646 CIRCULATION OF THE BLOOD AXD DORSAL VESSEL. 



The Structure of the Pericardial Septum (Pis. XLY., XLVI.V- 

 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 alae 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 

 fenestrae. 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. 



'TIUN OK Pl.ATK XLVI. 



A portion of the pericardial septum of an adult Blow-fly, seen with an oil immer- 

 :i i'-j- ft ft f at celK ; ///, in, muscle lilires of alar muscle; >/, , nerve fibres; 

 HI-, nerve cells ; /, fenestraled pericardial membrane. 



