise the condition described by Vrouik 
that the M. rectus overlies the M. pyramidalis of Dendrolagus inustus, 
such being not at all extraordinary for a part of the M. obliquus 
internus ”). | 
This has been clearly demonstrated in a Didelphys marsupialis (fig. 2). 
1) GEGENBAUER. Vergleichende Anatomie, I. 1898 p. 664. 
*) For the course of the abdominal muscles relative to the M. rectus, we refer 
to W. A. Mussera: “Over den bouw van den musculeuzen buikwand der Primaten.” 
Kon. Akad. v. Wet A'dam 14 Mei 1915. 
