646 CIRCULATION OF THE BLOOD AND DORSAL VESSEL. 
The Structure of the Pericardial Septum (Pls. XLV., XLVI.).— 
Graber has investigated the structure of the pericardial septum 
in various insects, and describes it as consisting of muscle 
fibres, some of which are derived from the alar muscles, and of 
connective tissues, elastic and tendinous fibres. These tissues 
are intermixed with various forms of cells, and he regards all 
these—except the muscles of the ale—as modifications of the 
tissue which forms the fat bodies generally. It is, in point of 
fact, a septum developed like the fat bodies from the meso- 
blast. 
The elastic tissue of the pericardial septum is well developed 
in the Blow-fly, and consists of a membrane with well-marked 
fenestra. Whether these are to be regarded as openings 
between the interlacing fibres which bound them is, I think, 
very doubtful. I am rather inclined to think that it is similar 
to the fenestrated membrane from the artery of a Vertebrate 
and that the so-called fibres are thickenings of a continuous 
membrane. Graber, however, who has examined the peri- 
cardial membranes of large insects, regards them as actual 
perforations. In surface views it is very difficult to decide 
whether they are perforations or not, owing to the transparency 
of the membrane; in sections I have failed to discover any- 
thing but a continuous membrane. 
The edges of the septum are fringed by fat-cells ; numerous 
tracheal vessels and muscle fibres are attached to the inferior 
surface of the membrane, and in many places fat-cells are 
interspersed between them. The upper, or pericardial surface 
appears to me to be covered by a layer of endothelial cells. 
The elastic fenestrated septum does not apparently extend 
backwards so far as the muscle fibres, and these are attached 
to the wall of the posterior chamber of the dorsal vessel itself, 
especially on its ventral surface. 
DESCRIPTION OF PLATE XLVI. 
A portion of the pericardial septum of an adult Blow-fly, seen with an oil immer- 
sion yy. f,/, fat cells; ™, m, muscle fibres of alar muscle; 7, 7, nerve fibres ; 
nc, nerve cells ; 7, fenestrated pericardial membrane, 
