648 CIRCULATION OF THE BLOOD AND DORSAL VESSEL. 
Graber means the subcutaneous adipose tissue of the insect ; 
and there is no doubt most of the cells are fat-cells in various 
stages of development. 
There are, however, other forms of cells in the pericardium, 
large stellate cells of the mesoblast connected with the tracheal 
vessels and with the muscular tissue. The processes of the 
latter are frequently seen to be striated muscle-fibres, and 
these cells, which are figured by Graber (813, Fig. 13], are 
precisely similar to the muscle-cells of the peritoneal coat of 
the ovary; they are probably the form of tissue which Weis- 
mann described as an intervisceral muscular network. These 
and the endothelioid plates on the surfaces of the pericardium, 
the cells of the tracheze, and the various forms of fat-cells are 
probably all modifications of the cellular mesoblast—parablast 
cells. Lastly, there are large pigmented branching ganglion 
cells and nerve-fibres, which will be further described 
hereafter. 
I am quite unable to discover any special pericardial cells 
containing ‘respiratory pigment’ such as Graber supposes ; 
the pigmented cells are either large ganglion cells, or fat bodies 
which contain bright green or red pigment. Cells of this kind 
are seen in many Lepidoptera, but when they occur the fat 
bodies elsewhere are similarly pigmented. 
The Aorta, or anterior part of the dorsal vessel, extends from 
the mesophragma to the posterior surface of the brain; it is 
cylindrical, not divided into chambers, and without any peri- 
cardial investment; it bifurcates into two lateral branches in 
the head, and each of these apparently ends in several branches 
which are distributed over the brain. Its average diameter is 
‘o7 mm. There are possibly two other vessels given off from 
the bulb, one on either side, which accompany the anterior 
venous sinuses of the pericardial septum and enter the thorax. 
It is, however, very difficult to trace such vessels, and until 
some means of injecting the dorsal vessel has been devised it 
would be rash either to assert or deny the existence of these 
lateral thoracic vessels, or of thoracic vessels given off from the 
aortic trunk in its course through the thorax. 
