648 CIRCULATION OF THE BLOOD AND DORSAL I'J-ISSEL. 



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 [313, 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 tracheae, 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- 

 carclial 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 

 07 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 

 late-nil thoracic vessels, or of thoracic vessels given off from the 

 aortic trunk in its course through the thorax. 



