1120 
Be this as it may, no evidence has as yet been brought forward 
for the homology of this muscle in placental and aplacental mam- 
malia, ie. of M. pyramidalis and M. marsupialis: the mere fact 
that the M. marsupialis of the aplacental mammalia occupies a place 
similar to that of the M. pyramidalis of tbe other mammalia, and 
that both are enclosed in the rectus-sheath, can hardly be deemed 
sufficient evidence for the homology. 
In this paper we publish the results of an inquiry into the 
validity of this homology-hy pothesis. 
The M. marsupialis generally arises from the medial border of 
the os marsupiale, the lower fibers of the muscle running more or 
less transversally, while those situated more proximally slope upward 
towards the linea alba. This muscle lies before the M. rectus, being 
separated from it only by a thin layer of loose connective tissue. 
However, it seems that the M. marsupialis does not always lie before 
the M. rectus, as W. Vrouk *) discovered that in Dendrolagus 
inustus the M. pyramidalis was covered by the right abdominal 
muscle. 
The other abdominal muscles of the marsupials run as follows: 
The M. obliquus externus always passes along in front of the M. 
rectus and in front of the M. marsupialis. The course of the M. 
obliquus internus, however, varies just as that of the M. transversus. 
A general view of it is given in the diagrams in Fig. 1, which 
| 
' i} 
i i i M. rectus 
} hii 
weer M. obl. ext. 
— M. obl. int. 
Sn M. transv. 
a. Petrogale. 
b. Didelphys. 
c. Phascologale. 
Belideus. 
=—= =—_=---- 
eene eene > = 
1) W. Vrouik. Ontleedkundige nasporingen omtrent Dendrolagus inustus. Kon, 
Akad. v. Wet. A’dam Afd. Nat. Deel V. 
