DORSAL VESSEL OF THE IMAGO OF THE BLOW-FLY. 647 



According to Graber, the alar muscles in most large insects 

 -Orthoptera, Coleoptera, Hymenoptera and Lepidoptera 

 are united with the heart itself, so that it is evident that they 

 cannot be separated from it by a continuous fenestrated layer, 

 as they are in the Blow-fly. In these insects the muscle-fibres 

 appear to lie in the substance of the fenestrated layer, and 

 probably perforate it obliquely. 



Graber, who has examined the structure and chemical rela- 

 tions of this membrane with great care, says it is not exactly 

 like any of the tissues of Vertebrates, but resembles most 

 closely the ligamentum pectinatum iridis. I regard it as a 

 cuticular membrane derived from the connective-tissue cells 

 of the septum, so that it is either continuous with the myo- 

 lemma of the muscle-fibres, or covers both their surfaces in the 

 median region of the septum. Graber also states that it is 

 sometimes continuous with the tunica adventitia of the heart. 

 So that it may be reflected over the dorsal vessel as a thin 

 subendothelial layer in some cases. 



The cells which are adherent to the pericardial septum are 

 termed by Graber ' pericardial ' cells. 



Pericardial Cells. I think it is evident from both the figures 

 and descriptions given by Graber, that he includes very dif- 

 ferent forms of cells under the term ' pericardial.' He says: 

 'In conjunction with R. Wagner, Leydig speaks of a special 

 cellular layer of the heart, which consists of large cells with 

 green or brown pigmented contents, and of a continuous trans- 

 parent ground or connective substance, in which the tendons 

 of the alar muscles are spread out and lost ; and Weismann 

 further states that the cellular layer is connected with the 

 heart by a loose muscular sheath in which the alar muscles are 

 imbedded.' Graber adds : ' My own observations lead me to 

 a view not consonant with the above, for the pericardial cells 

 form a single peculiar layer enclosing the space in which the 

 heart lies, and not infrequently have no connection either with 

 the heart itself or the alar muscles, but are continuous with the 

 connective layer of the hypodermis.' 



As there is no connective layer of the hypodermis, I suppose 



